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RESEARCH PRODUCT

I. Glass Transition. Theoretical concepts on the glass transition of polymers and their test by computer simulation

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Various organic molecules, in particular polymers, are extremely good glass formers and allow the study of supercooled melts near the glass transition in metastable equilibrium. Theories of the glass transition imply such an equilibrium (e.g. mode-coupling theory, or Gibbs-di Marzio theory) and can hence be tested in these systems. Simplified lattice models for polymer melts (e.g. the bond fluctuation model) have been developed that can very efficiently be studied by Monte-Carlo simulation, and although they fail to accurately describe the local structure, they describe many features of the experiments very well. In this model, the mechanism of the glass transition is a competition between the (energetically favorable) local stretching of effective bonds, and the (entropically favorable) dense packing of effective monomers (“geometric frustration”). It is shown that the model exhibits a dramatic slowing down upon cooling, and the properties of the frozen-in glass distinctly depend on the cooling rate. While the Gibbs-di Marzio theory predicts correctly the temperature region where the configurational entropy of the supercooled melt strongly decreases, the vanishing of the entropy is traced back to a severe underestimation of the entropy of the reference state (high temperature melt). The idealized version of the mode-coupling theory (involving critical singularities above the freezing transition) is found to be not very useful either, as the singularities are strongly rounded. But the extended version of the mode-coupling theory which describes this rounding, is in nice agreement with the simulations.