Mixed lamellar films: Evolution, commensurability effects, and preferential defect formation

The structure of lamellar poly(styrene-block-methyl methacrylate), denoted as P(S-b-MMA), diblock copolymer films on neutral poly(styrene-random-methy l methacrylate), denoted as P(S-r-MMA), brush surfaces was examined as a fu nction of annealing time and film thickness using neutron reflectivity and small-angle neutron scattering. Upon annealing, microphase separation occur s quickly with perpendicular and parallel lamellae emanating from the neutr al and air surfaces, respectively. This initial growth is followed by a slo w increase in the amount of parallel lamellae with a concurrent decrease in the amount of perpendicular lamellae as the samples are annealed further. After long annealing times, the amounts of perpendicular and parallel lamel lae do not change significantly. The mixed lamellar structures show a stron g commensurability trend with film thickness having a period equal to that of the natural lamellar period. This is explained through the preferential formation of s(1/2) disclinations having PS cores which dictate the observe d commensurability trend. The preferential formation of these defects was o bserved by field emission scanning electron microscopy on films predominant ly comprised of perpendicular lamellae.