thermochromic polymers

Vol. 12 THERMOCHROMIC POLYMERS 143

THERMOCHROMIC POLYMERS

Introduction

In recent years functional polymers changing their visible optical properties in re-sponse to an external stimulus have met with growing interest. According to theexternal stimulus which affects the optical properties, these so-called chromogenicpolymers 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 chro-mogenic 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 innovationsbased on chromogenic polymers. For all these applications laboratory prototypesdemonstrating the effect have been presented. A few of them have already reachedthe readiness for marketing and certainly others will follow in the near future.

This article focuses on thermochromic polymer systems. An overview ofthe different types of such polymer systems is given and the origin of the ther-mochromic effect, their specific material properties, and potential applications arediscussed. Table 1 displays the various generic types of thermochromic polymers.Following this classification, thermochromic polymer systems in which the colorappears owing to a Bragg reflection on a periodic structure are discussed first. Insuch systems the thermochromic effect is caused by a temperature dependence ofthe layer periodicity. Thus the wavelength of the reflected light can only changecontinuously. Another inherent feature of such thermochromic systems is thatthe visible color depends on the viewing angle. Because of these disadvantagesthe potential applications of thermochromic polymer systems based on Bragg re-flection are limited. The second classification is about thermochromic polymersystems varying their color or color intensity owing to temperature-dependentmolecular changes of the chromophoric group. This class of thermochromic poly-mers 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 LightCausing the Thermochromic Behavior

Effects Macroscopicon light Origin behavior Polymer class

Reflection Periodic structures �I, �λmax Cholesteric liquid crystalline polymersCrystalline colloidal arrays embedded

in a gel networkGels

Absorption Chromophoric groups �I, �λmax Conjugated polymersHydrogels containing indicator dyes

Scattering Areas with different �T% Polymer blends exhibiting LCSTa

refractive indices Hydrogles exhibiting LCSTLyotropic liquid crystalline hydrogels

aLCST: lower critical solution temperature.

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

144 THERMOCHROMIC POLYMERS Vol. 12

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

Thermochromic Polymer Systems Based on Bragg Reflection

Liquid Crystalline Polymers with Thermochromic Properties. Inseveral liquid crystalline phases a helical superstructure occurs if either a chiralcompound is added or the liquid crystalline compound itself consists of a chiralmolecular structure (1,2). In this way, the nematic phase, for example, transformsinto the so-called cholesteric phase. Depending on structure and concentrationof the chiral compound as well as on the phase type, the pitch (P) of the helicalsuperstructure can be in the range between 100 nm and infinite. Incident lightis selectively reflected, if the wavelength (λ) satisfies the Bragg condition λ = nPcos θ (n stands for the mean reflective index of the liquid crystal and θ for the anglebetween the incident light and the orientation of the helix). In many systems thewavelength of the reflected light is in the visible region; thus these liquid crystalsbecome intensively colored. The pitch generally changes with temperature andcan be modified by applying an electric field.

Thermochromic polymer foils based on cholesteric liquid crystals are al-ready commercially available from various manufacturers including HallcrestLtd., Merck KGaA, and Davis Liquid Crystals Inc. Such thermochromic sheetsconsist of a black backing layer, a layer of the cholesteric liquid crystal, and a pro-tective clear polyester layer. They can indicate temperature changes ranging froma fraction of a degree to over 20◦C by varying their color continuously through theentire visible region.

Chiral liquid crystalline polymers possessing thermochromic propertiesthemselves are described in numerous publications. Besides synthetic polymers,many cellulose derivatives have been found to possess cholesteric mesophaseswith selective reflection in the visible wavelength region. The pentyl ether of hy-droxypropyl cellulose is an example for such a cholesteric polymer (3). At roomtemperature it shows selective reflection at about 500 nm. With increasing tem-perature the wavelength of the selective reflection maximum smoothly increases,reaching 650 nm at about 80◦C. For applications in information technology, thosematerials are of special interest which allow fixation the cholesteric order at anyuser-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 cholestericphase range as well as cholesteric mixtures which are polymerizable and formcross-linked polymeric networks are suitable for this purpose. The preparationof patterned multicolor cholesteric liquid crystal polymer networks have been re-ported (12). The cholesteric liquid crystal polymer networks were obtained byphotopolymerization of a mixture containing 20% of a chiral monoacrylate CBC


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

(see Fig. 1), 25% of a nonreactive commercial chiral dopant CB15 (Merck), 54% of anonreactive commercial liquid crystal BL59 (Merck), 0.5% of a diacrylate C6M (seeFig. 1), and 0.5% Irgacure 651 photoinitiator at different temperatures. This mix-ture exhibits a cholesteric phase with a strong temperature dependence of thepitch. 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, and30◦C), red, green, and blue polymer networks with a temperature-independentcolor were obtained. In a next step the preparation of a patterned multicolor poly-mer network was carried out by using masks and performing the photopolymer-ization step by step in different regions at different temperatures. It was concludedthat by using a light source to induce patterned heating of the mixture, followedby fixing via UV-induced photopolymerization, it should be possible to producemuch more complicated structures.

