Theory for Enzymatic Degradation of Semicrystalline Polymer Particles

In enzymatic recycling or biodegradation of semicrystalline plastic waste, crystalline spherulites embedded into an amorphous matrix hinder and slow down depolymerization. When the enzymatic depolymerization temperature exceeds the glass transition temperature, these spherulites tend to grow. The depolymerization 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 depolymerization from a material with a low initial crystallinity.