From solutions to membranes: Structure studies of sulfonated polyphenylene ionomers

The structure of rigid sulfonated polyphenylene ionomers was investigated in bulk and in dilute organic solutions. The uniqueness of polyphenylene ionomers lies in their rigid backbone which prevents folding and therefore affects the partition into hydrophilic ionic domains and hydrophobic regions. This segregation dominates the structure of flexible ionic polymers. Small-angle neutron scattering studies of these ionomers have demonstrated that bundles of polymer molecules are formed in dilute organic solutions. This clustered building block persists in bulk dry and hydrated states of the ionomers. Hydration of these ionomers membranes results in segregation to hydrophilic and hydrophobic regions, where diffusion into interstitial spaces between the bundles of sulfonated and unsulfonated domains takes place followed by rearrangements of domains to yield locally bicontinuous regions. Only at very high sulfonation levels are fully bicontinues phases formed. With controlling the degree of continuity, the stiffness of the backbone offers a means to tune the transport in ionic polymers.