Surface-induced breakout crystallization in cylinderforming P(I-b-EO) diblock copolymer thin films

Abstract

We have investigated the slow crystallization in thin films of cylinder-forming poly(isoprene-b-ethyleneoxide) (P(I-b-EO)) diblock copolymers with PEO being the minority block. The film was crystallized at room temperature after melting at 62°C. We have combined optical and atomic force microscopy with grazing-incidence small-angle X-ray scattering and diffraction and X-ray reflectometry. Initially, the cylinders lie in the film plane, and PEO is amorphous. Only after 41 days at room temperature, we have observed breakout crystallization of PEO at the film surface, i.e. distinct crystalline domains having a circular, elevated rim, radii of ~250 μm and finger-like extensions at the growth front. After 148 days, crystallized, finger-like terraces were observed over the entire film surface. The terrace height is 20% higher than the repeat distance in the as-prepared film. Thus, at the film surface, the cylinders were destroyed by crystalline lamellae lying in the film plane. The PEO chain stems were perpendicular to the substrate surface. We attribute the crystalline state to once-folded and fully interdigitated PEO layers alternating with amorphous PI layers. Within the film, PEO is still amorphous, and the hexagonal morphology is presumably still present with an unchanged repeat distance.