Aggregation and Rheology of an Azobenzene-Functionalized Hydrophobically Modified Ethoxylated Urethane in Aqueous Solution

Abstract

Hydrophobically modified ethoxylated urethanes (HEURs) belong to an important class of telechelic associative polymers for improving solution rheological properties. We designed and prepared a novel azobenzene end-functionalized HEUR polymer (AzoHEUR), which was used to investigate the effects of hydrophobicity change of end hydrophobes induced by photoisomerization of azobenzene on the solution aggregation and rheological properties. The concentrated AzoHEUR solutions show a reversible rheological property change upon alternative exposure to UV and visible light. We have demonstrated that a reversible change in hydrophilic−lipophilic balance of polymer followed by photoisomerization of azobenzene induces a reversible rearrangement of micellar junctions through loop−bridge or bridge−loop transitions, which reversibly changes not only the network connectivity but also the solution relaxation behavior. Moreover, a structural model is proposed to describe the rearrangement of micellar junctions induced by photoisomerization of azobenzene. The work will not only provide new insights into the effect of hydrophobicity change of stimuli-responsive end groups on the aggregation and rheological behavior of HEUR aqueous solutions but also open a new perspective for development of some special applications of HEURs in fabrication and transmission of soft materials, medicines, cosmetics, inks for inkjet printers, and flow rate controlling systems. ■ INTRODUCTIONThe hydrophobically modified ethoxylated urethanes (HEURs) as a representative class of associative polymers have been widely used as thickeners in many fields in which careful control of the rheology of the solution is required, such as water-borne coatings, inks, medicines, and cosmetics over the past decades. 1 Like telechelic amphiphilic polymers, HEURs in aqueous solution can form flowerlike micelles composed of the flower loops (hydrophilic backbone) and the micellar cores (hydrophobic end groups) above a critical aggregation concentration (cac). When the polymer concentration (C) exceeds a critical percolation concentration (C p ), the extra hydrophobic end groups will come into the micellar cores through the bridge connection of the hydrophilic chains to form large aggregates of micelle, and a dynamical physical network of micellar junctions is formed eventually, leading to the solution viscosity rises sharply with increasing C. 2−6 Usually, HEUR aqueous solution possesses a Newtonian plateau at low shear rates, followed by shear thinning at high shear rates, with shear thickening at intermediate shear rates for some samples. The research interests include the mechanism of association, the association structure of the polymer in solution, and the response of this structure under shear stress.Up to now, a lot of theories or models have been developed to describe the aggregation and rheological behavior of HEUR aqueous solution, such as the loop−bridge model developed by Winnik 7,8 and the transient network theory developed by Tanaka and Edward et al. 3,9 Accord...

From Architectures to Cutting-Edge Properties, the Blooming World of Hydrophobically Modified Ethoxylated Urethanes (HEURs)

“…On one hand, photosensitive groups such as azobenzene and spiropyran, known for their photoisomerization ability, were grafted to the end of the HEUR hydrophobic groups. These two sensitive groups underwent a quick photoisomerization inducing an amphiphilic change, thus altering the hydrophobic/hydrophilic balance of modified HEUR chains. , Therefore, the aggregation and the rheological behavior of both photosensitive AzoHEUR and SP-HEUR were controlled by UV-irradiation.…”

“…25 Redox and light sensitive hydrophobic groups were as well synthesized with a similar method. In 2016, azobenzene and ferrocene end-functionalized HEURs were described as Azo-HEUR 26 and Fc-HEUR 27 , respectively. The addition of these stimuli-responsive groups to HEURs allowed controlling the rheological properties of the solution by applying external light irradiation or redox reactions.…”

Synthesis of High Molecular Weight Poly(glycerol monomethacrylate) via RAFT Emulsion Polymerization of Isopropylideneglycerol Methacrylate

“…12 − 14 Water-soluble polymers are also widely employed as viscosity modifiers. 15 − 17 For example, PEO 18 and poly(acrylic acid) (PAA) 19 are commonly used as thickening agents in cosmetics, while polyurethanes (PU) 20 and poly(vinyl alcohol) (PVA) are utilized in paint formulations. 21 In such applications polymers often confer the additional benefit of acting as steric stabilizers for other components of the formulation, e.g., oil droplets or pigments.…”

Light-Induced In Situ Chain Extension and Critical Gelation of Benzophenone End-Functionalized Telechelic Associative Polymers in Aqueous Solution

“…At a higher concentration (1 g/L), a second distribution with slow relaxation appears, indicating the formation of micellar clusters through the rearrangement of the loops to bridges that connect adjacent micelles, which is consistent with the previous research studies of typical TAPs. 53,54 Interestingly, both the relative intensity and relaxation time distribution of the slow peak increase after the solution is exposed to UV light, indicating that more micelles participate in the formation of larger clusters. The chemical crosslinking of micellar clusters after irradiation can sharply suppress the disengagement dynamics of end groups between adjacent micelles.…”