Configurational Characteristics of Thermotropic Polymers Comprising Rigid Groups Connected by Polymethylene Spacers

Abstract

The distribution of chain sequence extension, calculated by using rotational isomeric state (RIS) models, is compared with isotropic–nematic transition characteristics for a number of thermotropic polymers comprising rigid groups connected by polymethylene (PM) spacers. This distribution depends strongly not only on the even–odd character of the number of methylene units of the spacers, but also on the specific groups (or atoms) connected at the ends of PM spacers. When PM spacers are attached to a linear rigid unit (LR) by LR—O—PM or LR-O(O═)C—PM linkage, chains having even-numbered methylene spacers exhibit a significant population of highly extended conformers, favorable for packing, which allow nearly parallel alignment of rigid units along the major extension axis; they also lower significantly the internal (conformational) energy below the average energy. Chains with odd-numbered methylenes attached in the same way offer few conformers in which the polymer as a whole is highly extended; furthermore, the energies of the relatively extended conformers are only slightly lower than the average and they place the rigid units tilted by ca. 30° from the major extension axis. When PM spacers are attached by LR—C(═O)O—PM linkage, on the other hand, the population of extended conformers is reduced significantly for the chains with even-numbered methylene spacers. When compared with those of odd-numbered methylene spacers linked in the same way, they exhibit even smaller fractions of relatively extended conformers and the lowering in the energy of extended conformers relative to the average energy is smaller, reversing the typical even–odd effects. These characteristics of chain sequence extension and extended conformers relate very closely to the experimental results of isotropic–nematic transition characteristics of these polymers. This finding therefore shows very clearly the critical importance of highly extended conformers and their preferential selection in forming nematic states and thus supports the view that emphasizes the steric repulsive forces in considering the stability of ordered states of chain molecules. Furthermore, the detailed conformational order of PM sequences in nematic states is then deducible by matching the experimental results of enthalpy and entropy changes with those estimated from the conformational selection on the basis of chain sequence extension. © 1985, American Chemical Society. All rights reserved.