is used to predict viscoelastic scaling factors describing the effect of dissolved gas content on the viscosity curves of polystyrene melts swollen with dissolved carbon dioxide and dissolved 1,1difluoroethane. The predictions of the theory are compared to viscoelastic scaling factors measured by Kwag et al. 2 (J. Polym. Sci. B: Polym. Phys. 1999, 37, 2771 for each system at 150 and 175 °C, at concentrations up to 10 wt % of dissolved gas, and pressures ranging up to 22 MPa. The agreement between the theory and experiments is very good for the polystyrene-CO 2 system but only fair for the polystyrene-1,1-difluoroethane system. The experimental viscoelastic scaling factor values are also interpreted with the WLF equation to estimate the change in the underlying glass transition temperatures of the polystyrene-gas mixtures. The glass transition temperatures estimated from these rheological data are in very good agreement with values directly measured for polystyrene-CO 2 mixtures and with the theory of Condo et al. 3 (Macromolecules 1992, 25, 6119).
Partial graphitization of carbon nanofibers by high-temperature heat treatment can give improved composite properties. The intrinsic electrical conductivity of the bulk carbon nanofibers measured under compression is maximized by giving the fibers an initial heat treatment at 1500 °C. Similarly, for carbon nanofiber/polypropylene composites containing up to 12 vol% fiber, initial fiber heat treatments near 1500 °C give tensile modulus and strength superior even to composites made from fibers graphitized at 2900 °C. However, optimum composite conductivity is obtained with a somewhat lower heat-treatment temperature, near 1300 °C. Transmission electron microscopy (TEM) along with x-ray diffraction (XRD) explains these results, showing that heat treating the fibers alters the exterior planes from continuous, coaxial, and poorly crystallized to discontinuous nested conical crystallites inclined at about 25° to the fiber axis.
Cogswell's method for measurement of extensional viscosity by determining the entry pressure loss in capillary viscometry is critically examined. Comparisons are made between extensional viscosity values introduced in finite element calculations as input data and predictions based on the calculated entry pressure losses for polymer melts. The calculated pressure losses are in good agreement with experimental data by Laun and Schuch. An equation proposed by Cogswell and another by Binding are used in the extensional viscosity comparisons. The slope of the extensional viscosity versus stretch rate is predicted rather well, but the calculations can shift the results above or below the true values.
The cure kinetics of a dimethyl imidazole catalyzed diglycidyl ether of bisphenol A - methyltetrahydrophthalic anhydride resin system was examined. Degree of conversion versus cure time was obtained using isothermal DSC over the temperature range from 80 °C to 140 °C. The kinetic data were analysed using both nth order and autocatalytic kinetic models. The results indicated that the epoxy curing process could be described satisfactorily by Kamal's four-parameter generalised autocatalytic model modified with an exponential function of conversion defined as the diffusion factor. The curing process was predominantly autocatalytic in nature. The autocatalytic rate constant of the resin system was 10 to 60 times larger than the nth order rate constant. The corresponding activation energies obtained from Arrhenius equation were 78.1 kJ/mol and 67.6 kJ/mol, respectively. The approximate order of reaction was 0.39 for the autocatalytic reactions and 1.0 for the nth order reactions.
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