The formation of liquid crystalline phases with thermochromic properties ina poly(β-peptide) was reported in Reference 13. The color changes were observedby using polarizing optical microscopy. However, this method was not found to besuitable for determining thermochromism.

Thermochromic Gel Networks Based on Bragg Reflection. Polymergel networks with thermochromic properties, based on Bragg reflection, are fre-quently described in the literature, whereby most of the systems are composed ofa polymer and an organic solvent. It has been reported that poly-1-butene flakesswells in benzene, toluene, o-xylene, m-xylene, p-xylene, or tetrachloroethyleneand exhibits reversible color changes with temperature between the melting pointof the solvent and the sol–gel transition temperature (14). With increasing tem-perature 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 onthe microstructure of the gelled poly-1-butene. A similar behavior was observedby the same author for gelled isotactic polypropylene in benzene, toluene, andxylene (15). For example the color of a 3% isotactic polypropylene gel in benzenewas 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 monodispersepolystyrene spheres embedded in a poly(N-isopropyl acrylamide) (PNIPAM) hy-drogel, which was prepared by first dispersing highly charged and monodisperse


polystyrene spheres in an aqueous solution containing the PNIPAM monomer, fol-lowed by a photochemically initiated polymerization. A temperature-dependentstrong swelling or shrinking of the polymer matrix takes place. The embeddedpolystyrene spheres follow this volume change, causing thereby a modification oftheir lattice spacing and thus of their Bragg reflection wavelength. To character-ize the optical properties of the novel thermochromic material, films of differentlayer thickness were prepared and the temperature dependence of their diffractedwavelength determined. While the bulk material is translucent because of lightscattering on the colloidal particles, thin films become transparent. At room tem-perature 125-µm-thick hydrogel film was used to detect the thermochromic effectby absorption measurements. In this way, a continuous change of the diffractedwavelength from 704 nm at 11.7◦C to 460 nm at 34.9◦C was measured, which cor-responds to nearly the entire visible range. However, note that the thermochromiceffect is coupled with strong volume changes. An increase of the diffracted wave-length from 400 to 800 nm corresponds to a swelling in each direction by a factorof 2 and thus to an eightfold increase of the volume.

Crystalline colloidal arrays which vary the intensity of the Bragg reflectionin dependence on temperature without changing the wavelength of the diffractedlight have also been developed (16). Such materials were obtained by dispers-ing highly charged and monodisperse colloidal particles of PNIPAM with a size of100 nm in deionized water. In water these PNIPAM colloids self-assemble and forma body-centered cubic array which diffracts light following the Bragg diffractionlaw. Below their lower critical solution temperature of about 32◦C the PNIPAMcolloids begin to swell, leading to an increase of the sphere diameter from about100 nm above this temperature to about 300 nm at 10◦C. The lattice spacing ofthe cubic array and thus the reflected wavelength (at a constant glancing anglebetween the incident light and the diffracting crystal plane) depend only on theparticle density of the PNIPAM colloids. The intensity of the Bragg reflection ofcrystalline colloidal arrays, on the other hand, depends on the array ordering aswell as on the scattering cross section of the colloidal particles, which is influ-enced by the particle size of the colloids. As a result the reversible swelling orshrinking of the PNIPAM colloids with temperature causes pronounced changesin the intensity of the Bragg reflection. As an example the optical properties of acrystalline colloidal array of PNIPAM colloids (diameter: 100 nm) with a latticeconstant of 342 nm were reported. At 40◦C the nearest neighbor sphere distanceamounts to 242 nm and at the Bragg reflection wavelength nearly all incident lightis reflected. On the other hand at 10◦C the PNIPAM colloids are almost touchingand only a weak Bragg reflection occurs.

Thermochromic Polymer Systems Based on the Absorption of Light

Conjugated Polymers. The occurrence of thermochromic properties isfrequently observed in conjugated polymers (17,18) like polyacetylenes (19), poly-diacetylenes (19), polythiophenes (20), and polyanilines (21). Many conjugatedpolymers exhibit absorption of light in the visible range as well as high reflectivity.Hence, they often are colored and show a metallic appearance. Thermochromismin conjugated polymers has its origin in changes of the conformation as they


abruptly occur at phase transitions whereby even the slightest modification ofthe conformational structure can cause significant color changes. Especially theplanarity of the polymer backbone plays an important role. Any twisting of thechain 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 colorchanges of conjugated polymers are reversible. However, kinetic effects can causean irreversibility which can be useful for thermal recording (22). In Reference 23,synthesis and optical properties of a liquid crystalline polydiacetylene are de-scribed. At each of its mesogenic phase transitions a thermochromic effect wasdetected. That means thermochromic and mesogenic properties of the investi-gated polydiacetylene are coupled with one another. The structural and opticalproperties 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 liquidcrystalline 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. More-over, a thermochromic behavior was observed within the temperature range of25–140◦C. With increasing temperature the polymer changes its color contin-uously from yellow-green at 25◦C to blue at 130◦C. To characterize this ther-mochromic behavior in more detail, UV–vis absorption and fluorescence emissionspectra were measured at different temperatures. For both, a continuous evolutionwith temperature was found. In contrast to the results reported in Reference 23, inReference 24 even at the smectic H to smectic A phase transition no discontinuouschange of the UV–vis and fluorescence intensities could be detected.

