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.     

三维壳聚糖材料中水的状态与其性能的关系

利用原位沉析法制备了具有层状结构的高强度三维壳聚糖(chitosan,CS)材料,由于CS分子结构中存在大量的亲水基团,容易吸附水分子.通过热分析(TGA,DSC)测试发现吸水后的CS材料中的水分是以3种不同的状态存在,结合水、中间态水和自由水.结合力学性能的方法分析不同状态的水对材料性能的影响.结果表明,结合水和CS以氢键紧密结合,有较好的热稳定性,对材料的力学性能有增强作用;CS棒材在环境中吸附的中间态水相当于一种增塑剂,随着水含量的增加,材料力学性能的下降符合Fermi经验公式;中间态水达到饱和之后,自由水开始出现,自由水和CS之间的作用力很弱,温度较低时会结晶成冰.自由水的渗透与中间态水比对材料的力学性能没有明显的影响.     

热分析技术研究疏水缔合微区对含氟丙烯酸酯共聚物凝胶中水状态的影响

丙烯酸 (AA)与不同含量的 2 (N 乙基全氟辛烷基磺酰胺 )乙基甲基丙烯酸酯 (FMA)在叔丁醇中进行自由基共聚 ,合成了一系列疏水改性共聚物凝胶 .用示差扫描量热法 (DSC) ,热失重分析 (TGA)研究了含氟疏水改性凝胶中水的状态 ,证明FMA改性凝胶中存在 3种状态的水 ,即不可冻结水 ,可冻结结合水和自由水 .对于未改性凝胶和FMA含量为 1mol%的疏水改性凝胶 ,在凝胶中水含量分别低于 0 60 9mol%和 0 698mol%时 ,DSC升温曲线上观察不到熔融峰 ,说明凝胶中存在不可冻结水 .提出计算凝胶中 3种状态的水含量和熔融焓的理论假设 .计算出FMA含量为 1mol%的疏水改性凝胶中自由水和可冻结结合水的摩尔熔融焓分别为75 89 0 3和 6864 0 6J mol.计算结果表明疏水改性后 ,单位聚合物单元上结合的可冻结水数目 (n1 )和非冻结水的数目 (n0 )都增加     

吸水膨胀弹性体在不同介质中的溶胀行为

研究了由共硫化法制得的三种吸水膨胀弹性体（ＷＳＥ）在不同介质中的溶胀行为，得出了溶胀速率方程和动力学参数，确定了不同介质对平衡溶胀度（Ｑｅ）的影响规律．     

Design and analysis of the homogeneous and heterogeneous distribution of water confined within colloidal polymer particles

Water is known to distribute within polymeric films in multiple states differentiable by the energy of association. Potentiometric swelling of carboxylated latex samples and subsequent differential scanning calorimetry (DSC) and thermogravimetric analysis verified this distribution of water, specifically confined within colloidal nanoparticle dimensions. DSC cooling curves can delineate between the freezable bound and freezable unbound water at low total water content but become difficult to distinguish the freezable bound contribution at high total water content. Of note is that the ratio of weakly bound water in the secondary layer to the water strongly hydrogen-bound to the polymer is approximately constant regardless of carboxylic acid type and, in fact, is greater for the case of the hydrophobic base polymer. Aside from its distribution within the particles, the total water content also appeared to be more related to the hydroplasticized glass point of the polymer colloid as opposed to the polarity of the polymer.     

Determination of differences in free and bound water contents of beef muscle by DSC under various freezing conditions

The differences in bound water content of beef semimembranous muscle samples obtained from previously chilled (24 h at +4°C) middle-aged beef carcasses were determined by the use of DSC. Initially, samples obtained from fresh, unprocessed meat were frozen at –40, –50 or –65°C to determine their melting peaks for freezable water (free water) content with the use of DSC. The samples were then subjected to an environment with an ambient temperature of –30, –35, –40 or –45°C, with no air circulation, or with an air circulation speed of 2 m s^–1, until a thermal core temperature of –18°C was attained; this was followed by thawing the samples until a thermal core temperature of 0°C was reached. This process was followed by subjecting the samples to the ambient temperatures mentioned above, to accomplish complete freezing and thawing of the samples, with DSC, and thereby determination of the freezable water contents, which were then used to determine the peaks of melting. The calculated peak areas were divided by the latent heat of melting for pure water, to determine the freezable water contents of the samples. The percentage freezable water content of each sample was determined by dividing its freezable water content by its total water content; and the bound water content of each sample was determined by subtracting the percentage free water content from the total. In view of the fact that the free water content of a sample is completely in the frozen phase at temperatures of –40°C and below, the calculations of free and bound water contents of the samples were based on the averages of values obtained at three different temperatures.     

