Monomolecular Films of Arborescent Graft Polystyrenes

Abstract

Scanning force microscopy has been employed to visualize the molecular organization of arborescent graft polystyrenes with different branching densities as monolayer films cast on mica. The apolar polymers were depicted as distinct hexagonally packed globuli whose size was consistent with molecular dimensions obtained from viscosity and dynamic light scattering measurements. Distortions of the long-range order were observed and attributed to the inherent polydispersity of the polymers. The layer thickness, i.e., the particle diameter in the z-direction, depended on the branching density and indicated formation of a prolate shape in the vicinity of the flat substrate after evaporation of the solvent. Molecules with a high branching density (Mb ∼ 500 g/mol, average molecular weight between grafting points) recovered a spherical geometry after annealing above the glass transition temperature. In the case of a lower branching density, e.g., Mb = 2000 g/mol, the “pancake” structure remained stable upon annealing. The data demonstrate that arborescent graft polymers represent a peculiar type of colloidal particles which consist of one molecule possessing a topologically defined surface.