Morphology of Composites of Low-Molar-Mass Liquid Crystals and

Chapter 34

Morphology of Composites of Low-Molar-Mass Liquid Crystals and Polymer Networks 1

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Downloaded by STANFORD UNIV GREEN LIBR on October 8, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch034

C. V. Rajaram , S. D. Hudson , and L. C. Chien 1

Department of Macromolecular Science, Case Western Reserve University, Cleveland, OH 44106 Liquid Crystal Institute, Kent State University, Kent, OH 44242 2

A new class of liquid crystal/polymer network composite with very small amounts of polymer network (3 Wt%) is described. These composites are formed by photopolymerization of the monomers insitu from a solution of monomer dissolved in low-molar-mass liquid crystals. Several techniques have proven useful to characterize these polymer networks. This review describes polymer network structure and its influence on electro-optic behavior of liquid crystals. Structural formation in these composites begins with the phase separation of polymer micronetworks, which aggregate initially by reaction-limited, and then by diffusion-limited modes. The morphology can be manipulated advantageously by controlling: the crossover condition between such modes, the order of the monomer solution prior to photopolymerization, and the molecular structure of monomers or comonomers.

Liquid Crystalline Polymers are an important class of polymeric materials because they may exhibit optical properties similar to low-molar-mass liquid crystals and high mechanical properties of polymers. These polymers are broadly classified based on their molecular architecture, i.e. attachment of the mesogen to the polymeric backbone, as main-chain liquid crystal polymers (7) or side-chain liquid crystal polymers (2). In main-chain liquid crystal polymers, mesogens are incorporated into the backbone. The mesogens may be of different shapes and sizes, and are usually rodlike or disklike. Such polymers have not been used for opto electronic applications because it is very difficult to reorient these materials by electric field. Instead, these materials find applications that use their exceptional mechanical properties. Even side-chain liquid crystal polymers, whose mesogen is attached to the polymer backbone through a flexible spacer switch too slowly for Corresponding author © 1997 American Chemical Society

In Photonic and Optoelectronic Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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display applications, but are suitable for data storage. Considerable effort has been directed toward understanding the influence of the size and type of flexible spacers on mesomorphic properties (2). Another class of materials which are offering promise to be used as optical materials are side-chain liquid crystal elastomers (5). These are side chain liquid crystal polymers which are lightly crosslinked. These materials can be easily tailored to desired properties based on several modifications concerning molecular architecture, including the amount of crosslinking. These materials may range from translucent to opaque under unperturbed conditions, but can be switched to a transparent state by the application of small amounts of mechanical field. Another interesting class of materials based on the principle of liquid crystallinity and crosslinking, which is being studied in detail because of its potential use in optical components, is oriented polymer networh (3). In this class of materials, the monomer, which is usually a diacrylate having a mesogenic moiety and possessing the properties of low-molar-mass liquid crystals, along with a small amount of photoinitiator, is aligned in a desired orientation by either surface alignment or the electric or magnetic field. The entire system is photopolymerized by the application of UV light radiation of suitable intensity and wavelength. The photopolymerization fixes the orientation of the mesogens, resulting in densely crosslinked thermosets with unique anisotropic properties. A modification of the oriented polymer network systems are polymer stabilized liquid crystals (PSLC) (4) being studied in detail because of their application in flat-panel displays. In these materials, photopolymerizable diacrylate monomers are usually dissolved at a concentration less than 10% in non-reactive low-molar-mass liquid crystal solvents, commonly available, along with a small concentration of photoinitiator. Typically, the addition of small amounts of monomers and photoinitiator reduces the transition temperatures of the pure lowmolar-mass liquid crystals slightly, suggesting that the order in the system is not dramatically altered by the addition of monomers or initiator. In application, this solution is aligned in a particular desired state and then photopolymerized. Photopolymerization is preferred to thermal free-radical polymerization, because photopolymerizations are very fast and because the temperature of photopolymerization can be controlled more easily to optimize processing of the display. These formulations were developed to overcome low viewing angle, a disadvantage of conventional flat-panel displays. The liquid crystal orientation is perturbed by the phase separated polymer network. Since the polymer network is present only in small amounts spanning through the entire cell, the perturbation may be slight in all directions, thereby increasing the viewing angle without sacrificing greatly the contrast of the display. Due to low concentration of the polymer, the electro-optic response of the is dominated by the low-molar-mass liquid crystals in which they are made. These composites combine fast switching and low threshold voltage properties of the conventional low-molar-mass liquid crystals with higher angle of view (hazefree)and bistability arisingfromthe polymer network dispersed


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R A J A R A M ET A L .

Liquid Crystal-Polymer Network Composites

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in the liquid crystal matrix. The polymer networks formed in such systems containing mesogenic monomers have been extensively studied (5-14). To understand this new class of materials requires an interdisciplinary approach: free-radical photopolymerization chemistry, low-molar-mass liquid crystals physics, materials science of thermosets and display technology. This short review will touch on aspects of understanding of the morphology of these polymer networks formed in liquid crystal media.