Hydration and drying of various polysaccharides studied using DSC

The hydration of cellulose, chitosan, schizophyllan, hyaluronan, and carboxymethyl cellulose was studied using differential scanning calorimetry (DSC). In the first part, the classical freezing/thawing approach was used to determine the amount of non-freezing water. The inconsistency in enthalpies obtained during crystallization and melting of freezable water was discussed with respect to the DSC experimental conditions. Our interpretation questions the recent conclusions about competitive processes occurring during melting which are hypothesized to influence the determined melting enthalpy. In the second part, the hydration and drying were studied using the evaporation enthalpy of water. The dry mass normalized dependency of vaporization enthalpy on water content confirmed an abrupt break at low water content in hyaluronan sample which was attributed to the sudden appearance of a parallel process taking part during the drying. The rest of polysaccharide samples showed only a linear decrease in evaporation enthalpy. The renormalization of enthalpies by the water content revealed the increase in evaporation enthalpy with decreasing water content in most samples which was ascribed to the strong interaction between polysaccharide and water. The exceptions were carboxymethyl cellulose which showed a decrease in evaporation enthalpy. This indicates the existence of a simultaneous process occurring during drying, but unlike in hyaluronan, the processes do not appear abruptly but accompany the evaporation in the wide concentration range. Comparison of determined hydration numbers showed that part of non-freezing water in hyaluronan is not bound to sorption sites but occurs presumably in small temporary pores. In contrast, water-soluble schizophyllan forms temporary pores as well but presumably with higher dimension and the non-freezing water is formed mostly by water molecules interacting with sorption sites.     

Quantification of water in different states of interaction with wood pulp fibres

The state of water in wet pulps was studied over a wide moisture range, reflecting the conditions on the paper machine from the press section to the reel. Two methods were used: water retention value (WRV) and differential scanning calorimetry (DSC). To increase the measuring accuracy, a modification to the WRV procedure was introduced. The results were directly compared by plotting the values over the observed moisture range. Different types of water were defined from DSC data: free water, freezing bound and non-freezing bound water. The water types were plotted in water distribution charts against the solids content. These charts were compared with the WRV data. For all pulps, the three water fractions were found to be interdependent while the free water removal rate decreased. Water removal did not take place as an independent water fraction removal sequence. The presence of fines and pulp recycling were found to influence the amount of freezing bound water, but not non-freezing bound water. The results showed an irreversible decrease in WRV (hornification) for all investigated pulps in the range of 20–75% solids content. This pattern corresponded to the relative amount of free water present. Freezing bound water disappeared at a solids content of over 80–85%. With the absence of free water, no further hornification occurred. It was concluded that the WRV appears to be reduced by a closure of large voids by adhesion of the cellulosic pore walls. The softening effect of water is required to cause wet hornification of the structure.     

Studies on bound water in poly(vinyl alcohol). Hydrogel by DSC and FT-NMR

The phase transition of water bound in poly(vinyl alcohol)(PVA) hydrogel was measured by differential scanning calorimetry. The cross-link density calculated from the swelling ratio of PVA hydrogel ranged from 3 × 10^?5 to 1 × 10^?3 according to the total dose of ⁶⁰Co γ-ray irradiation. From the enthalpy of melting, it was found that water in PVA gel can be classified into at least three groups; ca. 1–1.5 mol non-freezing water, 5–6 mol freezing bound water, and the rest free water, corresponding to each hydroxyl group of PVA. The characteristic properties of polymer hydrogel were attributed to the large number of molecules of freezing bound water in the network. The amount of free water depended markedly upon cross-link density; however, it was not apparent in PVA gel with cross-link density >2.0 × 10^?4. The decreases in the nuclear magnetic resonance relaxation times of water and PVA were related to the cross-link density and were found to level off at a cross-link density 2.0 × 10^?4.     

Non-freezing water in protein solutions

Differential scanning calorimetric studies of protein solution provided the freezable water content as a function of concentration. Total water content was obtained by vacuum dehydration. The difference between the two gave the non-freezing water content in protein solutions. The values obtained by these techniques were 2–3 times larger than those obtained on the basis of NMR measurements on the same solutions.     

Water-activated cellulose-based electrorheological fluids

The performance of electrorheological (ER) fluids containing cellulose particles dispersed in lubricating oil was investigated as a function of particle water content, DC electric field strength, particle concentration, and temperature. Over a range of applied electric fields (0–3 kV/mm), yield stress was observed to increase with increasing cellulose moisture content up to 8.5 wt% followed by a decrease. Water adsorbed by cellulose particles used in these systems was shown to be non-freezing bound water. The maximum ER response for a cellulose-based fluid at 25 °C was observed at a moisture content near the transition of less mobile liquid-like (LM) water to more mobile liquid-like (MM) non-freezing water. At a constant moisture level, yield stress increased linearly with increases in either electrical field strength or particle concentration, while the ER effect decreased with increasing temperature. The present study concludes that the performance of water-activated ER fluids based on cellulose particles is influenced strongly by the mobility of non-freezing bound water adsorbed onto cellulose.     

