Chemical Disorder and Phase Separation: A Study of Two Liquid Crystal Polymers

We have followed by optical microscopy the gradual phase separation that occurs with increasing temperature in the nematic phase of a liquid crystal polymer containing chemically disordered chains. A biphasic fluid is first observed above 250 °C displaying optically isotropic droplets dispersed in the anisotropic medium. We have estimated the isotropic volume fraction as a phase inversion to birefringent droplets in an isotropic continuum occurs over a broad temperature range. In an effort to understand the origin of phase separation we used a computer simulation which generated apopulation of 10000 chemically disordered chains. In conjunction with this simulation we used the Landau-de Gennes theory of liquid crystals adapted to semiflexible chains by ten Bosch, Maissa, and Sixou. We introduce the concept of “polyflexibility”, a distribution of persistence lengths, and calculate the nematic to isotropic transition temperature for the 10000 hypothetical materials that would be generated if each of the chemical sequences were “cloned”. These calculations produce a curve which is qualitatively similar to that obtained from optical microscopy measurements of isotropic volume fraction. One of the hypothetical clones was actually synthesized in our laboratory and as predicted by theory this “monoflexible” but polydisperse liquid crystal polymer is found to have a sharp nematic to isotropic transition. We conclude that polyflexibility, produced in this case by chemical disorder of the experimental system, leads to the observed gradual phase separation. It is recognized that the relation between molar mass polydispersity and polyflexibility is not universal, and therefore a broad molecular weight distribution may lead to polyflexibility in some cases. In the context of the previous statement it is also recognized that not all types of chemical disorder will lead to polyflexibility. Our observations couldbe relevant to other polymeric systems which do not form liquid crystalline phases such as multistructural unit polymers or mixtures of biopolymers with different chemical sequences.