Thermomorphological and mechanical properties of vulcanized octenyl succinate/terpenoid-derivatized corn starch composites

Abstract

Modification of food starches with octenyl succinic anhydride (OSA) is a well-established protocol to endow the starches with olefin moieties that can subsequently undergo traditional polyolefin crosslinking and polymerization reactions. In the current work, olefin-modified starch derivatives are crosslinked with sulfur, through reactions mechanistically similar to vulcanization of the olefin moieties in rubber, thus yielding the analogous vulcanized starch composites. Given the importance of the olefin moieties in the vulcanization mechanism, OSA-modified corn starch was further derivatized by esterification with geraniol, a terpenol diene. The resulting geraniol-esterified starch (GES) thus contains three olefin moieties per esterified site, affording double the number of crosslinkable-carbon atoms compared to typical OSA-modified starches. GES was reacted with elemental sulfur to generate composites GSS_x (where x = wt% sulfur, either 90 or 95 in this work). The thermomorphological and mechanical properties of GSS_x were assessed by thermogrametric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy with element mapping by energy-dispersive X-ray analysis (SEM-EDX), mechanical test stand analysis, and dynamic mechanical analysis (DMA) and compared to those of previously-reported composites of OSA-modified starch and elemental sulfur (OSS_x). These tests revealed that GSS_x underwent significant intramolecular crosslinking to form particles that were well-dispersed in the sulfur-rich networks of the composites. As a result, GSS_x materials behaved as particle-reinforced network materials that deformed entirely plastically before breaking under a compressional stress. In contrast, OSS_x composites exhibited longer crosslinking chains and more interaction between domains. The result of these morphological differences confers upon GSS_x lower stiffness and greater strength relative to their OSS_x analogues.