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THERMOCHROMIC POLYMERS Introduction In recent years functional polymers changing their visible optical properties in response to an external stimulus have met with growing interest. According to the external stimulus which affects the optical properties, these so-called chromogenic polymers are classified as thermochromic (stimulus: temperature), photochromic (stimulus: light), electrochromic (stimulus: electric field), piezochromic (stimulus: pressure), ionochromic (stimulus: ion concentration), and biochromic (stimulus: biochemical reaction). Because of their advanced properties the demand on chromogenic polymers for future applications will become enormous. Smart windows, tunable light filters, large area displays as well as sensors, which can visualize, eg, temperature or pressure profiles, are the most important potential innovations based on chromogenic polymers. For all these applications laboratory prototypes demonstrating the effect have been presented. A few of them have already reached the readiness for marketing and certainly others will follow in the near future. This article focuses on thermochromic polymer systems. An overview of the different types of such polymer systems is given and the origin of the thermochromic effect, their specific material properties, and potential applications are discussed. Table 1 displays the various generic types of thermochromic polymers. Following this classification, thermochromic polymer systems in which the color appears owing to a Bragg reflection on a periodic structure are discussed first. In such systems the thermochromic effect is caused by a temperature dependence of the layer periodicity. Thus the wavelength of the reflected light can only change continuously. Another inherent feature of such thermochromic systems is that the visible color depends on the viewing angle. Because of these disadvantages the potential applications of thermochromic polymer systems based on Bragg reflection are limited. The second classification is about thermochromic polymer systems varying their color or color intensity owing to temperature-dependent molecular changes of the chromophoric group. This class of thermochromic polymers can in principle switch between any user-defined colors. The third class of Table 1. Generic Types of Thermochromic Polymers Classified by the Effect on Light Causing the Thermochromic Behavior Effects on light Reflection

Origin Periodic structures

Macroscopic behavior I, λmax

Absorption Chromophoric groups I, λmax Scattering

a LCST:

Areas with different refractive indices

T%

Polymer class Cholesteric liquid crystalline polymers Crystalline colloidal arrays embedded in a gel network Gels Conjugated polymers Hydrogels containing indicator dyes Polymer blends exhibiting LCSTa Hydrogles exhibiting LCST Lyotropic liquid crystalline hydrogels

lower critical solution temperature.

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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polymer systems reviewed are those changing their transparency with temperature by reversible switching between a transparent and a light scattering state. In the literature this subclass of thermochromic materials is generally denoted as thermotropic. The main interest in thermotropic polymer systems appears to be from their potential application in sun protecting glazing. Recently, a novel class of polymer systems changing both transparency and color with temperature has been described and will be reviewed.

Thermochromic Polymer Systems Based on Bragg Reflection Liquid Crystalline Polymers with Thermochromic Properties. In several liquid crystalline phases a helical superstructure occurs if either a chiral compound is added or the liquid crystalline compound itself consists of a chiral molecular structure (1,2). In this way, the nematic phase, for example, transforms into the so-called cholesteric phase. Depending on structure and concentration of the chiral compound as well as on the phase type, the pitch (P) of the helical superstructure can be in the range between 100 nm and infinite. Incident light is selectively reflected, if the wavelength (λ) satisfies the Bragg condition λ = nP cos θ (n stands for the mean reflective index of the liquid crystal and θ for the angle between the incident light and the orientation of the helix). In many systems the wavelength of the reflected light is in the visible region; thus these liquid crystals become intensively colored. The pitch generally changes with temperature and can be modified by applying an electric field. Thermochromic polymer foils based on cholesteric liquid crystals are already commercially available from various manufacturers including Hallcrest Ltd., Merck KGaA, and Davis Liquid Crystals Inc. Such thermochromic sheets consist of a black backing layer, a layer of the cholesteric liquid crystal, and a protective clear polyester layer. They can indicate temperature changes ranging from a fraction of a degree to over 20◦ C by varying their color continuously through the entire visible region. Chiral liquid crystalline polymers possessing thermochromic properties themselves are described in numerous publications. Besides synthetic polymers, many cellulose derivatives have been found to possess cholesteric mesophases with selective reflection in the visible wavelength region. The pentyl ether of hydroxypropyl cellulose is an example for such a cholesteric polymer (3). At room temperature it shows selective reflection at about 500 nm. With increasing temperature the wavelength of the selective reflection maximum smoothly increases, reaching 650 nm at about 80◦ C. For applications in information technology, those materials are of special interest which allow fixation the cholesteric order at any user-defined color (4). A fixing of the helical pitch takes place in a glassy state (5–7) as well as in cross-linked polymeric structures (3,8–12). Therefore, cholesteric polymers with a glass-transition temperature within the cholesteric phase range as well as cholesteric mixtures which are polymerizable and form cross-linked polymeric networks are suitable for this purpose. The preparation of patterned multicolor cholesteric liquid crystal polymer networks have been reported (12). The cholesteric liquid crystal polymer networks were obtained by photopolymerization of a mixture containing 20% of a chiral monoacrylate CBC

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Fig. 1. Molecular structure of the acrylates C6M and CBC.

