Self-Assembly of Block Copolymers for Photonic-Bandgap Materials
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Self-Assembly of
Block Copolymers for PhotonicBandgap Materials Jongseung Yoon, Wonmok Lee, and Edwin L.Thomas
Abstract Self-assembled block copolymer systems with an appropriate molecular weight to produce a length scale that will interact with visible light are an alternative platform material for the fabrication of large-area, well-ordered photonic-bandgap structures at visible and near-IR frequencies. Over the past years, one-, two-, and three-dimensional photonic crystals have been demonstrated with various microdomain structures created through microphase separation of block copolymers. The size and shape of periodic microstructures of block copolymers can be readily tuned by molecular weight, relative composition of the copolymer, and blending with homopolymers or plasticizers. The versatility of photonic crystals based on block copolymers is further increased by incorporating inorganic nanoparticles or liquid-crystalline guest molecules (or using a liquid-crystalline block), or by selective etching of one of the microdomains and backfilling with high-refractive-index materials. This article presents an overview of photonic-bandgap materials enabled by self-assembled block copolymers and discusses the morphology and photonic properties of block-copolymer-based photonic crystals containing nanocomposite additives. We also provide a view of the direction of future research, especially toward novel photonic devices. Keywords: block copolymers, nanocomposites, photonic crystals.
Introduction Block copolymers (BCPs), molecules comprising chemically distinct polymer chains connected to each other, selfassemble to create a variety of periodic structures.1 The self-assembly of BCPs is driven by a competition between the positive enthalpy of mixing of the respective block chains and the tendency for the polymers to desire a random coil configuration.2 When χN is larger than a certain value (e.g., 10.5 for symmetric diblocks), where χ is the Flory–Huggins interaction parameter between blocks and N is the total degree of polymerization (equal to the total numbers of A monomers and B monomers) of the BCP, microphase separation into well-defined domain structures occurs on the length scale of the
MRS BULLETIN • VOLUME 30 • OCTOBER 2005
respective blocks. For example, in the case of simple linear A–B diblock copolymers, χN and the volume fraction f determine the four equilibrium morphologies: lamellae, double gyroid networks, hexagonally packed cylinders, and bcc spheres. The diversity of BCP microstructures in terms of microdomain size and shape is greatly increased by changing the number of components, the architecture, or the persistence length (a measure of the local straightness) of the constituting chains, or by blending with additives (homopolymers, plasticizers, etc.). For example, A–B–C terpolymers, in which three chemically different blocks are either connected in a series via two junctions or connected to a single junction to form miktoarm
(“mixed arm”) star polymers, exhibit a range of more complex morph
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