Polymeric One-Dimensional Reflectors Based on Self-Organization of Comb-Shaped Supramolecules
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P6.18.1
Polymeric One-Dimensional Reflectors Based on Self-Organization of Comb-Shaped Supramolecules Harri Kosonen1,2, Sami Valkama1, Janne Ruokolainen1, Gerrit ten Brinke1,3, and Olli Ikkala1 1
Department of Engineering Physics and Mathematics and Center for New Materials, Helsinki University of Technology, P.O. Box 2200, FIN-02015 HUT, Espoo, Finland; 2 VTT Microelectronics, Technical Research Centre of Finland, P.O. Box 1208, FIN-02044 VTT, Finland; 3 Laboratory of Polymer Chemistry, Dutch Polymer Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
ABSTRACT Polystyrene-block-poly(4-vinylpyridine) (PS-block-P4VP) diblock copolymer was complexed with dodecylbenzenesulfonic acid (DBSA), leading to comb–coil polymeric supramolecules PS-block-P4VP(DBSA)y (y = 1.0, 1.5, and 2.0). Complexes formed hierarchical lamellar structure with long period of ca. 1400 Å. Due to periodicity and refractive index contrast between the layers, transmission and reflectance measurements showed a relatively narrow incomplete bandgap at ca. 460 nm. The angular dependence of the bandgap was also studied using different angles (θ) relative to the surface normal. The mid-gap was blue-shifted when the incident angle was increased indicating that the lamellar structure was partly oriented by the surfaces of quartz glasses. INTRODUCTION The manipulation the flow of light using periodic dielectric structures has received much attention over the recent years [1-2]. Photonic crystals are long-range ordered structures whose dielectric constant varies periodically. Under certain conditions (with a suitable structure and a sufficient dielectric contrast), photonic crystals may exhibit a forbidden bandgap within which no photons will be allowed to propagate [1]. In combination with controlled defect structures, a wealth of applications in photonics is expected: e.g. capability to confine, guide and control light. Such photonic crystals are not straightforward to construct. Detailed structures and defects, in principle, are possible to construct with sufficient dielectric contrast using lithographic and etching techniques but achievement of small structures down to the optical length scale is challenging. Spontaneous assemblies of colloids [3], synthetic opals [4-7], inverted opals [4,810], and block copolymers [11-14], on the other hand, allow the preparation of small enough structures. Although self-assembly leads to a well-defined local order and offers a low-cost method for the production of photonic crystals, it is nontrivial to achieve perfectly ordered structure with carefully engineered defects over the macroscopic length scale [15]. Block copolymers can self-organize into a wide variety of periodic structures (e.g. lamellar (1D), cylindrical (2D), spherical, and gyroid (3D)) typically at a length scale of 100 – 1000 Å, i.e. smaller than the optical length scale [15]. Increase in the long period up to 1000 – 2000 Å range, corresponding to λ/2n for optical wavelengths could, in principle, be achieved just by u
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