Article
Macromolecules, Vol. 43, No. 24, 2010 10229
with Dz of 250 nm at 35 °C (see heating curve). This means
that there may be the interaction of hydrophobic groups of
ibuprofen with hydrophobic PNIPAm chains. This was
supported by zeta-potentials of -14.0 mV. Above the LCST
the Dz of star-PNIPAm-CD aggregates in ibuprofen sodium
solution were smaller than those in distilled water and
aqueous ADA-NH3þ solution, but the change in its Dz with
temperature is similar to those in the two solutions. This is
because inclusion complex of CD with ibuprofen can lead to
a protrusion of the ibuprofen hydrophilic -COO- groups
from the CD cavity,66,67 causing the PNIPAm arm ends
carry negative charge (see Scheme 2c). This charge could
stabilize the formed star-PNIPAm-CD nanosized particles.
This was confirmed by the zeta-potentials of -11.9 mV (see
Table 2). This means that CD end groups have ability of
inclusion complex with guest molecules. In addition, as shown
in Figure 11, the formation and dissociation of the nanosized
aggregation of star-PNIPAm and star-PNIPAm-CD in aqueous
solution of Ibuprofen sodium salt can change reversibly by
changing temperature above and below the LCST.
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Novel star-PNIPAm and star-PNIPAm-CD have been synthe-
sized by core-first method successfully. In the synthesis, CD-core
with 21 initiation sites could be synthesized by the reaction of CD
with CPC. Star-PNIPAm could be synthesized by ATRP of
NIPAm using 21Cl-β-CD as initiator. Star-PNIPAm-CD could
be synthesized by ATRP of GMA-EDA-β-CD initiated via star-
PNIPAm. The molecular weight distributions of the star polymers
are narrow. By using ANS, ADA-NH3Cl, and ibuprofen-Na as
guest molecules, thermal sensitivity and inclusion behaviors of
the star polymers were investigated by fluorescence spectro-
photometer and DLS. It was found that the star polymers can
combine both thermal sensitivity of PNIPAm and inclusion
behavior of β-CD. Interestingly, the star polymers can self-
assemble to form nanosized aggregation above LCSTs of the
star polymers in aqueous solution. Below the LCSTs, the formed
nanoparticles could molecularly dissolve in aqueous solution again.
The aggregation of the star polymers in aqueous solution
shows molecular recognition capability. It is concluded that (1)
the size of self-assembled star-PNIPAm aggregates depends on
the interaction of PNIPAm arms with guest molecules above the
LCST and (2) the size of self-assembled star-PNIPAm-CD
aggregates depends on the property of inclusion complex of
CD end groups with guest molecules. Therefore, the selection
of appropriate guest molecule can modulate the aggregation
behavior of the star polymer in aqueous solution. These are very
useful for their application.
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Acknowledgment. We are grateful for the financial support
of the National Nature Science Foundation of China (Nos.
20674061 and 20974088).
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