Evaluation of the polymer-solvent interaction parameter χ for the system cured polybutadiene rubber and toluene

One of the most popular cured rubbers used in industrial applications is polybutadiene rubber (BR) and frequently its network structure, in the cured state, is studied by means of swelling in a solvent. Normally, toluene is used as solvent in this test. In order to estimate the crosslink density from the equilibrium volume fraction of rubber in the swollen state, the correct evaluation of the Flory-Huggins interaction parameter χ is necessary. This work covers the swelling behaviour of two types of cured BR in toluene. The rubbers used have sulphur and accelerator in their composition and were vulcanized at 433 K at different times in order to obtain several network structures and crosslink densities. The crosslink densities were evaluated from uniaxial stress-strain data measured at room temperature and analysed in the frame of the conformational tube model. Using this information and the swelling measurements in the frame of the Flory-Rehner relationship, a relationship between χ and vr, the polymer volume fraction at equilibrium (maximum) degree of swelling was established for the cured BR/toluene system. It was found that χ is not a constant but depends on the crosslink density of the sample.