Rheological and DSC changes in poly(vinyl alcohol) gels induced by immersion in water

Poly(vinyl alcohol) (PVA) gels were prepared by freezing and thawing aqueous solutions at temperatures from ?20 to 15°C. The temperature was varied periodically by use of a computer. The endothermic DSC peak was observed for the PVA gels at about 60°C for five specimens of different degrees of saponification (DS). Another endothermic peak was also observed in the range 67–80°C, and this peak shifted to higher temperature with increasing DS. These endothermic peaks shifted to lower temperature on immersion of the PVA gels in water. The dynamic Young's modulus E′ at room temperature was also decreased by immersion of PVA gels in water; E′ decreased monotonically with increasing temperature for PVA gels without immersion in water, while it increased up to a certain temperature and then decreased with increasing temperature for PVA gels in water. The X-ray diffraction showed a characteristic crystalline pattern for PVA gels of higher DS, and this peak was intensified by stretching the gel.     

Sorption of water by poly(vinyl alcohol)

The isotherms of water sorption by poly(vinyl alcohol) have been obtained by static sorption methods in a wide range of vapor activities. The properties of poly(vinyl alcohol) at various values of relative humidity have been studied by DSC, X-ray diffraction analysis, and mechanical testing. It has been shown that the correct thermodynamic analysis of sorption isotherms for sorbents with complex organization requires knowledge of their structural features. A method of allowing for the effect of osmotic pressure on the polymer sorption capacity is proposed. The pair interaction parameters estimated in this study are compared with the published data.     

Long branching in poly(vinyl acetate) and poly(vinyl alcohol). II. Polymerization of vinyl acetate in the presence of crosslinked poly(vinyl alcohol)

It is a common view that poly(vinyl acetate) has many branches at the acetyl side group, but that the corresponding poly(vinyl alcohol) has little branching. In order to study the branching in poly(vinyl acetate) and poly(vinyl alcohol) which is formed by chain transfer to polymer, the polymerization of ¹⁴C-labeled vinyl acetate in the presence of crosslinked poly(vinyl acetate), which was able to be decrosslinked to give soluble polymers, was investigated at 60°C and 0°C. This system made it possible to separate as well as to distinguish the graft polymer from the newly polymerized homopolymer. Furthermore, the degree of grafting onto the acetoxymethyl group and onto the main chain were estimated. It became clear that, in the polymerization of vinyl acetate, chain transfer to the polymer main chain takes place about 2.4 times as frequently at 60°C as that to the acetoxy group and about 4.8 times as frequently at 0°C.     

Liquid mulch based on poly(vinyl alcohol). PVA-soil interaction

This study is meant to provide a contribution in recent developments of biodegradable polymeric materials applied in agricultural practices with particular reference to the mulching segment. Special attention has been devoted to material based on renewable resources or utilization of waste products from agro-industrial activities, thus suggesting cost-effective and environmentally sound solutions to specific social needs. In the present contribution PVA solubility at room temperature has been improved by premixing with chemical additives and proteic materials. Waste materials from ethanol production by corn fermentation and from wood industries have been used as, respectively starch-based and lignocellulosic additives. The prepared formulations were applied on the soil by conventional spraying agricultural equipment. Time of permanence of the film formed on the soil was monitored as well as the effect on corn growth and yield production. Finally, in order to investigate the final fate of the PVA applied on the soil, a test was performed in lysimeters, with half of the lysimeters cropped with maize plants. PVA amount applied for the mulching test was in order of 5-10 g PVA/1 m²; under those conditions it resulted completely absorbed and no presence of PVA was recorded in the leached solutions.     

Structure–Property relationships of poly(vinyl alcohol). III. Relationships between stereo-regularity,crystallinity, and water resistance in poly(vinyl alcohol)

Structure–property relationships of poly(vinyl alcohol) have been studied by measuring the crystallinity and water resistance of films derived from samples of varying, known tacticity. Crystallinities of unannealed and annealed films were examined by means of density, infrared, and x-ray measurements. Higher tacticity did not lead to higher crystallinity. The apparent order of crystallizability was atactic ≧ syndiotactic-rich ? isotactic-rich. Water resistance of these films was determined by measuring the swelling index at 30°C. and solubility at 70 and 130–140°C. Water resistance increases as tacticity increases, with syndiotactic-rich PVA exhibiting the highest water resistance. Since water resistance also increases with crystallinity, both stereoregularity and crystallinity must be considered when evaluating structure–solubility relationships. Differential thermal and thermogravimetric analyses of these samples are also presented, together with a correlation of tacticity index as measured by an infrared technique with that of an NMR technique.     

