Aggregation and Rheology of an Azobenzene-Functionalized

Jun 28, 2016 - We have demonstrated that a reversible change in hydrophilic–lipophilic balance of polymer followed by photoisomerization of azobenze...
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Aggregation and Rheology of an Azobenzene-Functionalized Hydrophobically Modified Ethoxylated Urethane in Aqueous Solution Zhukang Du, Biye Ren,* Xueyi Chang, Renfeng Dong, Jun Peng, and Zhen Tong School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China S Supporting Information *

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.

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INTRODUCTION The 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 (Cp), 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 © XXXX American Chemical Society

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 Winnik7,8 and the transient network theory developed by Tanaka and Edward et al.3,9 According to the Winnik model,7,8 the polymer chains in solutions will undergo a series of bridge− bridge, loop−bridge, and bridge−loop steps under shear, leading to different rheological behavior. From the transient network theory,3,9 the network structure is influenced by the lifetime of the micellar junctions and the number density of effective elastic chains (the bridged chains). The end groups of Received: March 29, 2016 Revised: June 15, 2016

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DOI: 10.1021/acs.macromol.6b00633 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Routes of AzoC12OH End Group and AzoHEUR Polymer

trated solution of HEUR may occur through a rearrangement of associative structure induced by the photoisomerization of azobenzene, which will change the solution rheological behavior of HEUR solutions. It will be therefore very interesting to obtain new insights into the effects of hydrophobicity change of hydrophobic end groups on the solution aggregation and rheological behavior of an azobenzene-functionalized HEUR in relatively high concentration by alternating UV or visible light irradiation. However, to our knowledge, numerous works mainly focused on the reversible self-assembly and disruption of azobenzene-functionalized amphiphilic polymers in dilute solution in the past years. In this work, we designed and prepared an azobenzenefunctionalized HEUR polymer (AzoHEUR) by the step-growth polymerization of PEG with a slight excess of diisocyanates in toluene followed by the reaction of the terminal isocyanate groups with the lauryl alcohol-substituted azobenzene (AzoC12OH) (Scheme 1). The effect of hydrophobicity change of azobenzene hydrophobes on the aggregation and rheological properties of AzoHEUR aqueous solutions were investigated by UV−vis absorbance spectrophotometry (UV−vis), surface tension test, dynamical light scattering (DLS), transmission electron microscopy (TEM), and steady and oscillatory shear measurements. A reversible rearrangement of micellar junctions through the loop−bridge transitions or bridge−loop transitions was presented to understand the light-induced rheological property change of concentrated AzoHEUR aqueous solutions. The results may be interesting not only for the understanding of aggregation and rheological properties of photosensitive HEUR aqueous solution but also for fabrication and transmission of soft materials, medicines, cosmetics, ink for inkjet printers, and flow rate controlling systems.

HEURs can engage into and disengage from the micellar cores at the same time, so the network is dynamic and can relax in a finite time. Moreover, the concentration, temperature, molecular weight, hydrophilic backbone, and type of end groups will have a great impact on the solution aggregation and rheological behavior of HEURs.10−14 Although the hydrophobes only comprise a relatively small portion of the polymer chains, the feature of hydrophobes including the hydrophobic chain length, size, and structure has a disproportionately large influence on the aggregation and rheological behavior of the HEUR solution.15−17 In many cases it makes the solution properties of the HEUR polymers quite distinct different from those of similar unmodified polymers. Therefore, considerable research efforts have been focused on the effects of end hydrophobes in the past decades. For example, Elliott et al.16 studied the influence of terminal hydrophobe branching on the aqueous solution behavior of model HEURs. Recently, we reported several HEURs end-functionalized by substituted benzyl alcohols with one, two, and three nonyl tails, respectively.17 However, these HEUR polymers usually need complicated synthesis and purification processes. In terms of the effects of end groups, a facile and alternative method may be based on the hydrophobicity change of stimuli-responsive hydrophobes, rather than changing the type of end groups. As is well-known, azobenzene is one of the most interesting architectural motifs due to its reversible trans−cis and cis−trans photoisomerization and has been widely utilized for the construction of photoresponsive functional materials, such as drug release materials, 18−21 photosensors, 22 functional gels,23−26 and so on. The trans-azobenzene has a smaller dipole moment (0.5 D) than the nonplanar cis-form (3.0 D).27 The photoisomerization of azobenzene will lead to a remarkable hydrophilic−lipophilic balance change, which has been used to tune the reversible self-assembly and disassembly of azobenzene-functionalized amphiphilic copolymer micelles in dilute solution by light irradiation.28 Consequently, this stimulates us to incorporate alkyl-substituted azobenzene into HEUR polymer as hydrophobes, thereby a loop−bridge or a bridge−loop transition of micellar aggregates in the concen-

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EXPERIMENTAL SECTION

Materials. Aniline, phenol, acetic acid, dibutyltin dilaurate (DBTDL) (Aladdin, 99%), and 1,12-dibromododecane (J&K, 99%) were used as received. Potassium carbonate (Aladdin, 99.9%) was grinded and dried before used. Poly(ethylene glycol) with the molecular weight of 6000 (PEG 6000) was received from Aldrich

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Figure 1. UV−vis spectra of 0.1 wt % AzoHEUR aqueous solution upon exposure to 365 nm UV light (A) and 520 nm visible light (B) for different times. 7.90 (q, 4H, −ArH−). Anal. Calcd for AzoC12OH: C, 0.754; H, 0.089; O, 0.084; N, 0.073. Found: C, 0.748, H, 0.091; O, 0.086; N, 0.075. Synthesis of AzoHEUR Polymer. The synthetic route of AzoHEUR polymer is shown in Scheme 1 also. AzoHEUR was synthesized according to our previous work.17 First, the dry PEG 6000 was dissolved in dewatered toluene and excess amount of IPDI (4 equiv of NCO to 3 equiv of OH) along with 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 nitrogen, AzoC12OH was added into the reaction mixture as the end group. The reaction temperature further increased to 90 °C, and the reaction lasted for 4 h. Afterward, AzoHEUR was obtained by reprecipitating the warm polymer toluene solution in petroleum ether (3 vol of petroleum ether to 1 vol of toluene solution) to remove unreacted end groups and diisocyanate residues. Then, the solution was filtered. The product was dissolved in hot toluene and reprecipitated in petroleum ether again. Repeat this procedure three times. Finally, pure AzoHEUR was obtained by drying under vacuum at 40 °C for 24 h. Preparation of Samples. The polymer aqueous solutions of different concentrations were prepared according to our previous method.17 The trans-AzoHEUR solution were prepared under visible light and stored in the dark for 24 h.

and dried before used. 3-Isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI) from Aldrich was distilled under vacuum and stored in argon before use. Toluene was dried by molecular sieve and distilled under vacuum. 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) with a Waters 515 pump/M717 data module/R410 differential refractometer, using THF as the flow phase with a flow rate 1.5 mL/min and monodisperse polystyrene as the standard under a column temperature of 40 °C. The surface tension test was carried out by using Contact Angle System OCA with temperature controlling component at 20 °C. The solution was allowed to maintain an equilibrium value for 10 min before recording. The UV−vis spectra were recorded on a Hitachi UV-3010. The relaxation time distribution of micelles and aggregates of micelle were obtained on a Malvern Nano-ZS 90 Zetasizer using a monochromatic coherent He−Ne laser (633 nm) as the light source and a detector that detected the scattered light at an angle of 90°. TEM images were obtained from a JEM2100HR microscope with an acceleration voltage of 200 kV, and samples were taken through the homemade atomizer to spray onto cellulose-coated copper grids and then stained with 2.0 wt % uranyl acetate before observation. The rheological properties were measured on AR-G2 and ARES (TA Instruments Inc.). Both steady state shear and oscillatory measurements were recorded at 25 °C to obtain the shear rate versus viscosity profiles and the viscoelastic properties of the solution. Since the trans-azobenzene has a slightly lower energy than the cis-form, leading to a slow cis−trans thermal conversion.29 Therefore, all measurements were typically completed within 1 h to avoid the thermal conversion of azobenzene groups. Syntheses of AzoC12OH. The synthetic route of AzoC12OH is shown in Scheme 1. First, compound 1 was synthesized according to the reported method.30 After that, an excess amount of 1,12dibromododecane in acetone with dry potassium carbonate was added to 1. The reaction mixture was allowed to react for 12 h at 55 °C and then extracted with methylene dichloride, purified by silica gel column chromatography to yield 2. Second, an excess amount of acetic acid and potassium carbonate were added to 2 with a solvent of DMF. After being stirred for 6 h, the mixture was filtered and concentrated. Sodium hydroxide aqueous solution and THF were added into the reaction mixture and further stirred for 3 h. Finally, the resulting product was extracted with methylene dichloride and purified by silica gel column chromatography. 1H NMR, mass spectrometry, and FTIR spectra confirm that AzoC12OH is synthesized successfully, as shown in Figure S1. 1H NMR (CDCl3, TMS) δ (ppm): 1.27 (m, 16H, −(CH2)8−), 1.58 (m, 2H, −CH2−CH2−OH), 1.77 (m, 2H, −CH2− CH2−O−Ar), 3.62 (t, 2H, −CH2−OH), 4.06 (t, 2H, −CH2−O−Ar), 7.07 (d, 2H, −ArH−), 7.43 (t, 1H, −ArH−), 7.51 (t, 2H, −ArH−),

