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 Ca 2+, the complexation between one Ca 2+ 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 Ca 2+ and temperature as well as the sequence of adding Ca 2+ 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 Ca 2+ 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.