Mechanism of Degradation of Starch, a Highly Branched Polymer, during Extrusion

TLDR

Starch is an ideal model for mechanistic studies of polymer degradation because its branching distribution can be measured and it only undergoes scission, making its behavior analogous to shear‑degraded droplets. The study investigates the mechanisms of starch degradation during extrusion. Starch samples with varying amylopectin and amylose content were extruded with water and glycerol plasticizers across barrel temperatures from 50 °C to 140 °C. Extrusion preferentially cleaves larger starch molecules, narrowing the size distribution toward a maximum stable size, and shows that susceptibility to shear degradation increases with branching density and short branch length—short‑chain hyperbranched polymers are especially vulnerable—while in molten state branch length is less selective but longer branches are preferentially broken in semicrystalline granular form.

Abstract

An investigation of the mechanisms of degradation of a branched polymer in extrusion was performed using starch as substrate. Starch has the advantage that the distribution of degree of polymerization of individual branches can be readily obtained using a debranching enzyme and also that it does not undergo any reaction except scission during extrusion, thereby aiding mechanistic interpretation. Various starches, containing a range of the highly branched amylopectin component and the much less branched amylose component, were extruded in the presence of water and glycerol as plasticizers with extruder barrel temperatures ranging from 50 °C at the hopper zone through to 140 °C near the die exit. Analysis by size-exclusion chromatography of both whole and debranched samples subject to various levels of extrusion showed that the extrusion degradation process involved preferential cleaving of larger molecules, while causing the size distribution to narrow and converge toward a maximum stable size. This is analogous to a similar effect of shear degradation of droplets in emulsions. It was also found that the susceptibility of polymer molecules to shear degradation is not only dependent on the size of the molecule but also extensively influenced by the branching structure. High branching density and short branch length were associated with higher susceptibility to shear degradation. This is explained by the hypothesis that a short-chain hyperbranched polymer has a relatively inflexible structure, leading to a higher susceptibility to shear scission. The degradation process is not significantly selective toward the length of individual branches when the polymer is in a molten state but it preferentially breaks longer branches when the starch polymer is in a semicrystalline granular form. These inferences are generally applicable and use the additional information from the branch length distribution and absence of side reactions, which is generally not available for synthetic polymers.