Thermo-Induced Limited Aggregation of Responsive Star
Polyelectrolytes
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Abstract
Poly(<i>N</i>,<i>N</i>-dimethylaminoethyl methacrylate)
(PDMAEMA) star polyelectrolytes with dual thermo- and pH-responsive
properties have been studied by <i>in situ</i> 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 p<i>K</i><sub>a</sub>, 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