Phase separation behavior in polyvinyl alcohol/ethylene glycol/water ternary solutions

In this work, the effects of mixed solvent on the phase separation behavior in PVA/EG/water ternary solutions were studied through the time-resolved l ight scattering, pulsed NMR and X-ray measurements. The Cahn-Hilliard-Cook (CHC) linear theory at the early stage of spinodal decomposition was first used to discuss the phase separation behavior in PVR solutions. The results showed that an unusual behavior is observed in this system. At the water c ontent above 30 vol% in EG/water mixed solvent, the phase separation of PVA solutions was found to yield the mechanism of spinodal decomposition us de scribed by the linear CHC theory. On the contrary, phase separation may occ ur simultaneously with crystallization and then does not correspond to the linear CHC theory at water content below 30 vol'%. The apparent diffusion c oefficient of PVA chains in solution decreases rapidly and then approaches zero at water content above 50 vol'%, implying that the phase separation of PVA solutions is very difficult to achieve in this work. Finally, the stru ctural change in gel network with water content was investigated through X- ray and H-1 pulsed NMR measurements, The X-ray result shows that the crysta llinity of PVA gels decreases with water content. The (101) diffraction pea k related to PVA crystal vanishes for the gels with water content above 30 vol%, indicating that the gelation at this condition should form a low crys talline or an amorphous gel structure. The spin spin relaxation time T-2(s) in the polymer-rich phase of gel changes discontinuously with water conten t i.e. the increment of the T-2(s) value rises rapidly at about 30 vol%. Th is phenomenon must be related to the change of chain mobility in polymer-ri ch phase with various gel structures. (C) 2000 Elsevier Science Ltd. All ri ghts reserved.