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RESULTS AND DISCUSSION Characterization of AzoHEUR Polymer. The purified polymer was characterized in detail by GPC, 1H NMR, and FTIR spectra. The Mn determined by GPC is 19 600, and the polydispersity index (PDI) is 1.60. 1H NMR and FTIR spectra confirm the successful synthesis of AzoHEUR polymer, as shown in Figure S2. The degree of end-capping reaction defined as the endcapping ratio (ECR) is a very important parameter influencing the solution aggregation and rheological behavior of HEUR polymers.8 The calculated ECR is 95% from a standard curve obtained from the UV−vis measurement of AzoC12OH end group at different concentration (Figure S3). It means that the majority of polymers have two expected end-groups per chain, which is consistent with the previous results reported by us and other authors.8,14,17 UV−Vis Spectra. An UV−vis absorbance test was carried out for AzoHEUR aqueous solution at room temperature. Figure 1A shows the UV−vis spectra of 0.1 wt % AzoHEUR aqueous solution exposed to 365 nm UV (15 W) light for different time. As shown in Figure 1A, the trans-form solution shows a characteristic peak at 340 nm and a small peak at 440 nm. Upon exposure to UV light, the peak at 340 nm reduces dramatically due to the π to π* transition of azobenzene. C

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Dynamic Light Scattering of Dilute AzoHEUR Solution. As aforementioned, flowerlike micelles of HEUR solution at high concentration will associate into large micellar aggregates by bridging chains.32 DLS experiments were performed for AzoHEUR aqueous solutions at 25 °C. Figure 3 shows the characteristic relaxation time distributions of AzoHEUR aqueous solutions at indicated concentrations before and after UV light irradiation. As shown in Figure 3A, when the polymer concentration is 1.0 g/L, which is much greater than the caccis and cactrans of the polymer, the solution only shows a single fast relaxation mode, which should represent the individual micelles in the solution.33,34 As the concentration increases to 3.0 g/L, a slow relaxation mode is observed besides the fast mode. The slow relaxation mode should be attributed to the aggregates of micelle according to an open association mechanism suggested by Chasssenieux et al.33,34 When further increasing C, the peak of the slow relaxation mode broadens, indicating the formation of more and larger micellar aggregates. After irradiation by UV light, the peak of the slow relaxation mode corresponding to the micellar aggregates becomes narrow (Figure 3B). It means the larger structure breakup and smaller structure was re-formed. Interestingly, only a fast relaxation mode can be observed in the 3.0 g/L cis-AzoHEUR solution. This means that the initial slow relaxation mode of the micellar aggregates disappears after UV irradiation. In addition, the micelles in 1.0 g/L AzoHEUR solution are further confirmed by TEM image (Figure 4). As

Meanwhile, the peak intensity at 440 nm increases slightly due to the n to π* transition,27,31 indicating the azobenzene end groups gradually photoisomerize from trans- to cis-form. With increasing exposure over 120 s, the UV−vis spectra undergo no change, indicating the solution reaches its photostationary state. Immediately, the cis-form solution was exposed to visible light (22 W). As shown in Figure 1B, the peak at 340 nm gradually increases as time progresses and recovers its initial intensity after irradiation for 160 s, indicative of a completely reversible cis−trans conversion. Surface Tension Measurements. As aforementioned, the trans-azobenzene has a smaller dipole moment (0.5 D) than the nonplanar cis-form (3.0 D).27 This means that the transazobenzene groups are more hydrophobic than the cis-form. Hence, the trans−cis conversion of azobenzene and back will lead to a reversible hydrophobicity change of azobenzene end groups. In order to confirm the hydrophilic−lipophilic balance change of AzoHEUR, a surface tension test was carried out for AzoHEUR aqueous solution. As Figure 2 illustrates, both trans-

