Dissolution and aggregation of a poly(NIPA-block-sulfobetaine) copolymer in water and saline aqueous solutions

Thermal properties of the novel, double thermosensitive block copolymer, poly(N-isopropyl acrylamide)block-poly(3- [N-(3-methacrylamido-propyl)-N,N-dimethyl]-ammonio propane sulfonate) (PNIPA-b-PSPP) have been studied in pure and saline (NaCl) aqueous solutions by dynamic laser light scattering. The block copolymer [M-n(PNIPA) = 10 800 g/mol and M-n(PSPP) = 9700 g/mol] exhibits both upper (UCST about 9 degreesC) and lower (LCST about 32 degreesC) critical solution temperatures in pure water. The addition of NaCl enhances the solubility of the zwitterionic block, PSPP, leading to the disappearance of the UCST. On the other hand, the solubility of PNIPA in water decreases as NaCl is added. At 20 degreesC, the copolymer shows a bimodal size distribution through the NaCl concentration range of 0-0.93 M above a certain limiting polymer concentration. The slow and fast components of the diffusion coefficients of the polymer have been calculated. A gradual addition of salt turns the mutual interactions from zwitterionic attractions between PSPP blocks to hydrophobic attractions between PNIPA blocks. The formation of the aggregates and the aggregate sizes at T < UCST and T > LCST are influenced by polymer and salt concentrations. Below the UCST, the aggregates in saline polymer solutions are somewhat larger than those in pure polymer solutions. Above LCST, the aggregate size is determined by the salt concentration.