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Solvation structure of poly-m-phenyleneisophthalamide (PMIA) in ionic liquids
Abstract
Polyaramids are a class of high-performance polymers, known for their high mechanical strength and chemical and thermal stability. Their ability to create a network of intermolecular hydrogen bonds causes them to be very poorly soluble in conventional solvents. Hazardous solvents such as N-methylpyrrolidone (NMP) and dimethylacetamide (DMA), in combination with an inorganic salt such as CaCl2, are currently used for the synthesis and processing of polyaramids. Ionic liquids are proposed as suitable greener alternatives. In this work, we studied the solubility and dissolution mechanism of the meta-oriented polyaramid poly-m-phenyleneisophthalamide (PMIA) in a wide range of ionic liquids. It was found that, similarly to cellulose, PMIA could be dissolved readily and in large amounts in ionic liquids containing a strongly coordinating anion (such as chloride, acetate and dialkylphosphate) and an imidazolium cation. Hydrogen bonding between the anion and the amide NH of PMIA is the main solvent-solute interaction. An odd-even effect in solubility occurred when altering the length of the side chains on the imidazolium cation. Furthermore, it was found that the presence of hydrogen bond donating CH moieties on the cation is a necessary condition for dissolution. The exact role of these hydrogen bond donors was investigated by FTIR and 13C NMR spectroscopy. It was found that there is no significant interaction between the hydrogen atoms of the imidazolium ring and the amide carbonyl groups. Rather, the hydrogen bond donors are needed to stabilize the solvation shell around PMIA through alternating cation-anion interactions.
... In recent years, our research group has been investigating the potential of ionic liquids (ILs) as green alternative solvents for polyaramids. [9][10][11][12] ILs are compounds which consist entirely of ions, but are liquid at ambient to moderately elevated temperatures. 13,14 The green character of ILs stems from their negligible vapor pressure. ...
... They do not accurately simulate interactions with sterically hindered solute such as a polymer within a solvation shell. 33,53 ILs such as [P 14666 ][Cl], or substituted imidazoliums, 9 are too bulky to fit well within a solvation shell and so are unable to effectively interact with the aromatic parts of the polymer. GVL can take up this role and thus work synergistically with [P 14666 ][Cl], whereas ACN is less suitable for this task and only marginally increases the solubility. ...
Polyaramids are polymers that consist of aromatic repeating units that are connected via amide bonds. From their chemical structure, an extensive intermolecular hydrogen-bond network arizes, which makes them very difficult to dissolve in conventional organic solvents. A commonly used solvent system is N-methylpyrrolidone (NMP) mixed with CaCl2, where the chloride ions can break up the intermolecular hydrogen bonds. However, with NMP being a known teratogen, an alternative solvent is needed. Organic Electrolyte Solutions (OESs) are proposed as a green alternative solvent system. OESs are created by diluting ionic liquids (ILs) with an organic co-solvent. γ-Valerolactone (GVL) was selected as a green, renewable co-solvent. The solubility in OESs was tested for three commonly produced polyaramids: poly-p-phenylene terephthalamide (PPTA), poly-m-phenylene isophthalamide (PMIA) and copoly(p-phenylene/3,4′-diphenylether terephthalamide) (ODA/PPTA). Some OESs were excellent solvents for PMIA and ODA/PPTA, with [C8MIm][Cl]/GVL yielding solubilities as high as 23.7 wt% and 7.4 wt%, respectively, rivalling the currently used solvent systems. PPTA, on the other hand, was completely insoluble in all OESs. GVL was found to work synergistically with ILs, while acetonitrile and ethanol acted as a non-solvent and anti-solvent, respectively. OESs made from GVL and imidazolium ILs were suitable solvents for fiber spinning, as was demonstrated using a 3D-printed spinning setup. The Kamlet-Taft parameters of the OES were determined from which boundary conditions for dissolution were loosely defined as β > 0.85, α < 0.60 and π* > 0.85. A dissolution mechanism is proposed, in which the IL interacts with the polymer through hydrogen bonding via the anion, while GVL can undergo dispersion interactions with the aromatic parts of the polyaramid.
... 1−3 They have been applied as solvents in organic synthesis, 4,5 extractants for solvent extraction, 6−8 and solvents for the processing of (bio)polymers. 9,10 Because of the high variety of the possible cation−anion combinations, ILs can be tailor-made to each application and are, therefore, often referred to as "taskspecific" liquids. 11 Although different methods exist, the majority of ILs is generally prepared via a two-step synthesis method. ...
Following the initial cation formation, the synthesis of ionic liquids (ILs) often involves an anion-exchange or metathesis reaction. For hydrophobic ILs, this is generally performed through several cross-current contacts of the IL with a fresh salt solution of the desired anion. However, if a large number of contacts is required to attain an adequate conversion, this procedure is not economical because of the large excess of the reagent that is consumed. In this study, the metathesis of an IL, Aliquat 336 or [A336][Cl], to ILs with other anions ([A336][X] with X = HSO4–, Br–, NO3–, I–, and SCN–) was studied in a continuous counter-current mixer-settler setup. McCabe–Thiele diagrams were constructed to estimate the required number of stages for quantitative conversion. Significantly higher IL conversions were achieved, combined with reduced reagent consumption and waste production. This improvement in efficiency was most pronounced for anions placed low in the Hofmeister series, for example, HSO4–, Br–, and NO3–, which are difficult to exchange. The performance of the counter-current experiments was compared with the conventional multistep cross-current batch process by calculating the reaction mass efficiency (RME) and the environmental factor (E-factor). The RMEs of the cross-current experiments were notably smaller, that is, 38–78% of the values observed for the counter-current experiments. The E-factors of the counter-current experiments were a factor of 2.0–6.8 smaller than those of the cross-current experiments. These sustainability metrics indicate a highly efficient reagent use and a considerable, simultaneous decrease in waste production for the counter-current IL metathesis reactions.
... We, as well as other authors have recently investigated ionic liquids (ILs) as potential alternative solvents for the synthesis and processing of polyaramids [6][7][8][9][10][11][12]. Due to their ionic structure, they bear resemblance to the NMP/CaCl 2 solvent system. ...
N-methyl pyrrolidone (NMP) is a polar aprotic solvent that is critical for the production of polyaramids. However, due to its reprotoxicity and pending REACH restrictions, a benign alternative is needed. A mixture of N-butyl pyrrolidone (NBP) and the ionic liquid [C8MIm][Cl] is proposed as a promising candidate to replace NMP. This organic electrolyte solution provides a green approach to polyaramid synthesis.
... Ionic liquids (ILs) are known to be excellent green solvents for many types of synthetic polymers and biopolymers. [33][34][35][36][37][38][39][40][41] It has also been reported that Lewis-acidic chloroaluminate ionic liquids can be used to dissolve epoxy resins of tantalum capacitors. For these reasons, we decided to explore the use of ionic liquids for the recycling of bonded NdFeB magnets for the first time. ...
NdFeB permanent magnets are essential for modern day technology thanks to their excellent magnetic properties. Recycling of critical metals from sintered NdFeB permanent magnets has attracted a lot of attention over the last decade, whereas the recycling of (polymer or resin) bonded NdFeB magnets has been largely neglected. In this paper, an overview of the polymer or resin binders used in commercial bonded magnets is presented and different routes for the recycling of these magnets are explored. Three main types of polymers were found in commercial bonded NdFeB magnets: polyamides (PA6 and PA12), poly-p-phenylene sulfide (PPS) and epoxy. Both types of polyamide resins were easily dissolved by ionic liquids with coordinating anions (chloride, acetate or dialkylphosphate). Removal of the PPS resin was not possible by ionic liquid solvents, but only by using 1-chloronaphthalene and 1,3,5-triphenylbenzene at high temperatures. Although epoxy could be removed by several ionic liquids, reaction between the NdFeB powder and the ionic liquids was observed. A batch of PA6-bonded magnets was treated with an ionic liquid tributylethylphosphonium diethylphosphate, [P4442][Et2PO4], to selectively remove the polymeric portion. The PA-free magnet powder was found to retain >90% of its original magnetic properties. Two epoxy-bonded magnets produced with this recycled magnet powder showed magnetic properties that were close to those of commercial counterparts, proving the versatility of the process and that the materials loop could be successfully closed.
