Unusual Transient Network and Rheology of a Photoresponsive

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Unusual Transient Network and Rheology of a Photoresponsive Telechelic Associative Model Polymer in Aqueous Solution Induced by Dimerization of Coumarin End Groups Zhukang Du,† Xiaolong Yan,† Renfeng Dong,*,‡ Kang Ke,† Biye Ren,*,† and Zhen Tong† †

School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China School of Chemistry and Environment, South China Normal University, Guangzhou 510006, China

‡

S Supporting Information *

ABSTRACT: Telechelic associative polymers (TAPs) can form a dynamic transient network in water and have been widely used as rheological modifiers for improving solution rheological properties in many industrial fields. In this work, we designed and prepared a novel photoresponsive coumarin-functionalized telechelic associative model polymer (CouTAP), which was used to investigate the influence of light-induced dimerization of coumarin end groups on the polymer structure and solution rheological properties. The effects of light intensity, irradiation time, polymer concentration, and temperature on the transient network and rheological behavior of the CouTAP aqueous solution were studied in detail. The rheological properties of the dimerized CouTAP aqueous solution show weak temperature dependence, and the dynamics hardly depends on concentration due to the transformation of triblock to multiblock polymers. A novel transient network model composed of train loops and bridges covalently linked to train loops was proposed to describe the unique solution properties. This work will not only provide new insights into the influence of the light-induced dimerization of coumarin end groups on the network structure and rheological properties of CouTAP solution but also opens a new perspective for the controlled self-assembly of amphiphilic polymers and some special applications of TAPs in the manufacturing and transmission of soft materials, waterborne coating, inks, medicines, cosmetics, and so on.

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INTRODUCTION Telechelic associative polymers (TAPs), consisting of hydrophobic groups and hydrophilic backbone, can form so-called flower micelles composed of flower loops (hydrophilic backbone) and micellar cores (hydrophobic end groups) above a critical aggregation concentration (cac). As polymer concentration increases, some extra hydrophobic end groups may engage into different micelles, and the hydrophilic backbone connects the neighboring micelles; a dynamic physically cross-linked network is developed eventually, leading to the solution viscosity rise sharply. Consequently, TAPs have been widely used as rheological modifiers for improving solution rheological properties in many industrial fields.1,2 The end groups can dynamically attach to and detach from the cores of micelles. Accordingly, such a transient network can relax in a finite time. Importantly, the characteristic dynamics of the network strongly depends on the type of end groups, concentration, and temperature.3−17 Hydrophobic end groups only comprising a relatively small portion of the TAP polymer chains usually make the solution properties to be quite distinct. Therefore, considerable research efforts have been devoted to the influence of end groups on the aggregation and rheology of the TAP solution. © XXXX American Chemical Society

On the other hand, amphiphilic multiblock polymers containing alternating hydrophobic and hydrophilic blocks of monomers can self-assemble unique micellar structures with hydrophobic cores decorated by petals of hydrophilic blocks which can be regarded as flowerlike micelles also.18 The alternating multiblock polymers can lead to profound consequence on a wide range of applications due to the unique structures and properties.19 Usually, the multiblock polymers can be prepared by living polymerization, step condensation polymerization, or click chemistry coupling.20−25 Moreover, the influences of the ratio of hydrophobic to hydrophilic blocks, the structure of hydrophobic and hydrophilic blocks, and the solvent on the self-assembly behavior of the amphiphilic multiblock polymers have been studied.18,21,26,27 As is well-known, coumarin is one of the most interesting and biocompatible architectural motifs due to its photoinduced dimerization through [2 + 2] cyclization of the double bond under 365 nm UV light irradiation and has been widely utilized for the construction of photoresponsive functional materials, Received: July 17, 2017 Revised: January 27, 2018

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DOI: 10.1021/acs.macromol.7b01514 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Routes of CouC11OH End-Capper and CouTAP Model Polymer

network shows weak temperature dependence, and the dynamics hardly depends on polymer concentration. Furthermore, a unique dynamic network model composed of train flower loops and bridges covalently linked to train flower loops was presented to vividly describe the interesting aggregation and rheological properties. This work will be useful not only for providing a new method to prepare multiblock polymers from coumarin-functionalized TAPs by photoinduced dimerization of coumarin end group but also for understanding the rheology and network structure of coumarin-functionalized TAPs in aqueous solution before and after irradiation.

