ISSN:
0887-624X
Schlagwort(e):
ammonia/ammonium thiocyanate solvent
;
cellulose
;
dissolution mechanism
;
conformation
;
swelling
;
mercerization
;
hydrogen bonding
;
entropy
;
enthalpy
;
affinity
;
Chemistry
;
Polymer and Materials Science
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
Notizen:
In a previous article,1 we reported on the interaction of cellulose with NH3/NH4SCN by using solid state CP / MAS 13C-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 CH2OH 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 NH3 to partition in favor of the liquid phase. At the lower of the temperature cycling limits, enthalpy dominates and the situation is reversed 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. © 1994 John Wiley & Sons, Inc.
Zusätzliches Material:
3 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/pola.1994.080320204
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