Diffusion in polymers: Optical techniques

Abstract

Abstract Optical techniques have been applied to study gas diffusion in poly(methyl methacrylate) and Lexan polycarbonate resin[poly(diphenyl‐2,2‐propane carbonate)] for temperatures ranging from 20°C. to the glass transition temperatures. One method consists of observing the movement of a sharp colored diffusion boundary which occurs when certain radical scavenging gases (e.g., O 2 , I 2 ) penetrate a polymer which has been exposed to high energy irradiation. The theoretical basis and physical interpretation of the moving boundary are discussed. In essence, the color center concentration (produced by irradiation) is greater than the normal equilibrium concentration of scavenger molecules in the polymer so that the latter are trapped before they can move very far inward. The result is approximately a step‐function distribution of the remaining color centers. For O 2 penetration into Lexan below T g , D (cm. 2 /sec.) ⋍ 0.014 exp {7100 cal./mole/ RT }. This relation accounts for the effect of O 2 partial pressure on the penetration rate. For O 2 penetration into PMMA, below T g , an activation energy of 10 kcal./mole was observed. A spectrophotometric method was used to measure the uptake of J 2 vapor in polycarbonate films. The data indicate a large change (∼50 kcal./mole) in apparent activation energy near 114°C. Several possible applications of the moving boundary technique are mentioned and the results of its use to investigate the effect of stress on the diffusion rate in the polycarbonate are presented. Tensile stresses near the yield point produce a two‐fold increase in D , whereas compressive stresses of comparable magnitude cause only a slight reduction.