Effect of non-associated electrolyte solutions on the behaviour of poly(vinyl alcohol)-based hydrogels

The aim of this paper is to present the behaviour of the poly(vinyl alcohol) hydrogels [PVA-HG] in sodium and potassium chlorides aqueous solutions, due to their interactions. The tested [PVA-HG]-s have been obtained by repeated freezing and thawing cycles. White, heterogeneous hydrogels have been obtained. These hydrogels exhibit a mechanical active behaviour at their contact with electrolytes aqueous solutions, manifested by important changing in mass, volume and density of the hydrogel samples. These modifications could be explained by water elimination from the hydrogels that initially reached the equilibrium of swelling. The kinetic of the water desorption and the reversibility of this process, have been studied and some of the factors that influence this behaviour have been evidenced. The sensitivity of PVA hydrogels to electrolyte nature and concentration could be used in sensors design and also could explain some aspects of electrolytes diffusion through PVA membranes and targeted drugs delivery.     

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.     

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.     

Tailored preparation of dual phase concomitant methylcellulose/poly(vinyl alcohol) physical hydrogel with tunable thermosensivity

A novel dual phase concomitant, methylcellulose sol@poly(vinyl alcohol) (MC/PVA) hydrogel, was prepared via physical mixing and subsequent freezing/thawing. MC/PVA hydrogel was stable within a wide temperature range, and exhibited reversible thermoresponsivity. The initial sol-gel transition temperatures of MC/PVA hydrogels containing 40, 45 and 50 wt% MC were 45.9, 42.0 and 45.5 °C, respectively. It was found that the crystallinity of these samples was 41.1%, 38.3% and 40.3%, respectively; all of them were lower than that of MC and PVA. The thermal responding rates of MC/PVA hydrogel composed of 30, 40, 45 and 50 wt% MC were about 2.85, 3.17, 5.74 and 8.58%/min, respectively. The fluorescence micrograph and scanning electron microscopy of MC/PVA hydrogel revealed that the micro MC sol phases were dispersed in whole PVA network. Moreover, the thermal transition behavior and interior morphology of MC/PVA hydrogel could be tailored with its composition.     

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.     

The negative biodegradation of poly(vinyl alcohol) modified by aldehydes

Due to the wide application of PVA acetals, the biodegradation of PVA modified by formaldehyde, n-butyraldehyde, glyoxaldehyde and glutaraldehyde was conducted in an intensive biodegradation environment. Spectrophotometric analysis and weight loss were used to determine the biodegradation of PVA, and the changes of the mechanical properties of PVA acetals were also studied.An obvious decrease in biodegradation levels of all the modified samples was found, and a decrease in biodegradation level with increasing degree of acetals of PVA. The biodegradation of poly(vinyl formal) is better than poly(vinyl butyral) with the same degree of acetals whereas the biodegradation levels of poly(vinyl glyoxal) are lower than poly(vinyl glutaral) which has the same degree of crosslinking. The difference between the FT-IR of the samples before and after biodegradation indicated scission of residual PVA chain during the process.     

Ion-specific swelling behavior of poly(vinyl alcohol) gel prepared by γ-ray irradiation

Poly(vinyl alcohol) gel was prepared by γ-ray irradiation of an aqueous solution of the polymer and its swelling behavior in solutions of the alkali-metal and alkaline-earth-metal salts was investigated. The gel deswelled in solutions containing strongly hydrated anions and swelled in solutions containing strongly hydrated cations. The degree of swelling of the gel was in the following order: SO₄ ²⁻<Cl⁻<Br⁻ ≅ NO₃ ⁻<I⁻ for the anions and K⁺<Na⁺<Li⁺ and Sr²⁺<Ca²⁺<Mg²⁺ for the cations. These results were interpreted in terms of interactions between the polymer and the ions through the hydration layers. Received: 6 November 2000 Accepted: 24 May 2001     

Solid‐state 13C and 129Xe NMR study of poly(vinyl alcohol) and poly(vinyl alcohol)/lactosilated chitosan gels

Dry and hydrated poly(vinyl alcohol) (PVA) gels with 55% (a‐PVA) and 61% (s‐PVA) syndiotacticity and related PVA/lactyl chitosan (LC) blends have been investigated with ¹²⁹Xe and cross‐polarization/magic‐angle‐spinning ¹³C NMR techniques. Although the dry gels exhibit two broad ¹²⁹Xe resonances in the slow‐to‐intermediate exchange limit, both hydrated gels show three resonances. The corresponding dry blends exhibit two signals, the chemical shifts and line widths of which change with respect to those of pure PVA, whereas one (a‐PVA/LC) or two (s‐PVA/LC) signals appear in the spectra of the hydrated blends. A comparative analysis of the data demonstrates that LC rearranges the domains of the polymeric matrix in both the dry and hydrated blends according to the syndiotacticity of the PVA chains. Information on the molecular motions of the amorphous and swollen polymeric domains in the kilohertz range has been obtained from an analysis of the spin‐lattice relaxation times. These data indicate that the dynamics and arrangement of the PVA chains in the gels are strongly affected by their tacticity and the addition of the copolymer LC. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3123–3131, 2003     

