Morphological Studies of Block Copolymer under Confinement and Applications

Abstract

Block copolymers have been extensively investigated because of their unique microphase separated morphology, such as lamellae, gyroids, cylinders, and spheres, depending on volume fraction (f) of one of the block, degree of polymerization and the Flory-Huggins segment interaction parameter. Block copolymers have been also employed for templates for organic/inorganic hybrid materials, data storage media, micelles, and porous membranes. In addition to typical microdomains of lamellae, gyroids, cylinders and spheres, other nanostructures are required for unique optical property. One of the methods to achieve this objective is to confine block copolymers into certain geometries. Although previous studies have been done in one, two, and three-dimensional confinement, hemispherical cavity corresponding to 2.5 dimensional confinement has not been investigated. First, we investigated, via scanning and transmission electron microscopy, the arrangement of the microdomains of symmetric polystyrene-block-poly(methyl methacrylate) copolymer (PS-b-PMMA) confined in hemispherical cavities. The hemispherical cavities were prepared by using anodic aluminum oxide (AAO) template, where the inner surface of the cavities was modified by thin brush layers of PS, PMMA, and PS-ran-PMMA copolymer. When the inner surface of the cavity is strongly selective to PS chains, concentric rings consisting of PS and PMMA microdomains are formed, replicating the confined geometry of hemi-sphere. However, as the selectivity of a brush to PS chains becomes weaker, various interesting morphologies are formed, that have not been reported in the literature. The experimentally observed microdomain arrangement confined inside the hemispherical cavities was compared with the simulation results based on dynamic Metropolis Monte Carlo method.Secondly, we fabricated a high density array of concentric silver nanorings with uniform gap distance in a large area (over in2) by the confinement of symmetric PS-b-PMMA within hemi-spherical cavities. Silver nanoparticles with 6 nm height were selectively deposited only on the PS microdomains by thermal evaporation. The gap distance of two neighboring silver nanorings was controlled from 12 to 24 nm by changing the total molecular weight of PS-b-PMMAs. The substrate with high density array of silver nanorings showed excellent surface enhanced Raman scattering (SERS) enhancement factor as high as 4.3 x 107 with good reproducibility (± 7%). It could be used for biosensing, detection of trace-level explosive and hazardous chemicals, and reaction monitoring.Finally, we fabricated stacked and spiral structures by using lamellar-forming PS-b-PMMA. When cylindrical cavities made by AAO template are modified by thin brush layers of PS-ran-PMMA copolymer, PS and PMMA lamellae are stacked alternatively. After removal of AAO template, followed by silver deposition on PS microdomains by using thermal evaporation, high density array of stacked lamellae with silver nanoparticle shows very interesting optical properties. Namely, the absorption of visible (or near Infra-red) light depends significantly on irradiation angle. We also obtained spiral structures by confinement of lamellar-forming PS-b-PMMA into cylindrical AAO pores with very short length to diameter (L/D) of the cavity. Also, L is not commensurable to the lamellar domain spacing, block copolymer microdomains were formed into spiral structure to decrease entropy loss from chain shrinkage.