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Silica Nanoparticle Reinforced Composites as Transparent Elastomeric Damping Materials
Abstract
Inspired by the structure of the cornea, which is the transparent front part of the eye, we developed a transparent and tough silica composite elastomer consisting of poly[di(ethylene glycol)methyl ether methacrylate] (PMEO2MA) and 110 nm spherical silica particles without using an organic cross-linking agent. While filler composite elastomers, such as reinforced rubbers, have complex compositions containing multiple additives (dispersants, plasticizers, vulcanizing agents, etc.), the composition of our composite elastomer is very simple. With an increased amount of silica particles, the fracture energy of the composite elastomer (20.2 MJ m-3) was improved by 25 times compared to that of unfilled PMEO2MA (0.8 MJ m-3). Nanoscale mapping using atomic force microscopy elucidated the presence of an interface layer (IL) of approximately 15 nm thickness with a high elastic modulus near the silica particles in the composite elastomer. The fracture energy of the composite elastomer was found to be a maximum when the particle surface distance was approximately 30 nm. This particle surface distance meant that the ILs were just in contact with each other. The surface charge density and Hansen solubility parameter of the silica particles indicated the ionization of silanol groups and interactions caused by hydrogen bonding between polymer chains and the silica surface. The array of silica particles embedded with intervals of a few nanometers was expected to be able to effectively dissipate deformation energy as heat due to shear strain and friction between the particle surface and polymer matrices. Measurements of the vibration-damping properties revealed that the loss factor of the composite elastomer was significantly higher than that of the unfilled elastomer, indicating that the composite elastomer in this study could be applied as an interlayer film for laminated glass in automotive and architectural applications.
... PMEO2MA is a brush polymer structurally related to polyMEA, but with much larger side-chains. The new nanocomposite displayed improved elongation at break values (up to 670% in [68]), which in a wide range improved with increasing filler amount, except at highest silica loadings. At high silica contents (highly regular arrangement of the spheres), full transparency was achieved in the cornea-inspired material. ...
... At high silica contents (highly regular arrangement of the spheres), full transparency was achieved in the cornea-inspired material. In [68], also the role of the interfacial layer and the damping properties of PMEO2MA-silica were investigated. The same group finally also prepared a three-component composite elastomer with PMEO2MA matrix, multilayer graphene (25 µm wide, 6-8 nm thick) as 2D microfiller, and the 110 nm silica spheres [69]. ...
... An interesting finding was, that in spite of the periodic fluctuations in nanofiller concentration, which were close to the micrometre scale in the extreme case (see Figure 4b,e further above), no decrease in optical clarity was observed for the low-filled polyMEA/silica elastomers (see Figure 3 further above). This is in contrast with the results obtained for the distantly related nanocomposite based on PMEO2MA-silica (mentioned in Introduction: Asai, Takeoka and co-workers: [67,68]), where filler particles were much larger (110 nm) and where high optical clarity was observed only for highly regular distributions of the filler spheres. In case of polyMEA/nano-SiO2 studied in this work, the surprising independence of the nanocomposite transparency from fluctuations of filler distribution can be explained by the close match in the refraction indices of filler and matrix: both are given as 1.46 in the literature (amorphous SiO2: [70], polyMEA: [71]). ...
Novel stiff, tough, highly transparent and ultra-extensible self-assembled nanocomposite elastomers based on poly(2-methoxyethylacrylate) (polyMEA) were synthesized. The materials are physically crosslinked by small in-situ-formed silica nanospheres, sized 3–5 nm, which proved to be a very efficient macro-crosslinker in the self-assembled network architecture. Very high values of yield stress (2.3 MPa), tensile strength (3.0 MPa), and modulus (typically 10 MPa), were achieved in combination with ultra-extensibility: the stiffest sample was breaking at 1610% of elongation. Related nanocomposites doubly filled with nano-silica and clay nano-platelets were also prepared, which displayed interesting synergy effects of the fillers at some compositions. All the nanocomposites exhibit ‘plasto-elastic’ tensile behaviour in the ‘as prepared’ state: they display considerable energy absorption (and also ‘necking’ like plastics), but at the same time a large but not complete (50%) retraction of deformation. However, after the first large tensile deformation, the materials irreversibly switch to ‘real elastomeric’ tensile behaviour (with some creep). The initial ‘plasto-elastic’ stretching thus causes an internal rearrangement. The studied materials, which additionally are valuable due to their high transparency, could be of application interest as advanced structural materials in soft robotics, in implant technology, or in regenerative medicine. The presented study focuses on structure-property relationships, and on their effects on physical properties, especially on the complex tensile, elastic and viscoelastic behaviour of the polyMEA nanocomposites.
... 8 Among the plethora of materials reported in the literature, elastomers are considered to be effective VEMs, owing to their flexibility, tunability of loss factor, and ease of application. [11][12][13][14][15] In this context filled nitrile butadiene rubber (NBR) has been studied extensively owing to its intrinsically higher loss factor (tan δ) values as determined from dynamic mechanical analysis (DMA). [16][17][18] Blend of NBR with polyvinyl chloride (PVC) (miscible over a wide range of compositions) constitutes another class of VEM with tunable dynamic properties. ...
We report efficacious vibration damping of carbon black reinforced nitrile butadiene rubber‐polyvinyl chloride blend (NVC) vulcanizates in a constrained layer damping (CLD) configuration. The objective is to compare numerical simulations using finite element method (FEM) and the widely used Ross‐Kerwin‐Ungar (RKU) model with the experimental results. NVC blend (50:50 by weight), was compounded with carbon black as reinforcing filler and vulcanized using a sulfur‐based curative system. Significant reinforcement and toughening were observed for the carbon black reinforced vulcanizates compared to the pristine blend. The viscoelastic properties of the vulcanizates were assessed using dynamic mechanical analysis (DMA), exploring their behavior concerning temperature variations and different frequencies to generate the Prony series coefficients for input in FEM analysis. Vibration damping experiments were conducted using the Oberst beam method in a single cantilever mode, with steel as the vibrating substrate, aluminum as the constraining layer, and the NVC vulcanizates as viscoelastic materials (VEM). Numerical simulations were performed using FEM to analyze mode shapes and frequency response function (FRF), and the RKU model was employed to quantify CLD system loss factors (SLF). The salient finding of this study was the observed SLF values in the range of 0.08–0.20 in the frequency regime of 200–2000 Hz. These values qualify the studied material as effective VEMs for CLD treatment of structural vibrations. This study supports design optimization efforts in a wide range of engineering applications where effective vibration damping is critical.
Highlights
Enhancement of NVC blend properties through carbon black reinforcement.
Detailed analysis of viscoelastic behavior using DMA.
Validation of findings through experimental modal analysis.
Promising results in terms of system loss factors for CLD applications.
... Submicron and nanoscale fibers and particles are already known agents used to harden many polymers and polymer compositions [28,29]. We proposed using the same approach to strengthen gels (not solids), known in modern materials terminology as "soft matter", materials which, due to various physical properties, occupy an intermediate position between solids and liquids. ...
We propose a complex sealing compound for increasing the efficiency of shutoff operations based on natural materials processing for materials such as sand, peat, rice, and husks. We studied the influence of mechanical activation processes on the mechanical and rheological properties of the developed sealants. Through mechanochemical activation, sand dissolution in a low-concentrated alkali solution was possible, and gelling the resulting sodium silicate while reinforcing it with undissolved sand particles to obtain a sealant composition. We used this approach to produce a hybrid sealing compound based on activated rice husks with up to 20% biogenic silicon dioxide combined with mechanically activated peat: the maximum shear strain of the hybrid sealant was 27.7 ± 1.7 Pa. We produced hydrogels based on sodium silicate, polyacrylamide, and chromium acetate, reinforced with mechanically activated rice husks. We studied the sealants’ rheological and filtration properties and observed the respective viscoplastic and viscoelastic properties. An increase in the dispersion concentration from 0 to 0.5% increased the maximum strain value of undestroyed hydrogel’s structure in the range 50–91 Pa and the maximum shear strain from 104 to 128 Pa. The high residual resistance factor values of the ideal fracture model make the natural and plant-renewable raw materials very promising for repair and sealing work.
