INVESTIGATIONS OF THE MECHANOCHROMIC BEHAVIOR OF POLY(URETHANE DIACETYLENE) SEGMENTED COPOLYMERS

The optical properties of a number of structurally and morphologically different segmented polyurethanes containing a small fraction of conjugated poly(diacetylene) chains in their hard domains were examined during tensile elongation by visible dichroism techniques. In all cases, the poly(diacetylene) chains were found to orient into the stretch direction during elongation, implying that the hard segments orient transverse to the stretch direction during this process. All of the elastomers were found to exhibit mechanochromic behavior, visually observed during stretching as color changes from blue to red or yellow. The unique color changes observed during stretching were attributed to either a mechanically induced disordering of the hard domains or a stress-induced phase transition within the hard domains. In the latter case, it was found that the reversibility of the phase transition was influenced by the soft-segment molecular weight. Completely reversible phase transitions were identified in segmented polyurethanes containing crystalline hard domains. Evidence for significant irreversible hard-domain disruption was only found at strain levels greater than ca. 250%. In some cases, it was found that suitable mechanical treatments can actually improve the local molecular order of the hard domains by relieving localized stress concentrations.