Solvent-free esterification of poly（vinyl alcohol） and maleic anhydride through mechanochemical reaction

A solid-state mechanochemical processing,that is,pan-milling,was used to conduct the esterification of poly(vinyl alcohol) (PVA) with maleic anhydride (MA) through stress-induced reaction.FTIR spectrum study indicated the presence of ester linkages and olefinic double bonds in maleic anhydride cross-linked PVA.Thermal properties of the cross-linked product were characterized by DSC.The results showed its glass transition temperature was 20℃higher than the original linear PVA and the thermal stability was also improved.     

Modification of hydrophilic poly(vinyl alcohol) film via blending with hydrophobic poly(butyl acrylate-co-methyl methacrylate)

A series of poly(vinyl alcohol)/poly(butyl acrylate-co-methyl methacrylate) [PVA/P(BA-co-MMA)] blend films with different P(BA-co-MMA) content were prepared by the solution casting method. Surface morphologies of the PVA/P(BA-co-MMA) blend films were studied by scanning electron microscopy and atomic force microscopy. Thermal, mechanical, and chemical properties of PVA/P(BA-co-MMA) blend films were investigated by differential scanning calorimeter, thermogravimetric analysis, tensile tests, and surface contact angle tests. It was revealed that the introduction of P(BA-co-MMA) could affect the properties of the PVA films. The results also showed that, when P(BA-co-MMA) mole content is 3 %, the tensile strength and the surface contact angle of the polymer blend film are 20.4 MPa and 43.5°, respectively, suggesting that the polymer blend film holds both a better mechanical property and a better chemical property.     

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.     

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.     

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.     

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.     

Reaction of poly(vinyl alcohol) with superoxide anion

Reaction of pol(vinyl alcohol) (PVA) with superoxide anion was carried out in a dimethyl sulfoxide solution at 40°C under nitrogen and changes in the yield of recovered polymer, molecular weight, and chemical structure were examined. Appreciable decreases in the yield of recovered polymer and molecular weight were observed during the reaction, which indicated that a main chain scission of the polymer had occurred. Residual acetyl groups identified by the absorption peaks at ca. 1720 and 1250 cm^?1 in infrared (IR) spectra were removed rapidly and β-diketone groups identified by the peaks at ca. 1640 and 1560 cm^?1 in IR spectra and at ca. 255 nm in ultraviolet (UV) spectra were formed during the reaction. Formation of carboxylate groups was also assumed. The reaction mechanism was discussed on the basis of the results obtained.     

Improved oxygen barrier performance of poly(vinyl alcohol) films through hydrogen bond complex with poly(methyl vinyl ether-co-maleic acid)

Hydrogen bonding between poly(methyl vinyl ether-co-maleic acid) (PMVE-MA) and poly(vinyl alcohol) (PVOH) has resulted in films with lower oxygen transmission rates (OTR) than pure PVOH. In the range 20-30% (w/w) PMVE-MA, complexation between the two polymers in the blend was maximized, as shown by viscometry, Fourier Transform Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC) analysis. OTR measurements have shown that the maximum interpolymer complexation ratio also correlates with the lowest OTR values of the resulting film. The improved oxygen barrier properties are believed to be a combination of the relatively intact PVOH crystalline regions as shown with X-ray diffraction (XRD) and a higher degree of hydrogen bonding in the amorphous regions of the PVOH and PMVE-MA films as indicated by glass transition temperature (T_g) shifts. This leads to denser amorphous regions that reduces the rate of gases diffusing through the polymer film, hence the reduced OTR.     

The solvation of poly(vinyl alcohol)

The possible incorporation of water molecules within the crystal structure of poly(vinyl alcohol) is discussed. Modelling of the crystal structure suggested that water could be incorporated without severe disruption, and the effect on the X-ray powder diffraction trace was simulated. The effect of variation in tacticity is discussed in terms of the nature of the hydrogen bonding. Simulated traces are compared with experimental data from atactic samples in which a change in the diffraction peak intensities is observed for samples crystallised with water present. This is compared with samples produced from nonaqueous solutions.     