New polymer electrolyte membranes for low temperature fuel cells

Partially fluorinated proton exchange materials were synthesised by pre-irradiation grafting of styrene into poly(vinylidene fluoride) films with subsequent sulfonation. The grafted and sulfonated membranes, PVDF-g-PSSA membranes, have been studied with respect to water uptake, ion and water clustering, ion conductivity and water diffusion coefficients. Water associates with the membranes in three different ways: bound non-freezable water, freezable bound water and freezable free water. The proton conductivity of the membrane is strongly dependent on the hydration, it decreases more rapidly than the water self diffusion with decreasing water content. Ion clusters with a Bragg distance of 25 Å form the conducting channels in the membranes.     

Determination of freezable water content of beef semimembranous muscle DSC study

The freezable water contents of samples obtained from previously chilled semimembranous muscle of middle-aged beef carcasses after a 24 h cooling period a room at in 5±1C were determined by differential scanning calorimetry (DSC) at –5, –10, –15, –20, –30, –40, –50 and –65C. This was accomplished by freezing the samples at the above-mentioned temperatures, followed by thawing to 35C, and measuring the melting peaks of freezable water. The areas of these peaks were determined by using the peak integration method programs through a computer linked to the DSC, and they were then used to determine the latent heat of melting (H _m) in kJ kg^–1 at each freezing temperature. The resultant latent heat of melting per sample was divided by the latent heat for pure water to determine the amount of freezable water present in these samples. This amount of freezable water was divided by the total water content of the meat sample to determine the percentage of freezable water in the sample. The percentage of freezable water was subtracted from 100 to determine the percentage of bound water present in the sample.     

The States,Diffusion, and Concentration Distribution of Water in Radiation‐Formed PVA/PVP Hydrogels

Abstract

Hydrogels with various compositions of polyvinyl alcohol (PVA) and poly(1‐vinyl‐2‐ pyrrolidinone) (PVP) were prepared by irradiating mixtures of PVA and PVP in aqueous solutions with gamma‐rays from ⁶⁰Co sources at room temperature. The states of water in the hydrogels were characterized using DSC and NMR T₂ relaxation measurements and the kinetics of water diffusion in the hydrogels were studied by sorption experiments and NMR imaging. The DSC endothermic peaks in the temperature range ?10 to +10°C implied that there are at least two kinds of freezable water present in the matrix. The difference between the total water content and the freezable water content was referred to as bound water, which is not freezable. The weight fraction of water at which only nonfreezable water is present in a hydrogel with F_VP=0.19 has been estimated to be g_H2O/g_Polymer=0.375. From water sorption experiments, it was demonstrated that the early stage of the diffusion of water into the hydrogels was Fickian. A curve‐fit of the early‐stage experimental data to the Fickian model allowed determination of the water diffusion coefficient, which was found to lie between 1.5×10^?11 m² s^?1 and 4.5×10^?11 m² s^?1, depending on the polymer composition, the cross‐link density, and the temperature. It was also found that the energy barrier for diffusion of water molecules into PVA/PVP hydrogels was ≈24 kJ mol^?1. Additionally, the diffusion coefficients determined from NMR imaging of the volumetric swelling of the gels agreed well with the results obtained by the mass sorption method.     

Heat capacity of water in poly(methyl methacrylate) hydrogel membrane for an artificial kidney

The heat capacities of a poly(methyl methacrylate) hydrogel membrane for an artificial kidney have been determined over the range of temperatures from 228 to 298 K as a function of water content. At least two kinds of water were found: freezable water and nonfreezable water. The partial specific heat capacities of both waters were calculated from the dependence of heat capacity of the hydrogel on the water content. The heat capacities of freezable water were estimated to be 1.04 cal g⁻¹ K⁻¹ at 298 K and 0.47 cal g⁻¹ K⁻¹ at 228 K. The mobility must therefore be similar to that of bulk water at 298 K, though the melting temperature was lower than that of bulk water. Consequently, the freezable water was not assigned to bound water but to pore water for which the melting temperature was depressed due to interfacial tension. On the other hand, the heat capacities of nonfreezable water were estimated to be 1.02 cal g⁻¹ K⁻¹ at 298 K and 1.06 cal g⁻¹ K⁻¹ at 228 K. The mobility of the water would be similar to that of free water at both 298 and 228 K. © 1995 John Wiley & Sons, Inc.     