... However, there is no significant study on the interaction between the polymer surface and HUVECs by force measurement. In contrast, a recently published report described the design of a biocompatible elastomer using a PMEA-silica composite to obtain a tough and tube-like structure of ASDBV compared to the native vessel [35][36][37]. They reported that the mechanical properties of the PMEA-silica composites are comparable to those of native blood vessels and that the antithrombotic properties do not change with a slight increase in silica adhesion, although there is no evidence of endothelial cell adhesion ability. ...
Poly (2-methoxyethyl acrylate) (PMEA) is a US FDA-approved biocompatible polymer, although there is insufficient work on human umbilical vein endothelial cells (HUVECs) and plate-let interaction analysis on PMEA-analogous polymers. In this study, we extensively investigated HUVEC-polymer and platelet-polymer interaction behavior by measuring the adhesion strength using single-cell force spectroscopy. Furthermore, the hydration layer of the polymer interface was observed using frequency-modulation atomic force microscopy. We found that endothelial cells can attach and spread on the PMEA surface with strong adhesion strength compared to other analogous polymers. We found that the hydration layers on the PMEA-analogous polymers were closely related to their weak platelet adhesion behavior. Based on our results, it can be concluded that PMEA is a promising candidate for the construction of artificial small-diameter blood vessels owing to the presence of IW and a hydration layer on the interface.
... More importantly, clear effects on the specific heat capacity change across the T g (ΔC P ) were noticed. The value of ΔC P normalized by the weight fraction of polymer components (ΔC P,N ) decreased with an increase in ϕ P (Table 1), which is usually explained by the formation of the bound rubber phase around the SNPs [37,38]. The significant dispersion of SNPs in the polymer matrix was presented in the previous section, and thus, a clear bound rubber phase was formed because of the strong adhesion between the SNP and polymer chains. ...
We demonstrate the preparation and unique properties of composite vitrimer-like materials containing silica nanoparticles (SNPs). The matrix is composed of polyacrylate-based polymers crosslinked via a pyridine–bromo quaternization reaction, where the quaternized pyridine at the crosslinking point exchanges the bond pair via trans-N-alkylation at high temperatures. Homogeneous composite samples without a large aggregation of SNPs were prepared within the silica volume fraction range (ϕP) from 0.06 to 0.36. The formation of the bound rubber phase and the strong adhesion between the SNP and matrix are admitted in the data from structural, thermal, and mechanical investigations. The crosslinked composites exhibit great stress relaxation at high temperatures because of bond exchange, wherein unique bimodal relaxation behaviors are observed for the samples with high ϕP because of the bound rubber phase. In the final section, some useful functions, such as reprocessability and recyclability, derived from the bond exchange nature are exhibited. We demonstrate the preparation of composite vitrimer-like materials containing silica nanoparticles (SNPs), where the matrix is composed of polyacrylate-based polymers crosslinked via a pyridine–bromo quaternization reaction. The quaternized pyridine at the crosslinking point exchanges the bond pair via trans-N-alkylation at high temperatures. Thermal and mechanical investigations first reveal the formation of the bound rubber phase around the SNP, which eventually affects the relaxation behaviors at high temperatures. In the final section, some useful functions owing to the bond exchange nature, such as reprocessability and recyclability, are exhibited.
... Owing to the desirable mechanical properties of nanosilica, mixing it with another material improves the mechanical properties of the resulting hybrid. Hence, it can be used as an additive (Ding et al. 2018), UV-curable coating (Wu et al. 2013), composite material (Asai et al. 2021), and nanofiller (Khelifa et al. 2013). However, because of the large surface-area-to-particle-size ratio, it is challenging to disperse nanoparticles in polymers owing to aggregation, which subsequently reduces the mechanical properties of the hybrid materials (Mirabedini et al. 2008). ...
Water and wastewater treatment applications stand to benefit immensely from the design and development of new materials based on silica nanoparticles and their derivatives. Nanosilica possesses unique properties, including low toxicity, chemical inertness, and excellent biocompatibility, and can be developed from a variety of sustainable precursor materials. Herein, we provide an account of the recent advances in the synthesis and utilization of nanosilica for wastewater treatment. This review covers key physicochemical aspects of several nanosilica materials and a variety of nanotechnology-enabled wastewater treatment techniques such as adsorption, separation membranes, and antimicrobial applications. It also discusses the prospective design and tuning options for nanosilica production, such as size control, morphological tuning, and surface functionalization. Informative discussions on nanosilica production from agricultural wastes have been offered, with a focus on the synthesis methodologies and pretreatment requirements for biomass precursors. The characterization of the different physicochemical features of nanosilica materials using critical surface analysis methods is discussed. Bio-hybrid nanosilica materials have also been highlighted to emphasize the critical relevance of environmental sustainability in wastewater treatment. To guarantee the thoroughness of the review, insights into nanosilica regeneration and reuse are provided. Overall, it is envisaged that this work’s insights and views will inspire unique and efficient nanosilica material design and development with robust properties for water and wastewater treatment applications.
To solve the conventional problems of chemically cross-linked polymers, the vitrimer concept has attracted great attention because the associative bond-exchange nature in vitrimers provides sustainable functions. So far, various vitrimer designs have been reported using different polymer species with compatible bond exchange mechanisms. However, the biggest barrier for the practical application may be the elaborated functional group modification of the starting components and the limited availability of suitable monomers. As a much simpler preparation, recently, a one-shot preparation of vitrimers from commodity polyesters with no special reactive functional groups by blending and heating with tetra-functional epoxy molecules and base catalysts has been proposed. In this study, further one-shot preparation of composite vitrimers with silica nanoparticles (SNPs) is developed. The effects of SNPs on the cross-linking process and thermo/mechanical properties are first assessed, which reveals the formation of a bound rubber phase on the surfaces of the SNPs. The bound rubber phase also affects the bond exchange features of vitrimers, especially the distribution of relaxation time. Overall, the present facile preparation of composite vitrimers with enhanced mechanical properties could be a hint for practical application, whereas the knowledge on the unique relaxation behaviors of composite vitrimers should be beneficial in the fundamental sense.
Understanding the microstructure change of polymer nanocomposites (PNCs) under elongation deformation at the molecular level is the key to coupling structure-property relationships of PNCs. In this study, we developed our recently proposed in situ extensional rheology NMR device, Rheo-spin NMR, which can simultaneously obtain both the macroscopic stress-strain curves and the microscopic molecular information with the total sample weight of ∼6 mg. This enables us to conduct a detailed investigation of the evolution of the interfacial layer and polymer matrix in nonlinear elongational strain softening behaviors. A quantitative method is established for in situ analysis of (1) the fraction of the interfacial layer and (2) the network strand orientation distribution of the polymer matrix based on the molecular stress function model under active deformation. The results show that for the current highly filled silicone nanocomposite system, the influence of the interfacial layer fraction on mechanical property change during small amplitude deformation is quite minor, while the main role is reflected in rubber network strand reorientation. The Rheo-spin NMR device and the established analysis method are expected to facilitate the understanding of the reinforcement mechanism of PNC, which can be further applied to understand the deformation mechanism of other systems, i.e., glassy and semicrystalline polymers and the vascular tissues.
