Microcellular materials from rodlike macromolecules in solution

Date of Completion

January 1989


Chemistry, Polymer




The phase separation and gelation of poly($\gamma$-benzyl-L-glutamate) (PBLG) was studied in an effort to understand the mechanism by which microcellular materials are made via thermally induced phase separation processes. PBLG is a rigid-rod macromolecule, or lyotropic liquid crystal polymer (LCP). Previous workers have studied similarly prepared materials from molecules which exist as random coils in solution. The purpose of this work was to discover any unique properties offered by microcellular materials made from lyotropic LCP's, and to use this process to investigate the mechanism of phase separation and gelation in these systems.^ The microcellular materials were formed by lowering the solution temperature until either liquid-liquid or liquid-solid phase separation occurred, followed by solvent freezing. The solvent was removed by vacuum sublimation. An emphasis was placed on dilute isotropic solutions in this work to produce low-density materials or "foams". Both single (e.g. benzene) and two-component (e.g. dioxane/water) solvent systems were employed.^ Liquid-solid phase separation produced anisotropic ladder-like or sheet-like morphologies in a uniaxial quench. In this case, the polymer is pushed to the grain boundaries and the morphology is indicative of solvent crystallization.^ The morphology produced by liquid-liquid phase separation and gelation was an open cell, fibrous structure, which resembles a 3-D fishnet. The cell diameters were in the range of 1-10 $\mu$m and the fibers which comprise the struts were from 0.2 to 2.0 $\mu$m thick. The sign of birefringence in the fibers was positive, indicating that the rods are parallel to the fiber axis in the crystals which comprise the struts. This observation is discussed in terms of the theories and proposed mechanisms of phase separation in solutions of rod-like molecules. Calculations of the spinodal for the Flory theory of rodlike molecules were also made, to assess the possibility of placing the isotropic solutions into an unstable region.^ Thermoreversible gelation played a key role in the liquid-liquid phase separation processes studied in this work, as has been reported previously for PBLG in the wide biphasic region. The temperature-modulus response of the gels revealed a distinct two-stage decrease, which is discussed in terms of the coincidence of phase separation and gelation. ^