Structure-property relationships of novel cellulose esters

Abstract

Cellulose “waxy” (long-chain aliphatic) tri-esters of lauric (C₁₂), myristic (C₁₄), stearic (C₁₈), and eicosanoic (C₂₀) acids; mixed esters of lauric and acetic acids; and fluorinated mixed esters of 2,2,2-trifluoroethoxyacetic (TFEA) and acetic acids were synthesized in homogeneous DMAc/LiCl solution. Degree of substitution (DS) was conveniently controlled via stoichiometry, and fully-substituted cellulose derivatives were produced with little depolymerization. Thermal analysis of the C₁₂ - C₁₄ waxy tri-esters revealed separate melting transitions (T_m,L and T_{m, H}) for ester substituents and main cellulose chains, providing evidence for the existence of a separate (crystalline) phase formed by the ester substituents. Main-chain T_mS of waxy esters converged with the melting transition of the side-chain phase and/or disappeared as substituent length reached C₁₈. The effectiveness of melting point depression at low DS of large substituents increased with increasing substituent length for n-alkyl substituents up to lauryl (C₁₂); at mid-DS of large substituent, increasing the substituent length had no further effect in melting point depression above a length of C₆ (hexanoyl). The preferential sorption of ethanol from aqueous solution by cellulose (CE) and cellulose acetate trifluoroethoxyacetate (CATA) of varying DS was measured by liquid chromatography (LC); the preferential sorption of ethanol did not change when acetate substituents were replaced by trifluoroethoxyacetate substituents. The preferential sorption of ethanol by unmodified cellulose was stronger than the preferential sorption by cellulose acetate and trifluoroethoxyacetate esters.