Crystalline Structure Determination of Anisotropic Dimethyl Terephthalate Crystallites by Micro-Raman Spectroscopy
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heco Subdireccio´n Exploracio´n Produccio´n, Instituto Mexicano del Petro´leo, Apdo. Postal 14-805, Mexico DF 07730
S. Vargas Departamento de Físcia Aplicada y Tecnología Avanzada, Instituto de Física, Universidad Nacional Auto´noma de Me´xico, Apdo. Postal 0-1010, Quere´taro, Qro. 76001, Mexico
S. Jime´nez Laboratorio de Investigacio´n en Materiales, Centro de Investigacio´n y Estudios Avanzados, Apdo. Postal 1-1010, Quere´taro, Qro. 76001, Mexico
V.M. Castan˜o Departamento de Física Aplicada y Tecnología Avanzada, Instituto de Física, Universidad Nacional Auto´noma de Me´xico, Apdo. Postal 0-1010, Quere´taro, Qro. 76001, Mexico (Received 23 August 1999; accepted 23 March 2000)
A novel approach to determine the molecular orientation of dimethyl terephthalate molecules with respect to the direction of the crystal axis is reported. This determination was achieved by changing the crystal orientation with respect to the incident laser light of a micro-Raman spectrometer. Raman spectra were obtained at different incidence angles of the laser beam with respect to the crystal symmetry axis. The intensities of some specific bands were analyzed as a function of the tilting angle. With this information the molecular orientation with respect to the crystal axis was determined making use of a simple mechanical model.
I. INTRODUCTION
Raman spectroscopy represents an important analytical tool for characterizing both organic and inorganic materials. This is mainly due to the high sensitivity Raman has for various functional groups commonly present in polymers. Accordingly, recent studies have demonstrated the feasibility of this technique not only for determining some specific groups present in the polymeric chains but also to determine end groups, molecular structure, conformational characteristics, chain orientation, mechanical properties, etc. One of the greatest advantages of Raman spectroscopy is the possibility of analyzing samples in various aggregation states, ranging from powders, films, tubing, pipes, and fibers to sheets, liquids, solids, etc. Additionally, the amount of sample required can be extremely reduced in the case of micro-Raman spectroscopy (MRS), since it is a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 15, No. 6, Jun 2000
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possible to focus the laser beam into an area as small as one square micron. However, the main practical limitation of this technique is fluorescence, which is much larger than the Raman signal itself. Fluorescence can be due to the presence of fillers, catalyst, unreacted monomer, impurities, or the sample itself. This disadvantage can be overcome simply by switching the wavelength of the excitation to the near-infrared region. Raman spectroscopy is extremely suitable for the analysis of oriented polymer molecules in single crystals because the polarization of the laser beam can be varied with respect to the crystal symmetry axis. This allows one to select different contri
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