Another way to obtain thermochromic polymers is to incorporate a ther-mochromic material into a polymer matrix. Thermochromic polymers havebeen 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, exhibit-ing thermochromism according to temperature-induced changes of the conjuga-tion of the π -electron system. The thermochromic switching temperature of thepolythiophene pigments can be tailored by chemical modifications. In this way aset of pigments that visually and reversibly change colors at a prescribed temper-ature in the region of −35 to +125◦C was developed. These pigments are ther-mally stable until more than 200◦C. Commercially available paints, plastics, andrubbers were used as host polymers, whereby 0.1–1.0 wt% of the pigments werenecessary to obtain a visible thermochromic effect. However, a detailed descrip-tion of the chemical structure of the pigments or a list of the available switchingtemperatures and color changes has not been reported so far.

A thermoplastic polymer with thermochromic properties was recently pre-sented by Seeboth et al. (27). This polymer switches on heating at a certain tem-perature from blue to colorless. Figure 2 displays a photograph demonstratingthis thermochromic switching effect. A foil of the novel thermochromic polymer ispartially heated in hot water above the switching temperature. Clearly the coolblue and the hot colorless state can be seen. So far, neither the origin of the ther-mochromic effect nor the composition of this thermochromic polymer has beenreported.

Masterbatches of polyethylene and polypropylene containing microencapsu-lated thermochromic materials are already commercially available from variousmanufacturers. The thermochromic materials used in these microcapsules are


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, forexample, substituted phenylmethanes and fluorans. These so-called leuco dyesor color formers (29) are either colorless or weakly colored. However, on reactionwith an electron acceptor, as for example a phenol, an opening of the lactone ringoccurs. In the open ring state the conjugated π -electron system is extended, en-abling color formation. By the addition of appropriate solvents the color-formingreaction becomes reversible. In the molten state these solvents function as aninhibitor of the color-forming reaction. On the other hand during the crystalliza-tion of the solvent the donating chromogenic compound reacts with the electronacceptor; thus the crystalline state becomes colored. For example a mixture of1 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 stateand develops a vermilion color on cooling below 48◦C when the crystallization ofthe 1-hexadecanol takes place. The reversible lactone ring opening reaction of 2-chloro-6-diethylamino-3-methylfluoran, which is the origin for this thermochromiceffect, is shown in Figure 3. Although these masterbatches show excellent ther-mochromic switching behaviors, the use of microencapsulated thermochromic mix-tures results in a variety of limitations including poor thermal and shear stabilitywhich cause difficulties in processing.

Gel Networks. In the literature a few examples of thermochromic gel net-works consisting of conjugated polymers swelled in organic solvents are reported.The thermochromic effect described for these gels appears owing to changes of


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

the molecular structure of the polymer backbone at their respective gel–sol tran-sition temperature. One of the reported systems consists of polydiacetylene gelledin o-dichlorobenzene or other gel-forming solvents (30). Upon heating above thegel–sol transition temperature reversible pronounced color changes are observed.Colors and transition temperatures could be varied by using different solventsas well as by changing the composition. A weak thermochromic effect causedby temperature-dependent molecular changes of the chromophoric group in agel network has also been reported (31). On heating in benzene a poly[2-(3,7-dimethyloctoxy)-5-methoxy-1,4-phenylenevinylene] gel shows a gradually red toyellow color change at approximately 35◦C, which corresponds to the gel–sol tran-sition temperature. After cooling to room temperature the yellow state remainsmetastable. It eventually reverts after several hours to the red gel phase. Theauthors explained the thermochromic behavior of this system by a reduction ofinterchain π–π interaction at the gel to solution transition, at which the relationbetween aggregated and isolated chain sections abruptly changes.

The first example of a thermochromic effect of dyes embedded in a transpar-ent hydrogel was reported by Seeboth et al. (32). The described thermochromiceffect appears by the addition of suitable indicator dyes, with pKa values between7.0 and 9.4, to a definite poly(vinyl alcohol) (PVA)/borax/surfactant hydrogel net-work. With increasing temperature the used gel matrix shifts the equilibrium ofthe indicator dyes toward their deprotonated forms, leading thereby to gradualcolor changes with temperature. In dependence on the used indicator dye(s) aswitching between a colorless and a colored state or between two or even more dif-ferent colored states occurred. For example a gradual color change from colorlessat 10◦C to a deep violet at 80◦C was observed for a PVA/borax/surfactant hydro-gel containing the so-called Reichard betaine 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridino)-phenolate (DTPP), whose pH-sensitive equilibrium is shown in Figure 4.

