Highly isotactic poly(vinyl alcohol) derived from tert-butyl vinyl ether. Part IV. Some physical properties, structure and hydrogen bonding of highly isotactic poly(vinyl alcohol) films

Some basic physical properties, structure and hydrogen bonding have been characterized for different stereoregular PVA films including highly isotactic PVAs (HI-PVAs), which were recently succeeded in synthesis, as functions of the mm fraction by using different analytical methods. The melting temperature, degree of crystallinity, and ¹³C spin-lattice relaxation time of the crystalline component are found to have their own clear minima at the mm fraction of about 0.4-0.5. This fact suggests that structural disordering associated with the decrease in crystallinity may be most strongly induced at this mm fraction. The formation of the new crystal form of PVA has been reconfirmed for HI-PVAs with the mm fractions higher than about 0.55 by FTIR spectroscopy and the structure and hydrogen bonding have been investigated in detail by solid-state ¹³C NMR spectroscopy. It is found that all OH groups are allowed to form successive intramolecular hydrogen bonding along the respective chains in the crystalline region for HI-PVAs with the mm fractions higher than about 0.7. Since these chains should contain some amount of r units even in the crystalline region, a slightly helical structure with a considerably long period may be adopted by them as an energetically stable state. On the basis of the line shape analysis of the CP/MAS ¹³C NMR spectra of the crystalline components, structural causes of the appearance of the minima of the physical values described above are also discussed in relation to the introduction of disordered units mainly associated with hydrogen bonding to the syndiotactic or isotactic sequences forming successive intermolecular or intramolecular hydrogen bonding, respectively.     

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.     

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.     

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.     

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.     

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.     

Microstructural studies on BaCl2 doped poly(vinyl alcohol)

We have studied the effect of BaCl₂ dopant on the optical and microstructural properties of a polymer poly(vinyl alcohol) (PVA). Pure and BaCl₂ doped PVA films were prepared using solvent casting method. These films were characterized using FTIR, UV-visible, XRD and DSC techniques. The observed peaks around 3425 cm⁻¹, at 1733 cm⁻¹ and 1640 cm⁻¹ in the FTIR spectra were assigned to O-H, CC stretching and acetyle CO group vibrations, respectively. In the doped PVA shift in these bands can be understood on the basis of intra/inter molecular hydrogen bonding with the adjacent OH group of PVA. The UV-visible spectra shows the absorption bands around 196 nm and shoulders around 208 nm with different absorption intensities for doped PVA, which are assigned to n→π* transition. This indicates the presence of unsaturated bonds mainly in the tail-head of the polymer. Optical band energy gap is estimated using UV-visible spectra and it decreases with increasing dopant concentration. The powder XRD shows an increase in crystallinity in the doped PVA, which arises due to the interaction of dopant with PVA causing a molecular rearrangement within the amorphous phase of polymer. These modifications also influence the optical property of the doped polymer. The DSC study also supports increasing crystalline thickness and degree of crystallinity due to doping.     

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.     

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.     

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.     

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.     

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.     

Synthesis of end-group functionalized poly(octadecyl vinyl ether)

The cationic polymerization of octadecyl vinyl ether (ODVE) initiated by trimethylsilyl iodide and 1,1-diethoxyethane in the presence of ZnI₂ in toluene at 0°C and 10°C has been investigated. For molecular weights lower than 6000, a linearity of M̄_n with conversion was observed, but for higher molecular weights a strong deviation from calculated values, assuming a living mechanism, was found. Kinetic analysis of the polymerization and the variation of molecular weight as a function of conversion was in agreement with a transfer to monomer with k_tr/k_p ≌ 0.006 at 10°C. Analysis of the polymers obtained by termination with methanol provided evidence that the alkenyl ether end-groups formed by the transfer reaction lead to the same acetal end-groups as the active species. As a consequence, it is possible to prepare functionalized polyODVE polymers by end-capping with alcohols. This was confirmed by the synthesis of polyODVE macromonomers by end-capping with 2-hydroxyethyl methacrylate.     

Swelling and morphological properties of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) hydrogels in solution with high salt concentration

Understanding the swelling properties of hydrogels and how they affect the hydrogel's morphology is of fundamental importance in the development of hydrogel-based artificial muscles, bio-actuators, sensors and other devices. In this paper, the swelling behavior of PVA-PAA hydrogel films in saline water and in buffer solutions of different pH values was investigated. It was observed that the swelling factor of the hydrogel decreases when the ionic strength of the solvent solution increases. Scanning Electron Microscopy (SEM) revealed structures with different pore shapes and sizes depending on the type of solution used for hydration. In saline water, Energy Dispersive X-Ray (EDS) analysis indicated the formation of NaCl crystals within the polymeric network. Finally, the PVA-PAA hydrogel was used as an actuator to strain a fiber Bragg grating sensor, thus providing an indirect measurement of the pH value of the surrounding solution.     

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.     

A novel biodegradable poly(p-dioxanone)-grafted poly(vinyl alcohol) copolymer with a controllable in vitro degradation

A novel biodegradable copolymer was synthesized from poly(vinyl alcohol) and poly(p-dioxanone) by ring-opening polymerization. The molecular structure of the copolymer was characterized by one- and two-dimensional NMR spectroscopy. The results of differential scanning calorimetry (DSC) show that the amphiphilic and comb grafted structure of the copolymer make its crystalline behavior different from that of the poly(p-dioxanone) homopolymer (PPDO). The in vitro degradation rate of the copolymers can be controlled via adjusting the number and length of PPDO segments of PVA-g-PPDO copolymers. The copolymer has a potential application in biomedical materials or in the controlled release of drug.     

Synthesis and characterization of CdS quantum dots with carboxylic-functionalized poly (vinyl alcohol) for bioconjugation

In the present research it is reported the synthesis and characterization of CdS nanoparticles (NPs) prepared using carboxylic-functionalized poly (vinyl alcohol) (PVA) as the ligand via aqueous route at room temperature and ambient pressure. Different molar concentrations of carboxylic-PVA and PVA were investigated aiming at producing stable colloidal systems. Carboxylic-PVA was conjugated with BSA (bovine serum albumin) and used as capping ligand in the preparation of CdS nanocrystals. UV-visible spectroscopy, photoluminescence spectroscopy, and transmission electron microscopy were used to characterize the kinetics and the relative stability of polymer-capped CdS nanocrystals. The results have clearly indicated that the carboxylic-functionalized PVA was much more effective on nucleating and stabilizing colloidal CdS nanoparticles in aqueous suspensions compared to PVA. In addition, the CdS nanocrystals were obtained in the so-called “quantum-size confinement regime”, with the calculated average size below 4.0 nm and fluorescent activity. Thus, a novel simple route was successfully developed for synthesizing nanohybrids based on quantum dots and water-soluble chemically functionalized polymers with incorporated carboxylic moiety with the possibility of direct bioconjugation.