Monomer functionality effects in the anisotropic phase separation of liquid crystals

Holographic gratings formed through the anisotropic phase separation of liq uid crystals show promise as switchable optical elements. In order to form useful elements, however, it is necessary to control the nanoscale morpholo gies within the grating films. In this manuscript, we evaluate the role of monomer functionality on the morphology and the electro-optical properties of both gratings and conventional scattering polymer-dispersed liquid cryst al (PDLC) films. Both of these structures are formed using polymerization-i nduced phase separation (PIPS) of liquid crystals from a cross-linked polym er formed through free-radical photo-polymerization. Floodlit (uniform illu mination) films and holographic gratings (from non-uniform illumination cau sed by the interference of two laser beams) were made using monomers with 2 -5 acrylate groups, while keeping the LC concentration constant in the syru ps. The morphologies of these films were examined using low voltage scannin g electron microscopy (LVSEM). In all cases, very small LC domains were for med with little indication of growth or coalescence. Lowering monomer funct ionality reduced the volume fraction of phase-separated domains in the floo dlit samples. For the grating samples, the local volume fraction and the LC domain sizes decreased substantially as the monomer functionality was decr eased. Using detailed image analysis, differences in the anisotropy of the domains was also probed. A much stronger tendency to form anisotropically-s haped domains was observed for the higher functional syrups. These domain a nisotropy differences are correlated with the number of reactive double bon ds per monomer and are suggestive of local environmental differences exerte d at the time of the domain formation. (C) 1999 Elsevier Science Ltd. All r ights reserved.