Nano-silica (SiO 2 ) has been widely used to fill rubbers (crosslinked) and usual polyolefin elastomers (POEs). SiO 2 filled POE with crystalline structure can also be crosslinked. Crystallization, structure, and mechanical properties of crosslinked POE/SiO 2 composites can be affected by SiO 2 . In this paper, crosslinked POE/SiO 2 composites were obtained through two different methods: dynamic crosslinking in molten state and static crosslinking. For the non-crosslinked and static crosslinked composites, SiO 2 had a more significant effect on the nucleation in non-crosslinked POE than in static crosslinked POE. For the dynamic crosslinked composite, SiO 2 and crosslinking points hindered the mobility of POE chains and suppressed the POE crystallization, resulting in smaller and fewer crystals. Dynamic mechanical analysis showed that the SiO 2 and POE were compatible, as evidenced by the lower tan( δ ) value in SiO 2 -filled samples. The latter was more consistent with the higher tensile strength and elongation at break for the non-crosslinked and static crosslinked composites than for the non-filled samples. However, the dynamic crosslinked composite exhibited the worst elongation at break, resulting from the lowest number of crystals and shortened molecular chains due to the shearing that occurred during crosslinking process. The SiO 2 had no observable effect on the permanent deformation of samples.
ConspectusSoft materials, which are optically transparent and exhibit high mechanical performance, will be key materials in important research areas in our future lives, such as highly advanced medicine, soft robotics, and wearable displays. When soft materials are transparent, objects underneath the soft material can be observed by the naked eye. Additionally, because they allow light to pass through, such materials can be used in optical applications such as lenses and displays. Therefore, transparent materials make it possible to transmit light information. Such transparent soft materials have been widely used in our daily life, but their applications will be directed to higher value-added products in the future.One example of a great transparent and flexible soft material is transparent silicone rubber, but even this material is not without drawbacks. Silicone rubber has the property of not adhering to most materials. This feature can be an advantage, but it can also be a disadvantage in some applications. Moreover, its tensile strength, tear strength, and friction resistance are not always strong. The ability to develop transparent soft materials whose surface and mechanical properties differ from those of silicone rubber will lead to the development of various technologies that will enrich our lives.By use of colloid-sized molecules and materials as building blocks to form soft materials, the properties and functions of the resulting soft materials are expected to reflect those of the building blocks. In addition, if the soft material is formed such that the building blocks have a characteristic order, the physical properties derived from the ordered structure can be manifested in the soft material. If the size of the order is as large as the wavelength of light, it is possible to develop materials that can manipulate light. The authors have been working for many years on soft materials in which building blocks of various colloidal sizes are obtained in an ordered state. As a result, we have reported that we can obtain soft materials that are optically transparent and exhibit a variety of mechanical properties. In this Account, we introduce our various works on optically transparent soft materials consisting of colloidal building blocks that exhibit high mechanical performance. To help you understand our research, a basic description of the optical transparency of the materials and the mechanical properties of the materials is given in the Supporting Information, which you are invited to read if necessary.
The nanostructures of temperature-responsive and biocompatible gels were investigated by small-angle neutron scattering (SANS). The gels were copolymerized using two types of monomers with different ethylene glycol side chain lengths: diethylene glycol methacrylate (MeO2MA) (short side chain) and oligo-ethylene glycol methyl ether methacrylate (OEGMA) (long side chain). The temperature-responsive behavior was ascribed to the nanoscale structures and depended on the copolymerization ratio. The SANS profiles of swollen OEGMA-rich gels exhibited a characteristic peak, indicating a strong correlation with the hydrophobic main chain domains in the hydrophilic matrix. The long hydrophilic side chains of OEGMA acted as a cushioning material between the domains. On the other hand, the domains were randomly distributed in the MeO2MA-rich gels. As the temperature increased, the domains grew in the gels due to hydrophobic interactions between the dehydrated polymers. As a result, the peaks, that is, the domain periodicity, disappeared in the SANS profiles. The results of this study should lead to a synthesis strategy to control the physical properties and structures of such hydrogels for advanced applications, for example, biofilms, coatings, and carriers.
Significance
Many applications in engineering require stretchable materials that dissipate little energy during normal operation of cyclic loads (low hysteresis), but dissipate much energy to resist rupture (high toughness), and survive prolonged cyclic loads (fatigue resistant). However, existing stretchable materials cannot meet these requirements simultaneously. Here we present a principle to achieve this goal, and demonstrate the principle by a composite of two stretchable materials of low hysteresis, with large modulus contrast and strong adhesion. The composite retains the low hysteresis, but is much tougher and more fatigue resistant than the constituents. The same principle applies to elastomers, gels, and elastomer–gel hybrids. This class of materials provides opportunities to create high-cycle and low-dissipation soft robots and soft human–machine interfaces.
The current approach to regulating mechanical properties of elastomeric materials is predominately based on the exploratory mixing of different polymers, solvents, and fillers—which is both inflexible in application and imprecise in property control. Here we overview a new materials design approach that harnesses well-defined molecular codes of independently controlled architectural parameters to program grand variation of mechanical “phenotypes”. This design-by-architecture approach generates a set of universal correlations between the molecular architecture and the physical behavior of elastomers. In turn, this will lead to novel solvent-free materials that closely mimic the mechanical behavior of biological tissues, ranging from soft fat tissue to firm skin, and fundamentally change high-impact technologies such as soft robotics, wearable electronics, and biomedical devices.
An elastomer is a three-dimensional network with a cross-linked polymer chain that undergoes large deformation with a small external force and returns to its original state when the external force is removed. Because of this hyperelasticity, elastomers are regarded as one of the best candidates for the matrix material of soft robots. However, the comprehensive performance required of matrix materials is a special challenge because improvement of some matrix properties often causes the deterioration of others. For example, an improvement in toughness can be realized by adding a large amount of filler to an elastomer, but to the impairment of optical transparency. Therefore, to produce an elastomer exhibiting optimum properties suitable for the desired purpose, very elaborate, complicated materials are often devised. Here, we have succeeded in creating an optically transparent, easily fabricated elastomer with good extensibility and high toughness by using a polyrotaxane (PR) composed of cyclic molecules and a linear polymer as a cross-linking agent. In general, elastomers having conventional cross-linked structures are susceptible to breakage as a result of loss of extensibility at high cross-linking density. We found that the toughness of the transparent elastomer prepared using the PR cross-linking agent is enhanced along with its Young’s modulus as cross-linking density is increased.
The noise source of hybrid electric vehicle is varied, which makes the frequency band from low-frequency to high-frequency distribution. Using a single material to control the hybrid electric vehicle interior noise to improve the performance of vehicle, noise, vibration, and harshness performance is limited. In this article, the composite noise reduction materials which have the function of damping and acoustic absorption are used to control the vehicle interior noise. First, the noise characteristics of the hybrid electric vehicle are analyzed. The distribution of the hybrid electric vehicle interior noise is obtained. Then, the sound-absorbing properties of polyester-polypropylene bi-component fiber are analyzed. The low-frequency noise reduction principle of butyl rubber damping material is also analyzed. Finally, the noise reduction materials consisting of polyester-polypropylene and butyl rubber are added to the hybrid electric vehicle roof and floor. After improving material, the hybrid electric vehicle interior noise is test. As the result, the car driver’s right ear A-weighting sound pressure levels have been greatly improved. The low-frequency noise below 400 Hz reduced 1.5 dB(A), and the high-frequency noise above 400 Hz reduced 5.2 dB(A). The total noise reduction reached 3.3 dB(A) near the driver’s right ear, which significantly improve the noise, vibration, and harshness performance of hybrid electric vehicle.
In this paper, a micromechanically based constitutive model capturing anisotropic stress softening in reinforced elastomers under quasi-static loading is presented. A novel spatial distribution function of polymer chains taking into account its deformation induced anisotropic alignment is proposed. By this means, the anisotropic network averaging over the unit sphere can be evaluated analytically. The anisotropic alignment of polymer chains introduces non-affine deformation in the model. Furthermore, the anisotropic evolution of the Mullins effect and hysteresis are also derived analytically, so that no numerical integration is applied here. The model includes very few physically motivated material parameters and demonstrates good agreement with multi-dimensional experimental data as well as with molecular dynamics simulations.
