Langmuir 2007, 23, 12677-12681
12677
Stacking of Main Chain-Crown Ether Polymers in Thin Films Daniel R. Rueda,*,† Aurora Nogales,† Jaime J. Herna´ndez,† Mari-Cruz Garcı´a-Gutie´rrez,† Tiberio A. Ezquerra,† Stephan V. Roth,‡ Mikhail G. Zolotukhin,§ and Rosalı´a Serna| Instituto de Estructura de la Materia, CSIC, Serrano 119, 28006 Madrid, Spain, DESY-HASYLAB, Notkestrasse 85, 22607 Hamburg, Germany, Instituto de InVestigaciones en Materiales, UNAM, Coyoacan 04510, Mexico DF, and Instituto de Optica, CSIC, Serrano 121, 28006 Madrid, Spain ReceiVed June 12, 2007. In Final Form: September 12, 2007 Thin films (9-70 nm) of a series of polymers containing in the main chain dibenzo-18-crown-6 ether unit (DB18C6) linked to an aliphatic spacer of different length (10C and 14C) and nature have been prepared, from chloroform solutions, by spin coating on a silicon substrate. The quality and homogeneity of the polymer coatings was revealed by their reflectivity spectra and atomic force microscopy (AFM). The grazing incidence small-angle X-ray scattering (GISAXS) patterns show an out-of-plane structure correlation (interference maximum near the horizon) of scale size related to the polymer repeating unit length. Above this Bragg reflection, the shape of the scattering observed, in the GISAXS pattern, reveals an orientation of the stacked molecular columns in the coated polymer. A thermal treatment of the samples improves the nanostructure by increasing the lamellar coherence size (in y-direction) as well as the vertical orientation of the molecular columns.
Introduction The increasing interest in polymer thin films mainly comes from the great amount of technological applications appearing when they are used as functional surfaces.1-3 In particular, organic electronics are emerging as a viable alternative to silicon technology to create new, improved electrical and optical systems.4 For many of these potential applications, the structuring of the polymer film at the nanometer scale seems to be a necessary requisite to optimize its functionality. Nanostructuring in polymeric thin films can be mostly obtained by using either blends or block copolymers.1-4 However, in case of homopolymers, an external field is frequently required to produce a patterning.2 Another approach makes use of aromatic ring stacking to form one-dimensional columnar superstructures, which may possess semiconducting properties along the stacking direction.5-7 This procedure has been shown to be successful for low molecular weight materials,5-7 but no example in polymers has been yet reported. Cyclic polyethers, referred to hereafter as crown ether polymers, were first discovered by Luttringhaus8 and later developed by Pedersen.9 Crown ethers can be complexed with a great variety of cations and be used as starting materials for * Corresponding author. Tel.: +34915619800 ext 1111. Fax: +34915645557. E-mail:
[email protected]. † Instituto de Estructura de la Materia. ‡ DESY-HASYLAB. § Instituto de Investigaciones en Materiales. | Instituto de Optica. (1) Spatz, J. P.; Eibeck, P.; Mossmer, S.; Moller, M.; Herzog, T.; Ziemann, P. AdV. Mater. 1998, 10, 849-852. (2) Thurn-Albrecht, T.; Schotter, J.; Kastle, C. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126-2129. (3) Walheim, S.; Schaffer, E.; Mlynek, J.; Steiner, U. Science 1999, 283, 520-522. (4) Xiang, H. Q.; Lin, Y.; Russell, T. P. Macromolecules 2004, 3, 5358-5363. (5) McCulloch, I. Nat. Mater. 2005, 4, 583-584. (6) Sobel, A. Nat. Mater. 2003, 2, 643-644. (7) Kastler, M.; Pisula, W.; Laquai, F.; Kumar, A.; Davies, R. J.; Baluschev, S.; Garcia-Gutierrez, M. C.; Wasserfallen, D.; Butt, H. J.; Riekel, C.; Wegner, G.; Mullen, K. AdV. Mater. 2006, 18, 2255-2259. (8) Luttringhaus, A. Ann. Chem. 1937, 528, 181-233. (9) Pedersen, C. In Synthetic Multidentate Macrocyclic Compounds; Izattn, R., JJ, C., Eds.; Academic Press: New York, 1978.
the building of supramolecular assemblies, sensors for ions, and molecular templates.10-15 Recently, we have reported on the synthesis of a series of polymers containing in the main chain the dibenzo-18-crown-6 ether unit (DB18C6) linked to aliphatic spacers of different length and nature,16 where the crown moiety can act as a mesogen unit. A thermal and structural study on some of these materials has been also accomplished.17 It was found that all of these polymers exhibit a characteristic mesomorphic structure in which the molecular thickness is related to the polymer repeating unit length (