Figure 2. Plot of surface tension γ of AzoHEUR aqueous solution before and after UV light irradiation vs polymer concentration C at 20 °C.

and cis-polymer solutions have nearly the same surface tensions as water when their concentration is very low. As the concentration increases, the surface tension decreases sharply and then remain at a constant value. The cac values were determined to be about 0.05 g/L (cactrans) and 0.1 g/L (caccis) for trans- and cis-solutions, respectively. The results confirm the difference in hydrophobicity of the trans- and cis-polymers.

Figure 4. TEM images of AzoHEUR micelles: (a) trans-micelles and (b) cis-micelles.

seen in Figure 4, spherical micelles were observed in both the trans-solution and cis-solution, which are corresponding to the fast relaxation mode in the DLS measurements. For the trans-

Figure 3. (A) DLS results for trans-AzoHEUR (A) and cis-AzoHEUR (B) solution at indicated concentration. D

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Macromolecules solution, the size of the spherical micelles is about 30 nm in diameter, which is slightly smaller than spherical micelles (approximately 40 nm) of the cis-solution. It has been reported that amphiphilic block copolymers with more hydrophobic end groups usually holds smaller cac and fewer aggregation number of each flower micelle than those are less hydrophobic.35,36 As previous surface tension tests demonstrated, the cis-AzoHEUR holds a larger cac than the trans-AzoHEUR, suggesting that the trans-AzoHEUR solution should have a larger aggregation number of micelle than the cis-AzoHEUR. As a result, large spherical micelles are formed in the cis-solution due to the large aggregation number of cis-form micelle. When the transAzoHEUR solution is exposed to UV light, the initial transform micelles in the solution become unstable and need to reform a stable aggregation structure due to the aggregation number change. As a result, another hydrophobic end group of the bridged polymers may preferentially come into the same micelle cores to become the flower loops of the cis-form micelles. Therefore, the larger structure breakups and the slow relaxation mode are observed by DLS. Interestingly, the slow relaxation mode corresponding to micellar aggregates appears again when applying visible light. The azobenzene end groups disengage from the micellar core and engage into another micellar core again through the bridges between the micelles, and the slow relaxation mode is observed in the DLS again. A cycle test further confirms the multiple reversible aggregation behavior of AzoHEUR solution, as shown in Figure S4. From above discussions, the isomerization of azobenzene end groups should induce a bridge−loop transition or loop−bridge transition, leading to breakup of larger structures to smaller objects or re-formation of larger structures. According to previous researches, HEUR aqueous solution has the same aggregation behavior and association mechanism over a very broad range of concentrations.8,34 As a result, the origin of decrease in fluctuation time is the same as the decrease in relaxation time in rheological measurements. Thus, bridge− loop transition or loop−bridge transition will occur in the concentrated solution, and rheological response may be observed. Light-Induced Macroscopic Viscosity Changes of Concentrated AzoHEUR Solution. Previous observations demonstrate that the isomerization of azobenzene induces the rearrangement of micelles and micellar aggregates in the solution, which may influence the rheological behavior of AzoHEUR solution; thereby we can observe the macroscopic viscosity change induced by the isomerization of azobenzene. The time of the solution reaching its photostationary state is usually related to the concentration of the solution and the intensity of the light.29 Therefore, the more powerful light is employed to accelerate the photoisomerization of concentrated AzoHEUR solution. As shown in Figure 5, the 3.0 wt % solution is so viscous that it does not flow readily down to the bottom of the vial. It should be attributed to the formation of a physical network of micellar junctions. After UV irradiation (100 W), the solution changes color from pale yellow to dark yellow within about 6 min, and the viscosity decreases quickly. The sample performs as a watery-like solution, indicative of a significant change in network structures due to a trans−cis conversion of azobenzene groups. Thereafter, the solution is exposed to the green light (150 W) again, and the solution quickly recovers its initial viscosity and color within about 6 min. The solution can be cycled more than five times. The results confirm that the trans−cis isomerization of azobenzene

Figure 5. Macroscopic viscosity change of 3.0 wt % AzoHEUR aqueous solution upon alternative exposure to UV and visible light.

