Effects of Long-Chain Acyl Substituents on the Thermoplasticity and Mechanical Properties of Paramylon Mixed Esters

Abstract

The moldability and robustness of thermoplastic paramylon mixed esters are highly related to primary structural parameters, such as the length of the long-chain acyl group and its degree of substitution. Short chain lengths and lower substitution degrees provide more robust molded thermoplastics, whereas longer chains and higher substitution degrees improve moldability. Herein, the choice of an appropriate long-chain acyl group (lauroyl or myristoyl) and the effect of the substitution degree were investigated by examining the thermal and mechanical properties of a series of paramylon acetate laurates and myristates and comparing them with those of their curdlan analogues. Melt volume flow rate measurements (180–220 °C) indicate that myristoyl substitution in paramylon results in a local thermoplasticity maximum at a substitution degree of 0.35 at higher temperatures (above ~ 200 °C). These measurements and dynamic viscoelastic analyses suggest that the myristoyl substitution serves an “internal plasticizer” in the higher strain range and higher temperature region (~ 200–230 °C), while it serves as a “viscosity increaser” in the lower strain range and lower temperature region (170–200 °C). Moreover, tensile and bending testing showed that this product exhibits superior mechanical strength. The successful realization of a good balance between moldability and robustness suggests that fine-tuning the chain length and substitution degree is an effective approach for producing thermoplastic polysaccharides with superior injection moldabilities and mechanical robustness.