such as drug release materials, photosensors, functional gels, and so on.28−34 In view of the aggregation and rheological behavior of TAPs in aqueous solution, for TAPs functionalized by coumarin groups as end groups, such self-assembled flowerlike micelles may facilitate the photodimerization of coumarin groups in the cores of micelles. Using such a selfassembly assisted dimerization concept, amphiphilic multiblock polymers may be readily prepared from coumarin-functionalized TAPs by an in situ dimerization of coumarin end groups under 365 nm light irradiation. However, such a light-induced in situ transformation of the telechelic triblock into multiblock polymers has not been reported yet. Meanwhile, the bridged polymer chains will connect with the flower loops by the in situ covalent bonding of end groups in the cores of micelles under irradiation. Some TAPs as the flower loops of micelles will be connected end to end by the photoinduced dimerization of coumarin end groups to form large flower loops, thereby leading to an unusual transient network and solution rheological properties. Accordingly, it is important to understand and compare the unique aggregation and rheology of coumarin-functionalized TAPs in solution before and after irradiation, although the rheology of amphiphilic multiblock polymers has been reported and studied.23,25,35,36 Here, we designed and prepared a novel telechelic associative model polymer (CouTAP) end-functionalized by coumarin. The model polymer was prepared by the reaction of poly(ethylene glycol) with a large excess of diisocyanates followed by the end-capping of the terminal isocyanate groups with coumarin-substituted undecanol (CouC11OH) (Scheme 1). The effects of photoinduced dimerization of coumarin end groups on rheological behavior of CouTAP in aqueous solution were studied in detail. It has been demonstrated that the flower loops and bridges will be connected end to end through the dimerization of coumarin end groups in the cores of micelles to form multiblock polymers, leading to an unusual transient network. The solution performs interesting rheological and relaxation behavior after 365 nm light irradiation. The resulting

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EXPERIMENTAL SECTION

Materials. 7-Hydroxycoumarin, 11-bromo-1-undecanol, dibutyltin dilaurate (DBTDL), and 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI) 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 (PEG20000) was received from Aldrich and dried before used. Toluene and N,N′-dimethylformamide (DMF) were dried by Na and distilled under vacuum before used. The other chemicals are all analysis grade. Measurements. 1H NMR spectra were obtained on a Bruker 600 MHz spectrometer. Fourier transform infrared spectra (FTIR) 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 PS as standard with a column temperature of 40 °C. The UV−vis light spectra were recorded on a Hitachi UV-3010. The relaxation time distribution of micelles and aggregates of micelle was obtained on a Malvern Nano ZS90. The rheological properties were measured on an AR-G2 or ARES-G2 rheometer (TA Instruments Inc.) with a cone−plate geometry (40 mm diameter and 2° cone angle). Silicone oil was applied to seal the cone−plate in order to protect water from evaporation. Synthesis of Coumarin-Substituted Undecanol (CouC11OH). The synthetic route of CouC11OH end-capper is shown in Scheme 1. In brief, 7-hydroxycoumarin and excess amount of 11-bromo-1undecanol were dissolved in DMF; dry potassium carbonate as a B


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Figure 1. (A) UV−vis spectra of 3 wt % CouTAP aqueous solution upon exposure to UV light. (B) Plot of dimerization degree D vs irradiation time t for different light intensities. deacid reagent was added to the mixture. The reaction mixture was allowed to react for 12 h at 80 °C and then extracted with methylene dichloride, purified by a silica gel column chromatography to yield CouC11OH. 1H NMR and FTIR spectra confirm the successful synthesis of CouC11OH, as shown in Figure S1. 1H NMR (CDCl3, TMS) δ (ppm): 1.30 (m, 14H, −(CH2)7−), 1.58 (m, 2H, −CH2− CH2−OH), 1.81 (m, 2H, −CH2−CH2−O−Ar), 3.65 (t, 2H, −CH2− OH), 4.06 (t, 2H, −CH2−O−Ar), 6.25 (d, 1H, −ArH−), 6.80 (s, 1H, −ArH−), 6.85 (d, 1H, −ArH−), 7.35 (d, 1H, −ArH−), 7.62 (d, 1H, −ArH−). Anal. Calcd for CouC11OH: C, 72.26; H, 8.49; O, 19.25. Found: C, 72.57; H, 8.12; O, 19.31. Synthesis of CouTAP. The synthetic route of CouTAP is shown in Scheme 1 also. CouTAP was synthesized according to our previous work. First, the dry PEG 20000 was dissolved in dewatered toluene, and large excess amounts 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, CouC11OH was added into the reaction mixture, and the reaction time lasted for 12 h. Afterward, the target polymer was obtained by reprecipitating the warm polymer solution in diethyl ether (10 volumes of diethyl ether to 1 volume of toluene solution) for several times to remove unreacted end groups and diisocyanate residues. Then, the solution was filtered, and pure CouTAP was obtained by drying under vacuum at 50 °C for 2 days. Preparation of Samples. A weighted amount of CouTAP was individually dissolved in DI water and stirred at room temperature until it was fully dissolved. The samples were exposed to 365 nm UV light to dimerize CouTAP.

