Substituent Effects on a Photo-Induced Alternation of the Lower Critical Solution Temperatures of Poly(N-isopropylacryla
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Substituent Effects on a Photo-induced Alternation of the Lower Critical Solution Temperatures of Poly(N-isopropylacrylamide) with Azobenzene Units Haruhisa Akiyama, Nobuyuki Tamaoki Molecular Function Group, Institute for Materials & Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST) Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, JAPAN ABSTRACT We synthesized copolymers of N-isopropylacrylamide (NIPAM) and azobenzene-containing acrylates or acrylamides by free radical polymerization, and investigated the water solubility of these polymers upon irradiation with ultra-violet and visible light. The solubility depended on concentration and structure of photoreactive azobenzene unit in the polymers. The soluble polymers showed the lower critical solution temperature, which was varied along with light irradiation. Photo-induced large wettability alteration was observed in the film. INTRODUCTION Poly(NIPAM) is known to have the lower critical solution temperature (LCST) around 31-32 °C in water [1]. Moreover the transition did not much depend on the molecular weight and concentration [2]. Such unique and sharp thermo-response seems to be useful for new applications. Actually some applications such as drug delivery [3] and chromatographic separations [4] have been shown. If the polymer responds to other stimulation, it will have a much broader range of applications. In this study, we propose a photo-responsive polymer based on poly(NIPAM). A poly(NIPAM) co-polymerized with small amount of photoisomerizable azobenzene units has been already reported to show a photo-induced alternation of LCST in a water solution [5]. In such a polymer, it is thought that a polar cis form of azobenzene can increase solubility for water as compared with a non-polar trans form because a balance of polar and non-polar moieties is important to control the LCST. Since a polarity depends on the molecular structure, we investigated effects of substituents at azobenzene moieties in this kind of polymers to find an adequate molecular structure for the photoresponse. All polymers used in this study have isopropyl groups at azobenzene moieties because of the expectation that they can block stacks among azobenzenes moieties in water. In the last section, we report the photo-induced wettability alternation on the polymer films. EXPERIMENTAL Materials Chemical structures of azobenzene monomers are shown in Figure 1. These monomers were synthesized from 4-hydroxyazobenzene derivatives which were prepared by an usual diazo-coupling method from corresponding aniline and phenol derivatives 4-(4-isopropylphenylazo) phenylacrylate (isoAzac). The 0.76g of acryloyl chloride was added to the mixture of 2g of 4-(4-isopropylphenylazo) phenol and 0.84g of triethylamine in 150ml of dry dichloromethane. After stirred for 12hr, the reaction mixture diluted with ethyl acetate was washed with water and dried with magnesium sulfate. The concentrated solution was purified by silica-gel column DD6.13.1
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