“Chemically Shielded” Poly(ethylene oxide) Single Crystal Growth and Construction of Channel-Wire Arrays with Chemical and Geometric Recognitions on a Submicrometer Scale

A series of poly (ethylene oxide) -block-polystyrene (PEO-b-PS) diblock copolymers were used to generate nucleation sites for the crystal growth of a homo-PEO fraction in solution. The number-average molecular weights of the PEO blocks (MnPEO) were similar, and the number-average molecular weights of the PS blocks (MnPS) ranged from 4. 6K to 17K g/mol. In PEO-b-PS/ (chlorobenzene/octane) solutions, square-shaped single crystals bounded by four 120 planes were isothermally grown and observed with transmission electron and atomic force microscopy. A “sandwich” lamellar structure, constructed by a PEO single crystal layer covered by two tethered PS block layers on the top and bottom crystal basal surfaces, was found. These diblock copolymer single crystals were used as seeds to grow homo-PEO (MnPEO = 56K g/mol) single crystals in amyl acetate. When the MnPS in the block copolymer was 4. 6K g/mol, the edges and corners of the 120 bounded PEO-b-PS single crystals served as nucleation sites to initiate the further growth of the homo-PEO single crystal. As the MnPS of the block copolymers increased, the homo-PEO crystal growth was increasingly hampered along the 120 edges of the PEO-b-PS single crystals. When the MnPS of the block copolymer was 17K g/mol, only the four corners of the PEO-b-PS single crystal could still act as nucleation sites. The four edges were chemically “shielded” by the tethered PS blocks. This indicates that increasing the MnPS led to a higher reduced tethering density of the PS blocks on both the basal surfaces of the PEO-b-PS single crystals. The repulsion generated among the tethered PS blocks caused the PS blocks located near and at the edges to advance along the 120 direction. Interestingly, this local environment only accepted the PEO-b-PS molecules but rejected the homo-PEO molecules from further growth. As a direct result of this study, novel channel-wire arrays on a submicrometer length scale having chemical and geometric recognitions could be fabricated via alternating crystal growth of PEO-b-PS and homo-PEO. This fabrication provided robustly controlled arrays with spacing down to 50 nm.