Novel cellulose esters

Research in cellulose esters in recent years has concentrated on creating a variety of novel esters with a wide range of thermal and mechanical properties. These cellulose derivatives can be used for membranes, coatings, polymer blends, and other products. Novel cellulose solvent systems (DMAc/LiCl) introduced within the last twenty years have allowed the use of several esterification techniques that could not be used effectively in the past. The combination of the new cellulose solvent systems and esterification chemistry has enabled a large variety of novel cellulose esters to be produced.

Four groups of novel cellulose esters were produced in this study, cellulose trifluoroethoxy acetate (CT), cellulose acetate trifluoroethoxy acetate (CATA), cellulose esters of long chain linear aliphatic acids, and cellulose acetate laurate (CAL). The derivatives were synthesized in a homogeneous reaction medium of DMAc/LiCl. Several esterification chemistries were used including one based on reactions with TsCl (para-tolulene sulphonyl chloride). A wide range of degree of substitutions (DS) were produced without degradation to the starting polymer using a range of stoichiometric ratios. The fully substituted mixed cellulose esters were synthesized by consecutive reactions (work-up between reactions) and by "single pot" reactions. "Single" cellulose esters, with free hydroxyls, were produced by single pot reactions.

Structure property relationships were examined by several thermal and mechanical analyses. Mechanical response was studied with dynamic mechanical thermal analysis (DMTA) and thermal mechanical analysis (TMA). Thermal response was recorded by differential scanning calorimetry (DSC) and visual analysis. The CT derivatives revealed a linear relationship between T_g and DS between a DS range of 0.6 and 3.0, but no T_m was apparent. All the products with DS > 0.6 flowed without discoloration, therefore the materials were assumed to be mainly amorphous. The CATA derivatives revealed sharp T_gs and T_ms. Surprisingly, the T_gs did not change between the CT and CATA derivatives, and the CATA's T_ms decreased linearly with increasing DS of fluoro-ester. Substituent and main chain T_gs were observed for both CT and CATA derivatives, but the substituent T_g remaining constant throughout the DS range of the flouro-esters. Surprising solubilities were also observed for both CT and CATA derivatives.

The long linear aliphatic cellulose esters (with C₁₂ to C₂₀ alkyl substituents) revealed a wide range of thermal and mechanical responses. Side chain and main chain T_gs and T_ms were observed for C₁₂ and C₁₄ derivatives. Only one broad T_m with no T_g was observed for C₁₈ and C₂₀ derivatives by DSC analysis. After annealing, multiple crystalline responses were seen for all the highly substituted C₁₂ to C₂₀ cellulose esters. CAL derivatives revealed sharp T_ms with no T_g. The crystallization rate proved to be much faster for the CAL derivatives than any other derivative in this study.