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. (c) 2022 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Industrial Chemistry.

Automated summary

The dependence of free volume on temperature was investigated in copolymers and a terpolymer, with the lattice-hole model applied. Macroscopic behavior was analyzed using dilatometry, while positron annihilation lifetime spectroscopy provided insight into the microstructure of free volume. The coupling of these techniques allowed for the determination of hole density under specific assumptions about cavity geometry. The results showed that the hole shapes were more similar to discs than spheres, with aspect ratio decreasing as temperature increased. This radial growth was not influenced by the presence of acrylonitrile, unlike in butadiene-isoprene blends.