A STUDY ON THE MECHANISM OF DISSOLUTION OF THE CELLULOSE NH3/NH4SCN SYSTEM .2/

In a previous article,(1) we reported on the interaction of cellulose with NH3/NH4SCN by using solid state CP/MAS C-13-NMR, wide-angle X-ray and other techniques. It appears that during an imposed temperature c ycling sequence, specific cellulosic inter- and intramolecular hydroge n bonds are broken as polymorphic conversion and, ultimately, dissolut ion occurs. Cellulose is converted from the polymorph I to II to III a nd, finally, to amorphous. We speculate that these changes proceed via transformation of the polymorph conformations of CH2OH from trans-gau che, ''tg,'' to gauche-trans, ''gt,'' to gauche-gauche, ''gg.'' Remark ably, the temperature cycling effectively and rapidly effects these ch anges, seemingly, by invoking at two different temperatures (22 degree s C and -78 degrees C), a subtle but powerful temperature-related inte rplay of enthalpic and entropic forces. At the higher of the cycling t emperature limits, entropy and van der Waals forces dominate, causing NH3 to partition in favor of the liquid phase. At the lower of the tem perature cycling limits, enthalpy dominates and the situation is rever sed favoring NH3 partition toward the cellulose, resulting in breakage of inter- and intracellulosic H-bonds by the interpenetrating ammonia to form new H-bonds between cellulose and ammonia, leading ultimately to conformational changes and, ultimately, even dissolution. (C) 1994 John Wiley & Sons, Inc.