Thermo-Induced Limited Aggregation of Responsive Star Polyelectrolytes

Poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) star polyelectrolytes with dual thermo- and pH-responsive properties have been studied by in situ small-angle neutron scattering at different temperatures and pH conditions in order to reveal their conformational changes in semidilute solution. At pH values close to the pK(a), all PDMAEMA stars studied here are partially charged and show a core-shell quasi-micellar morphology caused by microphase separation with a collapsed core region with high monomer density and a hydrated loosely packed shell region. Upon increasing the temperature, the PDMAEMA star polyelectrolytes first experience a contraction in the shell region while the core size remains almost unchanged, and then start to form limited intermolecular aggregates. With decreasing pH values, the transition temperature increases and the size of the aggregates decreases (average aggregation number changes from 10 to 3). We suggest that these changes are triggered by the decrease in solvent quality with increasing temperature, which leads to the transition from an electrostatically dominated regime to a regime dominated by hydrophobic interactions. The observed phenomenon is in striking contrast to the phase behavior of linear PDMAEMA polyelectrolytes, which show macrophase separation with increasing temperature under the same conditions.