(see Fig. 1), 25% of a nonreactive commercial chiral dopant CB15 (Merck), 54% of a nonreactive commercial liquid crystal BL59 (Merck), 0.5% of a diacrylate C6M (see Fig. 1), and 0.5% Irgacure 651 photoinitiator at different temperatures. This mixture exhibits a cholesteric phase with a strong temperature dependence of the pitch. At 55◦ C it reflects red light, at 35◦ C green light, and at 30◦ C blue light. Thus, by performing the photopolymerization at different temperatures (55, 35, and 30◦ C), red, green, and blue polymer networks with a temperature-independent color were obtained. In a next step the preparation of a patterned multicolor polymer network was carried out by using masks and performing the photopolymerization step by step in different regions at different temperatures. It was concluded that by using a light source to induce patterned heating of the mixture, followed by fixing via UV-induced photopolymerization, it should be possible to produce much more complicated structures. The formation of liquid crystalline phases with thermochromic properties in a poly(β-peptide) was reported in Reference 13. The color changes were observed by using polarizing optical microscopy. However, this method was not found to be suitable for determining thermochromism. Thermochromic Gel Networks Based on Bragg Reflection. Polymer gel networks with thermochromic properties, based on Bragg reflection, are frequently described in the literature, whereby most of the systems are composed of a polymer and an organic solvent. It has been reported that poly-1-butene flakes swells in benzene, toluene, o-xylene, m-xylene, p-xylene, or tetrachloroethylene and exhibits reversible color changes with temperature between the melting point of the solvent and the sol–gel transition temperature (14). With increasing temperature the color of the poly-1-butene gels changes in the order of blue, violet, red, orange, and yellow. This thermochromic effect is explained by Bragg reflection on the microstructure of the gelled poly-1-butene. A similar behavior was observed by the same author for gelled isotactic polypropylene in benzene, toluene, and xylene (15). For example the color of a 3% isotactic polypropylene gel in benzene was found to change from blue at room temperature to yellow at about 70–80◦ C. A thermochromic hydrogel based on Bragg reflection has been reported (16). The described system consists of crystalline colloidal arrays of monodisperse polystyrene spheres embedded in a poly(N-isopropyl acrylamide) (PNIPAM) hydrogel, which was prepared by first dispersing highly charged and monodisperse

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polystyrene spheres in an aqueous solution containing the PNIPAM monomer, followed by a photochemically initiated polymerization. A temperature-dependent strong swelling or shrinking of the polymer matrix takes place. The embedded polystyrene spheres follow this volume change, causing thereby a modification of their lattice spacing and thus of their Bragg reflection wavelength. To characterize the optical properties of the novel thermochromic material, films of different layer thickness were prepared and the temperature dependence of their diffracted wavelength determined. While the bulk material is translucent because of light scattering on the colloidal particles, thin films become transparent. At room temperature 125-µm-thick hydrogel film was used to detect the thermochromic effect by absorption measurements. In this way, a continuous change of the diffracted wavelength from 704 nm at 11.7◦ C to 460 nm at 34.9◦ C was measured, which corresponds to nearly the entire visible range. However, note that the thermochromic effect is coupled with strong volume changes. An increase of the diffracted wavelength from 400 to 800 nm corresponds to a swelling in each direction by a factor of 2 and thus to an eightfold increase of the volume. Crystalline colloidal arrays which vary the intensity of the Bragg reflection in dependence on temperature without changing the wavelength of the diffracted light have also been developed (16). Such materials were obtained by dispersing highly charged and monodisperse colloidal particles of PNIPAM with a size of 100 nm in deionized water. In water these PNIPAM colloids self-assemble and form a body-centered cubic array which diffracts light following the Bragg diffraction law. Below their lower critical solution temperature of about 32◦ C the PNIPAM colloids begin to swell, leading to an increase of the sphere diameter from about 100 nm above this temperature to about 300 nm at 10◦ C. The lattice spacing of the cubic array and thus the reflected wavelength (at a constant glancing angle between the incident light and the diffracting crystal plane) depend only on the particle density of the PNIPAM colloids. The intensity of the Bragg reflection of crystalline colloidal arrays, on the other hand, depends on the array ordering as well as on the scattering cross section of the colloidal particles, which is influenced by the particle size of the colloids. As a result the reversible swelling or shrinking of the PNIPAM colloids with temperature causes pronounced changes in the intensity of the Bragg reflection. As an example the optical properties of a crystalline colloidal array of PNIPAM colloids (diameter: 100 nm) with a lattice constant of 342 nm were reported. At 40◦ C the nearest neighbor sphere distance amounts to 242 nm and at the Bragg reflection wavelength nearly all incident light is reflected. On the other hand at 10◦ C the PNIPAM colloids are almost touching and only a weak Bragg reflection occurs.

Thermochromic Polymer Systems Based on the Absorption of Light Conjugated Polymers. The occurrence of thermochromic properties is frequently observed in conjugated polymers (17,18) like polyacetylenes (19), polydiacetylenes (19), polythiophenes (20), and polyanilines (21). Many conjugated polymers exhibit absorption of light in the visible range as well as high reflectivity. Hence, they often are colored and show a metallic appearance. Thermochromism in conjugated polymers has its origin in changes of the conformation as they

