Synthesis and properties of curdlan branched and linear mixed ester derivatives

Abstract

Curdlan (CD) is a high-molecular-weight bacterial exopolysaccharide with linear β-(1 → 3)-glycosidic linkages. In a previous report, we showed that CD branched esters, namely, CD isobutyrate (CDiBu) and CD pivalate (CDPi), had very high melting temperatures (Tm = ca. 340 °C) that were similar to their thermal degradation temperatures (Td), which prevented their thermal processing [J Polym Res 2017; 25:181]. To solve this problem, a series of CD branched and linear mixed esters, CD isobutyrate heptanoate (CDiBuHep) and CD pivalate heptanoate (CDPiHep), were synthesized and characterized. No significant difference in Td was observed between CD homoesters and mixed esters. Tm values were observed for CDiBuHeps, but not for CDPiHeps, indicating that CDiBuHeps and CDPiHeps were crystalline and amorphous polymers, respectively. Tm values of CDiBuHeps were controlled from 280 °C to 72 °C by adjusting the degree of substitution ratios of branched and linear carboxylic acids. These Tm values were lower than the Td, suggesting that thermal processing was applicable to these mixed esters. All films of mixed esters were colorless and highly transparent, and their mechanical properties ranged from rigid to soft and rubbery states depending on the ratio of the linear ester group.