Gelation of poly(vinyl alcohol) in dimethyl sulfoxide/water solvent

Summary Gelation of poly(vinyl alcohol) (PVA) in dimethyl sulfoxide (DMSO)/water was observed at 23°C with viscometry, spectrophotometry, wide angle X-ray scattering and light scattering. Here transparent gel formation was found to take place prior to being turbid in some cases, whereas the solution became turbid prior to gelation in other cases. Whether transparent gel is formed at first or solution becomes turbid, depends on DMSO composition. PVA solution forms gel in the DMSO composition range from 20 to 80 wt.%. Below the boundary DMSO composition of 60–70 wt.%, gelation takes place at first (i.e. transparent gel is formed) and then becomes turbid eventually, while beyond this DMSO composition the solution becomes turbid and then opaque gel is formed.     

Formation Mechanism Analysis of Shear-Induced Microgel Filaments during Microfluidic Gelation

Dynamic microfluidic gelation enables the fabrication of bundle-structured multiple parallel microgel filaments from an aqueous two-phase system. The formation mechanism of shear-induced filaments from an alginate (Alg)/polyvinyl alcohol (PVA) blend is studied using red-colored PVA and a titanium alkoxide PVA crosslinker. Bundle-structured Alg microgel filaments are formed through contact with a Ca²⁺ crosslinker. In this process, the PVA acts as a sacrificial polymer to maintain the Alg gel filaments because approximately 90% of the red-colored PVA is released from the Ca²⁺-crosslinked Alg gel filaments into the wash water. In addition, the fabrication of PVA gel filaments shows that the sacrificial PVA is also transformed into fibrillar domains under shear. However, the filament structure cannot be formed from a single-phase PVA/Alg solution. These results clearly show that the bundle-structured gel filaments are maintained by preventing the fusion of filaments during gelation based on the tendency of the non-crosslinked filaments to cause splitting of the gelled filaments.     

Formation and destruction of physical crosslinks by mild treatments in chemically crosslinked poly(vinyl alcohol) gels

We report the formation, destruction, and re-formation of the microcrystallites in a chemical PVA gel, crosslinked slightly by glutaraldehyde. The dried gel, in which the microcrystallites were formed, was immersed in a poor solvent of a mixture of DMSO and water at 8 °C, where the gel stayed in its collapsed state. The gels swelled on heating to 50 °C, but did not return to the initial state on cooling to 8 °C. After washing the gel completely in water, a drying process caused the microcrystallites to be re-formed and the gel could return to the initial dried state at 8 °C. By using XRD and FT-IR measurements, it was concluded that the formation and destruction of microcrystallites in chemically crosslinked PVA gels could be controlled by the mild treatments of initial drying, temperature change, and washing and drying.     

Gel spinning of PVA/SWNT composite fiber

Single wall carbon nanotubes (SWNT), polyvinyl alcohol (PVA), dimethyl sulfoxide (DMSO) and water, homogeneous dispersion has been prepared by stirring and sonication. This dispersion was extruded into fiber via gel spinning. The modulus of the PVA/SWNT (3 wt%) composite fiber was 40% higher than that of the control PVA gel spun fiber. Fiber structure and properties have been studied. The PVA orientation in the control and the composite fibers were comparable while the composite fiber exhibited lower crystallinity.     

溶剂对聚乙烯醇纺丝原液及纤维结构和性能的影响

采用不同的溶剂溶解高相对分子质量聚乙烯醇(PVA),制备PVA纺丝原液及初生纤维,对其结构和性能进行了研究。结果表明:二甲基亚砜(DMSO)/H2O溶剂中DMSO的质量分数为92%时,对PVA的的溶解性能最好。相同条件下,与DMSO相比,N-甲基-N-氧化吗啉(NMMO)使PVA溶液粘度降低更快;DMSO/NMMO.2.5H2O混合溶剂中,当NMMO.2.5H2O质量分数为15%时,PVA纺丝的凝固性能较好,其初生纤维的结晶度较低。     

