Modification of poly(vinyl alcohol) with acid chlorides and crosslinking with difunctional hardeners

The reaction of acid chlorides with poly(vinyl alcohol) (PVA) leads to vinyl alcohol-vinyl ester copolymers. The esterification by Schotten-Baumann enables degrees of modification to be reached which depend on the chemical structure of the pendant unit introduced. High degrees of modification were obtained in the reaction with water-stable acid chlorides such as benzoyl or cinnamoyl chloride. The copolymers obtained were characterized by spectroscopic techniques, elemental analysis, thermal methods, and viscosity determinations. Since in all cases they were only partially modified, the remaining hydroxyl groups were reacted with difunctional hardeners, Epiclon B-4400, hexamethylene diisocyanate, and tolylene 2,4-diisocyanate, to obtain tridimensional networks. Vinyl alcohol-vinyl benzoate copolymers with different degrees of modification were crosslinked to study how the percentage of remaining hydroxylic groups affects the crosslinking process. The exotherm maximum of the curing process was lower when diisocyanates were used as hardeners and the degree of crosslinking was higher when hexamethylene diisocyanate was used according to the observed increase in T_g values. Whereas the TGA curves of linear polymers only showed one degradation, in crosslinked polymers two degradation steps were detected, suggesting several degradation mechanisms. © 1996 John Wiley & Sons, Inc.     

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.     

Fully-biodegradable poly(3-hydroxybutyrate)/poly(vinyl alcohol) blend films with compositional gradient

Fully-biodegradable bacterial poly(3-hydroxybutyrate) (PHB)/chemosynthetic poly(vinyl alcohol) (PVA) blend films with compositional gradient from one surface to the other surface of the films were prepared by a dissolution-diffusion technique. Three kinds of PVA samples, high- and low-molecular weight atactic PVA and highly syndiotactic PVA (s-PVA), were used in order to investigate the effects of molecular weight and tactic structure on the generation of compositional gradient. The solution of PHB in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), which is also a good solvent for PVA, was cast on the PVA film and then the solvent HFIP was evaporated. By selecting the optimum volume of solvent and the evaporation rate, the PHB/PVA blend film with compositional gradient was obtained. The formation of compositional gradient was confirmed by FT-IR microscopy and ATR-FT-IR analysis. The 50%/50% PHB/s-PVA blend film with a nearly ideal compositional gradient, that is, the composition of PHB (or PVA) in the film changing gradually from 100% at one surface to 0% at the other surface of the film was obtained by casting PHB/HFIP solution on to the s-PVA film. Positional dependence of the absorbance of C==O and OH stretching bands along the film thickness direction for the PHB/S-PVA cast films.     

Preparation of electrospun chitosan/poly(vinyl alcohol) membranes

Electrospinning of chitosan from its solutions in 2% aqueous acetic acid was studied by adding poly(vinyl alcohol) (PVA) as a “guest” polymer. Properties of the chitosan/PVA solutions including viscosity, conductivity, and surface tension were measured, and effects of the polymer concentration, chitosan/PVA mass ratio and processing parameters (applied voltage, flow rate, capillary-to-collector distance) on the electrospinnability of chitosan/PVA were investigated. Analyses of scanning electron micrographs and transmission electron micrographs suggested that the chitosan/PVA ultrafine fibers were often obtained along with beads, and chitosan was located in the elctrospun fibers as well as in the beads. Uniform chitosan/PVA fibers with an average diameter of 99 ± 21 nm could be prepared from a 7% chitosan/PVA solution in 40:60 mass ratio. Results of Fourier transform infrared spectroscopy and X-ray diffraction demonstrated that there were possible hydrogen bonds between chitosan and PVA molecules, which could weaken the strong interaction in chitosan itself and facilitate chitosan/PVA electrospinnability. The electrospun chitosan/PVA membranes showed higher water uptake and would have potential applications in wound dressings.     

New chemical hydrogels based on poly(vinyl alcohol)

The syntheses of two new types of chemical hydrogels based on poly(vinyl alcohol), PVA, are reported. Common to both synthetic routes is the preparation of a telechelic PVA (tel-PVA) obtained by periodate splitting of the vicinal diol units present in few percent along the chain. tel-PVA was then used as crosslinking agent with chitosan and PVA in two simple reactions in aqueous phase such as reductive alkylation of chitosan and acetalyzation of PVA. Both reactions yielded firm wall-to-wall networks showing marked differences in their swelling capability. © 1996 John Wiley & Sons, Inc.     

