Polymer-Dispersed Liquid-Crystal Materials Fabricated with Frontal Polymerization N. GILL, 1 J. A. POJMAN, 1 J. WILLIS, 1 J. B. WHITEHEAD, JR. 1,2 1 Department of Chemistry and Biochemistry, University of Southern Mississippi, Hattiesburg, Mississippi 39406-5043 2 Department of Physics and Astronomy, University of Southern Mississippi, Hattiesburg, Mississippi 39406-5043 Received 26 February 2002; accepted 17 October 2002 ABSTRACT: We report on the morphological and thermal properties of polymer-dis- persed liquid crystals (PDLCs) fabricated with frontal-polymerization-induced phase separation (FPIPS). Frontal polymerization is characterized by a rapid-conversion, high-temperature, and large-thermal-gradient environment. A comparison is made between the morphological and thermal properties of PDLCs fabricated with FPIPS and traditional thermal-polymerization-induced phase separation. Characterization includes differential scanning calorimetry to probe the glass and nematic-to-isotropic transitions and scanning electron microscopy to evaluate the phase-separated morphol- ogy. In addition, the frontal temperatures and velocities are reported for PDLCs fabricated with frontal polymerization. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 204 –212, 2003 Keywords: frontal polymerization; phase separation; morphology; dispersions; ther- mal properties INTRODUCTION Polymer-dispersed liquid crystals (PDLCs), fabri- cated with one of several phase-separation meth- ods, contain a dispersion of micrometer-sized or submicrometer-sized liquid-crystal droplets dis- persed in a solid polymer matrix. 1 The fabrication of epoxy-based PDLCs entails placing a sample in a laboratory oven and thermally curing it at tem- peratures ranging from 50 and 150 °C for 2–24 h. 2 The cure temperature is chosen to achieve the desired droplet size, and the cure time is chosen to ensure maximum conversion. The cure time, which ranges from several minutes to several hours, depends on both the curing agent and the cure temperature. PDLC materials are fabricated with thermally- induced phase separation (TIPS), solvent-induced phase separation (SIPS), or polymerization-in- duced phase separation (PIPS). 1,3 Each method begins with a homogeneous mixture of a low mo- lecular weight nonreactive liquid crystal, hence- forth simply called a liquid crystal, and a prepoly- mer or monomer. For TIPS, lowering the tempera- ture of the homogeneous liquid-crystal prepolymer/ monomer mixture induces phase separation. For SIPS, extracting a common solvent from the ho- mogeneous liquid-crystal prepolymer/monomer mixture induces phase separation. In the case of PIPS, a photoinitiator or thermal initiator is re- quired to initiate the polymerization of the pre- polymer/monomer component of the mixture. Phase separation commences after a sufficient amount of the monomer/prepolymer has been con- verted into polymer. Frontal polymerization entails the conversion of a monomer into a polymer via a localized exo- thermic reaction zone that propagates through the coupling of thermal diffusion and Arrhenius reaction kinetics. A schematic representation of Correspondence to: J. B. Whitehead, Jr. (E-mail: joe. whitehead@usm.edu) Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 41, 204 –212 (2003) © 2002 Wiley Periodicals, Inc. 204