On the use of solubility parameters to investigate phase separation-morphology-mechanical behavior relationships of TPU

This study proposes a thorough investigation, especially based on thermodynamics, to predict phase separation in a linear thermoplastic polyurethane, denoted TPU, prepared from fatty acid-based soft segments and MDI (4,4'-methylene bis(phenyl isocyanate))/BDO (1,4-butanediol) hard segments and specially designed for bitumen modification. Hansen' solubility parameters (HSP) of both segments are evaluated to predict their compatibility. The later ones are evaluated either individually from the corresponding segment synthesized separately or from a decomposition of the solubility diagram of the TPUs into two distinct spheres. In a second step, phase separation is experimentally analyzed by combining differential scanning calorimetry, microscopy techniques, and small angle X-ray scattering (SAXS). The microstructure of the TPUs is described considering one soft phase made of polyol chains and short miscible hard segments and a hard phase organized as semi-crystalline nanodomains either dispersed or assembled as ramified (nano)objects within the soft phase. The dynamic mechanical properties of the TPUs can be explained by the presence of such well-defined hard domains in the structure of the TPU, acting as reinforcing fillers while maintaining a thermoplastic elastomer mechanical behavior to the TPU above the glass transition of the continuous soft phase.