Facile fabrication of poly(vinyl alcohol) gels and derivative aerogels

A facile preparation of poly(vinyl alcohol) (PVOH) hydrogels and their derivative PVOH/montmorillonite clay aerogels is reported, using water as solvent and divinylsulfone as crosslinking agent, making use of an environmentally friendly freeze drying process. The materials exhibit significantly increased mechanical properties after crosslinking. The compressive modulus of an aerogel prepared from an aqueous suspension containing 2 wt% PVOH/8 wt% clay increased 29-fold upon crosslinking, for example. Crosslinking of the polymer/clay aerogels decreased the onset decomposition temperature as measured by thermogravimetric analysis, and generated a more continuous structure at higher clay contents. Such polymer/clay aerogels are promising materials for low flammability applications.     

Pure shear deformation of physical and chemical gels of poly(vinyl alcohol)

Pure shear deformation reveals the significant differences in elastic properties of the poly(vinyl alcohol) (PVA) gels with almost identical initial modulus, but with different types of crosslinks, physical crosslinks formed by microcrystallites and chemical crosslinks made of covalent bonds. The ratio of the two principal stresses steeply increases with elongation in the physical gels, while that remains almost constant independently of stretching in the chemical gels. The marked growth of the stress ratio with elongation in the physical gels leads to the negative values of the derivative of the elastic free energy (W₂) with respect to the second invariant of the deformation tensor in the whole range of deformation, which is firstly observed for elastomeric materials. By contrast, the chemical gels exhibit the positive values of W₂ like most chemically crosslinked rubbers. Among the existing theories of rubber elasticity, the classical non-Gaussian three-chain model considering the effect of finite chain-length is qualitatively successful in accounting for the steep increase of the stress ratio and the negative values of W₂ in the physical gels, although it fails to reproduce the large difference in the stress-strain behavior among uniaxial, pure shear and equi-biaxial deformations. These features of the physical gels are expected to stem from the structural characteristics such as fewer amounts of slippery-trapped entanglement along network strands compared to the chemical PVA gels.     

Miscibility and interactions in poly(methylthiomethyl methacrylate)/poly(vinyl alcohol) blends

Poly(methylthiomethyl methacrylate) (PMTMA) is miscible with poly(vinyl alcohol) (PVA) over the whole composition range as shown by the existence of a single glass transition temperature in each blend. The interaction between PMTMA and PVA was examined by Fourier transform infrared spectroscopy, solid-state nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy. The interactions mainly involve the hydroxyl groups of PVA and the thioether sulfur atoms of PMTMA, and the involvement of the carbonyl groups of PMTMA in interactions is not significant. The measurements of proton spin-lattice relaxation time reveal that PMTMA and PVA do not mix intimately on a scale of 1-3 nm, but are miscible on a scale of 20-30 nm. In comparison, we have previously found that PMTMA is miscible with poly(p-vinylphenol) and the two polymers mix intimately on a scale of 1-3 nm.     

Light scattering and viscoelasticity study of poly(vinyl alcohol)-borax aqueous solutions and gels

Poly(vinyl alcohol)-borate (PVA-borate) aqueous solutions properties with PVA concentrations ranging from 2 to 60 g/L and borax concentrations of 0.0 and 0.2 M were investigated at room temperature using static and dynamic light scattering (SLS and DLS), and dynamic viscoelasticity measurements. Light scattering and viscoelasticity data revealed that all the PVA-borate aqueous systems, except those with [PVA]≥40 g/L and [borax]=0.2 M, behaved as solutions. For PVA-borate aqueous systems with [PVA]≥40 g/L and [borax]=0.2 M, light scattering data revealed that these systems behaved like gels, but viscoelasticity data showed that these systems were in flow states. The experimental data suggest that PVA-borate aqueous systems with [PVA]≥40 g/L and [borax]=0.2 M are thermoreversible gels with finite equilibrium life time of thermoreversible borate-PVA di-diol crosslinks. The thermoreversible crosslinks can be observed by the non-perturbing light scattering technique but not by the pertubing rheometric method. These results indicate the advantage of light scattering relative to rheometers for studying the physical or reversible crosslink gels.     

Grafting and adsorption of poly(vinyl) alcohol in vinyl acetate emulsion polymerization

Poly(vinyl alcohol) is often used in vinyl acetate emulsion polymerization as a protective colloid, but its role is complex and controversial since it partakes in grafting reactions with the monomer, influencing process mechanisms, and affecting the colloidal properties of the latex. Furthermore, in industrial operations, the wide scatter of macromolecular properties of the commercial types of poly(vinyl alcohol) causes process irreproducibilities. In this work different types of polyvinyl alcohol were used to perform a series of polymerizations, and their kinetics were compared. A selective solubilization procedure was applied to separate the three fractions of poly(vinyl alcohol) in the final latex: free in the water phase, physically adsorbed onto the polymer particles and chemically grafted. These results were compared with those obtained from pure adsorption measurements of polyvinyl alcohol onto ‘emulsifier-free’ polyvinyl acetate dispersions. The rheological behavior of the different latexes was also compared, and the results were used to formulate an hypothesis on the interaction mechanisms acting in these systems.     

