A study on the mechanism of dissolution of the cellulose/NH₃/NH₄SCN system. II.

Abstract

Abstract In a previous article, 1 we reported on the interaction of cellulose with NH 3 /NH 4 SCN by using solid state CP / MAS 13 C‐NMR, wide‐angle X‐ray and other techniques. It appears that during an imposed temperature cycling sequence, specific cellulosic inter‐ and intramolecular hydrogen bonds are broken as polymorphic conversion and, ultimately, dissolution occurs. Cellulose is converted from the polymorph I to II to III and, finally, to amorphous. We speculate that these changes proceed via transformation of the polymorph conformations of CH 2 OH from trans‐gauche, “tg,” to gauche‐trans, “gt,” to gauche‐gauche, “gg.” Remarkably, the temperature cycling effectively and rapidly effects these changes, seemingly, by invoking at two different temperatures (22°C and −78°C), a subtle but powerful temperature‐related interplay of enthalpic and entropic forces. At the higher of the cycling temperature limits, entropy and van der Waals forces dominate, causing NH 3 to partition in favor of the liquid phase. At the lower of the temperature cycling limits, enthalpy dominates and the situation is reversed favoring NH 3 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. © 1994 John Wiley & Sons, Inc.