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


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

A Cresol Red containing PVA/borax/surfactant hydrogel on the other handswitches from yellow at 10◦C to wine-red at 80◦C and a Bromothymol Blue andCresol Red containing PVA/borax/surfactant hydrogel from yellow (below 5◦C) viagreen (between 15 and 25◦C) to violet (above 60◦C). A photograph displaying thethermochromic switching of the Cresol Red containing PVA/borax/surfactant hy-drogel is shown in Figure 5. The 3-mm-thick layer spacing of a double-glazingassembly is filled with this thermochromic hydrogel and partially heated in ahot water bath. While the cold zone remains yellow the color in the hot zone hasswitched to wine-red. In order to characterize the thermochromic effect in moredetail, UV–vis absorption measurements were carried out. As examples of theobtained results the UV–vis absorption spectra of the DTPP and of the Cresol Redcontaining PVA/borax/surfactant hydrogels measured at different temperaturesare displayed in Figures 6 and 7, respectively. One absorption maximum growingcontinuously with increasing temperature appears in the spectra of the DTPP con-taining PVA/borax/surfactant hydrogel (Fig. 6). This absorption at about 550 nmis caused by the phenolate form of DTPP and its growing intensity indicates arising phenolate concentration with increasing temperature. In Figure 7 for alltemperatures two absorption maxima, one at 419 nm, which corresponds to theabsorption of the phenol form of Cresol Red, and the other one at 581 nm, which cor-responds to the absorption of the phenolate form of Cresol Red, are observed. With


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 increaseswhile simultaneously the intensity of the absorption band at 419 nm decreases.All spectral curves meet at an isosbestic point at 486 nm, supporting thereby thesuggested model of a temperature-dependent equilibrium between two differentforms of the indicator dye as the origin of the observed thermochromic effect.

In addition to the transparency another important feature of these hydrogelsis that the thermochromic effect is not accompanied by volume changes. Thereforesuch hydrogels are applicable for smart windows, large area displays, and tunablecolor filters.

Fig. 7. UV–vis absorption spectra of a Cresol Red containing PVA/borax/surfactant hy-drogel in dependence on temperature (d = 1 cm). 16◦C (A); 60◦C (B); 80◦C(C).


Transparency/Scattering Switching

Some thermotropic polymer systems switch at a certain temperature reversiblybetween a highly transparent and a light scattering state. Such an optical effectcan be caused either by a phase transition between an isotropic and an anisotropic(liquid crystalline) state or by a phase separation process. Two different classesof 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 mis-cible with one another over the complete concentration range. There are alsopolymer pairs known which either mix or do not mix depending on tempera-ture; both an increase as well as a decrease of the miscibility (qv) with increas-ing temperature are possible. In the phase diagrams of such polymer pairs atwo-phase region appears either below a so-called upper critical solution temper-ature (UCST) or above a so-called lower critical solution temperature (LCST).Within a wide concentration range polymer blends of both these systems arethermotropic. Blends with an UCST in the phase diagram switch from translu-cent to transparent by increasing the temperature, whereas polymer blendsof systems with an LCST switch in the opposite manner. For application insun protecting glazing, a reduction of the transmission with increasing tem-perature is required and therefore the development of thermotropic polymerblends in recent years has focused on systems with an LCST. In 1993 Siol et al.(33) from the Roehm GmbH developed thermochromic polymer blends consist-ing of chlorinated rubber and polymethacrylates. Depending on the composi-tion 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) pre-sented polymer blends with a much better reversibility of the thermotropicswitching. Moreover, cloud points between 30 and 40◦C could be realized, whichis the required temperature range for sun protecting glazing. These polymerblends were derived from poly(propylene oxide) and a styrene–hydroxyethylmethacrylate copolymer cross-linked with a trifunctional cyclic isocyanate. Bycross-linking the copolymer in the presence of poly(propylene oxide) a semi-interpenetrating network is formed leading to an increase of the stability of the mi-crophase separation above the cloud point and thus to a much better reversibilityof the thermotropic switching. A similar cross-linked thermotropic polymer blendwas developed in 1995 by BASF (35). In the first step blends of poly(propylene ox-ide) and a copolymer of styrene, hydroxyethyl methacrylate, and a small portionof 4-acroyloxy butyl carbonato benzophenone were prepared. However, instead ofadding a cross-linking agent, radical polymerization under UV-light was used toform the semi-interpenetrating network. This method allows preparation of ther-motropic coatings first and polymerization to the semi-interpenetrating networklater on. In this way, the evenness of the coating surfaces and thus their opti-cal quality could be improved. The optical properties of the two polymer blendsprepared by W. Eck and by BASF respectively are presented in References 36and 37. For a 400-µm-thick layer of the polymer blend prepared by W. Eck onglass a change of the integrated normal-hemispherical transmission from 92% at20◦C to 30% at 90◦C and for a 600-µm-thick layer of the polymer blend prepared


by BASF on glass a change of the integrated normal-hemispherical transmissionfrom 89% at 30◦C to 38% at 85◦C were obtained.

