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Feb 7, 2017 - •S Supporting Information ... (6-ferrocenyhexyloxyl)-5-(6-azobenzenehexyloxy)benzyl alco- hol. ..... reversibility, as shown in Figure...
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An End-Bifunctionalized Hydrophobically Modified Ethoxylated Urethane Model Polymer: Multiple Stimuli-Responsive Aggregation and Rheology in Aqueous Solution Zhukang Du, Biye Ren,* Xueyi Chang, Renfeng Dong, and Zhen Tong School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: Hydrophobically modified ethoxylated urethanes (HEURs) belong to an important class of telechelic associative polymers for improving solution rheological properties in many industrial fields. In this work, we describe the synthesis and solution behavior of a novel hydrophobically modified ethoxylated urethane model polymer (AzoFcHEUR) end-bifunctionalized by Percec-type mini-dendron 3(6-ferrocenyhexyloxyl)-5-(6-azobenzenehexyloxy)benzyl alcohol. The telechelic polymer containing both azobenzene and ferrocene moieties in aqueous solution performs multiple stimuli-responsive aggregation and rheological properties dependent on the magnitude of amphiphicity change of functional end groups upon exposure to different external stimuli such as light irradiation and/or redox reaction. It has been demonstrated that the stimuli-responsive hydrophobicity change of end groups induces a reversible micellar transition or rearrangement of micellar aggregates in dilute AzoFc-HEUR aqueous solution and a rearrangement of physical network from a dense network to a sparse network in dense AzoFc-HEUR aqueous solution and vice versa. This work is the first report of such multiple stimuli-responsive rheological properties of telechelic polymers in dense solution induced by photo- and/or redox-stimuli. The results are therefore of interest not only for the fundamental science in the controlled self-assembly of telechelic polymer in aqueous solution but also for specific applications of HEURs in the control of viscoelasticity of polymer solution, fabrication, and transmission of soft materials, medicines and cosmetics, ink for inkjet printers, and flow rate controlling systems, where careful viscosity control is necessary.

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INTRODUCTION Telechelic polymers consisting of hydrophilic main chain and hydrophobic end groups can associate with each other in water to form various nanostructures. As one of such telechelic polymers, hydrophobically modified ethoxylated urethanes (HEURs) can form so-called flower micelles composed of flower loops (hydrophilic backbone) and micellar cores (hydrophobic end groups) in aqueous solution above a critical aggregation concentration (cac). As the concentration increases, the number density of flower micelles increases, and extra end groups of some chains will engage into different micellar cores to connect the micelles; a dynamical physically cross-linked network will be developed eventually.1−5 End hydrophobes can be dynamically attached to and detached from the micellar core, and thus such networks can relax in a finite time to exhibit interesting rheological properties. Consequently, HEURs have been widely used as rheological modifiers for improving solution rheological properties in many industrial fields. Hydrophobes only comprising a relatively small portion of the HEUR polymer chains usually make the solution properties to be quite distinct. Therefore, considerable research efforts have been devoted to demonstrate the effects of hydrophobes such as alkyl phenyls, fluorocarbons, and aliphatic alkyls by varying their chain lengths, sizes, and structures in the © XXXX American Chemical Society

past decades. However, a major question arises from that the rheological properties of these “classical” HEURs can be tuned only either by changing temperature, concentration, solvent or by synthesizing a new HEUR.6−12 On the other hand, stimuli-responsive groups, such as azobenzene (Azo), ferrocene (Fc), and spiropyran (SP), have been widely used as architectural motifs for fabrication of functional materials and devices due to their reversible physicochemical property changes in response to external stimuli.13−15 The amphiphilicity change of such stimuliresponsive groups to external stimuli has been used to tune the self-assembly and disassembly of functional copolymers in dilute aqueous solutions. In view of the effects of end groups in HEURs, the use of stimuli-responsive groups in HEURs as end hydrophobes will result in stimuli-responsive HEURs, which may be particularly appealing to tune the solution aggregation and rheological properties without changing the type of hydrophobes or synthesizing a new HEUR. Recently, we reported an azobenzene (Azo)-functionalized HEUR polymer (Azo-HEUR) with photoresponsive rheological properties.5 A Received: October 22, 2016 Revised: January 20, 2017

A

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Scheme 1. Synthetic Routes of 3-(6-Ferrocenyhexyloxyl)-5-(6-azobenzenehexyloxy)benzyl Alcohol (3) and AzoFc-HEUR Model Polymer

Scheme 2. Hydrophobic Network Change of AzoFc-HEUR Solution upon Exposure to Different Stimuli

materials due to its facile synthesis, reversible redox activity, and notable redox-responsive amphiphilicity change. When a redox reaction takes place, Fc can be oxidized by oxidants to yield ferrocenium cations (Fc+) immediately, meanwhile leading to a remarkable amphiphilicity change, and Fc+ can be reversibly reduced by reducers to give Fc again immediately.16−18 Thus, the introduction of end-cappers containing Azo and Fc moieties into HEURs is particularly attractive. The magnitude of amphiphilicity change of end groups can be readily controlled by light irradiation and/or redox reaction to a certain degree. The different aggregation and physical network structure corresponding to the change in the hydrophilic−lipophilic balance (HLB) of polymer can be obtained; thereby multiple stimuli-responsive rheological behavior of HEUR aqueous solution through a micellar transition and a rearrangement of network can be clarified in detail. As far as we know, the

light-induced reversible hydrophobicity change of Azo hydrophobes leads to a rearrangement of micellar junctions through loop-bridge and bridge−loop transitions in concentrated HEUR solution upon alternative exposure to UV and visible light and thus changes not only the network connectivity but also the relaxation behavior of the solution. Accordingly, the introduction of such stimuli-responsive hydrophobes into HEURs has proven a useful way to alter and control the rheological properties of HEUR aqueous solution. However, the Azo-HEUR solution shows a slow responsive rate and a small magnitude of rheological response to light irradiation due to its slow trans−cis isomerization rate and small dipolar difference between trans- and cis-Azo, which may limit its practical applications to a certain degree. Compared with the Azo moiety, ferrocene (Fc) is of particular interest for controlled self-assembly and functional B

