The film: poly(N-vinylcarbazole) (PVK) The fabrication of the cell:
1. Cell gap: 6 mm
2. The thickness of the fabricated PVK film was 0.2 mm.
Liquid Crystal
:
5CB (K15)
Liquid crystal PVK glass glass PVKThe LC cell with dual-PVK films
Thermally induced phase separation of liquid crystal and
poly(N-vinylcarbazole) substrates
Yuan-Di Chen,
1*
Andy Ying-Guey Fuh,
1,2,3
and Ko-Ting Cheng
2,3*
1
Department of Photonics, National Cheng Kung University, Tainan 70101, Taiwan.
2Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan.
3
Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan.
Electro-Optical Laboratory, National Cheng Kung University
Polymerization-Induced Phase Separation
Polymerization-induced phase separation, or PIPS, occurs when a liquid crystal is mixed with a solution that has not yet undergone polymerization (a prepolymer). Once a homogeneous solution is formed, the polymerization reaction is initiated. As the reaction progresses, the liquid crystal molecules come out of solution and begin to form droplets.
Thermally-Induced Phase Separation
Thermally-induced phase separation, or TIPS, can be used when the polymer binder has a melting temperature below its decomposition temperature. In this method, a homogeneous mixture of liquid crystal and a melted polymer is formed. The solution is cooled at a specific rate to induce phase separation. Liquid crystal droplets begin to form as the polymer hardens. The droplets continue to grow until the glass transition temperature of the polymer is crossed.
Solvent-Induced Phase Separation
The third common type of phase separation is called solvent-induced phased separation, or SIPS. This process requires both the liquid crystal and polymer to be dissolved in a solvent. The solvent is then removed (typically by evaporation) at a controlled rate to begin the phase separation. Droplets start growing as the polymer and liquid crystal come out of solution and stop when all of the solvent has been removed.
Introduction:
The methods of the phase separation based on PDLC light shutters
Conclusion:
In the current study, a novel ATIPS method of LCs and polymers was presented. The fabricated LC light shutter possessed the advantages of low driving voltage, fast response
in the order of milliseconds, independent of polarization, high CR (~300:1), and being polarizer free. Moreover, the electrically switchable LC light shutter in the scattering mode had extremely
promising potential applications, such as in energy-efficient smart windows and scattering mode LC displays.
Acknowledgement:
The authors thank the National Science Council of Taiwan for financially supporting this research under Grant No. NSC 98-2112-M-006-001-MY3 and NSC
99-2112-M-006-002-MY3. Additionally, this work is partially supported by Advanced Optoelectronic Technology Center as well.
Materials and sample fabrications
Experiment 2 : Polarized optical microscopic (POM)
References
Experiment 3 : Polarization Detection
Measured transmission of the fabricated scattering mode LC light shutter as a function of an applied AC (1 KHz) voltage. Insets show photographs of the LC light shutter at 25 °C with the applied AC voltages of (a) 0 and (b) 18 V.
(a)
(b)
SEM images of LC samples after being heated to 60 °C at a heating rate of 30 °C/min. The temperature of the LC sample was maintained at 60 °C for 8 min, and then cooled to 25 °C at the cooling rates of (a) 30, (b) 10, (c) 5, and (d) 1 °C/min.
[1] S. T. Wu and D. K. Yang,
“Reflective Liquid Crystal Displays” (Wiley, New York, 2001).
[2] Y.-D. Chen, Andy Y.-G. Fuh and K.-T. Cheng,
“Advanced thermally induced phase separation of liquid crystal and poly(N-vinyl carbazole) films,” under
peer-reviewed (2012).
*)
Electronic mail: [email protected], [email protected]
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In the current study, a novel method of advanced thermally induced phase separation (ATIPS) of liquid crystals (LCs) and polymers is presented. The process involves a combination of solvent- and thermal-induced phase separations. The LCs and PVK play the roles of solvent and solute, respectively, during the processes of ATIPS. The nematic LC sample fabricated by two substrates coated with poly(N-vinyl carbazole) (PVK) films is heated and then cooled, generating the rough PVK layers onto the surfaces of the substrates. The LC sample having rough PVK layers produces micron-sized, multiple domains of disordered LCs that can scatter incident light. Additionally, an application of a scattering mode light shutter fabricated by ATIPS is reported. The shutter has the advantages of low driving voltage, fast response, polarization-independent scattering, high contrast ratio, and being polarizer free. The electro-optical properties of the light shutter and the morphologies of the PVK layers are examined in detail. The present study is the first to report such a phase separation method and investigate the high performance of an LC device.
The transparent LC cell
Experiment 1 : Fabrication of the scattering LC cell
100％: dual-PVK cell filled with LC
Experiment 4 : Scanning electron microscope (SEM)
(a)
(b)
(c)
(d)
The multi-domains of the LCs with disordered alignments presented as different colors due to their different birefringence properties (phase retardations). The sizes of the multi-domains are measured to be in the order of micrometers. Experimentally, the POM images indicate that the LC directors were randomly dispersed.
