Effect of a polymer network on the alignment and the rotational viscosity of a nematic liquid crystal

Abstract

Polymer dispersions were made by photoinduced polymerization of 4,4'-bisacryloyl-biphenyl in liquid crystal 4-4'-cyano pentyl biphenyl as nematic solvent. Samples were prepared with 0.5, 1, and 1.5 wt 5% monomer. The polymerization was done in the isotropic phase. The rotational viscosities of the two lower concentration samples increased only rnderately akr UV curing ( 3 mw/cm2, 25 min). The 1.5 wt % sample gave under the same condition a nematic with a fairly rigid anisotropic network that did not realign in magnetic field up to 20 kG. We estimate that the mesh size d the network was in the order of 1 pm, significantly smaller than the magnetic coherence length d the solvent that is in the order of 5 pm. The diamagnetic anisotropies of the aligned samples were not affected significantly by the polymer. We studied electro-optical properties on thin films. The films were strongly light scattering but they could be switched to transparent state. We conclude that the polymer forms a network of loosely connected fibrils that interacts strongly with the director field. u I. INTRODUCTION Recently, new liquid crystal systems with interesting electrcwptical properties have been reported that contain a small amount of polymeric material. They were obtained by dissolving reactive monomers in nematic liquid crystals that were polymerized under UV light with the help of a photoinitiator. The polymers can form networks that strongly affect mechanical and optical properties. The sys- tems were first investigated at the Philips Research Lah- rato~ies'-~ mainly by ~ikmet.',~ He used sandwich cells and performed the polymerization in aligned nematic films.' The samples remained aligned and transparent but they changed to a strongly light scattering texture upon application of an electric field. . Hikmet studied the electrwptic properties of sand- wich cells that have a potential for display applications. Similar systems were studied by NMR in bulk It was confirmed that the orientation of the polymer strongly affects the orientation of the nematic director, even in the presence of a strong magnetic field (4.7 T) . The observa- tions indicate that the polymer forms a network that is strongly anisotropic when the polymerization takes place in the nematic state. The dynamics of the response to fields, the thresholds, and the relaxation processes are controlled by the rota- tional viscosity of the liquid crystal and the mesh size of the network. It is therefore of interest to explore the de- b pendence of the viscous properties upon network forma- tion and polymer concentration. In this article, we report