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applied to seal the cone−plate in order to protect water from evaporation. Synthesis of 3-(6-Ferrocenyhexyloxyl)-5-(6-azobenzenehexyloxy)benzyl Alcohol (3). The synthetic route of compound 3 is shown in Scheme 1. First, lithium aluminum hydride was dispersed in dry THF; methyl 3,5-dihydroxybenzoate was dissolved in dry THF also and slowly added dropwise into the flask. The reaction mixture was cooled to 0 °C with stirring under an argon atmosphere. After 3 h, water was added into the flask to quench the reaction. The reaction mixture was filtered and concentrated in a vacuum. The crude product was purified by column chromatography to yield 1. Second, 6bromohexylferrocene and an excess amount of compound 1 were dissolved in DMF. Potassium carbonate was added into the flask. The reaction mixture was kept in 80 °C under an argon atmosphere and stirred for 4 h. After that, diluted hydrochloric acid aqueous solution was added into the flask until the pH was adjusted to 2. The resultant solution was filtered and concentrated in a vacuum to obtain a crude product. The crude product was purified by column chromatography to give compound 2. Finally, 6-bromohexyloxyazobenzene, compound 2, and potassium carbonate with DMF were added into the flask. The reaction mixture was heated to 80 °C with stirring under an argon atmosphere. After 5 h, the reaction mixture was filtered and concentrated in a vacuum. The crude product was purified by column chromatography to give 3 eventually. 1H NMR and FTIR spectra confirm the successful synthesis of final product 3, as shown in Figure S1. 1H NMR (CDCl3, TMS) δ (ppm): 1.27 (m, 10H, −(CH2)3−, −(CH2)2−), 1.76 (m, 4H, −CH2−CH2−O−Ar), 1.82 (m, 2H, −CH2−CH2−O−Ar), 2.15 (t, 2H, −CH2−Cp), 3.93 (t, 4H, −CH2−O−Ar), 4.04 (t, 2H, −CH2−O−Ar), 4.31 (m, 9H, H(Cp)), 4.61 (s, 2H, Ar−CH2−OH), 6.38 (s, 1H, H(Ar)), 6.50 (s, 2H, H(Ar)), 7.00 (d, 2H, H(Ar)), 7.43 (t, 1H, H(Ar)), 7.50 (t, 2H, H(Ar)), 7.90 (q, 4H, H(Ar)). Anal. Calcd for AzoFc end group: C, 71.51; H, 6.98; Fe, 8.18; N, 4.07; O, 9.26. Found: C, 71.47 H, 7.01; N, 4.03; O, 9.34. Synthesis of AzoFc-HEUR. The synthetic route of AzoFc-HEUR polymer is shown in Scheme 1 also. AzoFc-HEUR was synthesized according to our previous work.11 First, the dry PEG 20000 was dissolved in dewatered toluene, and a large excess amount of IPDI (10 equiv of NCO to 1 equiv of OH) and DBTDL (0.2% of the total mass of reactants) as the catalyst were added to the flask. After 3 h of the reaction at 80 °C under argon, 3 was added into the reaction mixture. The reaction temperature further increased to 90 °C, and the reaction time lasted for 12 h. Afterward, the target polymer was obtained by reprecipitating the warm polymer toluene solution in diethyl ether (5 volumes of diethyl ether to 1 volume of toluene solution) for several times to remove unreacted end group and diisocyanate residues. Then, the solution was filtered and pure AzoFc-HEUR was obtained by drying under vacuum at 40 °C for 24 h. Preparation of Samples. A weighted amount of AzoFc-HEUR was individually dissolved in DI water and stirred at room temperature until it was fully dissolved. The trans-AzoFc-HEUR solution were prepared under visible light and stored in the dark for 24 h. The cisAzoFc-HEUR was prepared under 365 nm UV light. The oxidization state AzoFc+-HEUR solution was prepared by adding Fe2(SO4)3 (0.52 equiv to the total ferrocene units in polymer). To reduce the oxidized polymers, vitamin C (0.55 equiv) was added to the AzoFc+-HEUR solution and stirred until the yellow color was completely recovered.

stimuli-responsive rheology of polymer solutions through host−guest chemistry, π−π packing, and micellar formation and disruption has been reported,19−24 but multiple responsive rheological behavior of HEURs in dense solution induced by photo- and redox-stimuli through a micellar transition and a rearrangement of network has not been reported yet. In this work, we designed and prepared a novel hydrophobically modified ethoxylated urethane model polymer (AzoFcHEUR) end-bifunctionalized with Perce-type mini-dendron 3(6-ferrocenyhexyloxyl)-5-(6-azobenzenehexyloxy)benzyl alcohol (3). The model polymer was prepared by reaction of poly(ethylene glycol) with a large excess of diisocyanates followed by the end-capping of the terminal isocyanate groups with 3 (Scheme 1). The multiple stimuli-responsive aggregation and rheological behavior of AzoFc-HEUR in aqueous solution were studied in detail. It have been demonstrated that the multiple stimuli-responsive hydrophobicity change of end groups induces a reversible micellar transition or a rearrangement of micellar aggregates in dilute AzoFc-HEUR aqueous solution and a rearrangement of physical network from a dense network to a sparse network in dense AzoFc-HEUR aqueous solution dependent on the magnitude of amphiphilicity change of end groups upon exposure to different external stimuli and vice versa, leading to quite distinct solution aggregation and rheological behavior (Scheme 2). Consequently, this work will be useful not only for understanding the stimuli-responsive aggregation and rheological behavior of telechelic polymer in aqueous solution but also for correlating the magnitude of amphiphilicity change of end groups with the macroscopic viscoelastic properties of HEUR aqueous solution.

