Photoresponse of Complexes between Surfactants and Azobenzene-Modified Polymers Accounting for the Random Distribution of Hydrophobic Side Groups.

The design of photoresponsive macromolecules has opened the route to many applications, in particular to trigger macroscopic responses induced by light irradiation in complex fluids and polymer−surfactant formulations. In this report, we studied the association of three sets of azobenzene modified polymer (AMPs) derived from poly(acrylic)acid with varying integration levels of azobenzene and various azobenzene hydrophobic moieties, with the neutral surfactant Triton X 100 (TX 100). Binding isotherms in dilute aqueous solutions were determined by spectrophotometry (to measure the fraction of bound azobenzene) and capillary electrophoresis (to measure the amount of bound TX 100). The degree of binding of TX 100 to AMPs increases markedly with increasing azobenzene hydrophobicity and density in AMPs. A noticeable and reversible photoresponse of the associates was observed upon exposure to UV/visible lights, although the magnitude of the UV-triggered photodissociation and blue-triggered association depends on the chemical structure of both the azobenzene and AMPs. We introduce a critical distance lcthat accounts as single parameter for the balance between energy gain of hydrophobic binding and energy loss due to chain conformational constraints. Only segments of chains flanked with two azobenzene groups at their ends and shorter than lcare assumed to bind tightly. lcis used to fit both the maximum fraction of azobenzene transferred into TX 100 micelles (with saturation well below 100% despite the presence of excess free TX 100) and the amount of bound TX 100 as a function of the density of azobenzene in the chains. The model includes the effect of random distribution of azobenzene moieties along the chains. From this analysis, we find criterions for optimization of the photoresponse as a function of the azobenzene hydrophobicity and density in the chain, and the chain length. [ABSTRACT FROM AUTHOR]