Short branching in poly(vinyl alcohol). III. Some properties of model polymers of short-branched poly(vinyl alcohol)

Melting point, the iodine color reaction, and foam fractionation were studied on model poly(vinyl alcohol) (PVA) having short branches of one or two monomer units in length. An increase in the amount of short branching units caused a marked decrease in color intensity of the PVA–iodine reaction and in the melting point. These tendencies were more remarkable when the short branching was two monomer units in length than when it was one monomer unit. It was also found that foam fractionation of an aqueous PVA solution produced PVA fractions with different degree of short branching, the degree increasing with increase in the fraction number. The color intensity of the PVA–iodine reaction has been confirmed to decrease with increase in the fraction number, but this result cannot be explained solely in terms of the short branching. It is concluded that the phenomenon of foam fractionation of PVA and the iodine color reaction of the fraction appear to be governed by many factors such as molecular weight, stereoregularity, and short branching.     

Long branching in poly(vinyl acetate) and poly(vinyl alcohol). I. Crosslinking and crosslinking cleavage of poly(vinyl acetate)

A crosslinked network was formed by the reaction of partially saponified poly(vinyl acetate) and toluylene diisocyanate in benzene. The yield of gel was markedly dependent on the degree of saponification and the concentrations of polymer and diisocyanate. Crosslinked poly(vinyl alcohol) was obtained by treating the poly(vinyl acetate) with a catalytic amount of sodium hydroxide in methanol without any change of the urethane crosslinks. The crosslink based on the urethane linkage was quantitatively cleaved by acids, especially by hydrobromic acid, releasing polymers of the same molecular weight as the original.     

Short branching in poly(vinyl alcohol). II. NMR spectroscopy of model polymers of short-branched poly(vinyl alcohol)

In order to develop a method of measuring the amounts of short branches in PVA, an NMR study was made of a model poly(vinyl alcohol) having short branches, one or two monomer units in length. Detection and estimation of the short branches were shown to be possible by using the ¹³C-NMR spectra of PVA and the ¹H-NMR of acetylated PVA. In the ¹⁹F-NMR spectra of trifluoroacetylated model PVA, the resonance peaks of primary and tertiary alcohols in the branching structure were not well resolved from that of secondary alcohol of the main chain.     

Study on the nonisothermal crystallization behavior of poly(vinyl alcohol)/attapulgite nanocomposites by DSC analysis

Attapulgite (AT)‐reinforced poly(vinyl alcohol) (PVA) nanocomposite films were prepared by solution‐casting technique. The nonisothermal crystallization behaviors of PVA bulk and PVA/AT nanocomposites have been investigated by differential scanning calorimetry (DSC). It has been found that the uniformly dispersed AT nanorods in the matrix have great influence on the glass transition temperature and crystallization behavior of PVA matrix. The Jeziorny method has been employed to analyze the DSC data. The results show that Jeziorny method could describe this system very well. Comparing with the PVA bulk, PVA/AT nanocomposites have higher crystallinity X_t, shorter semicrystallization time t_1/2, and higher crystallization rate constant Z_c. It can be concluded that AT can be used as an effective nucleating agent and has effects on the growth of crystallites in the crystallization process of PVA matrix. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 534–540, 2006     

States of water in poly(vinyl alcohol) hydrogels

Poly(vinyl alcohol) (PVA) hydrogels with various water contents were prepared from 10 wt% aqueous solutions of mixtures of PVA and anionic poly(styrene sulfonic acid) sodium salt (NaPSS) by casting, dehydrating, and then extracting NaPSS. The existence of three types of ice were suggested by differential scanning calorimetry (DSC) measurements for every frozen hydrogel. In the frozen hydrogels the states of water, except unfrozen water, were ice of free water and disordered ice crystals. The mobility and activation energy for motion of water molecules in unfrozen hydrogels were investigated by using pulsed nuclear magnetic resonance (PNMR) measurements. It was concluded that there are two states of water in the unfrozen hydrogel, i.e. unfrozen water and disordered water which is mainly formed in narrow apertures in the hydrogel. The discrepancy between the DSC and PNMR measurements was explained by a structural transformation during cooling.     

Long branching in poly(vinyl acetate) and poly(vinyl alcohol). IV. Polymerization of vinyl trimethylacetate in the presence of crosslinked poly(vinyl acetate)

In order to clarify the grafting behavior of vinyl trimethylacetate (VTMAc) onto poly-(vinyl acetate) (PVAc), the polymerization of a radioactive VTMAc in the presence of a crosslinked PVAc gel was studied in accordance with the experimental technique described in the previous papers. It was found that, at 60°C, the grafting onto the main chain of PVAc takes place about 2.8 times as readily as that onto the acetyl side group on PVAc.     

