Thickness-induced morphology changes in lamellar diblock copolymer ultrathin films

We have investigated the morphology of lamellar polystyrene-polymethylmethacrylate 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.