Poly ( p ‐phenylene terephthalamide) (PPTA) polymer was synthesized and then modified to eliminate the processing difficulties arising from its insolubility in organic solvents. The chemical structures of the synthesized PPTA and N ‐butyl PPTA (BPPTA) were confirmed by elemental analysis, x‐ray diffraction, Fourier transform infrared (FTIR), and ¹³ C‐NMR spectroscopy studies. As a result of modification, an aramid derivative that is soluble in some common solvents was obtained and added into polyacrylonitrile (PAN) matrix in solution. PAN/BPPTA composite nanofiber mats (CNMs) were fabricated by electrospinning. Mechanical tests showed that tensile strength increased by 119% with 3% BPPTA content. The fabricated CNMs were combined with aramid fabrics to produce laminated hybrid armors and ballistic tests were performed following the NATO Stanag 2920 standard. This is the first report about the integration of CNMs into multilayer armor configurations and resulted in improved ballistic limit velocity (V50) with a relatively very small weight gain and emergence of new energy absorption mechanisms.
Highlights
Alkylation of the para‐aramid yielded an organo‐soluble aramid derivative.
Tensile strength of PAN nanofiber mat increased by 119% with 3% N ‐butyl PPTA.
Integration of the CNMs resulted in new ballistic impact absorption mechanisms.
CNMs contribute to V50 with a very low mass gain due to their high surface area/mass.
In this review, catalyst as well as solvent nature of ionic liquids in organic transformations has been described. Ionic liquids have been tremendously utilized for many years as a medium for organic solvents. Further ionic liquids are also used in different fields like electrochemistry, catalysis, spectroscopy material science. The current review discussed about the synthesis, properties, applications of ionic liquid and ionic liquid-based nanocomposites for organic transformations. The application of ionic liquids as acids, bases and soluble support for catalysts and organocatalysts are also discussed in this article. This review also discussed the application of ionic liquids in asymmetric reactions like aldol condensation, Diels–Alder reaction, enantioselective hydrogenation, Knoevenagel reaction, Michael addition reaction, miscellaneous reaction, transesterification and biodiesel production. It is easier to separate the product of a reaction from an ionic liquid than that of a traditional solvent, because the ionic liquids having polar nature. The green nature and reusable ability of ionic liquids make them more appropriate candidate in organic transformations.
This article focuses on the development of practical approaches to the preparation of benzo[1,2-d:4,3-d']bis(thiazoles) using blue light-induced photochemical cyclization of N,N'-(1,4-aryl)dithioamides in the presence of p-chloranil as a mild oxidant. The proposed method allows to obtain benzo[1,2-d:4,3-d']bis(thiazoles) containing donor substituents in the conjugated chain. Photophysical and (spectro)electrochemical properties of 2,6-di([2,2'-bithiophen]-5-yl)benzo[1,2-d:4,3-d']bis(thiazole) and -benzo[1,2-d:4,5-d']bis(thiazole) are studied in detail. The properties of the synthesized compounds suggest their potential applications for organic electronics.
Meta‐aramid paper is widely used in transformers because of its perfect heat resistance and dielectric properties. However, the loose structure and the voids of meta‐aramid paper may lead to lower breakdown strength, which will limit the applications of the insulated paper in harsh environment. Accordingly, it is urgent to enhance the dielectric breakdown strength of meta‐aramid paper. This work reports a simple and effective way to increase the breakdown strength of meta‐aramid paper. The paper was coated with salt‐free poly(m‐phenylene isophalamide) (PMIA) slurry and the solid contents of PMIA slurry were 4%, 6%, 8%, and 10%, respectively. To obtain salt‐free PMIA slurry, the meta‐aramid spun fibers were dissolved in dimethylacetamide. After coating 10% PMIA slurry on meta‐aramid paper, the paper demonstrated the best comprehensive performance. The tensile strengths in the cross and machine direction reached 40.67 and 90.76 MPa, which were increased by 29.93% and 20.16% compared with pristine paper. Meanwhile, the dielectric breakdown strength was up to 42.75 kV/mm (the breakdown strength of pristine paper was only 16.41 kV/mm). Because no crosslinking agent and adhesive were used, the coated meta‐aramid paper still maintained excellent properties.
PMIA fiber has a wide range of applications in flame retardant protection, high temperature filtration and other fields. However, its poor mechanical properties result in a shorter service life of products made from it. In order to improve the mechanical properties of poly(metaphenylene isophthalamide) (PMIA) fiber and prepare PMIA conductive fiber, a highly oriented PMIA/Poly(acrylonitrile-co-vinyl chloride) P(AN-VC) composite fibers with excellent mechanical properties, flame retardancy and thermal stability were successfully prepared by wet spinning. The breaking strength of PMIA fiber at the maximum water bath stretching ratio was 1.58 cN/dtex and 10%P(AN-VC) composite fiber at the maximum water bath stretching ratio was 2.97 cN/dtex. The limiting oxygen index (LOI) of PMIA/P(AN-VC) composite fiber increased from 28.2% to 29.2%, which benefited from the excellent flame retardant properties of P(AN-VC). It was the addition of P(AN-VC) that weakens the hydrogen bonding between PMIA molecular chains resulting in an increase of the breaking strength of the composite fibers compared to PMIA fibers. In addition, the composite fibers could be prepared into conductive fibers and possessed excellent electrical conductivity.
Ammonia (NH3) absorption has attracted much attention due to serious environmental pollution caused by ammonia in recent years. In this paper, the solubilities of ammonia (pressure-temperature-composition (pTx) data) in five dialkylphosphate-based ionic liquids (ILs) were determined at different temperatures (303.15 K-333.15 K) and pressures (about 0.1 MPa-0.6 MPa). It was found that the solubilities of ammonia increased with decreasing temperature and increasing pressure. NH3 dissolution could be improved in ILs with longer side chain lengths in the imidazolium ring of cations and shorter side chain lengths in anions, which could be related to the densities and viscosities of the ILs. Moreover, based on NH3 solubility data, Henry's law constants were obtained using the Peng-Robinson (PR) equation of state. Consequently thermodynamic properties including solution enthalpy ΔsolH, solution entropy ΔsolS and solution Gibbs free energy ΔsolG were calculated. The results indicated that higher absolute values of ΔsolH could generally demonstrate stronger interactions between NH3 and ILs, which corresponds to the smaller Henry's law constants and higher solubility of NH3 in the ILs. In addition these ILs can exhibit excellent reversibility, indicating that these ILs can be regarded as potential absorbents for NH3 in chemical industry processes. © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.
A high-molecular-weight aromatic polyamide resin incorporating furan-rings (f-resin) was prepared by low-temperature solution polycondensation of bio-based 2,5-furandiformyl chloride and 3,4′-diaminodiphenyl ether, in N,N-dimethylacetamide. Further, an aromatic polyamide fibre containing furan-rings (f-fibre) was obtained from the fresin by dry-jet wet spinning, and its structure and properties were investigated. The prepared f-resin showed good solubility and possessed good spinnability in solution. The mechanical, heat-resistance, and flame-retardant properties of the f-fibre were found to be excellent – comparable to or exceeding those of meta-aramid fibre (m-fibre). In addition, the furan acid chloride monomer is bio-based and is derived from abundant resources, unlike petroleum-based monomers. Therefore, f-fibre has good potential and broad application prospects as an environment-friendly and sustainable high-performance material.