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EXPERIMENTAL SECTION

Materials. Methyl 3,5-dihydroxybenzoate, lithium aluminum hydride, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI), and dibutyltin dilaurate (DBTDL) were purchased from Aladdin and used as received. Potassium carbonate (Aladdin, 99.9%) was grinded and dried before used. Poly(ethylene glycol) with the molecular weight of 20 000 (PEG 20000) was received from Aldrich and dried before used. Toluene, tetrahydrofuran (THF), and N,N′dimethylformamide (DMF) were dried by CaH2 and distilled under vacuum. The other chemicals are all analysis grade. 6-Bromohexylferrocene and 6-bromohexyloxyazobenzene were synthesized according to our previous works,5,17 and full experimental details can be found in the Supporting Information. The other chemicals are all analysis grade. Measurements. 1H NMR spectra were obtained on a Bruker 600 MHz spectrometer. Fourier transform infrared (FTIR) spectra were recorded on a Thermo Nicolet 6700 spectrometer using KBr pellet at room temperature (25 °C). Molecular weight and molecular weight distribution were measured by gel permeation chromatography (GPC), using THF as the flow phase with a flow rate 1.5 mL/min and monodisperse PEO as standard with a column temperature of 40 °C. The surface tension test was carried out by using the contact angle system OCA-15 at 25 °C. The solution was allowed to maintain an equilibrium value for 10 min before recording. The UV−vis light spectra were recorded on a Hitachi UV-3010. Cyclic voltammograms (CV) were obtained on a CHI 660C electrochemical workstation at room temperature using a three-electrode system. The relaxation time distribution of micelles and aggregates of micelle was obtained on a Malvern Nano ZS90. Transmission electron microscopy (TEM) images were obtained from a JEM-2100HR microscope with an acceleration voltage of 200 kV, and samples were taken on the carboncoated copper grids and then stained with 2 wt % uranyl acetate before observation. The rheological properties were measured on an AR-G2 or ARES rheometer (TA Instruments Inc.) with a cone−plate geometry (40 mm diameter and 2° cone angle). Silicone oil was

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RESULTS AND DISCUSSION The purified AzoFc-HEUR polymer was characterized by GPC, 1 H NMR, and FTIR spectra in detail. The Mn determined by GPC is 23 200, and the polydispersity index (PDI) is 1.24. 1H NMR and FTIR spectra further confirm the successful synthesis of AzoFc-HEUR, as shown in Figure S2. The end-capping ratio (ECR), a very important parameter of HEURs,25 is calculated to be 98% from a standard curve obtained by the UV−vis spectra of 3 in DMF at different concentration (Figure S3). The results mean that the majority of polymers have two expected end groups per chain, and another 2% none or C

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Figure 1. UV−vis spectra of AzoFc-HEUR aqueous solution upon exposure to different stimuli: (A) 365 nm UV light, (B) 520 nm visible light, and (C) oxidation and reduction.

monofunctionalized chains can effectively avoid the phase separation according to previous research.26 In order to demonstrate the stimuli-response of Azo and Fc groups, first, UV−vis spectra were carried out for 1 g/L transAzoFc-HEUR aqueous solution at room temperature before and after UV light irradiation and shown in Figure 1. As shown in Figure 1A, the trans-polymer solution shows a characteristic peak at 350 nm and a small peak at 440 nm. Upon exposure to UV light, the peak at 350 nm reduces dramatically and the peak intensity at 440 nm increases slightly due to the π to π* and n to π* transition of azobenzene, confirming a trans−cis photoisomerization of Azo group.27,28 When the cis-AzoFcHEUR solution is exposed to visible light, the peak at 350 nm gradually increases as time progresses and recovers its initial intensity, indicative of a fully reversible cis−trans conversion of Azo group (Figure 1B). Furthermore, UV−vis spectra were carried out for trans-AzoFc-HEUR aqueous solution before and after oxidation by Fe2(SO4)3, as shown in Figure 1C. After a small excess of oxidant was added, a new absorption peak appears at 625 nm immediately, indicating that the Fc moiety was oxidized into Fc+. On the contrary, the absorption peak at 625 nm disappears immediately after adding a small excess of reducing agent vitamin C, confirming a rapid and reversible redox response of AzoFc-HEUR in aqueous solution, which is in accordance with the results from cyclic voltammogram curves (Figure S4).17 The above observations indicate the multiple stimuli-responsive feature of AzoFc-HEUR in aqueous solution. As aforementioned, both the trans−cis conversion of Azo and the Fc-Fc+ conversion will change the amphiphilicity of end hydrophobes and the hydrophilic−lipophilic balance (HLB) of AzoFc-HEUR polymer.5,17,18 Hence, surface tension measurements were carried out for AzoFc-HEUR aqueous solution to validate the amphiphilicity change of end hydrophobes upon exposure to different stimuli at 25 °C, as shown in Figure S5. As can be seen, the polymer solutions show nearly the same surface tensions as water when their concentration is very low. As the concentration increases, the surface tension decreases sharply and then remains at a constant value. The determined cac values are listed in the third volume of Table 1 for AzoFcHEUR after exposed to different stimuli. It is therefore clear from Table 1 that the cac values gradually increase in the order: trans-AzoFc-HEUR, cis-AzoFc-HEUR, trans-AzoFc+-HEUR, and cis-AzoFc+-HEUR. This means that the HLB values of these polymers increase in the same order: trans-AzoFc-HEUR, cis-AzoFc-HEUR, trans-AzoFc+-HEUR, and cis-AzoFc+-HEUR, because they have the identical PEG segment.

