Controlled release of atenolol from freeze/thawed poly(vinyl alcohol) hydrogel

Physically crosslinked polymeric films containing atenolol drug were formulated and the release of the drug was evaluated with view to investigate the feasibility of these films as drug delivery systems. Freezing and thawing process for PVA was used to prepare a controlled release device for atenolol drug. The process included incorporation of the drug into PVA film during the freezing and thawing process. The PVA has used a molecular weight of 125 k and degree of saponification of 98. Various amounts of the atenolol drug were incorporated into the freeze/thawed PVA. The in vitro release behavior of atenolol from these films was investigated. The drug release profiles from the polymeric formulations indicated initial high rate of release followed by slow rate of the release. The release of atenolol increased with increasing drug concentration in the film. The results showed the feasibility of the use of freezing and thawing technique to control the release of atenolol drug from PVA.     

Analysis of solute diffusion in poly(vinyl alcohol) hydrogel membrane

Measurement of diffusion and partitioning of solutes having molecular weights ranging 180–66000 in PVA gel membranes with various crosslinking degree were carried out. With increasing solute size or decreasing number of average molecular weight between crosslinks of the membranes, both the solute permeability and partition coefficient decreased. In spite of similar solute sizes, the more hydrophilic solute ribonuclease showed higher permeability and partition coefficient than the less hydrophilic α-lactalbumin, probably due to interaction with the hydrophilic PVA. The solute diffusion through swollen gel membrane was analyzed by the equation based on free volume theory. In this analysis equation, the partition coefficient, which is defined as the ratio of solute concentration in gel membrane standardized by water volume in the membrane to that in bulk solution, was introduced as the probability of a diffusing species finding a mesh with a volume of at least the solute size. The efficiency of the proposed analysis equation was confirmed by the experimental results of the effects of solute size and water volume fraction in the membrane.     

Morphology assessment of chemically modified cryostructured poly(vinyl alcohol) hydrogel

The morphology of hydrogels based on poly(vinyl alcohol) (PVA) in their frozen hydrated state, modified with biologically active di- and multifunctional molecules was studied by scanning electron microscopy (SEM) with cryo-attachment. The porosity of samples was found to be more regular and ordered in the case of samples containing difunctional, and especially multifunctional carboxylic acids as compared to the neat PVA hydrogel. The morphology is dependent not only from the hydrogel composition but also the number of freezing-thawing cycles. Resulted highly porous and oriented structure has significant influence on materials properties, such as compressive stress and crosslinking density.     

Physical properties of gamma irradiated poly(vinyl alcohol) hydrogel preparations

Poly(vinyl alcohol) films from 15% w/w aqueous solutions and a thickness of 0.2 mm were selected for this study. The films were first humidified and then acetalized and/or gamma irradiated. Then, their physical properties were tested. Tensile strength of the hydrogel films reached its maximum value in samples irradiated with a 80 kGy dose, in the case of acetalized films the dose necessary for maximum tensile strength was only 40 kGy. The combination of acetalization with formaldehyde and gamma radiation produced an elastic hydrogel with good tackiness and excellent mechanical and thermal strength.     

Swelling behavior of a blend hydrogel made of poly(allylguanidino-co-allylamine) and poly(vinyl alcohol)

A hydrogel was prepared by mixing poly(allylguanidino-co-allylamine) hydrochloride (PAG) with poly(vinyl alcohol) (PVA) and repeatedly freezing and thawing the blend. The swelling behavior of the hydrogel was investigated as a function of the pH and ionic strength of the medium. In a salt-free aqueous medium, a size of the hydrogel was reduced below pH 3 and above pH 10, but the size was little affected in the pH range 3 ≈ 10. In a medium of constant ionic strength (μ = 0.1), the hydrogel's pH response was different: it was significantly reduced in size above pH 9, but the size was affected only moderately below pH 9. When the ionic strength of medium was varied at a fixed pH, the size change of the hydrogel was gradual. All these phenomena could be understood by observing that PAG displayed multiple protonation states due to pH and that the electrostatic interactions among the charges on the polymer backbone are shielded by the added electrolyte as the ionic strength of the medium is raised.     

Steric stabilization of thermally responsive N-isopropylacrylamide particles by poly(vinyl alcohol)

Poly(vinyl alcohol) (PVA) was used as a steric stabilizer for the dispersion polymerization of cross-linked poly(N-isopropylacrylamide) (PNIPAM) in water. A series of reactions were carried out using PVA of varying molecular weight and degree of hydrolysis. Under appropriate conditions, PNIPAM particles of uniform and controllable size were produced using PVA as the stabilizer. The colloidal stability was investigated by measuring changes in particle size with temperature in aqueous suspensions of varying ionic strength. For comparison, parallel colloidal stability measurements were conducted on PNIPAM particles synthesized with low-molecular-weight ionic surfactants. PVA provides colloidal stability over a wide range of temperature and ionic strength, whereas particles produced with ionic surfactants flocculate in moderate ionic strength solutions upon collapse of the hydrogel as the temperature is increased. Experimental results and theoretical consideration indicate that sterically stabilized PNIPAM particles resulted from the grafting of PVA to the PNIPAM particle surface. The enhanced colloidal stability afforded by PVA allows the temperature-responsive PNIPAM particles to be used under physiological conditions where electrostatic stability is ineffective.     