Gel electrolytes show certain advantages over conventional liquid and solid electrolytes, but their mechanical strength and surface adhesion to the electrode remain to be improved. To address the challenges, we design and fabricate herein the core-shell nanofiber mats in situ on the LiFePO4 electrode as matrices for gel electrolytes, in which the core is poly(m-phenylene isophthalamide) (PMIA) nanofiber and the shell are composite of Al2O3 nanoparticles and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP). The mechanical property of the core-shell polymeric nanofiber mats and their surface interaction with LiFePO4 electrode are characterized complementarily using dynamic thermomechanical analysis and scanning electron microscopy. The electrochemical properties of the gel electrolytes based on the as-prepared matrices after being loaded with lithium salt solution are studied systematically on half coin cells. It is found that the ultimate strength of the core-shell PMIA@PVdF-HFP/Al2O3 mat can reach 6.70 MPa, 2 times higher than that of the PVdF-HFP/Al2O3 nanofiber mat. Meanwhile, the shell PVdF-HFP/Al2O3 can ensure manifest surface affinity to the LiFePO4 electrode and enhance lithium-ion conductance. Thus, the as-assembled LiFePO4 half coin cells using PMIA@PVdF-HFP/Al2O3 gel electrolyte show good electrochemical performances, especially the long cycle stability with the capacity retention of 96.6% after 600 cycles under 1C.
The current synthetic procedures for polyaramids mainly involve the use of amide solvents such as N-methylpyrrolidone and N,N-dimethylacetamide. However, these solvents are suspected to be teratogenic and are considered ‘Substances of Very High Concern’ by the European Commission. Here we propose a benign alternative solvent system: an Organic Electrolyte Solution (OES) consisting of γ-valerolactone (GVL) and a small amount of the ionic liquid 1-methyl-3-octylimidazolium chloride, [C8MIm][Cl]. Three commercially relevant polyaramids were synthesized: poly-p-phenylene terephthalamide (PPTA), poly-m-phenylene isophthalamide (PMIA) and copoly(p-phenylene/3,4′-diphenylether terephthalamide) (ODA/PPTA). PMIA was successfully synthesized in the OES containing [C8MIm][Cl] in a molar fraction of xIL = 0.043, achieving an inherent viscosity of ηinh = 1.94 ± 0.064 dL.g-1, which is on par with the current industrial standard and the benchmark lab scale synthesis. The reaction mixture could also be directly used for the wet spinning of polyaramid fibers, and all components of the solvent could be recycled in good yields by a series of evaporation and distillation steps. ODA/PPTA could be synthesized, but only rather low inherent viscosities were achieved. The reaction mixture was too viscoelastic to be spun by our small-scale spinning setup. PPTA always instantly precipitated and could not be synthesized from a [C8MIm][Cl]/GVL OES. α-Picoline, the organic base which was added to capture the released HCl during the reaction, was found to play a pivotal role in the polymerization reaction. By undergoing an acid-base reaction with HCl, it forms a protic ionic liquid in-situ which increases the solubility of the polymer.
In order to improve the mechanical properties of the poly(m-phenylene isophthalamide(PMIA) fiber, poly(vinyl pyrrolidone)(PVP) was blended with PMIA as a functional component. High-strength PMIA/PVP composite fibers were prepared by wet spinning, and the properties of the solutions and fibers were characterized by polarized light microscopy (PLM) and rheology measurements, fourier transform infrared spectroscopy (FTIR), fiber mechanical strength tester, scanning electron microscope (SEM), thermogravimetric(TG), limiting oxygen index(LOI) instrument and so on. The results showed that the addition of PVP can effectively weaken the hydrogen bonding between the PMIA molecular chains. The breaking strength of the composite fiber (3.85 cN/dtex) was much higher than that of the pristine PMIA fiber (2.71 cN/dtex), and the elongation at break of composite fibers was 170% higher than that of pristine PMIA fiber. The heat resistance and flame retardancy of the composite fibers had hardly decreased. Composite fibers can still meet the requirements of flame retardant clothing. Compared with pristine PMIA fiber, PMIA/PVP composite fiber will have greater application prospects.
This paper presents the synthesis and characterization of two series of polymeric compounds comprising eight furan‐based polyamides prepared via melt polycondensation at low temperatures using various combinations of five aromatic raw materials. The chemical and physical structures and thermal stabilities of the obtained polyamides were investigated by various characterization methods. In addition, the polyamides were subjected to solubility testing in five common organic solvents. The results showed that the proposed furan‐based polyamides possessed thermal stabilities similar to those of conventional high‐performance aromatic polyamides, but with greatly improved solubility. Accordingly, the introduction of furan groups increased the solubility of the polyamides with respect to the solubility of their individual precursors, which is highly advantageous for subsequent polyamide processing and expanding their range of potential applications.
Organic Electrolyte Solutions – mixtures of a (room-temperature) ionic liquid with a neutral, organic, polar co-solvent – are attracting increasing attention as solvents for the regeneration and derivatisation of cellulose. Despite advantages (in comparison to simple ionic liquid analogues) associated with rapid or instantaneous dissolution, reduced viscosity, enhanced thermal stability and fine-tunable physicochemical properties, a firm understanding of the precise solvent–solute interactions and the relative kinetic versus thermodynamic contributions to dissolution remains elusive. The incorporation of a co-solvent introduces an additional layer of complexity, therefore an informed choice of both ionic liquid and co-solvent is necessary in order to achieve the desired properties. This article first provides an overview of the structure and bonding properties of native and non-native cellulose allomorphs, and a brief history of strategies for cellulose dissolution. Subsequently, organic electrolyte solutions are introduced as versatile solvents for cellulose, and the underpinning thermodynamic, kinetic and mechanistic phenomena behind cellulose solubility are critically discussed. The final sections summarize recent advances in the development of organic electrolyte technologies for derivatisation and regeneration of cellulose and critically discuss whether organic electrolytes can, or could be, rightly regarded as ‘green solvents’.
Cellulose, a well-known fascinating biopolymer, has been considered to be a sustainable feedstock of energy sources and chemical engineering in the future. However, due to its highly ordered structure and strong hydrogen bonding network, cellulose is neither meltable nor soluble in conventional solvents, which limits the extent of its application. Therefore, the search for powerful and eco-friendly solvents for cellulose processing has been a key issue in this field for decades. More recently, certain ionic liquids (ILs) have been found to be able to efficiently dissolve cellulose, providing a new and versatile platform for cellulose processing and functionalization. A series of cellulose-based materials, such as films, fibers, gels and composites, have been produced readily with the aid of ILs. This review article highlights recent advances in the field of dissolution and processing of cellulose with ILs. It is hoped that this review work will stimulate a wide range of research studies and collaborations, leading to significant progress in this area.
The effect of ionic liquids (ILs) on the solubility of cellulose was investigated by changing their anions and cations. The structural variation included 11 kinds of cations in combination with 4 kinds of anions. The interaction between the IL and cellobiose, the repeating unit of cellulose, was clarified through nuclear magnetic resonance (NMR) spectroscopy. The reason for different dissolving capabilities of various ILs was revealed. The hydrogen bonding interaction between the IL and hydroxyl was the major force for cellulose dissolution. Both the anion and cation in the IL formed hydrogen bonds with cellulose. Anions associated with hydrogen atoms of hydroxyls, and cations favored the formation of hydrogen bonds with oxygen atoms of hydroxyls by utilizing activated protons in imidazolium ring. Weakening of either the hydrogen bonding interaction between the anion and cellulose, or that between the cation and cellulose, or both, decreases the capability of ILs to dissolve cellulose.
