This work attempts to determine how percolation at an equilibrium state is correlated to percolation under experimental conditions. The dynamic process of forming conductive networks in carbon-black (CB)-filled poly(methyl methacrylate) composites was investigated by real-time tracing the time dependence of electrical resistivity during isothermal treatments. It was observed that the dynamic percolation curves maintains the same shape and shift to a shorter percolation time with increasing annealing temperature and filler concentration. An Arrhenius plot of the shift factor against the annealing temperature shows a linear relationship, irrespective of the filler concentration, and the activation energy of the percolation time is close to the activation energy of the zero-shear-rate viscosity of the polymer matrix. Furthermore, an increase in the thermodynamic interactions between CB and the polymer matrix causes a large reduction in polymer mobility, resulting in an increase in the percolation time. These results lead to the conclusion that percolation is delayed by the bulk mobility of polymer layers surrounding CB particles. An experimental approach for determination of the retardation time is proposed based on theoretical analysis of the dynamic movement of the carbon particles. It is suggested that the difference in the kinetic history with respect to percolation among different composite systems can be eliminated by normalizing the experimental conditions to the same value of retardation time.

No abstract

No abstract

Dispersion and electrical properties of fluorinated carbon black-filled poly-(vinylidene fluoride) (PVDF) composites were studied as a function of the fluorine content. It was found that with increasing the fluorine content carbon particles tend to stick together to form large aggregates. The percolation concentration increases to a high concentration, whereas the percolation process becomes gradual. The temperature dependence of resistivity measurements show that the fluorinated carbon black-filled PVDF composites exhibit a high PTC intensity and a low NTC effect. These phenomena were discussed in terms of thermodynamic interactions between fluorinated carbon and the PVDF matrix. The dielectric behavior was also investigated in this study.

Elastic and mechanical properties such as Young's modulus E,Poisson's ratio , Debye temperature D , Vickers hardness H v , fracture toughness K c , and fracture surface energies ␥ f of yCuO x ⅐(100؊y)P 2 O 5 glasses (y ‫؍‬ 45, 50, 55) with different copper valence states, i.e., R(Cu ؉ ) ‫؍‬ Cu ؉ /(Cu ؉ ؉ Cu 2؉ ), at room temperature (humidity 64%) have been examined. The following features have been found: (1) the glass transition temperature (218 -434°C), H v (2.7-4.4 GPa), E (50.6 -78.2 GPa), and D (358 -434 K) decrease largely with increasing R(Cu ؉ ); (2) the mean atomic volume, K c (0.56 -1.14 MPa⅐m 1/2 ), and ␥ f (1.9 -11.2 J⅐m ؊2 ) tend to increase with increasing R(Cu ؉ ); (3) 50CuO x ⅐50P 2 O 5 glasses with R(Cu ؉ ) ‫؍‬ 0.42 and 0.55 have a high resistance against crack formation in Vickers indentation tests and no crack is observed in the 45CuO x ⅐55P 2 O 5 glass with R(Cu ؉ ) ‫؍‬ 0.57 under an applied load of about 98 N. The results demonstrate that elastic and mechanical properties of yCuO x ⅐(100؊y)P 2 O 5 glasses depend strongly on the copper valence state and the CuO x /P 2 O 5 ratio. The unusal mechanical and elastic properties of copper phosphate glasses are well explained qualitatively by considering unique oxygen coordination and bonding states of Cu ؉ ions, i.e., lower coordination number and more covalent bonding compared with Cu 2؉ ions.