A molded film of single-component polymer-grafted nanoparticles (SPNP), consisting of a spherical silica core and densely grafted polymer chains bearing hydrogen-bonding side groups capable of physical crosslinking, was investigated by
in situ
ultra-small-angle X-ray scattering (USAXS) measurement during a uniaxial stretching process. Static USAXS revealed that the molded SPNP formed a highly oriented twinned face-centered cubic (f.c.c.) lattice structure with the [11−1] plane aligned nearly parallel to the film surface in the initial state. Structural analysis of
in situ
USAXS using a model of uniaxial deformation induced by rearrangement of the nanoparticles revealed that the f.c.c. lattice was distorted in the stretching direction in proportion to the macroscopic strain until the strain reached 35%, and subsequently changed into other f.c.c. lattices with different orientations. The lattice distortion and structural transition behavior corresponded well to the elastic and plastic deformation regimes, respectively, observed in the stress–strain curve. The attractive interaction of the hydrogen bond is considered to form only at the top surface of the shell and then plays an effective role in cross-linking between nanoparticles. The rearrangement mechanism of the nanoparticles is well accounted for by a strong repulsive interaction between the densely grafted polymer shells of neighboring particles.
We investigate the polymer packing around nanoparticles and polymer/nanoparticle topological constraints (entanglements) in nanocomposites containing spherical nanoparticles in comparison to pure polymer melts using molecular dynamics (MD) simulations. The polymer - nanoparticle attraction leads to good dispersion of nanoparticles. We observe an increase in the number of topological constraints (decrease of total entanglement length Ne with nanoparticle loading in the polymer matrix) in nanocomposites due to nanoparticles, as evidenced by larger contour lengths of the primitive paths. An increase of the nanoparticle radius reduces the polymer - particle entanglements. These studies demonstrate that the interaction between polymers and nanoparticles does not affect the total entanglement length because in nanocomposites with small nanoparticles, the polymer-nanoparticles topological constraints dominate.
The purpose of this paper is to determine the performance of damping provided by different viscoelastic materials (VEM). The damping performance of the viscoelastic material is investigated using constrained layer damping (CLD) treatment. Using the viscoelastic material as the core layer in the CLD structures, significant amount of damping can be obtained with minimum impact on the total mass of the structure. Viscoelastic polymeric materials have been used to damp vibrations for many years. Energy is dissipated through relaxation processes in the long chain molecular networks when a polymeric material is subjected to vibrations.
We propose a coarse-grained model able to describe filled entangled polymer melts. Our purpose is to study the reinforcement caused by the effect of fillers on the entanglement network, as speculated in previous experimental work, and also observed in molecular dynamics studies. In this work, the filler volume fraction effect, the distribution of the fillers (cubic lattice, randomly dispersed, and small clusters randomly dispersed) and the presence or absence of grafted chains on the fillers are investigated. Our model is based on a “slip-link” model initially developed to study the entanglements in pure polymer melts and offers a less costly computational method than molecular dynamics simulations for the study of entangled polymer melts. The polymer chains are described as Rouse chains of Brownian particles connected by Hookean springs, and are subject to friction and random forces. Entanglements are artificially imposed by objects (slip-links) exhibiting statistical fluctuations that do not modify the equilibrium statistics of the melt. In addition we introduced excluded volume interactions between chain segments, to take into account the incompressibility of the melt. These excluded volume interactions do not perturb the dynamics of the chains in the homogeneous limit as expected from theoretical considerations on short range interactions. Finally, the fillers are modeled by immobile spherical objects, with or without grafted chains, which interact with a repulsive potential with the chain monomers. The chains grafted onto the fillers are represented by “additional slip links” confined in the vicinity of each filler. We first present the effect of the filler distribution and filler volume fraction, considering only bare fillers. Then, the effect of grafted chains via the additional slip-links is also shown as a function of the same parameters. Our results show that the presence of grafted chains induces an important change in the viscosity, calculated by integrating the stress autocorrelation function. Both the plateau value and the terminal relaxation time depend on the density of fillers and on the number of grafted chains. Moreover, we find that a disordered filler configuration induces confinement effects that amplify reinforcement compared to the case of a perfectly ordered configuration.
We show that a bound layer composed of short and long chains can be exploited to regulate the elastic moduli of bulk polymer nanocomposites at same particle loadings and dispersion states. The bound layer thickness on particles with high coverage of long chains is reduced with oscillatory deformation in a model attractive nanocomposite system. Reversibility of the bound layer is, thus, possible for the short chains in the interphase. Compositional dynamic heterogeneity in the interphase has subsequent effects on the fragility of composites. With increasing amount of adsorbed long chains, the fragility index systematically improves from moderate to high values. Our results suggest that the interphase layer between adsorbed chains and free matrix directly governs the reinforcement in poly(methyl methacrylate)–silica nanocomposites and can be dynamically altered under large shear.
The static and dynamic properties of poly(2-vinylpyridine)/silica nanocomposites are investigated by temperature modulated differential scanning calorimetry, broadband dielectric spectroscopy (BDS), small-angle X-ray scattering (SAXS), and transmission electron microscopy. Both BDS and SAXS detect the existence of an interfacial polymer layer on the surface of nanoparticles. The results show that whereas the calorimetric glass transition temperature varies only weakly with nanoparticle loading, the segmental mobility of the polymer interfacial layer is slower than the bulk polymer by 2 orders of magnitude. Detailed analysis of BDS and SAXS data reveal that the interfacial layer has a thickness of 4–6 nm irrespective of the nanoparticle concentration. These results demonstrate that in contrast to some recent articles on polymer nanocomposites, the interfacial polymer layer is by no means a “dead layer”. However, its existence might provide some explanation for controversies surrounding the dynamics of polymer nanocomposites.
Scientists are exploring elastic and soft forms of robots, electronic skin and energy harvesters, dreaming to mimic nature and to enable novel applications in wide fields, from consumer and mobile appliances to biomedical systems, sports and healthcare. All conceivable classes of materials with a wide range of mechanical, physical and chemical properties are employed, from liquids and gels to organic and inorganic solids. Functionalities never seen before are achieved. In this review we discuss soft robots which allow actuation with several degrees of freedom. We show that different actuation mechanisms lead to similar actuators, capable of complex and smooth movements in 3d space. We introduce latest research examples in sensor skin development and discuss ultraflexible electronic circuits, light emitting diodes and solar cells as examples. Additional functionalities of sensor skin, such as visual sensors inspired by animal eyes, camouflage, self-cleaning and healing and on-skin energy storage and generation are briefly reviewed. Finally, we discuss a paradigm change in energy harvesting, away from hard energy generators to soft ones based on dielectric elastomers. Such systems are shown to work with high energy of conversion, making them potentially interesting for harvesting mechanical energy from human gait, winds and ocean waves.
This paper discusses the influence of surface energy on the contact between elastic solids. Equations are derived for its effect upon the contact size and the force of adhesion between two lightly loaded spherical solid surfaces. The theory is supported by experiments carried out on the contact of rubber and gelatine spheres.
Suspensions of spherical colloidal particles in a liquid show a fascinating variety of phase behaviour which can mimic that of simple atomic liquids and solids. `Colloidal fluids'1-4, in which there are significant short-range correlations between the positions of neighbouring particles, and `colloidal crystals'5-7, which have long-range spatial order, have been investigated extensively. We report here a detailed study of the phase diagram of suspensions of colloidal spheres which interact through a steep repulsive potential. With increasing particle concentration we observed a progression from colloidal fluid, to fluid and crystal phases in coexistence, to fully crystallized samples. At the highest concentrations we obtained very viscous samples in which full crystallization had not occurred after several months and in which the particles appeared to be arranged as an amorphous `colloidal glass'. The empirical phase diagram can be reproduced reasonably well by an effective hard-sphere model. The observation of the colloidal glass phase is interesting both in itself and because of possible relevance to the manufacture of high-strength ceramics8.
To provide insights into how nanoparticles (NPs) with different graft structures affect the moduli of polymer nanocomposites (PNCs), we have performed various surface modifications on silica nanoparticles, including the grafted small molecules, the long low-density grafted chains, and the cross-linked grafted shell. The nanoparticles can be well dispersed in a polymethyl methacrylate (PMMA) matrix using suitable mixing methods, and the inter-particle surface to surface distance follows the modified Woodcock model. The interfacial layer volume fraction was studied by the temperature-modulated differential scanning calorimetry (TMDSC), which showed a non-monotone variation with the increase of particle concentrations in the long low-density grafted particles filled PNCs. The effective interfacial thickness was also determined from the hydrodynamic enhancement of the rubbery plateau modulus at low particle loadings, which is smaller than that from TMDSC. The matrix entanglement contribution was then decoupled from the hydrodynamic contribution and the confined diffusion of nanoparticles. The change of mean tube diameter of polymer chains appeared when the interparticle surface to surface distance became comparable to the unconfined tube diameter. By adjusting the grafting phase structures, the interface structures, and the rubbery modulus of PNCs can be effectively controlled.
Poly(2-methoxyethyl acrylate) (PMEA) has attracted attention as a biocompatible polymer that is used as an antithrombotic coating agent for medical devices, such as during artificial heart and lung fabrication. However, PMEA is a viscous liquid polymer with low Tg, and its physical strength is poor even if a cross-linker is used, so it is difficult to make tough and freestanding objects from it. Here, we design and fabricate a biocompatible elastomer made of tough, self-supporting PMEA-silica composites. The toughness of the composite elastomer increases as a function of silica particle filling, and the stress at break of it is improved from 0.3 MPa to 6.7 MPa. The fracture energy of the composite elastomer with 39.5 vol% silica particles is up to 15 times higher than that of the cross-linked PMEA with no silica particles and the material demonstrates stress-strain behavior that is similar to that of biological soft tissue, which exhibits nonlinear elasticity. In addition, the composite elastomer shows the potential to be an antithrombotic property, while the results of the platelet adhesion test of the composite elastomer show that the number of adhered platelets is not significantly affected by the silica addition. Since the composite elastomer can be rapidly 3D-printed into complex geometries with high resolution features, it is expected to contribute to the development of medical devices from readily available materials.
The dynamic properties of filled elastomers have been a subject of both fundamental and applied research interest. We report on the tunable and synergistic effect of carbon black and anisotropic additives on the viscoelastic and vibration damping properties of a segmented polyurethane (PU). To this end, PU composites were prepared using aramid short fiber and graphite as micro-scale additives and organically modified nanoclay as the nano-scale anisotropic additive in conjunction with carbon black. The degree of hydrogen bonding, phase and filler dispersion morphology of the composites were ascertained by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Dynamic mechanical analysis of the composites revealed an additive induced decrease in the intensity of the dissipation factor, tan δ, implying immobilization of polymer chains in the vicinity of the filler surfaces and significant reinforcement effect of fillers on the PU matrix. The composite system loss factor evaluated from the modal analysis of frequency response, in constrained layer damping mode, revealed loss factor values in the range 0.1–0.2 (at 710 Hz), as against the corresponding value of 0.027 for the bare metal beam. A salient finding of the present study was the filler induced enhancement in system loss factor, while the tan δ magnitude decreased, which has been reconciled with several causal mechanisms.
Since almost a decade, a growing experimental evidence has revealed a strong correlation between the properties of nanoconfined polymers and the number of chains irreversibly adsorbed onto nonrepulsive interfaces, e.g. the supporting substrate of thin polymer coatings, or nanofillers dispersed in polymer melts. Based on such correlation, it has already been possible to tailor structural and dynamics properties – as the glass transition temperature, the crystallization rate, the thermal expansion coefficients, viscosity and wettability – of nanomaterials by controlling the adsorption kinetics. This evidence indicates that irreversible adsorption affects nanoconfinement effects. More recently, also the opposite phenomenon was experimentally observed: nanoconfinement alters interfacial interactions and, consequently, also the number of chains adsorbed in equilibrium conditions. In this review we discuss on this intriguing interplay between irreversible adsorption and nanoconfinement effects in ultrathin polymer films. After introducing the methods currently used to prepare adsorbed layers and to measure the number of irreversibly adsorbed chains, we analyze the models currently used to describe the kinetics of adsorption in polymer melts. We then discuss on the structure of adsorbed polymer layers, focusing on the complex macromolecular architecture of interfacial chains and on their thermal expansion; we examine the way the structure of the adsorbed layer affects the thermal glass transition temperature, vitrification, and crystallization. By analyzing segmental dynamics of 1D confined systems, we describe experiments to track the changes in density during adsorption. We conclude this review with an analysis of the impact of nanoconfinement on adsorption, and a perspective on future work where we also address the key ideas of irreversibility, equilibration and long-range interactions.
The addition of nanoparticles (NPs) to a polymer matrix, forming a polymer nanocomposite (PNC), is known to alter the microscopic dynamic processes of both species which leads to unique macroscopic material properties of the PNC. Because the NPs and polymers have overlapping characteristic length, time, and energy scales, the interactions within these materials are complex and the dynamics are interrelated. In this review, we present an overview of experimental, simulation, and theoretical results that probe multi-scale polymer and nanoparticle dynamics in polymer nanocomposites and navigate the dense parameter space presented by these multicomponent systems. Although a variety of PNC systems are mentioned, we focus this discussion on linear thermoplastics filled with hard spherical and cylindrical NPs in the melt state. We begin by introducing PNCs, the dynamic processes within them, and the importance of dynamics for properties and processing. At the smallest length and time scales, we discuss segmental dynamics in PNCs, including the role of polymer attributes, NP attributes, and NP-polymer interactions. Then, we present measurements of collective motions and intermediate (Rouse) dynamics in various PNC materials. At longer length and time scales, we discuss polymer center-of-mass diffusion in PNCs with either spherical or anisotropic NPs. We then explore the dynamics of the NPs in PNCs and polymer melts, including theoretical predictions, simulation results, and experimental observations. Finally, we note some of the remaining challenges in probing dynamics in PNC materials and fundamentally studying PNCs more generally.
We propose a strategy to develop a colorless, transparent and tough composite elastomer inspired by the cornea that is a transparent front portion of the eyeball. The composite elastomer in which 34 vol% of hard silica particles with a uniform particle size are dispersed as a filler in a low-crosslinking polymer network exhibits a fracture energy that is approximately 13.5 times higher than that of a system without the silica particles. This strategy also makes the elastomer optically transparent because the light scattered by each silica particle that forms an ordered structure in the polymer network is cancelled by interference. This research may pave the way for the development of optically transparent and durable materials for applications such as advanced medical devices and soft robots.