Thermotropic Polymer Gels. Thermotropic polymer gels can be basedeither on the appearance of lyotropic liquid crystalline phases or on phase sepa-ration processes. Similar to polymer blends, phase diagrams of polymer gels mayalso show LCST and UCST. Because of their greater practical relevance develop-ment in recent years has focused on polymer gels in which the phase separationprocess takes place on heating.

Since their first observation, lyotropic liquid crystalline polymers likepoly(benzyl glutamate) (38,39), hydroxypropyl cellulose (40,41), and fully aromaticpolyamides (42) have been extensively studied. Common properties of these poly-mers are their good solubility in water and the structural feature of a more orless rigid polymer backbone (stiff polymer chain). In certain concentration rangesthese polymers undergo in water as well as in some organic solvents a transitionfrom an isotropic phase to a polymer solvent system with pronounced long-rangeorder and thus a transition from a transparent into a light scattering state.

The preparation of lyotropic liquid crystalline polymer gel networks and thedetermination 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 de-gree of cross-linking were varied. Within a wide range of composition the PEGwas found to form lyotropic liquid crystalline domains in the PVA/borax hydrogelnetwork as detected by microscopic observations of schlieren and radial-dropletstructures between crossed polarizers. In dependence on the composition the tran-sition from the lyotropic liquid crystalline phase into an isotropic liquid phase(clearing temperature) could be shifted between about 15 and 90◦C. In all sam-ples this transition takes place without affecting the PVA/borax hydrogel network.As a consequence of this the mechanical properties are not influenced by the opti-cal switching process. On heating above their respective clearing temperature thetransparency of the hydrogels were found to increase from below 5% to more than70%, whereby contrast ratios of up to 85:1 were observed. This value is similar tothe contrast ratios of the nematic to isotropic phase transition of thermotropic orpolymer dispersed liquid crystals. The transparency versus temperature curvesof some selected examples of these PEG/PVA/borax hydrogels as well as of anethoxylated PVA/borax hydrogel are shown in Figure 8. With rising PVA concen-tration and increasing molecular mass of the PEG an increase of the clearingtemperature and thus of the translucent to transparent switching temperaturecan be observed. As can be seen, the molecular mass of the PEG also influencesthe steepness of the transparency changes. While the samples containing PEGwith a molecular mass of 20,000 change their transparency within a temperaturerange of a few degrees, a broadening of the switching temperature range to about30–40◦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 longtime. In the past two decades the interest in thermotropic gel networks has rapidlyincreased because of their potential application in sun protecting glazing. This dis-cussion focuses on recent developments in the preparation of thermotropic poly-mer gel networks based on synthetic as well as on biopolymers, the influence of


Fig. 8. Transparency versus temperature curves of (A) ethoxylated PVA 50 T; (B) 1% PEG20 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 so-lar and electrically controlled light filters. Moreover, polymer gel networks whichcombine thermotropic and color changing properties are reviewed.

Polymer Gel Networks Based on Synthetic Polymers. Gel networks arecomposed of a swollen cross-linked polymeric network and a solvent. Accordingto the nature of the cross-linker two categories—chemical and physical gels—canbe distinguished. In chemical gels the cross-linker connects two polymer chainsby covalent bonds. Such gels generally show a strong swelling or shrinking withtemperature. Physical gels on the other hand are formed by noncovalent inter-actions such as ionic interaction, hydrophobic interaction, and/or hydrogen bond-ing between the polymer chains and a cross-linking agent. Suitable cross-linkersfor this purpose are, for example, salts, polyvalent alcohols, borax, phosphonicacid derivatives, and other complex-building organic compounds. Compared withchemical gels the advantage of physical gels is that in most cases their volumedoes not change much with temperature, which is an essential condition, eg, fortheir use in sun protecting glazing.

For the preparation of thermotropic polymer gel networks mainly water-soluble polymers like poly(acrylic acid) derivatives, poly(vinyl alcohol), polyglycol,poly(vinyl acetal) resins, polyether, and cellulose derivatives have been success-fully used. The first observation of a thermoresponsive volume phase transitionin a nonionic hydrogel was reported by Tanaka in 1984 (44). It was found that aPNIPAM hydrogel exhibits a reversible collapse transition at about 34◦C, whichis accompanied by a transformation from a highly transparent into a translucentstate. Hydrogels of PNIPAM or PNIPAM copolymers (45–47) are still the most fre-quently studied class of temperature responsive hydrogels. The optical propertiesof the PNIPAM hydrogel and its suitability for the construction of sun protect-ing glazing were investigated in Reference 48. Although the optical propertiesof the PNIPAM hydrogel are quite promising, the volume changes and surface


