Solvent effect on structural change of poly(vinyl alcohol) physical gels

In this study, the relationship between the polymer-solvent interaction and the network structure of poly(vinyl alcohol) (PVA) gels prepared with organic solvents such as N-methylpyrrolidone (NMP) and ethylene glycol (EG) are investigated. The values of the intrinsic viscosity [eta] and Huggins constant k' of dilute PVA solutions indicate that the attractive interaction between PVA and NMP is higher than that between PVA and EG. The X-ray result shows that PVA-EG gels have a (101) diffraction peak of PVA crystal that appeared at about 2 theta = 19 degrees, while PVA-NMP gels only show a broad amorphous scattering peak. On the other hand, Fourier transform infrared results of PVA/EG gels also clearly show an intense peak at 1141 cm-l due to the crystalline absorption. The results of H-1 pulsed nuclear magnetic resonance show that the spin-spin relaxation time, T-2(s) and T-2(l), respectively, related to the polymer-rich and polymer-poor components decrease, and the fractional amount of the polymer-rich component, f(s), increases, while that of the polymer-poor component, f(l), decreases with an increase in the concentration of polymer. At a given concentration, the value of f(s) in the PVA-EG gel is larger than that in the PVA-NMP one. These facts indicate that the crystallinity in the PVA-EG gel is higher than that in the PVA-NMP gel, implying that the aggregation of PVA chains is much easier in the poor solvent, EG, than in the good solvent, NMP. The structural change with aging time in the PVA-EG gel is very remarkable because of the significant syneresis, indicating that the opaque PVA-EG gel with higher crystallinity has a comparatively heterogeneous and unstable network structure than the PVA-NMP gel does. (C) 1998 John Wiley & Sons, Inc.