Linear Alternating Associative Polymer with Ultrahigh Molecular

Feb 28, 2019 - ... 381 Wushan Road, Guangzhou 510641 , China. § Department of Material Technology, Jiangmen Polytechnic , Jiangmen 529090 , China...
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Letter Cite This: ACS Macro Lett. 2019, 8, 279−284

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Linear Alternating Associative Polymer with Ultrahigh Molecular Weight: Facile Preparation by Self-Assembly Assisted Dimerization of Anthracene and Rheology in Aqueous Solution Zhukang Du,†,‡ Yuke Shan,‡ Jintian Luo,‡ Ning Sun,§ and Biye Ren*,‡ †

ACS Macro Lett. Downloaded from pubs.acs.org by WEBSTER UNIV on 02/28/19. For personal use only.

South China Advanced Institute for Soft Matter Science and Technology, South China University of Technology, 381 Wushan Road, Guangzhou 510641, China ‡ School of Material Science and Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510641, China § Department of Material Technology, Jiangmen Polytechnic, Jiangmen 529090, China S Supporting Information *

ABSTRACT: Alternating associative polymers (AAPs) containing more than two species of alternating hydrophobic and hydrophilic units can form unique physical network and perform interesting rheological behavior in aqueous solution. In this work, an AAP was prepared through self-assembly assisted dimerization of an anthracene-functionalized telechelic associative polymer (AnTAP) in aqueous solution by light irradiation. It is demonstrated that AnTAP can in situ chain extend to AAP with well-defined linear structure and ultrahigh molecular weight through dimerization reaction of anthracene moieties in the core of micelle under light irradiation. Meanwhile, the solution changes from viscoelastic liquid to a free-standing gel, because a physical network that cannot relax in a finite time window has developed along with the dimerization process. The results are therefore of interest not only for understanding the network structure and rheological properties of AAP solution, but also for preparing AAPs with ultrahigh molecular weight by selfassembly assisted photodimerization reactions.

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solution exhibit complex aggregation and unique self-assembly behavior.14 Usually, the association behavior is controlled by the number and the type of alternating units, and the ratio of hydrophilic to hydrophobic segments of the AAPs.15 Sometimes, AAPs can form transient networks like TAPs, but the dynamic of these networks is usually characterized by a broad distribution of relaxation times, because the micellar cores and the coronas are less well-defined. As reported in the literatures, AAPs with different structures can be prepared by living polymerization, condensation polymerization, and click reaction.16−21 But the above methods are very complicated and difficult to obtain linear AAPs with ultrahigh molecular weight in situ. Thus, for in situ preparing linear AAPs with ultrahigh molecular weight, a facile synthetic strategy is needed. Moreover, compared to the booming research efforts in the aggregation and rheology of TAPs, the rheology of AAPs with ultrahigh molecular weight has been studied limitedly and even poorly understood up to now due to the limitation of synthesizing and complicated aggregation. Therefore, the study on aggregation and rheology of AAPs is still in its infancy. If an AAP holds ultrahigh molecular weight and a number of

ssociative polymers (APs) containing hydrophobic and hydrophilic segments have a tendency to associate with each other to form physical network structures in aqueous solution.1−6 Consequently, APs have been widely used for conveying useful rheological properties to solutions, such as gelation, increasing viscosity, providing shear-thinning, and thickening in aqueous media.7−9 Telechelic associative polymers (TAPs), consisting of hydrophilic backbone and two hydrophobic end groups, are a class of representative triblock APs and can form so-called flower micelles composed of coronas (hydrophilic backbone) and cores (hydrophobic end groups) above a critical micelle concentration (cmc). With the increase of the solution concentration, some hydrophobic end groups will individually attach into neighboring micelles by hydrophobic association. Meanwhile, the hydrophilic backbones bridge the adjacent micelles, and a physical network is developed finally. The end groups can dynamically engage into and escape from the micellar cores, so the dynamic of the network can be characterized by a single finite relaxation time.10−13 The polymers containing more than two species of alternating hydrophobic and hydrophilic units are usually called as alternating APs (AAPs). Featuring the alternating structures, the multi-intermolecular and multi-intramolecular hydrophobic associations coexist in the AAPs solution, the © XXXX American Chemical Society

Received: January 13, 2019 Accepted: February 21, 2019

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DOI: 10.1021/acsmacrolett.9b00028 ACS Macro Lett. 2019, 8, 279−284

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ACS Macro Letters hydrophobic segments in per chain, one polymer chain can bridge several micellar cores to form a unique transient network in aqueous solution, and interesting rheological behavior will be obtained. For this purpose, we attempt to develop a superior strategy to prepare AAPs with a well-defined linear structure and ultrahigh molecular weight from TAPs end-functionalized by photodimerization groups, such as coumarin, cinnamic acid, and anthracene through unique photodimerization reaction in micelle. As we know, these photodimerization groups can undergo reversible photochemical cycloaddition reaction, and have been widely utilized for construction of photoresponsive micelles.22−25 Thus, the incorporation of hydrophobic photodimerization groups into TAPs as end groups will result in chain-extended reaction under UV light irradiation. The hydrophobic association can effectively facilitate the photodimerization as the end groups are suppressed in the micellar cores. Based on such a self-assembly assisted dimerization strategy, linear AAPs can be readily prepared from TAPs by in situ chain-extended reaction through dimerization of end groups in the core of micelle. Meanwhile, interesting rheological behavior and network structure will be obtained along with the reaction. Recently, we reported the dimerization and rheological behavior of a coumarin end-functionalized TAP in aqueous solution.26 Due to a limited dimerization degree of coumarin, the average molecular weight of the resulting AAP is relatively small and few hydrophobic blocks in per polymer chain. So, the rheological behavior of the resulting network is more or less similar to typical TAP solutions. For preparing AAPs with ultrahigh molecular weight and more hydrophobic blocks in each polymer chain, a photodimerization TAP with higher dimerization degree is needed. Compared to coumarin, the dimerization degree of anthracene can reach more than 99% in its crystal state.27,28 It is therefore possible that the target AAP will be achieved by dimerization of anthracene in the core of micelles, which will lead to interesting network structure and solution rheological behavior. In this work, we report the preparation and solution rheological behavior of a linear AAP with ultrahigh molecular weight prepared from a TAP model polymer end-functionalized by anthracene (AnTAP) by the dimerization of anthracene groups in the core of micelle under light irradiation. The AnTAP model polymer was synthesized by the reaction of poly(ethylene oxide) with a large excess of hexamethylene diisocyanate followed by the end-capping of the terminal isocyanate groups with anthracene-substituted undecanol (Scheme S1).29,30 1H NMR and FTIR spectra confirm the successful synthesis of AnTAP, as shown in Figures S1−S4. The Mn determined by GPC is 21800 Da, and the dispersity (Đ) is 1.08 (Figure S5). As aforementioned, TAPs can self-assemble into flower-like micelles in aqueous solution above cmc. First, surface tension measurements and static light scattering (SLS) measurements were used to determine the cmc value and average aggregation number (Nagg) of micelles; the results are shown in Figures S6 and S7, respectively. As can be seen, the determined cmc value is 0.47 g/L and average Nagg is 16.4. In response to 365 nm UV light irradiation, the anthracene in the micellar cores can dimerize via a [4 + 4] cycloaddition reaction.31,32 The chains will be connected end to end through the covalent bonding of end groups in the micellar cores to form linear AAPs (Scheme 1a). Meanwhile, a sol−gel transition occurs along with the in

Scheme 1. (a) Chemical Structure of AnTAP and AnAAP; (b) Inverted Vial Tests for 2 wt % Polymer Solution before and after UV Light Irradiationa

a

The solution was colored by blue ink.

