Thermodynamic analysis on the cononsolvency of poly (vinyl alcohol) in water–DMSO mixtures through the ternary interaction parameter

Isothermal phase diagrams for the semicrystalline poly (vinyl alcohol) (PVA) in solutions composed of water and dimethylsulfoxide (DMSO) was studied at 25 °C. From the observed phase behavior, PVA was soluble in either water or DMSO individually but crystallization-induced gelation and liquid–liquid demixing were observed in water–DMSO mixtures. Flory–Huggins formalism including three binary interaction parameters and one ternary interaction parameter was used to study the phenomenon of the cononsolvency, i.e. the formation of nonsolvents by mixing two solvents. The equilibrium crystallization line in the DMSO-rich region and the total calculated binodals agreed well with the measured results when a composition-dependent ternary interaction parameter was included into calculations. In contrast, calculations yielded crystallization-induced gelation in the water-rich region, but experiments indicated that PVA remained well dissolved even 1 year after preparation. The discrepancy was explained by the temperature-induced changes in the relative interaction between water and PVA. In addition, the role of the ternary interaction parameter in the cononsolvent ternary polymer systems was discussed. It was found the contribution of the ternary interaction parameter in the cononsolvent system under study is to decline the degree of the cononsolvency. The driving force for cononsolvency is the strong interaction between water and DMSO to form the stable DMSO hydrate to exclude PVA segments in the vicinity of the hydrate.     

Time-resolving analysis of cryotropic gelation of water/poly(vinyl alcohol) solutions via small-angle neutron scattering

The structural transformations occurring in initially homogeneous aqueous solutions of poly(vinyl alcohol) (PVA) through application of freezing (-13 degrees C) and thawing (20 degrees C) cycles is investigated by time resolving small-angle neutron scattering (SANS). These measurements indicate that formation of gels of complex hierarchical structure arises from occurrence of different elementary processes, involving different length and time scales. The fastest process that could be detected by our measurements during the first cryotropic treatment consists of the crystallization of the solvent. However, solvent crystallization is incomplete, and an unfrozen liquid microphase more concentrated in PVA than the initial solution is also formed. Crystallization of PVA takes place inside the unfrozen liquid microphase and is slowed down because of formation of a microgel fraction. Water crystallization takes place in the early 10 min of the treatment of the solution at subzero temperatures, and although below 0 degrees C the PVA solutions used for preparation of cryogels should be below the spinodal curve, occurrence of liquid-liquid phase separation could not be detected in our experiments. Upon thawing, ice crystals melt, and transparent gels are obtained that become opaque in approximately 200 min, due to a slow and progressive increase of the size of microheterogeneities (dilute and dense regions) imprinted during the fast freezing by the crystallization of water. During the permanence of these gels at room temperature (for hours), the presence of a high content of water (higher than 85% by mass) prevents further crystallization of PVA. Crystallization of PVA, in turn, is resumed by freezing the gels at subzero temperatures, after water crystallization and consequent formation of an unfrozen microphase. The kinetic parameters of PVA crystallization during the permanence of these gels at subzero temperatures are the same shown by PVA during the first freezing step of the solutions.     

Gel spinning of poly(vinyl alcohol) from dimethyl sulfoxide/water mixture

Gel spinning of poly(vinyl alcohol) (PVA) was attempted from the PVA dope prepared from the mixture of dimethyl sulfoxide (DMSO) and water. The DMSO/H₂O = 80/20 (w/w) mixture and methanol were found to be the best solvent for the spinning dope and the coagulant, respectively, to give PVA fiber with the highest drawability. PVA fiber with the highest strength and Young's modulus were obtained from the undrawn gel fibers when subjected to hot two-stage drawing under conditions such as to produce maximum drawability. Furthermore, higher draw ratios of PVA fiber were attained at 6 wt % dope by lowering the coagulating temperature of methanol. In the present work, the highest tensile strength (2.8 GPa) and the highest Young's modulus (64 GPa) were realized, when the spinning dope was prepared from PVA with DP of 5,000 and the DMSO/H₂O (80/20) mixed solvent to have the PVA concentration of 6 wt %, the coagulating temperature of methanol was ?20°C, and the two-stage drawing was carried out at 160 (first) and 200°C (second). The PVA fiber prepared under this gel spinning condition could be elongated to 45 times draw ratio. The very high drawability of PVA fibers obtained from the DMSO/H₂O (80/20) mixture dope was ascribed to the ability of the DMSO/H₂O mixture to promote gelation. © 1994 John Wiley & Sons, Inc.     

