Monomer functionality effects in the formation of thiol-ene holographic polymer dispersed liquid crystals

Holographic polymer dispersed liquid crystals (HPDLCs) are a polymer/liquid crystal (LC) composite with wavelength selective diffraction that can be switched in microseconds with electric field. As such, HPDLCs are widely applicable in optical and photonic devices. Recently, replacing acrylate polymer with thiol-ene polymer as host for HPDLCs has improved the stability and electrooptic performance of these materials. This work examines the electrooptic performance and morphology of thiol-ene-based HPDLC reflection gratings. To further understand the relationships between the formation and performance of thiol-ene HPDLCs, electrooptic performance was characterized as a function of polymerization rate, gel point conversion, and the presence of excess monomer (stoichiometry). To this end, HPDLC formulations were examined to isolate the contribution of ene monomer functionality, thiol monomer functionality, and thiol-ene stoichiometry. In all formulations, the performance of HPDLC reflection gratings is correlated to the generation of morphology typified by well-defined polymer/LC lamellae and small LC droplet size. Increasing the rate of polymerization through increasing laser intensity, increasing ene monomer functionality, or thiol-ene stoichiometry improves the diffraction efficiency (DE) of HPDLCs by reducing LC droplet size. Gel point conversion is also critical to producing well-performing HPDLC reflection gratings. HPDLCs based on thiol-ene polymer with gel points from 40 to 60% monomer conversion show optimal optical behavior. This increase in performance is related to the impact of gel point on morphology, as low gel point conversion leads to the formation of small droplets and poorly defined grating structures while high conversion gel points lead to the formation of large LC droplets. Therefore, optimal HPDLC materials are formed with moderate conversion gel points that allow both well-defined grating structure and small droplet formation.