Sorption of Nickel Ions from Aqueous Solutions by Ion Exchangers

Sorption of Ni²⁺ions from aqueous solutions by ion exchangers differed in their chemical nature and structure was studied. Based on the endothermic curves of ice melting obtained by a differential scanning calorimetry, the amounts of freezing and non-freezing water present in free volumes (pores) of the studied ion exchangers were calculated. Comparison of results obtained from the kinetic curves of nickel ion sorption with data on differential scanning calorimetry indicates a role of structural factor in the sorption of nickel ions. It was found that, depending on the total amount of freezing and non-freezing water, the KU-2-8 sulfonated cationite is the most preferable ion exchanger for the sorption of nickel ions from aqueous solutions. Therefore, sorbent efficiency in this case is determined by its structure rather than by chemical nature.     

Enhanced dehumidification performance of PVA membranes by tuning the water state through incorporating organophosphorus acid

A novel thin film composite membrane with superior propylene dehumidification performance was prepared by coating a high hydrophilic organophosphorus acid ethylene diamine tetra(methylene phosphonic acid) (EDTMPA) doped poly(vinyl alcohol) (PVA) on polysulfone (PS) hollow fiber membranes. Experimental studies and molecular dynamics simulations were combined to probe the existing states and the transport mechanism of water in the membranes. Water vapor sorption experiments revealed that the enhanced dehumidification performance was governed by the diffusion process. Water states and water distribution were investigated by molecular dynamics simulation. At low EDTMPA content (<10 wt.%), states of the water were not obviously changed and the increase of water diffusion coefficient was mainly attributed to enlarged free volume of the membrane. At high EDTMPA content (10–30 wt.%), the increase in the water diffusion coefficient mainly arose from the variations in the water states. Strong interaction between PVA and EDTMPA reduced the amount of water that bounded to the PVA and increased the proportion of free water. The diffusion coefficients of water increased with increasing proportion of free water, since the mobility of free water was higher than that of bound water. The permeance of water reached 997.7 GPU for the PVA–EDTMPA/PS membrane with a 20 wt.% EDTMPA content when the proportion of free water was the highest, and the separation factor increased to infinity.     

快速响应的温敏性聚(N-异丙基丙烯酰胺)水凝胶 Ⅱ.热力学行为及水的状态研究

对使用CaCO3为成孔剂合成的快速响应的温敏性聚 (N 异丙基丙烯酰胺 ) (PNIPA)水凝胶进行了热力学行为和水的状态研究 .热力学研究表明 ,多孔结构的PNIPA水凝胶的平衡膨胀比随着反应物中CaCO3含量的增加而增加 ,随着交联剂浓度的增加而显著减小 ,但相转变温度均不受影响 .在水溶液中加入NaCl则使PNIPA水凝胶的相转变温度 (LCST)线性减小 .利用DSC分析了水凝胶中水的存在状态 ,证明了上述多孔PNIPA水凝胶中存在三种不同状态的水 ,研究了不同CaCO3粒子含量和离子强度对三种不同状态水的影响     

Determination of water states and the structural parameters of W/O microemulsions

The states of water in sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/isooctane/water reverse(W/O) microemulsions system have been investigated by using Fourier transform infrared spectroscopy(FT-IR) technique. The broad peak obtained for hydroxy(O-H) of water has been resolved by least square curve-fitting. It has been observed that the water solubilized in microemul-sion droplets has four states, i.e. bound water with sulfo-group, free water, bound water with sodium counterion in the water pool of microemulsion droplets and a small amount of trapped water in the palisade layer of microemulsion droplets. The following have also been determined: the aggregation number (n), the radius of the water pool in the microemulsions(rw), the thickness of the bound water with sulfo-group(d1), the thickness of the bound water with sodium counterion(d2), the total thickness of the bound water (d) and the effective area of head groups of AOT(AAOT).     

Gas permeation through hydrogels : I. Gel cellophane membranes

Permeabilities and diffusion coefficients of various gases, Ar, N₂, O₂, CO₂, CH₄, C₂H₄, C₃H₈ and C₄H₁₀, were measured for Water-swollen gel cellophane membranes. No dependence of permeabilities on gas pressure below 1 atm was found. It was observed that the permeability coefficients were not related linearly to the coefficients in bulk water. For the two states of water in the membrane, an analytical method is presented to estimate the diffusion coefficients and the solubilities in free water and non-freezing water. It was found that the diffusion coefficients in non-freezing water were lower than those in free water, and the solubilities in non-freezing water were higher than in free water for all gases studied except CO₂ and C₂H₄, which gave reverse results.