Micro- and mesoscopic structure of poly(vinyl alcohol) gels determined by neutron and light scattering

We studied the structure of poly(vinyl alcohol) (PVA) gels formed in mixtures of dimethyl sulfoxide (DMSO) and water using several scattering techniques such as wide-angle neutron scattering (WANS), small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (U-SANS) and light scattering (LS) to cover a very wide Q range from 10^-4 to 10 Å^-1. The WANS measurements have revealed that the cross-linking points of the gels are crystallites, and the size and its distribution have been evaluated by the SANS measurements. The SANS results have also shown that the structure observed in the low Q range below 10^-2 Å^-1 is dominated by a liquid–liquid-phase separation. The early stage of the phase separation has been studied in detail using the time-resolved LS technique, while the late stage has been investigated by the U-SANS technique because the LS measurements cannot access the opaque samples. On the basis of the results, we present a quantitative sketch of the structure of the PVA gel.     

Solute retention and the states of water in polyethylene glycol and poly(vinyl alcohol) gels

The states of water sorbed in a cross-linked polyethylene glycol (PEG) gel, TSKgel Ether-250, and cross-linked poly(vinyl alcohol) (PVA) gels of different pore sizes, TSKgel Toyopearl HW-40S, 50S, 55S and 75S, were investigated by means of differential scanning calorimetry (DSC). It was found that there were three types of water in these hydrogels, non-freezing water, freezable bound water and free water. The amount of water that functions as the stationary phase in the column packed with the each gel was also estimated by a liquid chromatographic method. The estimated amount of the stationary phase water is in good agreement with the sum of the amount of non-freezing water and that of freezable bound water for HW-40S, 50S and 55S, while it agrees with the amount of only non-freezing water for HW-75S and Ether-250. This means that the stationary phase water consists of non-freezing water and freezable bound water for HW-40S, 50S and 55S, while only non-freezing water functions as the stationary phase in HW-75S and Ether-250 gels. This result can be attributed to the difference in the structure of the gels; the PVA gels containing PVA at relatively high concentrations, HW-40S, 50S and 55S, have a homogeneous gel phase, whereas HW-75S and Ether-250 have a heterogeneous gel phase consisting hydrated polymer domains and macropores with relatively hydrophobic surface. The freezable bound water in Toyopearl HW-40S, 50S and 55S can be regarded as a component of a homogeneous PVA solution phase, while that in HW-75S and Ether-250 may be water isolated in small pores of the hydrophobic domains. The results obtained by the investigation on the retention selectivity of these hydrogels in aqueous solutions supported our postulated view on the structures of the hydrogels.     

Pulsed NMR study of the structure of poly(vinyl alcohol)-poly(sodium acrylate) composite hydrogel

The heterogeneous structure of poly(vinyl alcohol)-poly(sodium acrylate) composite hydrogels (PVA-PAA gels), prepared by repeatedly freezing and thawing mixed aqueous solutions of PVA and PAA, has been studied using pulsed NMR. The free induction decay (FID) signals of the transverse magnetization for the PVA-PAA gels were measured by both Carr-Purcell-Meiboom-Gill and solid-echo pulse sequences. The FID signal was decomposed into two or three decaying signals with various spin-spin relaxation times, based upon differences in the molecular mobility that reflected structural heterogeneity. The structural heterogeneity of the PVA-PAA gel is quite different from that of PVA gel free of PAA. We have made observations on “crosslinked” domains in the PVA-PAA gels. They are formed by PVA solid chains, probably phase-separated in the freezing-thawing process. The formation of crosslinked domains is influenced by the number of freezing-thawing cycles. © 1994 John Wiley & Sons, Inc.     

Effect of mixed solvent on solution properties and gelation behavior of poly(vinyl alcohol)

In this work, the static and dynamic light scattering measurements were used to investigate the solution properties and the aging effects on PVA/DMSO/water ternary system in dilute region at 25 °C. It was found that the phase separation and aggregate behavior occurs rapidly and obviously when DMSO mole fraction (X₁) in the solvent mixture is between 0.2 and 0.33, especially at 0.25. In this solvent composition range, a broad peak which indicates phase separation and chain aggregation can be observed from static light scattering measurement. However, when DMSO mole fraction is increased to 0.37, no such peak is present. For this ternary system, the gelation mechanism and the relationship between the phase separation behavior and the gelation of the formed physical gels were also investigated through the gelation kinetic analyses in the dilute and semi-dilute region. It is concluded that the cononsolvency effect in the dilute solution is not the sole origin that affects the phase separation, aggregation, and gelation behavior for the ternary system in a higher polymer concentration range. The hydrodynamic factors such as the higher viscosity and slower polymer chain diffusion that are resulted from higher polymer concentration should be also considered.     

