Photomechanical effect

In alkali halide crystals containing color-centers (F-centers) illumination with light of appropriate energy causes transient changes of hardness (Nadeau, 1964). This effect apparently results from changes in the effective sizes of the F-centers when they become excited. 

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Applications. Most of the stilbene-based polyquiaolines display photoresponsive (98) and photomechanical effects as manifested by a contraction ia polymer film samples upon kradiation. 

J. S. Nadeau, Two Photomechanical Effects in Alkah Hahde Crystals, Jour. Appl. Phys., 35, 669 (1964). 

Some fine review articles dealing with various aspects of photochromic polymers are reported in the literature,11-91 and several photoresponse effects have been described. These include light-induced conformational changes, photostimulated variations of viscosity and solubility, photocontrol of membrane functions, and photomechanical effects. Here we provide an overview of the photoresponse effects... 

Menzel1811 more recently described the properties and behavior of monolayers prepared from azobenzene-containing poly(L-glutamate)s possessing the structures XIX and XX (n= 2) shown in Scheme 11. These monolayers showed photomechanical effects opposite to those described above for azo-modified poly(L-lysine)s. In fact, they expanded when exposed to UV light (trans—>tis isomerization), and shrank when exposed to visible light (ds— trans isomerization). The expansion was found to be smaller than expected from comparison of the monolayer isotherms obtained from irradiated and nonirradiated solutions. This was attributed to the trans-cis photoconversion of the azo units, which occurs with lower yields in monolayers... 

Photomechanical effects have been also observed in monolayers obtained from poly(L-glutamic acid) modified with carbocyanine1831 and spiropyran dyes.184 In the latter case, irradiation at 254 nm produced changes in the molecular conformation, which in turn caused photomodulation of the surface pressure and surface area of the films. From all these examples, it appears that photoresponsive monolayers are quite fascinating systems, which may eventually come to be regarded as a machine to transform light into mechanical energy . 81 ... 

Yager K, Tanchak O, Godbout C, Fritzsche H, Barrett C. (2006) Photomechanical effects in azo-polymers studied by neutron reflectometry. Macromolecules 39 9311-9319... 

In this chapter we will consider to which extent conformational changes of the chromophores and connected chain segments of a photochromic polymer may induce changes of dimension of bulk polymers and thus generate reversible photomechanical effects. 

Upon irradiation trans -> ds isomerization causes conformational changes of adjacent network segments which are considered to be responsible for the photomechanical effect. The observed contraction is however small and amounts only to about 0.15-0.25%. 

The corresponding fatty acid could not be photoisomerized in the LBK film. By attaching the azobenzene chromophore to the hydrophilic backbone, however, the free volume in LBK films was increased and photoisomerization was possible (i.e., 50 to 70% cts-isomer compared to 0% for the nontethered azobenzene amphiphile and 90% cis-isomer in solution). However, concomitant with the increased free volume, there is a decrease in the orientational order of the chromophores. These polymers have been widely used as command surfaces to control the orientation of liquid crystals and to investigate the photomechanical effect. ... 

Another class of polymers equipped with azobenzene moieties comprises a-helical polypeptides, in particular pQly(L-glutamate)s and poIy(L-lysine)s. In solution, these azobenzene-modified polypeptides can undergo photoinduced helix-coil transitions. Polypeptides partially (30 to 50%) substituted with azobenzene moieties are surface active and form stable monolayers. Because of the partial substitution, there is sufficient free volume, and the azobenzene moieties can be isomerized in the monolayer. The photoisomerization changes the area per molecule, and the monolayer shows a photomechanical effect. LBK films of a photosensitive poly(L-lysine) with 31 mol... 

Menzel, H. Langmuir-Blodgett films of photochromic polyglutamates. 7. The photomechanical effect in monolayers of polyglutamates with azobenzene moieties in the side chains. Macromol. Chem. Phys., 195, 3747 (1994). 

Seki, T., and Tamaki, T. Photomechanical effect in monolayers of azobenzene side ch.iin polymers. Chem. Lett., 1739 (1993). 

Fig. 14. Schematic representation of the photomechanical effect induced in poly(ethyl acrylate) network with azoaromatic cross-links upon irradiation [35]...

The photoinduced deformation phenomenon of materials is called a photomechanical effect, and it has been so far reported for photoresponsive polymer films and gels [35-43]. When azobenzene is isomerized from the trans form to the cis form, the length of the molecule is shortened from 0.90 to 0.55 nm. The size change of the molecule on photoirradiation is expected to alter the shape of the polymers which contain the azobenzene molecules. However, it is not the case in polymer systems. The transformation in polymer films does not change the polymer shape because of the large free volumes of the polymer bulk. Suitable organization or assembly of the molecules is required for the photoinduced deformation of materials. 

Smets, G., Braeken, J. and Irie, M. (1978) Photomechanical effects in photochromic systems. Pure Appl. Chem., 50, 845—856. 

Eisenbach, C.D. (1980) Isomerization of aromatic azo chromophores in poly(ethyl acrylate) networks and photomechanical effect. Polymer, 21, 1175—1179. 

Matejka, L., Ilavsk,y M., Dusek, K. and Wichterle, O. (1981) Photomechanical effects in crosslinked photochromic polymers. Polymer, 22, 1511-1515. 

Generally, peak pressure amplitudes range from a few MPa at the ablation threshold up to several hundred MPa at high laser fluences [104-108]. During propagation through the substrate, these high-amplitude waves may induce structural modifications at areas away from the ablation spot. Thus, in contrast to the photochemical effects which are confined to the laser-irradiated area, the photomechanical effects of UV ablation can be much more delocalized. 

Aside from the mechanistic implications, the previous studies demonstrate that sequential holographic methodology is a powerful means for the examination of photomechanical effects over extended areas of the irradiated objects, and can be used for the on-line monitoring of UV laser processing schemes. Furthermore, the versatility of the technique allows its adaptation to the different detection requirements posed by the diverse types of processed objects. For instance, the technique can be directly employed in a reflection mode for the characterization of the effects in nontransparent... 

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