Cellulose is the most abundant and renewable organic compound on Earth, it is however not soluble in common organic solvents and aqueous solutions. Cellulose dissolution is a key aspect to promote its value-added applications. Ionic liquids (ILs) have been shown to solubilize cellulose under relatively mild conditions. The easy processability of cellulose with ILs and their environmental-friendly nature prompted research in various fields such as biomass pretreatment and conversion, cellulose fiber and composite production, and chemical conversion of cellulose in ILs. Progress has been made on understanding the mechanism of cellulose dissolution in ILs, including the structural characteristics of ILs that are cellulose solvents, however many details remain unknown. In light of rapid development and importance of cellulose dissolution in the field of IL-based cellulose and biomass processing, it is necessary to provide an overview of current understanding of cellulose dissolution in ILs and outline possible future research trends. Recent literature studies suggest that synergistic effects between the anions and the cations of ILs need to be revealed, which requires refining the structure of cellulose elementary fibrils, simulation of more realistic cellulose fibrils and detailed studies on the solution structure of cellulose in ILs. After analyzing literature studies, three interacting modules are identified, which are crucial to understand the process of cellulose dissolution in ILs: (1) the structure of elementary fibrils; (2) solvation of cellulose in ILs; and (3) solution structure of cellulose solubilized in ILs. A coherent analysis of these modules will aid in better design of more efficient ILs and processes.
One of the main drawbacks comprising an appropriate selection of ionic liquids (ILs) for a target application is related with the lack of an extended and well-established polarity scale for these neoteric fluids. Albeit a considerable progress has been made on identifying chemical structures and factors that influence the polarity of ILs, there still exists a high inconsistency on the experimental values reported by different authors. Furthermore, due to the extremely large number of possible ILs that can be synthesized, the experimental characterization of their polarity is a major limitation when envisaging the choice of an IL with a desired polarity. Therefore, it is of crucial relevance to develop correlation schemes and a priori predictive methods able to forecast the polarity of new (or not yet synthesized) fluids. In this context, and aiming at broadening the experimental polarity scale available for ILs, the solvatochromic Kamlet-Taft parameters of a broad range of bis(trifluoromethylsulfonyl)imide-([NTf2]-)-based fluids were determined. The impact of the IL cation structure on the hydrogen-bond donating ability of the fluid was comprehensively addressed. Based on the large amount of novel experimental values obtained, we then evaluated COSMO-RS, COnductor-like Screening MOdel for Real Solvents, as an alternative tool to estimate the hydrogen-bond acidity of ILs. A three-parameter model based on the cation-anion interaction energies was found to adequately describe the experimental hydrogen-bond acidity or hydrogen-bond donating ability of ILs. The proposed three-parameter model is also shown to present a predictive capacity and to provide novel molecular-level insights into the chemical structure characteristics that influence the acidity of a given IL. It is shown that although the equimolar cation-anion hydrogen-bonding energies (EHB) play the major role, the electrostatic-misfit interactions (EMF) and van der Waals forces (EvdW) also contribute, admittedly in a lower extent, towards the hydrogen-bonding acidity of ILs. The new extended scale provided for the hydrogen-bonding acidity of ILs is of high value for the design of new ILs for task-specific applications.
This work reports preparation of magnetic nanocomposites (MNCPPA)s from interfacial reaction between multifunctional poly(pyrimidine-amide)s (PPAs) and epoxide functionalized magnetic Fe3O4 nanoparticles. In this regard, organosoluble PPAs were synthesized selectively from polycondensation of bis(2-aminopyrimidine-4,6-diol) compound (DATPhP) with different aromatic and aliphatic dicarboxylic acids in a mixture of ionic liquid(IL)/triphenyl phosphate (TPP) at 110 °C for 2.5 h. The PPAs have weight-average molar mass (Mw) up to 49500 g mol−1, glass transition temperatures (Tg) up to 240 °C and 10% weight loss temperatures (T10%) up to 486 °C (in N2). The results showed that strong chemical bonding between modified Fe3O4 nanoparticles and PPAs chains reduced solubility and enhanced thermal and mechanical properties of the composites. Also, antibacterial and antioxidant activities of the monomer, PPAs and MNCPPA were studied.
Imidazolium-based ionic liquids having different anions 1-butyl-3-methylimidazolium ([BMIM]X: X = Cl(-), Br(-), I(-), and BF4(-)) and their aqueous mixtures were investigated by IR absorption and proton NMR spectroscopy. The IR spectra of these ionic liquids in the CHx stretching region differed substantially, especially for C-H bonds in the imidazolium ring, and the NMR chemical shifts of protons in the imidazolium ring also varied markedly for ILs having different anions. Upon the introduction of water to screen the electrostatic forces and separate the ions, both IR and NMR spectra of [BMIM]X (X = Cl(-), Br(-), I(-)) showed significant changes, while those of [BMIM]BF4 did not change appreciably. H-D isotopic exchange rates of C(2)-H in [BMIM]X-D2O mixtures exhibited an order: C(2)-HCl > C(2)-HBr > C(2)-HI, while the C(2)-H of [BMIM]BF4 was not deuterated at all. These experimental findings, supported by DFT calculations, lead to the microscopic bulk configurations in which the anions and the protons of the cations in the halide ionic liquids have specific, hydrogen-bond type of interaction, while the BF4(-) anion does not participate in the specific interaction, but interacts less specifically by positioning itself more above the ring plane of the imidazolium cation. This structural change dictated by the anion type will work as a key element to build the structure-property relationship of ionic liquids.
Cellulose is the most abundant biorenewable and biodegradable resource on the earth. However, the extent of its application is limited due to its inefficient dissolution in solvents. Thus, the development of new cellulose solvents continues to be an active area of investigation. In this work, a series of ionic liquids (ILs) have been synthesized by coupling the 1-N-butyl-3-methylimidazolium cation [C4mim]+ with the Brønsted basic anions [CH3COO]−, [HSCH2COO]−, [HCOO]−, [(C6H5]COO]−, [H2NCH2COO]−, [HOCH2COO]−, [CH3CHOHCOO]− and [N(CN)2]−. The solubilities of microcrystalline cellulose (MCC) in these ionic liquids were determined as a function of temperature. The effect of the anion structure on the solubility of cellulose has been estimated, and investigated by 1H NMR and a solvatochromic UV/visprobe. It was found that the solubility of cellulose increases almost linearly with increasing hydrogen bond accepting ability of anions in the ionic liquids. At the same time, novel [C4mim][CH3COO]/lithium salt (LiCl, LiBr, LiAc, LiNO3, or LiClO4) solvent systems have been developed by adding 1.0 wt% of lithium salt into [C4mim][CH3COO]. It was shown that the addition of lithium salts significantly increased the solubility of the cellulose. This observation was studied by 13C NMR spectra, and the results suggested that the enhanced solubility of cellulose originated from the disruption of the intermolecular hydrogen bond, O(6)HO(3) owing to the interaction of Li+ with the hydroxyloxygen O(3) of cellulose. Furthermore, the cellulose materials regenerated from the ionic liquids were characterized by scanning electron micrograph, thermogravimetric analysis and Fourier transform infrared spectroscopy, and the degree of polymerization of the original and regenerated cellulose materials was also determined. Good thermal stability was found for the regenerated cellulose. It is expected that the above information is useful for the design of novel ionic liquids and ionic liquid-based solvent systems for cellulose.
Ionic liquids (ILs) have become advantageous solvents for the dissolution and homogeneous processing of cellulose in recent years. However, despite significant efforts, only a few ILs are known for their capability to efficiently dissolve cellulose. In order to overcome this limitation, we screened a wide range of potentially suitable ILs. From our studies, some remarkable results were obtained, for example, an odd–even effect was found for different alkyl side-chain lengths of the imidazolium chlorides which could not be observed for the bromides. Furthermore, 1-ethyl-3-methylimidazolium diethyl phosphate was found to be best suitable for the dissolution of cellulose; dissolution under microwave irradiation resulted in almost no color change. No degradation of cellulose could be observed. In addition, 1-ethyl-3-methylimidazolium diethyl phosphate has a low melting point which makes the viscosity of the cellulose solution lower and, thus, easier to handle.
