Diffusion in amorphous polymers

Abstract

AbstractDiffusion and solubility coefficients are presented for He, Ne, Ar, and N2 in three ethylene‐propylene copolymers, two styrene‐butadiene copolymerg and a cis‐poly‐butadiene for five temperature levels in the range 0–50°C. The data were obtained by using a slightly modified Dow Gas Transmission Cell and analyzed by a new modified time‐lag technique which properly accounts for the increase in pressure on the downstream side of the cell. Activation energies were calculated for the twenty‐four diffusion systems and correlated with the polymer intermolecular forces and the gas molecule sizes by using ii cell theory for amorphous polymers which employs a potential energy depth parameter, ϵ*, a range parameter, p*, the number of degrees of freedom per structural unit (center), 3Cm, and the number of centers per unit cell, n. The first three parameters were evaluated from thermodynamic data, while n was evaluated from the diffusion data. It is believed that n is an indication of the number of backbone carbon atoms active in polymer segmental motion. The values obtained here (5–9) agree well with existing values estimated from polymer viscous flow data. A correlation for the solubility data is shown and a linear free energy relationship is discussed.