Sorption and flow of gases in polyethylene

Abstract

AbstractSolubilities and diffusivities of N2, O2, CO2, and He in a variety of polyethylenes were measured in the temperature range 0–50°C. Polyethylene films studied covered a range of crystallinities (43–82%) and branching indices, and were prepared under a variety of known thermal histories. Diffusivities were determined by the time‐lag method; solubilities, by time‐lag, and also by a newly developed and more accurate static method. Solubilities were found to obey Henry's Law; the solubility constants determined by both methods were found to agree within the limits of accuracy, confirming the applicability of the unsteady‐state diffusion equation to essentially isotropic crystalline polymers. For a given gas at constant temperature, the solubility constant is directly proportional to the volume fraction of amorphous material in a polyethylene sample (as determined by density), irrespective of its origin or thermal history; the concept of the crystallites as an impermeable, dispersed phase thus appears justified. Diffusivities were found to vary widely (nearly fivefold) depending on polymer crystallinity and thermal history and were as much as tenfold lower than the values estimated for completely amorphous polyethylene. Application of principles of flow through porous media to this system leads to a conclusion that abnormally low diffusivities arise predominantly from the impedance to gas flow offered by the dispersed crystalline phase. Variations in diffusivity with branching index and thermal history correlate qualitatively with expected corresponding variations in crystallite growth kinetics and shape; the existence of highly anisometric, laminar crystallites in annealed linear polyethylene is indicated from these studies. The combined influence of crystallinity on solubility and diffusivity permits as much as a tenfold variation in gas permeability of polyethylene depending on polymerization method or fabrication process.