Photofabrication of Surface Relief Gratings using Post Functionalized Azo Polymers
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ABSTRACT A series of azobenzene functionalized polymers has been synthesized by post polymerization azo coupling reaction. Photo-fabrication of surface relief gratings were studied on the polymer films. Epoxy based azo polymers were prepared by post azo coupling reaction to form polymers containing donor-acceptor type azo chromophores. The azo chromophores were designed to contain ionizable groups to impart self-assembling and photoprocessing capabilities to the polymers. The polymers containing 4-(4-(carboxylic acid)phenylazo)aniline chromophores can be directly photofabricated to form surface relief gratings with large surface modulations. Charge interactions had a strong influence on the details of the writing process. A new soluble polydiacetylene, post-functionalized with azobenzene groups was also prepared. Large amplitude surface gratings could be fabricated on this polydiacetylene film as well.
INTRODUCTION Polymers containing aromatic azo units have attracted considerable interest in various research fields. 1-8 The mesogenic azobenzene groups have widely been used in the design of liquid crystal polymer (LCP) studies. 1 The highly conjugated azobenzene groups can act as efficient nonlinear optical (NLO) chromophores. A variety of NLO azo polymers have been developed in the last decade. 2 -5 Optically induced trans-cis isomerization of aromatic azo groups is being investigated for diverse potential applications. 6 -8 Upon exposure to light, azobenzene groups undergo reversible photoisomerization between the generally more stable trans form and the less stable cis form. 9 -10 The rate of isomerization of the azo groups depend on the structure of azo compounds and polymer matrices incorporating the aromatic azo chromophores. 11,12 This photo-induced geometrical or conformational change can significantly influence the bulk and surface properties of the polymer. 6 -8 Therefore, this photoisomerization of the azobenzenes in polymer matrices and its related property changes are being actively explored for potential applications, such as in optical switching and optical information storage. 13-16 Upon exposure to a polarized light reorientation of the azobenzene groups in polymer matrices can be induced through repeated trans-cis photoisomerization and subsequent cis-trans relaxation of the azobenzene groups. 17-20 Reversible holographic phase gratings have been 141 Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
demonstrated using this optically induced orientation or birefringence. 19,20 Recently, our group 2 1-26 and several other groups 2 7 -2 9 have shown that large surface modulation can be photo-induced on azobenzene polymer films. The surface relief gratings were formed upon exposure to an interference pattern of Ar+ laser beam at modest intensities without any subsequent processing steps. We have found that this process is due to the large scale macromolecular translational motion, which may be attributed to the photoinduced trans-cistrans isomerization cycles of azobenzene chromo
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