Photomechanical

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

Photomechanical stencils are of two types direct coatings and transfer films. Direct coatings are either bichromated gelatin or bichromated poly(vinyl alcohol) (PVA). The coated screens are exposed through a positive, washed, and inspected. These screens are used for printing electronic components. They are not practical for commercial work because of the difficulty of reclaiming the screen after use. 

Lichtdruck, m. photomechanical printing photographic printing, -gelatine, /. photoengraving gelatin. 

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

Taking advantage of the sensitivity to light shown by the diazoazoles allows them to be used in the photomechanical reproduction process to produce light sensitive layers or in photocopying processes as precursor of the azo dyes to be generated in situ. 

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... 

The azo-modified, elastin-like polypeptide XIV illustrated in Scheme 9 exhibits a so-called inverse temperature transition" that is, the compound gives cross-linked gels that remain swollen in water at temperature below 25 °C but deswell and contract upon a rise of temperature. The trans-cis photoisomerization of the azo units, obtained through alternating irradiation at 350 and 450 nm, permits photomodulation of the inverse temperature transition.[S9] The result indicates that attachment of a small proportion of azobenzene chromophores is sufficient to render inverse temperature transition of elastin-like polypeptides photoresponsive, and provides a route to protein-based polymeric materials capable of photomechanical transduction. 

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 ... 

Due to the photomechanical reproduction process, some of the pictures shown in this chapter are unfortunately of considerably poorer quality than the originals, so that they no longer show all the details which are clearly visible on the original exposures. Online documents are available at www.air-photo.com/. 

International Military Tribunal, Trial of the Major War Criminals, IMT, 42 vols., Nuremberg 1947 photomechanical reprint Delphin, Munich 1984... 

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. 

The photomechanical behavior of the networks was studied under constant stress (load) by following at constant temperature the length contraction as a function of the period of irradiation in the dark, length recovery takes place although at a lower rate... 

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%. 

In the poly(ethyl acrylate)rubbers described above spirobenzopyran and aromatic azo chromophores were incorporated in the cross-links between the polyacrylic chains. In contrast, Matejika and al. studied cross-linked systems with azo side groups they considered especially the relative importance of the thermal effects occurring during photomechanical conversion. Most interesting are their results on copolymers ccmtaining low concentrations of chromophore (for which heat effects are less important), namely copolymers of )8-hydroxyethyl methacrylate (0.99) and azonaphthol methacrylate (XII) (0.01) swollen in water... 

From a similar point of view Osada and Katsumura examined the photomechanical energy conversion of a polyamide containing a stilbene diromophore in the backbone (XVIII) ... 

Arakawa K, Takahashi K (1987) In Proc. Int. Conf. Photomechanics and Speckle Metrology, San Diego... 

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... 

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