ROLE OF OXYGEN DIFFUSION IN POLYMER AGING - KINETIC AND MECHANICAL ASPECTS

For an ageing process involving the consumption of a small molecule (t ypically O2 or H2O) by reaction with the polymer, there are critical c onditions of reaction rate and/or thickness above which the process be comes kinetically controlled by the diffusion of the small molecule in the polymer. Suitable lifetime prediction models must then involve th e thickness distribution of reaction products. This latter can be pred icted from Fick's law, modified by a term relative to the rate of cons umption of the diffusing species by the chemical reaction. Some proble ms related to the use of this approach are examined here. It appears t hat, in the most frequent case, the thickness of the degraded layer is of the order of magnitude of D/k, where D is the diffusion coefficien t and k the pseudo-first-order rate constant for reactant consumption. Some examples of application related to photochemical, radiochemical and thermochemical ageing are examined. It can, for instance, be shown that in photochemical or radiochemical ageing, the thickness of the o xidized layer (TOL) is proportional to the reciprocal of I(beta), wher e I is the radiation intensity and beta an exponent depending essentia lly on the radical chain mechanism. It is generally expected that in t he case of thermal ageing, the TOL is a decreasing function of the tem perature. Some consequences of diffusion control on accelerated and na tural ageing methods are briefly examined. The consequences of the age ing-induced ''skin-core'' structure due to the diffusion control are e xamined. The main features of the observed polymer embrittlement can b e explained in terms of fracture mechanics.