end groups induces the macroscopic viscosity changes of the solution. It should be mentioned that the time scale of the isomerization in concentrated solution is longer than those in the UV−vis measurements. Previous researches have demonstrated that the time scale of the photoisomerization depends on the external stimulate and the microenvironment of azobenzene.37−39 Hence, the slow isomerization rate of azobenzene end groups in the 3.0 wt % AzoHEUR solution may be resulted from the high solution viscosity and the steric effect in the micellar cores. In order to deeply understand the influence of isomerization on the aggregation and rheological properties of AzoHEUR solution, the steady shear and oscillatory shear measurements were carried out for AzoHEUR solution by alternative exposure to UV and visible light. Steady Shear Behavior of AzoHEUR Aqueous Solution. Prior to measurement, an equilibrium time of 2 min is given at each experiment. Figure 6A shows a plot of the steady shear viscosity η against shear rate γ̇ for the trans-AzoHEUR solution exposed to UV light for different times. As we can see, the solution behaves as a typical Newtonian fluid at low shear rates and undergoes shear thinning at high rates. Interestingly, the solution viscosity in the Newtonian region decreases as UV irradiation time progresses; low or zero shear viscosity η0 (approximately represented by η at γ̇ = 0.01 s−1) in the Newtonian region strongly decreases by 13-fold (from 71 to 5.5 Pa·s). Meanwhile, the shear-thinning region shifts to higher shear rate. As discussed previously, when hydrophobic trans-azo end groups transform into less hydrophobic cis-form, the micelles would become unstable, and a rearrangement of micellar junctions is necessary to meet a new thermal equilibrium. As demonstrated by previous DLS measurements, large aggregates of micelle will break up into micelles and small micellar aggregates through bridge−loop transitions under UV light irradiation. In other words, the isomerization of azobenzene would change the relative population of bridging and looping chains through a bridge−loop transition. Consequently, the number density of bridged micellar junctions reduces to cause a low solution viscosity and a fast relaxation process as irradiation time progresses. After UV irradiation over 6 min, the viscosity of the solution remains nearly no change, indicating that the solution has reached its photostationary state. This means that the rheological properties of AzoHEUR E

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Figure 6. Plots of steady shear viscosity η vs shear rate γ̇ for 3.0 wt % AzoHEUR aqueous solution upon exposure to UV light (A) and visible light (B) for different times.

Figure 7. Plots of steady state shear viscosity η and steady state first normal stress coefficient Ψ1 vs shear rate for (A) trans- and (B) cis- solutions.

verify the non-Newtonian behavior of trans- and cis-solutions, the normal stress measurements are carried out over a wide shear rate range. The first normal stress coefficient Ψ1 can be obtained from eq 1:

solutions depend upon the percentage of trans-groups or cisgroups in the solution. Furthermore, the 3.0 wt % cis-solution is switched to visible light. Immediately, a steady shear measurement is carried out to illustrate the influence of cis−trans isomerization of end groups on the solution rheological behavior. Figure 6B shows a plot of steady shear viscosity η against shear rate γ̇ exposed to green light for different times. As can be seen, the solution viscosity in the Newtonian region increases as time progresses, and shear thinning appears at lower shear rates. Contrary to bridge−loop transitions under UV irradiation, a rearrangement of micellar junctions through loop−bridge transitions increases the number of micellar junctions, leading to a high solution viscosity and a slow relaxation process as time progresses. After irradiation for more than 6 min, the solution almost recovers its initial viscosity due to a completely reversible conversion of azo. Particularly, when trans-AzoHEUR solution was exposed to UV light again, the solution viscosity decreases as the first time. A cycle test further confirms the multiple reversible behavior of AzoHEUR solution, as shown in Figure 5. However, it is worth noting that a weak shear thickening is also observed at moderate shear rates before shear thinning for both trans- and cis-solutions, which is analogous to that of common telechelic associative polymers. Several models such as the finite extensible nonlinear elasticity (FENE) effect,40,41 the shear-induced reorganization of the network,42 or the simple transient Gaussian network model (anisotropy of creation of the HEUR strands)43,44 have been used to explain the shear thickening mechanism of HEUR aqueous solutions. In order to

