Mechanical response of dibenzyl-based polyurethanes with diol chain extension

A systematic investigation was made of the effects of varying hard and soft segment chemistry, crosslinking and preparation procedures, on the mechanical response of melt-cast polyurethane elastomers. In particular, two hard segments were compared, based on the diisocyanates: 4,4′-methylene bis(phenyl isocyanate) (MDI) and 4,4′-dibenzyl diisocyanate (DBDI). Rotation around the central -CH2-CH2- bridge in DBDI allows alignment of aromatic rings and hence crystallization within the hard phase, which is not available with MDI in melt-cast polyurethanes. Thus, new polymers were achieved, with a controlled ordering of copolymer hard segment blocks on the macromolecular chain. Wide angle X-ray diffraction of the as-moulded polymers revealed the presence of crystallinity in some cases, in the DBDI-based PU materials. Mechanical tests included load-unload cycles at constant rate of extension, with measurement of hysteresis and strain recovery, and stress relaxation tests. 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. The results were interpreted in terms of a physically-based constitutive model framework previously proposed. This revealed that the observed effects of varying hard segment could all be explained by the hard domains having a higher flow stress in the presence of DBDI relative to MDI, associated with increased hydrogen bonding in DBDI-based polymers, which is enhanced in some cases by hard segment crystallinity. Materials with mixed MDI and DBDI hard segments were found to give the optimum combination of high input strain energy, but minimum residual strain, compared to equivalent materials based on MDI or DBDI alone.