Effect of addition of saccharose on gelation of aqueous poly(vinyl alcohol) solutions

The properties of poly(vinyl alcohol) (PVA) hydrogels containing saccharose were examined. The effect of the addition of saccharose to atactic PVA (α-PVA) gels on their melting temperatures was larger than that for syndiotacticity-rich PVA (s-PVA) gels and the melting temperature was above 100°C for α-PVA gels with saccharose contents of 60 wt %. However, the fusion enthalpy (ΔH) of the α-PVA gels was at most 100 kJ/mol. The release of solvent (water/saccharose) from gels in air decreased with an increase in the saccharose content and the equilibrium was achieved after standing for 20 days for the α-PVA and s-PVA gels with saccharose contents above 40 and 20 wt %, respectively. © 1995 John Wiley & Sons, Inc.     

Gelation of poly(vinyl alcohol) solutions at low temperatures (20 to −78°C) and properties of gels

The flow points of atactic poly(vinyl alcohol) (a-PVA) gels with H₂O/dimethyl sulfoxide (DMSO) = 90/10 (v/v) chilled at 20 to ?78°C for 24 h depended on the chilling temperature and were 0–30°C for gels with the initial polymer concentrations (C_i) of 2–5 g/dL, whereas those for H₂O/DMSO = 50/50 chilled at 0 to ?78°C were independent of the chilling temperature and were 70–75°C. Syneresis occurred after eight cycles of freezing (?24°C) and thawing (20°C) for a-PVA hydrogels at concentrations above C_i = 4 g/dL and two such cycles for syndiotacticity-rich PVA (s-PVA) hydrogels at concentrations above C_i = 1 g/dL. The extent of syneresis per one cycle for s-PVA hydrogels was higher than that for a-PVA hydrogels at the initial cycles. In the a-PVA hydrogels with an initial polymer concentration of ca. 30 g/dL, syneresis was expected not to occur even after 20 cycles. If all the free water in the gels is assumed to have transuded by syneresis after 20 cycles, the residual water is bound water and is estimated to be six water molecules per one vinyl alcohol monomer unit. © 1994 John Wiley & Sons, Inc.     

Water stability of syndiotactic poly(vinyl alcohol)/iodine complex film

The desorption behavior of iodine in syndiotactic poly(vinyl alcohol) (s-PVA) with a syndiotactic diad content of 63.1% iodine film in water was investigated in relation to the solubility of s-PVA in water. Despite a low number-average degree of polymerization of 900, the degree of solubility of s-PVA film in water at 80°C was limited to about 10%, whereas atactic poly(vinyl alcohol) (a-PVA) film was completely soluble at a lower temperature, 50°C. The degree of iodine desorption of s-PVA/iodine film at a high temperature decreased remarkably compared with that of a-PVA/iodine film. The fastness of iodine of the drawn s-PVA/iodine film was far superior to that of the undrawn film. Over a draw ratio of 5, the iodine desorption was suppressed significantly with increasing drawing temperature rather than the draw ratio. In addition, the iodine desorption of postdrawn s-PVA/iodine film was lower than that of predrawn s-PVA/iodine film. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 108–113, 2001     

Properties of syndiotactic-rich poly(vinyl alcohol) thin film in water. VII. Elastic behavior of swollen film in water through heating/cooling or cooling/heating cycles

The thermoelastic behavior for highly swollen films of syndiotactic-rich poly(vinyl alcohol) (s-PVA) was ascertained under loads in water through heating/cooling or cooling/heating cycles (in the range of 25–70°C). The s-PVA films kept at the high temperature of 70°C behaved as a perfect elastomer through the cooling/heating cycle and had very low Young's modulus, 2–3.5 × 10⁶ dyn/cm². When highly swollen s-PVA films were kept at lower temperatures below about 50°C for a long time, microcrystals were formed or propagated in the s-PVA films and plastic deformation occurred in addition to elastic deformation through heating process. The microcrystalline growth and propagation at lower temperatures were supported by an increase in Young's modulus and a decrease in the molecular weight between junctions.     