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.     

Study on ketalization reaction of poly(vinyl alcohol) by ketones. VIII. kinetic study on acetalization and ketalization reactions of poly(vinyl alcohol)

The kninetics of acid-catalyzed acetalization and ketalization of poly(vinyl alcohol) (PVA) were systematically studied in completely homogeneous media with carefully selected solvents. Thus the acetalization reaction was run in water with six aldehydes [R₁CHO (R₁ = H, CH₃, C₂H₅, n-C₃H₇, i-C₃H₇, ClCH₂)], whereas the ketalization in dimethylslfoxide with 11 ketones [R₂CH₃CO (R₂ = CH₃, C₂H₅, n-C₃H₇, i-C₃H₇, n-C₄H₉, i-C₄H₉, tert-C₄H₉, C₆H₅CH₂, C₆H₅CH₂CH₂), cyclopentanone, and cyclohexanone]. The latter was difficult to proceed in aqueous media. Both reactions were reversible and bimolecular and, despite the use of different solvents, gave similar heats of reaction (7.5 kcal/mol) and activation energies (ca. 15 kcal/mol) except for the case of formaldehyde and chloroacetaldehyde; however the equilibrium constants at 25°C showed that the acetalization is thermodynamically much more favored than the ketalization (ca. 5000 vs. 0.01–0.9), probably because of steric hindrance of the ketone substrate. The rate constants of hydrolysis (reverse reactions) for the poly(vinyl acetal) and poly(vinyl ketal) followed the Hammett-Taft equation to give a single p* (=3.60) that is very close to that for the hydrolysis of diethyl acetal and ketal. From these and other data, it was concluded that the polymer hydrolysis, as well as PVA acetalization and ketalization, are all electrophilic reaction where the formation of hemiacetal or hemiketal is the rate-determining step. © 1996 John Wiley & Sons, Inc.     

Miscibility and orientation behavior of poly(vinyl alcohol) / poly(vinyl pyrrolidone) blends

The miscibility of poly(viny1 alcohol)/poly(vinyl pyrrolidone) (PVA/PVP) blends is investigated by differential scanning calorimetry (DSC) and wide-angle x-ray diffraction (WAXD). The molecular orientation induced by uniaxial stretching of the blends is also examined by WAXD and birefringence measurements. It is shown by the DSC thermal analysis that the polymer pair is miscible, since a single glass transition temperature (T_g) is situated between the T_gs of the two homopolymers at every composition. The T_g versus composition curve does not follow a monotonic function but exhibits a cusp point at a PVP volume fraction of a little under 0.7, as in a case predicted by Kovacs' theory. The presence of a specific intermolecular interaction between the two polymers is suggested by an observed systematic depression in the melting point of the PVA component. A negative value of the polymer-polymer interaction parameter, χ₁₂ = 0.35 (at 513 K), is estimated from a thermodynamic approach via a control experiment using samples crystallized isothermally at various temperatures. The extent of optical birefringence (Δn) of the drawn blends decreases drastically with increasing PVP content up to 80 wt %, when compared at a given draw ratio, and ultimately Δn is found to change from positive to negative at a critical PVP concentration of a little over 80 wt %. Discussion of the molecular orientation behavior takes into consideration a birefringence compensation effect in the miscible amorphous phase due to positive and negative contributions of oriented PVA and PVP, respectively.     

