Temperature dependence of the characteristic length scale for glassy dynamics: Combination of dielectric and specific heat spectroscopy

International audienceThe temperature dependence of characteristic length scales associated to the glass transition such as the cooperativity length scale introduced by Adam and Gibbs cooperative rearranging region CRR or the dynamic heterogeneity as estimated from the four point correlation function 4 , is at the center of large interests. Broadband dielectric spectroscopy and temperature modulated differential scanning calorimetry allow to study the CRR size temperature dependence in the temperature range of ergodicity loss for glass-forming liquids, starting from the onset of cooperativity in the crossover region down to the glass transition temperature. Furthermore, the correlation between these two techniques allows to explore a large frequency range from 1 mHz to 10 MHz. The goal of this work is to follow the cooperativity evolution along the Arrhenius plot for two different polymeric systems: polyethylene 1,4-cyclohexylenedimethylene terephthalate glycol and polybisphenol A carbonate

Fragility and molecular mobility in micro- and nano-layered PC/PMMA films

International audienceThe values of the fragility index of polycarbonate/poly(methylmetacrylate) multilayer films have been determined for materials having thicknesses varying from micro- to nano-scales. The influence of the layer thickness reduction down to 12 nm has been shown to have very different influence on the glass transition parameters of the two polymers. Polycarbonate has exhibited a huge sensitivity to such geometric scale modifications while no modification has been observed for PMMA. A large decrease of the fragility index m, has been observed for PC. This modification is proportional to length scale variations of molecular cooperative motions at Tg, ξ(Tg). In regard to the molecular structure of each polymer, these results show that this correlation between fragility index and ξ(Tg) is mainly associated to modifications of intermolecular interactions and local density. These material parameters are influencing the activation volume ΔV# which plays a role on the modification of the fragility index