Role of Increased Crystallinity in Deformation-Induced Structure of Segmented Thermoplastic Polyurethane Elastomers with PEO and PEO−PPO−PEO Soft Segments and HDI Hard Segments

The phase-segregated nature of polyurethanes allows meaningful connections to be made between morphological and physical properties. We have taken advantage of this behavior by synthesizing a series of polyurethanes with varying extents of crystallinity and studying their morphologies in both the unstrained and deformed states, going from a completely amorphous soft segment to one with similar chemistry that displays a high extent of soft domain crystallization, thus enhancing phase segregation. The presence of dispersed semicrystalline regions within the continuous soft domain has been shown to provide a reinforcing effect when compared to that of a non-crystalline soft segment polyurethane. Incorporating a semicrystalline soft segment (PEO, 1000 g/mol) has been shown to improve overall sample toughness; however, if higher molecular weight PEO soft segments are employed (4600 g/mol), extensibility and, consequently, toughness are adversely affected due to an increased continuous domain modulus. In-situ deformation experiments demonstrate two very different deformation responses. In the copolymer-containing polyurethane (PEO−PPO−PEO, 1900 g/mol), the hard domains retain a tilted configuration up to strains of ∼450%, with only a small fraction of the hard segments undergoing reshuffling. The PEO1000-containing polyurethane, on the other hand, begins to demonstrate meridional scattering at strains of 200%, with it being the dominant peak by a strain of 300%. These two deformation behaviors are indicative of the two primary responses to deformation, which are shear and tensile, respectively. Frequently, a tensile mechanism points to decreased polyurethane mechanical properties, though this phenomenon is not seen in the series of interest.