Table 1. Cac Values of AzoFc-HEUR Solutions and Viscoelasticity Characterization of 15 g/L AzoFc-HEUR Solution under Different External Stimuli stimuli no UV Ox. UV + Ox.

polymers trans-AzoFcHEUR cis-AzoFcHEUR trans-AzoFc+HEUR cis-AzoFc+HEUR

cac (g/L) 0.01

G0 (Pa)

τ (s)

125 15.6

v (1023 m−3)

v/n

0.316

0.102

0.201

0.065

0.03

79.5

0.08

24.9

1.06

0.063

0.02

0.12

14.8

0.027

0.037

0.012

The reversible amphiphilicity change of stimuli-responsive groups to external stimuli has been successfully used to tune the self-assembly and disassembly of functional copolymer in solutions.13,29,30 In order to verify the above observations, the reversible self-assembly of AzoFc-HEUR in aqueous solution was investigated by TEM and DLS. Figures 2A−D show TEM images of 0.2 g/L AzoFc-HEUR solution where the polymer concentration C surpasses its cac (0.12 g/L) value in the solution. As can be seen from Figure 2, different nanometersized spherical micelles are observed in solutions after being exposed to different external stimuli. The diameter of the spherical micelles is about 18, 30, 55, and 60 nm in transAzoFc-HEUR, cis-AzoFc-HEUR, trans-AzoFc+-HEUR, and cisAzoFc+-HEUR solution, respectively. It is clear that the size of the micelles increases orderly according to the hydrophilicity of polymers. As an illustration, a plot of the size of the micelles obtained from DLS against the cac values for polymer aqueous solution after exposed to external stimuli is shown in Figure 2F. This indicates that a micellar transition occurs in the polymer solution accompanied by the amphiphilicity change of hydrophobes upon exposure to different stimuli. It has been reported that amphiphilic block copolymers with more hydrophobic groups usually hold smaller cac and fewer aggregation number of each flower micelle.31,32 Considering the identical PEG backbone in the HEUR model polymer, thus, the cac values of the polymer will gradually increase in the order of the increasing hydrophilicity of end groups: trans-AzoFc-HEUR, cis-AzoFc-HEUR, trans-AzoFc+-HEUR, and cis-AzoFc+-HEUR upon exposure to different stimuli. The results are consistent with the previous surface tension measurements. More importantly, the aggregation number of each flower-like micelle will increase in the same order: trans-AzoFc-HEUR, cis-AzoFcHEUR, trans-AzoFc+-HEUR, and cis-AzoFc+-HEUR. Meanwhile, the electrostatic repulsion between Fc+ groups in the D

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Figure 2. TEM images of 0.2 g/L AzoFc-HEUR aqueous solution after exposed to external stimuli: (A) initial, (B) Ox., (C) UV, and (D) UV and Ox. DLS results of AzoFc-HEUR aqueous solution after exposed to external stimuli (E). The size dependence of the micelles obtained from DLS on the cac values for AzoFc-HEUR aqueous solution after exposed to external stimuli (F).

Figure 3. DLS measurements of AzoFc-HEUR aqueous solution (A) at indicated concentration (B) at 1.0 g/L upon exposure to different stimuli. (C) A structural model describing rearrangement of micellar aggregates through bridge−loop transitions.

HEUR solution under different stimuli (Figure 2E). This means that there are only individual flower-like micelles in the solution.33,34 However, it is worth noting that the sizes of micelles determined by DLS are slightly larger than those by TEM for the identical polymer solution. The reason is that the size determined by DLS reflects the dimensions of both the swollen core and the stretched shell (hydrodynamic diameter), but the size determined by TEM reflects only the conformation in the dry state. In order to further demonstrate the concentration dependence of aggregation behavior of the polymer solution, DLS measurements were carried out for AzoFc-HEUR aqueous solution at different concentrations C. As Figure 3A illustrates, the 0.1 g/L polymer solution only shows a single fast relaxation

charged micellar cores also increases the size of the micelles to a certain degree for the oxidation state polymers according to previous research.17,18 As a result, the enlargement of the micelles is mainly contributed by the lager aggregation number of each micelle and the electrostatic repulsion in the micelle core. Therefore, upon exposure to external stimuli, the small micelles in the initial trans-AzoFc-HEUR solution need to reform a stable aggregation structure through a micellar transition due to stimuli-induced hydrophilicity change of end groups, and the size of the micelles increases in ascending order of hydrophilicity of polymers. Accordingly, the smallest and largest micelles are observed in trans-AzoFc-HEUR and cisAzoFc+-HEUR solution, respectively. Moreover, DLS results indicate only a single fast relaxation mode for 0.2 g/L AzoFcE

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Figure 4. Dependence of G′ and G″ on angular frequency ω for 15 g/L AzoFc-HEUR aqueous solution before and after external stimuli: (A) initial, (B) UV, (C) Ox., and (D) UV and Ox.