The polarities of a wide range of ionic liquids have been determined using the Kamlet-Taft empirical polarity scales α, β and π*, with the dye set Reichardt's Dye, N,N-diethyl-4-nitroaniline and 4-nitroaniline. These have been compared to measurements of these parameters with different dye sets and to different polarity scales. The results emphasise the importance of recognising the role that the nature of the solute plays in determining these scales. It is particularly noted that polarity scales based upon charged solutes can give very different values for the polarity of ionic liquids compared to those based upon neutral probes. Finally, the effects of commonplace impurities in ionic liquids are reported.
The dissolution mechanism of cellulose in ionic liquids has been investigated by using cellobiose and 1-ethyl-3-methylimidazolium acetate (EmimAc) as a model system under various conditions with conventional and variable-temperature NMR spectroscopy. In DMSO-d(6) solution, NMR data of the model system clearly suggest that hydrogen bonding is formed between hydroxyls of cellobiose and both anion and cation of EmimAc. The CH(3)COO(-) anion favors the formation of hydrogen bonds with hydrogen atoms of hydroxyls, and the aromatic protons in bulky cation [Emim](+), especially the most acidic H2, prefer to associate with the oxygen atoms of hydroxyls with less steric hindrance, while after acetylation of all hydroxyls in cellobiose the interactions between cellobiose octaacetate and EmimAc become very weak, implying that hydrogen bonding is the major reason of cellobiose solvation in EmimAc. Meanwhile the stoichiometric ratio of EmimAc/hydroxyl is estimated to be between 3:4 and 1:1 in the primary solvation shell, suggesting that there should be one anion or cation to form hydrogen bonds with two hydroxyl groups simultaneously. In situ and variable-temperature NMR spectra suggest the above mechanism also works in the real system.
A process to produce fibers from Poly(m-phenyleneisophthalamide)(PMIA)solution in an ionic liquid via wet-spinning technology are described. The spinning processwas investigated on a small laboratory scale. Ionic liquid spinning solutions were firstprepared for PMIA fibers, followed by wet spinning. In the course of this research, thephysical properties of the PMIA fibers were estimated. We studied the dependence of themechanical properties of the obtained PMIA fibers on the composition of the coagulationbath, and on the choice of solvent in spinning solution. The morphology of the fibers fromionic liquid and traditional DMAc solvents via wet-spinning process were observed byscanning electrical microscopy(SEM). The differences of morphologies and properties ofthe PMIA fibers obtained from two different solvents are discussed.
The production cost and viscosity of certain ionic liquids (ILs) are among the major factors preventing the establishment of economically viable IL-based biomass pretreatment technologies. Recently, mixtures of an IL with an organic solvent have been proposed for cellulose processing and biomass pretreatment. DMSO is an inexpensive organic solvent that is industrially produced from lignin, a by-product of pulping process. We carry out a mechanistic study of dimethyl sulfoxide (DMSO)-assisted IL pretreatment of switchgrass. The physical structures of biomass samples are studied by x-ray diffraction (XRD), N2 adsorption analysis and small angle neutron scattering (SANS). Both dry and aqueous suspension of biomass samples are measured by SANS that provides unique information on biomass pretreatment. A mixture of 42 wt.% [C2C1Im][OAc] and 58 wt.% DMSO is proposed as the optimal pretreatment solution and the recycling and reuse of the mixture of solvents are also studied. The fermentability of the hydrolysates generated after pretreatment is evaluated using an E.coli strain engineered to produce isoprenol. This study suggests an avenue for developing more efficient and cost effective IL-based processes for the production of lignocellulosic biofuels and bioproducts.
Several ionic liquids (ILs) were tested for their suitability to synthesize the aramid polymer PPTA in an attempt to diminish the dependence on the toxic N-methylpyrrolidone (NMP) that is currently used in industry. The room-temperature IL 3-methyl-1-octylimidazolium chloride ([C8MIM][Cl]) showed the highest promise as with this medium the polycondensation reaction proceeds with a similar mechanism as it happens in the solvent mixture of NMP with CaCl2. With this IL, PPTA polymer with an inherent viscosity of 1.95 dL/g was obtained in a low-temperature polycondensation reaction. This is the highest reported molecular mass of PPTA to date that was obtained by polymerization in an ionic liquid. An EXAFS and solid state NMR spectroscopic study showed that [C8MIM][Cl] and the current industrial solvent of NMP and CaCl2 show similar characteristics when it comes to the synthesis of PPTA.
1-Octyl-3-methylimidazolium chloride ([C8MIM][Cl]) shows the greatest potential to replace N-methylpyrrolidone/CaCl2 as solvent for the synthesis of poly(p-phenylene terephthalamide) (PPTA), a high-strength material when spun into a fiber. Coordinating anions are crucial to prevent precipitation of the polymer during synthesis. Additionally, the imidazolium cations play an essential role in gelation of the reaction mixture and enable the polycondensation reaction to continue in a gel state. Analysis of Kamlet–Taft parameters and Gutmann donor and acceptor numbers of ionic liquids and various solvents showed that balanced interactions between the secondary amide bonds of the polymer, anion, cation, and solvent are required to form a network of hydrogen bond interactions throughout the reaction mixture. This hypothesis was supported by the fact that tri-n-butyl phosphate (TBP), containing similar Kamlet–Taft and Gutmann solvent parameters as N-methylpyrrolidone, was able to produce PPTA with an inherent viscosity as high as 1.86 dL/g in combination with [C8MIM][Cl]. Such a high value without the use of N-methylpyrrolidone or hexamethylphosphoramide has not yet been reported.
In recent years, ionic liquids (ILs) have become a promising solvent for cellulose pretreatment in biorefinery. However, almost all the ILs that can dissolve cellulose have a unsaturated heterocyclic cationic structure, while the ILs with cations of saturated ring can hardly dissolve cellulose. To reveal the underlying mechanism, 4 kinds of ILs composed of unsaturated and saturated cations (1-butyl-3-methylimidazolium, 1-butylpyridinium, 1-butyl-1-methylpyrrolidinium, and 1-butyl-1-methylpiperidinium) and acetate anion were explored as the solvents for a cellulose bunch by molecular dynamics simulation. It is shown that cellulose bunch was only dissolved in the ILs containing unsaturated heterocyclic ring of cations due to two aspects. One is the structure factor: the π electron delocalization of unsaturated heterocyclic ring makes the cation more active to interact with cellulose and provides more space for acetate anions to form hydrogen bonds with cellulose. The other is the dynamic effect: the larger volume of cations with saturated heterocyclic ring result in a slow transfer of both cations and anions, which is not beneficial to the dissolution of cellulose.
Cellulose was hydrolyzed using a novel biphasic system consisting of water and an acidic and hydrophobic ionic liquid. The biphasic system enabled a simple separation of the resulting glucose aqueous solution and ionic liquid. Additionally, a fermentation inhibitor, 5-(hydroxymethyl)furfural, could be removed from the aqueous phase into the ionic liquid phase. The yield of glucose in cellulose hydrolysis was 12.9% at 190 °C. The distribution ratio of glucose in the aqueous phase was 0.98 with an ionic liquid/water ratio of 0.13 (w/w), indicating that most of the glucose was recovered into the aqueous phase. 5-(Hydroxymethyl)furfural was absorbed into the ionic liquid phase from the aqueous phase. The concentration of 5-(hydroxymethyl)furfural in the aqueous phase decreased from 37 to 1.9 mM, which was lower than the concentration at which fermentation is inhibited (24 mM). The acidic and hydrophobic ionic liquids did not decompose during the cellulose hydrolysis and could be recycled four times.
