Effects of mixed solvent on gelation of poly(vinyl alcohol) solutions

The relationship between the polymer-solvent interaction and gelation behav ior of poly(vinyl alcohol) (PVA) solutions prepared from ethylene glycol/wa ter (EG/water) mixed solvents was investigated using a viscometer, light sc attering, FTIR, X-ray, and pulsed NMR analyses. The viscometric result show ed that the affinity to PVA for water is higher than that for EG. The light scattering result showed that the spinodal decomposition rate of the PVA s olution decreases rapidly as the water content in the EG/water mixed solven t is increased. On the other hand, the FTIR and X-ray results both indicate d that the crystallinity of the PVA gel decreases with water content. These results imply that the water molecules must improve the affinity of the so lvent to PVA to inhibit the aggregation or crystallization of PVA chains. T he pulsed NMR measurement results showed that the spin-spin relaxation time s related to the polymer-rich and polymer-poor phases of the PVA gel increa se, and the fractional amount of the polymer-poor phase increases while tha t of the polymer-rich phase decreases with increasing water content. These facts indicated that the increase in the mobility of PVA chains must give r ise to the difficulty in chain aggregation of PVA solutions with increasing water content. Two transition temperatures were found in the phase transit ion of the polymer-rich phase. The lower transition temperature was attribu ted to the destruction of the denser chain entanglements in the polymer-ric h phase and the higher transition temperature was mainly concerned with the melting of the crystallites. (C) 2000 John Wiley & Sons, Inc.