Phase Transitions in Triblock Copolymer Thin Films

Using scanning force microscopy, X-ray reflectivity and grazing incidence small angle X-ray scattering we investigate a phase transition in a thin film poly(styrene-b-ethylene-r-butylene-b-styrene) SEBS triblock copolymer (M$_w$ = 54kg/mol; f$_{PS}$ = 0.24). It has been shown for similar systems that the morphology of thin triblock copolymers is dependent on film thickness.\footnote{A. Knoll et al. Phys. Rev. Lett. 89, 035501 (2002)} A 37nm thick SEBS film on SiO$_2$ annealed at 180$^o$C shows a perforated lamella (PL) structure of PS embedded in an EB matrix with no islands. We investigate the phase transition of the PL into a cylindrical structure by further annealing the film at a higher temperature of 210$^o$C. In contrast to the flat PL film, the cylindrical phase shows islands of cylinders surrounded by a cylindrical monolayer. After a third annealing step at 180$^o$C, the islands return the PL morphology while the surrounding monolayer remains in the cylindrical morphology.