change in UV−vis spectra over time for the 3 wt % aqueous solution of CouTAP under 25 mW/m2 UV light at room temperature. The continuous decrease of the peak around 320 nm implies the photoinduced dimerization of coumarin end groups as the irradiation time progresses. With increasing exposure over 600 s, the spectra undergo no change, indicating the dimerization reaction should reach its photostationary state. At the same time, a sol−gel transition occurs along with the dimerization of coumarin when the solution is exposed to the UV light. Before irradiation, the CouTAP aqueous solution performs as a viscous liquid, and the initial solution can readily flow down to the bottom of the vial. After irradiation, the solution is so viscous that it can be free-standing on the top of the vial. The steep rise in viscosity should be mainly due to the formation of the multiblock polymers induced by the dimerization of coumarin. Meanwhile, the [2π + 2π] cycloreversion slows down the dynamics of the solution also.32 Consequently, a strong network is formed due to the increase of effective elastic chains and relaxation time in the solution. The time evolution for the degree of dimerization D can be approximately estimated by the equation D = (a − b)/a, where a is the initial peak intensity at 320 nm and b is the absorbance at 320 nm after UV light irradiation. Hence, the light intensity is varied from 11 to 25 mW/cm2 to investigate the effects of this variable on D, especially the kinetics of sol−gel transition and the time at which a final D is achieved during the irradiation process of 3 wt % polymer solution. Plot of D vs irradiation time t is shown in Figure 1B. As expected, a correlation between the light intensity and D is observed. As can be seen, there are three different regions during the network formation process under light irradiation. The D values increase linearly as irradiation time processes in the early state. As if the value of D reaches about 65%, the growth slows down and then reaches a plateau, no matter how long the irradiation time is. The degree of dimerization can reach its maximum 82% after 600 s of irradiation. It may be resulted from the dimerization balance of coumarin groups and the high viscosity of the 3 wt % polymer solution. Note that the incipient slope increases with the larger light intensity, which further confirms that the dimerization rate depends on the light intensity, whereas the final dimerization degree is irrelevant to the light intensity. The faster the dimerization rate is, the faster the network develops. Accordingly, the network formation rate can be modulated by changing light intensity.

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RESULTS AND DISCUSSION The purified CouTAP polymer was characterized by GPC, 1H NMR, and FTIR spectra in detail. 1H NMR and FTIR spectra further confirm the successful synthesis of CouTAP, as shown in Figure S2. The Mn determined by GPC is 20 300, and the polydispersity index (PDI) is 1.19. The end-capping ratio (ECR), a very important parameter of TAPs, is calculated to be 98.5% from a standard curve obtained by the UV−vis spectra of CouC11OH in DMF at different concentrations (Figure S3). The results mean that the majority of polymers have two expected end groups per chain, and another 1.5% none or monofunctionalized PEG chains can effectively avoid the phase separation according to the previous research.37 In response to 365 nm UV light irradiation, the coumarin moieties in polymers can exhibit photoinduced dimerization.38 The photochemical process of CouTAP can be observed by UV−vis absorbance measurements. Figure 1A shows the C


