Polymer film, isomerization

The conductivity of many metal modified systems is reportedly enhanced due to various factors such as charge transfer between metal ions and the electron-rich heteroatoms, elimination of impurities, and changes in the transport number of cations and anions due to environmental changes in the solid electrolytes. Even interesting cases have been reported where a polymer film can reach the electronically conducting metallic level by cis-trans isomerization. 

In a polar polymer, i.e., cellulose acetate (CA) or nitrocellulose (NC) 35E, 35Z, and 36 had a relatively longer absorption maximum wavelength than in less polar matrices. In NC the of 36 shifts to 528 nm, which is also longer than in organic solvents. The role of polymer films in the quantum yields of photoreactions is not clear. In a comparison of the photochemical properties of 35 in polymer films and in solvents, it was found that the E c in polymer matrices was substantially smaller than that in the corresponding solvent with similar polarity. However, the decoloration quantum yield Oc e in a polymer film was larger than that in solvents. In conclusion, the polymer matrix properties, such as polarity, viscosity, and glass transition temperature (Tg) are quite important for photochromic reactions and applications. The coloration, E — Z and Z —> E isomerizations were suppressed in polymer matrices. 

Positional isomerism is not generally an important issue in syntheses of polymers with backbones which do not consist exclusively of enchained carbons. This is because the monomers which form macromolecules such as poly(ethylcne terephthalate) (1-5) or nylon-6,6 (1-6) are chosen so as to produce symmetrical polymeric structures which facilitate the crystallization needed for many applications of these particular polymers. Positional isomerism can be introduced into such macromoicculcs by using unsymmetrical monomers like 1,2-propylene glycol (4-8), for example. This is what is done in the synthesis of some film-forming polymers like alkyds (Section 5.4.2) in which crystallization is undesirable. 

Polymer films have been obtained by plasma polymerization of hexafluorobenzene, N-vinylpyrrolidine, and chloracrylonitrile (Munro). Higuchi et al. have shown that irradiation of an azobenzene-modified poly(Y-methyl-L-glutamate-CO-L-glutamic acid) in bilayer membrane vesicles of distearyldimethylammonium chloride leads to trans-cis isomerization of the polymer this leads to transfer of the polypeptide from the hydrophobic bilayer membrane interior to the hydrophilic surface. As a result, there was a decrease in the ion permeability through the bilayer membrane and the formation of intervesicular adhesion. Eisner and Ritter have prepared photosensitive membranes from an aromatic polyamide and a cinnamate that incorporates a liquid crystalline component. 

These studies confirm that the BEM groups move together with the DRIM groups, and the kinetics of the process depends on copolymer composition. While the photoorientation and relaxation of the azo groups followed the already established biexponential behavior, the BEM groups photo-induced orientation could be described by a single exponential, and they did not relax. This suggests that the cis-trans isomerization that happens when the pump beam is turned off is the most important relaxation process in all polymer films. 

FIG. 13.16 Schematic repreaemation of the mechanism of surface gratings formation, involving pressure diertts. P is the pressure, I represents the iight intensity and isomerization, and x is the place on the polymer film. 

FIG. 14.8 Diffraction efficiency versus time while an SRG was being inscribed on a spin-coated side-chain azobenzene (CH-IA-CA) polymer film.The efficiency is seen to decrease when the film is coated vdth an ELBL film of poly(diallyl dimeti iammpnium chloride) (PDAC) alternated with suifonated polystyrene (SPS), whose molecules do not undergo transtrans isomerization. For an ELBL with 20 biiay-ers, the efficiency dropped considerably, and practically no SRG could be inscribed. From reference 55. 

Among these systems, polyimide with azobenzene chromophores in the backbone was first studied by Agolini and Gay in 1970 [30]. They reported 0.5% contraction of the polymer film at 200 °C taking place under ultraviolet irr liation. No correlation, however, was reported between the degree of isomerization and film contraction. At the high temperature they studied, the cis isomer content in the photo-stationary state would be very low, because a rapid thermal reverse reaction from the cis to the trans form should occur. Thus, the isomerization is not the sole origin of the contraction of the system. 

The presence of the p-methoxycinnamate moiety allowed the films of these polymers to be probed spectrophotometrically. The surprising difference in the solution vs film spectra of the l.c. polymers was accounted for by the unexpected intramolecular perturbation of the p-methoxycinnamate moiety by the phenyl ester group. The unexpected spectral changes during UV irradiation of the l.c. polymer films could be attributed to conformational changes, isomerization, and cyclobutane formation. 

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. 

The photochemical cis-trans isomerization of the azochromo-phore can be achieved by irradiation at the maximum absorption band of the trans isomer at A=378 nm the change in the absorption spectra of a polymer film before and after irradiation is given in Fig. 8. Similar fractions of cis-isomers can be obtained in samples quenched from the melt (i.e.,ofrom above the melting temperature T of the soft phase T =45 C) and in samples annealed at temperature some degrees below of the soft phase, by which some soft segment crystallization was caused. 

The reverse isomerization, viz. I —> II, occurs under photolytic conditions when I is embedded in a polymer film. A pseudovotation involving Ru-Ru bond cleavage is believed to bring about the rearrangement. The apparent conflict between the thermal and photolytic processes is probably a result of heterolytic and homolytic Ru-Ru bond fission brought about by the different initiation techniques. 

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