The effect of strain on the morphology and mechanical properties of thermoplastic polyurethane elastomers

The present investigation focused on the characterization of domain morphology of model polyurethane elastomers. Six model polymers were investigated. Each model polymer had hard segments comprised of 4,4{dollar}spprime{dollar}-methylenebis(phenyl isocyanate) (MDI) chain extended with 1,4-butane diol (BD).; The effect of uniaxial strain aging on domain morphology was investigated. Numerous analytical techniques including differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), small angle neutron scattering (SANS), and fourier transform infrared spectroscopy (FTIR) were utilized to thoroughly characterize the model polymers before and after strain aging and annealing experiments. It was determined that strain aging caused the hard domains to break into smaller, more closely arranged domains than were originally present in the unaged specimens. DSC indicated smaller domains following strain aging, by broadening of endothermic transitions associated with the hard domains, and lower endothermic peak temperatures with respect to increasing levels of strain. DMA showed a decrease in storage modulus, E', following strain aging at 100% elongation, indicating that domain disruption had resulted in reduced mechanical performance. DMA showed little change in glass transition temperature (Tg), indicating that the domains were not dissolved in the soft matrix but continued to exist as distinct entities. Model polymers strained at 100% elongation exhibited isotropic SANS patterns and smaller domain spacings than were present in the unstrained specimens, suggesting that the hard domains had become more closely spaced with no evidence of orientation. At higher elongations (200%-400%), DSC scans exhibited a reappearance of hard domain features, E' increased with respect to increasing strain level, and SANS indicated a continual decrease in domain spacing with the development of anisotropy. Thus, at high elongations, the smaller domains became oriented with the direction of strain.; Macroscopic mechanical investigations were performed to elucidate the structure-property relationships of the model polymers as a result of strain. Tensile and abrasion properties were measured. Ultimate elongation was found to be directly related to abrasion resistance and both properties decreased following strain aging. The mechanical properties were found to be directly related to changes in domain morphology as a result of aging. Hard domain disruption resulting from strain aging, seriously compromised the ultimate elongation of the model polymers. When the hard domains break up into smaller domains, they become more finely dispersed in the soft domain, resulting in less homogeneous soft domain available for further elongation. The plastic deformation of the hard domains in addition to their reduced domain spacing, inhibited relaxation of the model polymers following strain aging. Therefore, the polymers were unable to experience sufficient elongation when subjected to abrasion. A maximum value for ultimate elongation of the model polymers was found to be critical for optimized abrasion performance. (Abstract shortened by UMI.)...