Free volume expansion in some polybutadiene–acrylonitrile rubbers: comparison between theory and experiments

The dependence of free volume on temperature was investigated in three butadiene–acrylonitrile copolymers and in a butadiene–acrylonitrile–isoprene terpolymer, above the glass transition in order to apply the lattice-hole model. Macroscopic behaviour was highlighted by dilatometry; positron annihilation lifetime spectroscopy was used to shed light on the free volume microstructure. Coupling of the two techniques allowed us to obtain the number density of holes under specific geometrical assumptions about cavities. Comparison with the theoretical free volume fraction suggests hole shapes more similar to discs than to spheres, but with aspect ratio decreasing at increasing temperature. Such preferential growth in the radial direction is not influenced by the presence of acrylonitrile, contrarily to that recently found in butadiene–isoprene blends, where an increasing amount of isoprene makes such expansion less anisotropic.