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abruptly occur at phase transitions whereby even the slightest modification of the conformational structure can cause significant color changes. Especially the planarity of the polymer backbone plays an important role. Any twisting of the chain leads to a decrease of the effective conjugation length and thus to a blue (hypsochromic) shift of the absorption in the UV–vis region. In general the color changes of conjugated polymers are reversible. However, kinetic effects can cause an irreversibility which can be useful for thermal recording (22). In Reference 23, synthesis and optical properties of a liquid crystalline polydiacetylene are described. At each of its mesogenic phase transitions a thermochromic effect was detected. That means thermochromic and mesogenic properties of the investigated polydiacetylene are coupled with one another. The structural and optical properties of poly[2 ,5 -bis(hexadecyloxy)-1,4-phenylene-1,3,4-oxadiazol-2,5-diyl] have been reported in Reference 24. The polymer was found to possess liquid crystalline properties in a wide temperature range, whereby at 120◦ C an order– order transition from a smectic H into a smectic A phase takes place. Moreover, a thermochromic behavior was observed within the temperature range of 25–140◦ C. With increasing temperature the polymer changes its color continuously from yellow-green at 25◦ C to blue at 130◦ C. To characterize this thermochromic behavior in more detail, UV–vis absorption and fluorescence emission spectra were measured at different temperatures. For both, a continuous evolution with temperature was found. In contrast to the results reported in Reference 23, in Reference 24 even at the smectic H to smectic A phase transition no discontinuous change of the UV–vis and fluorescence intensities could be detected. Another way to obtain thermochromic polymers is to incorporate a thermochromic material into a polymer matrix. Thermochromic polymers have been obtained by dispersing poly(3-alkyl thiophene)s in host polymers (25,26). Poly(3-alkyl thiophene)s belong to the group of conjugated polymers, exhibiting thermochromism according to temperature-induced changes of the conjugation of the π-electron system. The thermochromic switching temperature of the polythiophene pigments can be tailored by chemical modifications. In this way a set of pigments that visually and reversibly change colors at a prescribed temperature in the region of −35 to +125◦ C was developed. These pigments are thermally stable until more than 200◦ C. Commercially available paints, plastics, and rubbers were used as host polymers, whereby 0.1–1.0 wt% of the pigments were necessary to obtain a visible thermochromic effect. However, a detailed description of the chemical structure of the pigments or a list of the available switching temperatures and color changes has not been reported so far. A thermoplastic polymer with thermochromic properties was recently presented by Seeboth et al. (27). This polymer switches on heating at a certain temperature from blue to colorless. Figure 2 displays a photograph demonstrating this thermochromic switching effect. A foil of the novel thermochromic polymer is partially heated in hot water above the switching temperature. Clearly the cool blue and the hot colorless state can be seen. So far, neither the origin of the thermochromic effect nor the composition of this thermochromic polymer has been reported. Masterbatches of polyethylene and polypropylene containing microencapsulated thermochromic materials are already commercially available from various manufacturers. The thermochromic materials used in these microcapsules are

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Fig. 2. Thermochromic switching from blue to colorless of a thermoplastic polymer foil.

composed of an electron-donating chromogenic compound, an electron acceptor, and a solvent (28). Suitable electron-donating chromogenic compounds are, for example, substituted phenylmethanes and fluorans. These so-called leuco dyes or color formers (29) are either colorless or weakly colored. However, on reaction with an electron acceptor, as for example a phenol, an opening of the lactone ring occurs. In the open ring state the conjugated π -electron system is extended, enabling color formation. By the addition of appropriate solvents the color-forming reaction becomes reversible. In the molten state these solvents function as an inhibitor of the color-forming reaction. On the other hand during the crystallization of the solvent the donating chromogenic compound reacts with the electron acceptor; thus the crystalline state becomes colored. For example a mixture of 1 wt% 2-chloro-6-diethylamino-3-methylfluoran, 5 wt% 2,2 -bis(4-hydroxyphenyl)propane, and 94 wt% 1-hexadecanol is colorless above 48◦ C in the molten state and develops a vermilion color on cooling below 48◦ C when the crystallization of the 1-hexadecanol takes place. The reversible lactone ring opening reaction of 2chloro-6-diethylamino-3-methylfluoran, which is the origin for this thermochromic effect, is shown in Figure 3. Although these masterbatches show excellent thermochromic switching behaviors, the use of microencapsulated thermochromic mixtures results in a variety of limitations including poor thermal and shear stability which cause difficulties in processing. Gel Networks. In the literature a few examples of thermochromic gel networks consisting of conjugated polymers swelled in organic solvents are reported. The thermochromic effect described for these gels appears owing to changes of

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Fig. 3. Reversible lactone ring opening reaction of 2-chloro-6-diethylamino-3methylfluoran.

the molecular structure of the polymer backbone at their respective gel–sol transition temperature. One of the reported systems consists of polydiacetylene gelled in o-dichlorobenzene or other gel-forming solvents (30). Upon heating above the gel–sol transition temperature reversible pronounced color changes are observed. Colors and transition temperatures could be varied by using different solvents as well as by changing the composition. A weak thermochromic effect caused by temperature-dependent molecular changes of the chromophoric group in a gel network has also been reported (31). On heating in benzene a poly[2-(3,7dimethyloctoxy)-5-methoxy-1,4-phenylenevinylene] gel shows a gradually red to yellow color change at approximately 35◦ C, which corresponds to the gel–sol transition temperature. After cooling to room temperature the yellow state remains metastable. It eventually reverts after several hours to the red gel phase. The authors explained the thermochromic behavior of this system by a reduction of interchain π–π interaction at the gel to solution transition, at which the relation between aggregated and isolated chain sections abruptly changes. The first example of a thermochromic effect of dyes embedded in a transparent hydrogel was reported by Seeboth et al. (32). The described thermochromic effect appears by the addition of suitable indicator dyes, with pK a values between 7.0 and 9.4, to a definite poly(vinyl alcohol) (PVA)/borax/surfactant hydrogel network. With increasing temperature the used gel matrix shifts the equilibrium of the indicator dyes toward their deprotonated forms, leading thereby to gradual color changes with temperature. In dependence on the used indicator dye(s) a switching between a colorless and a colored state or between two or even more different colored states occurred. For example a gradual color change from colorless at 10◦ C to a deep violet at 80◦ C was observed for a PVA/borax/surfactant hydrogel containing the so-called Reichard betaine 2,6-diphenyl-4-(2,4,6-triphenyl-1pyridino)-phenolate (DTPP), whose pH-sensitive equilibrium is shown in Figure 4.