situ dimerization. Before irradiation, the sample can quickly flow down to the bottom of the vial, performing as a viscoelastic liquid. After irradiation, the sample can freely stand on the top of the vial, performing as a gel (Scheme 1b). It means that a strong network is developed after the formation of AnAAP. In order to further understand the formation of AAP, UV− vis absorption measurements were carried out at room temperature to demonstrate the photochemical process. The time-course UV−vis spectra of 2 wt % polymer solution upon exposure to irradiation at 365 nm is shown in Figure 1a. As can be seen, the continuous decrease of the absorption peak indicates that the dimerization of anthryl end groups over time. After irradiation for more than 55 min, the spectra undergo no

Figure 1. (a) Time-course UV−vis spectra of AnTAP solution upon light irradiation at 365 nm. (b) The dimerization degree of the AnTAP solution after irradiation for different time. (c) 1H NMR spectra of AnTAP before and after UV light irradiation. 280

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molecular weight of each micelle (Mw(micelle) = Nagg × Mw(AnTAP)). It means that intramicellar and intramicellar dimerization occurs, some adjacent micelles can be covalently linked by the bridging polymer chains, and interesting rheological behavior and network structure should be obtained. For studying the viscoelastic properties and network structures of the TAP and AAP solutions, rheological measurements were carried out. As is well-known, TAPs can self-assembly into micelle, micellar aggregate and physical network with the increase of polymer concentration, leading to the solution viscosity rise sharply.35−37 First, steady shear measurements were conducted to determine the critical percolation concentration (Cp), and a plot of zero-shear viscosity (η0) versus C is shown in Figure S8. The Cp value is determined to be 0.6 wt % from the catastrophe point of the curve. As the concentration is smaller than Cp, the η0 values are nearly the same as pure water and increase with an extremely small slope, indicating the micelles and micellar aggregates are domain in the solution. As the concentration exceeds Cp, the η0 values dramatically increase, implying a network is formed and developed in this concentration range. The steady shear curves for the 0.4 wt % (Cp) solutions before and after irradiation are shown in Figure S9 also. As can be seen, the 0.4 wt % solution performs as typical Newtonian fluid in the range of the whole shear rates before and after irradiation. Since the micelles and micellar aggregates are domain in the 0.4 wt % solution, the in situ dimerization of anthracene will connect the telechelic chain to form alternating chain in an isolated micelle, and very few of adjacent micelles can be covalently linked by the bridging chains. As a result, the η0 value of the 0.4 wt % solution hardly changes after irradiation. On the other hand, the initial 2.0 wt % solution holds a Newtonian plateau at low shear rates and undergoes shear thinning at high rates, indicating a weak network is formed by AnTAP in this concentration. The η0 value is determined to be 10.5 Pa·s. But the AAP solution shows pronounced shear thinning over the whole shear range. The results indicate that the AAP forms a stronger network and shows much slower dynamic than initial TAP network. Furthermore, detailed oscillatory shear measurements were performed in the linear viscoelastic region to investigate the associative structure and relaxation behavior of the solutions at 25 °C. A plot of storage modulus G′ and loss modulus G′′ versus angular frequency ω is shown in Figure 3 for the 2 wt % AnTAP aqueous solution exposed to 365 nm UV light for different times. Before light irradiation, the values of G′ are almost smaller than G′′, while G′ and G′′ intersect at relatively high ω. It means that the transient network of AnTAP is very weak and the dynamics is fast. As the solution is irradiated by UV light for 2 min, the values of G′ are smaller than G′′ in the low ω region and crossover in high ω. Interestingly, the values of G′ and G′′ are numerically consistent in the mediate ω region, with G′ and G′′ ∝ ω0.5. The dynamic response of the solution is expected for the broad relaxation time distribution due to the large dispersity of the polymer chains. With increasing irradiation time, the dependence of G′ and G′′ on ω becomes weaker, while the high frequency moduli almost remain at consistent value. After irradiates for 55 min, G′ > G′′, and G′ hardly depends on ω in the whole ω. The above results suggest that the relaxation process in this solution is so slow that the solution performs non-Maxwellian relaxation in the experimental time window. It should be noted that the high frequency moduli or plateau moduli (G0) values slightly

