Hydrogels from ABA and ABC Triblock Polymers

Aqueous gels of telechelic poly(ethylene oxide) (PEO)-based triblock polymers, with homo and hetero combinations of 1,2-polybutadiene (PB) and poly(perfluoropropylene oxide) (PFPO) as hydrophobic end-blocks, were prepared by anionic polymerization and polymer polymer coupling reactions. The block molecular weights in kg/mol were (2.7-24-2.7) for PB-b-PEO-b-PB, (2.3-23-2.3) for PFPO-b-PEO-b-PFPO, and (1.9-26-2.3) for PB-b-PEO-b-PFPO. Gels with polymer concentrations ranging from 10 to 50 wt % were investigated using cryogenic scanning electron microscopy (cryo-SEM) and small-angle neutron scattering (SANS). The cryo-SEM micrographs revealed significant differences among the morphologies of the gels obtained, depending on the end-blocks used. Results from contrast matching SANS experiments were used to corroborate and refine the information obtained from microscopy. The PB-b-PEO-b-PB copolymers formed networks of spherical micelles at all concentrations as expected, albeit with significant spatial heterogeneity that diminished with increasing concentration. The PFPO-b-PEO-b-PFPO copolymers also formed networks by aggregation of the end-blocks, but the PFPO blocks tended to adopt like or even sheetlike structures. This is attributed to the extremely high interfacial tension of PFPO with water and is consistent with the super-strong segregation regime behavior. The heterotelechelic PB-b-PEO-b-PFPO terpolymers adopted a quite different structure, namely an intricate bicontinuous open-cell foam, with cells on the order of 500 nm in size and cell walls composed of PFPO disks embedded in PB sheets. These various network structures illustrate the potential of using end-block chemistry to manipulate both the morphology and the physical properties of polymer gels.