New Developments in Thermoplastic Polyurethanes of Variable Crystallinity: Sensitivity of Cyclic Stress-Strain Response to Chemical Structure

Abstract

A study was made of how aspects of the mechanical and thermal responses of thermoplastic polyurethane elastomers vary with composition: the hard segment, soft segment and chain extender were varied systematically with the aim of improving understanding of the relationship between molecular/supramolecular architecture at the nm-scale and macroscopic mechanical properties in such systems. Two hard segments were compared, generated from 4,4’-methylene bis(phenyl isocyanate) (MDI), or 4,4’-dibenzyldiisocyanate (DBDI). Rotation around the -CH2-CH2- bridge in DBDI allows alignment of aromatic rings and crystallization within the hard phase, which is not available with MDI. The physical structures were characterized by dynamic mechanical analysis (DMA) and by X-ray scattering (SAXS and WAXS) revealing signficant variations in degree of phase separation and degree of crystallinity, especially in the DBDI-based polymers. The presence of DBDI hard segments instead of MDI led systematically to increases in: the input strain energy to a given elongation, hysteresis and residual strain under cyclic loading, and stress relaxation. Stress-strain cycles reflect the resistance to plastic deformation occurring in the hard domains. This was enhanced by more pronounced hydrogen bonding achieved in the more mobile DBDI than in MDI. These differences between DBDI and MDI could be attributed to the greater flexibility of DBDI allowing a higher tendency to self-associate by hydrogen bonding. The degrees of hysteresis and stress relaxation were found to be greatly enhanced by hard-phase crystallinity, through its effect of increasing the flow stress. The results provide new insight into the physical origin of inelastic effects in reinforced elastomers. © 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11