粉煤灰固定化细胞去除聚乙烯醇的研究

前期研究分离到一株能高效降解并矿化聚乙烯醇(PVA)的黄单胞菌(Xanthomonas sp.)。考察了利用改性处理或未处理粉煤灰吸附该菌株去除PVA的特性。结果表明,实验所采用的5种粉煤灰对该菌细胞等温吸附方程与Langmuir方程、Freundlich方程的拟合都达到很高的水平。改性粉煤灰对菌体细胞的吸附量由大到小的顺序为HC l处理粉煤灰>H2SO4处理粉煤灰>Ca(OH)2处理粉煤灰>NaOH处理粉煤灰>未处理粉煤灰。在培养初期,粉煤灰固定化细胞对PVA的去除量略低于等量游离细胞,但其去除速率的变化与游离细胞基本相同。     

Synthesis of poly(vinyl alcohol) combs via MADIX/RAFT polymerization

Poly(vinyl acetate) combs have been prepared via macromolecular design via interchange of xanthate (MADIX)/reversible addition-fragmentation chain-transfer (RAFT) polymerization using xanthate functionalized polymer cores. The comb backbones were prepared using well-defined poly(vinyl alcohol) PVA polymers with a degree of polymerization of 20, 100 and 170, respectively. Functionalization with xanthates via R-group or a Z-group approach resulted in the formation of macromolecular MADIX agents. While Z group designed macromolecular xanthate agents appeared to inhibit the polymerization of vinyl acetate (VAc), R group designed macromolecular xanthate agents achieved to mediate efficiently the bulk polymerization of VAc affording PVAc combs. However, the growth of the combs was accompanied at low conversions by the formation of linear polymer chains as a result of the constant initiation (AIBN) and shoulders, which can be attributed to intermolecular coupling reactions. The proportions of single chains and termination products were observed to increase with the degree of polymerization of the macromolecular MADIX agents broadening the molecular weight distribution. As a result of a stable ester link between the branches and the PVA backbone, the branched PVAc architectures were finally hydrolyzed to afford poly(vinyl alcohol) combs.     

Water sorption in cross-linked poly(vinyl alcohol) networks

Poly(vinyl alcohol) (PVA) networks of different cross-linking densities were prepared by reaction with hexamethylene diisocyanate in solution and casting. The dynamic-mechanical properties of PVA films have been investigated in the temperature range of −150 to +150 °C. Two relaxations processes labeled α and β in order of decreasing temperature were observed. The α-relaxation shifts to lower temperature and the average molecular weight between cross-links decreases with increasing cross-linking density. Isothermal sorption from vapor and liquid water allowed determination of the Flory-Huggins interaction parameter between water and the polymer chain segments, which decreased with the water activity in the hydrogel and increased with the cross-linking density as a consequence of the hydrophobic character of the cross-linking agent. The water diffusion coefficients, D, in the networks obtained by means of dynamic sorption experiments increased with increasing water activity. This behavior is interpreted in terms of plasticization of the polymer by water molecules.     

Accelerated biodegradation of poly(vinyl alcohol) by a glycosidation of the hydroxyl groups

Biodegradability of N-acetyl-d-glucosamine (GlcNAc)-substituted poly(vinyl alcohol) (PVA) (1) in a soil suspension (pH 6.5) was investigated at 25°C for 40 days. Biochemical oxygen demand (BOD) of 1 with degree of substitution of 0.2-0.3 (DP=430-480) was higher than that of PVA under the degradation condition. Size exclusion chromatography (SEC), ¹H NMR, and FT-IR measurements of the recovered sample indicated that biodegradation of PVA main chain was accelerated by partial glycosidation of hydroxyl groups in PVA.     

Fabrication of poly(vinyl alcohol)s (PVAs) nanotubes through the fusion of nanocapsules composed of PVAs multilayer films

Poly(vinyl alcohol) (PVA) nanocapsules were fabricated by the deposition of PVA multilayer films onto the surface of silica particles followed by the removal of the silica cores. When a water dispersion of PVA nanocapsules was dried on a substrate, PVA nanotubes were formed through the one-dimensional fusion of the nanocapsules. This fusion behavior of the PVA nanocapsules was strongly affected by the molecular weights and acetylation degrees of PVA, the capsule film thickness of the PVA nanocapsules and the temperature to dry a water dispersion of the nanocapsules. When nanocapsules composed of 20 layered films of acetylated PVA with a 14% acetylation degree were used, nanotube formation via the fusion of these nanocapsules occurred effectively upon drying the water dispersion at 20 °C.     

