Swelling of sodium chloride filled polybutadiene networks in water, water/acetone and water/THF mixtures

The swelling of polybutadiene (PB) networks of different cross-link densities filled with finely divided sodium chloride (NaCl) particles are studied in water, water/acetone and water/THF mixtures. Swelling over periods of 2 months are recorded. The degree of swelling was observed to increase continuously in water during this period, whereas in water/acetone mixtures it reached a peak followed by deswelling and it converged to a maximal value in water/THF mixtures. The highly cross-linked samples exhibited unusually high degrees of swelling, and calculations indicated full stretching of the network chains. The enthalpic contribution to the chemical potential of the networks in equilibrium with solvent was determined using the phantom network model of rubber elasticity theory. The swollen network-salt system was approximated by the lattice model of polymer solutions. The positive contributions of water and acetone to the chemical activity of the swollen network were offset by the negative contribution of dissociation of salt in water, resulting in a small negative enthalpy of mixing. The highest rate of diffusion of solvent was observed for the PB/water/THF ternary system. Diffusion rates of pure water and water/acetone mixture were much smaller and of comparable magnitude. Mechanical measurements of stress and strain at break, and dynamic mechanical measurements of storage and loss moduli of filled and unfilled samples showed that filling with NaCl did not change the mechanical properties of the networks significantly, in spite of the dramatic change in the swelling behaviour. (C) 1998 Elsevier Science Ltd. All rights reserved.