Molecular Weight Dependence of Chain Orientation and Optical Constants of Thin Films of the Conjugated Polymer MEH-PPV Kaloian Koynov,* ,² Ayi Bahtiar, ²,§ Taek Ahn, ⊥,² Rodrigo M. Cordeiro, ² Hans-Heinrich Ho 1 rhold, ‡ and Christoph Bubeck ² Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany, and Institute for Organic Chemistry and Macromolecular Chemistry, UniVersity of Jena, Humboldtstr. 10, 07743 Jena, Germany ReceiVed May 18, 2006; ReVised Manuscript ReceiVed September 5, 2006 ABSTRACT: We have studied thin films of the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4- phenylenevinylene] (MEH-PPV), prepared from polymer samples whose weight-average molecular weight (M w ) was varied in the broad range of 10-1600 kg/mol. Anisotropic refractive index measurements by means of waveguide prism coupling and reflectometry as well as polarized infrared spectroscopy were used to analyze the polymer chain orientation in the films. We found that the film morphology depends significantly on the molecular weight, especially in the range M w < 400 kg/mol. Thin films of high molecular weight MEH-PPV have most polymer chain segments oriented parallel to the film planesin contrast to low molecular weight samples which have nearly random orientation of the chain segments. Appropriate choice of molecular weight enables fine- tuning of the refractive index of slab waveguides and reduction of their mode propagation losses to less than 1 dB/cm. Introduction Derivatives of poly(p-phenylenevinylene) (PPV) are inten- sively studied because of their outstanding semiconducting, luminescent, and nonlinear optical properties. 1-8 In particular, poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV, see Scheme 1 for its chemical structure) is frequently used as model material to gain basic understanding of the photophysics of a typical conjugated polymer. 1-4 It is soluble in common organic solvents and can be easily processed to thin films by spin-coating. It has been shown that such films exhibit uniaxial anisotropy due to preferred alignment of the polymer chains in the plane of the film. 9-15 Because the main electric polarizability and the transition dipole moment of the conjugated π-electron system are parallel to the chain direction of a conjugated polymer, the anisotropic orientation of chain segments is strongly correlated with significant birefringence of the films; i.e., the refractive indices at transverse electric (TE) and transverse magnetic (TM) polarizations differ considerably with n TE > n TM . 9-17 It is remarkable, however, that reports of basic optical properties of thin films of MEH-PPV such as their refractive index and absorption coefficient show significant disagreements. 6,10-13 This discrepancy of the published data is not limited to films of MEH-PPV only but is observed also for many other conjugated polymers. 18 There are two main reasons for inconsistencies in the reports of optical constants of thin polymer films: (i) Although a broad variety of techniques is currently used for the measurement of the refractive index and its anisotropy, their precise determination (especially of n TM ) in very thin films is still a major challenge. 18 (ii) The morphology of thin polymer films, and consequently the values of n TE and n TM , can depend significantly on the molecular weight, 16,19-21 the film thickness, 19,22 and the preparation conditions of the films. 4,23 It was reported recently that the molecular weight in particular has strong impact on morphology and optoelectronic properties of thin films of conjugated polymers. This is observed not only in the case of MEH-PPV 14,15,24 but also in polydiacetylene, 20 polythiophenes, 25-28 and polyfluorenes, 29-32 for example. The aim of this work is to present a comprehensive study on the influence of the molecular weight on the polymer chain orientation in thin spin-coated films of MEH-PPV by means of transmission and reflection spectroscopy, prism coupling of slab waveguides, and FTIR spectroscopy. We will show that higher molecular weight MEH-PPVs have an increased amount of polymer chain segments aligned parallel to the film plane as compared to low molecular weight samples. As a consequence, important optical constants of the films such as refractive index, absorption coefficient, and waveguide propagation loss coef- ficient depend significantly on the molecular weight of the polymer. Experimental Section It is necessary to apply different synthetic routes to obtain MEH- PPVs with a large variation of molecular weight. The frequently used, so-called Gilch dehydrohalogenation route 33,34 yields polymers which have a weight-average molecular weight M w on the order of 10 2 -10 3 kg/mol. However, M w can be significantly reduced by appropriate choice of end-cappers in the synthetic process. Another synthetic approach to MEH-PPV with very well-defined chain structure was realized by using the Horner-type polycondensation ² Max Planck Institute for Polymer Research. ‡ University of Jena. § Present address: Department of Physics, University of Padjadjaran Bandung, Jl. Jatinangor km. 21 Sumedang, 45363, Indonesia. ⊥ Present address: Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-600, Korea. * Corresponding author. E-mail: koynov@mpip-mainz.mpg.de. Scheme 1. Chemical Structure of Poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) 8692 Macromolecules 2006, 39, 8692-8698 10.1021/ma0611164 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/10/2006