Holographic polymer dispersed liquid crystals (HPDLCs) containing triallyl isocyanurate monomer

Abstract

The morphology and corresponding performance of holographic polymer dispersed liquid crystals (HPDLCs) based on thiol–ene polymer are dependent on a number of factors including the gel point conversion of the polymer, polymerization kinetics, and extent of liquid crystal (LC) phase separation. Previous research of HPDLC reflection gratings made from thiol–allyl ether polymer indicates that increasing polymerization rate in systems with moderate gel point conversion can improve diffraction efficiency (DE). This work examines HPDLC reflection gratings that contain the ene monomer triallyl isocyanurate (TATATO). In HPDLCs, thiol–TATATO polymerization is two times faster than the thiol–ene polymerization of triallyl ether. By substituting TATATO for triallyl ether, the LC droplet size within HPDLC reflection gratings decreases from 100nm to 25nm. The dramatic reduction in LC droplet size for thiol–TATATO HPDLCs increases baseline transmission from 55% in thiol–triallyl ether HPDLCs to 90% at 450nm. Unfortunately, the DE of thiol–TATATO HPDLCs is only approximately 10% due to poorly defined lamellae in the grating morphology. As determined with real-time IR (RTIR) spectroscopy, thiol–TATATO HPDLCs have significantly faster LC demixing kinetics in comparison to thiol–allyl ether HPDLCs. During holographic photopolymerization, the increased rate of LC demixing causes formation of LC droplets throughout the grating. The low DE of thiol–TATATO HPDLCs can be improved by mixing TATATO and allyl ether monomer. The morphology of ternary thiol–ene HPDLC formulations containing TATATO and allyl ether has a well-defined grating structure due to increased LC solubility in the system, an average LC droplet size of 50nm, and baseline transmission of nearly 85% at 450nm.