NMR AND X-RAY STUDIES ON THE MORPHOLOGY OF THERMOREVERSIBLE POLYACRYLONITRILE GELS

The morphology of thermoreversible polyacrylonitrile-propylene carbonate (PAN-PC) gels was examined using solid-state carbon-13 nuclear magnetic resonance (NMR) spectroscopy and x-ray diffraction. Following complete dissolution of the polymer at elevated temperature and cooling of the concentrated PAN-PC solutions, a gel was formed. The PAN-PC gels consisted of regions of mobile polymer chains, rich in PC, ''cross-linked'' by regions of rigid polymer. The mobile regions of the gels showed solution-type NMR spectra with resolution of tacticity effects. The rigid component detected by NMR would correspond to the crystallites detected previously by x-ray diffraction. Wide-angle x-ray diffractograms of the gels showed different peaks when compared with the dry polymer powder. After solvent extraction and drying of the gel, the diffractogram reverted to that of the original dry powder. This new result is the strongest evidence to support the view advanced earlier that the new peaks found in the diffraction pattern of the wet gels arises from solvated polymer crystallites rather than from ordinary polymer crystallites. (C) 1993 John Wiley & Sons, Inc.