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Substrate-Determined Shape of Free Surface Profiles in Spin-Cast Polymer Blend Films
Abstract
Sectional views of thin films of symmetric polystyrene/polyisoprene (PS/PI) blends spin-cast from toluene (C6H5CH3) onto CH3- and COOH-terminated self-assembled monolayers (CH3−SAM and COOH−SAM) show concave- (sharp-edged) and convex-shaped (round) protrusions, respectively, while other morphology features are identical. A 3-dimensional phase domain arrangement was determined with spectroscopic techniques (profiling and mapping mode of dynamic secondary ion mass spectrometry, dSIMS, and X-ray photoelectron spectroscopy). Surface topography was examined with atomic force microscopy and monitored, during the dSIMS analysis, with secondary electrons. In addition, solvent evaporation from PS, PI, and PS/PI layers cast on CH3−SAM and COOH−SAM was determined. The collected data were used to put forward a model of morphology formation and to elucidate the role of evaporation speed dependent on substrate chemistry in this process, demonstrated here for the first time.
... Another novel approach involves lithography without the prepatterned master [19][20][21][22][23][24][25][26][27]. Thin polymer films are used instead as etching or deposition masks with dnaturalT structural patterns formed by blend demixing [19,20], microphase separation [19,[21][22][23][24][25], or micellar self-organization [26,27]. ...
... Another novel approach involves lithography without the prepatterned master [19][20][21][22][23][24][25][26][27]. Thin polymer films are used instead as etching or deposition masks with dnaturalT structural patterns formed by blend demixing [19,20], microphase separation [19,[21][22][23][24][25], or micellar self-organization [26,27]. Pattern shape and (nanometer to micrometer) length scale are tuned by polymer molecular weight, chemical structure, molecular architecture, and composition [28,29]. ...
... In this paper, we present an extension of our earlier work [38], allowing, for the first time, to compare quantitatively surface patterns prior and after the transposition using a complete set of morphological measures. The integral geometry approach is demonstrated here for characteristic pattern types (island-, hole-dominated, and nearly bicontinuous) formed by spin-cast polymer blend films [20,38,40] and replicated into a substrate by ion milling. Three Minkowski measures ( F, U=2pv/K, v) plotted as a function of height characterize the patterns prior and after the transposition and specify their vertical extent Dh and characteristic lateral measures ( F, U, v). ...
An extension of the integral geometry approach, proposed to characterize pattern replication, was used to compare quantitatively the surface pattern of spin-cast polymer blend films with that replicated in their substrate by ion milling. The original and replicated patterns, recorded with atomic force microscopy (AFM) and analyzed in terms of the Minkowski measures plotted as a function of height, were characterized by the vertical extent and lateral morphological measures. Good, moderate, and far from ideal replication was discussed for various types (island-, hole-dominated, and bicontinuous, respectively) of the original pattern, formed by binary (deuterated) polystyrene mixtures with polyisoprene and poly(n-butyl methacrylate).
... Such morphology is commonly observed when there is a small pressure difference across the surface skin of the phase-separated domains caused by a slow solvent evaporation, resulting in the collapse of top surface. [46] In contrast, when the Ormostamp phase exceeds 75% in weight-ratio, an OrmoStamp matrix with nanoholes is observed instead of OrmoStamp nanopillars, as shown in Figure S3, Supporting Information. The change of the dispersed phase from OrmoStamp-rich to PS-rich, with change in blend composition, is known as phaseinversion. ...
In nanoimprint lithography (NIL), the imprinting stamp's fabrication is still a significant cost factor among the consumables. Bottom‐up lithography approaches based on a phase‐separation of polymer blends can provide a cost‐effective route for fabricating these stamps. Today's polymers used to prepare phase‐separated nanostructures (PSN), however, exhibit low glass transition temperatures. As a result, the PSN are prone to in‐plane stamp distortions in the presence of high imprinting pressure and temperature, limiting their practical relevance for NIL. Here, the realization of mechanically and thermally stable PSN‐based imprinting stamps for NIL systems via a phase‐separation of a homopolymer/inorganic–organic hybrid polymer blend is reported. It is demonstrated that these imprinting stamps are easily tunable and scalable by adjusting the formulation of homopolymer/hybrid polymer mixture and deposition conditions. Feature sizes in PSN ranging from a few μm down to 100 nm are achieved through an interplay of these factors. As demonstrations of the envisioned applications, the developed imprinting stamps are integrated into a roll‐to‐roll NIL system for patterning a polystyrene thin‐film. Moreover, light management is demonstrated by nanopatterning of a perovskite semiconductor in plate‐to‐plate process. The nanopatterned perovskite film achieves an integrated absorption and a photoluminescence emission peak increase of 7%rel and 121%rel, respectively.
... Based on the known general notions about the hardening process, it is not always possible to predict the surface composition and properties, therefore experimental diagnostics of the surface using XPS is widely used for these purposes. [1][2][3][4][5][6][7][8][9][10][11][12] The goal of the present work is to study the development of coatings with improved antifriction properties on the surface of model substrates such as mica, glass, and crystalline silicon. To accomplish this work, we studied the system epoxy resin-siloxane modifi er, amphiphilic in relation to epoxy resin. ...
New star-like polydimethylsiloxanes with a cis-tetraphenylcyclotetrasilsesquioxane fragment as the core (branching center) were synthesized and characterized. The thermophysical properties of the obtained polymers were studied by DSC and TGA. The epoxy resin surface modified with the obtained polymers was analyzed by XPS.
... Phase separation in thin polymer films, initiated by evaporating solvent during spin-coating process, may lead to formation of lateral phase-domain structure. This structure is very often reflected by surface topography as a consequence of different solidification rates of different phases, varied with solubility parameter [78] or glass temperature [79,80]. Moreover, the shape and dimensions of resulting morphology may be controlled by blend composition and total polymer concentration in solution, respectively [64,81], as presented in Fig. 4. ...
Systematical studies on the impact of the thickness of thin films composed of polystyrene (PS) or poly(ethylene oxide) (PEO) on the effective elasticity of polymer-decorated soft polydimethylsiloxane substrate were performed. For both investigated polymer films, elasticity parameter was determined from force-displacement curves recorded using atomic force microscopy. Effective stiffness of supported film grows monotonically with film thickness, starting from the value comparable to the elasticity of soft support and reaching plateau for polymer layers thicker than 200 nm. In contrary, for films cast on hard support no significant thickness dependence of elasticity was observed and the value of elasticity parameter was similar to the one of the substrate. Based on these results, non-conventional method to produce elasticity patterns of various shapes and dimensions induced by phase-separation process in symmetric and asymmetric PS:PEO blend films on soft support was demonstrated. Elevated PS domains were characterized by elasticity parameter 2 times higher than lower PEO matrix. In contrary, adhesion force was increased more than 3 times for PEO regions, as compared to PS areas.
... When a blend of immiscible homopolymers is deposited from a solution of the polymers in a common solvent, the morphology of the film after drying results from the superposition of complex phenomena such as phase separation process, dewetting and vitrification of the system. Depending on the evaporation rate, the phase separation and the dewetting can be stopped into more or less advanced stages [1][2][3][4][5][6][7][8]. This provides a wide variety of thin film morphologies including co-continuous or dis-continuous laterally phase-separated morphologies as well as layered systems. ...
In this work, smooth polymer films of PS, PLA and their blends, with thicknesses ranging from 20 nm up to 400 nm and very few defects on the surface were obtained by dip-coating. In contrast to the process of spin-coating which is conventionally used to prepare thin films of polymer blends, we showed that depending on the deposition parameters (withdrawal speed and geometry of the reservoir), various morphologies such as layered films and laterally phase-separated domains could be formed for a given blend/solvent pair, offering much more opportunities compared to the spin-coating process. This diversity of morphologies was explained by considering the superposition of different phenomena such as phase separation process, dewetting and vitrification in which parameters such as the drying time, the compatibility of the polymer/solvent pairs and the affinity of the polymer towards the interfaces were suspected to play a significant role. For that purpose, the process of dip-coating was examined within the capillary and the draining regimes (for low and high withdrawal speed respectively) in order to get a full description of the thickness variation and evaporation rate as a function of the deposition parameters.
... This kind of vertical phase separation can be used to fabricate sharpedged and round protrusions in polymer thin lms from polymer blend mixtures. 31 Aer solidication of amylose, PDP is still swollen in DMSO and further evaporation of solvent initiated interfacial instabilities between the amylose/PDP-rich phase (or PDP/DMSO) and lm/air interface, that led to a break up of the bilayer conguration and induced a secondary phase separation. 30 A continuous ow of nitrogen was used to create an up-and down ow motion of the uid which generated a temperature gradient between the upper (PDP-rich) and lower layer (amylose-rich) of the uid. ...
Carbon microrings were produced using a template based on phase separation of amylose / pentadecyl phenol (PDP) / dimethyl sulfoxide (DMSO) mixtures. The two-step phase separation of the mixture showed a micron-sized porous structure in which the rim of the pores was enclosed by rings of PDP. PDP was converted into a carbon precursor resin via reaction with formaldehyde vapor followed by pyrolysis under nitrogen atmosphere, leading to a carbon microstructure with ring morphology. The products before and after pyrolysis were characterized by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and Energy-dispersive X-ray spectroscopy (EDX). AFM and SEM confirmed the ring morphology of the samples obtained after pyrolysis. The presence and phase of carbon were confirmed by EDX and XRD, respectively. This study demonstrates a straightforward novel route to prepare carbon microrings from a phenolic precursor using amylose as a matrix material.
... It has been revealed that surface morphology and properties of the polymeric films depend on the molecular interactions with the substrate, on which the polymer is deposited and may significantly differ from those in the bulk [7][8][9][10][11]. However, surface morphology and adhesive properties of spincoated PMMA on different substrates not yet studied extensively. ...
In this work, the polymethyl methacrylate (PMMA) spin-coated on polyethylene terephthalate (PET), soda-lime-silica float glass and polymethyl methacrylate (PMMA2) acrylic substrates has been studied. For this purpose, atomic force microscopy (AFM) have been carried out to evaluate the PMMA films surface morphology and adhesive properties. It has been confirmed, by AFM, that the morphology of PMMA depends on the nature of the substrate: PMMA formed on PET exhibited highly asymmetric surface with densely distributed peaks and valleys; PMMA formed on glass exhibited relatively even surface with minor distributed local islands; PMMA formed on PMMA2 exhibited smooth surface with deep valleys. Adhesion forces measured by means of AFM force-distance curves revealed non characteristic adhesive behaviour of PMMA films on glass substrates, while PMMA films on PMMA2 substrates resulted in increased adhesive force due to presence of “stiction” phenomenon as considered from AFM study.
... For the same kind of solvent (e.g., good solvent for one component), the lower vapor pressure is, the longer the time to reach polymer solidification, and therefore the closer to the thermodynamic equilibrium state of phase separation process is [40][41][42]. Substrate dependent solvent evaporation can cause the formation of protrusions with either concave or convex topography, as shown in Fig. 3b and c, respectively [67]. Fast evaporation of solvent toluene from a developing film a on self-assembled monolayer (SAM) with COOH end-groups (COOH-SAM) substrate produces a positive pressure-difference p across the surface skin of a PSrich phase domain, resulting in the formation of convex . ...
