Phase transition, photostimulated

Several attempts have been made to construct stimuli-responsive polymer solution and gel systems which undergo isothermal phase transitions by external stimulation, such as photons or chemicals. Aqueous solutions of poly(A -isopropylacrylamide) having photoisomerizable chromophores or host molecules in the pendant groups showed reversible phase separations by photoirradiation or by the addition of specific metal or ammonium ions. The gels made of the polymers also underwent photostimulated or chemical-induced volume phase transitions. 

Photostimulated Volume Phase Transition of PNIPAM Gels. 59... 

PNIPAM containing pendant triphenylmethane leuconitrile groups also showed photostimulated phase transition from the phase separated state to the homogeneous state [13]. The triphenylmethane leuconitrile group is known to change the polarity more pronouncedly than the azobenzene chromophore by the ionic photodissociation of C-CN bond [14]. 

The concept of photostimulated phase separation can be applied to construct chemical-induced phase transition systems, which change the conformation reversibly in response to special chemicals. For the systems, host molecules are used as the receptor groups instead of photoisomerizable chromophores. Host molecules, such as crown ethers or cyclodextrins, are known to change the property by capturing guest chemicals in their cavity [16]. We employed benzo[l 8]crown-6 as the receptor molecule and incorporated it into the pendant groups of PNIPAM. 

The concept of photostimulated and chemical-induced phase transitions described in this chapter can be applied to other systems, such as proton-induced or electrochemical phase transitions. All of these systems have high energy conversion efficiency in principle and the stimuli-responsive polymer systems are sensitive. A serious disadvantage of these systems is that the working temperature is limited to a rather narrow range near the phase transition of the parent polymer. This point should be kept in mind when used in practice. 

In the first part of this article, the fundamental idea for molecular design of photoresponsive polymers is described. The second part deals with several examples of photostimulated property changes of the photoresponsive polymers. In the third part, photostimulated phase transitions of photoresponsive polymer systems are discussed. 

Figure 29 shows a schematic illustration of the photostimulated phase transition from the state X to the state Y. When the photochromic chromophores in the polymer... 

S. G. Odulov, Yu. A. Reznikov et al., Photostimulated Cange of Phase-Transition Temperature and Giant Optical Nonlinearity of Liquid Ci ystals, Sov. Phys. JEPT (1983) 224... 

Photostimulated change of phase transition temperature and giant optical nonlinearity of liquid crystals... 

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