Non-Crystalline Flexible Amorphous Thermoplastic Polyurethane Elastomer: Selecting the Appropriate Isocyanate and Chain Extender

The elastomeric performance of the polyurethanes (PUs) is heavily dependent on controlling segment crystallinity. This study aims to develop non-crystalline flexible amorphous PUs by employing asymmetric isocyanates and multi-functional chain extenders to achieve amorphous structures. PU samples were synthesized with Hexamethylene diisocyanate (HDI) and Isophorone diisocyanate (IPDI), and chain extenders including 1,4-butanediol (BDO) and glycerol (GL). Fourier-transform infrared spectroscopy (FTIR) confirmed successful synthesis, and atomic force microscopy (AFM) revealed clear morphological differences indicating varying phase separation. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses showed that IPDI-based PUs, particularly those using GL, effectively prevented crystallization in the soft segments. Mechanical testing revealed superior elongation and tensile strength in IPDI-based formulations compared to linear HDI-based samples, attributed to reduced phase separation and enhanced molecular network density.