AbstractMelting 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 decreasi
展开▼
