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

Abstract

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

Probing the Free Volume in Polymers by Means of Positron Annihilation Lifetime Spectroscopy

“…Similar findings were found in various elastomers [ 124 , 132 , 133 ]: in these cases, too, holes are well represented by flattened cylinders. Therefore, it seems that flattened holes are more common than spherical holes, in our investigated polymers.…”

“…Another result has been found in the three investigated butadiene-acrylonitrile rubbers and the terpolymer polyisoprene-polybutadiene-acrylonitrile [ 133 ], extrapolating the hole sizes at the glass transition: they are found in the range 0.6–0.7 nm. These values are comparable to the effective bond length l , defined as the square root of the ratio between the unperturbed mean-square end-to-end distance ⟨ R 2 ⟩ 0 of a chain and the number of its backbone bonds, which results in 0.5± 0.2 nm, according to Wang [ 135 ].…”

“…Since the Simha-Somcynsky theory is valid for amorphous polymers at equilibrium, we restricted our investigations to temperatures above the glass transition, although it is possible to also extend the theory in the glassy state [ 125 ]. We explored polystyrene with different molecular weight [ 126 ], perfluoropolyethers and polypropylene glycols with low molecular weights [ 127 , 128 ], atactic polypropylene [ 129 ], polyvinyl acetate [ 130 ], a thermoplastic polyurethane [ 131 ], and different rubber blends [ 124 , 132 , 133 ]. Our goal was to explore a possible influence of the investigated structure on the free volume.…”

“…However, the same procedure mentioned above, as applied to polybutadiene-polyisoprene blends [ 124 ] and acrylonitrile-butadiene rubbers [ 133 ], as well as a terpolymer acrylonitrile-polybutadiene-polyisoprene [ 133 ], did not produce satisfactory fits, for any geometrical choice of the cavity. Cylindrical or prismatic cavities reduce the systematic differences between the free volume fraction supplied by the theory and the same quantity evaluated from PALS and dilatometry, with respect to the spherical approximation; nevertheless, the fits, in terms of statistical test, are not acceptable.…”

“… Behavior of parameter p versus polyisoprene volume fraction in polybutadiene–polyisoprene blends (reproduced from ref. [ 133 ]. DOI: 10.1002/pi.6431).…”