Thermo-/pH-responsive behaviours of base-rich diblock polyampholytes in aqueous solution: experiment and simulation

We report on the thermo-responsive and pH-responsive behaviours exhibited by two poly(acrylic acid)-poly(2(dimethylamino)ethyl methacrylate) diblock copolymers with unbalanced acid-base compositions, PAA(40)-PDMAEMA(120) and PAA(40)-PDMAEMA(285), in water, a selective solvent. Using a combination of ultraviolet-visible (UV-vis) spectroscopy and dynamic light scattering, the dual-stimuli responsive behaviours can be clearly detected. The zeta-potential decreases sharply for a symmetrical counterpart PAA(60)-PDMAEMA(70) solution, while it changes evenly for PAA(40)-PDMAEMA(120) and PAA(40)-PDMAEMA(285) solutions because of the asymmetrical compositions. The lower critical solution temperature of the PAA(40)-PDMAEMA(285) solution is always less than that of the PAA(40)-PDMAEMA(120) solution at the same solution concentration and pH values. The molecular dynamics simulation is used to explore the molecular mechanism of the responsive behaviour. The simulation also evidences the presence of dual-stimuli responsive behaviour of the copolymer with respect to changes in temperature and pH in aqueous solutions. The radius of gyration R-g and the end-to-end distance R-e of the asymmetric polyampholyte A(20)-B-60 are smaller than those of the symmetric one A(40)-B-40 at all simulated temperatures, which indicates increased aggregation behaviour and the compact aggregates of asymmetric polyampholyte chains. The effect of polymer molecular mass M-W on the aggregation behaviour seems to be more profound than that of polymer composition, which may be attributed to the fact that increasing the molecular weight of the copolymer increases the variability of the micelle sizes. Additionally, hydrophobic interaction plays a key role in the aggregation process.