The Reverse Thermal Effect

Apicella et al. (1979) also assume that water within the polymer is divided between bound and free components that are associated with the polymeric chains and microcavities, respectively. They formulate a model for the growth of microcavities that enables them to explain the reverse thermal effect. 

Yet another explanation of the reverse thermal effect was offered by Suh et al. (2001) for a carbon/epoxy composite. They also consider that absorbed water consists of free and bound components, Mp and Mp, respectively. Similarly to previous considerations (Vieth and Sladek 1965 Vieth et al. 1976), the authors relate the bound water by means of Henry s law and the free water by means of Langmuir s law. They assume that the sorption of bound water is a reversible process and consider the uptake of free water to be irreversible - thereby not being squeezed out upon changes in hygrothermal conditions. It thus follows that... 

Fig. 27. Difference spectra due to reversible thermal effects only. Difference spectra of highly annealed PET film between elevated temperatures and 30 °C on the basis of one-to-one subtraction, from 1650 on"1 to 1320 cm"1...

Nie, G., and Aust, S. D., 1997, Effect of calcium on the reversible thermal activation of lignin peroxidase, Arch. Biochem. Biophys. 337 225n231. 

Useful insight into the mechanism of fluid ingress into polymers and polymeric composites can be gained by noting and interpreting the so-called reverse thermal effect. The earliest discovery of this phenomenon was made independently and interpreted differently (Apicella et al. 1979 Adamson 1980). 

Neither does the microbrownian motion of the amorphous mesh inhibit the liquid crystal phase, nor does the positional order of the molecules interfere with the elasticity. Hence, as a hybrid material that combines LC and rubber characteristics, LCEs have unique properties in which the molecular orientation of the liquid crystal is strongly correlated with the macroscopic shape (deformation) which is unparalleled to other materials. The most prominent example in the physical properties derived from this property is the huge thermal deformation. Figure 10.1 shows an example of the thermal deformation behavior of side-chain nematic elastomers (NE) [3]. When the molecules transform from the random orientation in the isotropic phase to the macroscopic planar orientation in the nematic phase, the rubber extends in the direction of the liquid crystal orientation and increases with decreasing temperature as a result of an increase in the degree of liquid crystal orientation. This thermal deformation behavior is reversible, and LCEs can be even considered as a shape-memory material. Figure 10.1 is from a report of the early research on thermal deformation of LCEs, and a strain of about 40 % was observed [3]. It is said that LCEs show the largest thermal effect of all materials, and it has been reported that the thermal deformation reaches about 400 % in a main-chain type NE [4]. 

The SPATE technique is based on measurement of the thermoelastic effect. Within the elastic range, a body subjected to tensile or compressive stresses experiences a reversible conversion between mechanical and thermal energy. Provided adiabatic conditions are maintained, the relationship between the reversible temperature change and the corresponding change in the sum of the principal stresses is linear and indipendent of the load frequency. 

From the colorless state it can be switched with light of short wavelength (A = 380 nm) via an electrocycHc ring opening and cis/trans rotation of one half of the molecule into a state with violet/purple color. The reverse reaction is effected by visible light (A = 580 nm). Since the system is metastable, one of the two reaction directions is matched by a rival thermal reaction, the thermoreversion. This progresses, however, in the case of benzospiropyran, at room temperature by a factor of 10 slower than the light-induced reaction. 

Since sulfoxides and sulfones are versatile synthetic intermediates, and since in both the thiolene oxide and dioxides the reverse dethionylation114 ( — SO), and cheletropic extrusion of sulfur dioxide296, respectively, readily take place thermally, these cycloadditions are expected to find a useful place in organic synthesis. It should be kept in mind, however, that the retrograde SO-diene reaction and interconversion of the thiolene oxides compete effectively against SO extrusion on heating, and that diene isomerization accompanies the forward reaction (SO + diene). 

From a process point of view, the direct neutralization is clearly preferred moreover, the product quality (color) and free oil content deteriorates with aging (Table lb). The fact that the free oil and the inorganic sulfate level increase simultaneously upon aging is due to the fact that the formation of p-sultones from olefins is a reversible reaction [28], in competition with thermal rearrangement to alkenesulfonic acid and y- and 8-sultone. The effects of the reverse reaction of p-sultones are less with AOS because the rearrangement rates of AO-derived sultones are higher [29,35]. 

The reverse process has also been examined. 2-Phenyloxazole is converted in a similar fashion to 3-phenyl-2//-azirine-2-carbaldehyde on irradiation in benzene or cyclohexane.128 Further rearrangement to the corresponding isoxazole can be effected thermally but not photochemically. A competing pathway leading to the formation of 4-phenyloxazole has also been observed and is thought to involve a bicyclic intermediate arising by 2,5-bonding. 

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