No abstract

For the first time, load and depth sensing indentation (DSI) was used in order to monitor physical aging of bisphenol A polycarbonate for 30 months at room temperature and for 1 month at an elevated temperature. The DSI experiments were combined with differential scanning calorimetry, gel permeation chromatography, and infrared spectroscopy. The endothermic peak of polycarbonate shifted toward higher temperatures upon aging at an elevated temperature and did not change its location upon aging at room temperature. The elastic modulus and hardness of polycarbonate increased in a stepwise fashion during aging at room temperature. The molecular weight distribution broadened slightly, and the trans−trans conformational population increased during annealing. No simple correlation between changes in the mechanical properties and the shift of the endothermic peak during annealing was found. These changes seem to be caused by phenomena of different nature; namely, the changes in the mechanical properties appeared to have a reasonable correlation with free volume relaxation of the polymer, whereas the changes in the endothermic peak may be associated with internal energy changes. The similarities and differences between our results and the results of others are discussed.

The electrical volume conductivity r of antimony-doped tin oxide (ATO)-acrylate nanocomposite hybrid coatings was investigated. The relation between r and the volume filler fraction p was analyzed for the ATO-acrylate coatings containing ATO nanoparticles grafted with different amounts of 3-methacryloxy-propyl-trimethoxy-silane coupling agent. Percolation thresholds were observed at very low filler fractions (1-2 vol %) for the coatings containing ATO nanoparticles with a low amount of surface grafting. A modified effective medium approximation (EMA) model was introduced. This model takes into consideration different distances between adjacent semiconductive particles in the particle network. The model elucidates how self-arrangement of the particles influences the location of the percolation threshold in the log r À p plot. The modified EMA model can successfully explain the multiple transition behavior and the variable percolation thresholds found for the ATO-acrylate nanocomposite hybrid coatings. V V C 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45:

Subsurface cracking after nano-and Vickers indentation on transparent silica-(meth)acrylate hybrid coatings deposited on a polycarbonate substrate was observed for the first time. It appeared that after initiation at the interface, these cracks propagated toward the surface and in radial directions as the indentation load increased. It was found that, for chemically identical coatings, the thicker the coating, the higher load necessary to initiate these cracks. Subsurface cracks formed during nanoindentation occur at lower loads than the normally observed surface cracks and are accompanied by a very small change in the slope of the loading curve.