change, implying the dimerization reaction has reached its photostationary state. Furthermore, the dimerization degree (D) can be roughly estimated from equation: D = (A0 − A)/ A0, where A0 is the initial peak intensity at 365 nm. The plot of D dependence on irradiation time t is shown in Figure 1b. The D values increase dramatically in the early stage, then the growth slows down and reaches a plateau. The D value can reach about 98.9% after 55 min of irradiation as hydrophobic association and π−π interaction in the micellar core can facilitate the photodimerization reaction. The results indicate that the majority of the polymer chains have chain-extended, and a linear AAP with ultrahigh molecular weight is retrievable. Furthermore, 1H NMR measurements were carried out to confirm the formation and structure of anthracene dimer in the solution after irradiation. From 1H NMR in Figure 1c, the anthracene groups in AnTAP show a series of characteristic proton peaks in the chemical shift (δ) range of 7.4−8.5 before UV light irradiation. On the other hand, the proton peaks corresponding to the initial anthracene groups disappear, and proton peaks in the chemical shift range of 5.7−7.0 for the anthracene dimer appear after irradiation. Furthermore, the δ of α-methylene slightly moves from 4.6 to 4.4 ppm. The above results indicate that the anthracene groups dimerize via a [4 + 4] cycloaddition reaction to form head-to-tail dimers.24,33,34 Since the topology of the polymer chains transforms from telechelic to alternating, the molecular weight and dispersity of the chains significantly increase as well. Thus, GPC measurements were conducted for the solution upon exposure to UV light versus time, and the results are shown in Figure 2. The

Figure 2. Chromatograms obtained by GPC in THF solution of AnTAP upon exposure to UV light vs time.

chromatograph of the initial solution shows only a narrow peak corresponding to AnTAP. As the irradiation process, another broad peak can be observed. It means that alternating chains with larger molecular weight were generated. As if the exposure time exceeds 55 min, the initial peak almost disappears while a wide peak can be observed. It is evident that the majority of the AnTAP have chain extended to AnAAP, and some linear chains can transform into cyclic chains according to previous research.24 This result is consistent with the previous observation from UV−vis measurements. Unfortunately, the elution time of the main peak is smaller than the elution time of 850 kDa PEO in the working curve. Thus, we cannot estimate the exact molecular weight of the resulting AAPs. However, it is no doubt that a linear AAP with ultrahigh molecular weight was obtained after irradiation. Furthermore, the molecular weight of AAP is much larger than the total 281

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Figure 3. Plots of G′ and G′′ vs angular frequency ω for the 2 wt % AnTAP aqueous solution after UV light irradiation for different times. (G′: black square; G′′: red cycle).

increase in the irradiation process. It indicates that the in situ dimerization of anthracene end groups slightly increases the number density of active junctions and slows down the network dynamic. On the other hand, each peak of G′′ can be roughly regarded as a relaxation mode. It is clear that there are two relaxation mode in the solution when irradiation time is larger than 5 min. The fast relaxation mode in the high ω region corresponding to the relaxation of AnTAP continue to decline and almost disappear in the irradiation process. It means that the relaxation of TAP becomes minor and the relaxation of AAP plays a dominate role in the solution after irradiation. In order to further understand the relaxation behavior of the solution with different irradiation time, the continuous relaxation time spectra H(τ) were obtained from fitting the following eqs 1 and 2):38−40 G′(ω) =

∫0

G″(ω) =

∫0

+∞

+∞

H (τ ) ω 2 τ 2 · dτ τ 1 + ω 2τ 2

(1)

H (τ ) ωτ · dτ τ 1 + ω 2τ 2

(2)

Figure 4. Continuous relaxation time spectra of 2 wt % AnTAP aqueous solution after UV irradiation for different times.

molecular weight can form strong association and numerous physical cross-linking in a chain. The results are in good agreement with the master curve obtained from time− temperature superposition (Figure S10). Moreover, an overview of the structural model is detailed in Scheme 2 to vividly describe the network formed by AnAAP. As previously discussed, the initial AnTAP can form a weak