High surface area nickel aluminate spinels prepared by a sol–gel method

The oxide spinel NiAl₂O₄ and spinel-type solid solutions Al₂O₃–NiAl₂O₄ (at Ni/Al=1:4, and Ni/Al=1:8) were prepared by controlled hydrolysis of mixed metal alkoxides, followed by calcination of the resulting gels. Powder X-ray diffraction showed that all samples prepared were single phase cubic materials having the spinel-type structure. The cubic lattice parameter, a_o, was found to decrease gradually with increasing aluminium content of the mixed metal oxides. The specific surface area (determined by nitrogen adsorption at 77 K) was found to be in the range of 200–300 m² g⁻¹. The materials were found to be basically mesoporous, the most frequent pore radius being in the range 3.2–6.4 nm. IR spectroscopy of CO adsorbed at liquid nitrogen temperature gave a main band at 2186–2195 cm⁻¹, which was assigned to the C---O stretching vibration of surface Al³⁺CO adducts where coordinatively unsaturated Al³⁺ ions act as Lewis acid centres.     

Anomalous stability of aqueous boehmite dispersions induced by hydrolyzed aluminium poly-cations

Aqueous dispersions of colloidal boehmite rods turn into strong gels when the concentration of (1–1) electrolyte concentrations becomes exceeds 50 mM. However, after addition of aluminium chlorohydrate (ACH) the rods remain stable up to salt concentrations as high as 2 M. Moreover the boehmite-ACH dispersions with an aspect ratio of 19 quickly separate into an isotropic and a liquid crystal nematic phase above a typical threshold concentration of 2 v/v%. It is known that ACH forms polynuclear cations at mild acidic conditions. The anomalous stability as encountered in these dispersions is explained by assuming that these hydrolyzed poly-cations cause a shift of the charge carrying surface.     

Swelling behavior of chemically crosslinked PVA gels in mixed solvents

We report the swelling behavior of chemically crosslinked polyvinyl alcohol (PVA) gels with different degrees of hydrolysis in water, several organic solvents, and their mixed solvents. The gels were dried after gelation and were put into their respective solvents. The gel volume in pure water decreased with increasing temperatures, and the total changes increased with decreasing degrees of hydrolysis. The swelling ratio depends on the solvent and its concentration. In the cases of mixed solvents of methanol–water, ethanol–water, and acetone–water, the gels shrank continuously with increasing concentrations of solvents and reached the collapsed state in the pure organic solvent. In the case of dimethyl sulfoxide (DMSO), on the other hand, the gels shrunk, swelled, and finally reached the swollen state in pure DMSO. Results of measurements using Fourier Transform infrared spectroscopy (FTIR) and X‐ray diffraction (XRD) suggested that crosslinks and microcrystallites were formed due to hydrogen bonds during the drying process after gelation. The hydrogen bonds were partly destroyed in a rich solvent, but the residual hydrogen bonds had an essential role in determining the swelling behavior in a poor solvent. The swelling behavior and the possible phase transition of the present system are discussed in terms of the solubility of polymers with different degrees of hydrolysis in given mixed solvents and in terms of the formation and destruction of physical crosslinks in the chemical PVA gels. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1978–1986, 2010     

P NMR investigation of a ringing gel phase and adjacent phases

The temperature-surfactant concentration phase diagram was examined for the dodecyltrimethylammonium dimethylphosphate/3-methyl-3-methoxybutanol/water ternary system. The phase diagram contained a highly elastic gel phase which is known as a “ringing gel phase”. The ringing gel phase and adjacent phases in the ternary system were investigated by polarized optical microscopy, freeze-fracture transmission electron microscopy, and ³¹P NMR. Globular textures were observed in an optically isotropic gel phase. Since the globules were larger than those found in an isotropic solution, the texture consists of domains of aggregated units in the cubic (I₁) phase. Structure units of domains are equivalent to microemulsions which are constructed by surfactant molecules and swollen by alcohol in the isotropic (L₁) phase. Characteristic polarized microscopic textures were visualized in two phases with higher surfactant concentrations. These phases were identified as being hexagonal (H₁) and lamellar (L) liquid crystals which was confirmed by transmission electron microscopy. The ³¹P NMR signal of the ringing gel showed a sharp singlet the same as that of the L₁ phase, indicating the fully averaged anisotropic interaction of the aggregates. The characteristic NMR signals of the anisotropic hexagonal and lamellar liquid crystal phases displayed chemical shielding with an asymmetric lineshape.     

