Orientational ordering of short LC rods in an anisotropic liquid crystalline polymer glass

The orientational phase diagram and ordering of guest liquid crystalline (LC) rods in a host liquid crystalline polymer (LCP) matrix quenched below the glass transition is determined by field theory. Microscopic anisotropic interactions can align the LC rods to each other and also align LCP matrix side chains and the LC rods in the plane normal to the local LCP chain contour. Our numerical analysis suggest ways to exploit host entropy, anisotropy of microscopic interactions and manipulate properties of LC rods for modern applications. We predict a nematic–nematic discontinuous orientational transition from a guest stabilized to a guest–host stabilized region and a reentrant transition from a guest stabilized nematic region to a host only stabilized regime. A detailed analysis of phase boundaries transitions and ordering is presented. 2004 Elsevier B.V. All rights reserved. Orientational ordering of LC rods in a host polymer matrix with microscopic anisotropy from molecular shape and/or interactions, quenched in a certain phase region, is relevant to many applications. These include liquid crystal technology, spatial light modulators and also high strength fibers such as Kevlar, a polymide. In chromatographic applications, a quenched anisotropy can be used to promote separation of similar compounds. In the present study, the host matrix is an amorphous polymer [1] with mesogen side chain quenched below the glass transition. Host anisotropic glass formers made of mesogens with side chains were studied recently for their remarkable polarization holographic data storage capabilities [2]. The electro-optical performance and response of LC guest rods immersed in a glassy polymer matrix is governed by the morphology and the relation among orientational ordering of the LC guest and the host disordered matrix. Previous studies centered primarily on effects of isotropic glasses on orientation of LC rods [3]. Computer simulations and theory showed that a * Corresponding author. Fax: +1-617-253-7030. E-mail address: [email protected] (J. Cao). 0009-2614/$ see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.02.086 quenched isotropic disorder can modify the order of the nematic–isotropic (N–I) transition and even suppress it [3]. Experiments on LC 8CB in silica gels show a glassy like orientational relaxation [4], results predicted earlier for rods in quenched isotropic disorder from spin models and simulations [5]. Motivated by the desire to optimize control of molecular orientation in liquid crystal displays, phase diagrams of a polymer dispersed liquid crystals (PDLCs) of polystyrene and EBBA were studied as a paradigm of PDLC without a liquid–liquid immiscibility region [6]. Experiments of C NMR and optical microscopy on LC droplets in PDLC of 5CB in a polymer matrix showed that ordering increased with decreasing pore size [7]. Polymer matrix features, droplet size and dispersity can be used to shift LC ordering to faster switching times at a lower voltage [8]; it was also found that application of an external field to the PDLC increases the nematic–isotropic temperature threshold [9]. Motivated by the theoretical and experimental considerations described above, we address the following question: what is the effect of an isotropic/anisotropic amorphous polymer glass on orientational ordering of LC rods? L. Gutman et al. / Chemical Physics Letters 389 (2004) 198–203 199 Here we consider a guest–host system made of a polymer matrix with strong backbone interactions and a guest made of low molecular weight rods. The polymer matrix carries mobile side chain free to move in the plane perpendicular to the polymer backbone contour. In this Letter we show that a glassy polymer matrix prepared at the N–I transition conditions, in fact, can cause a first order nematic–nematic transition in the guest LC rods that manifests as a spike in the orientational order parameter hSir. We also show that the anisotropic quenched LCP matrix suppresses completely the N–I phase transition expected from the guest LC rods alone in the absence of the anisotropic glass. Herein we construct a field theory for LC rods immersed in a host matrix of a homo-polymer with side chains quenched below its glass transition. The physical scenario described is depicted in Fig. 1. The worm like homo-polymer backbone (solid line) carries side chains (double headed arrows) that can rotate in the plane normal to the LCP local chain contour. The side chains in our model tend to align the LC rods (cylinders) in the plane normal to the LCP chain contour. Optical materials that display a normal guest–host microscopic orientational ordering were synthesized recently [10]. Optimization of free volume for alignment in these experiments was correlated to a larger normal guest–host ordering, an increase in the LC alignment, decrease in switching response times and overall, a better material performance in holographic data storage applications. The present study centers on the effect of quenching the LCP matrix in different regions of the phase diagram on orientational ordering of the LC rods. Our numerical analysis does not invoke the Landau expansion method used in other anisotropic systems [11], i.e., the present approach is suitable to make predictions for thermodynamical states not necessarily close to critical regions. Our model describes adequately the anisotropy in miFig. 1. LC rods in a LCP quenched matrix. Continuous solid line is LCP chain. Double headed arrow is LCP side chains. Cylinders are LC rods. croscopic pair interactions of LC, LCP, LC/LCP molecule pairs and it includes both an athermal repulsive contributions and a soft temperature dependent potential [13]. The Hamiltonian for the guest–host LC/LCP system is: