THE EFFECT OF DRAWING ON THE TRANSPORT OF GASES THROUGH POLYETHYLENE

A study has been made of the gas transport properties of polyethylene films of two different grades, Hizex 7000F and Rigidex 002-55, one-way drawn at 115-degrees-C to draw ratios in the range 1-20. Measurements of the permeability and diffusion coefficients of helium, oxygen, carbon dioxide and nitrogen have been made with a dynamic flow rate technique, utilizing a mass spectrometer detection system, and of oxygen using a commercial OXTRAN system. The samples were characterized by the measurement of density, birefringence and modulus and by wide-angle x-ray diffraction and differential scanning calorimetry. There is a large decrease in both the permeability and diffusion coefficients for all gases with increasing polymer draw ratio, with up to an 80-fold decrease in permeability for the larger permeants compared with the 10-fold decrease observed for helium. The solubilities of all the gases decrease only by a factor of approximately 2. The diffusion results are discussed in terms of geometric impedance and chain immobilization factors. The solubilities, on the other hand, appear to relate primarily to the amorphous volume fraction of the polymer.