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Novel Amphiphilic Styrene-Based Block Copolymers for Induced Surface Reconstruction
Funk, Lutz,Brehmer, Martin,Zentel, Rudolf,Kang, Huiman,Char, Kookheon WILEY-VCH Verlag 2008 Macromolecular Chemistry and Physics Vol.209 No.1
<P>This paper describes the synthesis of amphiphilic block copolymers by living radical polymerization (NMP) of new styrene-like monomers. The polar monomers (ethylene oxide side chains and free hydroxyl- or amino-groups after deprotection) were polymerized in a “protected form” to adjust the solubility of the monomers. In this way high molar mass polymers with a narrow polydispersity (around or below 1.2) were accessible. In the bulk state hydrophobic and hydrophilic domains demix. By exposing thin films of these polymers to vacuum (air) or alternatively to water or a hydrophilic surface it becomes possible to switch the surface polarity reversibly between contact angles of about 105° and 83° as a result of surface reconstruction. Through side chains of different length and with different functionalities, it was possible to adjust the glass transition temperatures to values between −2 °C to 140 °C for the hydrophilic blocks and −30 °C to 100 °C for the hydrophobic block. The wide range of the glass temperatures allowed it to find a block copolymer system with a slow kinetic concerning the surface reconstruction process, so that a mechanistic examination of the process by AFM was possible. It got, thereby, possible to detect the break-up of the hydrophobic surface lamella and the upfold of the hydrophilic lamella in contact with water.</P><P> <img src='wiley_img/10221352-2008-209-1-MACP200700312-gra001.gif' alt='wiley_img/10221352-2008-209-1-MACP200700312-gra001'> </P>
Kim, Hosub,Lyu, Yi-Yeol,Choi, Kyungsun,Zentel, Rudolf,Lee, Soo-Hyoung,Char, Kookheon WILEY-VCH Verlag 2009 Macromolecular Rapid Communications Vol.30 No.14
<P>A new method to tune both phosphorescence emission intensity and spectroscopic colors based on the alternatively structured host/guest multilayer thin films prepared by the spin-assisted LbL deposition is presented. Their emission characteristics are demonstrated with pairs of positively charged Ir-PEI complexes as guests and negatively charged CBZ-PAA as hosts. The phosphorescent emission of Ir-PEI complexes is enhanced by the energy transfer from the host to the guest and, additionally, this energy transfer can be finely tuned by the insertion of spacer layers between the phosphorescent donor and acceptor layers to vary the emission intensity as well as to render different emission colors.</P><P> <img src='wiley_img/10221336-2009-30-14-MARC200900023-gra001.gif' alt='wiley_img/10221336-2009-30-14-MARC200900023-gra001'> </P> <B>Graphic Abstract</B> <P>Tunable phosphorescent emission properties such as intensity and color are realized by control of energy transfer between different colored Ir(III)-complexes and a carbazole-based host material in multilayer thin films prepared by the spin-assisted layer-by-layer method. The fine tuning of emission properties is achieved by adding a non-emitting spacer layer between the Ir(III)-complex and host material of nanometer range. <img src='wiley_img/10221336-2009-30-14-MARC200900023-content.gif' alt='wiley_img/10221336-2009-30-14-MARC200900023-content'> </P>
Tuning the defect configurations in nematic and smectic liquid crystalline shells
Liang, Hsin-Ling,Noh, JungHyun,Zentel, Rudolf,Rudquist, Per,Lagerwall, Jan P.F. Royal Society 2013 Philosophical transactions. Series A, Mathematical Vol.371 No.1988
<P>Thin liquid crystalline shells surrounding and surrounded by aqueous phases can be conveniently produced using a nested capillary microfluidic system, as was first demonstrated by Fernandez-Nieves <I>et al.</I> in 2007. By choosing particular combinations of stabilizers in the internal and external phases, different types of alignment, uniform or hybrid, can be ensured within the shell. Here, we investigate shells in the nematic and smectic phases under varying boundary conditions, focusing in particular on textural transformations during phase transitions, on the interaction between topological defects in the director field and inclusions in the liquid crystal (LC), and on the possibility to relocate defects within the shell by rotating the shell in the gravitational field. We demonstrate that inclusions in a shell can seed defects that cannot form in a pristine shell, adding a further means of tuning the defect configuration, and that shells in which the internal aqueous phase is not density matched with the LC will gently rearrange the internal structure upon a rotation that changes the influence of gravity. Because the defects can act as anchor points for added linker molecules, allowing self-assembly of adjacent shells, the various arrangements of defects developing in these shells and the possibility of tuning the result by modifying boundary conditions, LC phase, thickness and diameter of the shell or applying external forces make this new LC configuration very attractive.</P>
Zorn, Matthias,Bae, Wan Ki,Kwak, Jeonghun,Lee, Hyemin,Lee, Changhee,Zentel, Rudolf,Char, Kookheon American Chemical Society 2009 ACS NANO Vol.3 No.5
<P>To combine the optical properties of CdSe@ZnS quantum dots (QDs) with the electrical properties of semiconducting polymers, we prepared QD/polymer hybrids by grafting a block copolymer (BCP) containing thiol-anchoring moieties (poly(para-methyl triphenylamine-b-cysteamine acrylamide)) onto the surfaces of QDs through the ligand exchange procedure. The prepared QD/polymer hybrids possess improved processability such as enhanced solubility in various organic solvents as well as the film formation properties along with the improved colloidal stability derived from the grafted polymer shells. We also demonstrated light-emitting diodes based on QD/polymer hybrids, exhibiting the improved device performance (i.e., 3-fold increase in the external quantum efficiency) compared with the devices prepared by pristine (unmodified) QDs.</P>