聚乙烯醇-碘复合物在水溶液中的形成

聚乙烯醇(PVA)在碘-碘化钾混合水溶液中能形成蓝色复合物,该复合材料在人们日常生活中已广泛应用.本文综述了在稀溶液条件下聚乙烯醇-碘复合物的形成,并对其复合物形成的可能影响因素,如:PVA的分子量、链结构的规整度、1,2-羟基结构、支链以及浓度、添加其它物质和反应时间等,进行了系统论述.从聚集模型观点看,在复合物中,碘主要是以线性多聚碘负离子形式存在的,如:T3-和I5-等;而PVA链通过分子间氢键作用形成链间聚集将多聚碘负离子包裹在内而形成复合物.     

Multilayer poly(vinyl alcohol)-adsorbed coating on poly(dimethylsiloxane) microfluidic chips for biopolymer separation

A poly(dimethylsiloxane) (PDMS) microfluidic chip surface was modified by multilayer-adsorbed and heat-immobilized poly(vinyl alcohol) (PVA) after oxygen plasma treatment. The reflection absorption infrared spectrum (RAIRS) showed that 88% hydrolyzed PVA adsorbed more strongly than 100% hydrolyzed one on the oxygen plasma-pretreated PDMS surface, and they all had little adsorption on original PDMS surface. Repeating the coating procedure three times was found to produce the most robust and effective coating. PVA coating converted the original PDMS surface from a hydrophobic one into a hydrophilic surface, and suppressed electroosmotic flow (EOF) in the range of pH 3-11. More than 1,000,000 plates/m and baseline resolution were obtained for separation of fluorescently labeled basic proteins (lysozyme, ribonuclease B). Fluorescently labeled acidic proteins (bovine serum albumin, beta-lactoglobulin) and fragments of dsDNA phiX174 RF/HaeIII were also separated satisfactorily in the three-layer 88% PVA-coated PDMS microchip. Good separation of basic proteins was obtained for about 70 consecutive runs.     

MMTCA recognition by molecular imprinting in interpenetrating polymer network hydrogels based on poly(acrylic acid) and poly(vinyl alcohol)

A novel IPN hydrogel designed to recognize MMTCA is prepared by applying the molecular-imprinting method. The IPN is characterized by FT-IR, DSC, and SEM. Langmuir analysis shows that an equal class of adsorption is formed in the hydrogel. The adsorption equilibrium constant and the maximum adsorption capacity are evaluated, and the effect of the pH on MMTCA adsorption is discussed. The selectivity of the imprinted polymer for MMTCA is studied in aqueous solutions of MMTCA/aspirin/riboflavin. The results suggest that the MMTCA-imprinted polymer shows superior selectivity for MMTCA as compared to riboflavin and aspirin. The reproducibility of the imprinted polymer to MMTCA is also studied.     

Fabrication and characterization of poly(vinyl alcohol)/chitosan hydrogel thin films via UV irradiation

A series of poly(vinyl alcohol)/chitosan (PVA/CTS) hydrogel thin films were prepared via ultraviolet (UV) irradiation, with acrylic acid (AA) monomer added as a crosslinker without the addition of any other photo-initiator. The swelling behaviors, intermolecular chemical bonds, molecular structures, thermal behaviors, degrees of crystallinity, morphologies of the surfaces and internal structure, and their relationship to the AA content were characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Poly(acrylic acid) (PAA) and its chemical crosslinks formed in hydrogel films via free-radical reactions were confirmed using FTIR and DSC analyses. The XRD patterns indicated that the degree of crystallinity of the hydrogel films decreased as the PAA content was increased. SEM micrographs showed that a uniform interconnected pore structure was formed through the entire hydrogel structure, and a gradient in the crosslinking density through the film thickness was observed to result from extended irradiation times. The swelling behaviors revealed that the formation of PAA and its crosslinking in the hydrogel thin films improved the pH stability and controlled the degree of swelling while retaining a high swelling rate. The successful formation of chemical crosslinking without any specific photo-initiator improves the natural characteristics of CTS and PVA and imparts the resulting PVA/CTS hydrogel thin films with properties that make them very promising in biomedical applications.     

Facile preparation of methylcellulose/poly(vinyl alcohol)physical complex hydrogels with tunable thermosensitivity

Novel complex hydrogels of methylcellulose(MC)and poly(vinyl alcohol)(PVA)with wide-spectrum thermoresponsivity were prepared via physical and mild process.Thermal phase transition of MC/PVA hydrogels exhibited two forms including sol/sol to gel/sol and sol/gel to gel/gel.The phase transition temperature of MC/PVA solution ranged from 38.7 to 60.6℃and was able to be adjusted by simply changing the feeding ratios of two components.The interior morphology of MC/PVA gels was examined with fluorescence analy...     

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.     