Materials in Polymer-Stabilized Liquid Crystals Molecular engineering of the monomers which form the polymer network in these composites is essential for controlling network structure and improving the electrooptic properties of the flat-panel displays. In order to control the polymer networks, diacrylate monomers were designed with varying amounts of flexibility by controlling the spacer between the mesogen and the polymerizable acrylate moiety, by varying the size of the mesogensfrombiphenyl, triphenyl etc., and by controlling the functionality. Often the building block for the mesogens has been a biphenyl core. A simple reactive monomer is 4,4' bisacryloyloxy Ι,Γ biphenylene (BAB) in which attachment of polymerizable acrylate moieties is on the 4,4' positions of the biphenylic mesogen. The polymer network resulting from these monomers gave displays with low threshold voltage and poor contrast. In order to further increase the alignment of the polymer network and possibly increase the sharpness of the electro-optic curve (transition from transmitting to scattering mode), a hexamethylene spacer was introduced between the biphenyl mesogen and the polymerizable acrylate group of the monomer BAB. This monomer, called BAB6 i.e., 4,4' -bis[6-acryloyloxy)-hexyloxy-] 1,1' biphenylene, forms polymer networks of higher orientation, because the flexible hexamethylene spacer gives more conformational freedom for the monomers during photopolymerization. The displays made from these highly aligned networks (fiber-like morphology) show high contrast ratio and hysteresis which provide gray scale with multiplexing capability (15). Further modification of the monomer was done; specifically, the biphenyl core of BAB6 was extended by another benzylic ester group on both 4,4' positions, resulting in the monomer 4,4'-bis-{4-[6-(acryloyloxy)hexyloxy]benzoate}-1,1 '-biphenylene (BABB6). This monomer has a rigid core which is more than twice the size of the mesogen of BAB. Unlike BAB which melts from crystal to isotropic liquid at 150 °C, this monomer exhibits several higher order smectic phases (S 108.5 °C; S 112.3 °C; S 119.2 °C and Ν 124.9 °C); S and S are unidentified. BABB6 materials exhibit high contrast ratio, low threshold voltage and haze-free display devices. Some interesting morphologies of these materials were found to provide a dramatic effect on the display device performance (16). A monomer with higher solubility in common low-molar-mass liquid crystals (such as 5CB [4'-pentyl 4 biphenyl carbonitrile], 8CB [4'-octyl 4 biphenyl carbonitrile], or E48 [a eutectic mixture of several low-molar-mass liquid crystals]) is C6M or RM82 [( 1,4-di-(4-(6-acryloy loxyhexy loxy)benzoy loxy)-2-methylbenzene], which was extensively used by Hikmet et al. (5-14) in studying these composites. Another xl

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monomer used in these studies is BMB (77) [4,4'-bis(2-methylpropenoyloxy)biphenyl]. Commonly used photoinitiators are BME (benzoin methyl ether) or Irgacure 651 (Ciba Geigy). Photopolymerizations are carried out upon exposure to UV light of wavelength 365 nm. The intensity of the UV lamp is varied depending on the requirements of the experiment. The molecular structures of these materials are shown in Chart I. Kinetics of Photopolymerization. For each of these monomers, the polymerization rate is relatively low at low monomer concentration (less than 1 Wt%). This is because dilution reduces initiator efficiency and prevents auto-acceleration (Tromsdorff effect), which is typical of bulk photopolymerizations. The polymerization rate increases with increasing monomer concentration (18). Similar observation was made by following the double bond conversion of the diacrylate (18). The maximum polymerization rate and the conversion at maximum rate increase with increasing monomer concentration, suggesting that, at low monomer concentration, the mobility in the mixtures is high and decrease of rate at later stages results from a depletion of monomers and a decrease in the mobility of the polymerrich phase as crosslink density increases (5). The polymerization rate is also dependent upon the architecture of the monomer. In dilute solution, differences in mobility are less, and factors such as the electronic structure of the monomers is important. Polymer Network Stabilized Liquid Crystal Displays. The alignment of liquid crystals plays an important role in the operation of a display. The alignment is conventionally induced by the display cell surfaces, but by distributing the surface of a polymer network through out the bulk of the liquid crystal, new properties are possible, and the performance of conventional devices can be improved. This short section will mention some of the conventional liquid crystal display devices modified by these polymer networks. Cholesteric Texture Displays. Cholesteric liquid crystals have unique optical properties because of their helical structure. When a cholesteric liquid crystal has a planar texture, it exhibits Bragg reflection; a focal-conic texture scatters light; and a homeotropic texture, in which the helical structure is unwound, is transparent. The primary disadvantages in using these materials in flat-panel displays, in spite of their brilliant colors and optical textures, are strong angular dependence of the reflected light intensity, instability of the focal conic texture, and lack of convenient means to switch between focal conic and planar texture. All of these disadvantages are overcome by the inclusion of a polymer network, which functions to perturb the alignment of the planar texture slightly and stabilize the focal conic (4, 19, 20). The wide viewing angle in these devices is due to the distribution of the helix axes of the cholesteric liquid crystals around the normal to the surface, resulting in reflected light distributed to a broad range of angles. The reflectivity of the cell is decreased for the above reason, but the contrast of the device remains high because the


34. R A J A R A M ET A L .

Liquid Crystal-Polymer Network Composites

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Morphology of Composites of Low-Molar-Mass Liquid Crystals and

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