The effect of heat treatment on the internal structure of nanostructured block copolymer films

We report on the temperature dependence of the nanostructure of thin block copolymer films, as studied using in situ grazing-incidence small-angle x-ray scattering (GISAXS). We focus on spin-coated poly(styrene-b-butadiene) diblock copolymer thin films featuring lamellae perpendicular to the substrate. In situ GISAXS measurements elucidate the structural changes during heat treatment at temperatures between 60 and 130 ◦C. Thermal treatment below 100 ◦C
does not destroy the perpendicular lamellar order. In contrast, treatment between 105 and 120 ◦C leads to a broad distribution of lamellar orientations which only partially recovers upon subsequent cooling. Treatment at 130 ◦C leads to severe changes of the film structure. We attribute the change of behavior at 100 ◦C to the onset of the glass transition of the polystyrene block and the related increase of long-range mobility. Our results indicate that the perpendicular lamellar orientation for high molar mass samples is not stable under all conditions.