No abstract

The 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 chi4, 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: poly(ethylene 1,4-cyclohexylenedimethylene terephthalate glycol) and poly(bisphenol A carbonate).

Interfacial and confinement effects on the evolution of cooperativity on approaching the glass transition have been studied in poly(propylenecoethylene) functionalized with diethylmaleate, polyethylene 1,4-cyclohexylenedimethylene terephthalate glycol and their nanocomposites with montmorillonite. A small increase of the structural dynamic cooperativity, a weak alteration of the temperature dependence of the characteristic relaxation frequency, and no changes in the glass transition temperature observed in poly(propylenecoethylene)-based samples can be rationalized in terms of interfacial interactions between polymer and exfoliated clay. On the other hand, confinement of polymer chains in the galleries of clay (intercalated nanocomposite) produces a strong reduction of cooperativity, of the temperature dependence of the characteristic relaxation frequency, and of the glass transition temperature in polyethylene 1,4-cyclohexylenedimethylene terephthalate glycol samples. Finally, by investigating the temperature dependence of a generalized fragility and of cooperativity, we evidenced that fragility of glass formers is determined not only by cooperativity.

Polyethylene/nanoclay specimens containing from 0 to 5% nanoclays were prepared from a commercially available premixed PE/nanoclay masterbatch containing 50% wt of nanoclay. The masterbatch was diluted to the desired concentration by adding PE along with various amounts of compatibilizer in order to achieve the best possible dispersion of the nanoclay platelets. The dielectric response of the compounded samples was investigated using a combination of time and frequency-domain spectroscopy in order to cover a wide frequency window. Both techniques were in good agreement when the time-domain data was transformed into frequency-domain data. Despite their low concentration, the addition of the dispersed nanoclays led to a significant alteration of the material dielectric response in the form of the appearance of various interfacial relaxation processes and an increase of charge carrier transport within the insulation material. Moreover, an onset of nonlinear charge transport process was observed at moderate fields for specimens containing a relatively low level of nanoclays. The high-field breakdown strength was shown to have been improved by the incorporation of the nanoparticles, particularly when the exfoliation was enhanced by the use of a maleic anhydride grafted polyethylene compatibilizer.

No abstract