Deswelling characteristics of poly(N-isopropylacrylamide) hydrogel

Poly(N-isopropylacrylamide) gels exhibit a lower critical solution temperature (LCST) behavior in aqueous solution. At temperatures below the LCST, the gel is more hydrated than at temperatures above the LCST. At the LCST, the volume change is sharp. It is shown here that the water content of the gel above the LCST depends upon previous states of the gel (e.g., dry or wet), the heating rate, and the gel thickness. Deswelling kinetics are also affected by the gel thickness. Caffeine release experiments indicate that microscopic water pockets are formed within the matrix during the rapid gel collapse procedure. © 1994 John Wiley & Sons, Inc.     

Synthesis and pH‐induced phase transition behavior of PAA/PVA nanogels in aqueous media

Polyacrylic acid (PAA) and polyvinyl alcohol (PVA) are well‐known FDA‐approved biocompatible polymers. A novel method for preparing PAA/PVA complex nanoneedles in PVA aqueous solution is presented in this article. The PAA/PVA complex nanogels are obtained via polymerization of acrylic acid monomer after PVA nanoparticles formed in water/acetone cosolvent. The results of TEM images showed that the PVA chains were aggregated to form gel particles with some erose nanoparticles. As AA monomers polymerized around PVA nanoparticles, PAA/PVA complex nanogels formed. The PAA/PVA nanogels had an average diameter of 300–100 nm with AA concentration of 0.5–2 g/100 mL. As acetone concentration varied, TEM images demonstrated that the morphologies of resulting nanogels are different. Without acetone in PVA aqueous solution, however, PAA/PVA complexes aggregated to form earthnut‐like particles. These results show that the shape and size of PVA/PAA nanogels can be tailored as a template or core for the formation of PAA/PVA nanogels. These PAA/PVA nanogels exhibited pH‐induced phase transition due to protonation of PAA chains. The novel PVA/PAA nanogels promise to be developed into pH‐controlled drug delivery system. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effects of solvent adsorption on solution properties of poly(vinyl alcohol)/dimethylsulfoxide/water ternary systems

In this study, the solvent adsorption phenomena of poly(vinyl alcohol) (PVA) in cosolvent mixtures of dimethylsulfoxide (DMSO; solvent 1) and water (solvent 2) were investigated. Typically, this cosolvent mixture could form hydrogen‐bonded DMSO/(water)₂ complexes, involving one DMSO and two water molecules. Because of the complex formation in the cosolvent mixtures, PVA chains preferentially adsorb water molecules at DMSO mole fraction X₁ < 0.33, but preferentially adsorb DMSO molecules at X₁ > 0.33. The preferential adsorption of DMSO (a good solvent for PVA) could cause the relatively extended conformation of PVA chains in solutions because of the increase in excluded volume effect. Because of various interactions between PVA chains and cosolvent mixtures, the aggregation and gelation behaviors of PVA solutions were significantly affected by the composition of cosolvent mixture. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3211–3217, 2004     

Cryostructuring of polymer systems. XXVI. Heterophase organic–inorganic cryogels prepared via freezing–thawing of aqueous solutions of poly(vinyl alcohol) with added tetramethoxysilane

Composite heterophase organic–inorganic hybrid cryogels of poly(vinyl alcohol) (PVA) containing silica constituents were prepared and studied. Such constituents were formed in the course of hydrolytic polycondensation (sol‐gel process) of tetramethoxysilane (TMOS) introduced in to the aqueous polymer solution prior to its freeze–thaw treatment. It was shown that moderate (over the range of ?15 to ?30°C) freezing, then frozen storage, and subsequent thawing of the water/PVA/TMOS systems resulted in the formation of macroporous composite cryogels filled with dispersed silica particles (discrete phase). The continuous phase of such gel materials represents the supramolecular PVA network, which is supposed to be additionally cured with the silicon‐containing oligomeric cross agents formed from TMOS in the course of hydrolytic polycondensation. The incorporated silica components influenced the morphology of cryogels. The effects of significant increase in gel strength and heat resistance with increasing TMOS concentration in the initial feed and with thawing rate decreasing have also been observed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Poly(vinyl alcohol) fiber radicals induced by photo-irradiation at room temperature

