On the viscoelastic properties of poly(vinyl alcohol) and chemically crosslinked poly(vinyl alcohol)

Poly(vinyl alcohol)(PVA) films chemically crosslinked with glutaraldehyde(GA) in the presence of HCl were prepared by casting from aqueous solutions. The PVA and PVA gels were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA); their swelling characteristics and tensile strength were also determined. The DSC results for the gels displayed depressions of the melting and crystallization temperatures, as well as a decrease of the heat of fusion, when compared to those of PVA free of crosslinker. The DMA analysis revealed that: (1) The glass transition temperature of the wet PVA was lower than that of the dry one, indicating that the water had a plasticizing effect. (2) The gels had a lower glass transition temperature than PVA. (3) The glass transition temperature of the wet gels increased with increasing crosslink density. Possible explanations are provided for these observations. Whereas the thermogravimetric curves of PVA exhibited a single degradation peak, two degradation peaks were detected for the crosslinked PVA. The wet PVA and PVA gels displayed lower tensile strengths and higher elongations than the dried ones. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1816–1823, 2001     

Exploitation of acetalization process of poly(vinyl alcohol) for the formation of crosslinked poly(vinyl formal) foams

In the present work, poly(vinyl formal) (PVF) foams were prepared using water as the pore-forming agent and formaldehyde as the crosslinking agent. The acetalization process of poly(vinyl alcohol) was exploited in depth in order to obtain a precise guidance on the preparation of PVF foams with adjustable properties. With the increasing of formaldehyde, the crosslinking degree was gradually increased and the prepared PVF foams changed to be amorphous. Morphology observation showed that the porous structure of PVF foams was successfully created using water as the blowing agent and it was strongly affected by the crosslinking agent and polymer concentration, leading to an easily tuned pore size from hundreds of microns to few microns. The prepared foams were proved to have small apparent density below 0.27 g/cm⁻³ and excellent mechanical strength, with the largest specific compressive strength of 11.54 MPa·cm³·g⁻¹. Based on the results, it is believed that this study can provide a scientific basis for the design and optimization of PVF foams with controllable structure and properties.     

Starch crosslinked with poly(vinyl alcohol) by boric acid

Starch was crosslinked with poly(vinyl alcohol) (PVA) by boric acid. A suitable plasticizer and defoamer were added to obtain the brei. A film from the starch and PVA (SP film) was prepared by casting. The effects of various factors, such as the crosslinking temperature, the PVA content, and the amounts of glycerol and boric acid, on the tensile strength and breaking elongation were studied. The results showed that the SP film prepared by boric acid crosslinking had excellent mechanical properties. The film‐forming properties, transmittance, and water resistance of the SP film were also investigated. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1394–1397, 2005     

Preparation of poly(vinyl amine)‐grafted crosslinked poly(vinyl alcohol) microspheres

Crosslinked poly(vinyl alcohol) (CPVA) microspheres were first prepared via the suspension polymerization of vinyl acetate and the alcoholysis of poly(vinyl acetate). Afterwards, a two‐step method involving graft polymerization and Hofmann degradation was used to prepare functional poly(vinyl amine)‐grafted crosslinked poly(vinyl alcohol) (PVAm–CPVA) microspheres, onto which poly(vinyl amine) (PVAm) macromolecules were grafted. The graft polymerization of acrylamide (AM) on CPVA microspheres was performed with cerium salt as the initiator in an acidic aqueous medium, resulting in polyacrylamide (PAM)‐grafted CPVA microspheres. Subsequently, the grafted PAM was transformed into PVAm via the Hofmann degradation reaction, and PVAm–CPVA microspheres were prepared. The effects of the main factors on the graft polymerization and Hofmann degradation were examined, and the reaction mechanisms were researched in depth. The experimental results showed that for the graft polymerization of AM on CPVA microspheres initiated by cerium salt, the acid concentration and the amount of cerium salt affected the grafting degree of PAM greatly. For the Hofmann degradation reaction of the grafted PAM, the amination degree of PVAm–CPVA microspheres was obviously affected by the amount of sodium hypochlorite in the presence of sodium hydroxide. The preliminary adsorption tests showed that PVAm–CPVA microspheres were multifunctional and had strong adsorption ability for Fe(III) ions by chelation action and for chromate ions (CrO) by strong electrostatic interactions. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mechanical properties of borate crosslinked poly(vinyl alcohol) gels