Fig. 4. Proton-transfer equilibrium between the phenol and phenolate form of the indicator dye 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridino)-phenolate.

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Fig. 5. Thermochromic switching of a 3-mm-thick Cresol PVA/borax/surfactant hydrogel layer placed between two glass plates.

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Red

containing

A Cresol Red containing PVA/borax/surfactant hydrogel on the other hand switches from yellow at 10◦ C to wine-red at 80◦ C and a Bromothymol Blue and Cresol Red containing PVA/borax/surfactant hydrogel from yellow (below 5◦ C) via green (between 15 and 25◦ C) to violet (above 60◦ C). A photograph displaying the thermochromic switching of the Cresol Red containing PVA/borax/surfactant hydrogel is shown in Figure 5. The 3-mm-thick layer spacing of a double-glazing assembly is filled with this thermochromic hydrogel and partially heated in a hot water bath. While the cold zone remains yellow the color in the hot zone has switched to wine-red. In order to characterize the thermochromic effect in more detail, UV–vis absorption measurements were carried out. As examples of the obtained results the UV–vis absorption spectra of the DTPP and of the Cresol Red containing PVA/borax/surfactant hydrogels measured at different temperatures are displayed in Figures 6 and 7, respectively. One absorption maximum growing continuously with increasing temperature appears in the spectra of the DTPP containing PVA/borax/surfactant hydrogel (Fig. 6). This absorption at about 550 nm is caused by the phenolate form of DTPP and its growing intensity indicates a rising phenolate concentration with increasing temperature. In Figure 7 for all temperatures two absorption maxima, one at 419 nm, which corresponds to the absorption of the phenol form of Cresol Red, and the other one at 581 nm, which corresponds to the absorption of the phenolate form of Cresol Red, are observed. With

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Fig. 6. Visible absorption spectra of a 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridino)phenolate containing PVA/borax/surfactant hydrogel in dependence on temperature (d = 1 cm). 10◦ C (A); 20◦ C (B); 30◦ C (C); 40◦ C (D); 50◦ C (E); 60◦ C (F); 70◦ C (G); 80◦ C (H).

increasing temperature the intensity of the absorption band at 581 nm increases while simultaneously the intensity of the absorption band at 419 nm decreases. All spectral curves meet at an isosbestic point at 486 nm, supporting thereby the suggested model of a temperature-dependent equilibrium between two different forms of the indicator dye as the origin of the observed thermochromic effect. In addition to the transparency another important feature of these hydrogels is that the thermochromic effect is not accompanied by volume changes. Therefore such hydrogels are applicable for smart windows, large area displays, and tunable color filters.

Fig. 7. UV–vis absorption spectra of a Cresol Red containing PVA/borax/surfactant hy16◦ C (A); 60◦ C (B); 80◦ C drogel in dependence on temperature (d = 1 cm). (C).

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Transparency/Scattering Switching Some thermotropic polymer systems switch at a certain temperature reversibly between a highly transparent and a light scattering state. Such an optical effect can be caused either by a phase transition between an isotropic and an anisotropic (liquid crystalline) state or by a phase separation process. Two different classes of thermotropic polymer materials have been extensively studied in recent years: polymer blends and polymer gels. Thermotropic Polymer Blends. Only a few pairs of polymers are miscible with one another over the complete concentration range. There are also polymer pairs known which either mix or do not mix depending on temperature; both an increase as well as a decrease of the miscibility (qv) with increasing temperature are possible. In the phase diagrams of such polymer pairs a two-phase region appears either below a so-called upper critical solution temperature (UCST) or above a so-called lower critical solution temperature (LCST). Within a wide concentration range polymer blends of both these systems are thermotropic. Blends with an UCST in the phase diagram switch from translucent to transparent by increasing the temperature, whereas polymer blends of systems with an LCST switch in the opposite manner. For application in sun protecting glazing, a reduction of the transmission with increasing temperature is required and therefore the development of thermotropic polymer blends in recent years has focused on systems with an LCST. In 1993 Siol et al. (33) from the Roehm GmbH developed thermochromic polymer blends consisting of chlorinated rubber and polymethacrylates. Depending on the composition the transition from the transparent into the translucent state (cloud point) could be varied between 60 and 140◦ C. In the same year Eck et al. (34) presented polymer blends with a much better reversibility of the thermotropic switching. Moreover, cloud points between 30 and 40◦ C could be realized, which is the required temperature range for sun protecting glazing. These polymer blends were derived from poly(propylene oxide) and a styrene–hydroxyethyl methacrylate copolymer cross-linked with a trifunctional cyclic isocyanate. By cross-linking the copolymer in the presence of poly(propylene oxide) a semiinterpenetrating network is formed leading to an increase of the stability of the microphase separation above the cloud point and thus to a much better reversibility of the thermotropic switching. A similar cross-linked thermotropic polymer blend was developed in 1995 by BASF (35). In the first step blends of poly(propylene oxide) and a copolymer of styrene, hydroxyethyl methacrylate, and a small portion of 4-acroyloxy butyl carbonato benzophenone were prepared. However, instead of adding a cross-linking agent, radical polymerization under UV-light was used to form the semi-interpenetrating network. This method allows preparation of thermotropic coatings first and polymerization to the semi-interpenetrating network later on. In this way, the evenness of the coating surfaces and thus their optical quality could be improved. The optical properties of the two polymer blends prepared by W. Eck and by BASF respectively are presented in References 36 and 37. For a 400-µm-thick layer of the polymer blend prepared by W. Eck on glass a change of the integrated normal-hemispherical transmission from 92% at 20◦ C to 30% at 90◦ C and for a 600-µm-thick layer of the polymer blend prepared