Drying of semicrystalline polymers: mathematical modeling and experimental characterization of poly(vinyl alcohol) films

A mathematical model was developed to predict the drying mechanism of semicrystalline polymers involving multiple solvents. Since drying of semicrystalline polymers can be accompanied by changes in polymer degree of crystallinity, the model integrates crystallization kinetics and the Vrentas-Duda diffusion model to provide a better understanding of the mechanism. The model considers the effect of external conditions such as temperature, film shrinkage and diffusion and evaporation of multiple solvents during drying. Poly(vinyl alcohol) (PVA)/water/methanol was chosen as a test system. The drying kinetics of PVA films swollen in water and methanol were investigated using gravimetric techniques. The model predicts that higher temperatures, lower film thicknesses and lower methanol to water ratios increase the drying rate. The model predictions were compared with experimental data and showed good agreement.     

Viscoelastic properties of poly(vinyl alcohol) hydrogels and ferrogels obtained through freezing-thawing cycles

We report the preparation of poly(vinyl alcohol) (PVA) hydrogels obtained through freezing-thawing cycles. The viscoelastic properties of these gels using parallel-plate shear mode were evaluated as a function of temperature, time, degree of swelling, concentration and the number of freezing-thawing cycles. The storage modulus was analyzed on the basis of a theoretical model based on the scaling approach. These results provide additional evidence for a non-crystalline nature of the structure of PVA cryogels.Furthermore, PVA ferrogels have been prepared from PVA aqueous solutions and a ferrofluid through freezing-thawing cycles. The viscoelastic properties of these materials have been evaluated. It is shown that the variation of the storage modulus with ferrofluid concentration cannot be fitted using classical theories what has been attributed to the small dimensions of the particles in the ferrogels and to the magnetic interactions between particles.     

偶氮苯改性聚乙烯醇的研究

通过三聚氯氰的氯原子和聚乙烯醇的羟基之间脱氯化氢的反应,用含有2个偶氮苯的三聚氯氰衍生物改性聚乙烯醇（PVA）1788。红外光谱和核磁共振分析表明,在二氧六环和水混合溶剂中可以将偶氮苯成功接枝到PVA上,PVA中约每12个羟基就有1个发生反应。含有偶氮苯的PVA可以溶解在水和二甲亚砜中。其紫外可见吸收光谱与偶氮苯相似,并表现出偶氮苯的典型光致异构现象。     

Transition of hydration states of poly(vinyl alcohol) in aqueous solution

A straightforward method for determination of the hydration number of polymer in aqueous solution based on ice-melting technique of DSC is proposed. The simple yet precise method has been applied to determine the hydration number of poly(vinyl alcohol) (PVA) in aqueous solution covering a wide range of concentrations, from 0.005 to 0.3 g(solute)/g(solution), for three samples with different molar masses. The hydration number of PVA maintains a constant lower value of 2.5 when the concentration exceeds 0.2 g(solute)/g(solution). It increases to a value of 7 when the concentration decreases to the overlap concentration C^∗ of the polymer, where C^∗ was estimated as the reciprocal of its intrinsic viscosity. For solutions of C < C^∗, the hydration number keeps constant again at the value of 7. This behavior evidently demonstrates that PVA has two hydration states, one occurs at the dilute regime and the other occurs at concentrated regime. The concentration dependent transition from one state to another is treated mathematically by a quantitative formula which involves two parameters: one denotes the transition concentration and the other denotes the width of the transition region. The transition concentration decreases linearly with increasing molar mass resembling the behavior of molar mass dependence of overlap concentration. The structural features for the two states of hydrated PVA are briefly discussed.     

Single wall carbon nanotube templated oriented crystallization of poly(vinyl alcohol)

Shearing of poly(vinyl alcohol) (PVA)/single wall carbon nanotube (SWNT) dispersions result in the formation of self-assembled oriented PVA/SWNT fibers or ribbons, while PVA solution results in the formation of unoriented fibers. Diameter/width and length of these self-assembled fibers was 5-45 μm and 0.5-3 mm, respectively. High-resolution transmission electron micrographs showed well resolved PVA (200) lattice with molecules oriented parallel to the nanotube axis. Nanotube orientation in the self-assembled fibers was also determined from Raman spectroscopy. PVA fibers exhibited about 48% crystallinity, while crystallinity in PVA/SWNT fibers was 84% as determined by wide angle X-ray diffraction. PVA and carbon nanotubes were at an angle of 25-40° to the self-assembled fiber axis. In comparison to PVA, PVA/SWNT samples exhibited significantly enhanced electron beam radiation resistance. This study shows that single wall carbon nanotubes not only nucleate polymer crystallization, but also act as a template for polymer orientation.     

