Star-to-Bottlebrush Transitional Behavior of Model Graft Polymers in Fast Extensional Flows

Polylactide (PLA) is a bio-based, industrially compostable polymer and represents one of the most prevalent sustainable materials in the plastics industry. PLA has poor melt strength (i.e., extensional viscosity), which limits its utility in processing methods that require uniaxial extension, such as film blowing. We report the synthesis and rheological characterization of poly((±)-lactide) with a graft molecular architecture with the goal of creating melt strain hardening to improve the melt strength. We demonstrate that a critical value of the backbone degree of polymerization (N_bb,c) is necessary for achieving melt strain hardening; which we attribute to a star-to-bottlebrush transition, where the graft polymer behaves star-like below N_bb,c and recovers bottlebrush behavior as N_bb increases. Ring-opening metathesis polymerization (ROMP) was employed to synthesize a library of polymers with varied N_bb and well-defined grafting densities and side-chain degrees of polymerization. Extensional rheological properties of the melts were determined using an ARES-G2 extensional viscosity fixture. The findings of this work will guide the design of graft polymers that exhibit melt strain hardening, enabling an expanded window for the processing of sustainable plastics.