... Unannealed samples represent more practical situations, because annealing is a further processing step, which may well have only a limited effect on highly immiscible films. There have been a few studies of the structure of spin-cast polymer blends [6,13,[26][27][28][29][30], but these have been largely qualitative, although more quantitative studies are now appearing [31,32]. A simple approach to understanding the mechanism of spin-casting is to consider it in several stages, the first of which is the removal of excess material (polymer and solvent) during spinning. ...
We have used helium-3 nuclear reaction analysis to study the morphology of spin-cast blends of
poly(9,-dioctylfluorene) (F8) and
poly(9,-dioctylfluorene-alt-benzothiadiazole) as a function of composition, casting solvent, and
initial solvent concentration. In blends cast from toluene, a surface segregated layer is
observed for a broad range of F8 compositions. The surface excess has a maximum of
nm at an F8 volume fraction = 0.5, dropping down to –2 nm for = 0.2
and 0.8. The existence of such surface segregated layers could play an important role in
charge transport in optoelectronic devices made from blends of conjugated polymers. Films
cast from chloroform show negligible surface segregation. Scanning probe microscopy
experiments show that films cast from toluene exhibit significant in-plane structure, in
contrast to those cast from chloroform, which show minimal lateral structure.
In this study, we blended two readily available polymers, polydimethylsiloxane (PDMS), a semi-crystalline polymer, and polystyrene (PS), an amorphous polymer, both having widely varying physical properties. The blend is then spin coated to form a thin film. We investigated the effects of relative polymer concentration, spin coating speed, and environmental factors, such as temperature, on the ultimate morphologies of the phase-separated thin films. It was found that it is possible to regulate the morphologies of the thin films to achieve desirable microstructures such as spherical droplets, holes, bi-continuous lamellar structures, and tubules by controlling the fabrication conditions. The polymer blend films with higher PS concentrations were shown to form a bilayer system with an upper PS-rich layer due to the thermodynamic instability of the film caused by the rapid evaporation of solvent, while films with higher PDMS concentrations exhibited cohesive forces that engendered microtubule formation and led to high surface roughness.
The morphology, phase separation, instability and dewetting of Polymer dispersed liquid crystal (PDLC) thin film comprising of nematic liquid crystal, 4-n-pentyl-4ʹ-cyanobiphenyl (5CB) and Polystyrene (PS) is reported in this paper. Thin film of PS-5CB PDLC is spin coated on flat and topographically patterned silica substrates from a common solvent (toluene). Films with two different thickness h ≈ 35 nm and h ≈ 85 nm are used in the study. The composition of PDLC is chosen as RB (PS: 5CB) = 1:2 and 1:1. On flat substrate, the thinner films (h ≈ 35 nm) dewet at room temperature (T ≈ 25 °C) over time, forming isolated droplets for both RB. In contrast, the h ≈ 85 nm thick films are stable at room temperature though they exhibit gradual phase evolution of the 5CB domains with time. These films dewet only when they are subject to thermal annealing at T ≈ 70 °C right after spin coating and results in an isotropic collection of nearly equal sized droplets, each of which comprises of an inner polymer core surrounded by a 5CB ring. Finally, we show that the instability patterns as well as the phase separated LC domains can be aligned by casting the films on topographically patterned substrate. While the 35 nm thin film results in spin dewetted array of aligned droplets along the substrates grooves, the 85 nm thick film remains continuous and undergoes progressive phase evolution, resulting in alternating domain of phase separated LC (within the grooves) and PS domains align along the contours of the substrate patterns.
The effects of confinement on the phase separation behavior of polymer-polymer mixtures have been frequently studied in morphologies such as thin films and rods, but little research exists with respect to the nanoscale droplet size regime. This paper addresses the phase separation of water-soluble polymers in submicron aerosol droplets. Atomized aerosol particles were prepared from aqueous solutions and dried using diffusion dryers. For poly(ethylene) glycol/dextran and poly(vinyl alcohol)/poly(4-styrene sulfonic acid) systems, small particles remain homogenous while larger particles undergo phase separation within a single particle. As the molecular weight of the polymers increases while maintaining a constant ratio between monomers of polymer A and polymer B, phase separation occurs in smaller diameter particles. These trends are modeled using a combination of equations describing the nucleation of a new phase and Flory-Huggins theory and provide qualitative agreement. These results provide insight into the phase separation of aqueous nanoscale polymer-polymer systems. Potential exists to make new polymer materials with unique properties due to the mixing of polymer combinations that normally undergo phase separation.
Substrate pattern guided self-organization of ultrathin and confined polymeric films on topographically patterned substrate is an useful approach for obtaining ordered meso and nano structures over large areas, particularly if the ordering is achieved during film preparation itself, eliminating any post processing such as thermal or solvent vapor annealing. By casting a dilute solution of two immiscible polymers Polystyrene (PS) and Polymethylmethacrylate (PMMA) from a common solvent (Toluene) on a topographically patterned substrate with grating geometry, we show the formation of self organized meso patterns with various degrees of ordering. The morphology depends on both the concentration of the dispensed solution (Cn) and blend composition (RB). Depending on the extent of dewetting during spin coating, the final morphologies can be classified into three distinct categories. At very low Cn the solution dewets fully, resulting in isolated polymer droplets aligned along substrate grooves (Type 1). Type 2 structures comprsising isolated threads with aligned phase separated domains along each substrate groove is observed at intermediate Cn. Continuous film (Type 3) is obtained above a critical concentration (Cn*) that depends on RB. While the extent of ordering of the domains gradually diminish with increase in film thickness for Type 3 patterns, the size of the domains remain much smaller than that on a flat substrate, resulting in significant downsizing of the features due to the lateral confinement imposed on the phase separation process by the topographic patterns. We finally show that some of these structures exhibit excellent broad band anti reflection (AR) properties.
We demonstrate a simple and robust procedure for the fabrication of rhodamine-doped nanoporous poly(methyl methacrylate) (PMMA) films, whose optical properties, such as anti-reflection, fluorescence and absorption can be tailored to specific applications. By exploiting phase separation of a binary polymer blend (PMMA and polystyrene), we fabricated foam-like nanoporous films that could be easily and cost-effectively integrated into the fabrication process of optical components. We link film morphology, studied by multifrequency atomic force microscopy (AFM), to the effective refractive index of the films for use as anti-reflection coatings. The film's morphology leads to superior broadband anti-reflection performance compared with homogeneous films. For applications involving optical filters and spectral conversion layers (e.g., for photovoltaic applications), we doped the films with the fluorescent molecule rhodamine, whereby simple variations in the fabrication process enabled us to prepare rhodamine-doped nanoporous PMMA with tunable fluorescence and absorption, without losing the anti-reflective properties. The above combination of optical properties makes the films attractive for a wide range of applications.
The occurrence of vertical phase separation has been reported for various spin-cast polymer films, including bulk-heterojunction films of polymer solar cells (PSCs). Focusing on real-space analysis, we conducted a study on the relationship between the morphology and processing conditions of PSCs for typical poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) cells. Our results demonstrated that spin-casting caused a localized reduction in the P3HT concentration in the bulk center. Thermal annealing after cathode formation enhanced the unevenness in concentration and created a multilayered vertical phase-separated morphology in which the P3HT domains were gathered near the electrodes, leaving only PCBM domains at the center of the film. Cells with this morphology had good power conversion efficiency (similar to 3%). (C) 2016 The Japan Society of Applied Physics
Dewetting pathways, kinetics and morphologies of thin films of phase separating polymer blends are governed by the relative mobilities of the two components.
We characterize the morphological transformations of the nanostructures of a PS/PMMA blend by annealing in toluene and chloroform vapors. Toluene leads to faster reorganization of PS, whereas chloroform engenders the opposite effect.
Spin coating produces a very rough PMMA rich layer that completely wets the substrate and forms a plethora of slender columns protruding through the continuous PS rich layer on top. The nanostructures were stable under long thermal annealing but in the vapor annealing, phase separation and dewetting occurred readily to form the equilibrium structures of dewetted droplets of PS on top of PMMA which also climbed around the PS droplets to form rims. Toluene and chloroform annealing required around 50h and 1h respectively to attain the equilibrium. Substantial differences are observed in the intermediate morphologies (heights of nanostructures, roughness and size). PMMA columns remained embedded in the dewetted PS droplets, whereas a high mobility of PMMA in chloroform allowed its rapid evacuation during dewetting to produce an intermediate swiss-cheese like morphology of PS domains.
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Distribution of polyaniline doped by camphorsulfonic acid, blended with polystyrene of different molecular weight and spin-cast from chloroform into (50–400 nm thick) films, was examined using secondary ion mass spectrometry (SIMS). Strong polyaniline segregation to the Au substrate was observed to be accompanied by depletion region or, for thicker films, by concentration oscillations. Segregation and substrate-directed phase separation, effective during solvent removal, form lamellar film structures.
Surface structures and compositions of poly(Styrene-block-Ethylene/Butylene-block-Styrene) (SEBS)/Poly(Methyl Methacrylate) (PMMA) blend films have been studied by Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). Substrates with different hydrophobicity and SEBS with and without Maleic Anhydride (MA) grafting were used to study the effect of polymer-substrate interactions. It is indicated that the surface energy of the substrate (substrate/air) plays a crucial role on the surface composition of the polymer component. For a fixed surface, the adsorption of polymer on the substrate is also important. The hydrophilic sites of SEBS-g-MA can prevent the dewetting of the SEBS-g-MA from the substrate. The dewetting of PMMA from the SEBS-g-MA will make the PMMA protrusions more pronounced, and the SEBS-g-MA phase domains are enlarged after annealing treatment. An adsorption scheme is suggested to explain the phase inversion and height difference observed in the various polymers used. In addition, SEBS triblock copolymers form wormlike and meshlike microphase separation morphologies on the hydrophilic and hydrophobic substrates, respectively.
A solution-processable interfacial bilayer is found to be able to significantly increase a short-circuit current and thus the overall performance of bulk heterojunction organic photovoltaic cells. The interfacial bilayer is comprised of surface-modified single-layer graphene oxide and a donor polymer. The use of the interfacial bilayer can minimize exciton quenching and inverse current and does not alter the energy level of photovoltaic devices, which is also applicable to other organic optoelectronic diode devices.
Interfacial materials play an important role in determining the efficiency of an organic photovoltaic (OPV) cell. They are not only responsible for establishing ohmic contact, but also determining different device parameters such as the internal electric field, the film morphology, and the carrier recombination rate which are important to the device performance. Here, we will present the material properties and requirements for these interlayers used in high efficiency OPV cells. This paper aims to reveal the different roles of interlayers, introduce techniques for characterizing their properties, and provide an insight into the future development of novel interlayers for high efficiency organic photovoltaic cells.
Pattern replication in solution-deposited thin films of
insulating and conjugated polymer mixtures might provide an
alternative to spatially resolved printing techniques for the fabrication
of polymer-based circuitries. Though it has been previously
shown that phase separation in the course of spin-casting
leads to the formation of domain structures resembling the
chemical patterns pre-set on the film substrate, finding optimal
casting conditions is a tedious process, which requires multiple
sample preparations. Here, we have demonstrated pattern replication
in a mixture of poly(3,3000-didodecyl quarter thiophene)
(PQT-12) and deuterated poly(styrene-co24-bromostyrene)
(dPBrS) deposited by horizontal-dipping on substrates, patterned
with self-assembling molecules by micro-contact printing.
