Reversible and irreversible thermochromic phase transitions in single crystals of polydiacetylenes substituted with alkyl-urethanes

Reversible transitions between the two spectroscopically distinct phases (A and B phases) have been investigated for a new family of polydiacetylenes (PDAs) substituted with side groups of alkyl-urethane [−(CH2)4OCONH(CH2)n−1CH3:n=1−10]. Measurements of optical spectra as well as x-ray and calorimetric studies have revealed the first-order-like phase transitions in a series of PDAs with n=1–10 which are associated with an n-dependent thermal hysteresis ranging from 0 to 60 K in width. It has been found that the thermochromic changes become irreversible in all these PDAs once the hydrogen bond chains in the side groups are interrupted by heating beyond the polymer melting temperature.