Letter pubs.acs.org/macroletters
Orthogonal Control of the Dynamics of Supramolecular Gels from Heterotelechelic Associating Polymers Jérémy Brassinne, Jean-François Gohy,* and Charles-André Fustin* Institute of Condensed Matter and Nanosciences (IMCN), Division of Bio and Soft Matter (BSMA), Université catholique de Louvain, Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium S Supporting Information *
ABSTRACT: One of the first examples of supramolecular gels presenting independent dual dynamics is built through a combination of hydrophobic and metal−ligand interactions. The associating building block consists in a water-soluble linear polymer terminated by a short hydrophobic sticker at one end, and a coordinating moiety at the other end. The distinct supramolecular nature of these noninterfering binding motifs allows the dynamics of the hydrogels to be finely tuned in an orthogonal fashion by the application of specific stimuli. Precisely, the solvent-induced plasticization of the hydrophobic associations and the acid-promoted dissociation of the metal− ligand complexes are used to control the network dynamics. By opposition to classically encountered binary gel−sol responses, we demonstrate that the stimuli-induced transition in material properties can be gradual, provided that the material structure is well designed and strong enough.
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reversed by stimuli (ON−OFF switch), thereby missing a wide spectrum of possible intermediate properties. At the core of our research lies a self-assembly based on a unique combination of hydrophobic interactions and metal− ligand coordination into supramolecular gels.27−29 We recently reported on the synthesis and stepwise assembly of heterotelechelic water-soluble polymers, as depicted in Figure 1.30,31 The latter consist of linear poly(N-isopropylacrylamide)s (PNIPAAm), with one chain-end terminated by a short polystyrene (PS) segment and the other bearing a terpyridine ligand (tpy). In practice, the first step of assembly is achieved by direct dissolution of the associating copolymers in aqueous media, leading to the aggregation of PS stickers into hydrophobic domains. The strength and dynamics of those associations can be finely tuned as a function of the hydrophobe length, from transient to frozen glassy micellar cores. Afterward, the second step of organization is achieved by the addition of transition metal ions to the micellar solutions. In presence of terpyridine ligands, metal ions form bis-complexes that effectively bridge the hydrophobic cores to form a percolated network. Furthermore, the strength and lability of these metallo-bridges can be varied according to the chelated cation. Finally, the thermoresponsive PNIPAAm sequence allows the tuning of the gel properties by controlling the collapse of the PNIPAAm onto the micellar cores as previously demonstrated.32
upramolecular polymer gels have attracted a lot of attention from the scientific community as a class of smart and functional materials showing stimuli-responsiveness and selfhealing abilities.1,2 They are generally constructed through the self-assembly of oligo- or polymeric building blocks via hydrogen bonding,3,4 metal−ligand coordination,5 host−guest interaction,6−8 hydrophobic association,9−13 or ionic bond.14,15 In this field, combining multiple noncovalent interactions has opened the way to hierarchically assembled materials, which lead to a parallel hierarchy of dynamic processes.16−18 Such systems hence show multiresponsiveness with the possibility of orthogonally targeting each individual association by appropriate stimulus.19−23 However, only binary gel−sol transitions are generally triggered upon applying the stimulus.24−26 Indeed, weak bonds are often dramatically affected by small environmental changes, which results in the disassembly of the supramolecular scaffold tethering the gel. In the abundant literature, the widely admitted tube inversion test provides facile qualitative visualizations of a gel−sol transition but does not afford any information regarding how the material properties can be finely affected in response to the applied stimuli. Hence, the possibility to use stimuli to more finely and progressively control the mechanical properties of materials build from robust and orthogonal associations is not clearly demonstrated yet. This missing quantitative insight would extend our fundamental understanding and control over supramolecular materials, and would further contribute to future developments of functional and smart devices. In essence, the fundamental goal of this study is to broaden the common opinion that gelation can only be triggered or © XXXX American Chemical Society
Received: November 2, 2016 Accepted: November 23, 2016
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Figure 1. Schematic representation of the stepwise assembly of heterotelechelic associating polymers into a transient coordination micellar gel.
on the rheological properties of hydrogels formed by poly(acrylic acid) end-capped with polystyrene blocks.34 They showed that addition of DMF mainly affects the magnitude of the linear viscoelastic response of these self-assembled materials. However, the associating networks were characterized by long relaxation times that could not be determined within the experiment time scale. For this investigation, a PS27-b-PNIPAAm300-tpy (numbers in subscript indicate the average degree of polymerization for each block) associating copolymer was synthesized according to a procedure described elsewhere (see Supporting Information).30,31 This copolymer was then dispersed in pure water followed by the addition of hydrochloric acid (HCl) or DMF to reach a given concentration ranging in the semidilute nonentangled regime. Thanks to the short PS sticker, direct dissolution of the copolymer was possible in a reasonable time scale. To induce gelation, half an equivalent, with respect to the terpyridine content, of nickel(II) ions as the chloride salt was finally added to the micellar solutions (Figure 1). This metal ion was selected so that the dynamics of metal−ligand bonds differs of orders of magnitude from the one of hydrophobic associations as schematized in Figure 2. The detailed characterization of this system without stimuli can be found in our previous papers.30,31 To probe the dynamic response of the materials, rheological characterizations were conducted in the form of frequency sweeps at a controlled, small amplitude oscillatory stress (see Supporting Information). In a first series of samples, increasing molar amounts, 0.001, 0.01, and 0.1 M, of HCl were introduced while keeping the final concentration in copolymer constant at 5% w/v. These strong acidic conditions compete with the metal−ligand coordination, but supramolecular gels that withstand the tube inversion test were nevertheless obtained even at pH = 1. This observation highlights the high robustness of the Ni(II)−terpyridine biscomplexes, as already noticed by Schubert and co-workers.33 Higher HCl concentrations were not investigated because they would damage the rheometer geometries and would induce irreversible chemical changes resulting in the destructuration of the material. While supramolecular gels are obtained in all cases, acidification of the gel media leads to a pronounced decrease in the slow relaxation time characterizing the self-assembled network (τ2). As shown in Figure 3, the modulus crossover that marks the terminal relaxation of the material is indeed gradually shifted by two decades to higher frequencies as the HCl concentration is increased from 0.001 to 0.1 M. In a chemical point of view, this pronounced variation can be rationalized by the study of Wilkins and collaborators who determined the relationship between pH and dissociation rate of metal− terpyridine complexes in water.35,36 Through kinetic measurements and theoretical calculations, they indeed demonstrated
Interestingly, the rheological signature of PS-b-PNIPAAmtpy hydrogels reflects the individual contribution from each transient junction, being readily tuned by the hydrophobe length and the nature of metal complexes.30,31 As depicted in Figure 1, their dissociation allows stress-relaxation at discrete rates that match the lifetimes of hydrophobic (τ1) and coordinative (τ2) associations. In turn, those variables determine the amount of deformation energy that is stored (G′) in entropic distortion of the polymer network or lost (G″) due to relaxations that occur on the time scale of the deformation (Figure 2). In the present study, the high
Figure 2. Schematic rheological response of a transient network constructed from associating polymers bearing a hydrophobic segment (τ1) and a coordinating moiety (τ2).
