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

The morphology of thermoreversible polyacrylonitrile-propylene carbona te (PAN-PC) gels was examined using solid-state carbon-13 nuclear magn etic resonance (NMR) spectroscopy and x-ray diffraction. Following com plete dissolution of the polymer at elevated temperature and cooling o f the concentrated PAN-PC solutions, a gel was formed. The PAN-PC gels consisted of regions of mobile polymer chains, rich in PC, ''cross-li nked'' by regions of rigid polymer. The mobile regions of the gels sho wed solution-type NMR spectra with resolution of tacticity effects. Th e rigid component detected by NMR would correspond to the crystallites detected previously by x-ray diffraction. Wide-angle x-ray diffractog rams of the gels showed different peaks when compared with the dry pol ymer powder. After solvent extraction and drying of the gel, the diffr actogram reverted to that of the original dry powder. This new result is the strongest evidence to support the view advanced earlier that th e new peaks found in the diffraction pattern of the wet gels arises fr om solvated polymer crystallites rather than from ordinary polymer cry stallites. (C) 1993 John Wiley & Sons, Inc.