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by BASF on glass a change of the integrated normal-hemispherical transmission from 89% at 30◦ C to 38% at 85◦ C were obtained. Thermotropic Polymer Gels. Thermotropic polymer gels can be based either on the appearance of lyotropic liquid crystalline phases or on phase separation processes. Similar to polymer blends, phase diagrams of polymer gels may also show LCST and UCST. Because of their greater practical relevance development in recent years has focused on polymer gels in which the phase separation process takes place on heating. Since their first observation, lyotropic liquid crystalline polymers like poly(benzyl glutamate) (38,39), hydroxypropyl cellulose (40,41), and fully aromatic polyamides (42) have been extensively studied. Common properties of these polymers are their good solubility in water and the structural feature of a more or less rigid polymer backbone (stiff polymer chain). In certain concentration ranges these polymers undergo in water as well as in some organic solvents a transition from an isotropic phase to a polymer solvent system with pronounced long-range order and thus a transition from a transparent into a light scattering state. The preparation of lyotropic liquid crystalline polymer gel networks and the determination of their thermotropic properties were reported in Reference 43. The investigated systems are poly(ethylene glycol) (PEG)/PVA/borax hydrogels, whereby the concentration of both polymers, their molecular masses, and the degree of cross-linking were varied. Within a wide range of composition the PEG was found to form lyotropic liquid crystalline domains in the PVA/borax hydrogel network as detected by microscopic observations of schlieren and radial-droplet structures between crossed polarizers. In dependence on the composition the transition from the lyotropic liquid crystalline phase into an isotropic liquid phase (clearing temperature) could be shifted between about 15 and 90◦ C. In all samples this transition takes place without affecting the PVA/borax hydrogel network. As a consequence of this the mechanical properties are not influenced by the optical switching process. On heating above their respective clearing temperature the transparency of the hydrogels were found to increase from below 5% to more than 70%, whereby contrast ratios of up to 85:1 were observed. This value is similar to the contrast ratios of the nematic to isotropic phase transition of thermotropic or polymer dispersed liquid crystals. The transparency versus temperature curves of some selected examples of these PEG/PVA/borax hydrogels as well as of an ethoxylated PVA/borax hydrogel are shown in Figure 8. With rising PVA concentration and increasing molecular mass of the PEG an increase of the clearing temperature and thus of the translucent to transparent switching temperature can be observed. As can be seen, the molecular mass of the PEG also influences the steepness of the transparency changes. While the samples containing PEG with a molecular mass of 20,000 change their transparency within a temperature range of a few degrees, a broadening of the switching temperature range to about 30–40◦ C takes place by increasing the molecular mass of the PEG to 70,000. Phase separation processes in gel networks have been well known for a long time. In the past two decades the interest in thermotropic gel networks has rapidly increased because of their potential application in sun protecting glazing. This discussion focuses on recent developments in the preparation of thermotropic polymer gel networks based on synthetic as well as on biopolymers, the influence of

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Fig. 8. Transparency versus temperature curves of (A) ethoxylated PVA 50 T; (B) 1% PEG 20 T/12% PVA 70 T; (C) 1% PEG 20 T/15% PVA 70 T; (D) 1% PEG 50 T/12% PVA 70 T; (E) 1% PEG 50 T/15% PVA 70 T.

the addition of salts on the material properties, and the construction of hybrid solar and electrically controlled light filters. Moreover, polymer gel networks which combine thermotropic and color changing properties are reviewed. Polymer Gel Networks Based on Synthetic Polymers. Gel networks are composed of a swollen cross-linked polymeric network and a solvent. According to the nature of the cross-linker two categories—chemical and physical gels—can be distinguished. In chemical gels the cross-linker connects two polymer chains by covalent bonds. Such gels generally show a strong swelling or shrinking with temperature. Physical gels on the other hand are formed by noncovalent interactions such as ionic interaction, hydrophobic interaction, and/or hydrogen bonding between the polymer chains and a cross-linking agent. Suitable cross-linkers for this purpose are, for example, salts, polyvalent alcohols, borax, phosphonic acid derivatives, and other complex-building organic compounds. Compared with chemical gels the advantage of physical gels is that in most cases their volume does not change much with temperature, which is an essential condition, eg, for their use in sun protecting glazing. For the preparation of thermotropic polymer gel networks mainly watersoluble polymers like poly(acrylic acid) derivatives, poly(vinyl alcohol), polyglycol, poly(vinyl acetal) resins, polyether, and cellulose derivatives have been successfully used. The first observation of a thermoresponsive volume phase transition in a nonionic hydrogel was reported by Tanaka in 1984 (44). It was found that a PNIPAM hydrogel exhibits a reversible collapse transition at about 34◦ C, which is accompanied by a transformation from a highly transparent into a translucent state. Hydrogels of PNIPAM or PNIPAM copolymers (45–47) are still the most frequently studied class of temperature responsive hydrogels. The optical properties of the PNIPAM hydrogel and its suitability for the construction of sun protecting glazing were investigated in Reference 48. Although the optical properties of the PNIPAM hydrogel are quite promising, the volume changes and surface