pattern formation at the transition temperature are a strong hindrance for com-mercial application. In 1995, a thermotropic hydrogel based on poly(methyl vinylether) cross-linked with methylene-bisacrylamide was presented by Chahroudi(49,50). A variation of the composition allows adjustment of the switching tem-perature in the range between about 10 and 65◦C. This thermotropic hydrogelsystem is commercially available as Cloud Gel (Suntek Corp.). Investigations ofthe temperature dependence of the optical properties of Cloud Gel were carriedout by Wilson (36) and by Wittwer et al. (51). In comparison with thermochromicpolymer blends much higher contrast ratios of the transparent to translucentswitching were obtained. A 1-mm-thick layer of Cloud Gel was found to changethe 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 ofthe transmission by about 65% was observed. Thermotropic hydrogels with ex-cellent switching properties have also been developed by Seeboth et al. (52). Insystems which tend to form anisotropic phases, improved switching propertiesof the transparent to translucent transition were found. The prepared hydro-gels consist of polyalkoxides with various polyethylene/polypropylene unit ratios,which were cross-linked by the addition of salts or complex-building organic aswell as inorganic compounds. These physical gels generally show no swelling orshrinking with temperature. Depending on the composition, switching temper-atures ranging from room temperature to 80◦C were obtained and the trans-parency of the translucent state could be adjusted within wide ranges. Maxi-mum contrast ratios comparable with those of Cloud Gel were obtained. Moreover,in concentration ranges in which anisotropic phases are formed multiple trans-parence minima and/or maxima occur in the transmission versus temperaturecurves.

Polymer Gel Networks Based on Biopolymers. Biopolymers, like polysac-charides, are nontoxic and inexpensive raw materials, are available in large quan-tities, and compared with synthetic polymers, have the benefit of environmentalcompatibility. Because of these advantages, the suitability of biopolymers for thepreparation of novel thermotropic hydrogels has been studied in recent years.Thermotropic hydrogels based on cellulose derivatives in combination with anamphiphilic component and various amounts of NaCl were developed by Watan-abe (53). By varying the composition, thermotropic switching temperatures rang-ing from room temperature up to 60◦C could be realized. Moreover, it was possibleto modify the rate of transparency change with temperature. With one of thesethermotropic hydrogels, an intelligent window a square meter in size was con-structed and successfully tested under practical conditions over a period of twoyears. In Reference 54 thermochromic hydrogels, which are also based on cel-lulose derivatives, were reported. It was shown that no amphiphilic componentis necessary to prevent an irreversible flocculation of suspended hydroxypropylcellulose, if hydroxyethyl cellulose is added. Although biological decomposition isoften mentioned as an advantage of biopolymers, it is also the most importanthindrance for their commercial use because contact of the biopolymers with mi-croorganism must be prevented during production and the total life time of theproduct.

Incorporation of Salts into Thermotropic Aqueous Polymer Systems. Theaddition of salts can strongly influence the material properties of aqueous polymer


systems. Even small amounts can lead to pronounced changes of the structuraland macroscopic properties. Often cross-linking of the polymer chains takes placeby the incorporation of the salt ions, whereby three-dimensional networks areformed. Systematic investigations of the effect of different salts on morphology,phase transition temperatures, water binding capability, as well as optical andrheological properties of thermotropic aqueous polymeric systems have been car-ried out by numerous authors in recent years. The influence of different salts andtheir concentration on the phase separation temperature of thermotropic aqueouspolymer systems was studied by Alexandridis et al. (55) as well as by other au-thors (56–58). For all investigated systems a linear shift of the phase separationtemperature dependent on the salt concentration was observed. Moreover, at agiven concentration different salts were found to shift the phase separation tem-peratures of the aqueous polymer systems according to their salting-in or salting-out strength, which is described by the so-called Hofmeister series. Salts with asalting-in phenomena lead to an increase of the phase separation temperature,whereas salts with a salting-out phenomena have the opposite effect. The influ-ence of LiCl on the water binding properties of an aqueous polyalkoxide system isreported in Reference 59. For four different LiCl contents the water binding prop-erties of the investigated polyalkoxide were determined by DSC measurements.The obtained results are shown in Figure 9. Each curve consists of three linearregions. At low water contents all water is so strongly bound to the polymer thatit cannot be frozen (nonfreezing bound water). Above the maximum content ofnonfreezing bound water additional water is more weakly bound to the polymer.This type of water freezes at low temperature (freezing bound water). Its meltingpoint and melting enthalpy is lower than that of pure water. Finally, at high watercontents the polymer is saturated with water and additional water builds a sepa-rate phase of free water. As can be seen in Figure 9, the addition of LiCl influencesthe water binding properties of the polyalkoxide. Proportional to the LiCl contentan increase of the non-freezing-bound water capacity of the polyalkoxide and anincrease of the binding enthalpy of freezing-bound water were observed. Both re-sults indicate that the interaction between water and polymeric system becomesstronger with increasing LiCl content. Reference 59 also reports an influence ofthe addition of LiCl on morphology, as well as optical and rheological proper-ties of the investigated polyalkoxide system. As an example Figure 10 shows thetemperature dependence of the optical transmission and of the dynamic viscosityof three samples with a fixed polyether/water mixing ratio of 4:1 and various LiClcontents of 0, 3, and 6 wt%. The observed changes of the optical transmission–temperature curves are caused by morphological changes through the addition ofLiCl. The pronounced viscosity changes, on the other hand, show the formation ofcross-linked structures, whereby in different phases gelation occurs in differentLiCl concentration ranges.