Poly-p-phenyleneterephthalamide (PPTA) is an aramid polymer with high tensile strength which is currently industrially synthesized in a solvent mixture of N-methylpyrrolidone (NMP) and CaCl2. Due to the toxicity of NMP and the need for a salt to increase the solubility, ionic liquids are suggested as suitable, alternative solvents. A whole series of ionic liquids (ILs) were investigated for their solubilization strength towards PPTA. For this study, small PPTA oligomers were synthesized and used as model compounds in solubility tests with ionic liquids. This study gave insights in the types of cations and anions required for optimal dissolving behavior. Ionic liquids with coordinating anions are a requirement to solubilize PPTA by disrupting the intermolecular hydrogen bond network, just as is the case for cellulose dissolution. Infrared and NMR-spectroscopic studies revealed the interaction of the anions with the hydrogen atoms of the secondary amides of the aramid chains. However, there is no one-to-one relationship between ionic liquids suitable for PPTA and cellulose dissolution. Cations with hydrogen atoms capable of hydrogen bond formation, like imidazolium cations, are poor solvents for PPTA. These cations hamper the anions in using their full potential for coordination with the oligomers. Ammonium and phosphonium ionic liquids which contain only sp3-bonded hydrogen atoms on the cation, do not show a tendency to form hydrogen bonds and dissolve PPTA oligomers much better than their imidazolium analogues. This hypothesis was further confirmed by the fact that substitution of hydrogen atoms by methyl groups on imidazolium and pyridinium cations improves the solvent power of the ionic liquid significantly. This screening test has identified several types of ionic liquids that are able to dissolve larger amounts of the PPTA oligomers on a molar basis than the currently used industrial solvent NMP/CaCl2.
As a kind of promising solvents, ionic liquids (ILs) have been used to dissolve cellulose and great progress has been made in recent years. However, the dissolution mechanism, especially the role of cations of ILs in the dissolution of cellulose, is still in debate. In this work, 13 kinds of ILs with a fixed anion [CH3COO]− but varied cationic backbones and alkyl chains have been prepared and characterized. The solubilities of cellulose in these ILs were measured at different temperatures. This allowed us to systematically study the effect of cationic structures on the cellulose dissolution at a given temperature. In order to investigate the dissolution mechanism, Kamlet–Taft parameters of these ILs in the temperature range from 25 to 65 °C and 13C NMR spectra of 1-benzyl-3-methylimidazolium acetate ([phC1mim][CH3COO]) + cellulose systems at 90 °C were also determined. It was found that acidic protons on the heterocyclic rings of the cations are essential for the dissolution of cellulose in the ILs, but the van der Waals interaction of cation with cellulose is not important. These protons may form C–HO hydrogen bonds with hydroxyl and ether oxygen of cellulose to increase cellulose solubility. Cations of the ILs may also decrease cellulose solubility by strong interaction with anions or steric hindrance effect of large size group in their alkyl chains. These interactions together with strong O–HO hydrogen bonds between the anion and hydroxyl protons of cellulose resulted in the disruption of the inter- and intra-molecular hydrogen bonds and thus effective dissolution of cellulose.
The dissolution of microcrystalline cellulose in 1-butyl-3-methylimidazolium acetate [C4C1Im][OAc] was studied using a solid–liquid equilibrium method based on polarized-light optical microscopy from 30 to 100 °C. We found that [C4C1Im][OAc] could dissolve as much as 25 wt% of cellulose at temperatures below 100 °C. The structure of the composite phase obtained after cooling a solution of 16 wt% of cellulose in [C4C1Im][OAc] was analyzed by low angle X-ray diffraction showing the absence of microcrystalline cellulose, but depicting an extensive long range isotropic ordering. With the aim of improving the dissolution of cellulose in the ionic liquid, dimethyl sulfoxide, DMSO, was added as a co-solvent. It was observed that it enhances the solvent power of the ionic liquid by decreasing the time needed for dissolution, even at low temperatures. In order to understand what makes DMSO a good co-solvent, two approaches were followed. Firstly, we studied experimentally the mass transport properties (viscosity and ionic conductivity) of [C4C1Im][OAc] + DMSO mixtures at different compositions and, secondly, we assessed the molecular structure and interactions around glucose, the structural unit of cellulose, by means of molecular dynamics simulations. As expected, DMSO dramatically decreases the viscosity and increases the conductivity of the mixtures, but without inducing cation–anion dissociation in the ionic liquid. These results were confirmed by molecular simulation as it was found that the presence of a 0.5 mole fraction concentration of DMSO does not significantly affect the hydrogen-bond network in the ionic liquid. Furthermore, molecular dynamics shows that in the [C4C1Im][OAc] + DMSO equimolar mixture, DMSO does not interact specifically with glucose. We conclude that DMSO improves the solvation capabilities of the ionic liquid because it facilitates mass transport by decreasing the solvent viscosity without significantly affecting the specific interactions between cations and anions or between the ionic liquid and the polymer. The behavior of DMSO as a co-solvent was compared with that of water and it was found that water molecules are more probably found near glucose than those of DMSO, thus interfering with ionic liquid–glucose interactions, which might explain the unsuitability of water as a co-solvent for cellulose in ionic liquids.
We present a systematic molecular dynamics study examining the roles of the individual ions of different alkylimidazolium-based ionic liquids in the solvation of cellulose. We examine combinations of chloride, acetate and dimethylphosphate anions paired with cations of increasing tail length to elucidate the precise role of the cation in solvating cellulose. In all cases we find that the cation interacts with the nonpolar domains of cellulose through dispersion interactions, while interacting electrostatically with the anions bound at the polar domains of cellulose. Furthermore, the structure and dimensions of the imidazolium head facilitate the formation of large chains and networks of alternating cations and anions that form a patchwork satisfying both the polar and nonpolar domains of cellulose. A subtle implication of increasing tail length is the dilution of the anion concentration in the bulk and at the cellulose surface. We show how this decreased concentration of anions in the bulk affects hydrogen bond formation with cellulose and how rearrangements from single hydrogen bonds to multiple shared hydrogen bonds can moderate the loss in overall hydrogen bond numbers. Additionally, for the tail lengths examined in this study we observe only a very minor effect of tail length on the solvation structure and overall interaction energies.
DTA, TG, and TMA curves of commercial Kevlar® 49 and Nomex® fibers have been used to assess their behavior at high temperatures. The fibers lost absorbed water around 100°C, and a glass transition was reflected in the DTA and TMA curves in the region of 300°C. Difficulties in the interpretation of DTA and TMA curves in the glass-transition region and in the assignments of Tv‘s for these high-performance fibers are discussed. Whereas Kevlar 49 showed both a crystalline melting point (560°C) and a sharp endothermal thermal decomposition (590°C), Nomex showed only the latter (440°C) and no evidence of melting from the DTA curves. The endothermal decomposition peaks apparently correspond to “polymer melt temperatures” reported for related materials, and correlate well with the TG and TMA features. During thermal analysis of Kevlar 49, oxidation occurs more readily than thermal decomposition, but the latter predominates for Nomex. Differences between dyed and undyed Nomex were due to differences in yarn constitution.
The dissolution behavior of chitin in a series of ionic liquids containing alkylimidazolium chloride, alkylimidazolium dimethyl phosphate, and 1-allyl-3-methyl-imidazolium acetate has been studied. Chitin with a low degree of acetylation and low molecular weight can be readily dissolved in ionic liquids, while those with a high degree of acetylation can also be dissolved in ionic liquids with various critical solution temperatures (CST) mainly depending on the structure of the chitin and the ionic liquids. The results indicated that the dissolution behavior of chitin in ionic liquids was affected by the degree of acetylation (DA), the crystallinity, and the molecular weights of chitin, as well as the nature of the anion of the ionic liquid.
1-Butyl-3-methylimidazolium chloride ionic liquid has been developed for the dissolution and regeneration of wool keratin fibers, which can be used to prepare wool keratin/cellulose blended materials directly.