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and loss moduli (G″) values of the CouTAP solution are not strain dependent in the range of 1−5% before and after 365 nm light irradiation. Hence, all the linear viscoelastic measurements were given under a 1% strain. As a comparison, the steady shear measurements were conducted for 3 wt % CouTAP and pure PEG20000 at 25 °C, and the data are shown in Figure S4A. As can be seen, the zero shear viscosity η0 value of the pure PEG20000 solution is 0.0025 Pa·s, which is close to that of pure water, whereas the η0 value of the CouTAP solution is 1.75 Pa· s, which is 700 times larger than that of pure PEG20000. Furthermore, a crossover of G′ and G″ of the initial solution is not observed until the temperature decrease to 5 °C, as shown in Figure S4B. The coumarin group and its dimer cannot dissolve in cool water but can dissolve in hot water,44 and the C11 alkyl chain in the end groups is not long enough to form a strong hydrophobic association. Consequently, the end groups can attach in and detach from the micellar cores quickly so that the network can relax in very short lifetime and dynamics in the initial solution is extremely fast at room temperature. Oscillatory shear measurements were performed in the linear viscoelastic region to investigate the associative structure and relaxation behavior of CouTAP solutions. A plot of G′ and G″ vs angular frequency ω is shown in Figure 3 for the 3 wt % CouTAP aqueous solution exposed to 25 mW/m2 UV light for different times. Before light irradiation, the values of G′ are consistently smaller than G″, with G′ ∝ ω1.96 and G″ ∝ ω0.995 in the frequency region. It means that the transient network is very weak and the dynamics is fast. When the solution is irradiated by 365 nm UV light for 60 s, the values of G′ are smaller than G″ in the low frequency region. Interestingly, the values of G′ and G″ are numerically consistent in the high frequency region, with G′ and G″ ∝ ω0.497. The dynamic response of the solution is expected for soft spheres (large clusters) formed by branching of micelles due to the dimerization of coumarin. As a result, the large micellar clusters are formed and the solution performs a well-defined longest relaxation time. As if the solution is irradiated by 365 nm light for 120 s, the values of G′ are smaller than G″ in the low frequency region, and a crossover of G′ and G″ is observed. In high frequencies, G′ ∝ ω0.29, while G′ > G″. This further indicates that a strong transient network is formed, and the network can relax in a finite relaxation time. Finally, when the solution is exposed to 365 nm light for 600 s, in the low frequency region (approximately from 0.01 to 0.1 rad/s), G′ ∝ ω0.95, G″ ∝ ω0.594, and G′ < G″. Moreover, with increasing ω (approximately from 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. This clear 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.45 Furthermore, plots of the dynamic change of G′ and G″ at 10 rad/s vs t and D is illustrated in Figure S5. Before light irradiation, G′ and G″ moduli are constant and G″ > G′. After UV light irradiation, both G′ and G″ increments are simultaneously observed. But the growth of G′ is much faster than G″, and the cross point of G′ and G″ is observed when the dimerization degree is 50% or exposure time is 90 s. After that, the increment slows down and G′ and G″ become constant, indicating a strong network is formed eventually. As can be seen, G′ and the dynamics dramatically increase as irradiation progresses. This should be due to the formation of multiblock polymer during the dimerization process. As a result, the exchange of hydrophobic chain from

Dynamic light scattering (DLS) measurements were conducted to understand the effect of dimerization on the self-assembled structures of CouTAP in the solution. The 3 wt % CouTAP aqueous solutions after irradiation for different times were diluted to 0.1 wt %. The CONTIN method has been used to analyze the measured autocorrelation functions from dynamic laser light scattering according to previous reports.39,40 As Figure 2 illustrates, the pure PEG20000 only

Figure 2. DLS measurements of 0.1 wt % pure PEG20000 and CouTAP aqueous solution upon exposure to UV light for different times.

