Hsiao Vincent K S, White Timothy J, Cartwright Alexander N, Prasad Paras N, Guymon C Allan
Department of Applied Materials and Optoelectronic Engineering, National Chi Nan University, Nantou 54561, Taiwan.
Department of Chemical and Biochemical Engineering, University of Iowa, Iowa City, IA 52446, United States.
Eur Polym J. 2010 May;46(5):937-943. doi: 10.1016/j.eurpolymj.2010.01.022.
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.
本文介绍了一种纳米多孔聚合物光栅的研究，其纳米结构孔隙率可控，研究内容包括光栅性能、光聚合动力学和形态学。通过加入非反应性溶剂对标准全息聚合物分散液晶（H-PDLC）体系进行改性，可得到层状纳米多孔形态，并制备出具有优异宽带反射光学性能的反射光学元件。如果在全息写入过程中使用足够的光能，在预聚物混合物中加入非反应性溶剂会导致形成以空穴/聚合物逐层结构为特征的形态。只有在改性体系中通过以下方式才能获得形成排列良好的纳米多孔结构所需的光曝光持续时间和强度：（a）在较长时间的全息干涉图案化（30分钟）下照明；或（b）在非常短时间的全息干涉图案化（30秒）下照明，然后使用均匀光曝光进行60分钟的后固化。相比之下，典型的H-PDLC在不到1分钟内形成。为了进一步了解这两种类似材料形成过程中的差异，研究了光引发和聚合（传播）动力学的时间动态。本文表明，写入曝光会产生在明亮区域更致密的交联聚合物网络。在丙烯酸酯转化率达到60%（甚至45%）时，写入后几乎不会留下游离单体。持续曝光主要用于增加交联。这往往会使网络坍塌，尤其是密度较小的部分，实际上会被粘到密度较大的部分。在溶剂增加聚合物网络流动性的程度上，这一过程将得到促进。同样重要的是，通过控制预聚物混合物中液晶的浓度或控制均匀后固化的时间，可以使周期性纳米孔的尺寸在10至50纳米之间变化。
