Synthesis and characterization of bioplastic curdlan esters with an introduced flexible carboxylic acid side chain

Curdlan, a linear β-1,3-glucan, was reacted with glutaric anhydride and heptanoyl chloride to afford thermoplastic curdlan esters (CrdE(HepGlu)) with a carboxylic acid side chain. CrdE(HepGlu) with a degree of substitution of the glutaric acid monoester moiety (DS_Glu) in the range of 0-0.58 and that of the heptanoate moiety (DS_Hep = 3 - DS_Glu) was prepared. The esterification of the hydroxy groups in the glucan skeleton effectively caused the cleavage of the interchain hydrogen bonds of curdlan and enhanced the formability of CrdE(HepGlu). Moreover, the flexible carboxylic acid side chain moderately affected hydrogen bonding. Thus, the glass transition temperature of CrdE(HepGlu), estimated by differential scanning calorimetry, increased with increasing DS_Glu. CrdE(HepGlu) with DS_Glu between 0 and 0.58 displayed high solubility in organic solvents and thermoplasticity, enabling the formation of homogeneous and free-standing films by solution casting. The mechanical properties of CrdE(HepGlu) films were evaluated by a stress-strain test, which showed that Young's modulus and the maximum stress increased with increasing DS_Glu. CrdE(HepGlu) exhibited higher mechanical strength than non-hydrogen-bonded curdlan triheptanoate and hydrogen-bonded curdlan alkylcarbamates, with thermal stability comparable to that of thermally stable curdlan esters. In addition, these properties can be regulated by controlling DS_Glu.