Sorption/Desorption and Diffusion Anomalies in Santoprene-Alkane Systems

Abstract

Kinetic curves for sorption/desorption cycles have been obtained by a gravimetric method for series of n-alkanes (C6 to C16), 2,2,4-trimethylpentane (TMP), cyclohexane and 1,2,3,4-tetra-hydronaphthalene (tetralin) into the miscible polymeric blend of ethylene-propylene random copolymer (60%) and isotactic polypropylene (40%) over the temperature interval of 25–70°C. Liquid diffusion coefficients have been calculated from mathematical relations derived from Fick's equation. A numerical method was used to describe the sorption and desorption processes. The kinetic curves obtained for these processes correlate well with the experimental results. The profiles of liquid concentration have been simulated for the Santoprene-alkane systems by solving Fick's diffusion equations and by Using numerical schemes. Activation parameters for diffusion and sorption were evaluated and these results are discussed in terms of molecular sizes and geometries of alkanes. Sorption and diffusion parameters exhibit dependence on size and shape of alkane molecules as well as temperature. The average values of diffusion coefficients have been analysed for concentration dependence. The diffusivity values ranged from a maximum of 2.93 x 10-6 cm2/s for heptane at 25° C to a minimum of 0.27 x 10-6 cm2/s for hexadecane. These values increase with increasing temperature. Activation energies for diffusion range from 6.4 to 18.8 kJ/mol, a value that is generally expected of rubbery polymer-organic solvent systems.