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pattern formation at the transition temperature are a strong hindrance for commercial application. In 1995, a thermotropic hydrogel based on poly(methyl vinyl ether) cross-linked with methylene-bisacrylamide was presented by Chahroudi (49,50). A variation of the composition allows adjustment of the switching temperature in the range between about 10 and 65◦ C. This thermotropic hydrogel system is commercially available as Cloud Gel (Suntek Corp.). Investigations of the temperature dependence of the optical properties of Cloud Gel were carried out by Wilson (36) and by Wittwer et al. (51). In comparison with thermochromic polymer blends much higher contrast ratios of the transparent to translucent switching were obtained. A 1-mm-thick layer of Cloud Gel was found to change the integrated normal-hemispherical transmission in the visible range from 92% at 25◦ C to 6% at 50◦ C; just 1◦ C above the transition temperature a drop down of the transmission by about 65% was observed. Thermotropic hydrogels with excellent switching properties have also been developed by Seeboth et al. (52). In systems which tend to form anisotropic phases, improved switching properties of the transparent to translucent transition were found. The prepared hydrogels consist of polyalkoxides with various polyethylene/polypropylene unit ratios, which were cross-linked by the addition of salts or complex-building organic as well as inorganic compounds. These physical gels generally show no swelling or shrinking with temperature. Depending on the composition, switching temperatures ranging from room temperature to 80◦ C were obtained and the transparency of the translucent state could be adjusted within wide ranges. Maximum contrast ratios comparable with those of Cloud Gel were obtained. Moreover, in concentration ranges in which anisotropic phases are formed multiple transparence minima and/or maxima occur in the transmission versus temperature curves. Polymer Gel Networks Based on Biopolymers. Biopolymers, like polysaccharides, are nontoxic and inexpensive raw materials, are available in large quantities, and compared with synthetic polymers, have the benefit of environmental compatibility. Because of these advantages, the suitability of biopolymers for the preparation of novel thermotropic hydrogels has been studied in recent years. Thermotropic hydrogels based on cellulose derivatives in combination with an amphiphilic component and various amounts of NaCl were developed by Watanabe (53). By varying the composition, thermotropic switching temperatures ranging from room temperature up to 60◦ C could be realized. Moreover, it was possible to modify the rate of transparency change with temperature. With one of these thermotropic hydrogels, an intelligent window a square meter in size was constructed and successfully tested under practical conditions over a period of two years. In Reference 54 thermochromic hydrogels, which are also based on cellulose derivatives, were reported. It was shown that no amphiphilic component is necessary to prevent an irreversible flocculation of suspended hydroxypropyl cellulose, if hydroxyethyl cellulose is added. Although 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 microorganism must be prevented during production and the total life time of the product. Incorporation of Salts into Thermotropic Aqueous Polymer Systems. The addition of salts can strongly influence the material properties of aqueous polymer

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systems. Even small amounts can lead to pronounced changes of the structural and macroscopic properties. Often cross-linking of the polymer chains takes place by the incorporation of the salt ions, whereby three-dimensional networks are formed. Systematic investigations of the effect of different salts on morphology, phase transition temperatures, water binding capability, as well as optical and rheological properties of thermotropic aqueous polymeric systems have been carried out by numerous authors in recent years. The influence of different salts and their concentration on the phase separation temperature of thermotropic aqueous polymer systems was studied by Alexandridis et al. (55) as well as by other authors (56–58). For all investigated systems a linear shift of the phase separation temperature dependent on the salt concentration was observed. Moreover, at a given concentration different salts were found to shift the phase separation temperatures of the aqueous polymer systems according to their salting-in or saltingout strength, which is described by the so-called Hofmeister series. Salts with a salting-in phenomena lead to an increase of the phase separation temperature, whereas salts with a salting-out phenomena have the opposite effect. The influence of LiCl on the water binding properties of an aqueous polyalkoxide system is reported in Reference 59. For four different LiCl contents the water binding properties of the investigated polyalkoxide were determined by DSC measurements. The obtained results are shown in Figure 9. Each curve consists of three linear regions. At low water contents all water is so strongly bound to the polymer that it cannot be frozen (nonfreezing bound water). Above the maximum content of nonfreezing bound water additional water is more weakly bound to the polymer. This type of water freezes at low temperature (freezing bound water). Its melting point and melting enthalpy is lower than that of pure water. Finally, at high water contents the polymer is saturated with water and additional water builds a separate phase of free water. As can be seen in Figure 9, the addition of LiCl influences the water binding properties of the polyalkoxide. Proportional to the LiCl content an increase of the non-freezing-bound water capacity of the polyalkoxide and an increase of the binding enthalpy of freezing-bound water were observed. Both results indicate that the interaction between water and polymeric system becomes stronger with increasing LiCl content. Reference 59 also reports an influence of the addition of LiCl on morphology, as well as optical and rheological properties of the investigated polyalkoxide system. As an example Figure 10 shows the temperature dependence of the optical transmission and of the dynamic viscosity of three samples with a fixed polyether/water mixing ratio of 4:1 and various LiCl contents of 0, 3, and 6 wt%. The observed changes of the optical transmission– temperature curves are caused by morphological changes through the addition of LiCl. The pronounced viscosity changes, on the other hand, show the formation of cross-linked structures, whereby in different phases gelation occurs in different LiCl concentration ranges. Hybrid Solar and Electrically Controlled Light Filters. Compared to electrochromic glazings, smart windows based on thermotropic hydrogels have the advantages of lower costs and a much higher transparency in the clear state. However, for specific applications switching on demand is required. The construction of a hybrid solar and electrically controlled transmission changing light filter based on thermotropic hydrogels has been described (60). The described arrangement consists of a 2- or 3-mm-thick thermotropic hydrogel layer placed between

