(53j) Mechanistic Understanding of the Thermal and Barrier Properties of PET and PEF Via Computation

Poly ethylene terephthalate (PET) is the most abundant polyester plastic manufactured in the world and is a major component in beverage bottles, clothing, packaging, and carpeting. Poly ethylene furanoate (PEF) has been proposed as an alternative to PET that can be sourced from biomass-derived sugars. The repeat unit of each polymer is quite similar, with the sole difference being the nature of the ring structure (benzene in PET and furan in PEF). This structural difference gives rise to significant functional differences, including superior barrier properties for PEF (e.g., order of magnitude lower permeability to molecular oxygen), as well as attractive thermal properties (e.g., glass transition temperature elevated by approximately 10°C over that of PET). In this contribution, we present mechanistic insight into the structural roots of the difference in thermal and barrier properties for PET and PEF. First, we focus on adequately describing critical intramolecular interactions of both PET and PEF. This is accomplished through employing parameterization schemes that are compatible with current popular forcefields (e.g. CHARMM) and utilizing adaptive force matching approaches to generate parameter sets that produce configurational degrees of freedom that replicate conformational preferences found at more rigorous levels of theory (e.g., semi-empirical quantum, density functional theory, etc.). Subsequent molecular dynamics simulations of both crystalline and amorphous systems allow for atomistic characterization of the dynamics and thermodynamics of various small molecules within each polymer matrix, thus giving insight into the permeability of each. We also test literature hypotheses regarding the structure-function roots of the decreased permeability of PEF, namely the increased resistance to ring-flipping due to the non-linearity of a PEF chain across the furan ring. Via contact angle experiments of water nanodroplets, we quantify the hydrophobicity of each polymer and give insight into the specific chemical interactions that impact the relative hydrophobicity/hydrophilicity of each polymer system. Finally, we probe the thermal properties of each polymer via molecular simulation.