Photopolymerization Behavior and Phase Separation Effects in Novel Polymer Dispersed Liquid Crystal Mixture Based on Urethane Trimethacrylate Monomer Nahid Hosein Nataj, 1 Ali Jannesari, 2 Ezeddin Mohajerani, 1 Farhood Najafi, 2 Hossein Jashnsaz 1 1 Laser and Plasma Research Institute, Shahid Beheshti University, Evin, Tehran, Iran 2 Institute for Color Science and Technology (ICST), Tehran, Iran Received 13 November 2011; accepted 10 February 2012 DOI 10.1002/app.37000 Published online 5 May 2012 in Wiley Online Library (wileyonlinelibrary.com). ABSTRACT: Polymer dispersed liquid crystals (PDLCs) are often formed by polymer induced phase separation, based on photopolymerization of multifunctional acrylate monomers. The emerged morphology is controlled by the interplay between polymerization rate and phase separation dynamics, which depends on different parameters such as monomer structure and functionality. In this work, a new PDLC formulation containing urethane trimethacrylate (UTMA) monomer is introduced, which has different mo- lecular weight evolution, polymer gel point, and polymer- ization kinetics in comparison with some common ester acrylate (such as TMPTA and DPHPA) based PDLC compo- sitions. UTMA is synthesized and characterized by Fourier transform infrared, 1 H-NMR, and 13 C-NMR spectroscopic techniques. Simultaneous examination of polymer evolution and LC phase separation by real-time infrared spectroscopy shows that the UTMA based PDLC, which contains trifunc- tional urethane acrylate monomer, has greater amount of bond conversion, polymerization rate, and liquid crystal (LC) phase separation in comparison with TMPTA based PDLC. In spite of the acrylate monomers, which show gel point conversions as low as 1.83–5.72%, UTMA reaches to its maximum rate at 19.5% conversion, which causes higher phase separation and therefore greater LC domain size. The experimental results are explained more precisely by means of SEM and optical microscopy analyses. The results are confirmed by electro-optics measurements. V C 2012 Wiley Periodicals, Inc. J Appl Polym Sci 126: 1676–1686, 2012 Key words: photopolymerization; phase separation; polymer dispersed liquid crystal; real time infrared spectroscopy; electro-optical properties INTRODUCTION Polymer dispersed liquid crystal (PDLC) composites have a wide range of applications, such as flexible displays, tunable lenses, and optical switches. 1–5 The formation of PDLCs is typically induced by photo- polymerization. 6,7 A typical photocurable PDLC con- tains liquid crystal (LC), monomer and initiator. It is known that during photopolymerization of mono- mer, the LC loses solubility in the polymer and separates into a distinct phase through either liquid- liquid or liquid-gel demixing. 6 Upon phase separa- tion, PDLCs take one of two morphologies: droplet (‘‘swiss cheese’’) or interconnected (polymer-ball). The control of the film morphology is very impor- tant as it serves to modulate the electro-optical prop- erties of PDLC films. Much work has been done to optimize the switching properties, to get a high con- trast at a low switching voltage, high switching speed, and low hysteresis. 7–9 The emerged morphol- ogy is controlled by the interplay between polymer- ization rate and phase separation dynamics, which depends on different parameters such as LC concen- trations, irradiation intensity, exposure time, reaction temperature, and polymer composition. In any case, the mixture composition plays the most important role in determining the morphology of the recorded structures and their final optical and electro-optical properties such as switching voltage and transmittance. The formation of PDLCs through photopolymeri- zation has been widely investigated in acrylate and thiol-ene systems. 10–14 In acrylate systems, mixture compositions contain acrylate monomers with vari- ous chemical structures and functionality. The poly- merization behavior of acrylate monomer in creating polymer with gel point conversion, as low as 3%, causes LC demixing in PDLC systems to predomi- nately occur through liquid- gel demixing. 6 The kinetics of acrylate polymerization dictates the mor- phology of a given PDLC. The variation of polymer- ization rate affect on the size of the LC domains and the size of the phase separated polymer areas. The electro-optical behavior of acrylate-based PDLCs is not optimal, as the anchoring of the LC to the poly- mer is quite strong. Ultimately, the significant anchoring energy in acrylate based PDLCs results in Correspondence to: E. Mohajerani (e-mohajerani@sbu.ac. ir). Journal of Applied Polymer Science, Vol. 126, 1676–1686 (2012) V C 2012 Wiley Periodicals, Inc. citation: Nataj, N. H., Jannesari, A., Mohajerani, E., Najafi, F. and Jashnsaz, H. (2012), Photopolymerization behavior and phase separation effects in novel polymer dispersed liquid crystal mixture based on urethane trimethacrylate monomer. J. Appl. Polym. Sci., 126: 1676–1686. doi:10.1002/app.37000