ON THE NATURE OF MESOPHASE TRANSITIONS IN POLYMERS .1. THE ISOTROPIC-NEMATIC TRANSITION

The purpose of this study is to examine the application of the meander model to the understanding of transition properties in the isotropic-nematic (paper I), and in the isotropic-smectic or - lamellar (paper II) phase transitions in polymers. This paper deals with the phase transition within the stable meander structure which will take place if the entropy of cube rotation is counterbalanced by a reduction of interfacial energy that may be realized by a parallel arrangement of the anisotropic meander cubes within the coarse grains (i/n-transition). But no change in the short-range order within the cubes will be assumed. Considering the free energy in the meander model, the only relevant term for this transition is DELTA-g(rot), which is derived, minimized to equilibrium, and discussed. By this procedure, the transition temperature, the heat and volume of transition, as well as their temperature dependences are deduced. The most important parameter is the excess volume upsilon(e) per chain of cube side length, caused by crossed-chains at the cube-interfaces, which can be determined by atomistic calculations. This model theory is compared with experimental data [15] on one low molecular weight LC and two LC side chain polymers, as well as on the hypothetic i/n-transition of polyethylene.