Article pubs.acs.org/Macromolecules
Direct Observation of Faceted Grain Growth of Hexagonal Cylinder Domains in a Side Chain Liquid Crystalline Block Copolymer Matrix Motonori Komura,* Hideaki Komiyama, Keiji Nagai, and Tomokazu Iyoda Division of Integrated Molecular Engineering, Chemical Resources Laboratory, Tokyo Institute of Technology, 4259-R1-25, Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan S Supporting Information *
ABSTRACT: Formation of a cylindrical microphase-separated structure (∼20 nm periodicity) of a liquid crystalline block copolymer comprising poly(ethylene oxide) (PEO) and poly(methacrylate) (PMA) bearing azobenzene (Az) mesogens in the side chains was investigated by in situ atomic force microscopy (AFM). The amphiphilic liquid crystalline block copolymer, PEO-b-PMA(Az), forms normally oriented PEO cylinders over the entire film by thermal annealing of various substrates without any pretreatment. Grazing incidence smallangle X-ray scattering (GISAXS) analysis with the simple compensation method of X-ray refraction effect confirmed an order−order transition from a ⟨110⟩-oriented bcc-sphere structure to normally oriented hex-cylinder structure, which is simultaneous liquid crystallization in a cooling process. The growth of cylindrical microphase separation grains in the liquid-crystallization-induced OOT was successfully visualized in the film surface by temperature-controlled AFM. The microphase-separated grains with hexagonally arranged cylinders grow primarily in hexagonal directions layer by layer in a region comprising bcc structures with an average growth rate of 1 nm/s. Moreover, formation of alignment defects of the microphase separation and collision of the grains to form grain boundaries were clearly observed. Furthermore, we succeeded in visualization of anisotropic formation of parallel oriented cylinders. A growth rate along the cylinders was 1−2 nm/s which was much higher than that along a perpendicular direction of the cylinders due to anisotropic growth of liquid crystallization. Such in situ AFM measurements are a significantly powerful tool to afford detailed visual information on structural changes during the formation process of the self-assembled nanostructures.
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INTRODUCTION Self-assembly of block copolymers, two chemically dissimilar polymers joined together, is emerging as a promising route for generating lithographic templates for the fabrication of photonic band gap materials,1 ultrahigh-density nanodots2,3 nanowire arrays,4,5 memory and capacitor devices,6 and nanopatterned substrates for biosensors. The feature size of phase separation between the polymer segments is restricted to molecular scale because of the chemical link between the incompatible polymer components. In general, phase separation depends on the thermodynamic interaction parameter χ, volume fraction of the different components, molecular weight, and molecular-weight distribution of the constituent component chains, and phase transition of the phase separation occurs in an appropriate temperature.7−10 Despite advantages such as parallel processing, molecular-level resolution, and the ability to generate three-dimensional structures, the practical application of block copolymer thin films has been limited owing to the lack of a strategy to control the structure formation in thin-film geometries. Various methods such as graphoepitaxy,11,12 chemical surface treatments,13,14 application of external fields,15−17 directional solidification,18,19 and solvent aging20,21 © 2013 American Chemical Society
have been developed to control the orientation of the block copolymer nanodomains. The authors of the paper previously developed amphiphilic liquid crystalline diblock copolymers comprising a hydrophilic block of poly(ethylene oxide) (PEO) and a hydrophobic block of poly(methacrylate) (PMA) with azobenzene (Az) liquid crystalline side chains, PEO-b-PMA(Az) (Figure 1a).22,23 On substrates such as silicon wafers, glass, mica, and poly(ethylene terephthalate) (PET), diblock copolymers showed a normally oriented cylindrical structure in their microphase-separated films just by thermal annealing (i.e., without any treatments such as surface neutralization and application of external fields).24,25 This phenomenon was also confirmed on a silicon wafer modified with a self-assembled monolayer showing wide range of water-contact angles (from
