Swelling in crosslinked natural rubber: experimental evidence of the crosslink density dependence of χ

Mechanical and swelling measurements were carried out on samples of dicumyl-peroxide-crosslinked natural rubber. The balance of the elastic free energy and the mixing free energy at swelling equilibrium was used to calculate the value of the Flory-Huggins χ parameter for a series of crosslinked rubbers swollen in an excess of different solvents. The results show that χ for the crosslinked rubber is greater than that of the uncrosslinked counterpart. Other evidence from the literature, based on swelling activity parameter measurements, which supports this conclusion, is discussed. We also find that the value of χ in the crosslinked rubber is a linear function of the crosslink density. We examine these results in the context of the Freed and Pesci lattice model.