Controlling Particle Location with Mixed Surface Functionalities in Block Copolymer Thin Films

The controlled placement of gold nanoparticles (AuNPs) in poly-(styrene-b-isoprene-b-styrene) [SIS] triblock copolymer thin films was achieved by tuning the surface chemistry of the AuNPs. Facile thiol ligand exchanges permitted quick and thorough exploration of the surface chemistry effects on AuNP segregation behavior. By using thiol-functionalized polystyrene (PS-SH) or 1-dodecanethiol (C12SH) ligands to tune nanoparticle surface chemistry, AuNPs were selectively incorporated into either the polystyrene (PS) or the polyisoprene (PI) domains. Nanocomposite polymer films were characterized by atomic force microscopy, transmission electron microscopy (TEM), and TEM tomography. AuNPs synthesized with a C12SH capping ligand preferentially segregated into PI-rich domains. However, after exchanging C12SH with PS-SH ligands to a C12SH:PS-SH molar ratio of approximately 5:1, AuNPs showed an affinity for PS-rich domains. The C12SH:PS-SH transition ratio was much higher than expected, based on molecule-averaged surface energy arguments that predicted a ratio of 0.4:1 to 0.8:1. The unexpected transition ratio was rationalized according to the area-averaged enthalpic contributions of the capping ligands. Furthermore, mixing and incorporating PI- and PS-preferential AuNPs created a well-mixed nanocomposite, which highlights the versatility of the AuNPs.