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.     

Catalytic poly(vinyl alcohol) functionalized membranes obtained by gamma irradiation

Polymeric catalytic membranes bearing sulfonic acid functions have been prepared by mutual gamma irradiation at a ⁶⁰Co source, of poly(vinyl alcohol) (PVA) membranes and methanesulfonic acid. The effect of various synthesis conditions on membranes' physical–chemical properties and catalytic activity in the esterification reaction between acetic acid and isoamyl alcohol to obtain isoamyl acetate (banana flavor), was evaluated. The membranes were characterized by ATR-FTIR, TPP, AFM and SEM. Water contact angle determinations were also performed. The obtained results showed that within the range of conditions studied the increase in sulfonic acid groups' content is accompanied by an enhancement in the membranes catalytic activity, while the increase in absorbed dose leads to a decrease in catalytic activity.     

Determining the vinyl alcohol distribution in poly(vinyl butyral) using normal-phase gradient polymer elution chromatography

The application of film-forming organic polymers, which are in common use in membrane technology, as chromatographic adsorbents for packed and capillary columns has been suggested. The chromatographic characteristics of poly[1-(trimethylsilyl)-1-propine] (PTMSP) as an adsorbent were studied. The film-forming properties of PTMSP simplify manufacturing of capillary and packed gas–solid columns. It was shown that separation of C₁–C₄ hydrocarbon gases on the columns with PTMSP is of practical interest. In the authors’ opinion, PTMSP is also promising for the separation of inorganic gases.     

Study of the miscibility of poly(vinyl pyridines) with poly(vinyl acetate), poly(vinyl alcohol) and their copolymers

Miscibility of poly(4-vinyl pyridine) (P4VP) and poly(2-vinyl pyridine) (P2VP) with poly(viny acetate) (PVAc), poly(vinyl alcohol) PVA and poly(vinyl acetate-co-alcohol) (ACA copolymers) has been investigated over a wide composition range. Differentiaal scanning calorimetry (DSC) results indicate that P2VP is immiscible with PVAC, PVA, and their copolymers over the whole composition range. In turn, P4VP appears to be immiscible with PVAC and PVA, but miscible with some ACA copolymers in certain range of composition. The P4VP-ACA phase diagram for different copolymer compositions has been determined. The variation of the glass transition temperature with composition for miscible mixtures was found to follow the Gordon-Taylor equation, with the parameter κ dependent upon copolymer composition. FTIR analysis of blends reveal the existence of specific interactions via hydrogen bonding between hydroxyl groups and the nitrogen of the pyridinic ring, which appear to be decisive for miscibility. © 1994 John Wiley & Sons, Inc.     

Dynamic mechanical and dielectrical properties of poly(vinyl alcohol) and poly(vinyl alcohol)‐based nanocomposites

In this article we report on the investigation of the dynamics of poly(vinyl alcohol) (PVA) and PVA‐based composite films by means of dielectric spectroscopy and dynamic mechanical thermal analysis. Once the characterization of pure PVA was done, we studied the effect of a nanostructured magnetic filler (nanosized CoFe₂O₄ particles homogeneously dispersed within a sulfonated polystyrene matrix) on the dynamics of PVA. Our results suggest that the α‐relaxation process, corresponding to the glass transition of PVA, is affected by the filler. The glass‐transition temperature of PVA increases with filler content up to compositions of around 10 wt %, probably as a result of polymer–filler interactions that reduce the polymer chain mobility. For filler contents higher than 10 wt %, the glass‐transition temperature of PVA decreases as a result of the absorption of water that causes a plasticizing effect. The β‐ and γ‐relaxation processes of PVA are not affected by the filler as stated from both dynamic mechanical thermal analysis and dielectric spectroscopy. Nevertheless, both relaxation processes are greatly affected by the moisture content. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1968–1975, 2001     

Fabrication and characterization of anhydrous polymer electrolyte membranes based on sulfonated poly(vinyl alcohol) and benzimidazole

In the present study, a new type of chemically cross-linked polymer blend membranes consisting of poly(vinyl alcohol) (PVA), sulfosuccinic acid (SSA) and benzimidazole (BnIm), as a dopant, at different stoichometric ratios were prepared and used as proton conducting polymer electrolytes. The proton conductivities of the membranes were investigated as a function of blending composition and the temperature. TGA indicated that the blend polymers were thermally stable up to approximately 175°C; differential scanning calorimetry (DSC) results illustrated the homogeneity of the materials. The local chain flexibility of the host polymer increased with BnIm concentration. The methanol permeability values of the membranes were much lower than that of a Nafion-membrane. The proton conductivity of these materials increased with BnIm and SSA concentration and the temperature.     

