Sugar-Based Thermoplastic Polyhydroxyurethanes: Effects of Sorbitol and Mannitol Diastereomers on Polymer Properties and Applications in Melt Blending

Abstract

Each of the three bio-sourced dicarbonates, sorbitol biscarbonate (SBC), mannitol biscarbonate (MBC), and diglycerol dicarbonate (DGC), is polymerized with 1,6-hexamethylenediamine (HMDA) or 1,10-diaminodecane (DAD) in dimethyl sulfoxide for 24 h at 100 °C to obtain highly crystalline (70-77%) thermoplastic polyhydroxyurethanes (TPHUs). Even though SBC and MBC are isomers, their respective TPHUs present similar (micro)structural, thermal, and rheological properties. However, MBC reacts faster with amines at room temperature and induces more flexibility into the TPHU chains than SBC. The glass transition temperatures (Tgs) of the MBC-based TPHUs are 7 °C lower than those of the SBC-based ones. Interestingly, the HMDA-based TPHUs exhibit liquid crystalline-like rheological behavior with their storage moduli increasing above their apparent melting points. Nevertheless, replacing SBC and MBC, whose structure contains a rigid furan ring, by DGC, a linear aliphatic dicarbonate, significantly alters the properties of the TPHUs, especially the rheological ones. The storage modulus of the DGC-DAD TPHU is ten-fold lower than those of the SBC and MBC analogues, when measured at similar conditions. MBC-HMDA, MBC-DAD, and DGC-DAD are then blended into poly(lactic acid) (PLA) (20/80 wt%/wt%), and the blends are predicted to be miscible from the Hoftyzer-Van Krevelen group contribution method. While PLA/MBC-HMDA and PLA/MBC-DAD show one Tg, the PLA/DGC-DAD presents two, despite the predicted miscibility of that blend. However, both Tgs are shifted lower compared to the homopolymers, indicating that the components of that blend act as plasticizers of a two-phase morphology, as implied by droplets no more than 5 μm in diameter in the PLA matrix observed from SEM. Despite their miscibility with PLA, the TPHUs agglomerate into droplets inside the blends caused by significant intramolecular hydrogen bonding interactions between their chains. These results expand the application of TPHUs as nontoxic additives, for example, plasticizers, reinforcing agents, rubber tougheners, and composites into different polymer matrices. © 2022 American Chemical Society.