Swelling behavior of partially cross-linked polymers: A ternary system

Cross-linked polymer networks, irrespective of the method of cross-linking, contain a fraction of unattached polymer (sol macromolecules). During swelling in a solvent of low molecular weight, the sol macromolecules participate in the swelling process as an athermal solvent. Also, sol macromolecules begin to diffuse out of the swollen polymer into the surrounding solvent bath until equilibrium is reached between the sol macromolecules within the polymer specimen and the molecules in the surrounding bath. Thus, a partially cross-linked polymer during swelling should be treated as a ternary system consisting of polymer network, a high molecular weight solvent, and a low molecular weight solvent. On the basis of the Flory-Rehner theory, a first model has been developed to predict the swelling behavior of such ternary system. Transient states are considered as quasi-steady state, and the final condition of equilibrium is evaluated. The effect of polymer molecules in the surrounding solvent bath on the swelling behavior was also studied. Swelling experiments of partially cross-linked high-density polyethylene in p-xylene were performed to evaluate the model predictions. The presence of sol macromolecules within the swollen gel increased the swelling while sol molecules in the surrounding bath decreased the swelling of the partially cross-linked polymer.