Heat capacity of a polydiacetylene single crystal from 3 to 300 K

Heat capacities C p of a polydiacetylene‐bis(toluene sulfonate) single crystal and its monomer have been measured in the temperature range from 3 to 300 K. The temperature dependence of C p for both monomer and polymer crystals differs from that for monoatomic solids. By applying a chain lattice model for a polymer crystal, the temperature dependence of the heat capacity can be described assuming a phonon density of states given by bending and stretching modes of the polymer backbone. With a combination of one‐dimensional and three‐dimensional elastic continuum approximations, the heat capacity has been calculated and a good fit to the data has been obtained. A small peak in C p was detected at 161 K for the monomer and at 198 K for the polymer. This may be ascribed to a lower‐temperature phase transition in the polydiacetylene crystals evidenced by previous x‐ray and spectroscopic measurements.