Conformational Rearrangement in the Nematic Phase of a Polymer Comprising Rigid and Flexible Sequences in Alternating Succession. II. Theoretical Results and Comparison with Experiments

Abstract

ABSTRRACTTheory predicts that the nematic state of a homopolymer comprising rigid and flexible sequences in alternating succession depends critically on the tendency of the flexible units to reconform to ordered extended conformations. When the relative statistical weight s* of such extended conformations is very small (very flexible), the chains undergo only a minor conformational rearrangement in the nematic state. Moreover, the clearing temperature and the orientational order of these nematic polymers are not much different from those of the monomeric liquid crystals. Hlowever, as the value of s* becomes larger than a certain critical value, an ordered (nematic) state exhibiting perfect orientational order and completely rodlike conformations becomes more stable than the state of only partial order. Hlence, an extensive conformational rearrangement is predicted to occur in nematic melts of the polymers when the flexible sequences are niot very flexible. These predictions are in good qualitative agreement with the conclusions from the thermodynamic and NMR measurements on two thermotropic polymers comprising identical rigid groups joined by flexible sequences of substantially different flexibility.