Boric acid does not introduce crosslinks in poly(vinyl alcohol) solutions, but gelation does occur in the presence of cations. In this experimental study, the dynamic mechanical properties of these gels were determined using test-tube torsion pendulums and an air-bearing torsion pendulum. The modulus at a fixed concentration of polymer and boric acid increases with increasing sodium ion concentration up to the point where the atom ratio of sodium to boron reaches 1. Higher sodium concentrations do not increase the modulus. The log decrement, on the other hand, decreases with increasing sodium concentration continuously without reaching a plateau at the equal atom ratio. Log decrements as low as 0.02 can be measured. The storage modulus depends on the logarithm of borate concentration and on the 4.7 power of poly(vinyl alcohol) concentration. Only a very small portion of the borates in solution take part in effective crosslinks. The activation energy for breaking individual bonds in a function of temperature and the cation to boron ratios. At a fixed cation concentration, this activation energy is more negative with increasing amount of boron ions due to a transformation of monomeric crosslinks into polymeric crosslinks, so that the storage modulus which measures crosslink density decreases as a temperature rises.     

Structure of poly(vinyl alcohol)-iodine complex formed in the amorphous phase of poly(vinyl alcohol) films

The effect of syndiotactivity of poly(vinyl alcohol) (PVA) both on the formation and thermal stability of the complex formed in the amorphous phase of PVA films is investigated, and then a model of the complex is presented. The amount of the complex formed in syndiotacticity-rich PVA is much larger than that formed in atactic PVA under a given iodine-soaking condition, and the former complex has a higher thermal stability in the soaking solution than the latter. The complex formed in the amorphous phase is proposed to have such a structure as that in which a linear polyiodine I₅^? or I₅^? with a 3.1 A? periodicity is enveloped by four PVA segments of syndiotactic configuration with extended conformation. In this model, these four PVA chains participating in a complex are supposed to be fixed by interchain hydrogen bonds. The observed X-ray meridional intensity curve of iodinated PVA film can be explained by a series of two I₅^?. © 1993 John Wiley & Sons, Inc.     

PH-induced structure change of poly(vinyl alcohol) hydrogel crosslinked with poly(acrylic acid)

The structure of the hydrogel of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) was investigated by small angle X-ray scattering (SAXS) of synchrotron radiation. A physically crosslinked blend gel, which was prepared by repetitive freezing and thawing of an aqueous solution of PVA and PAA, could be chemically crosslinked by esterfication of PVA with PAA even in the hydrogel state. The chemical crosslinking induced the destruction of physical crosslinks into a folded structure, indicating that the chemical crosslinking proceeds at the sites around the physical crosslinks that contain PVA and PAA in much higher concentration than other portion of the gel. The pH-induced structure changes of the PVA hydrogels, chemically crosslinked with poly(acrylic acid) (PAA) were investigated by SAXS on the samples of various chemical crosslinking time. The gels were shrunk at pH4, and swollen at pH8. The results of SAXS showed, that the Porod slope changed with chemical crosslinking time from -3.5 to ?2.9 at pH4, and from ?2.9 to ?2.4 at pH8. The results suggest that a folded structure as a structural domain, which is characterized by fractally rough interface, tends to change into the structure that corresponds to percolation cluster, particularly at pH8. The gels immersed in pH8 showed a remarkable structure change accompanying swelling. The results revealed that a conformational change of PAA chains, induced by the pH change, can be explained by the presence of a structural domain in the gel network, where both PVA chains and PAA chains get entangled and partially form a interpenetrating polymer network(IPN).     

