Microphase separation and morphological transitions at the surface of block copolymers

Tapping-Mode Atomic Force Microscopy is used to study the microphase separation in thin films of symmetric triblock copolymers synthesized via a two-step «living» radical polymerization of n-butylacrylate and methylmethacrylate (MMA). This straightforward synthetic pathway allows for strict control of molecular weight, molecular weight distribution, and composition. The presence in the copolymer of immiscible segments covalently bound to each other leads to phase separation on the nanometer scale. Measuring the phase of the oscillating cantilever simultaneously with the topography allows us to determine the microdomain morphology at the surface. Here this approach is applied to all-acrylate (PMMA-6-poly-n-butylacrylate-b-PMMA) thermoplastic elastomers. The data point to a strong contrast in the local mechanical properties, corresponding to the microphase morphology. Cylinders of the minority phase are found to orient perpendicular to the surface, due to the surface energy difference between the constituents. Lamellae are also arranged perpendicular to the surface, in contrast to what is usually observed in block copolymers. This particular orientation is thought to result from the symmetric character of these triblock systems, with the outer blocks more polar than the central sequence.