We study the strain-adaptive behavior of the self-assembled networks of linear-bottlebrush-linear (LBL) triblock copolymers using a combination of analytical calculations and molecular dynamics simulations. Interactions between immiscible blocks result in microphase separation and formation of soft and strain-adaptive composite networks. Such unique network properties are manifestations of the architectural asymmetry of the two blocks: (i) flexible linear chains that aggregate into domains and (ii) bottlebrush strands that form a soft matrix. The mechanical response of the networks is a two-stage process, which starts with the extension of the bottlebrush network strands (elastic deformation regime) followed by the pulling out of the linear chains from L-domains (yielding regime). In the elastic stage of deformation, the stress−strain curves are described by a nonlinear network deformation model, which considers bottlebrush strands as semiflexible chains with an effective Kuhn length. During the yielding stage, forces generated in bottlebrush strands become sufficient to pull linear chains from the aggregates. This pull out process creates a new interface between linear and bottlebrush blocks and occurs at a constant force. This is manifested in a linear dependence of the true stress on the network elongation ratio, σtrue ∝ λ, for uniaxial network deformations. The two-stage network deformation process is incorporated into a unifying model of strain-adaptive network deformation. The model predictions are confirmed by molecular dynamics simulations of uniaxial deformation of self-assembled LBL copolymer networks and by experimental results for copolymers consisting of poly(dimethylsiloxane) bottlebrush block and two poly(methyl methacrylate) linear chain blocks with different compositions and block lengths.
The dynamic and static properties of the interfacial region between polymer and nanoparticles have wide-ranging consequences on performances of nanomaterials. The thickness and density of the static layer are particularly difficult to assess experimentally due to superimposing nanoparticle interactions. Here, we tune the dispersion of silica nanoparticles in nanocomposites by pre-adsorption of polymer layers in the precursor solutions, and by varying the molecular weight of the matrix chains. Nanocomposite structures ranging from ideal dispersion to repulsive order or various degrees of aggregation are generated and observed by small-angle scattering. Pre-adsorbed chains are found to promote ideal dispersion, before desorption in the late stages of nanocomposite formation. The microstructure of the interfacial polymer layer is characterized by detailed modeling of X-ray and neutron scattering. Only in ideally well-dispersed systems a static interfacial layer of reduced polymer density over a thickness of ca. 2 nm is evidenced based on the analysis with a form-free density profile optimized using numerical simulations. This interfacial gradient layer is found to be independent of the thickness of the initially adsorbed polymer, but appears to be generated by out-of-equilibrium packing and folding of the pre-adsorbed layer. The impact of annealing is investigated to study the approach of equilibrium, showing that initially ideally well-dispersed systems adopt a repulsive hard-sphere structure, while the static interfacial layer disappears. This study thus promotes the fundamental understanding of the interplay between effects which are decisive for macroscopic material properties: polymer-mediated interparticle interactions, and particle interfacial effects on surrounding polymer.
Polymer segmental dynamics, center-of-mass chain diffusion, and nanoparticle (NP) diffusion are directly measured in a series of polymer nanocomposites (PNC) composed of very small (radius 0.9 nm) octa(aminophenyl) polyhedral oligomeric silsesquioxane (OAPS) NPs and poly(2-vinylpyridine) (P2VP) of varying molecular weight. With increasing OAPS concentration, both the segment reorientational relaxation rate (measured by dielectric spectroscopy) and polymer chain center-of-mass diffusion coefficient (measured by elastic recoil detection) are substantially reduced, with reductions relative to bulk reaching 80% and 60%, respectively, at 25 vol % OAPS. This commensurate slowing of both the segmental relaxation and chain diffusion process is fundamentally different than the case of PNCs composed of larger, immobile nanoparticles, where the motion of most segments remains relatively unaltered even though chain diffusion is significantly reduced. Next, using Rutherford backscattering spectrometry to probe the NP diffusion process, we find that small OAPS NPs diffuse anomalously fast in these P2VP-based PNCs, reaching diffusivities 10-10000 times faster than predicted by the Stokes-Einstein relation assuming the melt zero-shear viscosity. The OAPS diffusion coefficients are found to scale very weakly with molecular weight, M w-0.7±0.1 , and our analysis shows that this characteristic OAPS diffusion rate occurs on intermediate microscopic time scales, lying between the Rouse time of a Kuhn monomer 0 and the Rouse time of an entanglement strand e . Our findings suggest that transport of these very small, attractive nanoparticles through well-entangled polymer melts is consistent with the recently reported vehicle mechanism of nanoparticle diffusion.
Understanding the structure and dynamics of the bound polymer layer (BL) that forms on favorably interacting nanoparticles (NPs) is critical to revealing the mechanisms responsible for material property enhancements in polymer nanocomposites (PNCs). Here we use small angle neutron scattering to probe the temporal persistence of this BL in the canonical case of poly(2-vinylpyridine) (P2VP) mixed with silica NPs at two representative temperatures. We have observed almost no long-term reorganization at 150 °C (T g,P2VP + 50 °C), but a notable reduction in the BL thickness at 175 °C. We believe that this apparently strong temperature dependence arises from the polyvalency of the binding of a single P2VP chain to a NP. Thus, while the adsorption-desorption process of a single segment is an activated process that occurs over a broad temperature range, the cooperative nature of requiring multiple segments to desorb converts this into a process that occurs over a seemingly narrow temperature range.
The addition of nanoparticles to a polymer matrix is a well-known process to improve mechanical properties of polymers. Many studies of the mechanical reinforcement in polymer nanocomposites (PNCs) focus on rubbery matrices; however, much less effort concentrates on factors controlling mechanical performance of the technologically important glassy PNCs. This paper presents a study of the effect of the polymer molecular weight (MW) on the overall mechanical properties of glassy PNCs with attractive interaction by using Brillouin light scattering. We found that the mechanical moduli (bulk and shear) have a non-monotonic dependence on MW that cannot be predicted by simple rule of mixtures. The moduli increase with increasing MW up to 100 kg/mol followed by a drop at higher MW. We demonstrate that the change in the mechanical properties of PNCs can be associated with the properties of the interfacial polymer layer. The latter depend on the interfacial chain packing and stretching, as well as polymer bridging that vary differently with MW of the polymer. These competing contributions lead to the observed non-monotonic variations of glassy PNC moduli with MW. Our work provides a simple, cost-effective, and efficient way to control the mechanical properties of glassy PNCs by tuning the polymer chain length. Our finding can be beneficial for rational design of PNCs with desired mechanical performance.
This topical review discusses the theoretical progress made in the field of polymer nanocomposites, i.e., hybrid materials created by mixing (typically inorganic) nanoparticles (NPs) with organic polymers. It primarily focuses on the outstanding issues in this field and is structured around five separate topics: (i) the synthesis of functionalized nanoparticles; (ii) their phase behavior when mixed with a homopolymer matrix and their assembly into well-defined superstructures; (iii) the role of processing on the structures realized by these hybrid materials and the role of the mobilities of the different constituents; (iv) the role of external fields (electric, magnetic) in the active assembly of the NPs; and (v) the engineering properties that result and the factors that control them. While the most is known about topic (ii), we believe that significant progress needs to be made in the other four topics before the practical promise offered by these materials can be realized. This review delineates the most pressing issues on these topics and poses specific questions that we believe need to be addressed in the immediate future.
Atomic force microscope (AFM)-based nanomechanics is one of the most promising tools for accessing the rubber-filler interface while providing not only structural information but also mechanical-property evaluation. An AFM-based static modulus map is used to close in on the understanding of the filler reinforcement effect. As an example, a famous Guth-Gold equation is verified by comparing tensile testing and AFM. Two different novel methods are also introduced to visualize viscoelastic quantities such as storage and loss moduli, loss tangent, relaxation modulus, and viscosity. The difference in segmental dynamics between a rubber matrix and an interfacial region will be reviewed.