The fitting results are shown in Figure 4. As can be seen, the spectra can be divided into two regimes: a fast mode and a slow mode regime. For the initial solution, only a fast relaxation peak can be observed in the spectra. The fast relaxation peak can be assigned to the relaxation of AnTAP, since the hydrophobic end group can quickly engage into and escape from the micellar cores. As increasing irradiation time, another slow mode can be observed in the spectra. The slow relaxation mode can be assigned to the relaxation of the AAP chains. The intensity of the fast relaxation peak decreases, but the intensity of the slow relaxation mode increases, indicating that TAP polymers gradually dimer into AAPs as increasing irradiation time. It should be noted that the H(τ) in the slow mode can go down for the solution when irradiation time is less than 34 min. It implies that the solution can relax in finite time. As the irradiation time over 34 min, the H(τ) cannot go down but remain in a linear state. It suggests that the solution cannot relax in finite time as the linear AAP with ultrahigh

Scheme 2. Network Model of AnAAP Aqueous Solution

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ACS Macro Letters network in the solution. Because of the weak hydrophobic interactions between end groups, which can quickly attach into and detach from the micellar cores, the solution behaves as viscoelastic fluid. On the other hand, the in situ dimerization of anthracene end groups will concatenate AnTAP to form linear AnAAP with ultrahigh molecular weight after UV light irradiation. Meanwhile, more hydrophobic and less soluble anthracene dimer stickers in the micellar cores are generated, and the dimer can connect the flower loops to form “train like” loops in the coronas. Since there is a stronger hydrophobic association between the stickers, the relaxation is much slower than the initial solution since potential barrier for all the hydrophobic segments in each loop to detach from the micellar cores to relax the stress is much stronger. Moreover, the AAP chains can form several associating junctions with the other polymer chains in adjacent micelles, and a few of the AAP chains can entangle with each other to form linking points. Thus, AAP can bridge several micelles and a strong network is formed. Due to the strong hydrophobic association in the micellar cores and multiple junctions in a polymer chain, the dynamic of AAP is suppressed and the network cannot relax in finite time. Moreover, AnAAP can recover to TAP through the thermal cleavage reaction, as reported in the literature.41−43 The peaks for the anthracene monomer in the 1H NMR spectra reappear after heating at 150 °C for 1 h in bulk under vacuum (Figure S11). After repeating photodimerization and thermal cleavage five times and subsequently analyzing through UV−vis absorption measurements, the maximum and minimum absorbance intensities in the spectra at 365 nm hardly change (Figure S12). Furthermore, the profiles of rheological properties are similar to that of the original polymer solution. The G′, G′′, and η0 values almost completely recover to their initial values after heating (Figures S13 and S14). It suggests that the resulting network recover its initial state, performing excellent reversible photodimerization and thermal cleavage behavior. In conclusion, we reported a facile preparation of AAP with well-defined linear structure and ultrahigh molecular weight by photoinduced dimerization of an anthracene-functionalized telechelic associative polymer (AnTAP) in aqueous solution and its unusual solution rheological behavior. The photoinduced dimerization of anthracene end groups in the micellar cores can generate linear AAP with ultrahigh molecular weight when the solution concentration exceeds Cp. In the meanwhile, the solution changes from viscoelastic liquid to a free-standing gel. The dynamic of AAP in solution is suppressed by association and entanglement, and a physical network that cannot relax in a finite time window is formed. The results are therefore useful not only for understanding the network structure and rheological properties of the AAP solution, but also for developing a convenient strategy to prepare ultrahigh molecular weight AAP by photoinduced dimerization of responsive groups in the micelle.

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Experimental section, details on the synthesis and characterization of AnTAP, plots of cmc and Nagg, and additional rheological results of the solution (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhukang Du: 0000-0003-4730-0468 Biye Ren: 0000-0003-0131-8750 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We gratefully acknowledge the financial support from the NSFC (21674039), China Postdoctoral Science Foundation (2018M643065), and Jiangmen Science and Technology Bureau (No. [2017]368).

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00028. 283

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