Fluorescence Correlation Spectroscopy Study of Probe Diffusion in Poly(vinyl alcohol) Solutions and Gels

In this study, we demonstrate how the diffusion of probe particles in aqueous poly(vinyl alcohol) (PVA) solutions and gels is affected by: (i) the presence of cross-links, (ii) the cross-link density, (iii) the polymer concentration. We apply fluorescence correlation spectroscopy (FCS) to measure the diffusion time of a rhodamine-based fluorescent particle (TAMRA) and TAMRA-labeled dextran in PVA solutions and gels prepared at various polymer concentrations (1% to 8.6% w/v) and cross-link densities (1/400 to 1/50 cross-link monomers per PVA monomers). The measurements indicate that the probe particles are slowed down with increasing polymer concentration and with increasing cross-link density. Also, FCS can detect differences in the diffusion times measured in “fresh” and “aged” PVA solutions. We find that FCS provides a quantitative measure of network inhomogeneities.     

Morphology and crystallization behavior of poly(vinylidene fluoride)/poly(methyl methacrylate)/methyl salicylate,and benzophenone systems via thermally induced phase separation

Poly(vinylidene fluoride) (PVDF) blend microporous membranes were prepared by PVDF/poly(methyl methacrylate) blend (with mass ratio = 70/30) via thermally induced phase separation. Benzophenone (BP) and methyl salicylate (MS) were used as diluents. The phase diagram calculations were carried out in terms of a pseudobinary system, considering the PVDF blend to be one component. The crytallization behaviors of PVDF in the dilutions were detected by differential scanning calorimetry measurement. In these two systems, the melting and crystallization temperatures leveled off in the low polymer concentration (<40 wt %), but shifted to a higher temperature when the polymer concentration >40 wt %. The calculated crystallinity of PVDF for samples with low polymer concentrations was greater than those with high polymer concentrations, because of the limited mobility of polymer chains at a high polymer concentration. The membrane structure as determined by scanning electron microscopy depended on the phase separation mechanism. The quenched samples mainly illustrated the occurrence of crystallization on the same time scale as the liquid–liquid phase separated, resulting in the obvious spherulitic structure with small pores in the spherulites. As the polymer concentration increased, the size of the spherulites and pores within the spherulite was decreased. The evaluated porosity for BP diluted system was higher than that for MS diluted system, and decreased with the increased polymer concentration. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 248–260, 2009     

Microporous, fast response cellulose ether hydrogel prepared by freeze-drying

A simple and effective method of preparing fast-response gels is developed. The freeze-drying and subsequent rehydration of thermosensitive gels alters the microstructural properties of the gels in a way that leads to rapid shrinking rates. Microporous hydroxypropyl cellulose (HPC) gels were created by this method to investigate the influence of the process on the swelling and shrinking kinetics of the gels in response to temperature changes. Micropores of different size ranges were produced by freezing gels with different amounts of water at −20 °C. Water content was the key factor to control the microporosity and the shrinking rates of gels. After the freezing treatment, an effective diffusion coefficient for shrinking could be determined by fitting Fick's law to the data (5.2 × 10⁻⁴ cm²/s). This was an increase of two orders of magnitude over that of the untreated, non-porous gel (6.0 × 10⁻⁶ cm²/s). The magnitude of the shrinking coefficient indicates that the shrinking rate of the microporous gel is probably limited by the convective flow, as unsteady flow through porous media follows the same differential equation as Fick's law, but with much greater transport coefficients, as observed here. Physically, the shrinking rate may be determined by the level of interconnected-cells in a microporous structure present at the beginning of shrinking process because the convection through the interconnected-cells is estimated to be much slower than the polymer network diffusion rate of the struts of micropores (0.1–3.0 μm) as well as heat transfer.     

Microporous poly(vinyl methyl ether) hydrogels prepared by γ-ray irradiation at different heating rates

We prepared thermoresponsive and microporous polymer hydrogels by γ-ray irradiation of aqueous solutions poly(vinyl methyl ether) (PVME) at different heating rates. Under all temperature programs, opaque and heterogeneous PVME gels formed, which swelled at temperatures below the lower critical solution temperature and shrank at temperatures above it. All of the samples contained porous and phase-separated structures. The shape and size of the gel pores varied depending on the temperature programs. Gels having a sponge-like continuous porous structure formed only when the radiation-induced crosslinking was carried out at an optimum heating rate, which we found to be 0.11–0.13°C min⁻¹. For temperature changes between 10°C and 40°C, gels with this structure showed rapid volume transitions on a time scale of about a minute.     

