Search

BACKGROUND: To reduce the driving voltage of transmission holographic polymer-dispersed liquid crystals (HPDLCs), the polymer interface was modified by incorporating various amounts of vinyloxytrimethylsilane (VOTMS) monomer to the conventional grating formulation based on polyurethane acrylate (PUA). RESULTS: With the addition of increasing amounts of VOTMS, the contact angle of the film and the droplet size of the liquid crystal (LC) monotonically increased, implying that the VOTMS segments of the polymer are preferentially exposed to the interfaces and provide greater immiscibility with LC molecules. The operating voltage monotonically decreased with increasing VOTMS content with a minimum switching voltage of about 15 V with a response time of about 10 ms. However, in the presence of VOTMS, the droplets coalesced and caused random scattering, thus lowering the off-state diffraction efficiency below that of virgin PUA. CONCLUSION: It was found that VOTMS was essential to drive the film. The VOTMS segments of the polymer are preferentially exposed to the polymer/LC interfaces and decrease the surface anchoring strength and also influence the orientation of LC droplets. Also, lower molecular weight bifunctional polypropylene glycol generally gave greater diffraction efficiency, confirming that grating formation is favoured with higher crosslink density due to the greater polymer/LC phase separation. Copyright © 2008 Society of Chemical Industry

Films were made by incorporating various quantities of fluoroalkyl acrylate (FA) into the chain termini of polyurethane acrylate oligomer of polymer stabilized cholesteric liquid crystals (PSCLCs). The effects of switching voltage, scanning electron and polarized optical microscope morphologies, texture transition and reversibility, and contact angle of the films were then studied. It was found that the switching voltage of the film decreased with the addition of FA up to 0.8 wt%, beyond which it increased. The decrease and increase were respectively explained in terms of decreased interface energy as noted from the contact angle of the LC on polymer surface, and too small droplet size as noted from the SEM. Copyright © 2008 John Wiley & Sons, Ltd.

BACKGROUND: A slight change of polymer structure leads to a significantly different solubility parameter (SP), which is related to incompatibility and interface anchoring energy between a polymer and a liquid crystal (LC). RESULTS: Holographic gratings in polymer-dispersed liquid crystals have been fabricated based on various urethane acrylate oligomer sizes and type of reactive diluent at two LC loadings. A large SP gap, using N-vinylpyrrolidone as reactive diluent, gave much faster saturation of diffraction efficiency at higher level, but the grating failed to operate. On the contrary, films having a small SP gap, using 2-ethylhexyl acrylate, operated under 20 V with a response time of about 10 ms, although their saturation of diffraction efficiency was relatively low. The results were interpreted in terms of morphology (determined using scanning electron microscopy), anchoring energy and resin elasticity. CONCLUSION: Low driving voltage and high diffraction efficiency are obtained by controlling the SP gap between LC and polymer. Copyright © 2007 Society of Chemical Industry

The low surface energy and the great immiscibility of poly (dimethylsiloxane) (PDMS) with liquid crystals (LCs) are used in the fabrication of holographic polymer dispersed liquid crystals (HPDLCs). By adding increasing amounts of PDMS, the extent of the phase separation between the polymer and the LC, the LC channel width, and-eventually-also the diffraction efficiency of the film can be increased, while keeping the droplet size essentially the same. In addition, the presence of PDMS causes a decrease in the switching voltage and an increase in the response time. At an optimum content of PDMS (PUA40), a minimum switching voltage of 4 V microm(-1), a rise time of 0.20 ms, and a decay of 14.75 ms were obtained. Regarding the effect of the LC content, an overshoot of the diffraction efficiency was observed when the amount of LC exceeded 35 %, which can be attributed to droplet coalescence.

The effects of poly(vinyl carbazole) (PVK) doped to a poly(urethane acrylate) matrix in holographic-polymer-dispersed liquid crystals were studied. With the addition and increasing amounts of PVK, the driving voltage and rising time of the films decreased because of the increased effective local electric field across the liquid crystal (LC) droplet. Off-state diffraction efficiency was increased with the addition and increasing amounts of PVK presumably because of the increased elasticity of the polymer matrix, which augmented the phase separation of the polymer and LC by effectively squeezing the LC molecules out of the polymer matrix. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Rheological, thermal, and mechanical properties of polypropylene homo polymer (PPH)/amorphous poly alpha olefin (APAO) blends as a function of molecular weight, comonomer type and content, and blend composition have been investigated. Homo APAO grade showed better compatibility than copolymerized ones in terms of rheological and thermal properties. The mechanical strength showed strong dependence on APAO content and type, and the impact strength and melt index rapidly increased for certain types of APAO at and above 30 wt%. On comparison with commercially used PPH/ethylene–propylene rubber (EPR) blend system, it is supposed that PPH/APAO blend can be successfully used in thermoplastic polyolefin applications. Copyright © 2007 John Wiley & Sons, Ltd.

Effects of octanoic acid (OA) on the morphology, diffraction efficiency, and electro-optic properties of the transmission mode of holographic polymer-dispersed liquid crystals (HPDLC) are studied. Droplet size decreases with increasing OA content (0-9 %), and this leads to a monotonic increase in off-state diffraction with increasing OA content. However, on-state diffraction decreases with increasing applied voltage and shows a minimum at 6 % OA, for which minimum switching voltage (5 V microm(-1)) and maximum contrast ratio (10) are obtained. Rise time and decay time decrease with increasing OA content. Interposition of OA between polymer and LC droplet is theoretically predicted by the spreading coefficient (lambda>0) calculated on the basis of the solubility parameter, while the coalescence behavior of droplets is described by a dimensionless group (gamma d rho / mu(2)) called coalescence number.

Doping of conductive fullerene particles to the formulation of conventional holographic polymer dispersed liquid crystal-induced dual effects of reducing both droplet coalescence and operating voltage. Fullerene induced an induction period which otherwise does not exist, followed by a gradual increase of diffraction efficiency to a saturation value being increased with increasing fullerene content. The increased diffraction efficiency was caused by the decreased droplet coalescence which was due to the hindered migration of LC by the fullerene particles. On the other hand, doped fullerene particles augmented the conductivity of polymer phase and hence the local electrical field imposed on LC droplet, which overcome the threshold for driving and reduced operating voltage and response times. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5590–5596, 2007

Nanosized silicas added to holographic polymer-dispersed liquid crystals (HPDLC) provide the resin phase with increased elasticity, dimensional stability, and the high diffraction efficiency of the gratings. On the other hand, nucleation and growth of periodic modulation are delayed, especially with small-sized silica because of the increased viscosity of the resin mixture. Effects of the LC/resin composition and cell gap on the diffraction efficiency have also been studied.

The interposition of surfactants between polymer and liquid crystal (LC) droplets was theoretically predicted by the positive spreading coefficient (0