Excimer intrachain

Our interest in polymer photodegradation has led us to an investigation of the competition for migrating energy by chain impurities and "defects" such as excimer-forming sites. To isolate the intrachain phenomena, photooxidation of polymer solutions in dimethoxymethane have been carried out. As an example of a very photostable excimer-forming polymer, PS has been selected. Since poly(methyl methacrylate) (PMMA) is known to be far less stable than PS on a quanta-absorbed basis, MMA units have been incorporated into the chain to act as weak links. For energy sinks, 1-VN and 2-VN units have been made part of the chain. 

The excimer fluorescence of polystyrene (PS) solution has been widely studied since early sixties, predominately for intrachain excimer formation between adjacent pendant phenyl chromophores on the chain in dilute solutions, c < 5.10 g/dl. As the excimer interaction is a very short range interaction between an excited state and a ground state qhromophore at a parallel planar configuration of 0.33-0.35 nm apart, it should be useful to probe the molecular aggregation processes. For the present discussion only the concentration dependence of the intensity ratio < = of the excimer to monomer (isolated excited chromophore) fluorescence will be of interest. Typical fluorescence spectrum of PS in solution is shown in Fig. 2. The overlap between the monomer fluorescence peak (X = 285 nm) and the excimer fluorescence peak (X = 330 nm) is not severe so that simple peak intensity ratio is adequate for the evaluation of < within usual experimental errors. Results of first such investigation by Roots and Nystrom to show a sharp turn at the overlap concentration c (c [n] = 1) were not confirmed. Extensive studies carried out in the author s laboratory showed a peculiar concentration... 

This fractional power dependence of <(c) has shown to be a unique flexible polymer effect by experiments with PS samples of low molar mass ° and PS in solid miscible blends." When the molar mass of PS is less than 2 kD the concentration dependence <(c) showed a linear dependence without a region of fractional power dependence as shown in (Fig. 4). For small molecules in solution <(c) has been found to be proportional to the concentration in all cases and the value of < starts from zero. For low molar mass polymer PS because of the intrachain excimers existing in solution <(c) extrapolates to some finite value at zero concentration. A demarkation of the behavior of (c) for PS-DCE solutions of different molar mass samples has been found to lie between 1.7-4.2 kD. 

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