Phase behaviour of poly-(λ-benzyl-L-glutamate) in benzyl alcohol

Polarized optical microscopy has been used to investigate phase transitions in the poly-(λ-benzyl-L-glutamate) benzyl alcohol system and these have been compared with the predictions of Flory. All of the samples studied form gels at room temperature. The behaviour of the lowest concentration studied, 5 per cent by volume, shows transitions in the optical microscope compatible with the Flory phase diagram, becoming isotropic at elevated temperatures. Gels of higher concentrations exhibit bulk phase separation into an isotropic liquid phase and an anisotropic phase at room temperature, also in accord with the Flory predictions; the texture of the anisotropic phase varies with concentration. At higher temperatures these concentrations exhibit two coexistent anisotropic phases.     

Thermotropic Polymer Gels: Smart Gel Glass

Several attempts have been made recently in order to develop “smart” windows, which can moderate light and heat intensities. Thermotropic gels have met with growing interest because of their advanced properties. Based upon phase transitions of polymer gels, a novel thermally adjustable window, which includes a gel layer, has been developed in our laboratory. The so-called gel-glass becomes opaque when the temperature exceeds a certain value and goes back to its original transparent state when it is cooled below the critical value again. Intelligent “gel-glasses” made of these materials can moderate the amount of sunlight and radiated heat. The optical properties of the gel layer are also modified by the Joule heat of audio frequency AC current. Two types of gel-glasses have been developed and investigated. Phase transition temperature of a poly(N-isopropylacrylamide) based gel was adjusted by copolymerization and by changing of the solvent composition. The cloud point of a poly(methyl vinyl ether) – water system in a PVA gel was varied by modifying the composition of the swelling agent. In the case of electrically adjustable thermotropic windows, optical properties, energy consumption and temperature changes during the switching process were studied.     

耗散粒子动力学模拟Nafion膜和PVA/Nafion共混膜的介观结构

采用耗散粒子动力学模拟方法研究了水化Nafion膜和水化聚乙烯醇(PVA)/Nafion共混膜的微结构.模拟结果表明水化Nafion膜和水化PVA/Nafion共混膜均能形成相分离的微结构.在水化Nafion膜中,水与磺酸根混合形成管状的水团簇.随着膜内水含量增多,管状水团簇的尺寸逐渐变大并在膜内形成连续的水通道.在水化PVA/Nafion共混膜中,PVA、水、磺酸根混合形成亲水性区域.共混膜中PVA的质量分数和水含量共同影响膜的微结构.当膜中PVA质量分数较低时,PVA主要分布在Nafion的磺酸根基团周围;PVA质量分数升高后,PVA会在膜内单独成一相.当膜中的水含量相对较低时,水分子会溶解于PVA中,此时膜内不存在单独的水团簇;膜中的水含量增多后,膜内会形成接近于球形的水团簇.本文工作可为直接甲醇燃料电池用的PVA改性Nafion膜的开发提供参考.     

Poisson's ratio of polyacrylamide (PAAm) gels

Poisson's ratio (μ₀) of polyacrylamide (PAAm) gels was estimated. The value of μ₀ for PAAm gels was found to be 0.457, which is close to that for poly (vinyl alcohol) (PVA) gels swollen in the mixture of dimethylsulfoxide (DMSO) and water, but is higher than the value for PVA hydrogels.     

Thermal and photochemical stability of poly(vinyl alcohol)/modified lignin blends

A kraft lignin derivative (KLD) obtained by reaction with p-aminobenzoic acid/phthalic anhydride was blended with poly(vinyl alcohol) (PVA) by solution casting from DMSO. PVA and PVA/KLD films were exposed to ultraviolet radiation (24, 48, and 96 h) and analyzed by thermogravimetry (TG), differential scanning calorimetry (DSC), infrared spectroscopy (FTIR), hydrogen nuclear magnetic resonance (¹H NMR) spectroscopy, and scanning electron microscopy (SEM). PVA films show a loss of thermal stability due to irradiation. PVA/KLD reveals greater thermal stability than PVA and an increase in thermal stability after irradiation. These results suggest that the incorporation of KLD into PVA provides a gain in thermal and photochemical stability. FTIR, ¹H NMR, DSC, and TG results obtained for the blends suggest that intermolecular interactions between PVA and KLD chains are present. SEM micrographs revealed blend miscibility for a KLD blend content of up to 15 wt%, as observed at magnification of 1000 times.