Ψ1 =

N1 γ 2̇

(1)

where N1 is normal stress. The plots of η and Ψ1 against γ̇ for trans- and cis-solutions are shown in Figure 7. As can be seen, the thickening of both viscosity and first normal stress coefficient is observed in the trans-solution. Interestingly, the cis-solution exhibits the thickening of the viscosity while Ψ1 in the linear regime. According to previous works, the FENE effect might be coupled with the anisotropic creation of network strands. The shear thickening in the trans-solution may be contributed by the FENE effect coupling with the anisotropic creation of network strands.44 On the other hand, for the cis-solutions, Ψ1 stays as linear regime in the shear thickening region. This indicates that the FENE effect in the cissolution is so weak that the shear thickening is totally contributed by the anisotropic creation effect. In other words, the isomerization of azobenzene may reduce the FENE effect of the AzoHEUR solution. The nonlinearity in the chain tension and the dissociation rate of associative groups from junctions are two important factors determining the FENE effect of telechelic associating polymers.41 The dissociation rate increases when the trans−cis isomerization of azobenzene occurs. So, the weakening of FENE effect may result from the F

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The Em values of trans- and cis-form AzoHEUR solution obtained from the slope of the fitting lines are 98 and 75 kJ/ mol, respectively. The trans-form AzoHEUR solution holds a higher Em than the cis-form. As previously discussed, since the trans-azobenzene is more hydrophobic than the cis-form, it is therefore more difficult for the trans-azobenzene end groups to disengage from the micellar cores than the cis-form. In other words, the cis-azobenzene end groups have a faster disengagement rate than the trans-form, suggesting trans-azobenzene end groups possess stronger association interactions than the cisform at the same concentration. UV irradiation increases the disengagement rate of the end-aggregates; as a result, the shearthinning region shifts to higher shear rate. The onset of shear thinning at higher shear rates means a speeding up of the dynamics of the solution since the inverse of the shear rate at the crossover is roughly related to relaxation time.45 It further confirms that photoisomerization of azobenzene end groups will change the network connectivity. Moreover, the number of elastically effective chains ν is a very important parameter to describe the network connectivity and can be given by eq 3 in the Newtonian plateau:2

increasing dissociation rate of cis-azobenzene end-aggregates from micellar junctions. In order to understand the solution relaxation behavior of AzoHEUR polymer before and after UV light irradiation, the effects of temperature on the viscoelasticity of 3.0 wt % transand cis-form polymer solutions are also investigated in the temperature range of 15−40 °C (Figure S5). As expected, the η0 values of trans- and cis-form polymer solutions decrease with the increasing temperature. The temperature dependence of η0 determined from the Newtonian plateau can be fitted by the Arrhenius equation:2

η(T ) = A e Em / RT

(2)

where Em is the rheological activation energy representing the potential barrier to disengagement of the hydrophobic chain from micellar junction, R is the gas constant, and A is a preexponential constant. The η0 profiles as a function of temperature in Arrhenius form for trans- and cis-form polymer solutions are shown in Figure 8. Obviously, the plot shows a linear relationship between the ln(η0) and 1/T, implying that the η0 dependence on temperature follows the Arrhenius equation.

v=

η0 kTτ

(3)

where η0 is the zero-shear viscosity in Newtonian plateau, τ is the relaxation time, and k and T are the Boltzmann constant and the absolute temperature. So ν is a function of η0 and τ. It is therefore clear that the isomerization of azobenzene hydrophobes affects not only the network connectivity but also the solution relaxation behavior, leading to interesting photoreversible associative structure and rheological behavior changes. On the other hand, as aforementioned, when C > 2.0 wt %, the solution exhibits pronounced thickening property. Furthermore, the steady shear measurements are performed for AzoHEUR aqueous solution at different concentrations according to the same procedure. Plots of the steady shear viscosity against γ̇ and plots of η0 (Newtonian plateau) as a function of C are shown in Figures S6A and S6B, respectively. For both trans- and cis-AzoHEUR solutions, the viscosity increases and the relaxation time prolongs as the concentration increases. The viscosity dependence on concentration is likely as quadratic growth, which is in excellent agreement with common HEURs. This indicates that a more complete network

Figure 8. Arrhenius plot of ln(η0) against the reciprocal of absolute temperature for the 3.0 wt % trans- and cis-AzoHEUR aqueous solution.