irradiation, two peaks corresponding to individual micelles and large micellar aggregates are observed. As can be seen, both the scattering intensity and the relaxation time of micellar aggregates decrease, while the peak intensity of micelles increases and exceeds the peak of micellar aggregates, indicating large aggregates in trans-AzoFc+-HEUR solution further disrupt to small aggregates and more micelles due to a trans−cis conversion of Azo group (Figure 3C-3). The above observations indicate that a micellar transition and a rearrangement of aggregates induced by hydrophobicity change of end groups take place in polymer solution after UV irradiation and/ or by oxidation reaction. When a micellar transition and a rearrangement take place after UV irradiation and/or by oxidation reaction, the other hydrophobe of the bridged chain in the aggregates of micelle may preferentially come into the same micellar core to become the flower loops of the micelle through bridge−loop transitions to meet the larger aggregation number of each micelle; meanwhile, some HEUR polymers may become flower loops of larger micelles through a micellar transition, altering the relative population of bridging and looping chains in the solution. As a result, the larger aggregates break up to give more and larger micelles and smaller aggregates (Figure 3C). Furthermore, when reducer and/or visible light are employed, the DLS results of 1.0 g/L solution almost recover the initial values, confirming an excellent reversibility, as shown in Figure S6. In light of the same aggregation behavior and association mechanism of HEURs in aqueous solution over a very broad range of concentrations,25,34 a stimuli-responsive micellar transition or rearrangement of micellar junctions through bridge−loop transitions may alter not only the network connection but the relaxation behavior of the concentrated AzoFc-HEUR aqueous solution. It is therefore important to demonstrate the multiple stimuli-responsive rheological behavior of AzoFc-HEUR aqueous solution by rheological measurements.

mode corresponding to the individual micelles in the solution, which is q2-dependent.33,34 As C increases to 0.4 g/L, a slow relaxation mode is observed besides the fast mode. The slow relaxation mode should be attributed to the aggregates of micelle connected by the bridged HEURs according to the open association mechanism suggested by Chasssenieux et al.33,34 With further increasing C, the peak of the slow mode broadens, indicating that more and larger micellar aggregates are formed. It is worth noting that the scattering intensity depends strongly on the size of particles. Once the micellar aggregates appear, the DLS signal is heavily weighted by the larger component.35 As a result, the fast mode corresponding to the small micelles cannot be observed at a relatively high polymer concentration. Moreover, Figure 3B shows the DLS data of 1.0 g/L polymer solution under different stimuli. As the solution is exposed to UV light, the peak corresponding to micellar aggregates becomes narrow and average relaxation time decreases. The scattering intensity of aggregates decreases also. It means that the initial larger micellar aggregates in transAzoFc-HEUR aqueous solution slightly disrupt into smaller micellar aggregates in cis-AzoFc-HEUR aqueous solution (Figure 3C-1). On the other hand, there appear two peaks respectively corresponding to the fast mode and slow mode when oxidant is added into the trans-AzoFc-HEUR aqueous solution. This indicates that the larger structures disrupt to micelles and small aggregates when trans-AzoFc-HEUR is oxidized into trans-AzoFc+-HEUR (Figure 3C-2). After oxidation, the initial neutral micellar cores of Fe hydrophobes will transfer into the charged cores due to a conversion of Fc to Fc+, and the end hydrophobes will be more easily attached to and detached from the micellar core due to strong electrostatic repulsions between Fc+ side groups and weak hydrophobic interactions between end groups in micellar cores (Scheme 1). As a result, the width of the slow mode narrows and the average relaxation times decrease. Moreover, when the oxidization state trans-AzoFc+-HEUR solution was further exposed to UV light F

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Figure 5. Reversible cycle tests of 15 g/L AzoFc-HEUR aqueous solution upon exposure to stimuli: (A) UV and visible light irradiation, (B) oxidation and reduction, and (C) light irradiation and redox reaction.

single Maxwell model in the slow frequency region, except the high frequency region. Accordingly, the deviation in the high frequency region means that there is another fast relaxation mode besides the slow relaxation mode in the low frequency region.40 The slow relaxation mode is attributed by the thermal reorganization (dissociation/association) of the hydrophobic end group in the transient network.37,38 The fast mode corresponds to the local relaxation of PEG backbone.40 It is therefore clear that the cis-AzoFc-HEUR solution shows a single Maxwell slow relaxation in the low frequency region and a broaden non-Maxwell fast relaxation in the high frequency. The results further indicate that the solution behavior of cisAzoFc-HEUR changes from a viscoelastic gel to a viscoelastic liquid through a rearrangement of network via a micellar transition. Moreover, after oxidant was added into the initial reduction state trans-AzoFc-HEUR solution and stirred for 30 s, a large magnitude of rheological response is observed immediately (Figure 4C). Similar to the UV irradiation, the oxidization state trans-AzoFc+-HEUR solution behaves as a viscoelastic fluid, and the relaxation behavior can be well fitted by single Maxwell model in the low frequency region. The deviation at the high frequency region is contributed to the local relaxation of PEG chain also. It is worth noting that the G″ values at high frequency region (fast mode) are almost proportional to ω. This means that the solution performs Maxwellian relaxation since the separation between the slow and fast relaxation mode is relatively large. It is clear that the trans-AzoFc+-HEUR shows quite different relaxation behavior from the trans- and cisAzoFc-HEUR solutions. Furthermore, when the oxidization state trans-AzoFc+-HEUR solution was further exposed to UV light irradiation, the resulted cis-AzoFc+-HEUR solution performs a rather large magnitude of viscoelastic response as shown in Figure 4D. G′ values are consistently smaller than G″, with G′ ≈ ω2 and G″ ≈ ω, and the relaxation behavior can be well fitted by a single Maxwell model. The rheological characteristic of the solution is a viscous liquid. The relaxation time of the fast mode is so small that it cannot be observed in the measurement, suggesting the dynamic of network is very fast. It is clear that the relaxation time of slow mode decreases orderly accompanied by the increasing hydrophilicity of polymers when exposed to different stimuli. The relaxation time of slow mode can be described as the lifetime of the micelles in the network. Previous research has demonstrated that the decreasing hydrophobicity of end group of HEUR will shorten the lifetime of the micelles due to

