Polymer diffusion model. Molecular graphic showing the free volume distribution (coloured panel) in a cross-section through a matrix of amorphous poly- carbonate, a type of polymer. Wire-drawn polymer molecules fill the matrix (green = carbon; white = hydrogen; red = oxygen). Colours on the panel correspond to the key: red indicates high free en- ergy (low free volume); blue indicates low free energy (high free volume). This type of model is used to predict that rate at which gases diffuse through a polymer matrix.