(578e) Self-Assembly of Polystyrene-Polyisoprene STAR Copolymers

SELF-ASSEMBLY OF
POLYSTYRENE-POLY (N-ISOPROPYLACRYLAMIDE) STAR COPOLYMERS

Juan Pablo Hinestrosa,¹
Jason P. LeJeune,¹ S. Michael Kilbey II,^1,2 Jamie
M. Messman¹

¹Center for Nanophase Material Sciences, Oak Ridge
National Laboratory, Oak Ridge, TN 37834; ²Department of Chemistry,
University of Tennessee, Knoxville, TN 37996

A modular synthetic
process combining the utility of RAFT polymerization with ?click' chemistry
yields well-defined star copolymers suitable for fundamental studies of
self-assembly in selective solvents and for bio-related applications, such as
drug delivery, due to their temperature responsiveness at physiological
conditions. Copolymers composed of polystyrene (PS)-poly
(N-isopropylacrylamide) (PNIPAM) are synthesized by this modular approach
yielding three-arm stars as probed by FTIR, GPC and NMR measurements.
Self-assembly studies of these copolymers in H₂O, a selective
solvent for PNIPAM, show micellization into sphere-like structures with sizes ~
20 nm. For a diblock copolymer of PS (P)-block-PNIPAM (N), large
aggregates, i.e. micelle clusters, with sizes ~ 100 nm are detected in
coexistence with micelles, as can seen in the Figure. This behavior is accentuated
as temperature is increased closer to the coil-to-globule transition for PNIPAM
where this block becomes more hydrophobic.. Measurements at ~ 34 °C showed the
formation of clusters for the P₂N₁ star, i.e. two PS arm
and one PNIPAM arm, but not for star P₁N₂.

Figure. Normalized hydrodynamic radius, R_h,
distributions for PS (P)-PNIPAM (N) star copolymers in H₂O at 1
mg/mL and for the PNIPAM arm in THF at 5 mg/mL. The subscripts indicate the
number of PS or PNIPAM arms in each copolymer.