Can the rigidity of a side-chain liquid-crystalline polymer backbone influence the mechanism of distortion of its random-coil conformation?

The relationship between the isotropization transition temperatures and the associated entropy changes of two series of side-chain liquid-crystalline polymers based on three different polymer backbones (polymethacrylate, polyacrylate and polymethylsiloxane) and 4-methoxy-4′-hydroxy-α-methylstilbene and 4-hydroxy-4′-methoxy-α-methylstilbene mesogenic side-groups attached to the polymer backbone through different flexible spacers is discussed. In the case of polymers containing long flexible spacers, isotropization transition temperatures, which are mostly dictated by the side-groups, are higher for polymers based on flexible backbones, suggesting that they provide the highest degree of order in the mesophase. Therefore, their mesophase should exhibit the lowest entropy and the highest entropy change of isotropization. However, experimentally determined entropy changes of isotropization, which refer to the overall degree of order of the polymer, present the highest values for polymers based on the most rigid backbone. Two different mechanisms of distortion of the random-coil conformation of flexible and rigid polymer backbones are suggested to account for this contradictory result. A squeezed random-coil conformation, which in the case of smectic polymers is confined between the smectic layers, is considered for flexible backbones. An extended-chain conformation is considered for rigid backbones. The entropy associated with the squeezed random-coil conformation is higher than that associated with the extended conformation, and therefore the overall order that results from the combination of backbone and the organization of the mesogenic side-groups may explain the experimentally observed isotropization entropy changes. Polymers based on short flexible spacers exhibit similar entropy changes of isotropization irrespective of the nature of their backbone. This may suggest an extended backbone conformation for polymers based on short spacers and rigid or flexible backbones.