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Fig. 9. Water melting enthalpy as a function of water content of polyether/LiCl/water systems.

two tin-doped indium oxide (ITO) coated glass substrates, whereby the ITO layers were placed either inside or outside the double glazing item. When heated passively by solar energy or actively by applying an electric voltage on the ITO layers the thermotropic hydrogel switches from the transparent into the light scattering state. Even for the arrangement where the ITO layers are in direct contact with the hydrogel, no electrolysis was observed under the experimental conditions. Besides the position of the ITO layers, their thickness, the applied voltage as well as the thermochromic hydrogel material used was varied and the switching times of the glazing items were determined. Figure 11 shows the transmission–time curves of one of the investigated light filters at varying applied electric voltages. As can be seen, even 133-nm-thick ITO layers which reduce the maximum transparency of the light filter by about 10% allow fast switching times in combination with high contrast ratios. An increase of the ITO layer thickness decreases the wattage, which is necessary to achieve a constant switching time. On the other hand the transparency of the glazing item is thereby also reduced. For one of the investigated glazing items a transmission change from about 62% to ≤ 1% and a switching time of 5 min was achieved by applying a wattage of 0.246 W/cm2 . It was concluded that a further optimization of all components of the glazing item and especially of the layer thickness as well as the material properties of the incorporated thermotropic hydrogel will lead to a further significant reduction of the required wattage.

Gel Networks for Reversible Transparency and Color Control with Temperature. A novel class of polymer gel networks which change both their color and transparency with changing temperature has been developed (61). Starting from a

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Fig. 10. Temperature dependence of the optical transmission at 600 nm (d = 1 cm) and of the dynamic viscosity (measuring conditions: parallel plates of 25-mm diameter, frequency: 1 rad/s) of LiCl containing aqueous polyether samples with a fixed polyether/water mixing ratio of 4:1 and various LiCl contents ranging from 0 to 6 wt%.

thermotropic hydrogel composed of a polyalkoxide and an aqueous LiCl containing buffer solution the color changing properties were obtained by adding one of the pH-sensitive indicator dyes Chlorophenol Red, Nitrazine Yellow, or Bromothymol Blue. The addition of these dyes was found to have only a slight influence on the thermotropic switching behavior of the hydrogel matrix. The switching behavior appears due to a phase separation process as is typical for such polyalkoxide/LiCl hydrogel systems. The color changing effect on the other hand has its origin in temperature-induced pH changes of the gel network similar to the effect observed

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Fig. 11. Transmission versus time curves of a thermotropic gel containing light filter (12 × 15 cm) in dependence on the applied voltages; thickness of the gel layer d = 3 mm and thickness of the ITO layers = 133 nm. 8 V/4.8 W (A); 6 V/2.7 W (B); 4 V/1.2 W 3 V/0.7 W (D); 2 V/0.3 W (E); 1 V/0.1 W (F). (C);

earlier for phenol-substituted indicator dyes in a PVA/borax/surfactant gel network (32). As an example the UV–vis absorption spectra at two different temperatures and the transparency versus temperature curve of a hydrogel prepared by mixing 3.95 g polyalkoxide, 0.25 g LiCl, 0.8 g of an aqueous buffer solution (pH 10), and 0.12 g of a 2.2% aqueous solution of Bromothymol Blue are displayed in Figures 12 and 13, respectively. Both effects, the thermotropic switching at about 36◦ C and the temperature dependent cross over between an absorption band at λ = 408 nm and one at λ = 617 nm can be clearly seen. As a result of these effects the hydrogel changes its appearance from green transparent below 33◦ C, via yellow transparent until about 36◦ C to yellow translucent above this temperature. With increasing temperature the phenol–phenolate equilibrium of Bromothymol Blue in the polyalkoxide/LiCl/water system is shifted from the green colored phenolate form to the yellow colored phenol form. This result is in contrast to the behavior of phenol-substituted indicator dyes in a PVA/borax/surfactant gel network for which the opposite shift of the phenol–phenolate equilibrium with temperature was observed. Thus it can be concluded that the reversible color change of indicator dyes in gel systems depends on the specific composition of the gel networks. However, the origin of this effect on a molecular level is still under discussion. Another example of a color and transparency changing gel network is reported in Reference 62. Starting from a Phenol Red containing PVA/borax/surfactant hydrogel which changes its color but not its transparency in dependence on temperature, thermotropic properties were obtained by adding small amounts of a more hydrophobic polymer. For this purpose a polyalkoxide was used. At a content of 0.8 wt% polyalkoxide thermotropic behavior based on a phase separation process was found to appear without affecting the color changing behavior very much. With further increasing polyalkoxide content the degree of the reduction of transparency with temperature is increased and the starting temperature of this process is shifted to lower temperatures. Moreover, the

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Fig. 12. Transparency versus temperature curve of a Bromothymol Blue (BB) containing hydrogel composed of a polyalkoxide and an aqueous LiCl containing buffer solution (λ = 750 nm, d = 0.1 cm, [BB] = 1.9 × 10 − 4 mol/L). 1 = green transparent; 2 = yellow transparent; 3 = yellow opaque.

influence of the concentration of the zwitterionic sulfobetaine surfactant on the color changing and thermotropic behavior was studied. PVA/borax/Phenol Red hydrogels containing 1.1 wt% polyalkoxide and various sulfobetaine concentrations below and above their critical micelle concentration (cmc = 3.8 × 10 − 3 mol/kg) were prepared and the UV–vis absorption spectra as well as the temperature dependence of the transparency of these hydrogels were measured. In the UV– vis absorption spectra two bands occur. The first one at λ = 440 nm corresponds

Fig. 13. UV–vis absorption spectra of a Bromothymol Blue containing hydrogel composed of a polyalkoxide and an aqueous LiCl containing buffer solution in dependence on temperature (d = 0.1 cm). −5◦ C (a); 38◦ C (b).