shows a narrow relaxation mode, which is assigned to the PEG string.41 On the other hand, the 0.1 wt % polymer solution only shows a single q2-dependent fast relaxation mode before light irradiation, corresponding to the individual micelles in the solution, which is consistent with the previous researches of typical TAPs.40,42 Interestingly, as the irradiation time increases to 30 s, a slow relaxation mode is observed besides the fast mode. The slow relaxation mode is assigned to larger micellar aggregates according to an open association mechanism and shows independence at low q. With further increasing irradiation time, the peak of the slow mode broadens and moves to larger relaxation time distribution, and the relaxation time distributions gradually evolve, indicating that larger aggregates are formed.42,43 Furthermore, both the width of the fast and slow relaxation mode broadens, and the average relaxation times increase as polymer concentration increases. From above discussion, the micellar clusters cannot be formed before irradiation in this concentration. As a result, only a fast relaxation mode in the initial solution can be observed from the DLS measurements. Upon exposure to light irradiation, some coumarin end groups in the core of micelle will undergo dimerization to immobilize not only the micelles in the solution but also the transient bridges between micelles. As a result, some single flower loop in each micelle will be connected end to end to form some train loops; meanwhile, some transient bridges further will be connected with train loops by covalent bonding. Therefore, some large and branched clusters of micelles are formed, and the slow relaxation peak is observed from the DLS measurements until a complete physical network is developed in the concentrated solution. In order to understand the photodimerization behavior of CouTAP polymer in solution, detailed rheological characterizations were conducted for CouTAP aqueous solutions. Straindependent measurements show that the storage moduli (G′) D


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Figure 3. (A−J) Plots of G′ and G″ vs angular frequency ω for the 3 wt % CouTAP aqueous solution exposed to UV light for different times.

while the slow mode seems to follow a straight line of positive slope and then reach a peak. The slope and the τpeak value move to high values with increasing irradiation time.49 Accordingly, the fast mode can be attributed to the relaxation of polymer chain segment and the initial CouTAP chains, and the slow mode can be attributed to the relaxation of hydrophobic association. The length of hydrophobic group will be double, and the PDI will increase through chain extension reaction after irradiation. Hence, the slow peak of the relaxation time can be assigned to the hydrophobic association of train loops

the core of micelle slows down, which results in an increase of effective elastic strands and slowing down of the dynamics in the solution. In order to further understand the relaxation behavior of the solution with different irradiation time, the continuous relaxation spectra (BSW spectra) were obtained by fitting the data from the frequency sweep measurements and shown in Figure 4.46−48 As can be seen, the spectra can be divided into two regimes: a fast mode and a slow mode regime. The fast mode seems to group along a straight line of negative slope, E


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and the linear fitting in the terminal region is more and more significant after light irradiation. It may be resulted from the formation of multiblock polymers and the rise in PDI during the irradiation process. The above observations suggest that the CouTAP solution shows different self-assembled structures and viscoelasticity resulted from the different dimerization degree of coumarin as the irradiation time progresses. On the other hand, steady shear measurements were performed at 25 °C to understand the network formation process under 365 nm light irradiation. Figure 5A shows a plot of the steady shear viscosity η against shear rate γ̇ for the 3 wt % CouTAP aqueous solution exposed to UV light for different times. As we can see, the solution behaves as a typical Newtonian fluid in the range of the whole shear rates before the irradiation. As the dimerization of coumarin groups occurs, the solution viscosity increases dramatically, and a clear shear thinning at high shear rates is observed after irradiation for 90 s. After that, the Newtonian plateau becomes narrower and the shear thinning region shifts to lower shear rate. Accordingly, the crossover of Newtonian region and shear thinning region can be roughly defined as the relaxation time of the solution.50 It further indicates that the dynamics in the solution slows down after irradiation, which is consistent with the results previously discussed. Furthermore, a plot of the η0 (approximately represented by η at γ̇ = 0.01 s−1) vs irradiation time t is shown in Figure 5B also. Since the solution performs nonterminal region in the low frequency region, the complex viscosity, η* obtained from the [G″/ω]ω → 0 slightly deviates from η0. Similar to Figure 1B, η0 linearly increases quickly from 0 to 150 s; after that, the growth slows down and then reaches a plateau. As the irradiation time is over 600 s, the solution viscosity almost undergoes no change, indicating that the solution has reached its photostationary state. η0 strongly increases by about 4500-fold (from 1.5 to 6700 Pa·s), indicating a strong dynamic network has been formed. However, it is worth noting that no shear thickening is observed in the steady shear measurements. Usually, the typical TAPs will show reasonably shear thickening region in the moderate shear rate before the shear thinning region.51 Several mechanisms such as shear-enhanced formation of the network strands, the finite extensible nonlinear elasticity (FENE) of the strands, and anisotropic creation of strand under shearing were introduced into the model to describe the thickening mechanism. In the

