Effect of Hydrophilic Block-A Length Tuning on the Aggregation Behavior of α,ω-Perfluoroalkyl End-Capped ABA Triblock Copolymers in Water

Abstract

The tremendous influence of hydrophilic block length tuning on the aggregation behavior of novel water-soluble triphilic (i.e., hydrophilic, lipophilic, and fluorophilic) α,ω-perfluoroalkyl end-capped symmetric ABA triblock copolymers is demonstrated. The hydrophilic A and lipophilic B blocks are comprised of poly(glycerol monomethacrylate) (PGMA) and poly(propylene oxide) (PPO), respectively. The fluorophilic component consists of two “clicked” perfluoroalkyl segments (C9F19) at the ends of the block copolymers. Two of the different block copolymers synthesized, namely F9-PGMA24-PPO27-PGMA24-F9 (PB1) and F9-PGMA42-PPO27-PGMA42-F9 (PB2), differ only in the degree of polymerization of the hydrophilic PGMA blocks. Their critical micelle concentrations in water are determined from surface tension measurements. The aggregation behavior in aqueous medium studied by 19F NMR spectroscopy reveals that the fluorocarbon component forms part of the micelle corona of PB1, while in PB2 it aggregates to form part of the core. Furthermore, the aggregation behavior studied in aqueous medium by temperature-dependent 1H NMR spectroscopy and DLS measurements showed that PB1 forms only spherical micelles with hydrodynamic radius, Rh, of ∼18 nm in solution at all temperatures while PB2 forms mainly aggregate of micelles with Rh of 40 nm at 25 °C. The aggregates disintegrate into compact single “flowerlike” micelles with Rh of ∼17 nm at high temperatures. AFM and TEM investigations of the structures formed on solid supports after solvent evaporation also confirm the aggregation behavior of the two block copolymers. The marked difference in the aggregation behavior is a result of the inability of the shorter PGMA blocks of PB1 to loop during micellization and is explained based on random coil statistics.