Hybrid Solar and Electrically Controlled Light Filters. Compared to elec-trochromic glazings, smart windows based on thermotropic hydrogels have theadvantages of lower costs and a much higher transparency in the clear state.However, for specific applications switching on demand is required. The construc-tion of a hybrid solar and electrically controlled transmission changing light filterbased on thermotropic hydrogels has been described (60). The described arrange-ment consists of a 2- or 3-mm-thick thermotropic hydrogel layer placed between


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

two tin-doped indium oxide (ITO) coated glass substrates, whereby the ITO layerswere placed either inside or outside the double glazing item. When heated pas-sively by solar energy or actively by applying an electric voltage on the ITO layersthe thermotropic hydrogel switches from the transparent into the light scatteringstate. Even for the arrangement where the ITO layers are in direct contact withthe hydrogel, no electrolysis was observed under the experimental conditions. Be-sides the position of the ITO layers, their thickness, the applied voltage as wellas the thermochromic hydrogel material used was varied and the switching timesof the glazing items were determined. Figure 11 shows the transmission–timecurves 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 trans-parency of the light filter by about 10% allow fast switching times in combinationwith high contrast ratios. An increase of the ITO layer thickness decreases thewattage, which is necessary to achieve a constant switching time. On the otherhand the transparency of the glazing item is thereby also reduced. For one of theinvestigated glazing items a transmission change from about 62% to ≤ 1% and aswitching 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 glazingitem and especially of the layer thickness as well as the material properties of theincorporated thermotropic hydrogel will lead to a further significant reduction ofthe required wattage.

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


Fig. 10. Temperature dependence of the optical transmission at 600 nm (d = 1 cm) and ofthe 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 mixingratio of 4:1 and various LiCl contents ranging from 0 to 6 wt%.

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


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 andthickness of the ITO layers = 133 nm. 8 V/4.8 W (A); 6 V/2.7 W (B); 4 V/1.2 W(C); 3 V/0.7 W (D); 2 V/0.3 W (E); 1 V/0.1 W (F).

earlier for phenol-substituted indicator dyes in a PVA/borax/surfactant gel net-work (32). As an example the UV–vis absorption spectra at two different tem-peratures and the transparency versus temperature curve of a hydrogel preparedby 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 displayedin Figures 12 and 13, respectively. Both effects, the thermotropic switching atabout 36◦C and the temperature dependent cross over between an absorptionband at λ = 408 nm and one at λ = 617 nm can be clearly seen. As a result ofthese effects the hydrogel changes its appearance from green transparent below33◦C, via yellow transparent until about 36◦C to yellow translucent above thistemperature. With increasing temperature the phenol–phenolate equilibrium ofBromothymol Blue in the polyalkoxide/LiCl/water system is shifted from the greencolored phenolate form to the yellow colored phenol form. This result is in contrastto the behavior of phenol-substituted indicator dyes in a PVA/borax/surfactant gelnetwork for which the opposite shift of the phenol–phenolate equilibrium withtemperature was observed. Thus it can be concluded that the reversible colorchange of indicator dyes in gel systems depends on the specific composition of thegel networks. However, the origin of this effect on a molecular level is still underdiscussion.

Another example of a color and transparency changing gel networkis reported in Reference 62. Starting from a Phenol Red containingPVA/borax/surfactant hydrogel which changes its color but not its transparencyin dependence on temperature, thermotropic properties were obtained by addingsmall amounts of a more hydrophobic polymer. For this purpose a polyalkoxidewas used. At a content of 0.8 wt% polyalkoxide thermotropic behavior based on aphase separation process was found to appear without affecting the color chang-ing behavior very much. With further increasing polyalkoxide content the degreeof the reduction of transparency with temperature is increased and the start-ing temperature of this process is shifted to lower temperatures. Moreover, the


Fig. 12. Transparency versus temperature curve of a Bromothymol Blue (BB) contain-ing 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 = yellowtransparent; 3 = yellow opaque.

influence of the concentration of the zwitterionic sulfobetaine surfactant on thecolor changing and thermotropic behavior was studied. PVA/borax/Phenol Red hy-drogels containing 1.1 wt% polyalkoxide and various sulfobetaine concentrationsbelow 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 temperaturedependence 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 composedof a polyalkoxide and an aqueous LiCl containing buffer solution in dependence on temper-ature (d = 0.1 cm). −5◦C (a); 38◦C (b).


