Effects of spacer length on the side-chain micellization in random copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonate and methacrylates substituted with ethylene oxide-based surfactant moieties

Side-chain micellization in the copolymers of sodium 2-(acrylamido)-2-methylpropane-sulfonate and methacrylates substituted with HO(CH(2)CH(2)O)(m)C(12)H(25) (C(12)E(m)), where m = 2, 6, and 25, was investigated in 0.1 M NaCl aqueous solutions focusing on the effect of the ethylene oxide (EO) spacer length (m) on the apparent critical micelle concentration (cmc), aggregation number (N(agg)) of the sidechain hydrophobes in polymer-hound micelles, hydrodynamic radius of polymer assemblies, steady-shear viscosity, and viscoelastic behavior. The cmc decreased with increasing m, an opposite trend to that of the corresponding free C(12)E(m) surfactant molecules, whereas N(agg) decreased with increasing m, a similar trend to that of the free surfactant molecules. N(agg) values for the copolymers with m = 6 and 25 were virtually the same as those for micelles formed from free C(12)E(6) and C(12)E(25) molecules, respectively. Steady-shear viscosity increased gradually at low or intermediate polymer concentrations (C(p)) followed by a drastic increase at higher C(p). Viscosities for the copolymers with m = 25 are roughly 2 and 3 orders of magnitude higher than those of the copolymers with m = 6 and 2, respectively, at a given C(p). Dynamic viscoelastic measurements revealed that the number density of mechanically active chains for the copolymer with m = 25 is roughly 1 and 3 orders of magnitude larger than those for the copolymers with m = 6 and 2, respectively, at a given C(p), indicating that interpolymer side-chain micellization occurs more favorably for the copolymer with longer EO spacer length leading to a network structure of a higher cross-linking density. The rheological terminal relaxation times decreased with increasing m, indicating that the lifetime of the transient network is shorter for larger m.