Equilibrium swelling properties of lightly crosslinked isotropic networks: Natural rubber crosslinked in dry state

Lightly crosslinked natural rubber (NR) networks have been characterized by equilibrium swelling in toluene. A good agreement between the equilibrium swelling crosslink-junction densities (mu (E) ) and the values expected from the stoichiometry of dicumyl peroxide decomposition (mu (c)) has been obtained using Flory's early equation of state. The applicable value of chi = 0.37 was found to compare equally well and is in tandem with the previously reported literature values of chi = 0.391 and chi = 0.35 from swelling and heat of mixing measurements, respectively. At low crosslink-junction densities i.e., mu (E) less than or equal to 4.2 mM.l(-1) corresponding on average to 4 crosslink-junctions per cis 1,4-poly-isoprene chain and below, network formation is found to be incomplete. The approach used also provides a simple way of differentiating lightly crosslinked networks from the state of gelation.