Photo mechanism

The congruence of the handwritten notes on the photocopy of the original letter and on the carbon copy suggests that these notes were added photo-mechanically or in some other way. If this is correct, it would be another proof of forgery. 

The carbon copy bears the same notes at precisely the same places as the original letter. This is not only unusual, but also an impossible feat of handwriting. At least on the carbon copy, the notes can only have been added photo-mechanically. This too would indicate a forgery. 

Because of limitation of this chapter, only a few properties of photoresponsive polymers are described. Table II includes several other properties so far reported(27-50). All of these physical and chemical properties are found to be controlled reversibly by photoirradiation. It is now generally accepted that photochromic reactions are useful as a tool to photo-control the properties of synthetic polymers. The photoresponsive polymers have potential applications for many photoactive devices, such as sensors, switches, memories, photo-mechanical transducers and so on. 

Mention could be made of many debts to photography, including the discovery of radioactivity, which was directly due to the sensitiveness of the photographic plate, the discovery of stars too faint to be observed by the most powerful telescopes, the invention of photo-mechanical processes employed iu illustrating books and journals, photomicrography and its numerous applications, and the cinematograph, apart from its everyday uses in peace and war. 

Cviklinski J, Tajbakhsh AR, Terentjev EM. 2002. UV isomerisation in nematic elastomers as a route to photo mechanical transducer. Eur Phys J E 9 427 434. 

Kim HK, Wang XS, Fujita Y, Sudo A, Nishida H, Fujii M, Endo T. 2005a. Photo mechanical switching behavior of semi interpenetrating polymer network consisting of azobenzene carrying crosslinked poly(vinyl ether) and polycarbonate. Macromol Rapid Commun 26 1032 1036. 

The present report describes the observation and measurement of the photo-optical and photo-mechanical responses on polyurethane-acrylate block copolymers in which a photochrome (spiropyrane or w-spiropyrane) has been incorporated in the form of a simple solid solution or chemically bonded. 

A graphical representation for the measurements of the photo-mechanical response is shown in Figure 5. Upon irradiation, the initial stress applied to the sample decreases indicating an expansion of the sample, while in the dark, length recovery takes place by a subsequent contraction indicated by an increase in stress, the process can be repeated many times. Blair and Pogue, reported a photoexpansion in samples with photochromic material. 

It can be seen also that the higher the acrylate content the larger the photo-mechanical response. The acrylate is a rigid homopolymer, so can transmit the work more easy than the polyurethane (soft material), which can readily absorb the work applied by the photochromic... 

As a control, polyurethane-acrylate copolymers without photo-chromic agents were subjected to the same photo-mechanical test. No significant photomechanical response was observed. 

In general, it was observed that the higher the acrylate content of the copolymer, the more photo-response is observed. Rigid homopolymers can easier transmit the work than soft materials. It was also demonstrated that the spatial disruption generated by the photo-transformation of the photochrome is not the main driving force for the photo-mechanical and photo-optical effects. It is important to take into account that stereo-electronic interactions between the polymer matrix and the photochrome exist, mainly for the merocyanine form. 

From this study, it was concluded that an ideal photochromic spiropyrane molecule for inducing a large photo-mechanical effect, would be one that exhibits high spatial disruption, form strong dipoles in the merocyanine form, and is preferably bonded to the polymer backbone. 

The response prediction can be done effortlessly by computer modelling in order to establish the properties of the molecule in both forms (spiro and merocyanine). It also can help to understand, predict and even anticipate the photo-mechanical behavior of such compounds in a determinated polymer matrix. 

Figure 5.21. Photo-mechanical and photo-chemical transduction in a model protein that contains a proto-natable glutamic acid residue and a chromophore capable of being driven from trans to cis on absorption of 300 nm light, both existing within the same hydrophobic association domain. (A) Ultraviolet absorption spectra showing the changes in the spectrum on going from 100% trans to 90% cis on illumination with 300 nm light. (B) Light-driven shift of isomer from trans to cis increases the value of T,. (C) Light-driven shift of isomer from trans to cis lowers the pKa of Glu carboxy-late. (From Heimbach. ...

Previously, all the stencils were prepared manually but, now, indirect photo-mechanical processing and photocomposition are widely used. The information is entered into a computer, which arranges it in an appropriate layout and exhibits it on a video display terminal from which it is photographed to make the stencils (Nethercott and Nosal 1986). 

Photo-mechanical setup of the digital image correlation. 

Barrett C. J., Mamiya)., Yanger K. G., and Ikeda T, Photo-mechanical eifects in azobenzene-containing soft materials. Soft Matter, 3, 1249-1261... 

The photo-deformable gels have potential applications not only to photo-mechanical conversion but also to photo-driven molecular valve... 

On the other hand, LSCEs that contain isomerisable azobenzenes as light-sensitive molecules can also undergo anisotropic macroscopic deformations upon exposure to non-polarised light of the appropriate wavelength (i.e. photomechanical effect, Fig. 18.3b). Photo-mechanical effect in nematic LSCEs was first observed by Finkelmann et al. (2001) and, since then, it has been deeply investigated from both the theoretical and the experimental point of view. 

Fig. 18.3 Thermo- (a) and photo-mechanical (b) effect in LSCEs. Representative opto-mechanical experiment (c) in nematic liquid single crystal elastomers increase of the internal stress generated in the network upon irradiation with UV light (c, left) and decrease of the stress with the time in the dark at a constant temperature, T (c, right)...

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