Moreover, we show that casting conditions for
accomplishing pattern replication can be efficiently screened by
preparing thickness gradient samples. We have optimized the
reconstruction of the substrate pattern in the PQT-12:dPBrS
film. Our results prove that desired structures of semiconducting
and insulating polymers can be produced in a simple, highthroughput
technological process.
Spin-coated thin films of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (APFO-3) blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are used as the active material in polymer photovoltaic cells. Such blends are known for their tendency to phase separate during film formation. Tuning the morphology of the blend in a controlled way is one possible road towards higher efficiency. We studied the effect of adding chlorobenzene to chloroform-based blend solutions before spin-coating on the conversion efficiency of APFO-3:PCBM photodiodes, and related that to the lateral and vertical morphology of thin films of the blend. The lateral morphology is imaged by atomic force microscopy (AFM) and the vertical compositional profile is obtained by dynamic secondary ion mass spectrometry (SIMS). The profiles reveal compositional variations consisting of multilayers of alternating polymer-rich and PCBM-rich domains in the blend film spin-coated from chloroform. The vertical compositional variations are caused by surface-directed spinodal waves and are frozen in during the rapid evaporation of a highly volatile solvent. With addition of the low-vapour pressure solvent chlorobenzene, a more homogeneous vertical composition is found. The conversion efficiency for solar cells of this blend was found to be optimal for chloroform:chlorobenzene mixtures with a volume-ratio of 80:1. We have also investigated the role of the substrate on the morphology. We found that blend films spin-coated from chloroform solutions on PEDOT:PSS-coated ITO show a similar compositional structure as the films on silicon, and that changing the substrate from silicon to gold only affects the vertical phase separation in a region close to the substrate interface.
Microphase and macrophase separation phenomena can simultaneously appear in ABA/C copolymer blend systems due to the immiscibility among monomers A, B, and C. In this work, the surface morphologies and compositions of ABA/C blend thin films confined between two walls, which were used to mimic SEBS/PMMA films, have been simulated by a lattice Monte Carlo (MC) method. The effect of the polymer-wall interaction on the surface morphologies and compositions of thin films was investigated as a function of blend composition and film thickness. It is shown that the simulated surface morphologies of thin films resulting from the macrophase separation between copolymer ABA and homopolymer C and the microphase separation between block A and block B in ABA copolymer are similar to the experimental surface morphology of SEBS/PMMA polymer blend films observed by atomic force microscope (AFM). The effect of substrate on the surface morphologies by MC simulation is qualitatively consistent with the experimental results. The composition profiles of thin films are given to characterize the micro- and macrophase separation in thin films. It is indicated that the surface energy of the substrate (substrate/air) plays a crucial role on the surface composition. For a fixed surface, the adsorptions of polymer on the substrate and film thickness are also important.
The contact between the polymer active layer and the electrode is one the most critical interfaces in polymer solar cells. In this article, we report the progress of interface engineering in polymer solar cell research, where the multiple functions of the interfacial materials will be discussed. The vertical composition profile in polymer:fullerene blends is an emerging topic, and the interlayer effect on the vertical phase separation and device performance will be highlighted. We also discuss the energy level alignment at the bulk heterojunction (BHJ) interface, with the aim of providing a better understanding towards the route of high efficiency polymer solar cells.
Morphology of poly( styrene-block-dimethylsiloxane) (PS-b-PDMS) copolymer thin films was analyzed by atomic force microscopy (AFM) observation and transition electron microscopy (TEM) measurements. The asymmetric copolymer thin films spin-cast from toluene on mica presented a mesh-like structure, which was different from the sphericalop structure in the bulk. The annealing temperature affected the surface morphology of PS-b-PDMS copolymer thin films,PDMS microdomains at the surface were increased when the annealing temperature was higher than the PDMS glass transition temperature. The morphology of PS-b-PDMS copolymer thin films was different from solvent to solvent, for thin films spin-cast from toluene, the PS phase was pits in PDMS matrix. While the thin film spin-cast from cyclohexane solution exhibited an island-like structure, small separated PS phase appeared as protrusions over the macroscopically flat surface. The rnicrophase structures of the PS-b-PDMS copolymer thin films were also strongly influenced by the different substrates, for an asymmetric block copolymer thin film, the PDMS and PS phases on silicon substrate presented lamellae structure parallel to the film surface.
High quality, uniform thin polymer films are routinely produced by the technique of spin coating. Applications for such polymer films beyond photoresist layer fabrication include photovoltaics and light-emitting diodes, where device performance is dependent upon an appropriate phase separated morphology. Developing an understanding of the factors involved in the development of such morphologies is therefore an important goal. The spin coating of polymer blends is a high speed, non-equilibrium process and as such, it is difficult to monitor in situ, with most studies inferring structure development from the resultant final morphology. Over the past 20 years various in situ experimental techniques have been developed, providing insight into the details of the spin coating process itself and opening a route to understanding and controlling morphological development. The majority of studies have been based upon interferometry and light scattering, "based in reciprocal space" and have been able to verify theoretical models of spin coating and the associated phase separation of polymer blends, with direct real-space, in situ studies now a possibility.
A detailed understanding of the interface between living cells and substrate
materials is of rising importance in many fields of medicine, biology and
biotechnology. Cells at interfaces often form epithelia. The physical barrier
that they form is one of their main functions. It is governed by the properties
of the networks forming the cytoskeleton systems and by cell-to-cell contacts.
Different substrates with varying surface properties modify the migration
velocity of the cells. On the one hand one can change the materials
composition. Organic and inorganic materials induce differing migration
velocities in the same cell system. Within the same class of materials, a
change of the surface stiffness or of the surface energy modifies the migration
velocity, too. For our cell adhesion studies a variety of different,
homogeneous substrates were used (polymers, bio-polymers, metals, oxides). In
addition, an effective lithographic method, Polymer Blend Lithography (PBL), is
reported, to produce patterned Self-Assembled Monolayers (SAM) on solid
substrates featuring two or three different chemical functionalities. This we
achieve without the use of conventional lithography like e-beam or UV
lithography, only by using self-organization. These surfaces are decorated with
a Teflon-like and with an amino-functionalized molecular layer. The resulting
pattern is a copy of a previously created self-organized polymer pattern,
featuring a scalable lateral domain size in the sub-micron range down below 100
nanometers. The resulting monolayer pattern features a high chemical and
biofunctional contrast with feature sizes in the range of cell adhesion
complexes like e.g. focal adhesion points.
Spin casting
polymer blends of conjugated and dielectric macromolecules onto chemically patterned metal and oxidized silicon surfaces might provide a simple method to fabricate polymer-based circuitries that can be integrated with conventional electronics. Such solution-processing of the blend components offers simultaneous deposition and pattern-directed alignment of the phase separated polymer domains. The alignment is driven by self-organization guided by preferential surface segregation. Here we demonstrate that the laterally arranged domain structures in spin cast films of the conjugated poly(3-alkylthiophenes) (P3ATs): P3BT, P3DDT and regioregular R-P3HT, blended with dielectric polystyrene (PS), can be ordered by three different surface templates. The templates are formed by a patterned self-assembled monolayer (SAM), micro-contact printed on the surface of interest, i.e. hexadecanethiols on gold (for alignment of P3DDT/PS blend) and octadecyltrichlorosilanes on oxidized silicon (for R-P3HT/PS). Additionally gold lines are micro-patterned on SiO2 with photo-lithography (for P3BT/PS mixture). The forces driving pattern-directed self-organization of the polymers are discussed based on complementary studies of preferential surface segregation, observed for blend films spin cast on homogeneous surfaces that correspond to the different regions of the surface templates.
A Monte Carlo simulation using the bond fluctuation and cavity diffusion algorithms was adopted to investigate the micro-phase separation of ABC triblock copolymer in ultra-thin film on simple cubic lattice. Simulations reveal that the morphologies of ABC copolymer films are dependent on not only the volume fraction of the middle block B (f B) but also on the ratio of interaction between different kinds of blocks (ϵ(AC)/ϵ(AB)). As for the molecular orientation, the copolymers stretch parallel to the flat surface at lower f B, but tend to align perpendicularly along z direction at higher f B. Furthermore, the chain configuration was discussed in detail. Smaller ϵ(AC)/ϵ(AB) is beneficial to the formation of a “loop” configuration, whereas, larger ϵ(AC)/ϵ(AB) would result in a “bridge” configuration of ABC triblock copolymer chains. The formation of micro-phase structures was illustrated intuitively by the molecular orientation and the chain configuration.
A polymer vs solvent diagram of film structures, formed in polystyrene (PS) blends (1:1 w:w PS/PT) with poly(3-alkylthiophenes) PT [regioregular R-P3DDT, R-P3HT and regiorandom P3BT, P3DDT] spin-coated onto oxidized silicon surfaces from various common solvents [p-xylene, toluene, chloroform, chlorobenzene, cyclohexanone] is presented. The structures were determined with microscopic techniques (atomic, AFM and lateral, LFM, force microscopy, fluorescent microscopy FM) and dynamic secondary ion mass spectrometry (dSIMS). The diagram, arranged according to the solubility parameter of the PTs and the solvents, exhibits three main structural classes: dewetting, lamellar, and lateral (quasi-2-dim) morphology. Decrease in PT solubility parameter δPT inhibits dewetting of polymer films. It induces also a transition from lamellar to lateral film structure. Increase in solvent solubility parameter δS has similar effects. Such behavior is related to the stability of transient homogeneous films and multilayers in the course of spin-casting. The role of δPT and δS is elucidated based on the stability analysis performed in terms of spreading coefficient (dependent on δPT) and effective interfacial tension of solvent-rich polymer phase (dependent on δS).
In this study the morphology of spin-casted films of polymers blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM) has been studied. It was found that the lateral structure formation in the films is favored by rapid solvent evaporation and strong polymer−PCBM repulsion. The formation of homogeneous films is favored by slow evaporation and weak polymer−PCBM repulsion. The effect of solvent evaporation rate is the opposite of what is found for spin-casting polymer−polymer blends. The results can be explained by the kinetics of phase separation and the phase behavior involving limited solubility and crystallization of PCBM.
Two blends (1:1 and 5:3 w:w) of poly(2-vinylpyridine) (PVP) and partly brominated polystyrene were spin-cast (with constant inherent domain scale 2R = 4.2 ± 0.5 μm) onto a gold substrate which had been microcontact printed with stripes of hexadecanethiol SAM. Two chemical patterns on the substrate were used, one with symmetrical stripes (4 μm Au/4 μm hexadecane SAM) and the other with narrow gold stripes and wider SAM stripes (3 μm/5 μm). The resulting film morphologies were mapped with atomic force microscopy. Secondary ion mass spectrometry shows that raised regions on the film surface correspond to PVP-rich domains, and the surface polymer composition extends all the way down to the substrate. Fourier analysis reveals structural modes (λ/n) smaller than the pattern periodicity λ = 8 μm. PVP preferentially adsorbs to the gold regions of the pattern. For the asymmetric pattern, which has a relatively small Au area interacting with the PVP, the fundamental mode (n = 1) is extinguished and higher-order substructures (n > 1) are seen. Use of a PVP-rich blend increases the intensity of the n = 3 mode and reduces that of the n = 2 mode. When both blend and pattern were asymmetric, both λ/2- and λ/3-substructures were observed, and the fundamental pattern periodicity was extinguished.
