Tethered-Polymer Structures in Thin Film Polymer Melts

We show that polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer chains aggregate to form micelles, composed of an inner P2VP core and an outer PS corona, as well as adsorb onto the substrate, forming brush layers, in supported thin film PS/PS-b-P2VP mixtures. The degrees of polymerization of the chains that comprised the corona and core were N-PS and N-P2VP, respectively. The diameter of the micelle cores, D-core, increased with increasing degree of polymerization, P, of the PS host and became constant for large values of P. A decrease in the number density of micelles, n(micelle), accompanied the increase in D-core; n(micelle) reached a plateau in the same range of values of P where D-core became constant The organization of the micelles suggests the existence of attractive micelle-micelle interactions in the large P regime. Moreover, in this regime, the micelles preferentially migrated to the free surface. The morphology of this system is compared to thin film PS melts containing brush-coated nanoparticles. One fundamental difference between the two systems is that the micellar system has the ability to adjust the number of chains per micelle, and hence the number of micelles, in order to mediate the free chain/brush (micelle corona) interactions. This has important consequences on the location of the wet-brush to dry-brush transition.