The decay behavior of radicals produced on poly(vinyl alcohol) (PVA) fiber by photo-irradiation at room temperature and the graft copolymerization of methyl methacrylate (MMA) on the irradiated PVA fiber were investigated. Two kinds of stable radicals showing singlet and triplet spectra were indicated for both unsensitized and ferric ion-sensitized samples, especially with the emphasis of triplet component radical. The decay of radicals was promoted by contact with various organic solvent-water solutions, which effects were in the order of dimethyl sulfoxide (DMSO)–water > acetone–water > water > dioxane–water > methanol–water. On the other hand, graft copolymerization of MMA on the preirradiated sample was effectively initiated with the aid of a little water or a mixed solution of organic solvent and water. Methanol and dioxane, which decay radicals milder than acetone and DMSO do, contributed to give a higher per cent grafting. As no initiation took place with the unirradiated sample, it is concluded that the ability of preirradiated samples to initiate graft copolymerization should be caused by the PVA fiber radicals, which are smoothly produced by photo-irradiation at room temperature and show a triplet spectrum.     

Effects of cononsolvency on gelation of poly(vinyl alcohol) in mixed solvents of dimethyl sulfoxide and water

We studied the rates of gelation and phase separation of poly(vinyl alcohol) (PVA) solutions in mixtures of dimethyl sulfoxide (DMSO) and water at 25 °C and found that both the rates show a maximum at a volume fraction of DMSO φ_DMSO=0.60 while gelation was not observed either in pure DMSO or pure water, suggesting that water-DMSO is a cononsolvent system for PVA. On the basis of the data by Cowie [Can J Chem 36 (1961) 2240] we concluded that the 1:2 stable complex between one DMSO molecule and two water molecules is the main cause of this cononsolvency.     

Properties of gels obtained by freezing/thawing of poly(vinyl alcohol)/water/dimethyl sulfoxide solutions

Melting points of poly(vinyl alcohol) (PVA) gels obtained by repeated freezing/thawing cycles of the solutions in mixed solvents of water and dimethyl sulfoxide (DMSO) were measured. PVA solutions in 1–30 and 90 vol % DMSO froze at ?40°C. The melting point of a gel increased with an increase in freezing/thawing cycles for all the solutions frozen and with an increase in freezing time in 10–30 vol % DMSO. The solutions in 1–5 vol % DMSO gelled even at very low polymer concentration of 1.0 g/dL by repeated freezing/thawing. The melting point of all the gels with a 5% polymer concentration reached ca. 73°C after 14 freezing/thawing cycles except for the gels in 90 vol % DMSO. However, the melting point of the gels with the same polymer concentration prepared at ?40°C using an unfrozen mixed solvent in 60 vol % DMSO was ca. 77°C and the largest. The maximum apparent enthalpy of fusion of the gels prepared by repeated freezing/thawing was ca. 40 kcal/mol. The gels in 90 vol % DMSO were transparent independent of cycles where as those in 1–80 vol % DMSO were opaque. The degree of opaqueness increased with decreasing DMSO content.     

Study of cryostructuration of polymer systems. XX. Foamed poly(vinyl alcohol) cryogels

Foamed poly(vinyl alcohol) (PVA) cryogels, which are formed as a result of freeze–thaw treatment of whipped PVA water solutions (polymer with MW of 69,000 Da and DD ～99 mol % was used), were obtained and their properties were studied. The rheological characteristics and macrostructure of these gel materials were controlled by the same factors as for the ordinary nonfoamed PVA cryogels (initial polymer concentration and freezing–thawing regimes) and also by the conditions of generation of fluid PVA foams. The study of the kinetics of the freeze–thaw‐induced gel formation of these foams revealed that the temperature dependence of the efficiency of cryotropic gelation showed a maximum at about ?1.5°C. The presence of low molecular weight admixtures in the initial polymer solution appears to be a rather important factor because the admixtures were capable of decreasing the stability of fluid PVA foams and weakening both foamed and nonfoamed cryogel samples. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1609–1619, 2001     