Individual alkanethiol-functionalized gold nanorods act as morphological seeds that specifically template the growth and direction of large, uniform, cylindrical-phase domains from a polystyrene-block-polyisoprene (PS-b-PI) copolymer solution. Adding less than 0.0001 vol % nanorods switched the preference of domain growth from predominantly spherulitic to almost exclusively single crystalline, with the PI cylinders uniformly aligned in the same direction as each seeding nanorod. Polarized and dark-field optical microscopy on individual domains and small-angle X-ray scattering experiments on bulk material confirmed this specific seeding effect. By contrast, we find that spherical gold nanoparticles and other materials that are commonly used as heterogeneous nucleants in crystallization (e.g., carbon black, talc, and calcium carbonate) failed to promote single-crystalline domain growth in the same material. Mesoscale dynamic modeling of nanorod−copolymer mixtures illustrates these observations in terms of a specific interaction between the surface of the nanorods and the polyisoprene block, which orients the self-assembling polyisoprene cylinders in the nascent domain nucleus.
Blends of poly(2-vinylpyridine), polystyrene, and poly(methyl methacrylate) of four different compositions (2:3:0, 3:2:0, 1:1:0, and 2:2:1 w:w:w) were spin-cast onto periodically (λ = 4 μm) patterned substrate (with two alternating stripes: Au attracting PVP and neutral self-assembled monolayer), and resulting film morphologies (with inherent domain scale 0.2 λ ≤ R < 1.8 λ) were recorded with scanning force microscopy and examined with Fourier transform analysis and the integral geometry approach. The morphologies depend on not only spatial (R/λ) but also compositional commensuration between blends and symmetric pattern: λ/2-substructures are present, in addition to λ-structures, for isolated (2:3:0) but not for continuous (3:2:0) PVP domains. This explains also the data for 1:1:0 blends which present a transition from isolated to continuous PVP domains (for larger R values). In turn, interfacial compatibilizer (PMMA in 2:2:1) results in the well-ordered λ/2-substructures for both morphology types.
In this paper, we demonstrate the applicability of focused ion beam (FIB) preparation followed by tapping mode atomic force microscopy (TMAFM) to analyze model interphase thicknesses in high density polyethylene/polystyrene/poly(methyl methacrylate) (HDPE/PS/PMMA) ternary polymer blends prepared by melt mixing. Previous work has shown that, in a polyethylene matrix, this blend exhibits a dispersed phase composed of a well-segregated PMMA core and a PS shell. Control of the PS/PMMA composition ratio allows for the control of shell thickness and hence this blend provides an excellent model system to analyze interphase thicknesses. A focused ion beam preparation was applied to the melt blended samples to prepare very smooth surfaces without mechanical deformation (i.e., no plowing or interfacial debonding), while TMAFM was used to obtain high-resolution images of the composite droplets in order to measure the mean diameter of the droplets and PS shell thickness. It is shown that the three polymer components have different ion beam etching rates, which results in a topological contrast between the phases of the blends when viewed by tapping mode atomic force microscopy. In this case, PMMA has the highest etching rate, while PS has the lowest and HDPE is intermediate. This high level of contrast between the phases allows for a clear identification of the PS interphase. Even the fine features of particles in the process of coalescing can be clearly identified. To ensure that this procedure was not altering the blend phase sizes in any way, average composite droplet diameters obtained with FIB preparation and TMAFM measurements were compared with the classic technique of cryomicrotome preparation and SEM measurements. The dispersed phase size data from the two procedures compare well. The FIB/TMAFM approach allows for the estimation of the PS interphase thicknesses from 100 to 200 nm depending on the PS/PMMA composition ratio. The approach presented here avoids the pitfalls associated with microtomy such as microvoiding, deformation of the materials and debonding at the interphase. It also eliminates the need for extraction of polymers with selective solvents and staining techniques used to provide contrast. This technique provides significant advantages for the analysis of multicomponent polymer blends or blends with complex morphologies. It also provides a first step toward a new approach for analyzing interphase thickness in polymer blends.
The measurement of the kinetics of film formation and phase separation in a spin-cast polymer blend using in-situ reflectivity and light scattering techniques was discussed. The changes in film thickness during spinning were measured using specular reflectivity. The radially averaged light scattering exhibited pulsations in intensity with a periodicity observed in the specular reflectivity. The secondary phase separation in micrographs of the films after spinning was also elaborated.
An unusual polymer blend morphology of Voronoi Tessellations is discussed. AFM and phase-sensitive acoustic microscopy (PSAM) techniques are used for the purpose. Findings suggest that the congenitally entropy-driven phase separation process for polymer blend films may follow a quasi-Voronoi tessellation generation mechanism or engender a Voronoi tessellation stage. This propounds a newfangled way for modeling the phase-separation process during spin-coating.
For polystyrene blends with poly(butyl methacrylate), replacing poly(n-butyl methacrylate) by poly(tert-butyl methacrylate) results in modified topography of spin-cast films (holes dominating at a surface are exchanged with islands, or vice versa, such individual surface features are increased), decreased PBMA surface excess, but similar overall domain structure. Two series (PS/PnBMA and PS/PtBMA) of films with constant thickness and PBMA fraction 0 < Φ < 1, cast from toluene onto silicon wafers, were examined with atomic force microscopy, X-ray photoelectron spectroscopy, profiling and mapping mode of dynamic secondary ion mass spectrometry. Topography was analyzed with the integral geometry approach. Film structure formation was postulated to involve surface segregation, phase separation, and instability of the transient PBMA layer [a linear relation between Φ2 and averaged size of holes (islands) was observed for PS/PnBMA (PS/PtBMA)]. Structural changes caused by isomer exchange are related to glass transition temperature and blend incompatibility; both increased for PtBMA.
In this paper, a simple and inexpensive method of preparing nanosized titanium oxide (TiO2) pillars on glass substrates and ultrathin TiO2 layers is presented, utilizing a blend of commercially available polystyrene (PS) and poly(methyl methacrylate) (PMMA). The surface morphology of PS/PMMA blend films is investigated in terms of the processing parameters including solution concentration, blending ratio, and spin-coating speed. For the first time, a phase inversion was revealed for the PS/PMMA blend films spun coated on conducting substrates (ITO and TiO2), with increasing solution concentration. Atomic force microscopy studies show that PMMA forms vertical cylindrical structures in the matrix of PS on glass substrates and on ultrathin TiO2 layers deposited on indium tin oxide (ITO) substrates. After the PMMA phase is etched away by ultraviolet irradiation and acetic acid, TiO2 pillar structures are successfully created on both types of substrates by infiltration of the sol−gel mixture into the nanosized PS templates followed by calcination at an elevated temperature. The size and shape of the TiO2 pillars were found to be affected by the thickness and phase separation of the initial PS/PMMA films, which depend on the solution concentration and blending ratio of the two polymeric components.
Infrared (IR) thermography was used to monitor the drying of water from both smooth and rough surfaces. An analysis using the coefficient of variance (COV) of the temperature shows the point at which surface dryout begins. The dry spots that develop on a surface lead to temperature variations across the surface. The COV reflects these variations and is a sensitive measure of the early stages of dryout. The shape of the COV profile as the surface approaches dryness depends on the roughness of the surface. For rough surfaces such as wood, the COV climbs because differences in heat transfer to the heterogeneous wood surface are accentuated. Changes in the COV are much smaller for polymer films deposited on metal surfaces but are still dependent upon surface roughness.
A rapid and cost-effective lithographic method, polymer blend lithography (PBL), is reported to produce patterned self-assembled monolayers (SAM) on solid substrates featuring two or three different chemical functionalities. For the pattern generation we use the phase separation of two immiscible polymers in a blend solution during a spin-coating process. By controlling the spin-coating parameters and conditions, including the ambient atmosphere (humidity), the molar mass of the polystyrene (PS) and poly(methyl methacrylate) (PMMA), and the mass ratio between the two polymers in the blend solution, the formation of a purely lateral morphology (PS islands standing on the substrate while isolated in the PMMA matrix) can be reproducibly induced. Either of the formed phases (PS or PMMA) can be selectively dissolved afterwards, and the remaining phase can be used as a lift-off mask for the formation of a nanopatterned functional silane monolayer. This "monolayer copy" of the polymer phase morphology has a topographic contrast of about 1.3 nm. A demonstration of tuning of the PS island diameter is given by changing the molar mass of PS. Moreover, polymer blend lithography can provide the possibility of fabricating a surface with three different chemical components: This is demonstrated by inducing breath figures (evaporated condensed entity) at higher humidity during the spin-coating process. Here we demonstrate the formation of a lateral pattern consisting of regions covered with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) and (3-aminopropyl)triethoxysilane (APTES), and at the same time featuring regions of bare SiO(x). The patterning process could be applied even on meter-sized substrates with various functional SAM molecules, making this process suitable for the rapid preparation of quasi two-dimensional nanopatterned functional substrates, e.g., for the template-controlled growth of ZnO nanostructures [1].
Blends of the low-bandgap polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5- (4',7'-di-2-thienyl-2',1',3'-benzothiadiazole] (APFO-3) and the fullerene derivative [6,6]-phenyl–C61–butyric acid methyl ester (PCBM) were spin-coated from chloroform solution into thin films, which were examined with dynamic secondary ion mass spectrometry. For blends with high PCBM content, the depth profiles show composition waves that were caused by surface-directed phase separation during spin-coating. The formation of such multilayer structures by spontaneous self-stratification is likely to have implications for optimization strategies for the performance of organic solar cells.
