Azobenzene-modified polymers

Suzuki I, Sato K, Koga M, Chen Q, Anzai J I. 2003. Polyelectrolyte layered assemblies containing azobenzene modified polymer and anionic cyclodextrins. Mater Sci Eng C 23(5) 579 583. 

ASSOCIATION BETWEEN AZOBENZENE-MODIFIED POLYMERS AND SURFACTANTS OR NANOPARTICLES TO AMPLIFY MACROSCOPIC PHOTOTRANSITIONS IN... 

CHAPTER ASSOCIATION BETWEEN AZOBENZENE MODIFIED POLYMERS... 

Figure 7.4. Associative phase separation of complexes between surfactants and azobenzene-modified polymers. See page 251-253 for text discussion of this figure.

Surface gratings have been generated in various azobenzene-modified polymers epoxy polymers, polyacrylates, polyesters, conjugated polymers, poly(4-phenylazophenol), and cellulose [54-56]. 

The formation of responsive clusters of macromolecular chains, or gels of interconnected chains is achieved using hydrophobically modified polymers in presence of various additives, such as micelles of surfactants, CDs or proteins which bind to the hydrophobic side groups of the polymers. Hydrophobically modified polymers show the strong vlscoslflcatlon and high sensitivity to external stimuli. The use of mixtures of azobenzene-modified polymers with CD polymers allows to achieve photoviscosity swings by ca a decade at submillimolar concentrations of azo dyes [71]. 

Inclusion properties of molecular nanotubes composed of crosslinked a-cyclodextrin was investigated [47], Induced circular dichroism was used to probe the formation and dissociation of complexes between the nanotubes and azobenzene modified polyethylene glycol), either with or without a hydrophobic alkyl chain. The inclusion complex between the nanotubes and polymers formed at room temperature, and the polymers dissociated from the nanotubes with increasing temperature. 

Poly(L-lysine) containing azobenzene units linked to the side chains by means of a sulfonamide function (Scheme 4, Structure VI), was obtained by treating poly(L-lysine) with p-phenylazobenzenesulfonyl chloride. The poly(a-amino acid) was modified quantitatively conversion to the azo-lysine units of VI was effectively 100%. The azo-modified polypeptide was soluble in HFP, in which it exhibited an intense photochromism attributed to the trans-cis photoisomerization of the azobenzene units. Like other sulfonated azobenzene compounds, 33 azosulfonyl-modified polymers of L-lysine were found to be very stable in their tis form, and no thermal decay was observed at room temperature over periods of times as long as several weeks. Interconversion between the two forms at room temperature could only be effected by irradiation at appropriate wavelengths. This behavior allowed the authors to purify the trans and cis forms of the model compound NE-azobenzenesulfonyl-L-lysine (VII) by chromatography, and to measure the absorption spectra of the two pure photoisomers. 

A more remarkable and dramatic change in the conformation of helical polymers upon photoirradiation has been observed for the azobenzene-modified poly isocyanates. Polyisocyanates are typical dynamic helical polymers [59,60]. Even the optically inactive poly(n-hexyl isocyanate) (20), which is devoid of stereogenic centers, exists as an equal mixture of right- and left-handed helical conformations. Equilibrium exists in solution between both helices separated by... 

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. 

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... 

Figure 7.2. Photoswitch of the solubility of chains, (a) Schematic drawing of the phototriggered coiiapse and aggregation of azobenzene-containing polymers in poor soivent conditions or ciose to iow critical solubility temperature (LCST). (b) Typical variation of the radius of the chains as a function of solvent parameter, or temperature in the case of chains having a LCST in water. Bold line parent chain with no azobenzene dashed and dot-dashed lines azo-modified chains, respectively, exposed to UV and dark-adapted.

Figure 7.6. Indices of association of azobenzene onto hydrophobic domains, (a) Representative spectra of an azo-modified polymer in water (under blue exposure) with increasing concentration of surfactant Cl 2E4 (0-0.4 g/L). Polymer structure cf. left-hand side in Fig. 7.1, with n = 11 andx = 2%. (b) Variation of the fraction of bound azobenzene upon addition of Triton X 100 in a solution of polymer at fixed concentration (polymer similar as in (a), with n = 5 and x = 4%).

Figure 7.7. Association isotherms of micelles (TX 100) or the protein bovine serum albumin on azobenzene-modified poly (sodium acrylate) in dilute water solution, as measured from capillary electrophoretic analysis. Closed symbols dark-adapted samples, open symbols samples exposed to UV light (365 nm). (a) Polymer, as shown in Fig. 7.1, with n = 5 and x=7%, (b) n = 12 and x=3%.

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