Ice recrystallization inhibition and molecular recognition of ice faces by poly(vinyl alcohol).

The effects of poly(vinyl alcohol) (PVA) on the Ostwald ripening of polycrystalline ice samples are studied. At -6 degrees C, ice recrystallization in sucrose solutions is inhibited at PVA concentrations down to 0.005 mg mL(-1), with a recrystallization inhibition constant of 48.9 mL mg(-1). Ice growth-habit experiments reveal molecular recognition of the arrangement of water molecules in the ice by PVA molecules, and indicate that PVA molecules adsorb to the primary and secondary prism faces of hexagonal ice, Ih. Based on these observations, together with an analysis of the O-atom pattern in ice and the conformation of OH groups in PVA, an adsorption model is proposed. We suggest that PVA segments adsorb to the primary and secondary prism faces of ice parallel to the c axis with a linear misfit parameter of only 2.7 %, most likely via multiple hydrogen bonds. The proposed adsorption mechanism is discussed in the light of recent thermal hysteresis and scanning tunneling microscopy experiments.     

Dissolution mechanism of semicrystalline poly(vinyl alcohol) in water

Changes occurring in the degree of crystallinity and lamellar thickness distribution of poly(vinyl alcohol) (PVA) samples during dissolution in water were investigated. PVA samples of three different molecular weights were crystallized by annealing at 90, 110, and 120°C. The initial degrees of crystallinity measured by differential scanning calorimetry (DSC) and by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) varied from 43 to 60% and the average lamellar thicknesses measured by DSC ranged from 50 to 400 Å. PVA dissolution was followed at 25, 35, and 45°C from 30 s up to 195 min. Lamellar thicknesses were determined as a function of dissolution time using DSC. There was an initial drastic decrease in the degree of crystallinity, which leveled off to a fairly constant value before reaching zero by the time the polymer dissolved completely. Increase in molecular weight led to lesser number of crystals, but with larger average lamellar thickness, which were more stable in the presence of water. Increase in crystallization temperature or decrease in dissolution temperature led to larger average lamellar thickness. Based on these findings, a dissolution mechanism involving unfolding of the polymer chains of the crystal was proposed. © 1996 John Wiley & Sons, Inc.     

States of water in poly(vinyl alcohol) derivative hydrogels

We studied the behavior of absorbed water in equilibrium‐swollen poly(vinyl alcohol) derivative hydrogels by differential scanning calorimetry (DSC), ¹H nuclear magnetic resonance, and wide‐angle X‐ray diffraction. By DSC, three types of water were detected, and their relative fractions were estimated. With this technique we also calculated the pore size for every sample. From the nonexponential decay of the spin–spin relaxation data, we distinguished two environmental states of the absorbed water in the samples. The relaxation times were determined. From these data, we calculated the fractions of each type of water for every hydrogel and related them to the degree of crosslinking. The X‐ray study indicated that the water absorbed in these hydrogels forms a single crystalline phase on cooling. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1462–1467, 2003     

Short branching in poly(vinyl alcohol). IV. Determination of the short branches in poly(vinyl alcohol) polymerized under various conditions

The effect of the conditions of polymerization of vinyl acetate on the formation of short branching in the derived poly(vinyl alcohol) (PVA) was studied. ¹H-NMR combined with a CAT technique was employed for the determination of the short branches. It was found that the formation of short branching is favorable at low concentrations of monomer, at high conversions, and at high temperatures. These facts evidently support the idea that the mechanism of the formation of short branching is back-biting. Amounts of short branches in some commercial PVA were also estimated. The observed values lie in the range of 0.12–1.7 mole-%.     

Relaxation phenomena in poly(vinyl alcohol)/fumed silica affected by interfacial water

Interaction of poly(vinyl alcohol) (PVA) with fumed silica was investigated in the gas phase and aqueous media using adsorption, broadband dielectric relaxation spectroscopy (DRS), thermally stimulated depolarization current (TSDC), infrared spectroscopy, thermal analysis, and one-pass temperature-programmed desorption (OPTPD) mass-spectrometry (MS) methods. PVA monolayer formation leads to certain textural changes in the system (after suspension and drying) because of strong hydrogen bonding of the polymer molecules to silica nanoparticles preventing strong interaction between silica particles themselves. This strong interaction promotes associative desorption of water molecules at lower temperatures than in the case of silica alone. Interaction of PVA with silica and residual water leads to depression of glass transition temperature (T(g)). There are three types of dipolar relaxations at temperatures lower and higher than the T(g) value. A small amount of adsorbed water leads to significant conductivity with elevating temperature.     