In recent years it has become clear that the interfacial layer formed around nanoparticles in polymernanocomposites (PNCs) is critical for controlling their macroscopic properties. The interfacial layer occupies a significant volume fraction of the polymer matrix in PNCs and creates strong intrinsic heterogeneity in their structure and dynamics. Here, we focus on analysis of the structure and dynamics of the interfacial region in model PNCs with well-dispersed, spherical nanoparticles with attractive interactions. First, we discuss several experimental techniques that provide structural and dynamic information on the interfacial region in PNCs. Then, we discuss the role of various microscopic parameters in controlling structure and dynamics of the interfacial layer. The analysis presented emphasizes the importance of the polymer-nanoparticle interactions for the slowing down dynamics in the interfacial region, while the thickness of the interfacial layer appears to be dependent on chain rigidity, and has been shown to increase with cooling upon approaching the glass transition. Aside from chain rigidity and polymer-nanoparticle interactions, the interfacial layer properties are also affected by the molecular weight of the polymer and the size of the nanoparticles. In the final part of this focus article, we emphasize the important challenges in the field of polymernanocomposites and a potential analogy with the behavior observed in thin films.
Supramolecular elastomers are supramolecular soft polymeric materials with both transient cross-links and (semi)permanent cross-links, where transient cross-links have a finite relaxation time and are typically composed of noncovalent bonds such as hydrogen bonds, metal–ligand coordination bonds, etc., whereas (semi)permanent cross-links such as covalent bonds, crystalline segments and glassy hard domains have an infinite relaxation time. Excellent elastomeric properties were induced by all the above molecular features. However, if soft polymeric materials have only supramolecular/transient cross-links with a finite relaxation time, the materials eventually flow within a finite time and cannot behave as elastomers; in other words, they are merely supramolecular polymers that have only transient cross-links or bonds. Therefore, the key to preparing higher-performance supramolecular elastomers is their molecular design. Here, we firstly review the fundamental design of supramolecular polymers to make the differences between supramolecular polymers and supramolecular elastomers clear. Second, we discuss the simultaneous incorporation of (semi)permanent cross-links and supramolecular cross-links into polymeric materials to prepare supramolecular elastomers. Finally we summarized their properties and potential applications.
The field of polymer nanocomposites has been at the forefront of research in the polymer community for the past few decades. Foundational work published in Macromolecules during this time has emphasized the physics and chemistry of the inclusion of nanofillers; remarkable early developments suggested that these materials would create a revolution in the plastics industry. After 25 years of innovative and groundbreaking research, PNCs have enabled many niche solutions. To complement the extensive literature currently available, we focus this Perspective on four case studies of PNCs applications: (i) filled rubbers, (ii) continuous fiber reinforced thermoset composites, (iii) membranes for gas separations, and (iv) dielectrics for capacitors and insulation. After presenting synthetic developments we discuss the application of polymer nanocomposites to each of these topic areas; successes will be noted, and we will finish each section by highlighting the various technological bottlenecks that need to be overcome to take these materials to full-scale practical application. By considering past successes and failures, we will emphasize the critical fundamental science needed to further expand the practical relevance of these materials.
The mechanical reinforcement of polymer nanocomposites (PNCs) above the glass transition temperature, Tg, has been extensively studied. However, not much is known about the origin of this effect below Tg. In this Letter, we unravel the mechanism of PNC reinforcement within the glassy state by directly probing nanoscale mechanical properties with atomic force microscopy and macroscopic properties with Brillouin light scattering. Our results unambiguously show that the ‘glassy’ Young’s modulus in the interfacial polymer layer of PNCs is two times higher than in the bulk polymer, which results in significant reinforcement below Tg. We ascribe this phenomenon to a highly stretching of the chains within the interfacial layer. Since the interfacial chain packing is essentially temperature independent, these findings provide a new insight in the mechanical reinforcement of PNCs also above Tg.
For semicrystalline polymers there is an ongoing debate at what temperature the immobilized or rigid amorphous fraction (RAF) devitrifies (relaxes). The question if the polymer crystals are melting first and simultaneously the RAF devitrifies or the RAF devitrifies first and later on the crystals melt cannot be answered easily on the example of semicrystalline polymers. This is because the crystals, which are the reason for the immobilization of the polymer, often disappear (melt) in the same temperature range as the RAF. For polymer nanocomposites the situation is simpler. Silica nanoparticles do not melt or undergo other phase transitions altering the polymer–nanoparticle interaction in the temperature range where the polymer is thermally stable (does not degrade). The existence of an immobilized fraction in PMMA SiO2 nanocomposites was shown on the basis of heat capacity measurements at the glass transition of the polymer. The results were verified by enthalpy relaxation experiments below the glass transition. The immobilized layer is about 2 nm thick at low filler content if agglomeration is not dominant. The thickness of the layer is similar to that found in semicrystalline polymers and independent from the shape of the nanoparticles. Nanocomposites therefore offer a unique opportunity to study the devitrification of the immobilized fraction (RAF) without interference of melting of crystals as in semicrystalline polymers. It was found that the interaction between the SiO2 nanoparticles and the PMMA is so strong that no devitrification occurs before degradation of the polymer. No gradual increase of heat capacity or a broadening of the glass transition was found. The cooperatively rearranging regions (CRR) are either immobilized or mobile. No intermediate states are found. The results obtained for the polymer nanocomposites support the view that the reason for the restricted mobility must disappear before the RAF can devitrify. For semicrystalline polymers this means that rigid crystals must melt before the RAF can relax.
We report in situ nanostructures and dynamics of polybutadiene (PB) chains bound to carbon black (CB) fillers (the so-called "bound polymer layer (BPL)") in a good solvent. The BPL on the CB fillers was extracted by solvent leaching of a CB-filled PB compound and subsequently dispersed in deuterated toluene to label the BPL for small-angle neutron scattering and neutron spin echo techniques. The results demonstrate that the BPL is composed of two regions regardless of molecular weights of PB: the inner unswollen region of ≈ 0.5 nm thick and outer swollen region where the polymer chains display a parabolic profile with a diffuse tail. In addition, the results show that the dynamics of the swollen bound chains can be explained by the so-called "breathing mode" and is generalized with the thickness of the swollen BPL.
The modulus increase in rubbers filled with solid particles is investigated in detail here using an approach known widely as the Guth-Gold equation. The Guth-Gold equation for the modulus increase at small strains was reexamined using six different species of carbon black (Printex, super abrasion furnace, intermediate SAF, high abrasion furnace, fine thermal, and medium thermal carbon blacks) together with model experiments using steel rods and carbon nanotubes. The Guth-Gold equation is only applicable to such systems where the mutual interaction between particles is very weak and thus they behave independently of each other. In real carbon black-filled rubbers, however, carbon particles or aggregates are connected to each other to form network structures, which can even conduct electricity when the filler volume fraction exceeds the percolation threshold. In the real systems, the modulus increase due to the rigid filler deviates from the Guth-Gold equation even at a small volume fraction of the filler of 0.05-0.1, the deviation being significantly greater at higher volume fractions. The authors propose a modified Guth-Gold equation for carbon black filled rubbers by adding a third power of the volume fraction of the blacks to the equation, which shows a good agreement with the experimental modulus increase (G/G(0)) for six species of carbon black filled rubbers, G/G(0) = 1 + 25 phi(eff) + 14.1 phi(2)(eff) + 0.20(root S)(3)phi(eff) where G and G(0) are the modulus of the filled and unfilled rubbers, respectively; Teff is the effective volume fraction; and S is the Brunauer, Emmett, Teller surface area of the blacks. The modified Guth Gold equation indicates that the specific surface volume (root S)(3) closely relates to the bound rubber surrounding the carbon particles, and therefore this governs the reinforcing structures and the level of the reinforcement in carbon black-filled rubbers.
We performed a large-scale dissipative dynamics simulation to study the structural changes in unfilled and filled elastomers during uniaxial deformation, which helped to shed some light on the underlying reasons of filler reinforcement in rubber nanocomposites. Equilibrium stress–stain curves for different cross-linker concentrations and filler content were obtained, and their features were compared to the experimental data. Dependences of segmental orientation on deformation and true stress were studied; these dependences are discussed in the light of theoretical predictions and available experimental data. The structural changes in the deformed state were studied as well, namely, the dependences of mean end-to-end distance on the subchain length. For the filled elastomers it was found that in the matrix there are several sets of subchains with distinct properties. Part of the subchains which are not connected to the filler particles are deformed slightly more than in the unfilled matrix; the subchains connected to the filler particles are deformed significantly more. This “separation” is the main reason for reinforcement; its influence on the properties of the filled systems is discussed. In addition, the effect of the network topology (randomly cross-linked, end-linked, ideal diamond-like) on the mechanical properties of unfilled elastomers was studied.
