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

Abstract

The free volume dependence on temperature has been 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 behavior was highlighted by dilatometry; positron annihilation lifetime spectroscopy (PALS) 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 the 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 what recently found in butadiene-isoprene blends, where the increasing amount of isoprene makes such expansion less anisotropic. This article is protected by copyright. All rights reserved.