How cracks in SiOx-coated polyester films affect gas permeation

In this paper theoretical models have been established that can account for the gas transmission through nanocomposite laminates, consisting of an oxi de layer of finite permeability containing defects, on a polymer sheet of f inite thickness. The defect shapes can either be in the form of long cracks or rectangular holes. The models offer a choice of exact numerical calcula tions or fast and intuitive analytical approximations. The experimental mea surements of oxygen permeation through four different SiOx/poly (ethylene t erephthalate) samples that were strained to produce distributions or cracks showed good agreement when compared with predicted results from the approx imate analytic model. As a consequence of this observation, a key practical conclusion is that, because of the logarithmic dependence of transmission on the width of a crack, for a given strain it is better to have a small nu mber of large cracks rather than a large number of small cracks. (C) 2001 E lsevier Science B.V. All rights reserved.