Characterization of poly(vinyl alcohol) hydrogel for prosthetic intervertebral disc nucleus

In the recent years, development of intervertebral disc prosthesis has been of great concern to the world of medicine and science. Substitution of the spinal disc or its part being displaced or damaged due to trauma or a disease process for the artificial structure well imitating high tensile properties and elasticity of the real disc would highly improve the existing treatment techniques. In this work, the attempt to develop the PVA-based hydrogel material for artificial spinal disc has been made. The polymer was initially processed with the use of formaldehyde solution as a crosslinking agent and sulfuric acid as a catalyst. Then properties of the material have been altered by saturating the already existing PVA hydrogel with a mixture of hydrophilic and hydrophobic monomers (2-hydroxyethyl methacrylate and methyl methacrylate) and a subsequent exposure to ionizing radiation (⁶⁰Co source). In this way, interpenetrating polymer network has been built on the crosslinked PVA scaffold. Resulting structures were tested for their mechanical behavior at different loads. Series of measurements leading to the determination of the physicochemical properties of created gels including crosslink density and swelling abilities were also performed.     

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.     

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.     

Dilute solutions of poly(vinyl alcohol) derived from vinyl trifluoroacetate

The dilute-solution behavior of poly(vinyl alcohol) (PVA_VTFA), derived from vinyl trifluoroacetate, in water-dimethylsulfoxide (DMSO) mixtures was investigated. With solvent mixtures ranging from 10 to 20 vol % DMSO, the relation between the reduced viscosity η_sp/C and the polymer concentration C was linear for polymer concentrations above 0.2 g/dL, whereas in solutions in mixed solvents of other compositions the dependence was linear for polymer concentrations above 0.1 g/dL. The relation between the intrinsic viscosity [η] obtained for aqueous solutions of PVA_VTFA and the molecular weight M estimated from viscosity measurements in solutions of poly(vinyl acetate) (PVA_VTFA), obtained by acetylation of PVA_VTFA, was given by [η] = 7.34 × 10^?4 M^0.63. The value of [η] was greatest for the solvent mixture with 10 vol % DMSO and smallest for about 50 vol % DMSO, and Huggins constants k were smallest and greatest for these two cases, respectively. The turbidity of the solutions of low-molecular-weight PVA_VTFA, was higher than that of high-molecular-weight PVA_VTFA up to 30 vol % DMSO, and the reverse relation held for 40-70 vol % DMSO.     

Dye decolorisation by laccase immobilised in lens-shaped poly(vinyl alcohol) hydrogel capsules

In this study, laccase (from Trametes versicolor, 8.3 U mg _enz ^?1 ) was used for the decolorisation of Saturn Blue L4G (10 mg L^?1). The efficiency of the decolorisation (ratio between the amount of decolorised dye and initial amount of dye) by a free enzyme was 48 % and the decolorisation rate was determined at 2.11 × 10^?3 mg_dye mg _enz ^?1 min^?1. After immobilisation in lens-shaped poly(vinyl alcohol) hydrogel capsules LentiKats® Biocatalyst (LB) (concentration of immobilised enzyme: 4 mg per g of particles; volume-loading rate of LB: 10 g per 100 mL of medium), the enzyme retained 16.1 % of its original activity (1.34 U mg _enz ^?1 ). Immobilised laccase was used for the dye decolorisation in 130 repeated batch tests with 71 % efficiency (LB activity: 7 × 10^?3 mgdye min^?1 g _LB ^?1 ). In continuous mode (after 716.5 h), the efficiency of the dye decolorisation was 48 % (LB activity: 3.3 × 10^?4 mg_dye min^/?1 g _LB ^?1 ).     

ESR study on photodecomposition mechanism of oxidized poly(vinyl alcohol)

Photolysis behavior of oxidized poly(vinyl alcohol) (PVA) in which carbonyl groups were introduced by oxidation with sodium hypochlorite and periodic acid was investigated by using ESR spectrometry. Increased formation of PVA radicals was observed in the sample with low degree of oxidation, which showed an emphasized formation of triplet component radical. On the other hand, the formation of radicals tended to be depressed in the highly oxidized sample, and a singlet component was dominant in the ESR spectrum. By examining infrared and ultraviolet spectra of the oxidized sample, the sample was found to contain two structures, a saturated ketone group and an α,β-unsaturated group; the latter structure was intensified with increasing degree of oxidation. Based on ESR studies of the photoirradiated acetone and mesityl oxide, the models of saturated and α,β-unsaturated ketone structures in the oxidized PVA, respectively a photodecomposition mechanism of oxidized PVA was proposed.     

