Phase behaviour and electro-optic characteristics of a polymer stabilized ferroelectric liquid crystal

Abstract

The effects of adding a diacrylate monomer or its polymerized network to a ferroelectric liquid crystal have been characterized. The monomer lowers the temperatures of transition to the more ordered phases, whereas the polymer network phase separates into polymer rich and LC rich phases and has little effect on the LC phase behaviour. Ferroelectric polarization decreases comparably in both monomer and networked systems. As the network concentration increases, the size of LC domains decreases considerably. With low concentrations of polymer and, thus large LC domains, optical response and tilt angle remain fairly independent of polymer concentration, but as the polymer concentration increases, switching speed and tilt angle decrease dramatically. Polymerization rate maxima increase with monomer concentration until saturation of monomer in the liquid crystal is reached. The rate maxima then decrease as monomer must diffuse from monomer rich droplets. Double bond conversion during the polymerization is comparable for all monomer concentrations below 50 per cent.

Polymer Network-Stabilized Liquid Crystals

“…In contrast to non-stabilized samples, the effective tilt is strongly dependent on applied voltage, having a profound impact on the transmission±voltage characteristic of the cell [73,75] and allowing gray-scale generation with a device that is otherwise (in the non-stabilized state) bi-stable, thus allowing only monochrome information display. A similar behavior was reported for the spontaneous polarization, which is found to be smaller than for the pure FLC and decreases with increasing polymer concentration, [73,74] as exemplarily depicted in Figure 19. From this data, it is clear that the reduction of the spontaneous polarization is not solely due to dilution, which is a minor effect at these low network concentrations.…”

“…[74] As the response time, t, and spontaneous polarization, P S , are closely linked to the rotational viscosity, g, by the relation t » g/P S E, this might explain the observed decrease of the rotational viscosity reported for increasing network concentration. [74] It should also be pointed out that the introduction of a polymer network has an influence on the local phase symmetry of the liquid-crystalline material in the vicinity of the transition from the ferroelectric SmC* to the paraelectric SmA* phase, depending on the phase in which the polymerization was carried out. [77] Polymerization at large tilt angles in the SmC* phase tends to preserve the tilted order close to the network strands, which causes a contribution to the spontaneous polarization even when the bulk transition to the SmA* phase has occurred.…”

“…The effective tilt angle of the stabilized FLC was reported to decrease with increasing network density and to be smaller than for the pure FLC. [73,74] This is attributed to different microscopic regions within the sample: those that switch with the same tilt as the pure FLC (bulk switching, compare to PSCTs) and regions in the vicinity of the network strands that do not switch at all or only with a smaller tilt angle. The measured effective tilt is an average of the two regions.…”

Study of smectic layer structure of polymer-stabilized ferroelectric liquid crystal with grayscale memory

“…A polymer network is formed when a small amount of acrylate monomer [7] or polythiol/polyene monomer [8] is dispersed in the FLC. In our device, there is a high probability of dispersing the polymer network oriented in the rubbing direction as multi-functional monomers of the same type are polymerized in orientation.…”

Phase separation kinetics of an SA liquid crystal in a reactive epoxy-amine PDLC system: an X-ray diffraction study

“…Polymerization kinetics of reactive PDLC systems have been studied for chain polymerization (radical polymerization under UV or electron beam irradiation) [2][3][4][5] or for step polymerization [6][7][8][9][10] . The authors mainly describe the effects of the polymerization parameters (reaction temperature, reaction heat and monomer conversion) on the LC-phase separation in the fully cured polymer.…”