The mechanism of the solid state phase transitions in the polydiacetylene, poly-4bcmu: Thermal, x-ray diffraction and raman scattering studies

The urethane-substituted, soluble polydiacetylene, poly-4BCMU, undergoes two discontinuous thermochromic phase transitions in the solid state from, the pristine greenish-blue to a red phase at approximately 110 degrees C and to a yellow phase at approximately 130 degrees C. These transitions have been investigated in polycrystalline samples using differential scanning calorimetry, X-ray powder diffraction and Raman scattering techniques. These results suggest that the transition at approximately 110 degrees C involves a loss of long-range order between the hydrogen-bonded sheets containing the polymer chains, but a two-dimensional order within the planes is preserved. The X-ray diffraction data show that the second transition at approximately 130 degrees C involves a complete loss of long-range order. Both transitions have been shown by Raman spectroscopy to be associated with a discontinuous reduction in the chain conjugation parameter.