Polymer-Dispersed Liquid Crystals with Co-continuous Structures Generated by Polymerization-Induced Phase Separation of EBBA−Epoxy Solutions

Abstract

Polymer-dispersed liquid crystals (PDLCs), consisting of a dispersion of LC-rich domains in a polymer matrix, are used in different types of electrooptical devices. Their efficiency can in principle be increased if the LC domains exhibit a uniform characteristic size in the range of the wavelength of visible light. In an attempt to generate this type of morphology, a model PDLC system based on a 50 wt % solution of N-4-ethoxybenzylidene-4‘-n-butylaniline (EBBA) in an epoxy monomer (diglycidyl ether of bisphenol A, DGEBA) was analyzed. The polymerization-induced phase separation was performed at 80 °C, using a tertiary amine as initiator (benzyldimethylamine, BDMA). By selecting an initial concentration located close to the critical composition to promote spinodal demixing, co-continuous morphologies were obtained, which were rapidly fixed by gelation. The conversion of epoxy groups (p) was followed by near-infrared spectroscopy (NIR). At p = 0.28, phase separation took place as revealed by transmission optical microscopy (TOM) and by the acceleration observed in the isothermal cure rate. Gelation took place at p = 0.35, soon after the cloud point. Although the primary structure was arrested by gelation, the LC-rich phase was continuously enriched in pure EBBA, as revealed by the increase in T NI with conversion monitored by differential scanning calorimetry (DSC). Co-continuous structures remained unmodified after the storage of PDLCs for several months. The nematic range of the LC-rich phase at p = 1 was comprised between 34 °C (melting point) and T NI = 68 °C. A 57% of the initial LC was present in nematic domains at 40 °C, as determined by the variation of the FTIR absorbance of a characteristic LC peak between isotropic and nematic states. Therefore, a possible route to obtain PDLCs with a uniform characteristic size of LC domains is to start with a composition close to the critical one and select conditions to produce liquid−liquid demixing soon before gelation.

Polymer‐Dispersed Liquid Crystals Based on Polystyrene and EBBA: Analysis of Phase Diagrams and Morphologies Generated

“…The experimental value of the glass transition temperature of PS was T g,PS ¼ 356 K and the corresponding value of EBBA is a virtual value for an isotropic glassy state, which was estimated in a previous publication: [21] T g,EBBA ¼ 187.3 K. The Fox equation gave a good fitting of the experimental data, particularly considering that no adjustable parameters were used (the curve was forced to include the experimental point of pure PS but the decrease in T g when increasing w EBBA was strictly determined by the Fox equation using a literature value for T g,EBBA ). The maximum solubility of EBBA in the polydisperse PS is about 23 wt.-%.…”

“…When PDLC's are used in electrooptical devices such as reflective displays, optical switches, and variable transmittance windows, it is convenient to generate unimodal distributions of dispersed domains with sizes in the range of the wavelength of visible light. [21] This may be achieved by combining an adequate selection of the initial composition with the use of a fast cooling process.…”

Nonisothermal Model for the Direct Isotropic/Smectic-A Liquid-Crystalline Transition

“…Much of this interest stems from the fact that liquid crystal transformations occur on experimentally accessible timescales and are inherently anisotropic. In the latter case, the study of PDLC materials [3] has focused on different mechanisms of phase-separation [19][20][21][22][23][24][25][26][27][28] (polymerization, solvent, and thermally induced), where growth morphology of liquid crystal-rich domains and their texture are studied in order to control functional physical properties, such as electro-optical response. Subsequently, PDLCs composed of higher order liquid crystals have also been studied to a lesser extent [29][30][31].…”

Epoxies Modified by Palmitic Acid: From Hot‐Melt Adhesives to Plasticized Networks

“…This amount of initiator was the same as used in the previous studies to obtain an acceptable rate of epoxy homopolymerization and an almost full conversion of epoxy groups at reasonable reaction times. [16] The selected amounts of PA and DGEBA were mixed at 100 8C until a homogeneous solution was obtained. After cooling to room temperature, the adequate amount of BDMA was added.…”