Effect of thermal hysteresis on melting of syndiotacticity-rich poly(vinyl alcohol) hydrogel

The melting temperatures (T_M) of hydrogels prepared by chilling aqueous solutions of syndiotacticity-rich poly(vinyl alcohol) (s-PVA) at 0°C were measured rising temperature of gels from the initiative temperature (T_I) of 0–70°C (every 10°C). The apparent enthalpies of fusion of a junction ΔH's were estimated from the relation between the logarithm of polymer concentration (log C) and 1/T_M. ΔH depended on T_I, showing that the melting point of gels depended on a thermal hysteresis. The highest polymer concentration C_H in those of the gels which have no melting point above an initiative temperature was determined and ΔH was estimated from the relation between log C_H and the reciprocal melting point of the gels with C_H, 1/T_IM. The ΔH was 15.1 kJ/mol in the range of higher polymer concentrations and 43.9 kJ/mol in the range of lower concentrations.     

Gelation of poly(vinyl alcohol)/water/dimethylformamide solutions and properties of dried films

The properties of solutions for syndiotacticity-rich poly(vinyl alcohol) (s-PVA)/dimethylformamide (DMF)/water systems, the gelation of the s-PVA solutions, and the properties of the dried s-PVA gel films were examined. From the results of the dissolution temperature of the polymer, the gelation temperature of the solutions, the melting temperature of the gels, and the compressive modulus of the gels, the solubility of the polymer was the highest at DMF contents of 10–20 vol %. The maximum dynamic tensile modulus of the drawn (×18) films obtained from the dried gel films with a DMF content of 10 vol % was 54.9 GPa at 20°C. The orientation of the polymer chains in the amorphous regions was higher than that in the crystalline regions. The orientation of the polymer chains in the amorphous regions for the drawn films with a DMF content of 10 vol % was higher than that for the drawn films with a DMF content of 60 vol %. The orientation of the polymer chains in the amorphous regions was considered to play an important role in the properties of high strength and high modulus. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1661–1667, 1998     

Gels of syndiotacticity-rich poly(vinyl alcohol)–water/dimethyl sulfoxide or – water/ethylene glycol solutions

Gels of syndiotacticity-rich poly(vinyl alcohol) (s-PVA) in mixed solvents of water/dimethyl sulfoxide (DMSO) or water/ethylene glycol (EG) were made by chilling at the temperatures of 0–70°C from those solutions with the polymer concentrations below 10g/dL. The melting points of the gels were measured warming the gel from the gelling temperature (T_gel) at a constant heating rate. The apparent enthalpy of fusion of a junction of gel, ΔH was estimated from the relation between the apparent melting temperature and the polymer concentration. The s-PVA gels made from the mixtures of the water/lower contents of DMSO or EG had a minimum at lower T_gel and a maximum ΔH at a higher T_gel. On the other hand, the s-PVA gels made from the mixtures of the water/higher contents of them had nearly a maximum ΔH at a higher T_gel. From those results, it was considered that the former gels received a high thermal history while the latter gels received only slight thermal history.     

Effect of addition of saccharides on gelation of aqueous poly(vinyl alcohol) solutions

The properties of poly(vinyl alcohol) (PVA) hydrogels containing saccharides (D ‐xylose, D ‐fructose, D ‐glucose, and maltose) were examined. The effect of the addition of saccharides to PVA hydrogels on their melting temperatures was remarkable when the gels were chilled at 0°C with saccharide contents above 40 g/dL. Particularly, the melting temperature was the highest for PVA hydrogels with glucose and above 73°C at the polymer concentrations above 6 g/dL. Namely, the enthalpy of the thermal dissociation of the junctions of the spatial network ΔH was the highest of the four saccharides (glucose > fructose > maltose > xylose) and 150 kJ/mol for the hydrogels with the glucose content of 60 g/dL. The uniform preservation of saccharides and water in their gels were the highest for the gels with fructose during standing for a long time in air after freeze‐drying. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1298–1303, 1999     

