Predicting the Effects of Composition, Molecular Size and Shape, Plasticization, and Swelling on the Diffusion of Aromatic Additives in Block Copolymers

The rate of diffusion of small molecules within polymer matrices is important in an enormous scope of practical scenarios. However, it is challenging to perform direct measurements of each system of interest under realistic conditions. Free volume theories have proven capable of predicting diffusion coefficients in polymers but often require large amounts of physical constants as input. Therefore, we adapted a version of the Vrentas–Duda free volume theory of diffusion such that the necessary parameters may be obtained from a limited set of diffusion data collected at the temperature of interest using commercially available and automated sorption equipment. This approach correlates the size and shape of molecules to their trace diffusion coefficient, D, such that D of very large, solid diffusants can be predicted based on properties measured for condensable vapor diffusants. Our analysis was based on the volume-averaged transport properties of polyaromatic color additives within segmentally arranged poly­(ether-block-amide) (PEBAX) block copolymer matrices. At very high polyamide content the considerable plasticization effects due to absorbed water can be accommodated by increasing the available hole free volume as a function of water content. Alternatively, if the release rate of additives is measured for very high polyether content and degree of swelling, the release rate in the unswollen elastomer may be anticipated using the tortuosity model of Mackie and Meares. Agreement of these physical models to new experimental data provides a scientific basis for accurately predicting the in vivo leaching of aromatic additives from medical device polymers using accelerated and/or simplified in vitro methodologies.