Synthetic absorbent for dyestuffs based on gamma crosslinked poly(vinyl alcohol) (PVA)

This work reports on the characterization and uses of gamma radiation‐crosslinked poly(vinyl alcohol) (PVA) films as absorbents for different classes of dyestuffs normally released from textile factors. Dyestuffs were selected from different producers. These dyestuffs were Remacryl Blue (basic dye), Remazol Golden Orange (reactive dye), Solar Orange (direct dye), and Sandolan (acid dye). The percentage dye sorption by PVA was determined by spectroscopic analysis and color‐strength measurements of the PVA films after absorbing the different dyestuffs. The results showed that the concentration of dimethylbisacrylamide as a crosslinking agent has a great effect on the gel fraction and percentage swelling in water of the PVA polymer. In general, crosslinked PVA showed a low tendency to absorb the different dyestuffs under investigation at room temperature. This tendency was shown to increase with an increasing temperature of the dye solution from room temperature to 70°C. Meanwhile, the highest percentage of dye sorption was found in the case of the basic dye and the lowest one in the case of the reactive dye depending on the active groups on the dyes. Moreover, the percentage dye sorption by PVA was slightly higher in the acid medium rather than in the alkaline one. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 349–354, 2003     

室温交联PVA水凝胶的制备与性能研究

以环氧氯丙烷为交联剂,采用室温化学交联法制备了聚乙烯醇(PVA)水凝胶,研究了交联温度和交联时间对PVA溶胀性能和力学性能的影响,借助差示扫描量热法(DSC)和扫描电子显微镜(SEM)对PVA水凝胶进行分析表征。结果表明,当交联时间为3d且交联温度为50℃时PVA水凝胶的综合性能最佳。PVA的玻璃化转变温度为-54.50℃,PVA水凝胶中包含有自由水、束缚水和非冷冻水。     

Effect of chemical crosslinking on swelling parameters of modified poly(vinyl alcohol) hydrogel

Graft copolymerization of maleic anhydride (MA) onto partially and fully hydrolyzed poly(vinyl alcohol), PVA, was carried out in presence and in absence of an initiator. The structural features of these grafts were confirmed by 1 HNMR analysis. 1 HNMR analysis was also used to determine the percentage of grafting. These grafts were crosslinked using different concentrations of either 1,1,1-trimethylolpropane trimethacrylate (TPT) or methylene bisacrylamide (MBA) as Hexa- or tetra-functional crosslinker, respectively. The crosslinkers concentrations ranged from 1% to 30% (wt.%) based on the total weight of grafts. The final water content, volume fraction of the polymer and swelling capacity were determined for all grafts. The effect of MBA and TPT crosslinkers structure on swelling properties of both partially and fully hydrolyzed PVA grafts were also determined.     

Macroporous poly(vinyl alcohol) foam crosslinked with epichlorohydrin for microorganism immobilization

Macroporous poly(vinyl alcohol) foam with epichlorohydrin as a crosslinking agent was investigated. The average molecular weight between crosslinks, crosslinking density, and mesh size were determined through the equilibrium swelling theory. The characterization of foams with different crosslinking ratios was also investigated through the testing of the thermal properties, specific surface areas, and diffusion coefficients. The biomass densities and activity yields were measured by detection of the protein concentration and oxygen uptake rate. Thermogravimetry and differential scanning calorimetry tests showed an increase in the thermal stability and a decrease in the polymeric crystallinity with increasing crosslinking ratio. The biomass densities increased with increasing crosslinking ratio, with the highest value shown at 0.0638 ± 0.0093 g of volatile suspended solid (VSS)/g of carrier. However, the activity yields decreased with increasing crosslinking ratio, with the highest value at 69.38%. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Swollen behavior of crosslinked network hydrogels based on poly(vinyl alcohol) and polydimethylsiloxane

