Polyureas, azobenzene groups

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 the case of an azobenzene-modified poly(arylether ketone amide) (see Chart 5.2), a pronounced volume contraction due to photo-induced trans-cis isomerization of the azobenzene groups was evidenced by means of size-exclusion chromatography (SEC) [25]. When irradiated in dilute N,JV-diethylacetamide solution, this polymer underwent a reduction in its hydrodynamic radius by a factor of 2.7, corresponding to a contraction of the hydrodynamic volume by a factor of about 20. This pronounced shrinkage effect is believed to be due to a large number of conformationally restricted backbone segments, because other more flexible polyamides and polyurea polymers exhibit much weaker contraction effects. 

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. 

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