Figure 4. BSW spectra of 3 wt % CouTAP aqueous solution exposed to UV light for different times.

covalently bonded to bridges polymer chains. The number of the dimerized end groups in each train loop is different so that the relaxation of CouTAP can be defined as a broaden relaxation distribution rather than an accurate relaxation time. As a result, the resulting multiblock polymers show a relaxation time distribution similar to the molecular weight distribution in the slow mode. The rise in the peak intensity and τpeak may be attributed to the increasing average number of the dimerized end groups in each train loop as the irradiation time progresses, while the rise of slope may be resulted from the different PDI of the train loops. Moreover, the number density of elastic chains ν and the fraction of elastically active chains v/n are estimated according to the simple theory of rubber elasticity (G0 = vkT, where k and T are the Boltzmann constant and the absolute temperature, respectively). The number density of CouTAP chains (n), v, and v/n values are determined to be 8.9 × 1023, 2.67 × 1023, and 0.3, respectively. The above results suggest that a strong associative network is formed finally due to the increasing number density of elastic chains during the irradiation process. In general, the solution viscoelasticity of typical TAPs can be described by the single Maxwell model with G′ ∝ ω2 and G″ ∝ ω1 in the terminal region.16 As can be seen, the deviation of slope in the terminal region between the single Maxwell model

Figure 5. (A) Plot of the steady shear viscosity η vs shear rate γ̇ for 3 wt % CouTAP solution upon exposure to 365 nm light for different times. (B) Plot of the zero-shear viscosity η0 vs irradiation time t for 3 wt % CouTAP aqueous solution. F


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Macromolecules Scheme 2. Network Model of CouTAP Aqueous Solution after Exposure to UV Light

Figure 6. (A) Temperature sweep measurements (ω = 10 rad/s) for 3 wt % dimerized CouTAP aqueous solution. (B) Frequency measurements for 3 wt % dimerized CouTAP aqueous solution at indicated temperature.

result, the shear-induced orientation effect reduces and no shear thickening is observed in the CouTAP solution. In addition, an overview of the structural model is detailed in Scheme 2 to vividly describe the network development in the solution. As aforementioned, CouTAP can self-assembly into flowerlike micelles in the solution. As the concentration surpass critical percolation concentration, the end groups of extra CouTAP can dynamically attach in and detach from the neighborhood micelles. Because of the weak hydrophobicity of

anisotropic creation of strand model, the origin of the shear thickening is the elastic strands preferentially reassociate in the shear-gradient direction since the orientation of the strands. The reassociation and disengagement balance shifts to the reassociation side, leading to clear shear thickening.5 In the irradiation process, the dynamics of elastic strands reassociating in or disengaging out of the micellar cores slows down and a well percolated network with slow relaxation is formed. As a G


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Figure 7. (A) Plots of G′ (closed symbols) and G″ (open symbols) vs angular frequency ω. (B) Plots of steady shear viscosity η vs shear rate γ̇. (C) BSW spectra of CouTAP aqueous solution at indicated concentrations (1−5 wt %) after UV light irradiation.

in Figure S8. As can be seen, the time−temperature superposition works well for the CouTAP solution, indicating the temperature dependence of the solution follows Arrhenius behavior. From previous discussion, the G0 values will decrease 10−30 times among this temperature region for typical alkyl TAPs.8,11 It means that the descend range of G0 of CouTAP solution is much smaller than the typical TAPs as temperature increases, performing excellent thickening performence in high temperature. On the other hand, the steady shear measurements at indicated temperature are shown in Figure S9 also. Just like G0, η0 also exhibits a similar temperature dependence; η0 decreases as the temperature increases. The temperature dependence of the aT can be fitted by the Arrhenius equation expressed as52

