Chapter 2
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Cellulose Model Films: Challenges in Preparation Eero Kontturi and Monika Österberg Laboratory of Forest Products Chemistry, Helsinki University of Technology, P.O. Box 6300, FIN-02015 TKK, Finland
The peculiar supramolecular architecture of cellulose, responsible for its poor solubility, provides challenges when devising preparation methods for cellulose model films because ultrathin film deposition usually requires dissolving the material first. The poor solubility of cellulose is encountered in three different model film preparation methods: (i) the use of cellulose solvents, (ii) the use of colloidal dispersion of cellulose nanostructures, and (iii) preparation of a dissolving derivative that can be regenerated to cellulose after film deposition. A comprehensive literature review on all present methods for cellulose model film preparation is given in this chapter.
Introduction Model films are used to track down changes in chemistry and morphology of complex materials as well as monitoring the physicochemical interactions within complicated systems. A suspension of pulp fibers in papermaking, for example, is a highly complex system of a multitude of various components. Moreover, each wood fiber has a distinct morphology and it is, therefore, hard to draw a morphologically representative fiber sample. If one is able to take the main component of pulp fibers, cellulose, and reduce its morphology to a smooth film, fundamental research becomes easier and less ambiguous. Since model films are morphologically well-defined, ultrathin films of chemically pure materials, cellulose is not the most straightforward substance for model film preparation. Especially in its native form, cellulose possesses an © 2009 American Chemical Society In Model Cellulosic Surfaces; Roman, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
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58 exceptional supramolecular architecture (1, 2). The supramolecular character is the reason for the poor solubility of cellulose and solubility, in turn, is an essential factor in preparation of model films. In general, the main requirement for film preparation from any material is to dissolve the material and cast the film from solution by removing the solvent or regenerating the material in one way or another. To tackle the difficult solubility of cellulose, researchers have devised methods that can be roughly divided into three major categories: (i) face the difficulty and use direct cellulose solvents, which may seem rather “exotic” to many scientists, (ii) prepare a dispersion of cellulose nanomaterials (e.g., cellulose nanocrystals), and (iii) prepare a dissolving derivative which can be regenerated to cellulose after film deposition. This chapter is a literature review which consists of a short section on the required instrumentation, and a lengthier passage on these three main categories of preparation methods. The subject partially overlaps with our recent coverage on model films of cellulose and their applications (3), but this chapter does not intend to repeat that text. We intend to give a comprehensive and up-to-date review on the current status of the preparation of cellulose model films.
Instrumentation The thickness of model films is usually in the ultrathin regime (
