Photoviscosity effect

Neckers et al. [16, 17, 18] demonstrated that polyureas with backbone azobenzene groups (70) also underwent a photoviscosity effect when ultraviolet-irradiated. Stille et al. [19] reported that the intrinsic viscosity of polyquinoline (II) with backbone stilbene groups in di-m-cresyl phosphate/m-cresol decreased as much as 24% under ultraviolet light. The decrease was ascribed to the tram to cis isomerization of the stilbene groups. Because of its simplicity the mechanism (3) has been widely applied to other polycondensation or polyaddition polymers. 

In solution, the thermal reversion of methacrylate polymers substituted with spirobenzopyrans was often accompanied by a spectral shift of the photomerocyanines and a deviation from first-order kinetics. These observations were ascribable to intermolecular interaction between the merocyanine form and the ester residue of methyl methacrylate. This was supported by the emergence of photoviscosity effects in solutions of these photochromic polymers. 

Moniruzzaman M, Sabey CJ, Fernando GF. 2004. Synthesis of azobenzene based polymers and the in situ characterization of their photoviscosity effects. Macromolecules... 

Polymers prepared by condensation of 4,4 -diacetylstiblene as the bis(ketomethylene)monomer with 4,4 -diamino-3,3 -dibenzoylstilbene, a bi(amino ketone), exhibit photoviscosity effects in dilute solutions due to cis-... 

All the polymers exhibit a photoviscosity effect when exposed to UV irradiation at = 365 nm. This effect was attributed by them to conformational contraction of the polymer chains due to dipole-dipole interaction between neighboring chromophores. This conformational change might possibly be illustrated as follows ... 

Recently, Neckers et. al. demonstrated that polyureas with back-bone azobenzene groups also showed photoviscosity effects under ultraviolet irradiation. Stille et. al. reported that the viscosity of polyquinoline (2) with backbone stilbene groups in di-m-cresyl phosphate /m-cresol decreased by as much as 24 % in the intrinsic viscosity value under the influence of ultraviolet light.The decrease is ascribed to the trans to cis isomerization of the stilbene groups. Because of the simplicity of the mechanism, the concept to introduce photoresponsiveness to polymers may be applied to other poly-condensation or polyaddition polymers. 

One question still remains to be answered. Although the mechanism proposed by Lovrien seems correct, the photoviscosity effect he observed cannot be explained by that mechanism. The concentration of CHP in solution has to be above 0.001 M to see any effect on PMA at all. A viscosity decrease of 50 % would only be possible at much higher CHP concentrations, well above those employed by Lovrien. An almost complete withdrawal of the dye from the polymer domain would then have to be effectuated by light. This is unlikely even at a photo conversion of 80% of the dye, which is about the maximal obtainable. Comparison of our irradiation set-up with Lovrien s suggests that in his case the photoconversion will be lower than the 40% we obtained. The conclusion must be that Lovrien measured a different effect, about Ihe nature of which it is hard to even speculate on the basis of the experimental details given in his paper. 

The absence of noticeable photoviscosity effects by these polymers is possibly due to the inclusion of flexible segments in the polymer chain. 

Matejka and Dusek [7] examined the photoviscosity effect of polymers with pendant azobenzene groups in organic solvents. They used copolymers... 

The effect may be explained by the following mechanism radiation induces a strong dipole moment in the azo bond, and these dipoles may change intrapolymer and polymer-solvent interactions. The smaller photoviscosity effect in THF solution, compared with that in 1,4-dioxane, can be explained by the higher dielectric constant of the former, which may suppress the intrapolymer dipole-dipole interaction. 

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