ABSTRACT:The solubility and diffusivity of ethylene in semicrystalline polyethylene were experimentally measured using a magnetic suspension microbalance. The sorption measurements were carried out at temperatures up to 80°C and pressures up to 66 atm. The experimentally measured solubilities were found to decrease with increasing temperature and increased with ethylene pressure in good agreement with the predictions of the Sanchez-Lacombe latticefluid model. The diffusivity of ethylene in semicrystalline polyethylene films was estimated from the reduced sorption curves using the half-time method. The experimentally determined diffusivities were compared with theoretical values predicted by a new molecular hybrid model, which combines the characteristic features of the Pace-Datyner diffusion model with those of the Kulkarni-Stern free-volume model. The ethylene diffusion coefficient was found to increase with temperature and/or the ethylene-sorbed concentration.
ABSTRACT:The non-Fickian sorption kinetics of methanol vapor in poly(methyl methacrylate) films 8 and 51 m thick at 25°C are presented. The behavior of the system was studied in series of interval and integral absorption runs. The relevant diffusion coefficient and viscous relaxation processes were studied separately by kinetic analysis of the first and second stages of sorption kinetic curves. The sorption isotherm concaved upward at high activities, this being typical of Flory-Huggins behavior, whereas it exhibited a convex-upward curvature at low methanol vapor activities, this indicating sorption in the excess free volume of the polymer matrix. After excess free-volume fill-up, the concentration dependence of the diffusion coefficient was found to be well represented by the free-volume theory of Vrentas and Duda. Relaxation frequencies calculated from the second stage of two-stage curves exhibited a weak dependence on the concentration. Integral sorption experiments indicated that the system exhibited nearly case II kinetics at high methanol vapor activities.
ABSTRACT:The relaxation-dependent solubility model was applied to simulate the experimentally observed sorption kinetic behavior of the PMMA-vapor MeOH system at 25°C, to obtain the main transport parameters. Application of the model in series of successive sorption kinetic runs covering small concentration intervals revealed certain trends in the concentration dependence of the diffusion coefficient of the system, not detectable by a previous simpler analysis of the data. Following excess free volume fill up, relaxation frequencies exhibit a weak exponential dependence on concentration. The functional dependence of the thermodynamic diffusivity on the concentration, deduced from the aforementioned simulation procedure, was tested and found to reproduce reasonably well the series of sorption kinetic runs covering considerably larger concentration intervals. In addition, this analysis indicates a strong dependence of the relaxation mechanism on the concentration interval.
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