Effect of Ca2+ Ion and Temperature on Association of Thermally Sensitive PAA-b-PNIPAM Diblock Chains in Aqueous Solutions

Poly(tert-butyl acrylate)-b-poly(N-isopropylacrylamide) (PtBA-b-PNIPAM) was first synthesized by sequential reversible addition–fragmentation chain transfer polymerization of tert-butyl acrylate and N-isopropylacrylamide. Its hydrolysis led to amphiphilic poly(acrylic acid)-b-poly(N-isopropylacrylamide) (PAA-b-PNIPAM) that can form micelles in aqueous solutions at temperatures higher than 37 °C because PNIPAM is a thermally sensitive polymer. In the presence of Ca2+, the complexation between one Ca2+ and two COO– groups on different PAA blocks can induce the chain association. Using a combination of static and dynamic laser light scattering, we studied the effect of Ca2+ and temperature as well as the sequence of adding Ca2+ ions and heating the solution on such association. We found that (1) the association is controllable and reversible, (2) a distinct hysteresis is observed between the heating and cooling processes, (3) the time evolution of the average aggregation number (N agg) and the average hydrodynamic radius (⟨R h⟩) of the aggregates can be expressed by a single-exponential equation, (4) the aggregates have a fractal dimension of 1.5–1.9, suggesting a diffusion-limited process, and (5) adding Ca2+ before heating results in the aggregates with a more open and looser structure. The current study provides a model system to investigate a more complicate problem, namely, the effect of metal ions on the stability of protein chains.