One-Dimensionally Periodic Dielectric Reflectors from Self-Assembled Block Copolymer-Homopolymer Blends

Introduction. The well-ordered structures of block copolymer microdomains formed by self-assembly are potentially interesting optical materials, specifically as photonic crystals. Photonic crystals are structures with a periodic variation of refractive index.1-3 For light of the appropriate frequency, such crystals can be used to bend and steer the light, opening up many possible applications such as broad-band low loss waveguides for communication and efficient laser mirrors. The first photonic crystals were made by machining overlapping holes into a slab (millimeter length scales for microwave frequencies).4 Current work employs lithographic processing (at the micron and submicron length scale for IR and visible frequencies) to make dielectric structures.5 Very recent work has introduced self-assembly as a method to make photonic crystals with periodicities suitable for frequencies in the visible part of the spectrum. Materials used include colloidal crystal arrays,6 artificial opals and inverse opals,7,8 and hollow spherical micelles.9 Block copolymers can self-assemble into a wide variety of periodic structures including 1-, 2-, and 3-dimensional structures.10 There are several essential challenges to overcome in order to achieve desirable photonic crystal properties employing block copolymers. These include obtaining the correct size of domains for the optical frequencies of interest, attainment of long-range domain order and appropriate orientation, as well as providing sufficient dielectric contrast between the domains. Accessing large periodicities in block copolymers is of course possible using very high molecular weight blocks, but attaining well-ordered structures is difficult due to the very high viscosity of such systems. An alternative approach is to utilize a liquid crystalline block for which the domain thickness varies linearly with block molecular weight. In this way, spacings into the micron range can be produced.11 Another approach is to blend block copolymers with homopolymers and thus swell the microdomains. Block copolymer/homopolymer blends have been investigated both experimentally12-14 and theoretically.15,16 Such blends offer tailorable domain spacings, as well as the processing advantage afforded by somewhat lower molecular weight components. The dependence of the lamellar repeat in bulk cast blends of homopolystyrene and polystyreneb-polyisoprene block copolymers has been analyzed as a function of the percent homopolymer added and its molecular weight.14 The lamellar thickness depends on the many factors including homopolymer volume fractions and the block composition and molecular weight. A regular lamellar morphology having a difference in the index of refraction between the respective domains is similar to a layered optical filter or quarter-wave stack. In this article we demonstrate that a diblock copolymer/homopolymer blend, which can self-assemble into a lamellar microdomain structure, can exhibit a limited angular range stop band at visible frequencies.