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 de nsities filled with finely divided sodium chloride (NaCl) particles ar e studied in water, water/acetone and water/THF mixtures. Swelling ove r periods of 2 months are recorded. The degree of swelling was observe d to increase continuously in water during this period, whereas in wat er/acetone mixtures it reached a peak followed by deswelling and it co nverged to a maximal value in water/THF mixtures. The highly cross-lin ked samples exhibited unusually high degrees of swelling, and calculat ions indicated full stretching of the network chains. The enthalpic co ntribution to the chemical potential of the networks in equilibrium wi th solvent was determined using the phantom network model of rubber el asticity theory. The swollen network-salt system was approximated by t he lattice model of polymer solutions. The positive contributions of w ater 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 sma ller and of comparable magnitude. Mechanical measurements of stress an d strain at break, and dynamic mechanical measurements of storage and loss moduli of filled and unfilled samples showed that filling with Na Cl did not change the mechanical properties of the networks significan tly, in spite of the dramatic change in the swelling behaviour. (C) 19 98 Elsevier Science Ltd. All rights reserved.