Gelation of a new hydrogel system of atactic‐poly(vinyl alcohol)/NaCl/H2O

a‐PVA/NaCl/H₂O hydrogels have been prepared by gelation of aqueous atactic‐poly(vinyl alcohol) (a‐PVA) solutions in the presence of NaCl. The gelling temperature, melting temperature and the preservation of water of the hydrogels have been measured. The effect of the addition of NaCl to the hydrogels on gelling temperature and melting temperature is marked when the NaCl concentration is over a certain value. High NaCl concentration favours high‐melting‐point hydrogels. When the NaCl concentration is high enough (such as 11%), high‐melting‐point and white turbid opaque a‐PVA/NaCl/H₂O hydrogels can be prepared, regardless of the PVA concentration. Similarly, the low gelling temperature of a‐PVA/NaCl/H₂O solutions comes from low NaCl concentration, while high gelling temperature ranges from 50 to 70 °C when the NaCl concentration is 11%. In appearance, the types of syneresis of a‐PVA/NaCl/H₂O are χ‐type or a mixture of χ‐ and n‐types; water release of the hydrogels is slowed down by the addition of NaCl. © 2002 Society of Chemical Industry     

Properties of hydrogels of atactic poly(vinyl alcohol)/NaCl/H2O system and their application to drug release

In order to prepare cost‐effective physically cross‐linked hydrogels including food salt sodium chloride, samples, were prepared with various concentrations of NaCl and respective atactic poly(vinyl alcohol) (a‐PVA), and were evaluated. It had been observed that hydrogels containing NaCl concentration (9–11 wt%) along with a‐PVA concentration 9–5% respectively exhibited higher melting points (91.5–95.1 °C). A higher melting point characterizes the hydrogel composition of a system like a‐PVA(7%)/NaCl(11%)/H₂O. The swelling degree of this hydrogel was found to be comparatively better at 37 °C than at any other temperature studied here. However, irregular Fickian swelling was found at this temperature. The UV light absorption maximum at 362–364 nm and minimum at 351 nm for this hydrogel had been found as evidence of physical cross‐linking. A drug, theophylline was loaded by solvent‐sorption and feed‐mixture dissolving methods. The feed‐mixture dissolving method is better than solvent sorption because of high drug loading, comparatively low fraction release rate and more sustained‐release of drug than that of solvent‐sorption. Theophylline was released twice as fast from the hydrogel after solvent‐sorption drug loading (3 h) than from that which used the feed‐mixture dissolving method (6.5 h). Theophylline‐loaded hydrogels of this system (feed‐mixture dissolving) were then prepared at high temperature (60 °C) thawing for 6 h followed by chilling at 0.4 °C for 3 h as one cycle. And the drug release behaviour and mass transfer were found almost the same as for chilling (24 h at 0.4 °C)–thawing (48 h at room temperature). Drug release behaviour was studied as apparently irregular Fickian diffusion (Higuchi Matrix Dissolution Model). © 2002 Society of Chemical Industry     

Mechanical denaturation of high polymers in solutions. XXXIX. Flow-induced crystallization of high molecular weight syndiotactic-rich poly(vinyl alcohol)

The crystallization of syndiotactic-rich poly(vinyl alcohol) (s-PVA) with a molecular weight of 6.73 × 10⁵ from aqueous solutions was carried out under Couette flow with or without seed crystals. The crystallization of s-PVA from aqueous solutions occurred below the surface of the solutions even in the presence of seed placed in the solution; whereas, in the case of aqueous s-PVA solutions containing 0.02% octyl alcohol (antifoaming reagent) only the longitudinal growth of seed occurred. The lengths of the fibers obtained were 2.5 to 3.0 cm.     

