Effect of Microgravity on the Distribution of Liquid-Crystal Droplets Dispersed in a Polymer Matrix

Abstract

The average droplet size and their distribution in polymer-dispersed liquid-crystal (PDLC) materials, prepared under microgravity and terrestrial environments, are studied experimentally as a function of cure time. The PDLC films are prepared using a polymerization-induced phase-separation (PIPS) technique. A theoretical kinetic model, based on the birth-death type of differential equation, is developed. (This amounts to ignoring the coalescence produced by hydrodynamic motion.) Two important observations are reported in this study. The first, we found that microgravity plays a decisive role in establishing a Gaussian distribution of the liquid-crystal droplets which is in good agreement with theory. This behavior is maintained even if the droplets are allowed to grow substantially under terrestrial conditions provided the final stages of growth takes place under microgravity environment. The growth under microgravity conditions is diffusion dominated with an estimated effective spatial diffusion coefficient D = 3.78 × 10-12 cm2 s-1, which seems appropriate for polymer matrix systems with high viscosities. The second result shows that the average particle size grows faster than the theoretically predicted t 1/3 dependence. We believe that the coalescence processes that take place during the growth of droplets at 1 g during the cure time are responsible for this deviation.

Thiol–enes: Chemistry of the past with promise for the future

“…To date, the liquid crystals are mixtures of several cyanobiphenyl molecules, and the crosslinked networks are formed by the copolymerization of a thiol–ene mixture (Norland 65 or a related thiol–ene system, or another similar thiol–ene mixture from Norland Optical Adhesives, Inc.) and a critical concentration of the nonreactive small‐molecule liquid crystal. A very large number of publications (several hundred) have appeared since 1991 dealing with every aspect of the phase‐separation process, morphology, and optical switching performance of many types of PDLCs based on multifunctional thiol–enes, virtually all from Norland Optical Products, and we only list a few of the references in this review to provide lead references to the interested reader 140–211. In traditional PDLCs, the photocurable resin, initially optically clear because of the mixing of the liquid crystal with the thiol–ene, which yields a homogeneous solution, becomes opaque as the polymerization proceeds upon illumination with a nontemporally and nonspatially coherent light source, eventually resulting in a crosslinked network with phase‐separated liquid‐crystal‐rich regions and a crosslinked network matrix that has a much smaller impurity concentration of liquid‐crystal molecules in the network.…”

“…NOA65, the thiol–ene mixture typically used in PDLC applications, is reputed140 to consist of AE4, which is a tetrafunctional allyl ether formed from the reaction of trimethylolpropane diallyl ether and isophorone diisocyanate, TriThiol1, and 5 wt % BP as a photoinitiator. In films formed by the photopolymerization of NOA65 with a liquid‐crystal component,140–171 the switching efficiency of the film is no doubt dictated by the nature of the basic crosslinked network. As we have stated previously, the uniform crosslink density of the film formed by the polymerization of multifunctional thiol–ene mixtures is characterized by very well‐defined phase‐separated liquid‐crystal‐rich regions with relatively narrow dispersity in the shape and size of the liquid‐crystalline regions.…”

Rheological Behavior during Phase Separation Induced by UV Curing

“…The basic step in the preparation of such composite films is the phase separation in which the microdroplets of the insoluble phase are generated in the polymer matrix. The most frequently used method is the photopolymerizationinduced phase separation [1][2][3][4]. When the molecular weight increases during curing, the microdroplets are produced as a result of a sharp decrease of solubility.…”