Heterogeneous dynamics at the glass transition in van der Waals liquids, in the bulk and in thin films

It has been shown over the last few years that the dynamics close to the gl ass transition is strongly heterogeneous, both by measuring the diffusion c oefficient of tagged particles or by NMR studies. Recent experiments have a lso demonstrated that the glass transition temperature of thin polymer film s can be shifted as compared to the same polymer in the bulk. We propose he re first a thermodynamical model for van der Weals liquids, which accounts for experimental results regarding the bulk modulus of polymer melts and th e evolution of the density with temperature. This model allows us to descri be the density fluctuations in such van der Waals liquids. Then, by conside ring the thermally induced density fluctuations in the bulk, we propose tha t the 3D glass transition is controlled by the percolation of small domains of slow dynamics, which allows to explain the heterogeneous dynamics close to T-g We show then that these domains percolate at a lower temperature in the quasi-2D case of thin suspended polymer films and we calculate the cor responding glass transition temperature reduction, in quantitative agreemen t with experimental results of Jones and co-workers. In the case of strongl y adsorbed films, we show that the strong adsorption amounts to enhance the slow domains percolation. This effect leads to 1) a broadening of the glas s transition and 2) an increase of T-g in quantitative agreement with exper imental results. For both strongly and weakly adsorbed films, the shift in T-g is given by a power law, the exponent being the inverse of that of the correlation length of 3D percolation.