The formation, structure and properties of urethane-diacetylene copolymers as optical strain-sensitive surface coatings

Segmented copolyurethanes comprising ductile polyether-urethane (PE-U) and rigid diacetylene-urethane (DA-U) phases, have been prepared via a one-shot, bulk polymerisation process. Subsequent thermal treatment (100 °C/40 h) of the as-prepared linear copolymers effected solid-state topochemical reactions within the DA-U phase, producing crosslinked and intensely coloured materials. The cross-polymerised copolymers had values of T_g between -32°C and 70°C depending on composition, and were elastically isotropic with tensile moduli and strengths ranging, respectively, from 20-1800 MPa, and 8-67 MPa. The copolymers exhibited optical properties similar to those of polydiacetylene single crystals, and produced intense resonance Raman spectra. In particular, the CC triple bond stretching Raman band at ~ 2080 cm^-1 was well defined and shifted to lower frequency under the application of tensile stress/strain. The copolyurethanes may be readily applied as surface coatings to various substrates, and their use as optically stress/strain-sensitive materials (optical strain sensors) is demonstrated.