to the phenol form and the second one at λ = 563 nm to the phenolate form ofPhenol Red. With increasing sulfobetaine concentration a decrease of the inten-sity of both UV–vis absorption bands occur. It is well known that above the cmcsurfactants can influence the UV–vis absorption behavior of water-soluble dyes.However, here this effect takes place at the lowest sulfobetaine concentration of2.5 × 10− 3 mol/kg, which is significantly below the cmc. Also, a small but signif-icant and reproducible influence of the surfactant concentration on thermotropicbehavior is reported even below the cmc. To explain this behavior the authorssuggested the formation of complexes between dye molecules and aggregates ofsulfobetaine, but also discussed an interaction of the dye with single sulfobetainemolecules as an alternative model.

Application of Thermochromic Polymers

The potential applications of thermochromic material include temperature tun-able light and heat radiation filters, large area displays, and temperature indica-tors visualizing the temperature of the surrounding medium or the temperatureprofile of a surface which is coated with the thermochromic material.

Materials changing their color with temperature are already used in a varietyof commercial products such as textiles, toys, baby spoons, coffee pots, aquariumthermometers, novelty items, pans with a thermospot, and labels for wine bottles,indicating the proper drinking temperature (63–65). However, the commercial-ization of thermochromic materials is just at the beginning and certainly thismarket will rapidly grow in the near future. Potential industrial and processingapplications are, eg, monitoring of overheating of machine parts, observation ofthermal leaks, and controlling processing temperature. In the field of storage andtransportation thermochromic materials with an irreversible switching could as-sure the observance of required temperature ranges. For example heat-sensitivematerials require a storage and transportation temperature below a maximumvalue and for frozen food the temperature has to kept below 0◦C. In medicaltechnology skin temperature indicators might find application for diagnostic pur-poses. Another big market addresses security aspects. Road signs with integratedthermochromic effects could warn of icy conditions. Thermochromic coatings ofheating plates, fire doors, radiators, and other parts of domestic appliances whichbecome 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 isactually in progress and in some cases prototypes have already been presented.

The interest in thermotropic materials on the other hand appears more be-cause of their optical properties than because of their ability to detect temperaturechanges. Such materials are optical shutters for light as well as for heat radia-tion. The major market for such materials arises from their potential applicationas functional layers in the construction of sun protecting glazing (66,67). In mod-ern architecture glazing has become a high-tech-component with a wide rangeof properties and functions. Besides the classic function as viewing window glaz-ing elements enable the use of daylight for the illumination of the building andthe passive use of the solar energy for space heating. To realize these functionsmore and more parts of the building facade are substituted by large area view


and illumination windows, skylights, and sunroofs. In many modern buildingsmore than 80% of the facade area consists of glazing elements. In buildings withlarge glazing areas the incident solar radiation plays an important role in energyperformance. While on cold days, such as during the winter, the incident solarradiation reduces the energy consumption for space heating, on hot days, such asduring the summer, solar radiation causes overheating inside the buildings andthus an enormous increase of the energy consumption for space cooling. To reducethis negative effect regulation of the incident solar radiation is necessary. Today,this is achieved by mechanical optical shutters like venetian blinds, awnings, andsun blinds. An innovative solution which is under development is the use of sunprotecting glazings whose optical properties can either be actively adjusted orautomatically adjust themselves according to changes of the climatic conditions.Thermotropic glazing in which a functional layer of a thermotropic material is in-corporated into the glazing assembly is the most promising type of sun protectionglazing and closest to reaching market readiness.

As early as 1950 sun protecting glazing based on a thermotropic gel networkwas set up and tested in the residence of the Munich zoo for a period of about 10years. This glazing assembly consists of a mixture of 5 wt% of poly(vinyl methylether) in agar-agar filled between a double glazing. However, the transmissionchange 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 a2-mm-thick thermotropic hydrogel layer at 20◦C (left side) and slightly above the switchingtemperature of 30◦C (right side).


years advanced prototypes of sun protecting glazing based on thermotropic hydro-gels with strongly improved optical properties have been presented by Chahroudi(49,50), Watanabe (53) and Seeboth (52). These glazings possess a transparencyof about 90% in the transparent state and can reach a transparency of less than10% in the translucent state. Moreover, the transparency change takes place ho-mogeneously without the appearance of streaks. As an example, the switching ofa 100 × 50 cm large prototype of a thermotropic glazing presented by Seeboth isshown in Figure 14. To commercialize thermochromic glazings based on hydrogelstechnology transfer from the laboratory scale to an industrial scale has to takeplace. Thus, for example, an automated production line for putting the polymerbetween the double glazing could be established.

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ARNO SEEBOTH

DETLEF LOTZSCH

Fraunhofer-Institut fur Angewandte Polymerforschung

thermochromic polymers

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