Chapter 8
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Chromogenic Polymer Gels for Reversible Transparency and Color Control Arno Seeboth, Jörg Kriwanek, André Patzak, and Detlef Lötzsch Fraunhofer Institute for Applied Polymer Research, Richard-WillstätterStrasse 12, 12489 Berlin, Germany
A current overview about the preparation and characterization of novel chromogenic polymer materials is given. The topic of chromogenic materials has developed extremely rapidly in the last few years. Among them, thermotropic and thermochromic polymer gel networks have met with growing interest, because of their advanced properties. These novel polymer gels exhibit pronounced changes in transparency and/or color over a modest temperature range. Some of them possess a temperature independent volume, which is an essential condition for many future technical applications.
Introduction The preparation and characterization of chromogenic polymer gels changing their optical properties in response to temperature has met with growing interest in the last decade. Two classes of such gels are generally distinguished in literature: thermotropic polymer gels, which switch in dependence on temperature reversible between a transparent and a translucent state and thermochromic polymer gels which change their color or color
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© 2005 American Chemical Society In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
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strength upon changes in temperature. However, so-called chromogenic polymer gels combining thermotropic and thermochromic properties in one material have been developed. From the practical point of view, temperature sensitive chromogenic polymer gels seem to be promising not only as temperature indicators, but also for use in smart windows, temperature tunable light filters and large area displays. Especially their potential application as thermally self-adjusting light and heat filters in the external glazings of buildings has motivated their development in this field. For most of the desired applications a constant volume of the chromogenic material is required. Thus gels whose optical switching is accompanied by considerable shrinking or swelling will not be considered in this chapter.
Thermotropic Gel Networks for Reversible Transparency Control with Temperature Thermotropic gel networks transform at a certain temperature from a highly transparent into a light scattering state. Such an optical effect can be either caused by a phase separation process or by a phase transition between an isotropic and an anisotropic lyotropic liquid crystalline state. With the presentation of advanced prototypes of sun-protective glazing by Charoudi (7), Watanabe (2) and Seeboth (5) the interest in thermotropic gel networks has rapidly increased in recent years. A detailed description of the material development in thisfieldwas recently reviewed (4). In this chapter we will focus on new results on the preparation of thermotropic hydrogels based on biopolymers, the usefulness of calorimetric measurements for die characterization of thermotropic gel networks, the influence of the addition of salts on the material properties and the construction of hybrid solar and electrically controlled light filters.
Preparation of Thermotropic Polymer Gel Networks Based on Biopolymers For commercial use of hydrogels, non-toxic and inexpensive raw materials are required. Biopolymers, like polysaccharides, offer tlie possibility to fulfil all these requirements and, compared with synthetic polymers, they have the benefit of their environmental compatibility. Therefore, the suitability of
In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
112 biopolymers for the preparation of novel thermotropic hydrogels was extensively studied in recent years. Thermotropic hydrogels based cm cellulose derivatives in combination with an amphiphilic component were developed by Watanabe (2). By using one of these thermotropic hydrogels a window of lm size was constructed and successfully tested under practical conditions over a period of two years. Thermochromic hydrogels, which are also based on cellulose derivatives have been reported (5). It was shown, that no amphiphilic component is necessary to prevent an irreversible flocculation of suspended hydroxy propylcellulose, if hydroxy ethylcellulose is added. Although the biological decomposition is often mentioned as an advantage of biopolymers, it is also the most important hindrance for their commercial use because contact of the biopolymers with microorganisms must be prevented during production and throughout the lifetime of the product.
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Calorimetric Measurements on Aqueous Polymer Gel Networks In order to characterize the properties of polymer gel networks optical and dielectric spectroscopy, rheological investigations and calorimetric measurements are commonly used. Calorimetric measurements are well known to provide information on the water binding properties of the polymeric systems. Recent published results show that differential scanning calorimetry (DSC) is also suitable to detect phase transitions and phase separation processes of hydrogels (6-8). Whereas optical techniques can only determine phase transition temperatures, DSC measurements also provide the transition enthalpy data. Alexandridis et al. investigated the influence of the addition of salts on the phase separation and on the unimer-to-micelle transition temperatures in aqueous solutions of a polyethylene oxide)-block-poly(propylene oxide)-blockpoly(ethylene oxide) copolymer (