Characterization of poly(vinyl alcohol) during the emulsion polymerization of vinyl acetate using poly(vinyl alcohol) as emulsifier

During the emulsion polymerization of vinyl acetate (VAc) using poly(vinyl alcohol) (PVA) as stabilizer and potassium persulfate as initiator, the VAc reacts with PVA forming PVA-graft-PVAc. When the grafted polymer reaches a critical size it becomes water-insoluble and precipitates from the aqueous phase contributing to the formation of polymer particles. Since particle formation and therefore the properties of the final latex will depend on the degree of grafting, it is important to quantify and to characterize the grafted PVA. In this work, the quantitative separation and characterization of the grafted water-insoluble PVA was carried out by a two-step selective solubilization of the PVAc latex, first with acetonitrile to separate PVAc homopolymer, followed by water to separate the water-soluble PVA from the remaining acetonitrile-insoluble material. After the separation, the water-soluble and water-insoluble PVA were characterized by Fourier Transform Infrared (FTIR) spectroscopy and ¹H and ¹³C nuclear magnetic resonance (NMR) analyses, from which the details of the PVA-graft-PVAc structure were obtained. © 1996 John Wiley & Sons, Inc.     

Radiation effects on the thermal degradation of poly(vinyl chloride) and poly(vinyl alcohol)

Radiation effects on the formation of conjugated double bonds in the thermal degradation of poly(vinyl chloride) (PVC) and poly(vinyl alcohol) (PVA) were investigated. Thin films of PVC and PVA were either irradiated with γ-rays at ambient temperature (pre-irradiation) and then subjected to thermal treatment, or irradiated at elevated temperatures (in situ irradiation). An extensive enhancement of the thermal degradation was observed for the pre-irradiation of the PVC films, which was more effective than the effect of the in situ irradiation at the same absorption dose. For the PVA degradation, however, the effect of the in situ irradiation was larger than that of the pre-irradiation. The results were explained and related mechanisms were discussed based on radiation-induced chemical reactions and their individual contributions to the thermal degradation behaviors of the two polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 3089–3095, 1998     

PVA-g-PS复合微球的制备与粒径控制研究

由链转移自由基聚合与端基置换反应法,合成了苯乙稀基单封端的聚醋酸乙烯酯(PVAc)大分子单体,使其与苯乙烯在乙醇/水的混合介质中进行自由基分散共聚,得到了表面以PVAc为接枝链的聚苯乙烯(PVAc-g-PSt)微球。将所得微球在碱性条件下醇解,形成了以亲水性聚乙烯醇(PVA)为壳、聚苯乙烯为核的复合微球(PVAc-g-PSt)。用核磁共振对聚合物的结构进行表征,定出了PVAc末端双键的含量;并用激光光散射、扫描电子显微镜对微球的粒径与形态进行了表征。研究结果表明,在共聚反应体系中大分子单体的分子量与浓度、苯乙烯浓度、引发剂浓度及溶剂的组成对微球的形态和粒径大小有明显影响。     

Hydrophilic membranes based on poly(vinyl methyl ether-crosslinked-polystyrene) semi-interpenetrating networks

Microporous membranes were prepared from poly(vinyl methyl ether - crosslinked-polystyrene) (PVME-cross-PS) semi-IPN's by extraction of poly(vinyl methyl ether) (PVME) with water. Membrane morphology and properties are fixed during the process of swelling, phase separation, PVME extraction and vitrification which occurs upon immersing the samples in water. The membranes are characterized by swelling (water uptake), the relative amounts of free and bound water, and their permselectivity towards KCI solution.     

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.     

Radiotracer studies on ion-selective membranes based on poly(vinyl chloride) matrices

Radiotracer studies with (45)Ca, (89)Sr and (133)Ba have provided evidence that the permeation of magnesium, strontium and barium ions through PVC membranes containing Orion 92-20-02 liquid ion-exchanger is inhibited by their low affinity for the liquid ion-exchanger sites. Experiments with (7)Be indicate a strong affinity of the membrane for beryllium ions with corresponding inhibition of permeation. When acid is present in the solution on one side of the membrane, preferential permeation by protons may lead to transport of ions against their concentration gradient in order to maintain the balance of charge.