Swelling and Mechanical Properties of Polymer Gels: Salt, Macroion and Network Topology Effects

Abstract: The work in this thesis concerns polymer gels, especially their swelling and mechanical properties modelled using a coarse-grained model of polyelectrolytes. The method used to study polymer gels was molecular simulation utilising the simulation package MOLSIM. Particular focus is on the effect of salt, macroions and network topology effects (polydispersity, loose-ends and interpenetrating networks) upon swelling and mechanical properties. The deswelling of polyelectrolyte gels in salt solution was studied and compared to the Flory-Rehner-Donnan theory. All gels shrank as the concentration of salt in the reservoir increased, however networks made of stiffer polymers, or gels containing divalent counterions did not shrink as much. The agreement with theory was good, but the importance of taking into account the excess chemical potential was stressed. Gels deswelled when network counterions were replaced by large, highly charged macroions. The deswelling was strongly dependent upon the macroion charge density and the properties of the network. The effect of polydispersity upon gel deswelling was studied and a pronounced deswelling upon increasing polydispersity was observed. The effect was weaker for uncharged polymer gels and was attributed to the energetically unfavourable strong-stretching of short chains. The effect of loose-ends in the gel was studied and a swelling effect was seen as chains were successively removed from one end in the gels due to a decrease in the elastic pressure of the network. Interpenetrating networks made from combining two networks of (i) a polyelectrolyte network, (ii) an uncharged polymer network of identical crosslink density and (iii) a polyelectrolyte network of higher crosslink density were studied. The influence on swelling was described in terms of a competition between counterion osmotic pressure and the network restoring force. Upon uniaxial extension gels decreased in volume as a function of strain. An increased stiffness in a polyelectrolyte gels made from one loose and one highly crosslinked network was observed due to the strongly-stretched short chains of the highly crosslinked network.