Water resistance, mechanical properties and biodegradability of methylated-cornstarch/poly(vinyl alcohol) blend film

The main shortcomings of biodegradable starch/poly(vinyl alcohol) (PVA) film are hydrophilicity and poor mechanical properties. With an aim to overcome these disadvantages, cornstarch was methylated and blend films were prepared by mixing methylated-cornstarch (MCS) with PVA. The mechanical properties, water resistance and biodegradability of the MCS/PVA film were investigated. It was found that MCS/PVA film had higher water resistance than the native starch/PVA film. However, the water resistance of MCS/PVA films did not have significant difference with the increase in the degree of substitution (DS) of the methylated starch from 0.096 to 0.864. Enzymatic, microbiological and soil burial biodegradation results indicated that the biodegradability of the MCS/PVA film strongly depended on the starch proportion in the film matrix. The degradation rate of starch in the starch/PVA film was hindered by blending starch with PVA. Both tensile strength and percent elongation at break of the MCS/PVA film were improved as DS of the methylated starch increased. Conversely, increasing the methylated starch proportion in film matrix deteriorated both tensile strength and percent elongation at break of the film.     

Physically crosslinked poly(vinyl alcohol)-hydroxyethyl starch blend hydrogel membranes: Synthesis and characterization for biomedical applications

Poly(vinyl alcohol), PVA is a polymer of great importance because of its many appealing characteristics specifically for various pharmaceutical and biomedical applications. Physically crosslinked hydrogel membranes composed of different amounts of hydroxyethyl starch (HES) in (PVA) and ampicillin were prepared by applying freeze–thawing method. This freezing–thawing cycle was repeated for three consecutive cycles. Physicochemical properties of PVA–HES membrane gel such as gel fraction, swelling, morphology, elongation, tensile strength, and protein adsorption were investigated. Introducing HES into freeze–thawed PVA structure affected crystal size distribution of PVA; and hence physicochemical properties and morphological structure have been affected. Increased HES concentration decreased the gel fraction %, maximum strength and break elongation. Indeed it resulted into a significant incrementing of the swelling ability, amount of protein adsorption, broader pore size, and pore distribution of membrane morphological structure. Furthermore, an increase in HES concentration resulted in better and still lower thermal stability compared to virgin PVA and freeze–thawed PVA. The maximum weight loss of PVA–HES hydrogel membranes ranged between 18% and 60% according to HES content, after two days of degradation in phosphate buffer saline (PBS), which indicates they are biodegradable. Thus, PVA–HES hydrogel membranes containing ampicillin could be a novel approach for biomedical application e.g. wound dressing purposes.     

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.     

Preparation of poly(vinyl pivalate) microspheres by dispersion polymerization in an ionic liquid and saponification for the preparation of poly(vinyl alcohol) with high syndiotacticity

We report here a successful free-radical dispersion polymerization of vinyl pivalate (VPi) in an ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][TFSI]) using poly(vinyl pyrrolidone) (PVP) as a stabilizer. Morphological analysis by FE-SEM revealed that poly(vinyl pivalate) (PVPi) obtained from dispersion polymerizations were in the form of spherical particles. Micron-sized, PVPi particles with a number-average molecular weight (M_n) of 166,400 g/mol could be obtained using 5% stabilizer (w/w to monomer) at 65 °C for 20 h. The effects of varying concentration of stabilizer, initiator and monomer upon polymer yield, molecular weight, and morphology of PVPi were also investigated. Analogous polymerizations in dimethyl sulfoxide (DMSO) and bulk served as references. In addition, the preparation of poly(vinyl alcohol) (PVA) by saponification of the resultant PVPi was described.     

淀粉-丙烯酸钠接枝共聚物的固体高分辨核磁共振研究

运用固体高分辨核磁共振技术,通过测量13C魔角旋转/交叉极化(CP/MAS)谱、1H自旋-晶格弛豫时间T1及旋转坐标系中的自旋-晶格弛豫时间T1ρ,对一系列淀粉-丙烯酸钠接枝共聚物的相结构进行了研究,并与淀粉、均聚丙烯酸钠及两者共混物的实验结果进行了比较.结果表明,接枝共聚导致了淀粉结晶度的明显降低;在共混物和接枝共聚物中,淀粉和聚丙烯酸钠组分都具有纳米尺度的相容性,由于接枝的效应,接枝共聚物中两个组分表现出比共混物更高的相容水平.     

Swelling Kinetics of Interpenetrating Polymer Hydrogels Composed of Poly(Vinyl Alcohol)/Chitosan

Interpenetrating polymer network (IPN) hydrogels based on poly(vinyl alcohol)/chitosan were prepared by UV irradiation. The swelling behavior of the IPN hydrogels was studied by immersion of the films in deionized water at various temperatures and in buffer solutions at various pHs. IPN3 exhibited a relatively high swelling ratio. The swelling ratio increased with an increase in the content of chitosan and were higher in acidic rather than in alkaline pHs. The overall swelling process was anomalous diffusion due to polymer relaxation. The diffusion coefficient values increased with an increase in temperature and the content of chitosan.