Polymers thermochromic properties

The first soluble doped polymer was the polyfp-phenylene. sulphide) with AsFs- in AsFs [44]. Even if a quite unusual solvent was needed, it was the first to show the possibility of having a solution of doped polymers. Later, the discovery that Poly-n-alkylthio-phene was soluble in a common organic solvent [45], provided a new route. Besides, the films show thermochromism properties that varj as a flinction of the chemical structure of the side groups [8]. The stability and the great variety of substituents that can be added at the thiophene ring ensure that many structural studies are still being carried out [46]. 

Liquid Crystailine Polymers with Thermochromic Properties. In... 

The recent interest in substituted silane polymers has resulted in a number of theoretical (15-19) and spectroscopic (19-21) studies. Most of the theoretical studies have assumed an all-trans planar zig-zag backbone conformation for computational simplicity. However, early PES studies of a number of short chain silicon catenates strongly suggested that the electronic properties may also depend on the conformation of the silicon backbone (22). This was recently confirmed by spectroscopic studies of poly(di-n-hexylsilane) in the solid state (23-26). Complementary studies in solution have suggested that conformational changes in the polysilane backbone may also be responsible for the unusual thermochromic behavior of many derivatives (27,28). In order to avoid the additional complexities associated with this thermochromism and possible aggregation effects at low temperatures, we have limited this report to polymer solutions at room temperature. 

Figure 9 Thermochromic UV properties of (a) end-graft polysilane 79, (b) dilute solution, and (c) film analogs 80.65 Reprinted with permission from Ebata, K. Furukawa, K. Matsumoto, N. Fujiki, M. Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.) 1999, 40, 157-158, 1999 American Chemical Society.

Abstract We describe mechanochromic and thermochromic photoluminescent liquid crystals. In particular, mechanochromic photoluminescent liquid crystals found recently, which are new stimuli-responsive materials are reported. For example, photoluminescent liquid crystals having bulky dendritic moieties with long alkyl chains change their photoluminescent colors by mechanical stimuli associated with isothermal phase transitions. The photoluminescent properties of molecular assemblies depend on their assembled structures. Therefore, controlling the structures of molecular assemblies with external stimuli leads to the development of stimuli-responsive luminescent materials. Mechanochromic photoluminescent properties are also observed for a photoluminescent metallomesogen and a liquid-crystalline polymer. We also show thermochromic photoluminescent liquid crystals based on origo-(/ -phenylenevinylene) and anthracene moieties and a thermochromic photoluminescent metallocomplex. 

Typically, polygermane-polysilane copolymers have similar UV properties as compared with polygermane and polysilane homopolymers . The thermochromic behaviour of the copolymers depends on their monomer-to-monomer ratios in the polymer chain and on the identity of the alkyl substituents. In principle, the fine tuning of the absorption maximum can be achieved by a controlled copolymerization. Unfortunately, the copolymers reported so far often have relatively low molecular weights, which in turn results in the kmax values being even lower than those observed for high molecular weight polysilanes (Table 4). 

In this chapter, the theory of conformation-dependent polymer-solvent interactions, which was developed in detail by Schweizer (20-22) for soluble TT-conjugated polymers, will be used to explain both qualitatively and quantitatively a large body of observations on the polysilylenes (23, 24). The same theory has been used recently to interpret qualitatively order-disorder phenomena and the electronic thermochromism of TT-conjugated-polymer solutions and films (25, 26). The study presented in this chapter represents part of an ongoing effort to understand in a unified fashion both the optical properties (27-30) and order-disorder transitions (20-24) of flexible, conjugated-polymer solutions. 

The second volume of this new treatise is focused on the physicochemical properties and photochromic behavior of the best known systems. We have included chapters on the most appropriate physicochemical methods by which photochromic substances can be studied (spectrokinetic studies on photostationary states, Raman spectroscopy, electron paramagnetic resonance, chemical computations and molecular modeling, and X-ray diffraction analysis). In addition, special topics such as interactions between photochromic compounds and polymer matrices, photodegradation mechanisms, and potential biological applications have been treated. A final chapter on thermochromic materials is included to emphasize the chemical similarities between photochromic and thermochromic materials. In general, the literature cited within the chapters covers publications through 1995. However, in several cases, publications from as late as 1997 are included. 

PolyTCDU (X = C5H5) can be synthesized so that the red, high-temperature phase is stable at room temperature (12). Conversion to the blue phase can be achieved at low temperatures (at least partially) (13) or under a strain field (14). The optical properties of the red aTi blue phases of polyTCDU are nearly identical to those of polyETCD and polylUPDO (9,J ). The X-ray structure of polyTCDU (2 ) suggested that thTs polymer existed in the butatriene conformation, -(R)C C=C=C(R)-, rather than the commonly observed (and theoretically more stable) acetylenic conformation, =(R)C-C5C-C(R)=. This led to the suggestion that thermochromism involved an acetylenic-to-butatrienic conformational change (10). 

The optical properties of conjugated polymers are a direct consequence of the electronic structure, which is determined both by the chemical and the geometrical structure of the polymer. As the thermochromic transition is reversible, we do not expect to find covalent bonds formed, or destroyed, in the thermochromic transition. It is rather the geometry that is the candidate for changing the electronic structure of the polymer in a reversible manner. 

Summarizing, thermochromism only occurs above the glass temperature in amorphous polymer solids, and above the melting temperature for crystalline polymers [45], The crystallization of side chains, which may or may not occur, is not a necessary condition for the thermochromic transition. A more crucial requirement for thermochromism is that of the necessary level of regioregularity of the polymer. This will help define the local geometry of the polymer, which is crucial for the optical properties. Long-range order, like that found in crystalline phases, is not necessary for expression of the chromatic transitions. 

The synthesis of substituted polythiophenes for investigations of thermochromism has brought many examples of how the regioregular side chain substitution is central to the chromic phenomena. Some of these materials show two-phase behaviour with an isobestic point in the sequence of the spectra. As the optical absorption only reflects local properties, we have to understand what mechanism it is that will always keep the material in just two states, with no intermediate states. As the torsion of the main chain is the cause of the chromic behaviour, we have to understand more specifically what is the essential physics of a cooperative phase transition which takes the chain, or parts of the polymer chain, from one phase to the other. In many ways this might look like the falling of a row of dominoes, upon one fluctuation of one of the dominoes. In our case this would be... 

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