the end group, the CouTAP solution cannot form a strong network at room temperature, and the solution behaves as viscous fluid. On the other hand, the in situ dimerization of coumarin end groups will concatenate the CouTAP polymers in the solution to form multiblock polymers after UV light irradiation. The dimerization of coumarin in the micellar cores will connect the flower loops to form some train loops. However, not all the flower loops in the micelle can be connected with each other to form an entire loop because of a limited dimerization degree of coumarin end groups after irradiation. Meanwhile, some transient bridges will be connected with the train loops of neighborhood micelles by the covalent bonding. As a result, a unique transient network composed of train loops and bridges covalently bonded to train loops is formed. The network show a wide relaxation time distribution in the slow mode which is contributed to the relaxation of train loops because the number of the dimerized end groups in each train loop is different. In order to further understand the influence of temperature on unique dimerization network and the relaxation behavior of CouTAP aqueous solution after UV light irradiation, the temperature sweep measurements were investigated for the 3 wt % CouTAP aqueous solution and shown in Figure 6A. As can be seem, both G′ and G″ slightly decrease with increasing temperature, but G′ surpasses G″ in the whole temperature range, and the solution shows an elastic behavior even though temperature increases until 60 °C. It indicates that the CouTAP solution behaves as a viscoelastic gel in the whole measurement temperature range. Furthermore, the frequency measurements for 3 wt % dimerized CouTAP at indicated temperature are shown in Figure 6B. The G0 decreases simultaneously as temperature increases, but G0 platues are observed clearly. The G0 values slightly decrease from 1300 to 500 Pa as the temperature increases from 15 to 50 °C. On the other hand, BSW spectra for 3 wt % dimerized CouTAP at indicated temperature are shown in Figure S6. As can be seen, the peak intensity decreases and the τpeak value moves to low relaxation time with increasing temperature, while the slope describes parallel lines. It means that the relaxation of train loops will be accelerated with increasing temperature. Moreover, the horizontal and vertical shift factors are denoted by aT and bT, respectively, and plots of aT and bT against temperature are shown in Figure S7. The master curves are constructed by shifting G′ and G″ data at different temperatures horizontally and vertically to fit the low-frequency data. The master curves of the G′ and G″ for 3 wt % dimerized CouTAP aqueous solution at the reference temperature of T = 25 °C are shown

ln(aT ) = Em(T −1 − Tr −1)/R

(1)

where Em is the activation energy representing the potential barrier to disengagement of the train loops from micellar cores, T is the thermodynamic temperature, Tr is the reference temperature, and R is the gas constant, equal to 8.31 J mol−1 K−1. The aT profile as a function of temperature in Arrhenius form for 3 wt % dimerized CouTAP aqueous solutions is shown in Figure S10. Clearly, aT follows an Arrhenius temperature dependence. The Em value obtained from the slope of linear fittings is 116 kJ/mol for the dimerized solution. Furthermore, the Em values for 3 wt % CouTAP solution with different irradiation times t are estimated also, and a plot of Em as a function of t is shown in Figure S11. Em values increase quickly; after that, the growth slows down and then reaches a plateau. The Em values dramatically increase from 62 to 116 kJ/mol as the irradiation time increases from 0 to 600 s, as the number of average hydrophobic segments in each train loop will increase with exposure time. A higher Em value suggests a stronger association interaction at a given polymer concentration.53 The results indicate that the potential barrier energy for train loops to disengage from the micellar cores is larger than the hydrophobic end groups of typical alkyl TAPs, which is attributed to the formation of multiblock ploymer during the irradiation process. The association of train loops is stronger than the typical end groups so that the dynamics of train loops is much slower. It further suggests that the unique network of CouTAP solution is less temperature dependent and performs significant thickening performnce at high temperature. In order to further understand the unusual transient network, rheological measurements were performed for the CouTAP aqueous solution at different concentrations before and after UV light irradiation. The frequency measurements of CouTAP H


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be contributed to the relaxation of such train loops. The high polymer concentration will increase the number of such bridges in the soltuion, leading to the growth of v in the network. On the other hand, the hydrophobic segments in each train loops should be consistent because coumarin end groups have almost the same final dimerization degree for all the concentration. The enchanging rate of train loops should be contributed to the number of hydrophobic segments in each train loops, and the relaxation time of the network can be regarded as the relaxation of the train loops, which depends on the dimerization degree of end groups rather than the concentration. As a result, the relaxation behavior hardly shows concentration dependence.