Figure 9. Storage modulus G′ and loss modulus G″ dependence on angular frequency ω for 3.0 wt % AzoHEUR aqueous solution upon exposure to UV light (A) and visible light (B) for different times. G

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Macromolecules is developed with increasing C. After UV irradiation, the solution viscosity of the cis-AzoHEUR sharply decreases, and the relaxation time gets shorter because less hydrophobic cisform induces bridge−loop transitions to decrease the density of micellar junctions. For example, η0 decreases from 7.5 to 1 Pa·s for 2.0 wt % polymer solution, while η0 decreases from 450 to 100 Pa·s for 5.0 wt % polymer solution. Oscillatory Shear Measurement of AzoHEUR Aqueous Solution. The above steady shear measurements have demonstrated that the light irradiation induces the solution aggregation and rheological behavior change of AzoHEUR polymer. In order to quantitatively verify the associative structure and relaxation behavior change, the oscillatory shear measurements are performed in the linear viscoelastic region to investigate the solution viscoelastic behavior of AzoHUER. The storage modulus G′ and loss modulus G″ as a function of angular frequency ω are shown in Figure 9A for the AzoHEUR aqueous solution of 3.0 wt % exposed to UV light for different times. Before UV irradiation, in the low frequency region (approximately from 0.1 to 6 rad/s) G′ increases with a slope of 2, G″ with a slope of 1, and G′ < G″. Moreover, with increasing ω (approximately from 6 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. This plateau modulus is analogous to a classical rubber plateau modulus (G0). This plateau implies that the solution behaves as an elastic body in this region. The above results suggest that an associative network is formed due to a large number of mechanically active micellar junctions.46 As UV irradiation time progresses, G0 value decreases and the point of intersection G′/G″ comes out at higher ω. After irradiation for more than 6 min, the solution reaches its photostationary state. G′ and G″ intersect at ω close to the maximum one. G″ > G′ almost over the frequency range, suggesting the change of dynamical network structure induced by UV light irradiation. It further indicates the decrease in the number density of the effective elastic chains through bridge− loop transitions and the shortening of the relaxation time. Furthermore, the solution is exposed to visible light for different time again. As shown in Figure 9B, G′ and G″ almost completely recover their initial values after irradiation. The crossover frequency is observed at 6 rad/s again. The results indicate that the solution behaves as a viscoelastic fluid again due to a large number of micellar junctions induced by a rearrangement of networks through loop−bridge transitions. In general, the solution viscoelasticity of HEURs can be described by the following single Maxwell model:7 G′(ω) =

G″(ω) =

Table 1. Quantitatively Calculation of the 3.0 wt % AzoHEUR Solution after Irradiation by UV or Visible Light for Different Times t (min)

a

v/n

φ (%)

τ (s)

549 347 248 151 151 350 443 534

1.33 0.84 0.60 0.37 0.37 0.85 1.08 1.30

0.231 0.146 0.104 0.064 0.064 0.147 0.187 0.226

0c −8.5c −12.7c −16.7c 0d 8.3d 12.3d 16.2d

0.125 0.058 0.036 0.028 0.028 0.06 0.104 0.119

UV irradiation. bVisible light. cφ(blt) values. dφ(lbt) values.

to 0.028 s after UV irradiation. Since the solution performs Maxwellian relaxation in the linear regime, the τ obtained by eq 3 from η0 is identical to τ obtained in fitting the single Maxwell model. Furthermore, the relaxation time of HEUR related strongly to the lifetime of flower micelles formed from HEUR molecules.3,47 Previous researches have demonstrated that methanol can weaken the aggregation of hydrophobic groups in the cores of flower micelles, shortening the lifetime of the transient cross-link points.48 Thus, it is clear that the UV irradiation alters the hydrophobic interactions between the azobenzene end groups, thereby weakening the aggregation of hydrophobic end groups in the micellar cores and shortening the lifetime of the transient cross-link points, as confirmed by the Em values obtained from fitting Arrhenius equation. Therefore, the lifetime of the flower micelles reduces, and the relaxation time of the transient network shortens. It should be noted that the relaxation time of the network is much faster than the irradiation time (6 min). As a result, the responsive time of AzoHEUR solution mainly depends on the time scale of the photoisomerization. According to the simple theory of rubber elasticity, G0 = νkT, where k and T are the Boltzmann constant and the absolute temperature, respectively.49 Moreover, the number density n of polymers can be calculated by eq 6 from the polymer concentration C, the molecular weight Mw, and Avogadro constant NA:50 n=

CNA Mw

(6)