Strain-dependent measurements show that the storage moduli (G′) and loss moduli (G″) values of the AzoFcHEUR solution are not strain dependent in the range of 0.1− 5%, even though the external stimuli applied. Hence, all the linear viscoelastic measurements are given a 1% strain. It is surprisingly noted that AzoFc-HEUR solution form a viscoelastic gel even though the concentration is as low as 8.0 g/L, which is much lower than the concentration of common HEURs end-capped by long-chain alkanols previously reported (Figure S7).31,36−38 As the concentration C increases, a strong growth of modules is observed, indicating an increase of the elastic strands and the development of a more complete network.39 As can be seen, the initial reduction state transAzoFc-HEUR solution is so viscous that it does not flow readily down to the bottom of the vial. Figure 4A shows plots of G′ and G″ vs ω for 15 g/L AzoFc-HEUR solution. In the whole ω range, G′ > G″, and G′ hardly depends on ω. The above results suggest that the relaxation process in the initial AzoFc-HEUR solution is very slow so that the solution performs nonMaxwellian relaxation in the experimental time window (0.01− 100 rad/s), which is consistent with our previously reported Perce-type mini-dendron-functionalized HEURs. It is clear that the initial AzoFc-HEUR solution has developed a transient network (G0 = 125 Pa) with a very long relaxation time, leading to an excellent thickening performance. In order to confirm the stimuli-responsive solution rheological behavior of AzoFc-HEUR, the solution was exposed to UV light for 10 min, and the light-treated cis-AzoFc-HEUR solution was tested in dark environment. As Figure 4B illustrates, in low frequency region (0.01−0.1 rad/s), G′ ≈ ω2, G″ ≈ ω, and G′ < G″. Moreover, with increasing ω (0.1 to 100 rad/s), G′ asymptotes to a constant value higher than G″, while G″ exhibits a maximum and a crossover of G′ and G″ is observed, indicating the solution behaves as a viscoelastic fluid. It should be noted that the G″ values in the high frequency region hardly depend on ω, which is similar to the initial solution. According to previous reports, the linear viscoelasticity of HEUR solution can be described by a single Maxwell model: G′(ω) =

G0ω 2τ 2 2 2

1+ωτ

G″(ω) =

G0ωτ 1 + ω 2τ 2

(1)

where G0 is the plateau modulus, ω is the angular frequency, and τ is the relaxation time. As can be seen in Figure 4, the solid line represents the result of best fitting of the G′ and G″ data with the single Maxwell model. The data are fitted well by a G

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Macromolecules the decrease in hydrophobic interactions of end groups.5,10,37,38 As previously discussed, the stimuli-responsive conversion of Azo and Fc groups will decrease the hydrophobicity of end group in various degrees. As a result, the decreasing lifetime of micelles will accelerate the dynamic of the transient network, leading to different relaxation behavior. Interestingly, when both reductant and visible light were employed to the cis-AzoFc+-HEUR solution, G′ > G″ in the whole frequency range, indicating the network recover its initial state. Cycle tests further confirm the excellent multiple reversible behavior (Figure 5). As shown in Figure 5, the magnitude of rheological response can be reversibly controlled in various degrees when different stimuli are applied. The above results indicate a dynamic and reversible network rearrangement induce by UV light irradiation and/or redox reaction. In order to further quantitatively describe the rheological response of the 15 g/L solution, the plateau module (G0) and characteristic relaxation time of the slow mode (τ) are obtained from the fitting single Maxwell mode and listed in Table 1. According to the transient network theory developed by Tanaka and Edward et al.,37,38 the end groups of HEURs can engage into and disengage from the micellar cores at the same time. The relaxation time of network relates strongly to the lifetime of flower micellar junctions.37,38 These data indicate that the network becomes weaker and the lifetime of micellar junctions reduces due to photoisomerization and/or oxidation−reduction state conversion of the end group. Moreover, the number density of elastic chains ν was estimated according to the simple theory of rubber elasticity (G0 = vkT, where k and T are the Boltzmann constant and the absolute temperature, respectively) and shown in Table 1 also.41 The number density of HEUR chains (n) is determined to be 3.1 × 1023, and the number density of elastic chains (v) decreases in the order: trans-AzoFc-HEUR, cis-AzoFc-HEUR, trans-AzoFc+-HEUR, and cis-AzoFc+-HEUR. As can be seen from Table 1, the v values of trans- and cis-AzoFc+-HEUR are much smaller than n, but the v values of the trans- and cis-AzoFc-HEUR are close to n. It is clear that the elastic chains decrease in various degrees when different stimuli are applied. According to the dense network and sparse network model, n will be close to v in the dense network and v is much smaller than n in the sparse network. In view of this, the trans- and cis-AzoFc-HEUR in the solution may form a dense network, and the trans- and cisAzoFc+-HEUR may form a sparse network. Moreover, the efficiency of forming elastic chains defined as v/n was evaluated and is shown in Table 1 also, where n is the number density of polymer chains.42,43 As can be seen, the G0 values slightly decrease from 125 to 79.5 Pa, and the τ values reduce to 15.6 s for the cis-AzoFc-HEUR solution. On the other hand, for the trans- and cis-AzoFc+-HEUR, G0 values decrease to 24.5 and 14.8 Pa, and τ values reduce to 1.06 and 0.027 s, respectively. It is therefore clear that the magnitude of rheological response gradually increases in the order: cis-AzoFc-HEUR, transAzoFc+-HEUR, and cis-AzoFc+-HEUR. The above results suggest that the magnitude of rheological response is in accordance with the magnitude of amphiphilicity change of polymer upon exposure to different stimuli. Furthermore, a plot of G0 values vs concentration C before and after exposure to external stimuli is shown in Figure 6. Both G0 values linearly depend on C and can be well fitted by power laws, which is similar to previous reports.1,40 For the trans- and cis-AzoFc-HEUR solutions, G0 ∝ c2.38 and G0 ∝ c2.47, respectively. For the trans- and cis-AzoFc+-HEUR solutions,