A new and highly efficient direct solvent, 1-allyl-3-methylimidazolium chloride (AMIMCl), has been used for the dissolution and regeneration of cellulose. The cellulose samples without any pretreatment were readily dissolved in AMIMCl. The regenerated cellulose materials prepared by coagulation in water exhibited a good mechanical property. Because of its thermostable and nonvolatile nature, AMIMCl was easily recycled. Therefore, a novel and nonpolluting process for the manufacture of regenerated cellulose materials using AMIMCl has been developed in this work.
The dilute solution properties of two aromatic polyamides, poly(1,4-phenylene terephthalamide) (PPDT) and poly(p-benzamide) (PBA) in 96% sulphuric acid, have been investigated by measurements of the intrinsic viscosity, by light scattering and by gel permeation chromatography (g.p.c.). The Mark—Houwink relation for PPDT indicates that the conformation is intermediate between a coil and a rod-like particle. The conformations of both aromatic polyamides have been determined precisely by coupling g.p.c., light scattering and viscosity and it was found that PPDT and PBA in 96% sulphuric acid are not very rigid particles. The rigidity has been characterized in terms of a worm-like chain. The persistence lengths q which evaluate the rigidity of the chain are for PPDT and for PBA has the more rigid polymer chain.
Multinuclear NMR spectroscopy and conductivity measurements showed that the 1-ethyl-3-methylimidazolium cation, [emim] + not only forms strong hydrogen bonds (using all three ring protons H-2, H-4 and H-5) with halide ions in polar molecular solvents ( e.g. ethanenitrile) and ionic liquids, but that it exists in a quasi- molecular state, [emim]X, in non-polar solvents ( e.g. trichloro- and dichloromethane), showing a conventional aromatic stacking phenomenon.
Wholly aromatic polyamides (aramids) are considered to be high-performance organic materials due to their outstanding thermal and mechanical resistance. Their properties arise from their aromatic structure and amide linkages, which result in stiff rod-like macromolecular chains that interact with each other via strong and highly directional hydrogen bonds. These bonds create effective crystalline microdomains, resulting in a high-level intermolecular packing and cohesive energy. The better known commercial aramids, poly(p-phenylene terephthalamide) and poly(m-phenylene isophthalamide), are used in advanced technologies and have been transformed into high-strength and flame resistant fibers and coatings, with applications in the aerospace and armament industry, bullet-proof body armor, protective clothing, sport fabrics, electrical insulation, asbestos substitutes, and industrial filters, among others. Owing to their chemical structure, they exhibit extremely high transition temperatures that lie above their decomposition temperatures, are sparingly soluble in common organic solvents and, accordingly, can only be transformed upon solution. Research efforts are therefore underway to take advantage of their properties, enhance their processability and solubility, and incorporate new chemical functionalities in the polyamide backbone or lateral structure, so that their applicability is expanded and remains on the forefront of scientific research. (C) 2009 Elsevier Ltd. All rights reserved.
Structure parameters of various poly(p-phenylene terephthalamide fibers have been investigated using WAXD and correlated with mechanical properties. The mechanical properties examined were modulus E and strength sigma; the pertinent structural parameters include orientation angle phi (300), lattice constants a, b, c, paracrystalline parameter g(H) apparent crystal sizes ACS(110), ACS(200), ACS(001), intensity ratio I-110/ I-200 and transverse crystallinity X. The parameters, c, g(H) and I-110/I-200 are found to be interrelated and to provide indications of nonreversible chain conformational changes due to post-treatment. It is concluded that the fiber modulus is determined by the combination of the orientation of the crystallites and the paracrystalline parameter through the following equation:
1/E-f = (1/E-0 + D(1)g(H)(2)) + A(sin(2) phi) (10)
in which E-f is the fiber modulus; g(H) the paracrystalline parameter; phi the orientation angle; and E-0, D-1 and A the material constants. This relationship is derived from our proposed morphological model in which crystallites are: (a) formed from chains have nonlinear conformations; and (b) packed with an orientation distribution. The correlation of structure with strength has also been studied. In addition, different types of Kevlar (R) fibers, Kevlar (R) 119, Kevlar (R) 29, Kevlar (R) 49 and Kevlar (R) 149 show slight, systematic, variations in structure. In particular, all Kevlar (R) fibers except Kevlar (R) 149 show the forbidden 001 diffraction reflection, which has been related to conformational differences.
Ultraoriented polymer fibers have elastic modulus E as large as 350 GPa and tensile strength σb as large as 7 GPa in materials with a density p ≈ 1200 Kg/m3. Keys to achieving these properties are near perfect orientation of polymer chains along the fiber axis and reduction of the number of chain ends. The two materials that have been most thoroughly studied are polyethylene (PE) and poly(p-phenylene terephthalamide) (PPTA). Various schemes for calculating the elastic modulus are reviewed, together with estimates of effects of imperfect chain orientation and the presence of chain ends. Fracture of fibers is treated in terms of covalent bond scission and/or chain slip originating at chain ends. Under laboratory conditions the experimental modulus E can be > 90% of the theoretical modulus. It appears that fracture is more sensitive to chain end defects, limiting practical strength to less than 25% of the ultimate strength predicted from bond scission models.
The use of ionic liquids as novel solvents for the synthesis of aromatic copoly(ester-amide)s, containing a 9,10-anthraquinone moiety in the main chain, from the polycondensation reaction of terephthaloyl chloride and various ratios of p-phenylenediamine and 1,4-dihydroxyanthraquinone is reported. 1,3-Dialkylimidazolium-based ionic liquids are suitable reaction media for the synthesis of copoly(ester-amide)s. These copolymers exhibit color characteristics and thermal stability. The presence of the amide groups in the backbone of these polymers enhances their thermal stabilities. Inherent viscosities of the polymers obtained in 1,3-dialkylimidazolium bromide range from 0.28 to 0.42 dL/g.
Ionic liquids (ILs) are subject to an enormous research effort due to their unique properties, such as non-volatility, high solution and reactivity ability, etc. For the first time ILs have been used as a solvent for preparing polymers via direct polycondensation. The influence of IL's nature and reaction parameters upon the polymer formulation has been investigated. It is shown that direct polycondensation is successfully proceeded in ILs and triphenyl phosphite (condensing agent) without any additional extra components, such as LiCl and pyridine, using in similar reactions in ordinary molecular solvents. Various polyamides (ηinh=0.11–1.10 dl/g), polyamide imides (ηinh=0.48–1.41 dl/g), -hydrazides (ηinh=0.56–0.60 dl/g) and polyhydrazides (ηinh=0.71–1.32 dl/g) have been obtained in quantitative yield and high molecular weight.
Fourier transform infrared temperature studies of a semicrystalline polyamide, nylon 11, are presented. From previous studies of an amorphous nylon, we demonstrated that the absorptivity coefficient of the hydrogen bonded N-H stretching mode was a very strong function of the strength of the hydrogen bond. Accordingly, area changes in the N-H stretching envelope as a function of temperature were not solely the result of hydrogen bonded N-H groups transforming the "free" groups. This result is also applicable to semicrystalline nylons. As a result, significant errors exist in the previously reported thermodynamic parameters obtained from infrared studies. Emphasis has been placed in this study on the N-H stretching and amide I modes of nylon 11. The two regions yield different information. The hydrogen bonded N-H stretching frequency does not exhibit separable features attributable to ordered and disordered hydrogen bonded conformations but rather reflects the overall distribution of hydrogen bonded strengths. In contrast, the amide I mode is conformationally sensitive through dipole-dipole interactions, and separate bands can be identified that are assigned to ordered and disordered hydrogen bonded conformations. Finally, in both regions of the spectrum, bands attributable to "free" (non-hydrogen bonded) amide groups are discernable.