Change in molecular weight distribution by elution of polymers from PVA cast gel

Poly(vinyl alcohol) (PVA) cast gels with high degrees of polymerization and hydrolysis were prepared at room temperature. After sufficient exchange of the outer solvent by fresh pure water, the gel was re-dissolved in pure water at a high temperature. Using this solution, the cast gel was prepared again. This re-dissolution process was repeated again, and a third cast gel was prepared. During each swelling process after each cast-drying process, the amount of elution from the gel was measured and the molecular weight distributions of PVA in the gel as well as in the outer solvent were quantitatively evaluated. The molecular weight distribution of the eluted PVA was found to depend on that of the PVA in the gel; the average molecular weight gradually increased as the re-dissolution process was repeated, while the polydispersity indices of both the PVA from the gel and the eluted PVA decreased.     

Effects of poly(vinyl alcohol) suspension medium on critical current density of Ba2Y1Cu3 oxide superconducting filaments produced by suspension spinning method

Superconducting Ba₂Y₁Cu₃ oxide filament was produced by means of the suspension spinning method. The effects of poly(vinyl alcohol) (PVA) suspension medium on structure and critical current density (Jc) of the filament sintered were explored with PVA of various degrees of polymerization (DP) in dimethyl sufoxide (DMSO), hexamethylphosphoric triamide (HMTA) and the mixed solution of DMSO and HMTA (1 : 1). Although the PVA was removed as a volatile component by heating treatment of the filament, the dispersion behavior of the oxide powder in the filament was dependent on the suspension medium, hence the microstructure and superconducting properties of the filament obtained was affected by the suspension medium. The filament spun through the mixed solution of DMSO and HMTA had a high Jc of 77 K more than 100 A/cm² at the wide range of DP and powder content. The crystal grains of the oxide in the sintered filament grew finer with increasing the DP of PVA. On the other hand, the Jc of the filament produced by pure solution of DMSO and HMTA was not detected at DP of 3000 and 2450, respectively, due to the influence of entanglement of the molecular chains of PVA.     

Molecular weight effect on theta-gel formation in poly(vinyl alcohol)–poly(ethylene glycol) mixtures

Injectable hydrogel formulations that undergo in situ gelation at body temperature are promising for minimally invasive tissue repair. This work focuses on the investigation of injectable poly(vinyl alcohol) (PVA) and poly(ethylene glycol) (PEG) mixtures. The injectable PVA–PEG aqueous solutions form a hydrogel as temperature is reduced to near body temperature, while filling a defect in the injection site. Gamma sterilization of these solutions compromises injectability presumably due to crosslinking of PVA. We hypothesized that by modifying the PEG molecular weight and its concentration, injectability of radiation sterilized PVA–PEG hydrogels can be optimized without compromising the mechanical properties of the resulting gel. The use of a bimodal mixture of higher and lower molecular weight PEG (600 and 200 g/mol) resulted in lower PVA/PEG solution viscosity, better injectability, and higher gel mechanical strength. The PVA/bimodal-PEG had a lower viscosity at 2733 ± 149 cP versus a viscosity of 5560 ± 278 cP for PVA/unimodal-PEG (400 g/mol). The gel formed with the bimodal PEG mixture had higher creep resistance (61% total creep strain under 0.5 MPa) than that formed with unimodal PEG (84%). These hydrogel formulations are promising candidates for minimally invasive tissue repair. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Characterization of Poly(Vinyl Alcohol) and Poly(Vinyl Pyrrolidone) Co-polymer Blend Hydrogel Prepared by Application of Gamma Radiation

Hydrogel is prepared from a poly(vinyl pyrrolidone) (PVP) and poly(vinyl alcohol) (PVA) blend solution by gamma radiation with a ₆₀Co λ source at room temperature. Properties of the prepared hydrogel, such as gel fraction, gel strength swelling ratio, equilibrium water content, and water absorption in room temperature, were investigated. Blending hydrogel with PVP and PVA obviously increased the gel strength and decreased the swelling ratio of hydrogel. It was observed that the gel fraction increased while the swelling ratio and water content decreased with increased radiation dose, but gel strength increased up to a certain radiation dose and then decreased. The percentage of water absorption at room temperature increased with time but after a certain time it became steady and decreased with radiation dose.     

