Yumiko Hirashima

Effects of preparation temperature on swelling and mechanical properties of PVA cast gels

Poly(vinyl alcohol) cast gels (PVA cast gels) were prepared at different gelation temperatures, Tgel, and the swelling ratio and mechanical properties were characterized by the equilibrium weight measurement, a uniaxial loading test, a tearing force measurement, and a cyclic compression test at room temperature. The results showed that the swelling ratio decreased and both the breaking stress and the tearing energy increased with an increase in Tgel. The compression deformation energy of 50% reduction of its original size increased monotonically, while the hysteresis loss was not affected by Tgel. To understand these macroscopic properties, the network structures of the samples were examined by X-ray diffraction and Fourier transform infrared measurements. As a result, the size of the microcrystallites increased with an increase in Tgel and the average distance between the microcrystallites also increased. These results suggested that the structures of PVA networks, crosslinked by the microcrystallites, have essential roles in determining the mechanical properties of PVA cast gels. The macroscopic mechanical properties are discussed on the basis of the size, number, and distribution of microcrystallites as crosslinkers and the homogeneity of amorphous PVA networks, which were determined at gelation.

Elution of polymers from physically cross-linked poly(vinyl alcohol) gels

Elution of polymers from physically cross-linked poly(vinyl alcohol) (PVA) gel in its swollen state was investigated by measuring quantitatively the total carbon in the outer water using a Total Organic Carbon (TOC) analyzer. The dried samples, prepared by a cast-drying method, were immersed in pure water under different conditions. As a result, the elution was detected, and it gradually stopped in a limited amount of water. On the other hand, the polymer was successively released by exchanging the outer water repeatedly at long intervals (more than 21 h), which was suppressed by adding a drying treatment at each water exchange. Effects of the degree of polymerization of PVA on the elution ratio were also presented.

Roles of Hydrogen Bonding on the Volume Phase Transition of Ionized Poly(N-isopropylacrylamide) Gels

We report the volume phase transition of poly( N -isopropylacrylamide-co-sodium acrylate) gel. The diameter decrement was observed under the continuous water flow (or exchange) for the cylindrical ...

Salt effects on formation of microcrystallites in poly(vinyl alcohol) gels prepared by cast-drying method

Abstract The preparation method of poly(vinyl alcohol) (PVA) gels by a simple cast-drying technique using salt solutions was reported in the present investigation. The effects of the salt concentration on the macro- and microscopic properties were examined in detail, and the relationships between macroscopic properties, such as swelling ratio, mechanical strength, and the network microstructure were presented. It was found that the swelling ratio of PVA cast gel increased largely by adding a small amount of salt, and decreased rapidly, hereafter it increased with increasing the salt concentration above 2% (mass fraction). The changes in the breaking stress and strain corresponded to the changes in the swelling ratio. The macroscopic properties were found to be correlated with the microcrystallites and hydrogen bonds, which were characterized by the measurements using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR).

Irreversible Swelling Behavior and Reversible Hysteresis in Chemically Crosslinked Poly(vinyl alcohol) Gels

We report the swelling properties of chemically crosslinked poly(vinyl alcohol) (PVA) gels with high degrees of polymerization and hydrolysis. Physical crosslinking by microcrystallites was introduced in this chemical PVA gel by a simple dehydration process. The equilibrium swelling ratio was measured in several mixed solvents, which comprised two-components: a good solvent (water or dimethyl sulfoxide (DMSO)), and a poor organic solvent for PVA. In the case of aqueous/organic solvent mixtures subjected to a multiple-sample test, the swelling ratio decreased continuously when the concentration of the organic solvent increased, reaching a collapsed state in the respective pure organic solvents. In the case of DMSO, starting from a swollen state, the swelling ratio rapidly decreased by between 15 and 50 mol % when the concentration of the organic compound increased in a single-sample test. To understand the hysteresis phenomenon, the swelling ratio was measured in a DMSO/acetone mixed solvent, starting from a collapsed state in acetone. The reversibility of swelling in response to successive concentration cycles between DMSO and acetone was examined. As a result, an irreversible swelling behavior was observed in the first cycle, and the swelling ratio in acetone after the first cycle became larger than the initial ratio. Subsequently, the swelling ratio changed reversibly, with a large hysteresis near a specific molar ratio of DMSO/acetone of 60/40. The microstructures were confirmed by Fourier transform infrared spectroscopy during the cycles. The irreversible swelling behavior and hysteresis are discussed in terms of the destruction and re-formation of additional physical crosslinking in the chemical PVA gels.