Thickness-induced morphology changes in lamellar diblock copolymer ultrathin films

We have investigated the morphology of lamellar polystyrene-polymethylmetha cry late diblock copolymers in the ultrathin-film limit, spin-cast and annealed on silicon nitride substrates. Our experiments show evidence for a morphology change from lamellar domains parallel to the substrate to perpendicular domains that appears to be unique to a thickness of one lamellar repeat spacing and low annealing temperatures. We find that in these ultrathin films phase separation occurs well before thickness quantization, suggesting effective confinement as a possible mechanism. A model for the free energy of this system is developed which takes into account termination ("capping") of the perpendicular lamellae inside the film and at its top surface