Theory of the rod‐to‐coil transition in polydiacetylene

Abstract A theory is proposed for the rod to coil transition in polydiacetylene 4BCMU and related polymers which is based on the hypothesis that the high‐temperature (yellow) phase consists predominantly of the cis structure, while the low‐temperature (red) phase is trans. Because the occurrence of a cis‐trans interface is energetically costly, the correlation length for either isomer remains long and the transition is sharp, much like the helix‐coil transition in the theory of Zimm and Bragg. The transition is driven by the higher entropy of the cis isomer, which is free to coil, unlike the trans or butatriene forms. The theory gives excellent agreement with optical absorption measurements and is consistent with all other experimental data for this system.