Understanding the evolution of the cooperative molecular mobility as a function of time and temperature remains an unsolved question in condensed matter research. However, recently great advances have been made within the framework of the Adam–Gibbs theory on the connection between cooperatively rearranging regions, or dynamic heterogeneities, i.e., domains of the supercooled liquid whose relaxation is highly correlated. The growth of the size of these dynamic domains is now believed to be the driving mechanism for different experimental parameters like relaxation times and viscosity of supercooled liquid approaching the glass transition. Recent studies have shown the evolution of cooperative motions in supercooled liquids using different experimental tools and models. In this work, broadband dielectric spectroscopy and modulated temperature differential scanning calorimetry were carried out on six different amorphous glass-forming systems in order to scan a wide range of relaxation times and temperatures. Two different models based on four point dynamic susceptibilities and the thermodynamic fluctuation approach, have been used to compare the temperature evolution of the number of molecules dynamically correlated during the α-relaxation process. Divergences and convergences between these two models are discussed.

New experimental results focused on Flash DSC, DSC and broadband dielectric spectroscopy investigations are reported in this work. The fictive temperatures and fragility indexes are estimated from Flash DSC experiments and compared to values obtained from classical DSC. The consistency of the Tool-NarayanaswamyMoynihan model and fragility concept is then investigated over a large range of cooling rates. Indeed, the Flash DSC allows exploring thermal properties of materials over a continuous and broad range of heating and cooling rates, complementary to rates usually available with DSC. The reliability of investigations is also demonstrated by comparing results obtained from two model amorphous polymeric systems: polystyrene and poly(ethylene terephthalate)-glycol. The temperature dependence of the cooling rate obtained by Flash DSC and DSC is also compared to the temperature dependence of the relaxation times obtained from broadband dielectric spectroscopy, experiment considered as the reference concerning the fragility measurements. The comparison of these two dependencies implies a better understanding about the origin of the temperature dependence of the cooling rate.

A Saiter, et al.. Segmental mobility and glass transition of poly(ethylene-vinyl acetate) copolymers : Is there a continuum in the dynamic glass transitions from PVAc to PE?. Polymer, Elsevier, 2015, 76, pp.213-219. 10.1016/j.polymer.2015 The segmental dynamics of amorphous poly(ethylene-vinyl acetate) copolymers (from PVAc to EVA50) were studied. In that sample set with similar backbone stiffness and different amount of dipoles, the dynamic glass transition was investigated by Modulated Temperature Differential Scanning Calorimetry and Broadband Dielectric Spectroscopy measurements. A decrease of the cooperativity length scale was obtained with the vinyl acetate (VAc) content decreasing. On the other hand, there was no modification of the temperature dependence of the relaxation time. Thus, the fragility value is quite constant whatever the VAc content. These results show that fragility and cooperativity have two different origins. An extrapolation to nonconstrained polyethylene amorphous phase was proposed and new glass transition temperature and fragility values were determined. Highlights 1.Amorphous phase mobility is studied for poly(ethylene-vinyl acetate) copolymers 2.Fragility is constant from PVAc to EVA50 while cooperativity length decreases 3.New extrapolated T g and fragility values are proposed for PE 4.Fragility and cooperativity are not governed by the same macromolecular properties Keywords Amorphous phase, dynamic glass transition, poly(ethylene-vinyl acetate) . Therefore, structural relaxation temperature dependences can be defined as Super-Arrhenius, due to a possible behavior between two extreme limits: "strong" glass forming liquids for which the viscosity variations (or relaxation time) are very slow and follow an Arrhenius law, and the "fragile" glass forming liquids for which a very abrupt and steep non-Arrhenian variations can be observed.The fragility index (m) quantifies the steepness of the temperature dependence of the relaxation time () close to T g (defined at τ = 100s) and can be calculated as follows:The fragility values for different materials such as polymers, metallic glasses, organic and inorganic ionic glasses, and for small organic molecules were summarized Several approaches have been proposed to explain the correlation between the fragility and the molecular mobility near T g [10,16,17]. According to the theory proposed by Adam and Gibbs [18], it is well accepted that the α relaxation process is cooperative in nature: a structural unit can move only if a certain number of neighboring structural units move also. Besides, the molecular motions are mainly governed by the intermolecular interactions having important effects in the viscous slowing down of molecular dynamics when the glass-forming liquid is cooled-down close to T g . Thus, the notion of Cooperative Rearranging Region (CRR) was introduced, and the CRR size can be estimated according to different models and theories in terms of characteristic length scale or in terms of structural unit number [16,19-22]. Accordi...

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This work highlights the influence of layer thicknesses on glass transition and molecular mobility in polycarbonate (PC) and poly­(ethylene terephthalate glycol) (PETg) multilayered films obtained by the layer-multiplying coextrusion process. By combining modulated temperature scanning calorimetry (MT-DSC) and dielectric relaxation spectroscopy (DRS) measurements, the average values of the cooperative rearranging region (CRR) size in a wide range of relaxation times and temperatures have been calculated. The size reduction from micro- to nanoscale is accompanied by a significant deviation of the structural and dynamical properties compared to the bulk. Furthermore, we have evidenced significant differences in PETg and PC behaviors: PETg plays the role of a “hard-confined” polymer, while PC behaves as a “free-confined” polymer, implying opposite variations of the glass transition temperature. The determination of the relaxation parameters for each individual polymer has been possible even for very low layer thicknesses (below 10 nm), for which the multilayer structure is not observed. This result shows that even when PETg and PC are brought into very intimate contact, they maintain their own structural behavior and segmental relaxation, suggesting the formation of immiscible nanodroplets when the multilayer structure disappears.