Nanoimprint Directed Assembly of Associating Polymer-Grafted Nanoparticles for Polymer Thin Films with Enhanced Stability

We find that changing the chemistry of the inorganic core of polymer-grafted nanoparticles (PGNPs) from Au to TiO2 in a polymer matrix having the same chemistry leads to a qualitative change in the propensity of the PGNPs to self-assemble in the polymer matrix, pointing to the significance of enthalpic interactions between the NP cores and the polymer matrix. Specifically, PGNPs having polystyrene (PS) chains grafted to a Au nanoparticle core disperse well in a PS matrix, while PGNPs under similar thermodynamic conditions having instead a TiO2 core of about the same PS grafted chains and polymer matrix, exhibit large scale self-assembly into noncompact "clusters of PGNPs" (PGNPCs) (Nat. Mater. 2009, 8 (4), 354-359). We then investigated whether we could still use the method of nanopatterning established in our previous study of nonassociating PGNPs, for also directing the self-assembly of PGNPCs. We find nanoimprinting the associating PS-TiO2 particles still allows us to localize the PGNPCs to patterns through the entropic localization effect that we investigated in previous work, but the nanoimprinting process also allows us to direct the PGNPC self-assembly process, resulting in significant property changes in PGNPC nanocomposite films in comparison to films containing nonassociating PGNPs. For example, the associating PGNPC films become significantly stabilized against dewetting in comparison to the nonassociating PGNPs. We quantify the partitioning of the PGNPCs to the patterned regions by estimating the cluster partition coefficient Ic of the PGNPCs to the thicker film regions, demonstrating that selective segregation of PGNPCs to the imprinted patterns still arises from an entropic segregation of the PGNPCs associated with the alteration of the conformation fluctuations of the grafted polymers under confinement induced by the imprinting process. This form of pattern-directed self-assembly of PGNPCs can be expected to be useful for "writing" large scale patterns in thin polymer films having designed optical, electronic, mechanical, frictional properties, while at the same time enhancing the stability of the polymer film.