Chapter 6
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Optical Characterization of Cellulose Films via Multiple Incident Media Ellipsometry Ufuk Karabiyik1,3, Min Mao1,3, Maren Roman2,3, Thomas Jaworek4, Gerhard Wegner4, and Alan R. Esker1,3,* 1
Department of Chemistry, 2Department of Wood Science and Forest Products, and the 3Macromolecules and Interfaces Institute, Virginia Polytechnic Institute and State University, Blacksburg VA 24061 4 Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
Ellipsometry measures the relative intensity and the phase shift between the parallel and perpendicular components of polarized light reflecting from a surface. Single wavelength ellipsometry measurements at Brewster’s angle provide a powerful technique for characterizing ultrathin polymeric films. In this study multiple incident media ellipsometry is utilized to simultaneously obtain the refractive indices and thicknesses of thin films of trimethylsilylcellulose (TMSC), regenerated cellulose, and cellulose nanocrystals. Experiments were conducted in air and water for TMSC, and in air and hexane for regenerated cellulose and cellulose nanocrystals. The refractive indices of TMSC, regenerated cellulose, and cellulose nanocrystals are found to be 1.46 ± 0.01, 1.51 ± 0.01, and 1.51 ± 0.01, respectively.
Introduction Model cellulose surfaces are important for elucidating how cellulose, hemicellulose, and lignin self-assemble to form the hierarchical structure of cell walls (1–4). Likewise, model surfaces provide model substrates for studying the enzymatic degradation of lignocellulosic materials (5, 6). On the other hand, the swelling behavior of cellulose surfaces in aqueous media attracts great attention © 2009 American Chemical Society In Model Cellulosic Surfaces; Roman, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
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138 from both fundamental and applied sciences in terms of performance and potential applications of cellulose based materials in the paper and textile industries (7, 8, 9). It is clear that in such applications where the cellulose is in contact with a liquid medium, techniques that are applicable for in situ characterization are desirable. In addition, prior to further surface treatment and subsequent surface analysis, it is important to characterize and explore initial surface characteristics, e.g. film thickness and refractive index. Many techniques have been developed that can be used to measure the thicknesses and refractive indices of thin films, such as refractometry (10), waveguide prism couplers (11), surface plasmon resonance spectroscopy (12, 13), polarizing interference microscopy (14), variable-angle single wavelength ellipsometry (15, 16), and spectroscopic ellipsometry (17). Of these, ellipsometry as a rapid, non-contact, and non-destructive method is ideal for measuring thickness and refractive index in nanoscale coatings through changes in polarization upon the reflection of light from the surface. In addition, the simultaneous determination of film thickness and refractive index is possible trough multiple incident media (MIM) ellipsometry (18, 19). Thickness determinations via ellipsometry are complicated by the need to know the film’s optical properties. Refractive index and thickness are correlated parameters in ellipsometry, hence, it is not possible to uniquely obtain both parameters through a single measurement at a constant wavelength for thin films (20). Spectroscopic ellipsometers overcome this problem by conducting measurements at multiple wavelengths. However, the refractive index of the film needs to be optically modeled as a function of wavelength. As a consequence, some prior knowledge of the refractive index of the film at some point in the sampled wavelength window is usually desired. Another complication is that the bulk refractive indices may not be applicable for thin films with thicknesses