The morphologies of poly(styrene-block-di-methylsiloxane) (PS-b-PDMS) copolymer thin films were analyzed via atomic force microscopy and transition electron microscopy (TEM). The asymmetric copolymer thin films spin-cast from toluene onto mica presented meshlike structures, which were different from the spherical structures from TEM measurements. The annealing temperature affected the surface morphology of the PS-b-PDMS copolymer thin films; the polydimethylsiloxane (PDMS) phases at the surface were increased when the annealing temperature was higher than the PDMS glass-transition temperature. The morphologies of the PS-b-PDMS copolymer thin films were different from solvent to solvent; for thin films spin-cast from toluene, the polystyrene (PS) phase appeared as pits in the PDMS matrix, whereas the thin films spin-cast from cyclohexane solutions exhibited an islandlike structure and small, separated PS phases as protrusions over the macroscopically flat surface. The microphase structure of the PS-b-PDMS copolymer thin films was also strongly influenced by the different substrates; for an asymmetric block copolymer thin film, the PDMS and PS phases on a silicon substrate presented a lamellar structure parallel to the surface at intervals. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1010–1018, 2007
The surface compositions of a series of polystyrene-b-polydimethylsiloxane (PS-b-PDMS) and polystyrene-g-polydimethylsiloxane (PS-g-PDMS) copolymers were investigated using ATR-FTIR and XPS technique. The results showed that enrichment of PDMS soft segments occurred on the surface of the block copolymers as well as on that of graft copolymers. And the magnitude order of the enrichment was as follows: PS-b-PDMS > PS-g-PDMS, which was attributed to the facilitating of the movement of the PDMS segments in PS-b-PDMS copolymer. Meanwhile, the solvent type and the contact medium had influence on the accumulation of PDMS on the surfaces. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci, 2006
Film blends of poly(3-butyltiophene-2,5-diyl) (PT) and polystyrene (PS; 1 : 1 w/w) were spin-coated onto silicon wafers from chloroform, tetrahydrofuran, and cyclohexanone at a controlled relative humidity between 4 and 86%. The film morphologies were determined with atomic and lateral force microscopy and mapping and depth profiling modes of dynamic secondary-ion mass spectroscopy. Independently, white light interferometry was used to examine the expansion and response time (τ) of pure polymer layers exposed to solvent vapors and moisture. The higher PS solubility, in comparison with the PT solubility, in chloroform and tetrahydrofuran resulted in PS/PT//Si bilayers, which were the final structures for coatings from chloroform [with much larger τ(PS)/τ(PT) ratios]. For tetrahydrofuran, these bilayers were destroyed, most likely by surface and interface instabilities, yielding hierarchic lateral structures. For cyclohexanone (with the largest τ values), a large-scale component of the lateral structures was absent, and this suggested the leveling of surface instabilities. The humidity changed the structural scales and thickness of the films cast from tetrahydrofuran (because it had the best solubility with water). The humidity effects of chloroform and cyclohexanone [reported earlier for polyaniline and poly(vinyl pyridine) blends] were practically absent. Moisture was not easily absorbed by PT and PS [in contrast to polyaniline and poly(vinyl pyridine)] and probably adsorbed merely at the surfaces of blend films rich in tetrahydrofuran. © 2007 Wiley Periodicals, Inc. JAppl Polym Sci 105: 67–79, 2007
Monte Carlo simulations were used to identify the microphase morphologies of ABA triblock copolymer melts confined in a cylindrical nanotube. The influences of the volume fraction of mid‐block B ( f B ), the radius of nanotube ( R ) and the asymmetry of ABA triblock copolymer chain were discussed in detail. When f B varies, a series of double‐continuous, three‐layer concentric cylinder barrel, porous net, double helixes and the new multiplex structures were observed under different conditions. In addition, the stacked disk, catenoid‐cylinder and multi‐layer concentric cylinder barrel structures occur in turns at changing R . The relation between circular lamellae period L and layer number N layer of concentric cylinder barrel with the increase of R was investigated to further explain the put‐off phenomenon of microphase transition of the multi‐layer concentric cylinder barrel structures. As for the increase of the asymmetry of ABA triblock copolymer chain, it was concluded that the short A I segments tend to site at the interface between rich A and B circular lamellae.
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Organic photodiodes are presented that utilize solution-processed perylene diimide bulk heterojunctions as the device photoactive layer. The polymer (9,9′-dioctylfluorene-co-benzothiadiazole; F8BT) is used as the electron donor and the N,N′-bis(1-ethylpropyl)-3,4,9,10-perylene tetracarboxylic diimide (PDI) derivative is used as the electron acceptor. The thickness-dependent study of the main device parameters, namely of the external quantum efficiency (EQE), the short-circuit current (ISC), the open-circuit voltage (VOC), the fill factor (FF), and the dark current (ID) is presented. In as-spun F8BT:PDI devices the short-circuit EQE reaches the maximum of 17% and the VOC value is as high as 0.8 V. Device ID is in the nA mm−2 regime and it correlates with the topography of the F8BT:PDI layer. For a range of annealing temperatures ID is monitored as the morphology of the photoactive layer changes. The changes in the morphology of the photoactive layer are monitored via atomic force microscopy. The thermally induced coalescence of the PDI domains assists the dark conductivity of the device. ID values as low as 80 pA mm−2 are achieved with a corresponding EQE of 9%, when an electron-blocking layer (EB) is used in bilayer EB/F8BT:PDI devices. Electron injection from the hole-collecting electrode to the F8BT:PDI medium is hindered by the use of the EB layer. The temperature dependence of the ID value of the as-spun F8BT:PDI device is studied in the range of 296–216 K. In combination with the thickness and the composition dependence of ID, the determined activation energy Ea suggests a two-step mechanism of ID generation; a temperature-independent step of electric-field-assisted carrier injection from the device contacts to the active-layer medium and a thermally activated step of carrier transport across the device electrodes, via the PDI domains of the photoactive layer. Moreover, device ID is found to be sensitive to environmental factors.
Two‐dimensional periodic patterns were observed for PS/PVP blend films and the corresponding homopolymer films on tilted glass substrates. They were attributed to the Marangoni instability resulting from surface tension non‐uniformity. For the 1:1 PS‐1/PVP‐1 blend films, two‐dimensional regular arrays of the separated phases were observed, similar to those observed on homopolymer films from thickness variations. In addition, for the PS‐2/PVP‐2 blend films of 30/70 composition, one‐dimensional strip patterns of separated phases were observed. The pattern formation mechanism is discussed and a characteristic parameter, defined as the ratio of the final film thickness to initial solution viscosity, is found to correlate well with the pattern regularity through variation in polymer concentration.
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Distribution of polyaniline doped by camphorsulfonic acid, blended with polystyrene of different molecular weight and spin-cast from chloroform into (50–400 nm thick) films, was examined using secondary ion mass spectrometry (SIMS). Strong polyaniline segregation to the Au substrate was observed to be accompanied by depletion region or, for thicker films, by concentration oscillations. Segregation and substrate-directed phase separation, effective during solvent removal, form lamellar film structures.
The phase morphology of multi-component polymer blends is governed by the interfacial interactions of its components. We discuss here the domain morphology in thin films of model binary and ternary polymer blends containing polystyrene, poly(methyl metacrylate), and poly(2-vinylpyridine) (PS, PMMA, PVP). When sandwiched between two non-polar surfaces, characteristic lateral phase morphologies are observed after the film formation by spin-coating. We discuss here two techniques, by which hierarchical lateral structures in polymer films can be made. The first method makes use of two simultaneously occurring interfacial instabilities. The second technique employs the effect of a variation of the enthalpic interaction parameters in a ternary polymer mixture on its lateral polymer phase morphology.
Topography and composition images of model thin films of deuterated polystyrene (dPS) and polyisoprene with different blend compositions were analyzed with an extension of integral-geometry approach. Surface patterns, formed in the course of spin-casting from toluene onto self-assembled monolayers (SAM), were recorded with scanning force microscopy. Their relation with lateral phase domain structures was demonstrated by dynamic secondary ion mass spectrometry, yielding maps of dPS distribution. Morphological measures, which cannot be provided by Fourier transform analysis (FTA), characterize individual images, compositional series of the surface patterns and individual features of the patterns. Different morphologies (nucleation- and spinodal-type and hole- and island-dominated ones) are consistently characterized by the Minkowski measures and related parameters. For instance, the latter can measure circular character of the individual features and estimate dominant lateral length (determined rigorously with FTA). Lateral morphologies are hardly affected when CH3-terminated SAM is exchanged for SAM with COOH end groups.
Phase separation in bulk mixtures commonly leads to an isoropic, disordered morphology of the coexisting phases. The presence of a surface can significantly alter the phase-separation process, however. Here we show that the domains of aphase-separating mixture of polymers in a thin film can beguided into arbitrary structures by a surface with a prepatterned variation of surface energies. Such a pattern can be imposed on a surface by using printing methods for depositing micro structured molecular film, thereby allowing for such patterns to be readily transferred to a two-component polymer film. This approach might provide a simple means for fabricating polymer-based microelectronic circuits or polymer resists for lithographic semiconductor processing.
NATURE | VOL391 | 26FEBRUARY1998 877-979
Optical surfaces coated with a thin layer to improve light transmission are ubiquitous in everyday optical applications as well as in industrial and scientific instruments. Discovered first in 1817 by Fraunhofer, the coating of lenses became standard practice in the 1930s. In spite of intensive research, broad-band antireflection coatings are still limited by the lack of materials with low refractive indices. A method based on the phase separation of a macromolecular liquid to generate nanoporous polymer films is demonstrated that creates surfaces with high optical transmission.
External surfaces can significantly alter the phase decomposition (PD) of polymer mixtures in thin films. The surface mode of PD orders two coexisting phases in organised structures with laminated domain morphology. Such structures are employed in new polymer-based technologies of photoelectronic devices or microelectronic circuits. We studied the surface mode of PD in thin films composed of various binary mixtures of polystyrene with its deuterated- and partially brominated- counterpart. We have examined how PD is influenced by: (a) surface active diblock copolymers admixed to decomposing blends, (b) substrate surface modification, and (c) finite film thickness. Thin films were studied by nuclear reaction analysis (NRA) and secondary ion mass spectroscopy (SIMS). The results provided composition profiles as a function of depth in the film with a nanometer precision, comparable with the polymer chain dimensions. Lateral morphology was investigated by means of the atomic force microscope and optical microscope. Various laminated structures composed of 2-, 3-, or 4- layers or column-like structures self-stratified from initially homogenous films were observed.
The microdomain morphology of block copolymers at the free surfaces of solution-cast films was investigated by means of transmission electron microscopy and ultramicrotomy. The free surfaces of block copolymers consisting of amorphous components obey the thermodynamic requirements, i.e. the constituent block chains with lower critical surface tension always cover the free surface even for block copolymer films in which the component with higher critical surface tension forms the matrix. On the other hand, the free surfaces of block copolymers with crystalline components are governed by the crystallization kinetics, and their morphology depends on the orientation of the crystalline lamellae near or at the free surface. Therefore, the surface composition may vary with the method of preparation, and both amorphous and crystalline components can have a chance to be exposed to the free surface.
This in-depth treatment of the instrumentation, physical bases and applications of x-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectroscopy (SSIMS) contains a specific focus on the subject of polymeric materials. XPS and SSIMS are widely accepted as the two most powerful techniques for polymer surface chemical analysis, particularly in the context of industrial research and problem solving. The author describes the techniques and applications of XPS and SSIMS. He also includes details of case studies, emphasizing the complementary and joint application of XPS and SSIMS in the investigation of polymer surface structure and its relationship to the properties of the material. This book will be of value to academic and industrial researchers interested in polymer surfaces and surface analysis.
Thin polymer films with lateral structures are expected to play an important role in future applications (e.g. plastic-based electronic devices). Such structures could be formed when blend films are spun-cast onto substrates patterned with self-assembled monolayers (SAM). The spin-coating process results in composition variations accompanied by surface undulations. We have studied both phenomena for PVP/dPS and PVP/PBrS blends, composed of poly(vinylpyridine) (PVP) and deuterated (dPS)- or brominated (PBrS)-polystyrene. SAM stripes of HS(CH2)15CH3 on Au substrate (‘bare’ or covered with HS(CH2)15COOH) were used as the pattern with periodicity of 4μm. Transfer of the pattern from the substrate to the film interior and to the film surface was examined with secondary ion mass spectroscopy (SIMS) and atomic force microscopy (AFM) combined with selective dissolution of blend components. Characteristic size D of the phase domains corresponding to given spin-casting conditions was determined for the blends cast on homogeneous SAM substrate. Fourier transform analysis (FTA) of topographic (AFM) and compositional (SIMS) maps was performed. FTA confirms that the pattern-directed composition variations coincide with the surface undulations. It reveals also that most effective pattern transfer is achieved for the size D commensurate with the pattern periodicity for the carefully adjusted polymer–substrate interactions.
