Molecular dynamics study of the role of the free surface on block copolymer thin film morphology and alignment

Next-generation applications of block copolymer thin films will require a better understanding of the driving forces unique to thin film coatings, specifically those arising from the polymer-air interface. Previous modeling studies of film morphology have treated rigidly confined films, neglecting free surface considerations altogether. We report in this article the first systematic molecular dynamics investigation of block copolymer thin film ordering for unconfined films. We investigate the molecular basis of the formation of a number of experimentally relevant coating features, including surface islands and vertical lamellae. Surface islands are found to form in response to film incommensurability, whereas commensurability considerations are insufficient to explain vertical lamellar formation. Dynamics of lamellar formation presented herein demonstrate that vertical lamellar orientation is initiated in the surface regions of the film, most strikingly at the free surface. We conclude that the free surface plays a pivotal role in the free energy balance determining overall film morphology, and that confinement models provide an incomplete explanation of the physical basis of morphology selection in block copolymer coatings.