Chapter 13
Downloaded by UNIV MASSACHUSETTS AMHERST on October 9, 2012 | http://pubs.acs.org Publication Date: July 9, 1996 | doi: 10.1021/bk-1996-0632.ch013
Thermal-Induced Phase Separation in a Mixture of Functional Poly(methyl methacrylate) and Low-Molar-Mass Liquid Crystals Thein Kyu, I. Ilies, C. Shen, and Z. L. Zhou Institute of Polymer Engineering, University of Akron, Akron, OH 44325-0301 Miscibility phase diagram and phase separation dynamics of a mixture of a flexible polymer and a monomeric liquid crystal, hereafter called a polymer dispersed liquid crystal (PDLC), have been investigated. A theoretical calculation was carried out for predicting an equilibrium phase diagram of a PDLC system by taking into consideration Flory-Huggins (FH) free energy of mixing of isotropic liquid phases in combination with Maier-Saupe (MS) free energy of nematic ordering of the liquid crystal. The combined FH-MS theory predicts a "tea-pot" phase diagram in which liquid-liquid phase separation is overlapped with an isotropic-nematic coexistence region. A temperature-composition phase diagram of a mixture of hydroxyl functionalized polymethyl methacrylate (PMMA-OH) and eutectic nematic liquid crystals (E7) was established by light scattering and differential scanningcalorimetry.This phase diagram can be characterized as an upper critical solution temperature exhibiting liquid-liquid phase separation between the polymer and the isotropic phase of the liquid crystal. The nematic-isotropic transition occurs at high liquid crystal compositions. The calculated phase diagram was found to conform well with the cloud point phase diagram. The dynamics of phase separation was investigated by means of light scattering and optical microscopy. The time-evolution of structure factor was analyzed in terms of a power law.
Inhomogeneous composite films comprising of a polymer binder and low molar mass liquid crystals, hereafter termed polymer dispersed liquid crystals (PDLC), are of interest because of their potential for applications in optical devices such as optical switches, flat panel display screens, and privacy windows (1-4). The PDLC devices are operated on the principle of electrical switching from an opaque to a transparent state (4). For a successful application of PDLC devices, the optimization of various parameters including maximum opacity in the off-state of the electric field, high transparency in the on-state, fast electrical switching and response times, is of paramount importance. The electro-optical properties depend on various parameters such as size, shape, uniformity of liquid crystal dispersions in the polymer matrix. In general, polymer dispersed liquid crystals are produced via phase separation of initially homogeneous binary mixtures of polymer and liquid crystals either by thermal quenching or chemical reactions such as thermal or photo-polymerization (4-7). The 0097-6156/96/0632-0201$15.00/0 © 1996 American Chemical Society In Liquid-Crystalline Polymer Systems; Isayev, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
Downloaded by UNIV MASSACHUSETTS AMHERST on October 9, 2012 | http://pubs.acs.org Publication Date: July 9, 1996 | doi: 10.1021/bk-1996-0632.ch013
202
LIQUID-CRYSTALLINE POLYMER SYSTEMS
understanding of mechanisms of phase separation and pattern formation driven by thermal or chemical processes is essential for the elucidation of the relationship between the structure and electro-optical properties of PDLC. Our effort in this area has been directed to the elucidation of phase equilibria and dynamics of phase decomposition of various PDLC systems (5-7). Recently, we have derived a theoretical scheme by combining the Flory-Huggins free energy (FH) for describing isotropic mixing and the Maier-Saupe free energy (MS) for the nematicisotropic phase transition of a nematic liquid crystal (8). We found that this combined FH/MS theory is capable of predicting an equilibrium phase diagram of PDLC systems and tested favorably well with the cloud point phase diagrams of polymethyl methacrylate/E7 (eutectic liquid crystals) and polybutyl methacrylate/E7 (8.9). fn this paper, phase behavior of a polymer dispersed liquid crystal consisting of functional polymethyl methacrylate having hydroxy groups (PMMA-OH) and eutectic main chain liquid crystals (El) have been investigated without the involvement of cross-linking. The emphasis is placed on mutual interference between an upper critical solution temperature (UCST) associated with liquid-liquid phase separation and the nematic-isotropic transition of the liquid crystals. The experimental phase diagram has been tested with the theoretical prediction. Pattern formation and kinetics of phase decomposition driven by thermal quenching into a liquid-liquid region and a nematicliquid region have been investigated by time resolved small angle light scattering and optical microscopy. The growth behavior is analyzed in the framework of a power law. Theoretical Model A theoretical phase diagram for mixtures of polymer and liquid crystals has been calculated by combining Flory-Huggins free energy for isotropic mixing (10) and Maier-Saupe free energy for nematic ordering (11.12) as follows; g =