Poly(vinyl alcohol) (PVA) was dissolved in the water to make a 10 wt % aqueous solution, and polydimethylsiloxane (PDMS) was mixed with 1 wt % 2,2‐dimethyl‐2‐phenylacetophenone (DMPAP) and 0.5 mol % methylenebisacrylamide (MBAAm) in isopropyl alcohol. This mixture was added to a PVA aqueous solution and heated at 90°C for 7 h. Various crosslinked networks were prepared at different molar ratios of PVA/PDMS (1:1, 1:3, and 3:1). The characterization of PVA/PDMS crosslinked networks was determined by Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), a universal testing machine (UTM), and the equilibrium water content (EWC). The DSC melting endotherms showed, at 219.49°C, a sharp endothermic peak of PVA, and PVA/PDMS crosslinked networks had melting peaks close to this point. The value of EWC increased with the content of PVA in the crosslinked networks, simultaneously depending on the temperature. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 957–964, 2002     

Recovery of heavy metals from aqueous solution by using radiation crosslinked poly(vinyl alcohol)

Crosslinked poly(vinyl alcohol) (PVA) and PVA/acrylic acid hydrogels, prepared by γ‐irradiation, were used in the removal of heavy metals from aqueous solution. Comparative studies of the properties of the two hydrogels were made. The application of the prepared hydrogels as adsorbent materials for Cu²⁺, Co²⁺, and Ni²⁺ from aqueous solution was studied. The chemical and physical properties of the hydrogels, before and after adsorption of the heavy metal ions, were investigated by means of thermogravimetric analysis, differential scanning calorimetry, and scanning electron microscopy. The efficiency of such hydrogels for the recovery of metal ions was determined by atomic absorption and UV spectroscopic analysis. The effect of changing pH on the metal uptake was also studied. It was found that the prepared hydrogels have a substantial ability to adsorb metal ions from their solution. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1649–1656, 2004     

Preparation and characterization of high loading porous crosslinked poly(vinyl alcohol) resins

Porous crosslinked poly(vinyl alcohol) resin beads have been prepared by a two-step pathway involving a pre-crosslinking reaction followed by a reverse suspension crosslinking reaction with epichlorohydrin as crosslinker. The reaction conditions have been optimized for the two steps in order to obtain resins with various pores sizes. The crosslinking density of these resins was calculated from their swelling properties by a modified Flory equation. High loading of hydroxyl groups is one of the important characteristics of these resins (attaining ca. 17 mmol/g). The porosity, pore volume, pore structure and the swelling behavior in water of these resins have been studied.     

Preparation of anion exchange membranes based on pyridine functionalized poly(vinyl alcohol) crosslinked by 1,4-dichlorobutane

A series of anion exchange membranes [pyridine functionalized-poly(vinyl alcohol)-1,4-dichlorobutane (PVA-PY-DL_x)] were synthesized by using PVA-PY as polymer matrix and DL as crosslinker and iodomethane as quaternization reagent. During the experiment, pyridine groups grafted on PVA were transformed into quaternary ammonium group during the formation process of the crosslinked structure and the quaternization routine by iodomethane. The characterization results revealed that the PVA-PY-DL_x membranes have been successfully prepared and the crystallinity increases with increase of DL. PVA-PY-DL_x membranes have smooth and uniform morphology. The introduction of crosslinked structure improves the mechanical properties and dimensional stability of the PVA-PY-DL_x membrane, enhances the alkali resistance. When the mass content of DL was 4.0%, composite membrane had the maximum tensile strength (44.2 MPa), and the OH⁻ conductivity reaches 1.05 × 10⁻² S cm⁻¹ at 70 °C. The accelerated aging experiment was carried out in 3 mol L⁻¹ potassium hydroxide (KOH) solution for 120 h at 80 °C, which revealed that the anionic conductivity of PVA-PY-DL_4.0 membrane retains 79.6% of its initial conductivity, showing better stability of alkali stability. Methanol permeability of PVA-PY-DL_x membranes was only the 0.37–0.72% of the Nafion-117 membrane in 3 mol L⁻¹ methanol at 60 °C. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47395.     