Figure 6. Concentration dependence of G0 for AzoFc-HEUR aqueous solutions before and after exposure to different stimuli.

G0 ∝ c3.06 and G0 ∝ c3.11, respectively. As a result, the trans- and cis-AzoFc+-HEUR solutions seem to have stronger G0 dependence on c than the trans- and cis-AzoFc-HEUR solutions. These further suggest that different network structures must be in operation when different stimuli are applied. From the above observations, it is therefore very clear that both trans- and cis-AzoFc+-HEUR solutions perform single Maxwellian relaxation and strong G0 dependence on C, while the cis-AzoFc-HEUR solution performs non-Maxwellian relaxation in the high frequency region and weaker G0 dependence on C. Furthermore, the initial trans-AzoFc-HEUR solution shows non-Maxwellian relaxation over the whole frequency. The conversion of broad non-Maxwellian relaxation to single Maxwellian relaxation occurs as the solution is exposed to different stimuli. The single Maxwellian relaxation can be well described by the sparse network consisting of super bridges. On the other hand, the non-Maxwell relaxation can be explained by the dense network composed of individual HEUR elastic chains and no intrastrand dissociation sites.40,44 In view of the above discussion, it can be concluded that a rearrangement of network from the dense network to the sparse network through bridge−loop transition leads to interesting multiple stimuli-responsive rheological behavior of AzoFc-HEUR aqueous solution. The network in the initial reduction state trans- and cis-AzoFc-HEUR solutions is more likely a dense network. On the contrary, the network in the oxidization trans- and cis-AzoFc+-HEUR solutions is a sparse network. Steady shear measurements were performed in 25 °C in order to further demonstrate the network rearrangement. Figure 7 shows plots of the steady shear viscosity η against shear rate γ̇ for the 15 g/L solution under different stimuli. The initial reduction state trans-AzoFc-HEUR solution shows pronounced shear thinning over the whole shear range (Figure 7A). Interestingly, after UV light irradiation, a relatively narrow Newtonian plateau appears in low shear rate, and a rapid shear shinning follow-up (Figure 7B). In contrast, a relatively broad Newtonian plateau followed by slight shear thickening before thinning was observed in the trans-AzoFc+-HEUR solution (Figure 7C). Furthermore, when the trans-AzoFc+-HEUR solution was exposed to UV light irradiation, a strong shear thickening appears before the shear thinning in the cis-AzoFc+HEUR solution (Figure 7D). The results demonstrate that the nonlinear shear thickening behavior is more remarkable in the H

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Figure 7. Comparison of steady state viscosity (plots) and magnitude of complex viscosity (solid line) for 15 g/L AzoFc-HEUR aqueous solution before and after exposure stimuli: (A) initial, (B) UV irradiation, (C) oxidation, and (D) UV irradiation and oxidation.

Figure 8. Plots of steady state shear viscosity η and steady state first normal stress coefficient Ψ1 vs shear rate for 15 g/L(A) trans- and (B) cisAzoFc+-HEUR solutions.

oxidation state trans- and cis-AzoFc+-HEUR solutions than in the reduction state trans-and cis-AzoFc-HEUR solutions. In addition, the AzoFc-HEUR solutions show different shear thinning behavior upon exposure to different stimuli. Usually, the η dependence on γ̇ in the shear thinning region of HEURs can be well described by power law.1 The initial trans-AzoFcHEUR solution shows shear thinning over all the shear range, and η ∝ γ̇ −0.89. After exposure to UV light irradiation, η ∝ γ̇ −0.92 . For the oxidation state trans-and cis-AzoFc+-HEUR solutions, η ∝ γ̇ −0.97 and η ∝ γ̇ −1.02, respectively. The transand cis-AzoFc+-HEUR solutions show more sensitive shear thinning to γ̇ in high frequency region than those reduction state solutions. Accordingly, the shear shinning is mainly attributed to the flow-induced fragmentation of the HEUR network and shear orientation of the fragmented network. This further indicates that network disruption are different in shear thinning upon exposure to different stimuli. Accordingly, the

traditional HEURs, which usually hold a sparse network, follow η ∝ γ̇ −1 in the shear thinning region.43 On the other hand, the shear thinning of entangled polymers follows η ∝ γ̇ −0.82 for monodisperse linear chains.45,46 As can be seen, the trans-and cis-AzoFc+-HEUR solutions show a stronger γ̇ dependence on η than those entangled polymer solutions and can be well described by the sparse network model. For the reduction state trans- and cis-AzoFc-HEUR solutions, the γ̇ dependence on η is less remarkable, which is more like the solution behavior of the entangled polymers in the shear shinning region. Furthermore, the entangled polymers are known to obey the Cox−Merz law and can be described as47 η(γ )̇ ≅ |η*(ω)|ω = γ ̇

(2)

where η is steady shear viscosity and η* is complex viscosity. Figure 7 shows a comparison of steady state viscosity (data points) and magnitude of complex viscosity (solid line) for 15 I

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CONCLUSION In conclusion, we reported a novel end-bifunctionalized hydrophobically modified ethoxylated urethane model polymer (AzoFc-HEUR) with interesting stimuli-responsive aggregation and rheological behavior in aqueous solution. The aggregation and rheological properties of such telechelic polymer in solution are strongly dependent on the magnitude of amphiphicity change of functional end groups upon exposure to external stimuli such as light irradiation and/or redox reaction. The isomerization of Azo can slightly reduce the hydrophobic interactions of end groups, and Fc−Fc + conversion can largely suppress the hydrophobic interactions, leading to different magnitude of rheological response. The multiple stimuli-responsive amphiphilicity change of end groups induced a reversible rearrangement of the hydrophobic network in dense polymer solution from a dense network to a sparse network. The results are therefore of interest not only for the fundamental science in multiple stimuli-responsive selfassembly of telechelic polymer in aqueous solution but also for the control of solution viscoelasticity, flow rate of fluids, manufacture and transmission of soft materials, and so on.

g/L AzoFc-HEUR aqueous solution before and after external stimuli. Clearly, the Cox−Merz law is ill-suited for all the solutions, since |η*(ω)| deviates from the η(γ̇) in the shear region. This confirms the difference between the AzoFc-HEUR network and the entanglement network of entangled polymers. It is worth noting that the deviation between the steady shear viscosity and complex viscosity for trans- and cis-AzoFc-HEUR solutions is rather slight. On the contrary, the deviation is more significant for the trans- and cis-AzoFc+-HEUR solutions. This indicates that the dynamic in initial and cis solution is somehow similar to that in the entangled polymers solution. The above results means that the trans-AzoFc-HEUR solution will selfassembly into two quite distinct network when different stimuli are applied. For trans- and cis-AzoFc-HEUR solutions, the network is dense while the network in the trans- and cisAzoFc+-HEUR solutions is sparse. It further indicates the network rearrangement through bridge−loop transition in the solution. In order to further understand the origin of shear thickening behavior, plots of η and first normal stress coefficient Ψ1 against γ̇ for the trans-AzoFc+-HEUR solution are shown in Figure 8. As can be seen, the solution exhibits the thickening of the viscosity while Ψ1 in the linear regime. Accordingly, the shear thickening is attributed to the anisotropic creation under shear.43,48 For the sparse model, the elastic of the network mainly owes to the superbridges; the reassociation and disengagement rates are balanced in the low shear rate. In the intermediate shear rate, the strands preferentially reassociate in the shear-gradient direction since the orientation of the strands. The reassociation and disengagement balance shifts to the reassociation side; thus, shear thickening is observed in the trans- and cis-AzoFc+-HEUR solutions. On the other hand, the trans- and cis-AzoFc-HEUR solutions form a dense network, in which individual chains having no internal dissociation sites become the bridge chains. The effect of the anisotropy creation under shear is weaker in a dense network than in a sparse network. Hence, no shear thickening is observed in the trans- and cis-AzoFc-HEUR solutions. Finally, an overview of the structural model is detailed in Scheme 2 to vividly describe the network rearrangement. The AzoFc-HEUR solutions can form a dense interpenetrating network, and individual HEUR chains having no internal dissociation sites become the network strands, which is more likely the entangled network since the deviation between the |η*| and η is slight. In this case, the solution exhibits nonMaxwell relaxation associated with indistinctive C dependence of G0 and shear thinning in the whole shear rate. After irradiation by UV light, a rearrangement of network takes place; the cis-AzoFc-HEUR solution forms a dense network also. The G0 and τ values slightly reduce, and a small magnitude of rheological response is obtained due to a small polar difference between trans- and cis-Azo moieties. Furthermore, when the oxidant is added into the trans- and cis-AzoFc-HEUR solutions, the dense network rearranges into a sparse network composed of super bridges. The resulted trans- and cis-AzoFc+-HEUR solutions exhibit single Maxwellian relaxation associated with significant C dependence of G0, and shear thickening is observed in the intermediate shear rate. The G0 and τ values reduce largely, and a large magnitude of rheological response is obtained due to a great polar difference between Fc and Fc+ moiety.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02301. Synthesis and characterization of 6-bromoundecylferrocene and 6-bromohexyloxyazobenzene; UV−vis spectra and cyclic voltammograms of 3-(6-ferrocenyhexyloxyl)-5-(6-azobenzenehexyloxy)benzyl alcohol in DMF and H2O; the standard curve obtained from the maximum absorbance at 350 nm of 3-(6-ferrocenyhexyloxyl)-5-(6-azobenzenehexyloxy)benzyl alcohol in DMF; 1H NMR and FT-IR spectra of AzoFc-HEUR; surface tension tests of AzoFc-HEUR aqueous solution. DLS data of 1.0 g/L AzoFc-HEUR upon exposure to different stimuli (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86-20-87112708 (B.R.). ORCID

Biye Ren: 0000-0003-0131-8750 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the NSFC (21674039) and Guangzhou Science and Technology Innovation Commission (201607010212).

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REFERENCES

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