Influence of non-reactive solvent on optical performance, photopolymerization kinetics and morphology of nanoporous polymer gratings

Abstract

A study of nanoporous polymer gratings, with controllable nanostructured porosity, as a function of grating performance, photopolymerization kinetics and morphology is presented. Modifying the standard holographic polymer dispersed liquid crystal (H-PDLC) system, by including a non-reactive solvent, results in a layered, nanoporous morphology and produces reflective optical elements with excellent optical performance of broadband reflection. The addition of the non-reactive solvent in the pre-polymer mixture results in a morphology typified by void/polymer layer-by-layer structures if sufficient optical energy is used during the holographic writing process. The duration and intensity of optical exposure necessary to form well-aligned nanoporous structures can only be obtained in the modified system by (a) illumination under longer time of holographic interference patterning (30 min) or (b) illumination under very short time of holographic interference patterning (30 s) and followed by post-curing using homogeneous optical exposure for 60 min. Comparatively, a typical H-PDLC is formed in less than 1 min. To further understand the differences in the formation of these two analogous materials, the temporal dynamics of the photoinitiation and polymerization (propagation) kinetics were examined. It is shown herein that the writing exposure gives a cross-linked polymer network that is denser in the bright regions. With 60% (or even 45%) acrylate conversion, almost no free monomer would be left after the writing. Continued exposure serves primarily to add cross-links. This has the tendency to collapse the network, especially the less dense portions, which in effect get glued down to the more dense parts. To the extent that the solvent increases the mobility of the polymer network, this process will be aided. Equally important, the size of the periodic nanopores can be varied from 10 to 50 nm by controlling either the LC concentration in the pre-polymer mixture or by controlling the time of the homogeneous post-cure.

One‐Step Fabrication of Graded Rainbow‐Colored Holographic Photopolymer Reflection Gratings

“…Although the thickness, t , of each layer was only around 200 nm, the intersection region to the top surface is relatively large due to the small angle, ϕ , between these two planes. On the other hand, these lateral surface patterns were not observed in our previous work fabricated using the triangular prism setup4–7, 16 because the interference pattern was parallel to the two interfaces and did not intersect with the top surface of the H‐PDLC film (see Figure S1).…”

“…However, due to the intrinsic porous properties of the photopolymer material employed in our fabrication, it was difficult to measure the thickness of the periodic layers accurately from the SEM images. In our previous report,16 high resolution transmission electron microscope (TEM) was employed to characterize the detailed structure of the reflection gratings. A complicated process was required to cut the structure into ultra‐thin slices for TEM characterization, which was extremely demanding in sample preparation 2.…”

The effect of solvent on postcuring in free radical photopolymerization

“…As we know, the solvent cannot be photopolymerized in the photopolymerization system. However, this method may help the diffusion of free radicals and the unreacted double bonds; thus, the conversion of double bonds would increase further during the postcuring stage . In this work, the commercial poly(methyl methacrylate) (PMMA) and 1,6‐hexanediol dimethacrylate (HDDMA) with bifunctionality were used in the formulation.…”

The color properties of a reflective CMYK module using polymer‐dispersed liquid crystal (PDLC)

“…PDLC films could be easily prepared by the methods of photo‐induced phase separation (PIPS), solvent‐induced phase separation, temperature‐induced phase separation, and reaction‐induced phase separation . Because of the simple fabrication methods and good electro‐optical properties, PDLC are widely used in a variety of applications, such as light shutters, grating device, switchable windows, and display …”

“…[6][7][8][9][10] Because of the simple fabrication methods and good electro-optical properties, PDLC are widely used in a variety of applications, such as light shutters, grating device, switchable windows, and display. [11][12][13][14][15] In recent years, many research concerning color PDLC displays has been reported. A multicolor PDLC device was fabricated by packing both LCs and dichroic dyes into microcapsules for multicolor performance, 16 and the dyedoped PDLC could display rich color by mixing the three primary colors RGB (red, green, and blue).…”