Release of diclofenac through gluteraldehyde crosslinked poly(vinyl alcohol)/poly(acrylic acid) alloy membranes

A controlled‐release preparation of diclofenac sodium for transdermal administration has been developed. Poly(vinyl alcohol) (PVA) and PVA/poly(acrylic acid) (PAA) alloy membranes were prepared from a solvent‐casting technique using different PVA/PAA (v/v) ratios. The release of the drug from the membrane was evaluated under in vitro conditions at pH 7.4. The delivery system provided linear release without time lag, burst effect, and boundary layer resistance. Effects of variables such as film thickness and PVA/PAA ratio on the permeation behavior of the polymeric membranes were discussed. The optimal PVA/PAA was determined as 50/50. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 72–77, 2004     

Synthesis of poly(vinyl alcohol-co-vinyl gallate) by the chemical modification of poly(vinyl alcohol)

Summary Poly(vinyl alcohol-co-vinyl gallate) was successfully synthesized utilizing an esterification reaction in which poly(vinyl alcohol) was transesterified with methyl gallate under alkaline conditions. Aqueous solutions of this copolymer exhibited a sharp critical micelle concentration value, indicating that surface activity is exhibited by the copolymer. Most aqueous solutions of this copolymer possess a black coloring, which indicates that the gallate group is complexing with metal ions present in solution.     

Characteristics of poly(vinyl alcohol) films crosslinked by cinnamaldehyde with improved transparency and water resistance

Poly(vinyl alcohol) (PVA) films crosslinked by cinnamaldehyde (CIN) at various concentrations were prepared. Their physicochemical and antibacterial properties were compared with those of uncrosslinked films. The results suggest that the crosslinked films exhibited higher transparency, tensile strength, and elongation at break in a certain CIN concentration range. They also displayed decreased water vapor permeability and water solubility. Although the antibacterial activity of the crosslinked film‐forming solution was less slightly than that of the uncrosslinked film‐forming solution, the crosslinked film‐forming solution still demonstrated strong antibacterial activity when the concentration of CIN was 2% v/v. The aldol condensation reaction between the aldehyde group of CIN and the alcoholic hydroxyl group of PVA was determined by Fourier transform infrared spectroscopy. By the crosslinking reaction, the heat stability of the crosslinked film was enhanced compared with that of the uncrosslinked film. Meanwhile, a compact, smooth, and continuous cross‐sectional microstructure present in the crosslinked film was observed by scanning electron microscopy. The higher water resistance and transparency gave the crosslinked films priority for use in food packaging. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45324.     

Effect of filler concentration on the phase separation in poly(vinyl acetate)-poly(methyl methacrylate) mixtures

The temperatures and kinetics of phase separation have been studied as a function of filler concentration in a polymer mixture of poly(vinyl acetate)-poly(methyl methacrylate). The filler was shown to have a marked effect on the cloud point position and the phase separation kinetics at various temperatures. The effects observed are dependent upon the component ratio.     

Thickening of poly(vinyl alcohol) by borate

Viscosity and pH of aqueous borate–poly(vinyl alcohol) mixtures were determined under different conditions toward a clarification of the thickening mechanism. Boric acid has little effect on dilute poly(vinyl alcohol) (PVOH), but sodium borate causes sharp viscosity increases which are temperature sensitive and apparently stem from labile exothermic crosslinking of PVOH by borate ions. The pH data indicate that the complexing of borate with simple polyols such as mannitol and glycerol is a different process from borate crosslinking of PVOH. The PVOH thickening and gelling effect probably arises from the replacement of labile waters of hydration on borate ion by OH groups of PVOH rather than from borospirane formation.