Isothermal phase transitions

The specific heat is constant for each stream (or if either stream undergoes an isothermal phase transition). 

Structural polymorphism has been already reported as a peculiar solid-solid phase transition with a large spectral shift in the cast film of CgAzoCioN+ Br (chapter 4). The type 1 spectrum was thermally transformed to the type VI spectrum and then backed to the type I by the isothermal moisture treatment. The reversible spectral change between the type I and VI is a good experimental evidence of Okuyama s prediction on the molecular packing. Since the type VI state is assumed to be a metastable state, the isothermal phase transition to the type I state is expected to be induced by some external stimuli. Water molecules adsorbed to cast bilayer films might act as an accelerator of the phase transition. 

Organic compounds which show reversible color change by a photochemical reaction are potentially applicable to optical switching and/or memory materials. Azobenzenes and its derivatives are one of the most suitable candidates of photochemical switching molecular devices because of their well characterized photochromic behavior attributed to trans-cis photoisomerization reaction. Many works on photochromism of azobenzenes in monolayers LB films, and bilayer membranes, have been reported. Photochemical isomerization reaction of the azobenzene chromophore is well known to trigger phase transitions of liquid crystals [29-31]. Recently we have found the isothermal phase transition from the state VI to the state I of the cast film of CgAzoCioN+ Br induced by photoirradiation [32]. 

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. 

Fig. la, b. Schematic illustrations of chemical-induced phase transition of a polymer system. Thermal phase transition from state X to Y in the presence and absence of chemicals (a), and isothermal phase transition by chemicals (b)... 

Fig. 6. Isothermal phase transitions from the phase separated to homogeneous state of the aqueous solution of the copolymer containing 11.6 mol % pendant crown ether groups by the addition of (O) potassium and (9) sodium chlorides at 32 °C and 37 °C. Polymer concentration was 1 mass %...

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. 

Tkeda, T., Horiuchi, S., Karanjit, D. B., Kurihara, S., and Tazuke, S. Photochcmically induced isothermal phase transition in polymer liquid crystals with mesogenic phenyl K nzoate side chains. 1. Calorimetric studies and order parameters. Macromolecules 23. 16 (1990). 

Pressure-area ( r(A)) isotherms Phase transitions, packing densities, compressibilities, thermodynamic cheiracteristics. Molecular Interpretation very limited. 

Most of the liquid crystalline main-chain polymers synthesized are polyesters. For the present study where isothermal phase transition kinetics studies should be performed, this was a less suitable alternative since they undergo thermally induced reactions, i. e. transesterifications, and for this reason instead polyethers were chosen. Polyethers also have other advantages like lower transition temperatures and higher solubilities. 

S. Kurihara, T. Ikeda, and S. Tazuke, Photochemically induced isothermal phase transition in liquid crystals. Effect of interaction of photoresponsive molecules with matrix mesogens, Mol. Cryst. Liq. Cryst. 178, 117-132(1990). 

S. Kurihara, T. Ikeda, T. Sasaki, H.-B. Kim, and S. Tazuke, Tune-resolved observation of isothermal phase transition of liquid crystals triggered by photochemical reaction of dopant, J. Chem. Soc., Chem. Commun. 1990, 1751-1752. 

Photochemically induced isothermal phase transition behaviors, Macromolecules 23, 42-48... 

Each stream has a constant specific heat. Isothermal phase transitions (corresponding to an effective specific heat of infinity) satisfy this requirement and, in fact, simplify the solution. 

Ikeda T, Horiuchi S, Karanjit DB, Kurihara S, Tazuke S. 1990. Photochemically induced isothermal phase transition in pol5uner liquid crystals with mesogenic phenyl benzoate... 

The phototuning of BPs can also be fabricated in a pure material system [147]. Das et al. reported a light-induced stable blue phase in photoresponsive diphen-ylbutadiene based mesogen 37. This compound was found to exhibit SmA and N during heating. When the temperature was kept at 118 °C, the photoisomerization induced an isothermal phase transition from SmA to N. Photoirradiation of the SmA film held at a higher temperature (124 °C) for 100 s resulted in transition to a phase with a characteristic classical BP texture showing in Fig. 5.30. The BP was thermodynamically stable and could be maintained at this state for several hours. The characteristic sharp reflection bands compared to the rather broad reflection bands observed for the chiral nematic phase confirmed the formation of BP. The photoinduced formation of the BP exhibited a reflection centered at 510 nm. Subsequent irradiation led to the blue shift to 480 nm in the reflection band. 

Fig. 5.30 Photoinduced isothermal phase transition from SmA to BP at 124 °C upon irradiation with 360 nm light. Reproduced with permission from [147]. Cop5fright 2010 Royal Society of Chemistry...

Tamaoki and coworkers reported some chiral azobenzenes and diphenylbutadiene-based LC dimers 13-15 exhibiting light-induced phase transition behavior starting from smectic phases [26, 51-55]. For example, dimers 15, which were synthesized by connecting a cholesteryl group with 1,4-diphenylbutadiene unit, were found to experience isothermal phase transition from SmA to N and further to the isotropic state upon UV light irradiation at 366 nm [26]. 

Very recently, chiral 1,4-diphenylbutadiene-based mesogens 16 have been found to show isothermal phase transition behavior from the SmA phase [56]. As shown in Figure 5.13, the thin film of 16 exhibited an SmA phase with a focal conic texture at 118 °C, and the photoirradiation with 360 nm light resulted in the disappearance of the focal conic texture accompanied with the appearance of an oily streak texture, indicating the phase transition from SmA to N (Fig. 5.13). Upon further irradiation, the complete conversion was observed. 

FIGURE 5.13 Photo-induced isothermal phase transition of 16 at 118 °C upon irradiation with 360 nm light for different time (a) 0 s, (b) 50 s, (c) 170 s, (d) 370 s. Reprinted with permission from Reference 56. Copyright 2010 Royal Society of Chemistry. 

Fig. 8.20 Schematic diagram for a Carnot cycle utilizing a pure polymer as the working substance, (a) Polymer always in amorphous state (b) intervention of an isothermal phase transition.

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