Synthesis, characterization and swelling properties of a chemically cross-linked poly(vinyl alcohol) hydrogel

A poly(vinyl alcohol) hydrogel was prepared by coupling poly(vinyl alcohol) with epichlorohydrin as the cross-linking agent. The structure of the hydrogel was characterized by FTIR and GPC techniques. Various amounts of water were added into the dry gel to swell it, and the quantity of water in various states in the partially swollen hydrogel was determined by DSC technique. The analytical results indicate that the water introduced into the dry gel first combines with the hydrophilic groups of the network chains through hydrogen bond forming non-freezable water. The weight ratio of the non-freezable water to dry gel in the hydrogels is about 0.20. After the non-freezable water is saturated, the additional water penetrates the network space and exists simultaneously both in the freezable and free water states until reaching equilibrium swelling. Translated from Acta Polymeric Sinica, 2006, (5): 671–675 (in Chinese)     

Morphology and structure of highly elastic poly(vinyl alcohol) hydrogel prepared by repeated freezing-and-melting

Morphology and structure of poly(vinyl alcohol) (PVA) hydrogel prepared by the repeated freezing-and-melting method have been investigated by X-ray diffraction, scanning electron microscopy, light-optical microscopy, and simple tension test. The PVA aqueous solution gelled highly by using this method to show rubber-like elasticity, reflecting the gel network in which the amorphous chains are physically cross-linked by the crystallites. The gel morphology was characterized by the porous structure, which was originated from the gelation of continuous PVA-rich solution phase segregated around copious ice crystal phases formed upon freezing. The high gelling ability involved in this method was closely related to the segregation mechanism.     

Polydopamine- and polyDOPA-coated electrospun poly(vinyl alcohol) nanofibrous membranes for cationic dye removal

Mussel adhesive proteins including special functional groups, such as dopamine and 3,4-dihydroxy-l-phenylalanine (DOPA), exhibit strong adhesion and have thus been used in numerous applications. As a novel dye adsorbent for wastewater treatment, this study examineed poly(vinyl alcohol) (PVA) nanofibrous membranes (NFMs) fabricated via electrospinning and then coated with polydopamine (pDA) or polyDOPA through a simple dip coating process in dopamine or DOPA solution to examine. The surface morphology, chemical composition and hydrophilicty of PVA NFMs coated with pDA or polyDOPA were compared using scanning electron microscopy (SEM), UV photoelectron spectrometry (XPS) and contact angle analyzer, respectively. The thermal degradation temperatures of the PVA NFMs were increased significantly by about 100 °C due to the radical scavenging ability of pDA and pDOPA. Also, the differences in the adsorption performance toward a cationic dye, methylene blue (MB), for polydopamine- or polyDOPA-coated PVA NFMs were evaluated using a UV–Visible spectrophotometer. Finally, a recyclability test was conducted to confirm the applicability as a dye adsorbent.     

Kinetics of the complex process of thermo-oxidative degradation of poly(vinyl alcohol)

The thermo-oxidative degradation of poly(vinyl alcohol) (PVA) has been investigated by TG+DTG+DTA simultaneous analysis performed in static air atmosphere, at four heating rates, namely 3, 5, 10 and 15 K min⁻¹. TG, DTG and DTA curves showed that, in the temperature range 25–700°C, four successive processes occur. The first process consisting in the loss of physical adsorbed water is followed by three processes of thermal and/or thermo-oxidative degradations. The processing of the non-isothermal data corresponding to the second process (the first process of thermo-oxidation) was performed by using Netzsch Thermokinetics — A Software Module for Kinetic Analysis. The dependence of the activation energy evaluated by Friedman’s isoconversional method on the conversion degree shows that the investigated process is complex one. The mechanism of this process and the corresponding kinetic parameters were determined by Multivariate Non-linear Regression Program and checked for quasi-isothermal experimental data. It was pointed out that the first process of thermo-oxidation of PVA consists in three consecutive steps having Avrami-Erofeev kinetic model. The obtained results can be used for prediction of the thermal lifetime of PVA corresponding to a certain temperature of use and an endpoint criterion.     

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.     

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.     

Temperature-sensitive poly(vinyl methyl ether) hydrogel beads

Temperature-sensitive hydrogel beads were prepared by radiation crosslinking of poly(vinyl methyl ether) PVME spheres wrapped in Ca-alginate. The obtained gel beads have diameters in the sub-millimeter or millimeter range (depending on the PVME concentration). They were characterized by sol-gel analysis, swelling measurements, and differential scanning calorimetry. The gel content g increases with increasing radiation dose D. The swelling degree Q_v decreases with increasing PVME concentration c_p and increasing D. In comparison to PVME bulkgels the phase-transition temperature of the synthesized PVME gel beads is a little decreased.     

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.