Spectroscopic and light scattering studies of the conformational (rod-to-coil) transition of poly(diacetylene) in solution

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Abstract
A rod‐to‐coil conformationaltransition has been demonstrated for polydiacetylene, 4‐butoxy‐carbonyl‐methylurethane (4BCMU) in solution. The transition can be induced by changing either the temperature or the quality of the solvent. The light scattering and spectroscopic data as a function of polymer concentration have shown that the transition is a single chain (intramolecular) phenomenon. However, because of the large end‐to‐end length (L≂1.2 μm) of the fully extended polymer, the dilute limit is not reached until concentrations below 10− 5 g/cm3. At higher concentrations evidence of cluster growth and aggregation are observed prior to gelation which occurs above a critical concentration c 0≂5×10− 4 g/cm3. This cluster growth occurs as a result of the rod‐like conformation of the individual molecules, but it is not the cause of the transition. The large increase in scattering intensity (at fixed polymer concentration) on going from coil to rod follows directly from the change in dielectric constant due to the spectral shift of the π–π* absorption; no significant increase in molecular weight is implied by the data. A theoretical model of the transition has been developed in which the ordered rod‐like conformation is the low temperature phase. Conformational kinks (to a coil phase) cost energy through interruption of the π‐electron delocalization and through the breaking of H bonds between R groups. Nevertheless, the increase in entropy associated with the many degrees of freedom of the coil‐like conformation is sufficient to lead to the observed transition.