Development of gelatin/chitosan/PVA hydrogels: Thermal stability,water state,viscoelasticity, and cytotoxicity assays

Gelatin/chitosan/poly(vinyl alcohol) (PVA) hydrogels were fabricated with different polymer ratio using the freeze-drying process. The thermal stability, water state, rheological, and cytotoxic properties of the hydrogels were evaluated. Thermogravimetric/differential scanning calorimetry analyses showed a decomposition onset temperature below 242.7 ± 2.7 °C. The samples did not show statistical differences (p < 0.05) on the onset temperature values. Nonfreezing water reached a constant value around of 1 g water/g polymer. Freezing water increased linearly with the increase of the water content independently of the polymer ratio. The hydrogels showed an equilibrium water content from 9 to 13 times their mass. The hydrogels exhibited a solid-viscoelastic behavior. The elastic modulus was higher with the increase of chitosan concentration (G′ = 22 170 ± 85 Pa) independently of the temperature (5–55 °C). In vitro assay showed that hydrogels are nontoxic in the HT29-MTX-E12 cell line. These results indicate that the gelatin/chitosan/PVA hydrogels could be considered for biomedical applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47149.     

Interaction of self‐nucleation and the addition of a nucleating agent on the crystallization behavior of isotactic polypropylene

The relationship between heterogeneous or homogeneous nucleation and self‐nucleation of polypropylene (PP) and PP nucleated by an organic phosphate salt (PPA) was studied by DSC. For pure PP, it homogeneously nucleated during cooling after melting at the selected temperature (T_s) of 170–200°C for 3 min, but at the T_s of 160–168°C self‐nucleation occurred; PPA only nucleated heterogeneously at the T_s of 168–200°C, and there existed self nucleation at the T_s of 160–168°C. The double melting peaks of PP and PPA at the T_s of 162°C were observed. Once the self‐nucleation occurred, the change of the crystallization temperature and heat of fusion of PP is more significant than that of PPA with the change of the T_s, depending upon the crystallization conditions. Results were explained by homogeneous nucleation, heterogeneous nucleation, self‐nucleation, and annealing crystallization. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 78–84, 2001     

Physicochemical evaluation of nanocomposite hydrogels with covalently incorporated poly(vinyl alcohol) functionalized graphene oxide

To avoid the negative effect of graphene oxide (GO) nanosheets aggregation in aqueous solutions on physicochemical properties of GO incorporated nanocomposite hydrogels, poly(vinyl alcohol)-functionalized GO (GO-es-PVA) are synthesized and are used for preparation of nanocomposite hydrogels. By graft copolymerization of GO-es-PVA with poly(AA-co-AAm) chains, the nanocomposite hydrogel samples with covalently incorporated GO-es-PVA are achieved. FTIR spectroscopy, XRD analysis, and SEM and EDAX techniques confirm successful synthesis process. It is clear that GO-es-PVA content has significant effect on physicochemical properties of nanocomposite hydrogels, such as improvement of the water uptake properties, porosity, and gel strength. The hydrogel sample with 1:80 mass ratio of GO-es-PVA/AAm has the best physicochemical properties due to the optimum amount of GO-es-PVA, which gives the hydrogel proper viscoelasticity as well as fine porosity and water uptake rate. Interpenetration of PVA chains into the polymeric networks makes the movement of the polymer chains easier, which leads to softer polymeric networks. This phenomenon is called plasticizing effect. The plasticizing nature of PVA and its high hydrophilicity are the main reasons for the fine physicochemical properties. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48025.     