Morphology-rheology relationship in hyaluronate/poly(vinyl alcohol)/borax polymer blends

In this work, we have prepared bioartificial polymer blends using hyaluronate (HA) as a biological component and poly(vinyl alcohol)-borax association (PVAs) as a synthetic component, and investigated the rheological properties as well as morphology of the blends. When plotted against the blend composition, the rheological properties showed both positive and negative deviation from the linear additive mixing rule depending on thermal history. The blend showed enhanced viscosity at the composition of 20 wt% of HA and 80 wt% of PVAs, when PVA was dissolved at high temperature. The viscosity enhancement was caused by the network formation of HA aggregates in the micrometer scale. In addition, the network structure of HA aggregates was found to be fractal with the fractal dimension of 1.7. As PVA system also forms a network structure in the nanometer scale between hydroxyl groups of PVA and borate anions, the blend system is unique in that it has network structures in both micrometer and nanometer scales in one material. On the contrary, HA formed aggregates but not any network structure in the blend of the same composition but of the negative deviation. In conclusion, we showed that HA/PVAs blend system may have diverse morphology as well as very broad spectrum of rheological properties, and could suggest that the rheology and morphology of HA/PVAs blends can be designed not only by controlling composition but also by controlling thermal and deformation history of the components.     

Synthesis and characterization of hydrophobically modified poly(vinyl alcohol) hydrogel membrane

Hydrophobically modified poly(vinyl alcohol), [PVA] was synthesized by graft copolymerization of N-tertiary butyl acrylamide [NTBA] onto PVA by free radical polymerization. The incorporation of NTBA onto PVA chains was confirmed by elemental analysis, FT-IR and NMR spectroscopy. A series of graft copolymers with different contents of NTBA were prepared and membranes were casted from these copolymer solutions in dimethyl sulfoxide. The increase in hydrophobicity with an increase in NTBA content was investigated by contact angle measurements. The swelling behaviour of membranes as a function of temperature, hydrophobic content, annealing temperature and period was studied. Permeability of solutes through these membranes was investigated as a function of solute size, membrane hydrophobicity and temperature. The swelling behaviour of the copolymer membranes showed that the lower content of NTBA gives discontinuous volume transition with respect to temperature whereas, the presence of higher amounts of NTBA showed decreased swelling ratios with very little influence of temperature on the swelling. The permeabilities of solutes through these membranes were strongly dependent on the size of the solute, solution temperature and hydrophobicity of the membrane. The copolymer membranes were further characterized using DSC, DMA and XRD. The peak becomes broader as the NTBA content increases.     

Poly(vinyl alcohol) hydrogel fixation on poly(ethylene terephthalate) surface for biomedical application

A surface modification technique was developed for the covalent immobilization of poly(vinyl alcohol) (PVA) hydrogel onto poly(ethylene terephthalate) (PET) to improve the biocompatibility of the film. The PET film was first graft copolymerized with poly(ethylene glycol) monomethacrylate (PEGMA) in the presence of ethylene glycol dimethacrylate (EGDMA) as crosslinker, and then oxidized with a mixture of acetic anhydride (Ac₂O) and dimethyl sulfoxide (DMSO) to produce aldehyde groups on the PET surface. Finally, the prepared PVA solution was cast onto the film and covalently immobilized on the film through the reaction between the aldehyde groups on the PET film and the hydroxyl groups of PVA. The good attachment of the PVA layer to the PET film was confirmed by observing the cross-section of the PET-PVA film using scanning electron microscopy (SEM). Heparin was immobilized on the PVA layered PET using two different methods, physical entrapment and covalent bonding, to further improve the biocompatibility of the film. Attenuated total reflectance (ATR) FT-IR spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize the chemical composition of the surface modified films. The biocompatibility of the various surface modified PET films was evaluated using plasma recalcification time (PRT) and platelet adhesion.     

Effect of crosslinking on the mechanical and thermal properties of poly(vinyl alcohol)

Poly(vinyl alcohol) was crosslinked with hexamethylene diisocyanate in solution. A broad range of degrees of crosslinking, from 1.7 up to 74 mol% of reacted hydroxyl groups, was achieved. The variation of the thermal and mechanical properties of PVA with the crosslinking density show an initial decrease due to the diminution of the crystallinity of the system, caused by the crosslinking. After an abrupt rise at about 20%, the properties tend to level off independently on the increase of the crosslinking. This behaviour is explained as a result of the competitive action of at least three factors during the crosslinking: (i) weakening of the existing physical network due to hydrogen bonding; (ii) formation of a chemical network; and (iii) introduction of flexible moieties. The last factor is closely connected with the specific chemical structure of the crosslinker itself.