A neutron reflectivity study on a terraced lamellar morphology in a block copolymer thin film

A microphase-separated structure of a poly(deuterated styrene-block-2- vinylpyridine) (dPS-b-P2VP) block copolymer thin film was studied by neutron reflectivity (NR). The spun-coated dPS-b-P2VP block copolymer (on a Si substrate) showed the coexistence of a minority thin lamellar layer (holes) dispersed in a majority thick layer. The "terraced structure" was formed due to the mismatch between the initial film thickness and the intrinsic lamellar periodicity of the dPS-6-P2VP block copolymer. The amount and height of the holes were evaluated using atomic force microscopy (AFM), transmission electron microscopy (TEM) and transmission electron microtomography (TEMT). The three-dimensional (3D) images obtained from the TEMT experiments clearly showed that the microphase-separated structure inside the thin film was homeotropically aligned. The thickness of the thick and the thin lamellar layers corresponded to (5/2)L₀ and (3/2)L₀, respectively (L₀ is the lamella periodicity in the bulk state). The NR profile from the dPS-b-P2VP thin film, R_exp, showed many distinctive scattering peaks. Based on the structural information obtained from the microscopy, a scattering length density profile along the depth direction, i.e., the direction normal to the film surface, b/v_TEMT was evaluated, which was then used as an initial profile in the conventional model fitting method. An excellent best-fit to R_exp was obtained using b/v_TEMT, even though the thin film had the terraced structure. The surface coverage of the holes was estimated from the resulting b/v, which was in good agreement with the estimated value from TEM.