ACTIVATED RATE THEORY TREATMENT OF OXYGEN AND WATER TRANSPORT THROUGHSILICON-OXIDE POLY(ETHYLENE-TEREPHTHALATE) COMPOSITE BARRIER STRUCTURES

Poly(ethylene terephthalate) substrates were coated with thin films of silicon oxide deposited by magnetically enhanced chemical vapor depos ition. The rates of oxygen and water vapor transport through the coate d and uncoated film systems were obtained as a function of temperature . Activated rate theory treatment of oxygen transmission rates reveale d that the silicon oxide coatings were imperfect; the apparent free en ergies of activation (Delta E(p)) for transport through film substrate s which were coated on a single side were statistically identical to u ncoated controls. However, coating both sides of the polymer substrate with identical oxide layers resulted in a 54 kJ/mol increase in the D elta E(p) value. A simple empirical model for the change in transport mechanism is offered to explain this unanticipated result. Analogous t reatment of water vapor transport rates for these same film systems sh owed no obvious change in transport mechanism. However, Delta E(p) val ues obtained for water vapor permeation through silicon oxide-coated p oly(ethylene terephthalate), polystyrene, polypropylene, and polycarbo nate substrates were identical within experimental error, suggesting a ttractive interaction between the oxide layer(s) and water.