Annealing effect on the microstructure of poly(vinyl chloride)

The effect of annealing on the microstructure of commercial grade poly(vinyl chloride) was investigated by calorimetric, X-ray and viscoelastic measurements. The degree of crystallinity increases with increasing annealing temperature from above the glass transition temperature up to 130°C, at which point the degree of crystallinity takes on a maximum value. Also, the crystal melting temperature increases with increasing annealing temperature. Thermal analysis and X-ray study suggest that the crystallite of poly (vinyl chloride) decomposes by thermal degradation when annealed, above 170°C. The isothermal crystallization process is analyzed using Avrami's equation employing the degree of crystallinity as a function of annealing time at various annealing temperatures. The crystallization rate has a maximum value at around 140°C. It is expected that the crystalline texture grows in the shape of a lineal-like habit, judging from the magnitude of Avrami's constant and from a study of the X-ray intensity distribution. The α_f-transition was observed to occur at temperatures 5 to 10°C lower than the crystalline melting temperatures for annealed specimens of poly(vinyl chloride) using a dynamic spring analysis. The α_f-transition may be attributed to thermal molecular motions with a long time scale, resulting from the cross-link points introduced by the small crystallites.     

Swelling behavior of poly(acrylamide‐co‐N‐vinylimidazole) hydrogels under different environment conditions

A series of random copolymers of acrylamide and N‐vinylimidazole, poly(AAm‐co‐NVI), with various compositions were prepared using redox copolymerization. The influence of environmental conditions such as pH, temperature, and ionic strength on the swelling behavior of the copolymeric hydrogels was investigated. The hydrogels exhibited the highest equilibrium swelling in basic medium at high temperature. Equilibrium swelling decreased with rising ionic strength at pH 5.0. As pH increased, equilibrium swelling of the hydrogels increased at pH 11.0 and I = 0.20 M. Swelling kinetics of the hydrogels was found to be non‐Fickian at 25°C. The process tended to be Fickian at higher pH and temperature. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1783–1788, 2005     

Rheology of low nematic transition temperature thermotropic liquid crystalline copolyester HBA/HQ/SA

The rheology of a liquid crystalline copolyester of hydroxybenzoic acid, hydroquinone, and sebacic acid (HBA/HQ/SA copolyester) was studied on both a rotational and a capillary rheometer. DSC studies show that the copolyester has a crystalmesophasic and a broad mesophasic-isotropic transition at 170°C and 220°C. Optical texture observations show the mesophase is characterized by line defect textures, which are characteristic of a nematic structure. At 220°C, both isotropic and nematic phases coexist with the latter being the major. As temperature reaches 250°C, a clear dominance of isotropic phase is observed. At this temperature, the nematic phase of irregular shapes randomly disperses within the isotropic matrix. Subsequent rheological studies were thus conducted in crystal/nematic biphase, single nematic phase, nematic/isotropic biphase, and the near single isotropic phase. Dynamic strain sweep measurements show that a linear viscoelastic region exists at all temperatures tested. The maximum strain amplitude for the linear viscoelastic region is found to be highly structure dependent; it is > 100% in the nematic phase, ∼20% in the biphases, and only about 5% in the isotropic phase. The concurrence of curves obtained at different temperatures in a Cole-Cole plot of G′ vs. G″ indicates similar structures in the nematic phase and biphases. Measurements of steady shear viscosity using a rotational rheometer and a roundhole capillary rheometer show that in the nematic phase the copolyester behaves as a shear thinning fluid for a wide shear rate range of 1 ∼ 10,000 s⁻¹, in which the power law index is about 0.6 ∼ 0.8, and the viscosity is < 10 Pa.s at shear rates >1 s⁻¹.     