用DSC研究线形多嵌段聚氨酯与聚氯乙烯、氯化聚氯乙烯共混相容性

用示差扫描量热法（DSC）研究了线形多嵌段聚氨酯（PU）与聚氯乙烯（PVC）、氯化聚氯乙烯（CPVC）共混相容性，说明了PU／VC、PU／CPVC的相容是由于共混物中形成了新的氢键的缘故．聚酯型聚氨酯与PVC、CPVC的相容性要好子聚酸型聚氨酯，CPVC与PU的相容性又要好于PVC．聚氨酯中硬段的引入不利于PU／PVC、PU／CPVC的相容性．     

Structure–property relationships of poly(vinyl alcohol). I. Influence of polymerization solvents and temperature on the structure and properties of poly(vinyl alcohol) derived from poly(vinyl acetate)

The solubility properties of poly(vinyl alcohol) (PVA) vary with the method of preparation of the poly(vinyl acetate) (PVAc) from which it is derived. PVAc was prepared with free-radical catalysts over a range of temperatures from ?78 to 90°C. with solvents of varying chain-transfer ability. The corresponding PVA samples varied in their resistance to dissolution in water. Their high-resolution proton nuclear magnetic resonance spectra showed on differences in tacticity. Data on 1,2-diol content showed only minor differences. Hence, the increase in resistance of PVA to dissolution in water arising from changes in chain-transfer activity of the solvent used in vinyl acetate polymerization is largely attributable to a decrease in molecular weight, and the increase in resistance of PVA to dissolution in water arising from a decrease in the temperature of the vinyl acetate polymerization is largely attributable to a decrease in both long and short branches. Evidently, with polar polymers having small side groups, tacticity is not the only factor influencing property variation; that is, variations in stereoregularity influence more the crystallinity of the sample as measured by density or x-ray methods than the ultimate crystallizability under conditions of mechanical and thermal treatment. In this regard polar polymers having small side groups differ from nonpolar polymers.     

Hydrogel membranes with mesh size asymmetry based on the gradient crosslinking of poly(vinyl alcohol)

Poly(vinyl alcohol) (PVA) hydrogel membranes with mesh size asymmetry were prepared and their transport properties were studied. Homogeneous membranes with water contents of 82%, 76% and 72% were prepared by crosslinking PVA with glutaraldehyde. These membranes were then modified to create asymmetry by establishing a glutaraldehyde concentration gradient across the hydrogel thickness. The reaction time and magnitude of the glutaraldehyde concentration gradient were varied to determine the optimum values of permeability and selectivity. Permeation experiments with creatinine, Fab and IgG were performed in a stirred diffusion cell through homogeneous and asymmetric PVA hydrogels. A modified version of the multiple-membrane technique was used to determine boundary layer resistance in order to determine the intrinsic membrane permeability. As expected, the selectivity of creatinine over IgG increased as the modification time increased. However, the selectivity of Fab over IgG initially increased as the modification time increased, but then decreased at longer times, indicating that the increased crosslinking at the surface effectively blocks both proteins. At a given value of IgG rejection, the asymmetric membranes had higher creatinine and Fab permeabilities than the corresponding homogeneous membranes. This indicates that creating mesh size asymmetry in a hydrogel can result in a high-flux, high-selectivity membrane for cell encapsulation or bioseparations.     

Heterotactic poly(vinyl alcohol)

Bulky substituents in vinyl trialkylsilyl ethers and vinyl trialkylcarbinyl ethers led to heterotactic polymers (H = 66%). The polymers were converted into poly(vinyl alcohol) (PVA) and further to poly(vinyl acetate), and tacticity was determined as poly(vinyl acetate). Vinyl triisopropylsilyl ether in nonpolar solvents yielded a heterotactic polymer with a higher percentage of isotactic triads than syndiotactic triads (Hetero-I). Vinyl trialkylcarbinyl ethers in polar solvents gave a heterotactic polymer with more syndiotactic triads than isotactic (Hetero-II). Heterotactic PVA was soluble in water and showed characteristics infrared absorptions. Interestingly, Hetero-I PVA showed no iodine color reaction, but Hetero-II showed a much more intense color reaction than a commercial PVA. The mechanism of heterotactic propagation was discussed in terms of the Markóv chain model.