Surface relief terraces in double-gyroid-forming polystyrene- block -polylactide thin films

This study describes the thin film self-assembly behavior of a polystyrene-block-polylactide diblock copolymer, which undergoes melt self-assembly in bulk into a double gyroid (DG) network phase with a cubic unit cell parameter a=52.7nm. Scanning electron microscopy and grazing-incidence small-angle x-ray scattering reveal that thermally annealing 140-198 nm thick copolymer films on SiO2 substrates below the morphological order-to-disorder transition temperature yields polydomain DG structures, in which the (422) planes are oriented parallel to the surface. Bright-field optical microscopy and atomic force microscopy analyses further reveal the film thickness-dependent formation of topographical terraces, including islands, holes, and bicontinuous features. The occurrence of these features sensitively depends on the incommensurability of the as-prepared film thickness with the (211)-interplanar spacing (d211) of the DG unit cell. While the step heights between adjacent terraces exhibiting characteristic "double wave"patterns of the DG (422) planes coincide with d211, previously unreported transition zones between adjacent terraces are observed wherein "boomerang"and "droplet"patterns are observed. These intermediate patterns follow the expected sequence of adjacent termination planes of the bulk DG unit cell along the [211] direction, as confirmed by comparisons with self-consistent mean-field theory calculations.