The v/n can be defined as the efficiency of forming the bridges and shown in Table 1. As seen in Table 1, the v/n values of the solution decrease from 0.231 to 0.064 after UV irradiation. This indicates that the trans−cis isomerization of azobenzene greatly weakens the bridge efficiency of hydrophilic chains. In order to calculate the percentage φ of the bridge−loop transition (φ(blt)) and loop−bridge transition (φ(lbt)), we define νtrans and νcis as the number density of the effective elastic chain before (0 min) and after UV irradiation (6 min), and then φ can be described as v − vtrans φ(blt) = × 100% (7) n v − vcis φ(lbt) = × 100% (8) n

(4)

G0ωτ 1 + ω 2τ 2

v (1023 m−3)

0 2a 4a 6a 0b 2b 4b 6b

G0ω 2τ 2 1 + ω 2τ 2

G0 (Pa)

a

(5)

where G0 is the plateau modulus, ω is the angular frequency, and τ is the relaxation time. The fitting data according to the Maxwell model are shown in Figure 9. The solid line represents the best fittings to the single Maxwell model. The fitting lines possess similarity to the data point. G0 and τ can be obtained from the fitting data and listed in Table 1. As seen in Table 1, both the G0 and τ values decrease as UV irradiation time progresses, while green light irradiation has exactly the reverse effect, and the G0 and τ values nearly recover the initial values again. The G0 value obtained from eqs 4 and 5 decreases from 549 to 151 Pa, and the τ value dramatically reduces from 0.125

The νtrans and νcis values can be obtained by v = G0/kT from the corresponding G0 values of AzoHEUR aqueous solutions before and after UV irradiation. The calculated φ values are H

DOI: 10.1021/acs.macromol.6b00633 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules shown in Table 1. After 6 min UV irradiation, the φ(blt) value decreases by 16.7%. It means that 16.7% of the HEUR polymers change from the bridge to the loop segments after UV irradiation. The Cole−Cole plot of G′ vs G″ is another way to investigate the solution viscoelastic behavior of HEURs. The Cole−Cole plots of G″ against G′ can be obtained by the equation2 G″ = [G′G0 − G′ 2]m

describe the solution aggregation and rheological behavior change induced by photoisomerization of azobenzene hydrophobes. The work will be useful not only for understanding the influence of hydrophobicity change of stimuli-responsive end groups on the solution associative structure and rheological property of HEURs but also for design of the photoresponsive viscosity system, control of the release rate of substances and flow rate of fluids, manufacture and transmission of soft materials, waterborne coating, inks, medicines, cosmetics, and so on.

(9)

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The data of a Maxwell fluid should be in the form of a semicircle. Figure S7 shows the Cole−Cole plots for the transand cis-form AzoHEUR solution at 25 °C. Obviously, for transAzoHEUR solution, the experimental dynamic data fit perfectly with the semicircle and perfect semicircle is observed, indicating the trans-form AzoHEUR solution performs as the viscoelastic fluid before UV-light irradiation. However, for the cisAzoHEUR solution, no perfect semicircle is observed, indicating the solution is more likely as the viscous fluid. The above results further show that the isomerization of azobenzene induces aggregation and rheological behavior changes. Finally, an overview of the structural model is detailed in Figure 10 to vividly describe the aggregation and rheological

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00633. Additional figures including 1H NMR and FTIR of azobenzene end-capper; 1 H NMR and FTIR of AzoHEUR polymer; the standard curve obtained from UV−vis measurement; the steady shear viscosity η against shear rate for the 3.0 wt % AzoHEUR solution at different temperatures; the Cole−Cole model for 3.0 wt % AzoHEUR solution (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project received financial support from the NSFC (21274047), the Specialized Research Fund for the Doctoral Program of the Education Ministry (20120172110005), and the Specialized Scientific Research Fund of Guangzhou Science and Technology Innovation Commission (201607010212). The authors thank the editor and referees for their valuable comments, which greatly improve the quality of the article.

Figure 10. Structural model of AzoHEUR aqueous solution upon exposure to UV or visible light.

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behavior change of AzoHEUR aqueous solution. In this model, AzoHEUR in solution will form micelles, micellar aggregates, and physical network as increasing C. After irradiation by UV light, a bridge−loop transition takes place; the effective elastic chain of the network and lifetime of the micelles reduce to lead to a weaker network. On the contrary, a loop−bridge transition happens when the solution is exposed to visible light. The weaker network recovers to its initial physical network, performing excellent reversibility.

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K

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