Neutron Reflection Studies on Segment Distribution of Block Chains in Lamellar Microphase-Separated Structures

Segment distribution of styrene in an alternating lamellar structure of styrene−2-vinylpyridine diblock copolymer was investigated by neutron reflection. Block copolymers having three types of deuterium-labeled styrene block chains consisting of fully labeled, partially junction-labeled, and partially end-labeled block chains were used in this study. The degree of junction and end labeling was varied to obtain a more complete picture of segment distribution. Spin-coated thin films of these block copolymers on silicon wafers exhibited lamellar structures which were oriented preferentially parallel to the silicon surface. Our results indicate that the poly(2-vinylpyridine) block chain exists at the silicon surface, while polystyrene appears at the air surface in all of the film specimens. The segment distribution at the interface between polystyrene and poly(2-vinylpyridine) lamellae could be well described by an error function. The thickness of the lamellar interface (defined as a full-width half-maximum value of the error function) is evaluated to be about 4.5 nm, suggesting that the blocks are strongly segregated. Accordingly, the segments of a block chain in the vicinity of the chemical junction point between two block chains are determined to be strongly localized near the lamellar interface. However, the free ends are broadly distributed throughout the lamellar microdomain with their net maximum distribution at the center of each microdomain. Such an end-segment distribution of the block chains is shown to be consistent with predictions from a mean field theory.