NOVEL CELLULOSE DERIVATIVES .3. THERMAL-ANALYSIS OF MIXED ESTERS WITH BUTYRIC AND HEXANOIC ACID

Cellulose derivatives with low degrees of substitution (i.e., DS < 1.5) often fail to reveal glass transition temperatures (T-g) by virtue of their tenacious adherence to moisture, thus preventing systematic analysis of substituent effects (size and DS) on T-g and T-m transitions. On the other hand, cellulose triesters have T(m)s that decline with acyl substituent size except when the substituent size becomes very large (i.e., > C-6), and they have T(g)s within 5-20 degrees C of their T(m)s. This proximity is unusual for a semicrystalline material, and it interferes with the crystallization process that occurs between T-m and T-g. Triesters of cellulose with mixed acyl substituents (one smaller and one larger) allow not only unambiguous observation of T(g)s and T(m)s but also an adjustable Delta(T-m-T-g) window that depends upon the size and the DS of the larger substituent. The materials studied including cellulose acetate butyrate triesters (DSbbu 0.8-2.6), cellulose acetate hexanoate triesters (DShex 0-3.0), and cellulose acetate (DSac 2.44), revealed that only the mixed esters, in which the bulkier acyl group is in the range of DS 0.3-1.0, had a Delta(T-m-T-g) value in excess of 40 degrees C. Although the T-m of cellulose acetate hexanoate declined by ca. 150 degrees C per unit of DShex as DShex rose from 0 to 1, this was only ca. 25 degrees C between DShex of 1 and 3. Frequently observed dual-melt endotherms were attributed to two separate crystal morphologies. (C) 1995 John Wiley & Sons, Inc.