Uncertainties in the Determination of Cross-Link Density by Equilibrium Swelling Experiments in Natural Rubber

Abstract

Equilibrium swelling is a feasible and simple experiment to determine the cross-link density of networks. It is the most popular and useful approach; however, in most of the cases, the given values are highly uncertain if not erroneous. The description of the complex thermodynamics of swollen polymer networks is usually based on the Flory-Rehner model. However, experimental evidence has shown that both the mixing term described by the Flory-Huggins expression and the elastic component derived from the affine model are only approximations that fail in the description and prediction of the rubber network behavior. This means that the Flory-Rehner treatment can only give a qualitative evaluation of cross-link density because of its strong dependence on the thermodynamic model. In this work, the uncertainties in the determination of the cross-link density in rubber materials by swelling experiments based on this model are reviewed. The implications and the validity of some of the used approximations as well as their influence in the relationship of the cross-link densities derived from swelling experiments are discussed. Importantly, swelling results are compared with results of a completely independent determination of the cross-link density by proton multiple-quantum NMR, and the correlation observed between the two methods can help to validate the thermodynamic model.

Discrimination of the roles of crosslinking density and morphology in the swelling behavior of crosslinked polymers: Poly(N‐vinylimidazole) hydrogels

“…The affine model and gaussian chains were considered for P el . 22,24,31 Equation (5), with 16 v ¼ 0.496 þ 0.308v 2 , affords the m e (S) values plotted in Figure 5 for PVI swollen in deionized water.…”

UV Radiation Induced Cross-Linking of Whey Protein Isolate-Based Films

“…is determined with the help of swelling experiments and calculated with the equation of Flory-Rehner [14][15][16]:…”

ESR investigation of NR and IR rubber vulcanized with different cross-link agents

“…Equilibrium swelling experiments in toluene were performed according to the protocol described elsewhere [16]. The solvent density !…”

“…s = 0.87 g/mL was used for toluene. The influence of insoluble components (such as zinc oxide) was corrected according to Equation (1) [16]: (1) where w d is the sample weight after drying, ! rubber is the density of NR (0.92 g/cm 3 ); w ZnO is the ZnO weight, w compound is the recipe weight and !…”

“…It is well known that the # parameter depends on the solvent-rubber pair and the volumetric fraction of rubber. For this reason a variable # parameter has been used according to the recommendations made elsewhere [16].…”