Theory for enzymatic degradation of semi-crystalline polymer particles

TL;DR

This paper develops a geometric model to understand enzymatic degradation of semi-crystalline polymers, predicting degradation yield and kinetics by considering competition between amorphous erosion and spherulite growth, influenced by additives and impurities.

Contribution

It introduces a novel extension of Voronoi tessellation to model the competition between amorphous erosion and spherulite growth during enzymatic polymer degradation.

Findings

Model accurately predicts degradation yield and kinetics.

Nucleating agents and impurities can significantly hinder depolymerisation.

The approach provides insights into pretreatment strategies for polymer recycling.

Abstract

In enzymatic recycling or biodegradation of semi-crystalline plastic waste, crystalline spherulites embedded into an amorphous matrix hinder and slow down depolymerisation. When the enzymatic depolymerisation temperature exceeds the glass transition temperature, these spherulites tend to grow. The depolymerisation process is thus controlled by a competition between erosion of the amorphous matrix from the particle surface and the growth of recalcitrant spherulites within the particle bulk and at its surface. We present a geometric model that captures this competition, together with an algorithm to solve the equations numerically. Our algorithm introduces a new extension of Voronoi/Delaunay tessellation in space. We extract the parameters for the model from experimental data on the enzymatic depolymerization by hydrolase LCC-ICCG of PET bottle flakes and textile waste, in order to make a prediction of the observed degradation yield as a function of time. Both the final yield and the degradation kinetics are correctly predicted. Most importantly, the model clarifies how and to which extent nucleating agents, impurities, additives, and/or rapid crystal growth present in the waste can undermine pretreatment efforts aiming to initiate depolymerisation from a material with a low initial crystallinity.