Study of cryostructuration of polymer systems. XIV. Poly(vinyl alcohol) cryogels: Apparent yield of the freeze–thaw‐induced gelation of concentrated aqueous solutions of the polymer

Poly(vinyl alcohol) (PVA) cryogels, which are formed as a result of freeze–thaw treatment of concentrated solutions of the polymer, were studied in respect to the amount of gel and sol fractions in these heterogeneous macroporous gel materials depending on the conditions of the thawing step of similar cryotropic gelation. It was shown that the yield of gel fraction (the efficiency of the gelation process) was not quantitative; this was controlled by the initial PVA concentration in the solution to be frozen, and to a higher extent, by the thawing rate, when the yield increased with slowing of the defrostation process. The sol fraction could be extracted from the PVA cryogels by their rinsing with water at room temperature; the extraction of the sol was accompanied with the variations of the swelling parameters of the gels (initial slight upswelling and subsequent gradual deswelling), as well as with analogous, in their character, variations of the gel strength. It was also demonstrated that at the evaluation of the fusion enthalpies of PVA cryogels with the aid of the Eldridge–Ferry equation a consideration of the values of gel‐fraction yield gave rise to the significantly higher ΔH values than in traditional cases commonly used for the thermoreversible gels, where such an yield was not taken into account. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1822–1831, 2000     

Preparation and characterization of membranes obtained from blends of acrylonitrile copolymers with poly(vinyl alcohol)

New microfiltration and ultrafiltration membranes were obtained using acrylonitrile‐vinyl acetate copolymers in mixture with poly(vinyl alcohol) (PVA). Thus, a blend polymer solution was prepared in dimethylsulfoxide (DMSO) and used to obtain bicomponent polymer membranes by phase inversion. The rheological behavior of the DMSO polymer solutions was, mostly, dilatant at low shear gradients and pseudo plastic with quasi Newtonian tendency at higher gradients. Membranes were characterized by Fourier transform infrared spectrometry (FTIR), optical microscopy, atomic force microscopy, thermal gravimetric analysis‐differential thermal gravimetry, and pure water flux (PWF). FTIR spectra displayed the characteristic bands for acrylonitrile, vinyl acetate, and PVA. The morphology and the porosity can be tailored by the preparation conditions. PVA allows controlling the size of the pores and enables, in principle, to use the resulted membranes as supports for enzyme immobilization. PVA content influences the thermal stability. PWF values depend on the copolymer, on the content in PVA, but also on the coagulation bath composition. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41013.     

Reactivity of lime and related oxides. XVII. Sulphating of lime with magnesium and aluminium sulphates

Lime water and solid hydrated lime have been sulphated with magnesium and aluminium sulphate solutions. Changes in phase composition and sulphating rates are followed from pH (glass electrode) and conductivity measurements. The crystallinities of the products have been compared with calcium sulphate dihydrate precipitated directly from solution by double decomposition and subsequently aged. The reactions with the solid hydrated lime are not appreciably impeded by the insoluble products, magnesium and aluminium hydroxides, which are very finely divided. These compounds provide additional surface on which most of the gypsum can form a gel when the gypsum solution saturation ratios have become sufficiently high. The small amounts of gypsum ultimately separating out from the lime water have crystallinity inferior to the larger amounts formed from the solid hydrated lime. Both are still inferior to gypsum from acid-sulphated lime, and are more comparable with gypsums precipitated from calcium nitrate in acidic media containing concentrations (±0.1 %) of albumen, or from calcium chloride in neutral media. Slower gypsum precipitation from calcium chloride with sulphuric acid (as in the manufacture of ‘Pearl hardening’) gives a more consistent degree of crystallinity.