Thin films, formed by polymer blends spun-cast from a blend/solvent solution onto a rigid substrate, are used in many practical applications (e.g. photoresist layers, dielectric coatings). Film preparation process is often accompanied by phase decomposition (PD) during the rapid evaporation of the solvent. PD is reflected in undulations formed on an air/film interface. We have studied the topography of surface undulations and the phase domain morphology in thin film blends of polystyrene (PS) and polyisoprene (PI) using atomic force microscopy combined with selective dissolution of blend components. Gold covered with self-assembled monolayers [HS(CH2)15COOH]y[HS(CH2)15CH3]1−y (SAMy) was used as a substrate. For films of PS and PI (50% by mass) cast from toluene, the PS-rich domains protrude high above the PI-rich matrix forming concave or convex islands for hydrophobic (SAM0)- or hydrophilic (SAM1)-support, respectively. Different substrates (e.g. SAM0.5 and Si with a native oxide layer), solvents (CCl4, chloroform) and PS mass fractions were used to evaluate the extent of this novel effect.
The phase separation kinetics of ultrathin deuterated poly(styrene)/poly(butadiene) polymer blend films spun cast onto striped self-assembled monolayer (SAM) substrates is studied by atomic force microscopy (AFM). Fourier transform analysis of the AFM topographic data at various stages of the film pattern development reveals the presence of quantized surface deformation modes. These modes are excited by the phase separation process when the scale of phase separation becomes commensurate with the period of the striped surface pattern. Thus, higher frequency modes become excited at early stages of phase separation, and these excitations decay with time as the phase separation pattern further coarsens. The film ultimately self-organizes into a periodic structure in which the fundamental mode has the largest amplitude. The influence of film thickness on the film morphology in this late stage is also investigated. A decrease in the film thickness leads to surface patterns that match those of the SAM substrates with increasing resolution. However, these film patterns break up into droplet arrays along the SAM stripes if the films are made too thin. This phenomenon is attributed to a capillary wave instability.
The process of pattern formation and droplet coarsening has been studied for ternary polymeric fluids in which phase separation was induced by solvent evaporation. For a particular range of solvent evaporation rates and thicknesses of liquid layers, ordered hexagonal patterns were formed at the liquid film-air interface. We ascribe this effect to Bénard-Marangoni convection induced by solvent evaporation and estimate conditions generating periodic two-phase structures in polymeric films. Droplet coarsening rate featured a crossover from R~t0.89 to R~t0.67 coinciding with the onset of convection and was explained by convection-induced stabilization of droplets against coalescence.
We investigate a wetting reversal transition in thin films of two-phase mixtures of poly(ethylene-propylene) (PEP) and its deuterated analog (dPEP) on substrates covered by self-assembled monolayers (SAM) whose surface energy, gammaSAM, is tuned by varying the SAM composition. As gammaSAM increases from 21 to 24 mJ/m2, a transition from a dPEP/PEP/dPEP/SAM to a dPEP/PEP/SAM structure occurs at increasing TC-T, where TC and T are the critical and transition temperatures, respectively. The dependence of T on gammaSAM is predicted by a simple model from surface and interfacial energies of PEP/dPEP.
Both liquid and solid foams, together with analogous cellular materials, have distinctive mechanical or rheological properties which find many applications. This review concentrates on the search for a basic understanding of the underlying mechanisms, in terms of specific structural models. Computer simulations play an increasing role and are beginning to be applied to three-dimensional models.
Experimental research on wetting in polymer films is a subject that is reaching maturity. We review progress from the past few years in research into the influence of a boundary in polymer blends, concentrating largely on the wetting transition, and the growth of wetting layers, where we pay particular attention to blends in which hydrodynamic flow plays a dominant role. A summary of work over the same period concerning the dewetting of polymer films is also included, along with a discussion of the role of pattern formation caused by dewetting and topographically and chemically patterned substrates. We conclude by summarising some experiments that we believe may inspire future research.
The process of spin coating is described, with particular attention to applications in microelectronics. The physical mechanisms involved in the process are discussed and those mechanisms that affect the final state are identified, viz., centrifugal and viscous forces, solute diffusion, and solvent evaporation: A model is proposed that incorporates only the latter mechanisms, with viscosity and diffusivity depending on solute concentration. The evaporation of solvent during spinning causes the solution viscosity to increase and the flow is reduced. The thickness of the final solid film is related to the thickness of a diffusion boundary layer near the free surface. The model predicts the final dry film thickness in terms of the primary process variables, spin speed, and initial polymer concentration. A similarity boundary‐layer analysis leads to a simple approximate result for the final film thickness that is consistent with limited experimental data, hf ∼KC0(ν0D0)1/4/Ω1/2, where K is a number of order unity and the other quantities are, respectively, the initial polymer concentration, the kinematic viscosity, the solute diffusivity, and the spin speed. The dependence on diffusivity has not previously been described theoretically. The total spin time is shown to be proportional to Ω−1, in agreement with experiment. The rate of solvent evaporation is shown to be proportional to Ω, which contradicts previous assumptions.
The morphology of cellulose acetate/poly(methyl methacrylate), CA/PMMA, blends of various compositions was studied. Optical microscopy and DSC conclusively showed that the polymers form incompatible blends. However, partial miscibility between the phases was noted since the glass transition temperature of the PMMA phase increased with the increase in the amount of the CA fraction in the blend, while the glass transition temperature of CA decreased with the increase in the amount of the PMMA fraction in the blend. ATR-FTIR spectra and surface energy measurements showed that the CA/PMMA film surfaces were comprised primarily of PMMA polymer. These results suggest that the CA/PMMA blends form a layered morphology with surface layers comprised essentially of PMMA polymer and the interior layer comprised of phase-separated blend. He, O2 gas transport properties of CA, PMMA and CA/PMMA blends of several compositions have been measured at 35°C. The data were analyzed in terms of parallel and series resistance models. The high He/O2 and He/N2 gas separation factors were best described by a combination of the parallel and series models that is consistent with the proposed layered film morphology.
This brief review has three general aims. The first is to develop the necessary thermodynamic and statistical thermodynamic background to understand the unusual phase behavior of polymer blends. The second is to examine the mechanisms of phase separation, especially spinodal decomposition. The third and final purpose is to review the various theories of polymer interfaces. Only physical blends of homopolymers are considered in this paper.
THE range of materials now available for polymer-based
light-emitting diodes (LEDs) is such that electroluminescence can be obtained
throughout the visible spectrum112. Here we show that, by
blending polymers with different emission and charge-transport characteristics,
LEDs can be fabricated in which the emission col-our varies as a function of
the operating voltage. This phenomenon arises from the self-organizing
properties of the blends, in which entropy drives phase separation of the
constituent polymers and gives rise to submicrometre-sized domains having a
range of com-positions and emission characteristics. Emission from domains of
different composition is controlled by the ease with which charge is injected,
which in turn depends on the applied voltage.
THE photovoltaic effect involves the production of electrons
and holes in a semiconductor device under illumination, and their subsequent
collection at opposite electrodes. In many inorganic semiconductors, photon
absorption produces free electrons and holes directly1. But in
molecular semiconductors, absorption creates electroná¤-hole pairs (excitons)
which are bound at room temperature2, so that charge collection
requires their dissociation. Exciton dissociation is known to be efficient at
interfaces between materials with different electron affinities and ionization
potentials, where the electron is accepted by the material with larger electron
affinity and the hole by the material with lower ionization
potential3. A two-layer diode structure can thus be used, in
which excitons generated in either layer diffuse towards the interface between
the layers. However, the exciton diffusion range is typically at least a factor
of 10 smaller than the optical absorption depth, thus limiting the efficiency
of charge collection3. Here we show that the interpenetrating
network formed from a phase-segregated mixture of two semiconducting polymers
provides both the spatially distributed interfaces necessary for efficient
charge photo-generation, and the means for separately collecting the electrons
and holes. Devices using thin films of these polymer mixtures show promise for
large-area photodetectors.
Thin polymer blend films of deuterated polystyrene (dPS) and poly(p-methylstyrene) (PpMS) with several blend compositions between 95:5 and 5:95 were prepared on top of glass substrates. The samples were examined right after preparation and after annealing. Their surface morphology was investigated with scanning force microscopy (SFM). A statistical analysis of the SFM data yielded the rms roughness as well as the most prominent in-plane length parallel to the sample surface. Information about the density profile perpendicular to the sample surface is gained from neutron reflectivity measurements in the region of total external reflection. Right after preparation dPS segregates to the air interface, while during annealing at a temperature above the glass transition temperature of both polymers, PpMS builds up a top layer toward the interfaces. Depending on blend composition and annealing time, different surface topographies are evolving. The examined film thicknesses are below a critical thickness, ensuring that composition fluctuations directed normal to the surface are suppressed. However, the small film thickness constrains kinetics largely to two dimensions. The surface morphology is controlled by an interplay between phase separation and dewetting.
A series of thin films of blends of poly(styrene-d8) and poly(styrene-co-p-bromox-styrene), where 1 ≥ x ≥ 0, cast on to silicon wafers are examined by atomic force microscopy, X-ray photoelectron spectroscopy, and static secondary ion mass spectrometry. Films deposited on wafers stripped of the native oxide are smooth. The poly(styrene-d8) component segregates to the polymer−air interface, and the extent of segregation increases with the degree of bromination. An inverse linear correlation is obtained between the extent of segregation and the polymer compatibility, the latter measured by the interfacial width of bilayer films. For films deposited on wafers retaining the oxide, topographical features are observed with dimensions depending on the blend composition and degree of bromination of the polymer. The bromopolymer forms islands that are raised. The most pronounced topography is found with blends containing the fully brominated styrene. Reducing the degree of bromination increases the polymer compatibility thereby suppressing the formation of topographical features. Changes in topography and surface composition are observed when the films are annealed, leading to dewetting of the substrate.
Atomic force microscopy (AFM), neutron reflection (NR) and secondary ion mass spectroscopy (SIMS) are used to examine phase separation in symmetrically segregating thin polymer blend films (≤1000 Å). Phase separation in the film leads to undulations of the liquid−air interface, provided the film is sufficiently thin to suppress surface-directed spinodal decomposition waves. Flattened droplets are formed at a very late stage of phase separation, and the aspect ratio of these droplets can be rationalized by an interfacial free energy minimization argument.
Surface structure, obtained from atomic force microscopy and X-ray reflectivity, and surface chemical analysis data, obtained from X-ray photoelectron and static secondary ion mass spectroscopy, are reported for blends of poly(p-bromostyrene) with poly(deuteriostyrene). When high speeds are used in the spin-coating process, the atomic force microscopy measurements reveal that the surface structure consists of islands, the distribution and number changing with the poly(bromostyrene) content. A ribbon structure is observed at just above 50% (w/w) poly(bromostyrene) in the mixture. These ribbons merge to form more continuous structures, leaving voids at higher concentrations. X-ray reflectivity data from the films were consistent with the topographical features observed with the AFM. At low spinning speeds, continuous films with little or no topographical structure are formed. The islands observed at high spinning speeds are predominately poly(bromostyrene) and reflect the importance of thermodynamic and kinetic driving forces in their formation.
