Surface Behavior of PSn(P2VP-b-PtBA)n Heteroarm Stars

We report the surface behavior and morphologies of two series of pH-responsive amphiphilic heteroarm star block copolymers. In this respect, we studied polystyrene/poly(2-vinylpyridine)/poly(tert-butylacrylate), PSn(P2VP-PtBA)(n), heteroarm star block terpolymers and their precursors, PS(n)P2VP(n), star copolymers. These star block polymers differ in architecture (copolymer vs terpolymer), block topology, arm length (molecular weight of PtBA segments varies from 8 900 up to 15 250 Da), and number of arms (n = 9, 22, and 28). The pi-A isotherms at different subphase pH (pH 5.8 and 2.0) exhibited strong pH dependence, leading to different limiting molecular area and surface micelle stability. Because of the pH-induced ionization of the P2VP block, the surface morphology of star copolymers bearing the free P2VP arms was strongly dependent on the pH of the subphase, whereas the star terpolymers containing the protonated hydrophilic P2VP block as midblocks maintained the same circular morphology at low pH and high pressures. The surface morphologies suggested that the high number of arms promoted the formation of unimolecular micelles, which are stable under varying deposition conditions.