Polymer brushes on nanoparticles: their positioning in and influence on block copolymer morphology.

Polymers brushes grafted to the nanoparticle surface enable the precise positioning of particles within a block copolymer matrix by determining the compatibility of nanoparticles within a polymeric matrix and modifying the interfacial properties between polymers and inorganic nanoparticle. Short thiol terminated polystyrene (PS-SH), poly(2-vinylpyridine) (P2VP-SH) and PS-$r$-P2VP with the molecular weight (M$_{n})$ of 3 kg/mol were used to control the location of Au nanoparticles over PS-b-P2VP diblock copolymer template. We will discuss further the approach of varying the areal chain density ($\Sigma )$ of PS-SH brushes on the PS coated particles, which utilizes the preferential wetting of one block of a copolymer (P2VP) on the Au substrate. Such favorable interaction provides the strong binding of Au particles to the PS/P2VP interface as $\Sigma $ of PS chains on the Au particle decreases. We find that at $\Sigma $ above a certain value, the nanoparticles are segregated to the center of the PS domains while below this value they are segregated to the interface. The transition $\Sigma $ for PS-SH chains (M$_{n}$ = 3.4 kg/mol) is 1.3 chains/nm$^{2}$ but unexpectedly scales as M$_{n}^{-0.55}$ as M$_{n}$ is varied from 1.5 to 13 kg/mol. In addition, we will discuss changes in block copolymer morphology that occur as the nanoparticle volume fraction (\textit{$\phi $}) is increased for nanoparticles that segregate to the domain center as well as those that segregate to the interface, the latter behaving as nanoparticle surfactants. Small \textit{$\phi $} of such surfactants added to lamellar diblock copolymers lead initially to a decrease in lamellar thickness, a consequence of decreasing interfacial tension, up to a critical value of \textit{$\phi $} beyond which the block copolymer adopts a bicontinuous morphology. I thank my collaborators G. H. Fredrickson, J. Bang, C. J. Hawker, and E. J. Kramer as well as funding by the MRL as UCSB from the NSF-MRSEC-Program Award DMR05-20418.