Design, Methodology and Preparation of Novel Polymers for Nonlinear Optics
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II. EXPERIMENTAL 2.1 Post azo-coupling reaction The diazonium salts of appropriate chromophoric units were prepared by adding the corresponding amines directly into the nitrososulfuric acid solution at 5 'C, followed by decomposition of excess nitrous acid by addition of suitable amount of sulfamic acid in water. The freshly prepared diazonium salt solution was added dropwise into a solution of appropriate precursor polymer in N,N'-dimethylformamide (DMF) at 0 'C. The solution was stirred at 0 'C for 12 h. Chromophore functionalized polymers were obtained by precipitation in water followed by repeated washing of the polymer with acetone and drying under vacuum at least for 24 h. 2.2 Post tricyanovinylation reaction Recrystallized tetracyanoethylene was added slowly into the precursor polymer solution in DMF under vigorous stirring. After the addition of tetracyanoethylene, the reaction temperature was raised to 45 'C, at which the reaction solution was stirred for 12h. The solution was subsequently precipitated in water. The precipitate was collected by filtration. The polymer was repeatedly washed with water, dried and further purified by repeated extraction with toluene. The polymer was dried under vacuum at least for 24 h before use. III. RESULTS AND DISCUSSION The precursor polymers were functionalized to introduce different chromophores at the final stage of the polymer preparation, using post azo coupling reaction as well as post tricyanovinylation reaction. Heteroaromatic chromophore functionalized epoxy based NLO Post polymers synthesized by post azo-coupling reaction are shown in Figure 1. tricyanovinylation reaction was employed as shown in Figure 2, to synthesize tricyanovinyl functionalized NLO polymers. Second order NLO properties and other physical properties of the synthesized epoxy polymers are listed in Table 1. The first part of the polymer nomenclature is an abbreviation to distinguish between precursor polymers from diglycidyl ether of bisphenol-A (BP) and other epoxide compounds (CH). The following parts refer to the different conjugation bridges and electron acceptor moieties of the chromophores. The glass transition Most of the synthesized polymers are solution processable. temperatures (Tg's) of the polymers were studied using differential scanning calorimetry (DSC) and are listed in Table 1. Polymer, BP-IM-DC exhibit the highest Tg of 179 'C, among the series and the high Tg is attributed to possible presence of intermolecular hydrogen bonds between imidazole units and hydroxyl groups, causing physical networks. The precursor polymers, BPAN and BP-TA, have very similar thermal stability as studied by thermogravimetric analysis (TGA) and are stable up to 330 °C under nitrogen atmosphere. Thermal stability of the formed NLO polymers declines, upon functionalization. The magnitude of the decrease depends on the type of heteroaromatic chromophores present in the polymer. BP-TA-TC starts to decompose in the temperature range between 270 and 280 'C, which is as stable as the epoxy based polymer c
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