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

Abstract

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 polymersurfactant 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 l c that 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 l c are assumed to bind tightly. l c is 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.

Non-monotonous variation of the LCST of light-responsive, amphiphilic poly(NIPAM) derivatives

“…7C), the spectrum displayed a red-shift of the band around 350 nm, and a decrease of the apparent extinction coefficient, which are signatures of the aggregation of azobenzenes. 36,37 Interestingly, relaxation toward the initial spectrum did not occur upon simple incubation below the LCST. In Fig.…”

“…It is known that the absorbance properties of azobenzene can be used as a reporter of the local hydrophilic/phobic environment. 36,37 To get evidence on the presence of inter-azobenzene associates, we thus carried out absorbance measurements in PBS buffer at T ¼ 25 C. In addition, spectrophotometry enabled us to assess the cis-trans state of azobenzene under exposure to UV or visible light. Fig.…”

Reverse Variation of Cloud Points of Light-responsive Assemblies of Azobenzene-modified Amphiphilic Polymers

“…In order to estimate the fraction of azobenzene present in the corona of clusters, we extended here to self-assemblies of polymers the principles described in ref. 28 for complexes between polymers and micelles of surfactant. The equilibrium transfer of azobenzene side groups from water to intracluster (hydrophobic) cores is accordingly assumed to obey the scheme shown in Figure 1, i.e., tightly bound segments of chains have a length shorter than the threshold length l c .…”

“…It was tentatively ascribed to competition between aggregation and self-assemblies of chains into micelle-like globules. 26,27 In a recent paper, 28 we proposed a model to describe the impact of micellization on the photoresponse of complexes of surfactant-and azobenzene-modified poly(acrylic acid). The binding of azobenzene in micelles was analyzed by taking into account their partition between water and hydrophobic cores of micelles.…”

Novel Chromatic Transitions of Azobenzene‐Functionalized Polydiacetylene Aggregates in 1,2‐Dichlorobenzene Solution

“…As we know, azobenzene group is a well‐known photosensitive chromophore, which undergoes trans to cis isomerization upon 365 nm irradiation, or the cis to trans isomerization upon 435 nm irradiation 34–36. Thus, it was anticipated that the photo‐isomerization of the azobenzene chromophore may influence the conformational transition of the side chains and fruit PDAs with novel photo‐responsive property.…”