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CPP: Chemische Physik und Polymerphysik
CPP 13: Glass Transition
CPP 13.2: Talk
Thursday, April 5, 2001, 18:05–18:25, 111
Relaxations in poly(n-alkyl methacrylate)s: Crossover region and nanophase separation — •Mario Beiner — FB Physik, Universität Halle, D-06099 Halle/Saale, Germany
The relaxations in the poly(n-alkyl methacrylate) (PnAMA) series depend systematically on side chain length. This allows a detailed study of special parts of the relaxation spectrum. Two effects are discussed: (i) Separate onset of the cooperative α relaxation in the crossover region. The systematic shift of the crossover region, where the relaxation times of α and β processes approach, was used to study it by dielectric, shear, and heat capacity spectroscopy. The dynamic glass transition consists of two distinct parts - the high temperature process a with small and constant cooperativity above the crossover and the conventional α relaxation with increasing cooperativity below the crossover. Both processes are separated by a saddle-like peculiarity in cp"(ω,T). The influence of chemical modifications on this scenario is discussed. (ii) Nanophase separation in higher PnAMAs. Two coexisting glass transitions, a polyethylene-like glass transition αPE at low temperatures (<-80oC) and the a-α process, are observed for the higher PnAMAs. The αPE process is related to cooperative motions in the alkyl part of the side chain. Its intensity and glass temperature increase with side chain length. Its existence indicates a nanophase separation of incompatible main and side chain parts in our homopolymers in analogy to the microphase separation in block copolymers. Such static nanodomains (d=0.5...1.5nm) are confirmed by SAXS. The domain sizes are compared with the size of dynamic heterogeneities estimated from calorimetric data using the fluctuation approach.