"Mechanical Impedance Method" is a measuring method of the loss factor of damped beam specimens widely adopted for practical use. In this method, an impedance head driven by a shaker is attached in the center of a beam. When the loss factor is evaluated for the anti-resonance, this method is considered to be a kind of double cantilever beam measurement. In order to confirm this, the equation for an Euler beam driven at the center is expanded with a linear combination of the natural modes of a double cantilever beam and the translational displacement. Thus, the equivalent mass of the natural modes is obtained. A frequency responce of mechanical impedance calculated using this equivalent mass is shown to be almost complete fitting with that calculated using free-free beam natural-modes. As one of remarkable applications of the modal expansion, an expression only using measurement in the center for estimating vibration transmissibility from the center to the tip is derived.
The loss factor of a viscoelastic damping material is commonly measured by the half-power method using a composite beam with the viscoelastic layer. However, the method involves problems of large measurement errors and of the beam shape, which does not represent practical applications. This paper discusses the relatinship between vibration modes and measurement accuracy, and presents specimen dimensions which materialize the bending mode for the accurate loss factor. The vibration modes are classified into torsion, warping, torsion and warping, and bending. According to the discriminants this study induces, the beam shape has satisfy (l/b)(d1/b)≥0.06 to materialize the bending mode. The loss factor of the viscoelastic damping material obtained by the composite beam within the dimension limits and the Oberst theory had good agreement with the loss factor of the same material measured by the nonresonance forced vibration method, and was certified to be sufficiently accurate.
We investigate the influence of casting solvent on the final spatial distribution of nanoparticles (NPs) in polymer nanocomposites (PNCs). The nanocomposites formed from bare silica NPs and poly(2-vinylpyridine) (P2VP) were cast from two different solvents: methylethylketone (MEK) and pyridine, which are either theta or good solvents for both the NPs and the polymer. In MEK, we show that P2VP strongly adsorbs onto the silica surface, creating a temporally stable, bound polymer layer. The resulting “hairy” particles are sterically stabilized against agglomeration and thus good NPs dispersion is achieved at a low silica loading. With high silica content, due to the local bridging of silica NPs by P2VP chains, the composites demix into NP-rich and NP-lean phases. On the contrary, in pyridine, P2VP does not adsorb onto the silica surface. As a result, the phase behavior in such a case is governed by a subtle balance among electrostatic repulsion, Van der Waals attraction, polymer-induced depletion forces, and the kinetic slowdown of diffusion-limited NP aggregation. These initial, solvent-driven non-equilibrium agglomerated NP states are difficult to completely anneal away within a reasonable experimental time scale, especially for PNCs containing high molecular weight (MW) matrices. We further show that, in the case of using low MW matrices, the NP structure starts to approach its thermodynamic equilibrium state upon sufficient thermal annealing. These results emphasize the crucial role played by the casting solvent in the spatial dispersion of NPs in a polymer matrix.
A study was conducted to investigate the chain dynamics within the polymer matrix in a nanocomposite with nonattractive interactions, ranging from the initial Rouse dynamics to entanglement controlled motion. The use of neutron spin echo spectroscopy made it possible to directly observe the single chain dynamic structure factor of a polymer chain in the environment of the nanocomposite. The study investigated poly(ethylene-alt-propylene) (PEP) filled with hydrophobically modified silica particles as a function of filler concentration. The researchers engaged in the study first focused on the short time dynamics that needed to reflect the underlying Rouse motion of the chains when analyzing the data. Strong evidence for chain disentanglement at high particle fractions was found and quantified by an increase of the respective confinement length dtube using a mean field approach.
There has been considerable interest in characterizing the polymer layer that is effectively irreversibly bound to nanoparticles (NPs) because it is thought to underpin the unusual thermomechanical properties of polymer nanocomposites (PNC). We study PNCs formed by mixing silica nanoparticles (NPs) with poly-2-vinylpyridine (P2VP) and compare the bound layer thickness δ determined by three different methods. We show that the thickness obtained by thermogravimetric analysis (TGA) and assuming that the bound layer has a density corresponding to a dense melt clearly underestimates the real bound layer thickness. A more realistic extent of the bound layer is obtained by in situ measurements of the interaction pair potential between NPs in PNCs via analysis of TEM micrographs; we verify these estimates using Dynamic Light Scattering (DLS) in θ solvent. Our results confirm the existence of long-ranged interactions between NPs corresponding roughly in size to the radius of gyration of the bound chains.
Even in a simple quasi-static loading, filled elastomers demonstrate distinct inelastic phenomena, such as the Mullins effect, deformation induced anisotropy, permanent set, hysteresis, and strain-induced crystallization. So far, these phenomena have not been described by a single constitutive model. In the present paper we propose a generalized micro-mechanical framework capturing all aforementioned effects with a single constitutive formulation. It is based on the decomposition of the rubber matrix into five parallel networks considered to be the sources of the purely elastic behavior, damage, hysteresis, and stress hardening (due to the strain-induced crystallization). The model is described by a relative small number of physically interpretable material constants and predicts the quasi-static response of filled elastomers in good agreement with experimental data.
The results from two polymer-nanoparticle pairs, namely poly(methyl methacrylate)-silica and poly(ethyl methacrylate)-silica, where strong hydrogen-bonding interactions between the we have chosen three polymers, namely poly(2-vinylpyridine), poly-(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA) were chosen. All polymers were synthesized using free radical techniques and therefore have polydispersity indices of ̃2. The nanocomposites were prepared by dissolving the polymer and Irganox 1010 in MEK. Various concentrations of the silica suspension were added to the polymer solutions, and the polymer/silica suspensions were vigorously shaken for 2 h followed by 1 min of ultrasonication, poured into PTFE drying dishes, and allowed to sit overnight in a fume hood. The results suggest that annealing thin polymer films can lead to an almost complete absence of a transition temperature shift even when the interactions between the polymer and surface are attractive.
Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.
Energy expended irreversibly in stretching filled rubbers is calculated for a simple two-phase series model: a soft phase resembling the corresponding unfilled rubber and a hard phase in series with the soft phase. It is assumed that the rubber is initially wholly in the hard state and that it changes progressively into the softened state on stretching, as proposed by Mullins and Tobin. For a wide range of model parameters, the dissipation of mechanical energy is predicted to rise to about 40% of the input energy at large strains. This predicted behavior is in reasonably good agreement with observations of extra energy dissipation in carbon black-filled rubbers, in comparison with corresponding unfilled rubbers, suggesting that the proposed mechanism of hysteresis by phase transformation is valid. A method of combining energy losses from two simultaneous dissipative processes is also proposed.
The solubility of C60 Fullerene has been correlated with Hansen solubility parameters (HSP) in a database of 89 organic solvents. The parameters δD, δP, and δH for C60 were determined as 19.7, 2.9, and 2.7, respectively, with units MPa1/2. These parameters are indicative of an essentially nonpolar solid, and have been used to identify 55 additional predicted good solvents for C60. Under the assumption that polymers with similar HSP to C60 itself will show enhanced compatibility with C60, 30 polymers that have the highest predicted solubility affinity for C60 have been selected from a HSP database of polymeric materials. There is a tendency for those polymers with greatest affinity for C60 to have the same types of functional groups as the better solvents. These include aromatic rings and atoms significantly larger than carbon, such as chlorine and sulfur.