Oxidative Extrusion of Polypropylene Melts: Impact of Chain-End and In-Chain Functionalization on Thermal Properties, Crystal Structure, and Crystallization Kinetics

The melt oxidation of isotactic polypropylene (iPP) was used to impart functionality relevant for upcycling and repurposing the polymer for new applications. Terminal vinylidene groups (due to chain scission) and oxidized functional groups such as ketones, aldehydes, and hydroxyl groups were present in the melt oxidized samples, forming PP copolymers with functional co-repeat units. Two regimes were identified: a scission regime that occurred in which chain molecular weight was reduced with only TVD functionality added to the chain end, and an oxidation regime, in which molecular weight changes were small and the level of in-chain functionalization increased more dramatically. The oxidation regime, which created in-chain defects that disrupted the isotactic order of the chains, brought about more substantial changes in melting, crystallization, and glass transition temperatures as well as crystallinity as compared to the scission regime (with chain-end defects). While the unit cell of the iPP crystals was unchanged, the lamellar long period and thickness were reduced through oxidation due to the presence of in-chain defects. The reduction in crystallinity and thermal transitions (melting, crystallization, and glass transition temperatures) went well beyond that which could be accounted for based on molecular weight changes and therefore was driven by increased functionalization of the chains. Both nucleation and growth were slower for the oxidized samples, presumably due to the presence of in-chain defects. This study provides key insights into distinct regimes in melt functionalization under which chain scission and oxidation dominate, as well as their impact on crystal structure, thermal transitions, and crystallization kinetics, essential features for future applications of the materials.