Hierarchical Organization of Poly(ethylene oxide)-<i>block</i>-poly(isobutylene) and Hydrophobically Modified Fe₂O₃ Nanoparticles at the Air/Water Interface and on Solid Supports

Abstract

Langmuir monolayers and Langmuir-Blodgett (LB) film morphologies of block copolymers and hydrophobically modified iron oxide nanoparticles were studied by surface pressure-mean molecular area (pi-mmA) measurements and by tapping mode atomic force microscopy (AFM). The amphiphilic diblock copolymers consisted of a hydrophilic poly(ethylene oxide) (PEO) block and a hydrophobic poly(isobutylene) (PIB) block. The pi-mmA isotherm of PEO(97)-b-PIB(37) (the subscripts refer to the respective degrees of polymerization) at the air/water interface had an extended plateau reflecting the extension of PEO chains into the water subphase at a surface pressure of 10 mN.m(-1), which is absent for the more hydrophobic PEO(19)-b-PIB(130). Iron oxide (Fe(2)O(3)) nanoparticles capped with oleic acid ligands as the shell were dispersed in the amphiphilic block copolymers at the air/water interface to prevent macroscopic aggregation of the particles. When the nanoparticles were mixed with PEO(97)-b-PIB(37), using a particle to polymer chain ratio of 1:100, macroscopic aggregation of the nanoparticles was not observed, and the pi-mmA isotherm was dominated by PEO(97)-b-PIB(37). Monolayers of block copolymers were transferred at different surface pressures from the air/water interface to hydrophilic silicon substrates using the Langmuir-Blodgett technique. The AFM images of PEO(97)-b-PIB(37) LB films depicted not only the typical finger-like morphology of the crystallized PEO blocks but also PIB blocks arranged in vertical columns growing perpendicular to the substrate surface. The columns are characteristic for PEO(19)-b-PIB(130) LB films after transfer at high surface pressures and can be assigned to a mesomorphic PIB phase with ordered chains. Finally, it was observed that small clusters of a few Fe(2)O(3) nanoparticles occupy the top of PIB phases after compression and transfer of the block copolymer nanoparticle mixtures to solid supports.