The film thickness dependence of surface structure for immiscible polystyrene/poly(methyl methacrylate) (PS/PMMA) films was investigated on the basis of atomic force microscopic observation and X-ray photoelectron spectroscopic measurement. In the case of the PS/PMMA film of 25 μm thickness, the air−polymer interfacial region was covered with a PS rich overlayer due to its lower surface free energy compared with that of PMMA and a well-defined macroscopic phase-separated structure was formed in the bulk phase. Also, in the case of the PS/PMMA thin film of 100 nm thickness, the phase-separated structure, in which the PMMA rich domains separated out of the PS rich matrix, formed at the film surface. The formation of the surface structure for the PS/PMMA thin film can be attributed to either the chain conformation or chain aggregation structure being frozen at the air−polymer interfacial region before the formation of a PS rich overlayer due to the fairly fast evaporation of solvent molecules. On the other hand, the two-dimensional PS/PMMA ultrathin film of 10.2 nm thickness did not show distinct phase-separated structure. When the film thickness became thinner than 10.2 nm, the two-dimensional PS/PMMA ultrathin film of 6.7 nm thickness showed fine and distinct phase-separated structure with the domain size of a few hundred nanometers. This structure can be designated as “mesoscopic phase-separated structure”. The surface phase state for the two-dimensional PS/PMMA ultrathin films can be explained by the film thickness dependence of both the interaction parameter and the degree of entanglement among polymer chains.
The interplay between phase separation in polyfluorene blends which show photoinduced charge transfer and photovoltaic performance in photodiodes has been investigated. Phase separation length scales have been varied from several microns to tens of nanometers by limiting the time allowed for solvent-enhanced self-organization through several different processing routes. Concurrent with the decrease in feature size, an increase in maximum photovoltaic efficiency of nearly 1 order of magnitude was observed in photodiodes incorporating the phase-separated blends as the active layer. The structure of the blend films was investigated using fluorescence microscopy, fluorescence scanning near-field optical microscopy, and atomic force microscopy. In some cases, a hierarchy of micron- and nanometer-scale phase separation was observed which may explain the unexpectedly high photoresponse in devices with up to micron-scale phase separation structure. This result along with in situ fluorescence microscopy studies of the transformation process highlights the complex, multistage nature of the conjugated polymer blend formation process which generally exhibits spinodal behavior.
The critical role of solvent evaporation on the roughness of spin-cast polymer films was studied. Solvent-rich films were demonstrated to possess insufficient time to level and heal surface roughness created by Marangoni instabilities when the solvent was rapidly evaporating. The role of solution leveling was also emphasized by dynamic contact angle measurements. Analysis showed that it was possible to control the surface roughness of dip coated or spin cast films from good solvents by varying the evaporation rate of the solvent.
With specular and off-specular X-ray scattering the surface morphology in terms of surface roughness, film quality, and roughness correlation in thin polymer films of polystyrene and fully brominated polystyrene is measured. During the preparation of the thin films on top of silicon substrates, the common solvent was varied. We investigated eight different solvents and three different solvent mixtures to depict the influence of typical solvent parameters. In the regime of a small solvent vapor pressure, we observed correlated roughness as the ultimate lower limit of accessible surface smoothness. The resulting films are homogeneous, and the surface roughness is given by the substrate. In an intermediate vapor pressure regime marked surface morphologies are detected, while at a high vapor pressure smoother films result again.
Nuclear reaction analysis (NRA) was used to study the segregation of an asymmetric diblock copolymer, consisting of polyisoprene (PI, molecular weight M = 10(4))/deuterated polystyrene (dPS, M = 10(5)) blocks, to the interfaces formed by polystyrene (PS) homopolymer with various phases. PS/vacuum, PS/silicon, and PS/PI homopolymer interfaces were investigated for different M values of the PS matrix (M = 1.7 X 10(3)-330 x 10(3)), and segregation isotherms were established as a function of temperature and of diblock concentration within the PS homopolymer. In all cases the diblocks attach to the interfaces by their PI moieties alone, to form brushlike structures of end-attached PS tails. The high spatial resolution of the NRA technique enabled studies of the brush conformation as a function of the attachment density at the PS/vacuum surface and was used to characterize the extent of penetration of the PS matrix chains into the diblock brushes. Detailed analysis of the brush conformation and of the segregation isotherms, mainly in terms of a Flory-type mean field model based on those due to de Gennes and to Leibler, provided a consistent description of our data; it enabled the extraction of the PI/PS segmental interaction parameter chi(PIPS), yielding values in accord with scattering studies, and of the attachment energies of the PI diblock moiety to the PS/air and PS/silicon interfaces. The values of chi(PIPS) extracted from our data using this Flory-type model were found to increase at lower M values of the PS matrix, in qualitative accord with previous results.
Polystyrene (PS) and poly(methyl methacrylate) (PMMA) thin films (<100 nm thickness) have been spin-cast from chloroform solution onto cleaved mica surfaces (roughness within 0.2 nm). An algorithm for calculating the film thicknesses based on the relative intensities of the C 1s peak of the films and the Si 2s peak of the mica from angle-resolved X-ray photoelectron spectroscopy (XPS) is presented. The film thickness changes as a function of casting conditions. Data from this approach are comparable with thickness measured by an atomic force microscopy (AFM) tip-scratch method in the range 1.5−5.5 nm. Thicknesses of the films are shown to increase linearly with concentration of cast solutions.
Dispersion forces are present everywhere. Their importance, however, is largely neglected because directly at a surface or at an interface they are mostly weak compared with specific interaction of short range. Here, we show that these forces are nonetheless extremely relevant and may have drastic consequences on the stability of thin films. We demonstrate that a force (per unit area) of <1 Pa is capable of “destroying” 100 nm (!) thick films, even if they are “glued” to the substrate by end-grafted polymers. We present the temporal evolution of different morphologies of unstable thin liquid polymer films caused by destabilizing intermolecular forces.
UV−excimer laser photoablation was used, in combination with surface blocking techniques, to pattern proteins on the surfaces of polyimide and poly(ethylene terephthalate). This technique involves physical adsorption of avidin through laser-defined openings in low-temperature laminates or adsorbed protein blocking layers. Visualization of biomolecular patterns were monitored using avidin and fluorescein-labeled biotin as a model receptor−ligand couple. Adsorbed proteins could be shown to bind to UV-laser-treated polymer surfaces up to three times higher than on commercially available polymers. UV-laser photoablation was also used for the generation of three-dimensional structure, which leads to the possibility of biomolecule patterning within polymer-based microanalytical systems. The simplicity and easy handling of the described technique facilitate its application in microdiagnostic devices.
We have used laser confocal fluorescent microscopy and atomic force microscopy to study surface and bulk morphologies in solvent cast poly(methyl methacrylate) (PMMA)/polystyrene (PS) films. The solvent evaporation rate strongly affects the surface morphology. Sufficient suppression of solvent evaporation results in a periodic distribution of highly monodisperse PMMA-rich domains. Formation of the surface pattern is interpreted in terms of diffusion-driven coarsening of the minor phase and flow induced by the diffusive transport of PMMA molecules from the bulk of the liquid film to the air−film interface.
Sum frequency generation (SFG) vibrational spectroscopy and atomic force microscopy (AFM) have been applied to study the poly(n-butyl methacrylate) (PBMA)/polystyrene (PS) blend surfaces. SFG showed that PBMA tended to segregate to the blend surfaces because of its lower surface tension. The phenyl groups on the pure PS surface orientated closely to the surface normal with a narrow angle distribution. Presence of PBMA dramatically affected the orientation angle of the phenyl groups on the blend surface. For example, on the surface of the blend with only 4 wt % PBMA in the bulk, the phenyl groups would tilt more toward the surface, but they did not completely lie down on the surface. Because of the larger orientation angle of the phenyl groups versus the surface normal, no ppp signal of PS could be detected on the PBMA/PS blend surfaces with our experimental geometry. AFM results showed that pure PS and PBMA surfaces were flat, but domain structures existed on the polymer blend surfaces. Selective solvent cyclohexane has been used to identify species on the PBMA/PS blend surface. Annealing and solvent effects on the blend surface morphology have also been investigated.
Blending is a technique known in polymer technology that takes advantage of the processibility of polymers to produce new solid materials or composites with specific structural and physical properties, distinct from the ones of their components. In thin films of polymer blends interesting morphologies are formed because of phase separation. For conjugated polymers, i.e. solution-processible semiconductors, blending also opens a way to optimize the performance of opto-electronic devices, bringing about technological benefits. It is therefore crucial to achieve understanding of the effect film morphology has on the device performance, and, ultimately, to achieve control over the phase separation in a blend, so that structures can be designed that yield the desired device performance. Light-emitting diodes (LEDs) made of polymer blends have shown strongly enhanced electroluminescence (EL) efficiencies, as compared to pure homopolymers. Colour conversion, white light emission, polarized light emission, emission line narrowing, and voltage-tunable colours are other effects that have been observed in blends containing light-emitting polymers. Although the enhanced EL efficiency is attributed to Förster-type energy transfer in numerous reports, the exciton dynamics behind this effect is not well understood. Here we review the formation and morphology of thin films of conjugated polymer blends, as well as modern microscopic and spectroscopic techniques to study them. Furthermore, we attempt to link the film morphology to the electronic performance of electroluminescent and photovoltaic devices and discuss energy and charge transfer phenomena at the interfaces. We also report some new results, specifically for polyfluorene blends in LEDs.
This article was originally intended for publication in Issue 42 of this volume, which was a special issue on Conjugated Polymers: Issue 42
We present the results of a study of the morphology of phase separation in thin films of two different polymer blend systems: polystyrene/polyisoprene and polystyrene/poly(methyl methacrylate). For each blend system, the two polymer components are dissolved in a common solvent. Spin coating of the ternary solutions (polymer blend/solvent) is used to confine the blends to a thin film geometry and to produce phase separation because of rapid evaporation of the solvent (solvent quench). As a quantitative measure of the phase separation morphology the average domain area of the minority component is measured as a function of the polystyrene mass fraction. For both blend systems we identify a small range of composition corresponding to a large increase in the average domain area. We show that the strong dependence of the average domain area on spin speed allows control over the quench time of the polymer blend thin films.
The domain structure in thin films of an immiscible polystyrene/poly(methyl methacrylate) (PS/PMMA) blend was studied after spin-casting from a common solvent. Atomic force microscopy (AFM) combined with selective dissolution was used to obtain three-dimensional information on the domain morphology in thin films. Three different common solvents and three different substrate surfaces were studied. Distinct differences in the thin film domain structure and surface topography are observed depending on the substrate surface energy and the solubility of the two polymers in the three solvents. The topographic modulation can be explained by a different rate of solvent evaporation during spin-coating for the two phases. The normal and lateral organization of the phase-separated domains is governed by a complex interplay between preferential aggregation of one phase at the substrate and phase segregation in the film. Additionally, some of the results suggest that a dewetting process may be involved in the domain formation. The structures obtained after spin-casting are far from thermodynamic equilibrium. The equilibration of the films during annealing depends strongly on the phase morphology, and long-lived metastable configurations are found.
The domain morphology of a polystyrene/poly(methyl methacrylate)/poly(2-vinylpyridine) (PS/PMMA/ PVP) polymer blend was studied after spin-coating a film from a common solvent. The strongly incompatible polymer mixture phase-separates during spin-coating. Atomic force microscopy, combined with selective dissolution of the different polymer phases was used to obtain information on the polymer distribution in the film. By changing the relative composition of the mixture, a great variety of different morphologies was observed. The common feature of all observed morphologies is the compatibilizing function of poly-(methyl methacrylate), which intercalates between the more incompatible polystyrene and polyvinylpyridine domains to prevent the formation of high-energy interfaces.
The surface morphology of thin polymer blend films of deuterated polystyrene (dPS) and polyparamethylstyrene (PpMS) is investigated
with scanning force microscopy (SFM) and optical microscopy. From a statistical analysis of the data the most prominent in-plane
length picturing the domain size as a function of the blend film thickness is determined. In ultra-thin films surface patterns
directly after preparation are absent, whereas for thicker films a linear dependence is observed. After a relaxation towards
equilibrium, resulting from annealing or storage under toluene vapor, the power law observed changes for ultra-thin films
and remains unchanged for thicker films.
