A study of aggregation behavior of a sulfobetaine copolymer in dilute solution

A copolymer poly(acrylamide-co-3-[N-(2-methacroyloylethyl)-N, N-dimethylammonio]-propane sulfonate) (P(AM/DMAPS)) was synthesized via conventional free-radical polymerization in aqueous media. Its aggregation and disaggregation in aqueous solution were studied as a function of copolymer concentration, ionic strength and ageing time at different temperature by gel permeation chromatography combined with laser light scattering (GPC-MALLS) technique, static and dynamic laser light scattering. GPC-MALLS analysis shows that at low copolymer concentrations (below 0.4 g.L⁻¹), a decrease of both apparent weight-average molecular weight and radius of gyration is observed due to the dissociation of the interchain association. With increasing copolymer concentration, the interchain association is enhanced and both apparent weight-average molecular weight and radius of gyration increase. Correspondingly, the exponent ρ of the radius of gyration/molecular weight (R_g−M_w) relationship shows an increase from 0.29 to 0.6 with increasing copolymer concentration (below 0.4 g.L⁻¹), and then decreases continuously to 0.33 with further increasing copolymer concentration. Dynamic laser light scattering studies reveal that the addition of a small amount of salts (below 0.1 mol.L⁻¹) leads to the disaggregation of the intra- and interchain aggregation. Further addition of salts results in the enhancement of interchain aggregation. The influence of various anions on the aggregation behavior increases in the order Cl ⁻ < Br ⁻ < I ⁻. The prolonging of the ageing time at 25 °C and 70 °C leads to the disaggregation of the interchain association of P(AM/DMAPS) copolymer in both deionzied water and 0.15 mol.L⁻¹ NaCl solution.