Fourier transform infrared temperature studies of an amorphous polyamide are presented. The results strongly suggest that prior interpretations of the changes occurring in the N-H stretching region of the spectra of polyamides and polyurethanes with temperature were greatly oversimplified. In essence, these spectral changes were interpreted to be solely due to hydrogen-bonded N-H groups transforming to "free" N-H groups. Subsequent use of these data to obtain thermodynamic parameters associated with hydrogen bond dissociation must now be considered erroneous. The primary factor not taken into account concerns the very strong dependence of the absorption coefficient with hydrogen bond strength. With increasing temperature, the average strength of the hydrogen bonds decreases, which is observed in the infrared spectrum by a shift to higher frequency. Concurrently, the absorption coefficient decreases, leading to a reduction in the absolute intensity of the hydrogen-bonded N-H band. In this study we present experimental results in the N-H stretching and amide I, II, and V regions of the infrared spectrum of an amorphous polyamide. In addition, we present a model, justified by theoretical considerations, which we believe advances our understanding of the strong dependence of absorption coefficient with the strength of the hydrogen bonds. The ramifications of this work to hydrogen-bonded polymers are discussed.
Wood cellulose can be used for producing biofuels and biopolymers, thus offering a solution to global concerns on the excessive use of fossil fuels. This requires a cellulose solvent that also allows the ecofriendly processing of selective wood components. Some ionic liquids (ILs) have shown promising results as cellulose solvents with many advantages over traditional approaches. It is agreed that their ionic nature is responsible for cleaving hydrogen bonds between cellulose chains, resulting in dissolution of the biopolymer. However, it is still necessary to establish a structural relationship between IL cations and anions, which explains why only certain ion combinations show the ability to dissolve cellulose. This work aims to analyze the structural similarities displayed by common cellulose solvents focusing on requirements for ionic liquids to qualify as such. A mutual relationship between IL anions and cations is postulated that offers an explanation for the ability or disability of certain ion combinations to dissolve the biopolymer.
A novel high-resolution thermogravimetry (TG) technique in a variable heating rate mode that maximizes resolution and minimizes the time required for TG experiments has been performed for evaluating the thermal degradation and its kinetics of Kevlar fiber in the temperature range ∼ 25–900°C. The degradation of Kevlar in nitrogen or air occurs in one step. The decomposition rate and char yield at 900°C are higher in air than in nitrogen, but the degradation temperature is higher in nitrogen than in air. The initial degradation temperature and maximal degradation rate for Kevlar are 520°C and 8.2%/min in air and 530°C and 3.5%/min in nitrogen. The different techniques for calculating the kinetic parameters are compared. The respective activation energy, order, and natural logarithm of preexponential factor of the degradation of Kevlar are achieved at average values of 133 kJ/mol (or 154 kJ/mol), 0.7 (or 1.1), and 16 min−1 (or 20 min−1) in air (or nitrogen). The technique based on the principle that the maximum weight loss rate is observed at the minimum heating rate gives thermal degradation results that were in excellent agreement with values determined by traditional TG experiments. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 565–571, 1999
The use of ionic liquids as novel solvents for the synthesis of condensation polymers was investigated. A series of ionic liquids including new ones was synthesized and purified. 1,3-Dialkylimidazolium-based ionic liquids seem to be suitable reaction and activating media for the synthesis of high-molecular-weight aromatic polyimides and polyamides. Inherent viscosities of the polymers obtained in 1,3-dialkylimidazolium bromides range from 0.52 to 1.35 dL/g.
Cornhusk cellulose was regenerated using the ionic liquids viz., 1-allyl-3-methylimidazolium chloride (AmimCl) and 1-ethyl-3-methylimidazolium acetate (EmimAc). The cast cellulose films were characterized by FTIR, WAXD and SEM techniques. Their mechanical properties were also studied. These studies indicated that AmimCl and EmimAc are good solvents for the regeneration of cornhusk cellulose. The regenerated cornhusk cellulose (RCC) was found to be cellulose (II) with dense structure. The films cast from AmimCl exhibited good mechanical properties; the tensile modulus and strength were as high as 6 GPa and 120 MPa respectively, whereas these values for those films cast using EmimAc were found to be 4.1 GPa and 47 MPa respectively. Further, it was observed that after regeneration, the solvents could be effectively recycled. Thus a novel nonpolluting process of forming RCC films from agricultural waste was developed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010
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Interrelation between the hydrogen bonding energy in cellulose, its solubility in aqueous and nonaqueous solvents is examined. Factors controlling the solubility and selection criteria for nonaqueous solvents for cellulose are analyzed.
Native skin collagen fibers were successfully dissolved in the ionic liquid, 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), and regenerated in different precipitators. The observation by polarized optical microscopy showed that the crystal structure of collagen fibers had been destroyed by [BMIM]Cl during the heating. Temperature-dependent FTIR was applied to detect the structural change of collagen/[BMIM]Cl during dissolving. The structure of regenerated collagen was characterized by FTIR and XRD. It showed that the triple helical structure of collagen had been partly destroyed during the dissolution and regeneration. The film forming ability and the thermostability of the regenerated collagen was highly dependent on the precipitating treatment. The possible mechanisms of dissolving of collagen in [BMIM]Cl and the regeneration in the precipitators have been proposed. The collagen/cellulose composite with different forms (film, fiber, gel) can be successfully prepared by using [BMIM]Cl as medium.
Ionic liquids with physico-chemical special characteristics such as the low melting point, adjustable acidity and good solubility have been used widely as the environment-friendly solvents; Cellulose are the most abundant natural renewable resources. Non-derivative cellulose solvents which being one category of ionic liquids have attracted enormous studies in cellulose recently. This review summarizes the dissolution and functional modification of cellulose as ionic liquids based on previous researches.
We report here initial results that demonstrate that cellulose can be dissolved without activation or pretreatment in, and regenerated from, 1-butyl-3-methylimidazolium chloride and other hydrophilic ionic liquids. This may enable the application of ionic liquids as alternatives to environmentally undesirable solvents currently used for dissolution of this important bioresource.
beta-D-glucose dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate in a 6 : 1 molar ratio (ionic liquid : glucose) has been studied by neutron scattering, NMR and molecular dynamics simulations. Good agreement was found between simulated neutron scattering profiles generated for isotopically substituted liquid systems and those experimentally determined as well as between simulated and experimental diffusion coefficients obtained by Pulsed Field Gradient NMR spectroscopy. The overriding glucose-ionic liquid interactions in the liquid are hydrogen-bonding between acetate oxygens and sugar hydroxyl groups. The ionic liquid cation was found to play only a minor role in the solvation of the sugar and does not participate in hydrogen-bonding with the sugar to any significant degree. NOESY experiments lend further evidence that there is no direct interaction between sugar hydroxyl groups and acidic hydrogens on the ionic liquid cation.
Recycling of “green” solvents : Recycling of ionic liquids with high efficiency is of key importance on going from the laboratory‐scale to large‐scale industrial application of these solvents. magnified image
Recyclability is one of the reasons why ionic liquids (ILs) are attracting the attention of a growing number of scientists and engineers, but do we understand the recyclability of ILs in a real sense? For this purpose, this review focuses on the methods need for their separation from their “working” environment. Here we proposed that the appropriate separation method should be selected according to different systems. To better understand the separation of ILs, fundamental research on the existence forms (ions, ion pairs or supermolecule) of ILs in solvents is vitally important.
A force field has been refined for the antiparallel chain-rippled sheet structure of polyglycine I. Transition dipole coupling and hydrogen bonding are explicitly taken into account. Amide I and amide II mode splittings are well accounted for, the latter also providing a quantitative explanation of the amide A and amide B mode frequencies and intensities. In addition to predicting other features of the vibrational spectrum of polyglycine I, this force field is completely transferable to other β polypeptides, even though these have the antiparallel chainpleated sheet structure.