Diffusion mechanisms in solid and molten polyethylene

The diffusion coefficient D and solubility coefficient k of small molecules [C 3 H 6 , C 4 H 10 , (CH 3 ) 4 C] are determined at very low solute concentrations in annealed linear polyethylene over a wide range of temperature above and below the melting point T m . For measurements above T m the specimen was lightly crosslinked by irradiation from a 60 Co source. The diffusion data fit equations of the form D = D 0 exp {–Δ H D / RT }. An abrupt change in Δ H D occurs at T m : representative values (for C 4 H 10 ) are 4.53 and 14.9 kcal/mole above and below T m . At T m , D 0 also changes abruptly: representative values (for C 4 H 10 ) are log D 0 = −2.65 above T m and log D 0 = +2.70 below T m . The mechanism of diffusion therefore changes at the melting point. The melt exhibits typical liquidlike characteristics (negative values of activation entropy Δ S D ). The ratio Δ S D /Δ H D = 4β (β denoting the isobaric coefficient of volume expansion) holds below but not above T m . Equations of the form k = k 0 exp {–Δ H k / RT } fit the solubility data. The log k versus T −1 plots above and below T m are parallel but separated by a step at T m . If crystallization followed by annealing is assumed to leave a weight fraction of polymer α k (the amorphous fraction) in which the solute can absorb and if the specific solubility coefficient of the amorphous fraction is identical to that of the melt, then log α k Equals the magnitude of the step at T m . Values of α k determined from the observed step are very close to values of amorphous fraction determined by measurement of density. The solubility experiments support the concept of polyethylene as a two‐phase solid with the amorphous fraction of specific volume equal to the extrapholated specific volume of the melt. The passage of a solute molecule from one potential well to another, however, occurs by processes in the melt and the amorphous fraction which are entirely different.