Novel cellulose derivatives. III. Thermal analysis of mixed esters with butyric and hexanoic acid

Abstract

AbstractCellulose derivatives with low degrees of substitution (i.e., DS < 1.5) often fail to reveal glass transition temperatures (Tg) by virtue of their tenacious adherence to moisture, thus preventing systematic analysis of substituent effects (size and DS) on Tg and Tm transitions. On the other hand, cellulose triesters have Tms that decline with acyl substituent size except when the substituent size becomes very large (i.e., > C6), and they have Tgs within 5–20°C of their Tms. This proximity is unusual for a semicrystalline material, and it interferes with the crystallization process that occurs between Tm and Tg. Triesters of cellulose with mixed acyl substituents (one smaller and one larger) allow not only unambiguous observation of Tgs and Tms but also an adjustable Δ(Tm − Tg) window that depends upon the size and the DS of the larger substituent. The materials studied including cellulose acetate butyrate triesters (DSbu 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 Δ(Tm − Tg) value in excess of 40°C. Although the Tm of cellulose acetate hexanoate declined by ca. 150°C per unit of DShex as DShex rose from 0 to 1, this was only ca. 25°C between DShex of 1 and 3. Frequently observed dual‐melt endotherms were attributed to two separate crystal morphologies. ©1995 John Wiley & Sons, Inc.