Effects of temperature and salt concentration on the structural and dynamical features in aqueous solutions of...

Effects of temperature and salt addition on the association behavior in aqueous solutions of a series of charged thermosensitive methoxypoly(ethylene glycol)-block-poly(N-isopropylacrylamide)-block-poly(4-styrenesulfonic acid sodium) triblock copolymers (MPEG(45)-b-P(NIPAAM)(n)-b-P(SSS)(22)) with different lengths of the PNIPAAM block (n=17, 48, and 66) have been studied with the aid of turbidity, small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS). Increasing temperature and salinity as well as longer PNIPAAM blocks are all factors that promote the formation of association structures. The SAXS data show that, for the copolymers with n=48 and n=66, increasing temperature and salt concentration induce interchain associations and higher values of the aggregation number, whereas no aggregation was observed for the copolymer with the shortest PNIPAAM chain. However, DLS measurements reveal the presence of larger association clusters. The cloud point is found to decrease with raising salinity and longer PNIPAAM block. The general picture that emerges is the delicate interplay between repulsive electrostatic forces and hydrophobic interactions and that this balance can be tuned by changing the temperature, salinity, and the length of the PNIPAAM block.