This
article reports on the synthesis of doubly responsive binary
heterografted, three-arm star molecular brushes and the study of their
stimuli-induced star-to-globule shape transitions in aqueous solution.
The star brushes were prepared using the highly efficient copper(I)-catalyzed
azide–alkyne cycloaddition reaction to simultaneously graft
an alkyne end-functionalized thermoresponsive polymer of ethoxydi(ethylene
glycol) acrylate (PDEGEA), with a lower critical solution temperature
(LCST) of 9 °C, and a pH-responsive polymer of 2-(N,N-diethylamino)ethyl methacrylate (PDEAEMA), with a pK
a of 7.4 at room temperature, onto an azide-bearing three-arm
star backbone polymer. The star architecture of the formed brushes
was confirmed by atomic force microscopy. The use of two different
stimuli-responsive polymers as side chains allowed for the stabilization
of collapsed brushes against aggregation when one side chain polymer
became insoluble in water and for the formation of two distinct globular
states with different polymers in the core. At temperatures below
the LCST of PDEGEA, upon increasing the pH from 5.00 to 9.40, the
brushes underwent a shape transition from an extended starlike to
a collapsed globular state stabilized by the hydrated PDEGEA side
chains. On the other hand, at pH values slightly lower than the pK
a of PDEAEMA, upon heating across the LCST of
PDEGEA, the brushes also exhibited a star-to-globule shape transition,
forming a globular state distinct from the one induced by pH changes.
The collapse of inner pH-responsive blocks drives cylindrical-to-globular shape transition while outer thermoresponsive blocks provide additional control of solution state.
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