Isocyanate-free, thermoplastic polyhydroxyurethane elastomers designed for cold temperatures: Influence of PDMS soft-segment chain length and hard-segment content

Polydimethylsiloxane (PDMS)-based, segmented polyhydroxyurethanes (PHUs) was synthesized from Bisphenol A (BPA) dicarbonate and diamine-terminated PDMS. Two PDMS samples were employed with 875 g/mol and 2500 g/mol number-average molecular weight (M_n) values. Due to the inherent hydrophobic nature of PDMS, the PDMS-based PHUs exhibit excellent surface hydrophobicity with a water contact angle >100°. Because of the strong thermodynamic incompatibility of PDMS with BPA-based hard segments, hard-segment content and soft-segment M_n are critical in ensuring macroscopic homogeneity and effective nanophase separation leading to thermoplastic elastomer (TPE) character extending from room temperature to temperatures as low as −100 °C. Nanophase separation was confirmed via small-angle X-ray scattering and dynamic mechanical analysis. With 20–25 wt% hard-segment content, the TPEs derived from 2500 g/mol PDMS exhibit quasi-rubbery plateau regions with tensile storage modulus in the range of 1–10 MPa and the absence of a yield point in tensile tests with strain at break values that can exceed 1000%. With 40–50 wt% hard-segment content, the TPEs derived from 875 g/mol PDMS exhibit quasi-rubbery plateau regions with tensile storage modulus in the range of 100–1000 MPa but yield points in tensile tests at 25–40% strain. This work presents a pathway to achieving hydrophobic, non-isocyanate thermoplastic polyurethane elastomers with highly tunable rubbery moduli that extend to extraordinarily low temperatures.