Transmission holographic gratings produced using networked polyurethane acrylates with various functionalities

Abstract

Holographic gratings have been fabricated in networked polyurethane acrylates with various cross-linking densities. It was found that low (f = 2) and high (f = 6) functionality monomers gave narrow and clean polymer–liquid crystal (LC) phase separation due presumably to the insufficient migration of LC molecules out of the polymer domains. However, the mechanisms leading to poor phase separation seem different, i.e., reaction at a slow rate for f = 2, and entrapment of LC molecules within highly networked polymer for f = 6. With intermediate monomer functionality (f = 3), relatively large channels with large LC droplets gave the greatest diffraction due to a large difference in refractive index between polymer and LC phases.

Effects of Monomer Functionality on Switchable Holographic Gratings Formed in Polymer‐Dispersed Liquid‐Crystal Cells

“…As Park and Kim have shown, although the diffraction efficiency increases monotonically with irradiation intensity (100 < I < 250 mW cm À2 ), it is the film composition which produces an optimum diffraction efficiency at a particular value, and the microstructure of the grating depends on the functionality of the acrylate monomers. [42] Similarly, Sarkar et al have presented detailed results on the morphology, diffraction efficiency, and switching characteristics of transmission gratings formed in mixtures of UV-curable acrylate monomers having monomer functionalities ranging from 1.3 to 3.5 and nematic LC TL203. In spite of substantial differences in the compositions of the mixtures and the experimental conditions, there are marked similarities between some of the major results of the present work and those of Sarkar et al [43] For example, the diffraction efficiency in both studies begins to increase with increasing functionality, but whereas it stays at the high value for f !…”

“…[40] Other important results have been published on similar but significantly different systems. [41][42][43][44] Because of the differences…”

“…The microstructure and diffraction efficiency are sensitive to functionality; both improve with increasing functionality. [42] ). Our choice of low laser intensity was based on the intuition that the polymerization and diffusion rates in effect at such low curing intensities should result in well-defined and "cleaner" morphology of the gratings.…”

“…Our choice of low laser intensity was based on the intuition that the polymerization and diffusion rates in effect at such low curing intensities should result in well-defined and "cleaner" morphology of the gratings. Whereas Bowley et al [41] and Sarkar et al [43] utilized an aliphatic urethane acrylate system cured with an Ar + laser, Park et al [42] studied a novel system containing polyurethane acrylates. As Park and Kim have shown, although the diffraction efficiency increases monotonically with irradiation intensity (100 < I < 250 mW cm À2 ), it is the film composition which produces an optimum diffraction efficiency at a particular value, and the microstructure of the grating depends on the functionality of the acrylate monomers.…”

Electro-optical switching of the holographic polymer-dispersed liquid crystal diffraction gratings

“…A comprehensive review of HPDLC is given by Bunning et al [14]. Though there are a number of impressive reports describing the performance parameters of these switchable holographic materials [14][15][16][17][18][19][20][21][22][23][24] there is still a growing need to produce materials that offer high diffraction efficiencies (DE), low switching fields and fast response times. Recently, new kinds of holographic gratings (POLICRYPS and POLIPHEM) were proposed that avoid LC droplet formation and realize a channel structure in which the polymer and LC molecules are almost completely phase separated [25,26].…”