Emiko Otsuka

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.

Network Microstructure of PVA Cast Gels Observed by SAXS Measurements

We report the relationship between macroscopic properties (swelling ratio, breaking stress and strain) and the network microstructure of PVA cast gel. Swelling in water solvent and drying in air were alternately repeated, where water was exchanged by new water before each swelling. The effects of the number of water exchange, N on the macro- and microscopic properties were examined in detail. As a result, the swelling ratio of PVA cast gel decreased, and both of breaking stress and strain increased with increasing N. The measurements using X-ray diffraction (XRD) indicated that the average distance between the microcrystallites decreased due to the formation of the additional microcrystallites during the repeated processes of water exchange with drying. On the other hand, the measurements using Fourier Transform Infrared (FT-IR) Spectroscopy suggested that the microcrystallites and hydrogen bonds between polymers increased during the repeated processes.

Swelling Properties of Physically Cross-linked PVA Gels Prepared by a Cast-drying Method

We report the swelling properties of physically cross-linked polyvinyl alcohol (PVA) hydrogels prepared by a cast-drying method. The swelling ratio of PVA cast gel in its swollen state decreased after repeated processes of water exchange with drying. The measurements using a Fourier Transform Infrared Spectroscopy and X-ray diffraction suggested that the hydrogen bonds were additionally formed during the process of water exchange with drying. We concluded that non-cross-linked polymers with low molecular weight eluted into the solvent by water exchange and additional hydrogen bonds were formed during the dehydration, which resulted in the decrement of the water content in the swollen state.

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).

Effect of Residual Swelling Solvent on Nanopore Formation in Replication of Swollen Hydrogel Network

Hydrogel network of poly N, N′ – isopropyl acrylamide (PNIPAM) was replicated into a silica matrix in the coexistence of PNIPAM hydrogel and solidifying preform sol of silica. During the drying process, the nanoscopic network structure was gradually immobilized in the solidifying silica matrix. The condition for the drying was shown to be highly influential on the total volume of the nanopore and pore size distribution. When the drying was carried out near the saturated vapor pressure of water in order to retard the evaporation of the contained water, the total volume and cross-sectional diameter of the nanopore as the replica of PNIPAM hydrogel were significantly larger than those in the case of normal air-drying open to air. This result indicates that the residual water enveloped PNIPAM polymer chains as a “water robe”, and as a result of that, the size and total volume of the nanopore significantly enhanced. The comparison of the formed volume of the nanopore to the water content in the preform revealed that a nonnegligible proportion over the half of the water evaporated during the retarded drying without contributing to the formation of the nanopore. Thus, only the water that remained in the hybrid of PNIPAM and silica at the stage of the entire solidification of the silica matrix could play the role of enhancing the nanopores.