Segmental orientation of thermoplastic polyurethanes investigated by 1H double-quantum NMR. Correlation with thermodynamic and mechanical properties

The segmental orientation of a series of thermoplastic polyurethane (TPU) samples with different content in hard segments and different number average molecular weights of soft segments was investigated by 1H double-quantum (DQ) NMR. The residual dipolar couplings of the hard segments were measured via the residual second van Vleck moments. A simple theoretical approach based on Rouse modes was developed to explain the orientation dependence of the hard segments on the number average molecular weight of the soft segments. The effective transverse relaxation rates of the hard segments were correlated to the 1H residual dipolar couplings. Moreover, the correlation between the segmental orientation of the hard segments and the thermodynamic and mechanical properties of the TPU samples is investigated. The glass transition temperatures of the soft segments represent the relevant thermodynamic quantities. The mechanical parameters used for discussing the correlations are Young's modulus, yield stress, and rebound resilience angles from tensile testing and rebound resilience measurements.