Synthesis and Characterization of Temperature-Sensitive Poly(N-isopropylacryamide) Hydrogel with Comonomer and Semi-IPN material

A series of temperature and pH sensitive hydrogels were synthesized using N-isopropylacrylamide (NIPAAm) as main monomer, sodium alginate (SA) as semi-IPN material, ethyl acrylate (EA) and acrylic acid (AA) as comonomer, and N-maleyl chitosan (N-MACH) as cross-linker. The temperature and pH sensitive behavior, swelling/deswelling kinetics of the hydrogels were investigated. And the mechanism of the phase transition was summed up. Sodium alginate/Poly(N-isopropylacryamide) semi-interpenetrating polymer network (SA/PNIPAAm semi-IPN) hydrogels exhibited a lower critical solution temperature (LCST) at about 32 °C with no significant deviation from the conventional PNIPAAm hydrogels. Poly(N-isopropylacryamide-co-ethyl acrylate) (P(NIPAAm-co-EA)) hydrogels exhibited LCST at 29–31°C, increasing the amount of EA in the hydrogel gradually decreased the LCST. Poly(N-isopropylacryamide-co-acrylic acid) [P(NIPAAm-co-AA)] hydrogels exhibited LCST at 34–39°C, with decreasing NIPAAm/AA from 96/4 to 92/8 and 90/10, the LCST increased from 34°C to 37°C and 39°C. In the swelling/deswelling kinetics, all the dried hydrogels exhibited fast swelling/deswelling behavior, which might be attributed to macroporous structures of the hydrogels.     

Microstructure and mechanical properties studies of poly(vinyl alcohol)–lead salts complexes

Poly(vinyl alcohol) (PVA) was found to form complexes with PbCl₂, Pb(NO₃)₂, and Pb(CH₃COO)₂. Different complex compositions were prepared by casting technique. Microdomains were observed in the case of the PbCl₂-PVA system. Complexes of PbCl₂-PVA system are somicrystalline over the entire composition range. PbCl₂-PVA complexes are stable to temperatures greater than 350°C in comparison to 225°C of pure PVA. The general features of the microdomains depends upon the temperature and the salt concentration. The hardness properties of the binary mixture were found to change in a nonlinear mode exhibiting a maximum value with the increase of the PbCl₂ content and in a linear mode for the other salts. © 1995 John Wiley & Sons, Inc.     

Terpolymers. III. Isochronal viscoelastic parameters for terpolymers of vinyl stearate,vinyl acetate,and vinyl chloride

Isochronal viscoelastic parameters were collected for many of the copolymers, terpolymers, and diluent mixtures whose mechanical properties at ambient temperatures were reported in the preceding paper. In the polymeric systems, vinyl stearate, acting as the primary internal plasticizer, was introduced into terpolymers by displacing vinyl acetate from base copolymers of vinyl acetate and vinyl chloride, across the range of composition. In the diluent mixtures, poly(vinyl chloride) was plasticized by di-2-ethylhexyl phthalate across the range of compositions. For direct comparison with the mixtures, vinyl chloride was plasticized by copolymerization with vinyl stearate across the same range of compositions. Moduli for the co- and terpolymers reached the low values characteristic of soft materials at room temperature only through a short range of vinyl stearate composition. At higher internal plasticizer compositions, side-chain crystallization stiffened the samples and raised their moduli. In contrast, moduli for the mixtures decreased steadily with increase in diluent at ambient temperature. The effective use temperature ranges were narrow for the co- and terpolymers but broad for the mixtures. Curve broadening was similar for both types of systems, but reached a maximum at about 40 weight-% plasticizer for the diluent mixtures. The slopes of the glassy modulus with decreasing temperature at 50°C below T_g for the vinyl stearate copolymers were relatively large. However, moduli close to that of poly(vinyl chloride) were reached only near the temperature range associated with the γ-transition. Consequently, this behavior was attributed to motions of the side chains in the glassy matrix. Room temperature moduli, which could be obtained before the onset of melting, were correlated with the fractional side-chain crystallinity for polymers having a high vinyl stearate content. From this relation, the modulus for the hexagonal crystal form of the side-chain crystallites of poly(vinyl stearate) was estimated to be 1.2×10¹⁰ dynes/cm². Moduli for the glassy amorphous phase of this same polymer appeared to have one sixth of this value at 40°C below the glass transition. The glass transition temperature occurred about 10° below the inflection temperature at 10⁹ dynes/cm², as an average for all of the systems studied.