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to the phenol form and the second one at λ = 563 nm to the phenolate form of Phenol Red. With increasing sulfobetaine concentration a decrease of the intensity of both UV–vis absorption bands occur. It is well known that above the cmc surfactants can influence the UV–vis absorption behavior of water-soluble dyes. However, here this effect takes place at the lowest sulfobetaine concentration of 2.5 × 10 − 3 mol/kg, which is significantly below the cmc. Also, a small but significant and reproducible influence of the surfactant concentration on thermotropic behavior is reported even below the cmc. To explain this behavior the authors suggested the formation of complexes between dye molecules and aggregates of sulfobetaine, but also discussed an interaction of the dye with single sulfobetaine molecules as an alternative model.

Application of Thermochromic Polymers The potential applications of thermochromic material include temperature tunable light and heat radiation filters, large area displays, and temperature indicators visualizing the temperature of the surrounding medium or the temperature profile of a surface which is coated with the thermochromic material. Materials changing their color with temperature are already used in a variety of commercial products such as textiles, toys, baby spoons, coffee pots, aquarium thermometers, novelty items, pans with a thermospot, and labels for wine bottles, indicating the proper drinking temperature (63–65). However, the commercialization of thermochromic materials is just at the beginning and certainly this market will rapidly grow in the near future. Potential industrial and processing applications are, eg, monitoring of overheating of machine parts, observation of thermal leaks, and controlling processing temperature. In the field of storage and transportation thermochromic materials with an irreversible switching could assure the observance of required temperature ranges. For example heat-sensitive materials require a storage and transportation temperature below a maximum value and for frozen food the temperature has to kept below 0◦ C. In medical technology skin temperature indicators might find application for diagnostic purposes. Another big market addresses security aspects. Road signs with integrated thermochromic effects could warn of icy conditions. Thermochromic coatings of heating plates, fire doors, radiators, and other parts of domestic appliances which become hot during their use could indicate that the surface is too hot to be touched. The development of thermochromic polymers suitable for all these applications is actually in progress and in some cases prototypes have already been presented. The interest in thermotropic materials on the other hand appears more because of their optical properties than because of their ability to detect temperature changes. Such materials are optical shutters for light as well as for heat radiation. The major market for such materials arises from their potential application as functional layers in the construction of sun protecting glazing (66,67). In modern architecture glazing has become a high-tech-component with a wide range of properties and functions. Besides the classic function as viewing window glazing elements enable the use of daylight for the illumination of the building and the passive use of the solar energy for space heating. To realize these functions more and more parts of the building facade are substituted by large area view

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and illumination windows, skylights, and sunroofs. In many modern buildings more than 80% of the facade area consists of glazing elements. In buildings with large glazing areas the incident solar radiation plays an important role in energy performance. While on cold days, such as during the winter, the incident solar radiation reduces the energy consumption for space heating, on hot days, such as during the summer, solar radiation causes overheating inside the buildings and thus an enormous increase of the energy consumption for space cooling. To reduce this negative effect regulation of the incident solar radiation is necessary. Today, this is achieved by mechanical optical shutters like venetian blinds, awnings, and sun blinds. An innovative solution which is under development is the use of sun protecting glazings whose optical properties can either be actively adjusted or automatically adjust themselves according to changes of the climatic conditions. Thermotropic glazing in which a functional layer of a thermotropic material is incorporated into the glazing assembly is the most promising type of sun protection glazing and closest to reaching market readiness. As early as 1950 sun protecting glazing based on a thermotropic gel network was set up and tested in the residence of the Munich zoo for a period of about 10 years. This glazing assembly consists of a mixture of 5 wt% of poly(vinyl methyl ether) in agar-agar filled between a double glazing. However, the transmission change of the glazing assembly was insufficient for commercial use. In more recent

Fig. 14. Switching behavior of a sun protecting glazing (100 × 50 cm) containing a 2-mm-thick thermotropic hydrogel layer at 20◦ C (left side) and slightly above the switching temperature of 30◦ C (right side).

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years advanced prototypes of sun protecting glazing based on thermotropic hydrogels with strongly improved optical properties have been presented by Chahroudi (49,50), Watanabe (53) and Seeboth (52). These glazings possess a transparency of about 90% in the transparent state and can reach a transparency of less than 10% in the translucent state. Moreover, the transparency change takes place homogeneously without the appearance of streaks. As an example, the switching of a 100 × 50 cm large prototype of a thermotropic glazing presented by Seeboth is shown in Figure 14. To commercialize thermochromic glazings based on hydrogels technology transfer from the laboratory scale to an industrial scale has to take place. Thus, for example, an automated production line for putting the polymer between the double glazing could be established.

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ARNO SEEBOTH ¨ DETLEF LOTZSCH ¨ Angewandte Polymerforschung Fraunhofer-Institut fur