Interfacial properties and diffusion
Polymer networks are an important class of material due, for example, to their resistance to solvents. (A network can only swell in a good solvent; withdraw the solvent and the network should recover its original form.) My research has consisted of looking at the form of interfaces involving networks and high molecular weight linear polymers (or two different networks) to see how they compare to immiscible homopolymer interfaces . This has particular relevance to issues involving adhesion; and further work along these lines was performed using neutron reflectometry to investigate the growth of an interface between polymer networks and polymer chains tethered to a silicon substrate . I have also looked at diffusion of linear polymers into networks, as well as within networks . Another topic has been the segregation of linear polymers from within a network to the surface .
Swelling of polymer networks
The swelling of polymer networks is an issue that has been the subject of great debate for the past sixty years. The standard experiment has been to swell a polymer in a good solvent and to measure how much solvent has been absorbed by the network. Our approach has been to swell the network (crosslinked polystyrene) with linear chains of the same chemical species (polystyrene). This has advantages over the earlier methods:
- The network strands and linear chains are essentially the same.
- The total swelling of the network can be varied by changing the molecular weights of the linear polymer used to swell the network.
- The networks are not swollen by large amounts; we can probe the unfolding of clusters of crosslinks as the network is swollen.
We observed greater swelling in the networks than that predicted by simple mean-field theory. A scaling theory has been developed to demonstrate this, providing good agreement with our data. Consider the following argument:A linear chain extends in a good solvent, because an extended chain maximises the number of chain-solvent contacts if it is stretched. If the chain is polystyrene, it would be extended in styrene. However, a polystyrene chain mixed with polystyrene is not extended; there is no benefit in providing increased contacts between chains. A network is a heterogeneous object consisting of many clusters of crosslinks, weakly interconnected by weaker clusters or linear strands to other clusters. The linear polymer in our swelling experiment acts as a good solvent for the network clusters. These clusters therefore extend, resulting in increased swelling [5,6].
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 T. Russ, R. Brenn, and M. Geoghegan Macromolecules 36 127-41 (2003).