Influence of site-specific sulfonation on acrylic graft copolymer morphology

Novel selectively sulfonated graft copolymers, poly(methyl methacrylate)-g-(poly(sulfonic acid styrene)-b-poly(tert-butyl styrene)), poly(methyl methacryl ate)-g-(poly(tert-butylstyrene)-b-poly(styrenesulfonic acid)), and the corresponding sodium sulfonate salts were successfully synthesized. The weight-average molecular weights (M,) of the graft copolymers were similar to 200 000 g/mol and the polydispersity ranged from 1.86 to 2.22. The graft copolymers contained similar to 9-10 branches on average and 4 wt % of sulfonic acid or sodium sulfonate blocks adjacent to the backbone or at the branch terminus. The mobility of the sulfonated blocks located at the branch terminus enabled the sulfonated blocks to more readily interact and form ionic aggregates. The glass transition temperatures of the sulfonated graft copolymer with sulfonated blocks at the branch terminus were higher than values for copolymers with sulfonated blocks adjacent to the backbone. This behavior was attributed to the sulfonated blocks at the branch terminus more easily forming ionic aggregates. In general, appearance of multiple scattering maxima in the small-angle X-ray scattering (SAXS) data indicated the formation of lamellar morphologies. More facile aggregation of sulfonated blocks at the branch terminus resulted in the appearance of ionomer peaks in SAXS whereas ionomer peaks were not observed in sulfonated graft copolymers with sulfortated blocks adjacent to the backbone. Reasonable fits of the Yarusso-Cooper model to the SAXS data strongly supported the identification of the nonlamellar maxima as ionomer peaks. Strong ionic interactions with Na+ counterions reduced the Bragg spacings, d, of the sodium sulfonate graft copolymers and increased the size of the ionic aggregates. The location of sulfonated blocks in both sulfonic acid and sodium sulfonate graft copolymers significantly affected thermal properties and morphologies.