We have studied surface-directed phase separation in thin films of deuterated polystyrene and poly(bromostyrene) (with 22.7%
of monomers brominated) using 3He nuclear reaction analysis, dynamic secondary ion mass spectroscopy and atomic force microscopy combined with preferential
dissolution. The crossover from competing to neutral surfaces of the critical blend film (cast onto Au) was commenced: polyisoprene-polystyrene
diblock copolymers were added and segregated to both surfaces reducing in a tuneable manner the effective interactions. Two
main stages of phase evolution are characterised by i) the growth of two surface layers and by ii) the transition from the
four-layer to the final bilayer morphology. For increasing copolymer content the kinetics of the first stage is hardly affected
but the amplitude of composition oscillations is reduced indicating more fragmented inner layers. As a result, a faster mass
flow to the surfaces and an earlier completion of the second stage were observed. The hydrodynamic flow mechanism, driving
both stages, is evidenced by nearly linear growth of the surface layer and by mass flow channels extending from the surface
layer into the bulk. The final bilayer structure, formed even for the surfaces covered by strongly overlapped copolymers,
is indicative of long-range (antisymmetric) surface forces.
Polymer mixtures and block copolymers in thin film geometry find much recent attention, both theoretically [1–63] and experimentally [5,7,8,29,64–122]. This interest arises because of various applications of thin polymer films in materials science (adhesive properties, lubrication,
coatings, etc.) [123,124], but also from the point of view of basic science: e.g., from suitable measurements of thin films of polymer mixtures one
can extract information on bulk phase behavior (e.g. [70,81]), interfacial widths (e.g. [84,125]), and surface properties controlling surface enrichment or the formation of wetting layers (e.g. [5,8,65,69,74–82, 126]). In such thin films, one may also observe very interesting kinetic phenomena and associated structure formation, e.g. growth
of surface enrichment layers and adjacent depletion layers [66,69,83], dynamics of phase inversion in unmixed films [77], and — last but not least — surface-directed spinodal decomposition [5,127–158]. Of course, also the dynamics of ordering in block copolymer films is a topic of great current interest (e.g. [101]), but will not be considered further in this article.
Blends of polystyrene and poly(4-bromostyrene) phase-separate during spin-casting onto silicon wafers to give a thin film
with islands of poly(4-bromostyrene) in a sea of polystyrene. Variation of the molecular weights of the blend components shows
that the poly- (4-bromostyrene) and polystyrene influence the film structure in different ways. For poly(4-bromostyrene) of
a given molecular weight, the ratio of the observed feature height to the overall film thickness remained constant as the
film thickness increased. Moreover, the mean height of the topographical features was independent of the polystyrene but decreased
with the molecular weight of the brominated polymer. It is concluded that the substrate–poly(4-bromostyrene) interaction dominates
the formation of topography and consequently, though the islands are poly(4-bromostyrene), the mean height of the topographical
features is greater the lower the molecular weight of the brominated polymer. The polystyrene has a secondary role, altering
the thermodynamics or viscosity of the blend, thereby controlling the number of islands formed: the higher the molecular weight
of the polystyrene the greater the number of islands.
Thin films of blends of polystyrene (PS) and poly(n-butyl methacrylate) (PBMA) were prepared by spin-casting onto silicon wafers in order to map the lateral distribution of
the two polymers. The surfaces were examined by atomic force microscopy (AFM) secondary ion mass spectroscopy X-ray photoelectron
spectroscopy (XPS) and photoemission electron microscopy (PEEM). Films with PBMA contents of 50% w/w or less were relatively
smooth, but further increase in the PBMA content produced, initially, protruding PS ribbons and then, for PBMA ≥80% w/w, isolated
PS islands. At all concentrations the topmost surface (0.5–1.0 nm) was covered by PBMA, whilst the PBMA concentration in the
near-surface region, measured by XPS, increased with bulk content to eventual saturation. PEEM measurements of a PS–PBMA film
at the composition at which ribbon features were observed by AFM also showed a PS-rich ribbon structure surrounded by a sea
of mainly PBMA.
By a direct numerical integration of the dynamical evolution equations, we study phase separation of thin polymer blend films in two-dimensional models. The focus of our work is on the late-time morphology of the films. We first investigate morphological transitions in a thin film on a flat, homogeneous substrate. In particular, we probe the transition between a completely wetting and a partially wetting surface morphology. This transition is controlled by a competition between surface interactions and the interfacial energy in a thin film. This competition becomes even more pronounced when a chemically patterned substrate is used. For films cast on a patterned substrate, we consider several different substrate pattern widths and surface interaction strengths in our simulations. We also probe effects of thermal fluctuations on the late-time film morphology by carrying out simulations to many different final quench temperatures. Along with quench simulations, we further study film morphology by slow cooling the film from an initial high temperature. Comparing the morphology of films for a quench case to a slow-cooling case, we identify possible barriers to reaching equilibrium in experimental situations under a quench.
Films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) blends of two different thicknesses have been examined by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Blends with different compositions were spin-cast onto a mica substrate with chloroform as the mutual solvent. XPS measurements revealed surface enrichment of PMMA in all compositions. The thicker (66 nm) films exhibit a higher degree of PMMA surface enrichment than the thinner (17 nm) films. AFM imaging allows distinctions to be drawn between blends with differing compositions. The blend films with less than 50% PMMA bulk concentration generally exhibit pitted surfaces; the pit size varies with film thickness and bulk composition. When the PMMA bulk concentration is greater than 50%, the film surface changes to show island-like phase-separated structure. The surface segregation and morphology are explained in terms of solubilities of the two polymers in the solvent and dewetting of PMMA relative to PS. The phase domains on the film surface have also been resolved by frictional force microscopy (FFM) using hydrophilic tips bearing hydroxyl groups.
The effect of boundary surfaces on phase separation and microphase separation in binary polymer blends and block copolymers respectively, has gained increasing attention over the last 5 years. It has been realized that the complex interplay between wetting and phase separation may severely influence the phase separation process thereby leading to near-surface domain structures, which differ distinctly from the respective bulk morphologies. In the present article, we try to summarize the basic features of surface directed (micro-) phase separation in immiscible polymer systems. For both polymer blends and block copolymers a brief review of the historical development is given, followed by a list of selected examples representing the large number of current research activities in these fields. Particular attention is given to the possibility to actively control the domain morphologies via surface interactions in view of possible technological applications.
Nanometer films composed of model ternary blend of deuterated polystyrene (dPS), poly(2-vinylpyridine) (PVP) and poly(methyl methacrylate) (PMMA) were studied after spin-coating from a common solvent. Surface undulations and the distribution of phase-separated domains at the surface and in the bulk are closely related as revealed by atomic (AFM) and lateral (LFM) force microscopy. For the first time the chemical sensitivity of LFM is demonstrated for a ternary polymer mixture. In this case PMMA intercalates between dPS and PVP leading to extended interfaces and surface patterns with two dominant length scales (∼1 μm and ∼100 nm). Both of these length scales as well as the film thickness increase linearly with total polymer concentration in the solvent. Phase separation on two length scales is concluded.
The phase behaviour of a simple fluid or Ising magnet (at temperatures above its roughening transition) confined between parallel walls that exert opposing surface fields h2 = -h1 is found to be markedly different from that which arises for h2 = h1. Whereas critical wetting plays little role for confinement by identical walls, it is of crucial importance for opposing surface fields. Analysis of a Landau functional and other mean-field treatments show that if h1 is such that critical wetting occurs at a single wall (L = ∞) at a transition temperature Tw, then phase coexistence, for finite wall separation L, is restricted to temperatures T < Tc, L, where the critical temperature Tc, L lies below Tw. In the temperature range Tc,b >T >Tw there is a single soft mode phase that is characterized, for zero bulk field and large L, by a +- interface located at the centre of the slit, a transverse correlation length ξ∼≈eL and a solvation force that is repulsive. For large h1, Tw can lie arbitrarily far below the bulk critical temperature Tc,b. Scaling arguments, whose validity we have confirmed in two dimensions by comparison with exact solutions for interfacial Hamiltonians, predict that such behaviour persists beyond mean-field for systems with short-ranged forces. They also predict similar phase behaviour for long-ranged forces, but with ξξ∼ increasing algebraically with L in the soft mode phase. The solvation force t̃fs changes from repulsive to attractive (at large L) as the temperature is reduced below Tw, i.e. the sign of t̃fs reflects wetting characteristics.
Thin films of incompatible polymer blends can form a variety of structures on preparation. For the polymer blend system consisting of two poly(styrene-co-para-bromo-styrene)s at different degrees of bromination, PBrxS/PBryS, the compatibility can be tuned through a variation of the difference in the degree of bromination. Within this blend system, two series of samples with different compatibilities were investigated at various blend compositions. The surface morphology of the thin films was investigated by atomic force microscopy (AFM) measurements, while diffuse X-ray scattering provided additional depth sensitivity at a comparable lateral resolution. The results are indicative for phase separation lateral, as well as perpendicular, to the sample surface.
Partially compatible symmetrical ($N_{\rm A}=N_{\rm B}=N$) binary mixtures of linear flexible polymers (A, B) are considered in the presence of two equivalent walls a distnace $D$ apart, assuming that both walls preferentially adsorb the same component. Using a Flory-Huggins type mean field approach analogous to previous work studying wetting phenomena in the semi-infinite version of this model, where $D\to\infty$, it is shown that a single phase transition occurs in this thin film geometry, namely a phase separation between a A-rich and a B-rich phase (both phases include the “bulk” of the film). The coexistence curve is shifted to smaller values of the inverse Flory-Huggins parameter $\chi^{-1}$ with decreasing $D$, indicating enhanced compatibility the thinner the film. In addition, due to the surface enrichment of the preferred species (B), the critical volume fraction of A monomers is shifted away from $\phi_{\rm crit}=0.5$ (where it occurs for $D\to\infty$ due to the symmetry of the model) to the B-rich side. This behavior is fully analogous to the results established previously for the Ginzburg-Landau model of small molecule mixtures and to Monte Carlo simulations of corresponding lattice gas models. We argue that for symmetric walls the stable solutions always are described by volume fraction profiles $\phi(z)$ that are symmetric as function of the distance $z$ across the film around its center, but sometimes the system is inhomogeneous in the lateral direction parallel to the film, due to phase coexistence between A-rich and B-rich phases. Antisymmetric profiles obtained by other authors for symmetric boundary conditions are only metastable solutions of the mean field equations. The surface excess of B, whose logarithmic divergence as $\ln |\phi-\phi_{\rm coex}|$ signals complete wetting for $D\to\infty$, stays finite (and, in fact, rather small) for finite $D$: hence studies of wetting phenomena in thin film geometry need to be analyzed with great care.
Summary The spin coating process was investigated using solutions of polystyrene dissolved in toluene. The residual film thickness
depends not only on the spinning velocity and concentration, but also on molecular weight. Specific scaling exponents were
determined. The molecular weight dependence was investigated in detail to reveal the type of molecular weight average. This
enables a fast determination of the molecular weight by use of spin coating. The use of model molecular weight distributions
yields a relation to number Mn and weight Mw average molecular weight.