Recent theoretical models for the nematic phase of semiflexible polymer chains predict a strong coupling between order and the conformational degrees of freedom of the chain. The presence of order in the nematic phase results in a strong preference for linear or rodhyphen;like conformations over flexible, random coil conformations. This conformational selection or induced rigidity is predicted to be general phenomenon associated with semiflexible chains. We have tested these predictions using a soluble polydiacetylene (4BCMU) as a probe. The 4BCMU chain undergoes a conformational transition (rodndash;coil) as a function of temperature in toluene which is accompanied by a large change in optical properties allowing the conformational transition to be followed spectroscopically in extremely dilute solutions. 4BCMU is miscible with both isotropic and nematic solutions of polyhyphen;(nhyphen;hexyl isocyanate) in toluene. If current models of induced rigidity are accurate, there should be a large shift in the transition temperature for the 4BCMU transition in nematic polyhyphen;(nhyphen;hexyl isocyanate) solutions. Experimentally we find no shift in the transition for nematic solutions when compared to dilute isotropic solutions. Possible explanations for the discrepancy between theory and experiment are discussed.
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