Surface structure of thin asymmetric PS-b-PMMA diblock copolymers investigated by atomic force microscopy

Asymmetric poly(styrene-b-methyl methacrylate) (PS-b-PMMA) diblock copolymers of molecular weight Mn = 29,700 g mol−1 (MPS = 9300 g mol−1 MPMMA = 20,100 g mol−1, PD = 1.15, χPS = 0.323, χPMMA = 0.677) and Mn = 63,900 g mol−1 (MPS = 50,500 g mol−1, MPMMA = 13,400 g mol−1, PD = 1.18, χPS = 0.790, χPMMA = 0.210) were prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization. Atomic force microscopy (AFM) was used to investigate the surface structure of thin films, prepared by spin-coating the diblock copolymers on a silicon substrate. We show that the nanostructure of the diblock copolymer depends on the molecular weight and volume fraction of the diblock copolymers. We observed a perpendicular lamellar structure for the high molar mass sample and a hexagonal-packed cylindrical patterning for the lower molar mass one. Small-angle X-ray scattering investigation of these samples without annealing did not reveal any ordered structure. Annealing of PS-b-PMMA samples at 160 °C for 24 h led to a change in surface structure.