in indicated concentration before and after irradiation are shown in Figure S12. As can be seen, before irradiation, G″ > G′ in the whole frequency for all the solution, even though the concentration is 5 wt %, indicating the solution is viscous fluid. On the other hand, all the solution can form strong transient network and show clear crossover after irradiation. The G′ increases dramatically, and a clear plateau is observed, indicating the solution behaviors as viscoelastic gels, even the concentration is as low as 1 wt %. Furthermore, plots of the steady shear viscosity of the solution at indicated concentration before and after irradiation are shown in Figure S13 also. Before irradiation, all the solutions perform clear Newtonian behavior in the whole shear rate, but η0 increases as the concentration increases. On the other hand, the solution viscosity dramatically increases upon exposure to 365 nm light, and a pronounced shear thinning region is observed. The dependence of viscosity on shear rate in shear thinning region can be described by a power law relationship, similar to the behavior found in concentrated polymer solutions.54 Here the exponent of the power law is close to −1. Furthermore, plots of the frequency sweep measurements and steady shear measurements at indicated concentration after light irradiation are shown in Figures 7A and 7B. As Figure 7 illustrates, G0 and η0 both expectedly exhibit a huge rise as the concentration increases, indicating the elasticity of the solution increases with increasing polymer concentration. It is worth noting that the crossover frequency of G′ and G″ in Figure 7A and the onset shear rate of shear thinning region in Figure 7B seem to remain no change with increasing concentration. It seems that the maximum relaxation time of the solution is independent of the concentration. In order to further understand the relxation behavior of the solution with different concentration, the BSW spectra for CouTAP aqueous solution at indicated concentrations (1−5 wt %) after UV light irradiation are shown in Figure 7C. As can be seen, all the spectra show clear both fast and slow modes. Both the fast and slow modes decrease or increase with nearly the same slope and show parallel lines. The same τpeak values with different Hi are observed. From above discussions, we can draw a conclusion that the relaxation behavior of the dimerized CouoTAP hardly depends on the concentration, which only influences the G0. Furthermore, the η0 can be obtained from η0 =

∫0

τmax

H(τ ) dτ

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CONCLUSIONS In conclusion, we reported the photoinduced dimerization of a novel coumarin-functionalized telechelic associative model polymer (CouTAP) in aqueous solution and its unusual solution rheological behavior. The photoinduced dimerization of coumarin end groups will suppress the end groups in the micellar cores, resulting in a unique transient network. Furthermore, on the basis of the effect of irradiation time, light intensity, concentration, and temperature on the rheological behavior of the CouTAP solution, a novel transient network model composed of train loops and bridges covalently linked to train loops is proposed to describe the unique solution properties. The dynamics of the network relaxation can be regarded as the rate of attachment in and detachment from the micellar cores of the train loops. The results are therefore useful not only for understanding the formation, transition, and evaluation of dynamic transient networks in TAP aqueous solution induced by external stimuli but also for preparing multiblock polymers from coumarin-functionalized TAPs by photoinduced dimerization of coumarin end groups.

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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.7b01514. Characterizations of CouC11OH end group and CouTAP polymer; the standard curve obtained from the maximum absorbance at 320 nm of CouC 11OH in DMF; rheological data of CouTAP aqueous solution before and after 365 nm light irradiation; Arrhenius plots for 3 wt % CouTAP solution after 365 nm light irradiation (PDF)

(2)

It shows that the viscosity rise with increasing concentration is principally caused by the incerasing effective elastic chains, since the BSW spectra for all concentrations are nearly the same. Normally, the transient network of TAPs shows a strong concentration dependence; the v and τ values will grow along with the increasing concentration. The characteristic relaxation time is associated with the necessary time for the end group to exchange among junction points. Annable et al. had suggested that the concentration-dependent relaxation behavior of typical TAPs network should be contributed to the formation of superbridges. Thus, increasing polymer concentration will result in longer relaxation time. In the present case, v markedly depends on the concentration, but relaxation behavior remains constant with increasing concentration. As previously disscussed, the dimerization of coumarin will form multiblock polymer with double alkyl chain length in the solution and lead to some train loops and bridges covalently linked to train loops in the network. The relaxation of network under stress should

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AUTHOR INFORMATION

Corresponding Authors

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

Biye Ren: 0000-0003-0131-8750 Author Contributions

Z.D. and X.Y. contributed equally. 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 I


Article

Macromolecules

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Innovation Commission (201607010212). We thank Prof. Weixiang Sun and Dr. Jintian Luo for his helpful discussions.

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Unusual Transient Network and Rheology of a Photoresponsive

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