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Phase separation and pattern instability of laser-induced polymerization in liquid-crystal-monomer mixtures

Chandroth P. Jisha, Kuei-Chu Hsu, YuanYao Lin, Ja-Hon Lin, Kai-Ping Chuang, Chao-Yi Tai, and Ray-Kuang Lee

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Abstract

Directly written by a ultra-short femto-second laser pulse, we report the phase separation and pattern formation induced by polymerization in a liquid-crystal-monomer mixture. By varying the scanning speed of optical fields along a line, pattern transitions of photon-induced polymer structures are illustrated in shapes of double-humped, single-humped, and broken stripes. The experimental data collected by optical microscopic images are in a good agreement with numerical simulations based on a modified set of coupled 2 + 1 dimensional diffusion equations. The demonstration in this work provides a step for controlling phase separation morphologies as well as transferring patterns in polymer-dispersed liquid crystals.

© 2011 Optical Society of America

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Figures (3)
Fig. 1
Fig. 1 Illustration of our experimental setup, where a frequency-doubled Ti:sapphire laser is focused on the sample mixed with liquid crystals (LC) and monomer molecules. A half-wave plate (HWP) is used to rotate the polarization of writing pulses. The translational stage is used to control the scanning rate of our focused beam.

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Fig. 2
Fig. 2 Optical microscopic images of our liquid-crystal-monomer mixture, taken after the laser writing at the scanning speeds of (a) 0.1 and (b) 3mm/sec, respectively. The corresponding scanning electron microscope (SEM) images, taken after washing out the liquid crystal molecules for the samples, are shown in (c) and (d), respectively. The close-up SEM images in higher resolutions are shown in (e) and (f), respectively.

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Fig. 3
Fig. 3 Optical microscopic images of a pattern transition in photo-induced polymerization processes by a laser pulse writing at different scanning speeds of (a) 0.1, (b) 0.5, (c) 2, and (d) 3mm/sec, respectively. Dark regions represents the polymer structures, while bright region corresponds to the liquid crystal rich area. Simulation results based on a modified diffusion model for the concentration of polymer molecules are demonstrated in (e–h) for different scanning parameters: f = (e) 0.001, (f) 0.01, (g) 1, and (h) 5, respectively. The insets shown in (e–h) correspond to the transverse profile of polymer molecules along the x-axis, at a fixed position y = 0. Other parameters used in simulations are D0 = 1, K0 = 6, α = 0.7, and A0 = 1.

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Equations (3)

Equations on this page are rendered with MathJax. Learn more.

∂ ϕ m ∂ t = ∇ [ D ( x , y , t ) ∇ ϕ m ( x , y , t ) ] − F ( x , y , t ) ϕ m ( x , y , t ) ,
∂ ϕ p ∂ t = F ( x , y , t ) ϕ m ( x , y , t ) ,
F ( x , y , t ) = K 0 exp [ − α ϕ p ( x , y , t ) ] I ( x , y ) ,
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