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.