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

Abstract

We investigate the effects of monomer functionality on the formation and switching characteristics of holographic transmission gratings in polymer-dispersed liquid-crystal cells fabricated by using the 633 nm wavelength of an He-Ne laser. We present results for the microstructure, diffraction efficiency eta, and switching characteristics of gratings formed with acrylate monomers of functionalities ranging from 2 to 4. The microstructure and diffraction efficiency are sensitive to functionality; both improve with increasing functionality. For functionalities of 2.5 or more, eta approaches 34% and can be switched off with electric fields of about 20 MV m(-1).

A review of the electro-optical properties and their modification by radiation in polymer-dispersed liquid crystals and thin films containing CdSe/ZnS quantum dots

“…13 the maximum diffraction efficiency and switching characteristics of transmission gratings as functions of the monomer functionality. These gratings are characterized by functionality-dependent maximum diffraction efficiency (≥30%), operating ac (1 kHz) voltages on the order of 0.5 MV/m, and response times ∼1-4 ms [63]. These experiments have demonstrated that the maximum diffraction efficiency of the cured gratings increases in a non-linear fashion with increasing monomer functionality used for the synthesis of the grating structure.…”

“…Again, we used liquid crystal (E8), monofunctional oligomer (CN135), tetrafunctional crosslinker (SR295), photoinitiator (Methylene Blue), and co-initiator (p-toluene sulphonic acid). The functionality of the polymerizable mixture was calculated by using the relative percentages of each monomer used in the mixture [63]. In this context, we show in Fig.…”

Highly diffractive, reversibly fast responsive gratings formulated through holography

“…A 441.6 nm He-Cd laser was split into two equal beams, each with an intensity of 8.8 mW cm À2 , and then directed at the LC cells for 20 s at an angle of q set ¼ 24 before being blocked by a timer shutter. All recorded holograms were post-cured under a mercury lamp for 300 s. The grating period L and Klein-Cook parameter Q are 1060 nm and 16.5 respectively according to eqn (1) and ( 2), indicating that our gratings belong to the Bragg regime type (volume gratings) and their properties can be predicted by Kogelnik's coupled-wave theory, 32,43,53…”

“…[39][40][41][42] One effective way is to change the monomer type and the average functionality. 40,[43][44][45][46][47][48] Besides the crosslinking density and the elastic force of the polymers giving a sufficient chemical difference from the LCs, the main hindrance for the holographic polymerization induced phase separation is monomer gelation. Only if the LC diffusion is much faster than the LC nucleation, and if the latter occurs earlier than the polymer network gelation, can a perfect segregation structure be formed.…”

Low-voltage-driven and highly-diffractive holographic polymer dispersed liquid crystals with spherical morphology

“…Only the diffusion of LCs is faster than their nucleation and the nucleation is faster than the polymerization gelation, can a well-dened grating be formed, affording high diffraction efficiency. Through careful selection of LC concentration, 17,18 monomer type and average functionality, 16,[18][19][20][21][22][23][24][25] sensitizer and co-initiator, 13,14,20,26 or judicious optimization of the processing conditions such as recording temperature, light intensity and exposure time, 17 the micromorphology of HPDLCs can be well controlled to offer high diffraction efficiency. Nevertheless, the electro-optical properties of HPDLCs are also closely dependent on their micromorphology, and HPDLC gratings with high diffraction efficiency usually need a high driving voltage to switch.…”