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.