Diffusion mechanisms in solid and molten polyethylene

Abstract

AbstractThe diffusion coefficient D and solubility coefficient k of small molecules [C3H6, C4H10, (CH3)4C] are determined at very low solute concentrations in annealed linear polyethylene over a wide range of temperature above and below the melting point Tm. For measurements above Tm the specimen was lightly crosslinked by irradiation from a 60Co source. The diffusion data fit equations of the form D = D0 exp {–ΔHD/RT}. An abrupt change in ΔHD occurs at Tm: representative values (for C4H10) are 4.53 and 14.9 kcal/mole above and below Tm. At Tm, D0 also changes abruptly: representative values (for C4H10) are log D0 = −2.65 above Tm and log D0 = +2.70 below Tm. The mechanism of diffusion therefore changes at the melting point. The melt exhibits typical liquidlike characteristics (negative values of activation entropy ΔSD). The ratio ΔSD/ΔHD = 4β (β denoting the isobaric coefficient of volume expansion) holds below but not above Tm. Equations of the form k = k0 exp {–ΔHk/RT} fit the solubility data. The log k versus T−1 plots above and below Tm are parallel but separated by a step at Tm. If crystallization followed by annealing is assumed to leave a weight fraction of polymer αk (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 αk Equals the magnitude of the step at Tm. Values of αk 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.