Structural irregularities in poly(vinyl alcohol)

Poly(vinyl alcohols) derived from the product of polymerization of vinyl acetate in methanol have been characterized by various physical and chemical methods before and after NaIO₄ cleavage. The 220-MHz ¹H-NMR spectra confirm the reliability of NaIO₄ titrimetry for estimating 1,2-glycol content and help explain the tendency for viscometry to grossly underestimate the 1,2-glycol content for low molecular weight polymers. The spectra and related chemical evidence indicate that the major endgroups are HOCH₂CH₂? and CH₃CH(OH)CH(OH)CH₂? . ß-Hydroxyethyl groups also occur as short chain branches, mainly attached to α carbon atoms in the normal head-to-tail polymer chain sequence. The concentrations of the branch and endgroups depend on polymerization conditions and help explain polymerization “solvent” effects on physical properties.     

Pervaporation separation of water-acetic acid mixtures through poly(vinyl alcohol) membranes crosslinked with glutaraldehyde

Poly(vinyl alcohol) (PVA) membranes crosslinked with glutaraldehyde (GA) were prepared by a solution method for the pervaporation separation of acetic acid-water mixtures. In the solution method, dry PVA films were crosslinked by immersion for 2 days at 40°C in reaction solutions which contained different contents of GA, acetone and a catalyst, HCl. In order to fabricate the crosslinked PVA membranes which were stable in aqueous solutions, acetone was used as reaction medium in stead of aqueous inorganic salt solutions which have been commonly used in reaction solution for PVA crosslinking reaction. The crosslinking reaction between the hydroxyl group of PVA and the aldehyde group of GA was characterized by IR spectroscopy. Swelling measurements were carried out in both water and acetic acid to investigate the swelling behavior of the membranes. The swelling behaviour of a membrane fabricated at different GA content in a reaction solution was dependent on crosslinking density and chemical functional groups created as a result of the reaction between PVA and GA, such as the acetal group, ether linkage and unreacted pendent aldehydes in PVA. The pervaporation separation of acetic acid-water mixtures was performed over a range of 70–90 wt% acetic acid in the feed at temperatures varying from 35 to 50°C to examine the separation performances of the PVA membranes. Permeation behaviour through the membranes was analyzed by using pervaporation activation energies which had been calculated from the Arrhenius plots of permeation rates.     

Interaction of polyoxometalate Mo132 with poly(vinyl alcohol)

Some aspects of the interaction of (NH₄)₄₂[Mo ₇₂ ^VI Mo ₆₀ ^V O₃₇₂(H₃CCOO)₃₀(H₂O)₇₂] · 30H₃CCOONH₄ · 250H₂O (polyoxometalate Mo132) with a water-soluble nonionic polymer (poly(vinyl alcohol) or related poly(vinylpyrrolidone)) were studied in aqueous solutions and in films. A set of methods was used: spectrophotometry, EPR spectroscopy, visual microscopy, X-ray powder diffraction, scanning probe microscopy, and potentiometry. The complex-formation features of the system were revealed. The miscibility of the components and their influence on crystallization phenomena were considered. The effect of UV and X-rays on poly(vinyl alcohol) + polyoxometalate films was studied. Poly(vinyl alcohol) stabilizes the polymer. Mutual radiation and thermal stabilization of the polymer and polyoxometalate was discovered. The utility of ion-sensitive electrodes for Mo132 determination in solution was demonstrated.     

Reaction of poly(vinyl alcohol) with tetravalent ceric ion

Poly(vinyl alcohol) (PVAI) was oxidized by ceric ion, Ce(IV), in aqueous HNO₃ medium at different temperatures and found to be degraded as a result of selective cleavage of the 1,2-glycol unit existing in PVAl. The rate of oxidation increased with increasing temperature. The aldehyde groups formed at the ends of the degraded polymer upon oxidation were relatively stable at 0°C. With rise of temperature, the aldehyde groups reacted either with excess of Ce(IV) to carboxylic acids or with hydroxyl groups of PVAl molecules to give acetal linkage. When the acetalization predominated over the oxidation to carboxyl group, gelation of the reaction mixture was observed. Based on these results, a plausible mechanism of oxidation of PVAl with Ce(IV) and the subsequent reactions is discussed.