Monomer Functionality Effects in the Formation of Thiol−Ene Holographic Polymer Dispersed Liquid Crystals

Abstract

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.

Characterization of volume holographic recording in photopolymerizable nanoparticle-(thiol-ene) polymer composites at 404 nm

“…32,33 The thiol-ene photopolymerization mechanism was also used to record electrically switchable reflection gratings in holographic polymer-dispersed liquid crystals at Ar-ion and solid-state laser lines of 364, 442, 488, and 532 nm. 31,[34][35][36] Thanks to the recent development of highly coherent blue diode lasers for HDS, 37 it is possible to implement compact HDS systems in the blue (at and near the 405-nm diode laser line). 38,39 The use of a blue diode laser provides higher storage capacity in HDS, [18][19][20][37][38][39] but the effect of grating distortions due to shrinkage on degradation in readout fidelity may become more significant than the case with a green laser.…”

Phototunable Azobenzene Cholesteric Liquid Crystals with 2000 nm Range

“…[1] To date, these materials have seen most extensive application in the visible spectra in displays, [2] filters, [3][4][5][6] and in a wide-range of specialty optical applications. [7][8][9][10][11][12] The emergence and growing utilization of infrared sources and their application in optical communication, the need for infrared optical materials and in particular, dynamic infrared optical materials are evergrowing.…”

High Performance Holographic Polymer Dispersed Liquid Crystal Systems Formed with the Siloxane-containing Derivatives and Their Applications on Electro-optics

“…Much attentions have been attracted to HPDLC systems due to their unique switching property in electric field to make them applicable to information displays, optical shutters, and information storage media [6][7][8][9][10][11][12][13][14][15]. Many research groups have made efforts to realize useful recording materials for high performance holographic gratings [16][17][18].…”

“…Photo-polymerizable materials, typically multifunctional acrylates, epoxy, and thiol-ene derivatives have been mostly studied because of their advantages of optical transparency, large refractive index modulation, low cost, and easy fabrication and modification [19][20][21][22][23][24][25]. T.J. Bunning group has reported investigation that the correlation between polymerization kinetics, LC phase separation, and polymer gel point in examining thiol-ene HPDLC formulations to enable more complete understanding of the formation of thiol-ene HPDLCs [26]. Kim group has developed that the doping of conductive fullerene particles to the formulations based on polyurethane acrylate oligomers in order to reduce the droplet coalescence of LC and operating voltage [27].…”