robustness of the network associations, that is, the polystyrene cores and metal−terpyridine bridges, presumably allows them to withstand environmental changes. The supramolecular hydrogels should thus exhibit a much more subtle response to stimuli than a simple binary gel−sol transition. Hence, such materials constitute formidable model systems to validate the possibility of controlling the molecular dynamics of each transient junction separately, and to evaluate the effects on the macroscopic mechanical properties. To orthogonally address the dynamic mechanical properties of PS-b-PNIPAAm-tpy hydrogels, two common stimuli have been selected that are pH and a plasticizing cosolvent. On one hand, protonation of the terpyridine ligand at low pH should compete with metal−ligand coordination. In this frame, Schubert et al. already investigated the stability of different bis-terpyridine macro-complexes regarding pH variations. In their study,33 the possibility to open such complexes by protonation of the ligands was demonstrated, suggesting that material properties could be tuned accordingly. On the other hand, the presence of a plasticizing cosolvent should dramatically enhance chain mobility in the hydrophobic domains, that is, the micellar cores. In this context, Tsitsilianis et al. reported on the effect of N,N-dimethylformamide (DMF) 1365
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as evidenced by the shift of the local maximum in G″ toward higher oscillatory frequencies. On the other hand, the dynamics of the materials remains essentially unaltered in the lowfrequency regime, therefore, attesting the specificity of the plasticizing agent for hydrophobic associations. Indeed, adding DMF to the gel media should mainly increase the mobility of the PS stickers. The reduced interfacial tension between the hydrophobic cores and the surrounding media also enhances the detachment of the chain-ends from the network nodes,37 leading to a decrease of the fast relaxation time (τ1). Interestingly, such findings can be correlated to dynamic tensile measurements on lightly modified polystyrenes by Hara et al., who demonstrated that exposition to DMF also enhances chain relaxation in the bulk PS materials.38 To the best of our knowledge, the exchange kinetics of unimers between PS-bPNIPAAm micelles in aqueous media has not been reported in literature. Furthermore, comparison with such an independent determination would be complicated by the fact that the PS escape dynamics from the micellar core is affected by many parameters (presence of charges, degree of swelling/collapse of the corona, degree of swelling of the core, etc.).30 Above 10% v/v DMF, a dramatic change in the viscoelastic response of the gel was observed. Under these conditions, it is thought that DMF is not acting simply as a plasticizer for the micellar cores, but dissolves them, hence, the loss of the gel-like properties. As shown in Figure S3 (see Supporting Information), a weak viscoelastic response was still measured in the low frequency regime, evidencing a residual network-like structure. However, the moduli are much lower showing the dramatic loss in network connectivity. Finally, the degree of orthogonality between the coordination and hydrophobic interactions was further examined by simultaneously imposing the two stimuli, that is, pH and cosolvent. As shown in Figure 5, the frequency response of this hydrogel can be decomposed to match, respectively, the highfrequency (τ1) and low-frequency (τ2) dynamics of samples prepared in the presence of cosolvent or under acidic conditions only. Essentially, this observation is evidence of the possibility to selectively disrupt metal−ligand or micellar associations by chemical means, allowing the material relaxations to be activated orthogonally. To conclude, a controlled transient network is here formed through the assembly of a heterotelechelic associating polymer. This tailored material is one of the first examples of supramolecular polymer systems with noninterfering binding motifs whose dynamics can be orthogonally tuned by application of stimuli in the gel state. The associating polymer consists in a water-soluble linear polymer terminated by a short hydrophobic sticker at one end, and a coordinating moiety at the other end. Solvent-induced plasticization of the hydrophobic associations and acid-promoted dissociation of the metal−ligand complexes are tested as simple routes to control the network dynamics. By opposition to classically encountered binary gel−sol responses, we demonstrate here that the stimuliinduced transition in material properties can be progressive provided that the material structure is well designed and strong enough. Here, the stimuli targeted to the network nodes are not simply used to disassemble the material scaffold but rather to finely tune its dynamics and, hence, network relaxation, as quantitatively accessed by rheology. The investigated materials are particularly versatile since the two relaxation modes are regulated in an orthogonal fashion, without changing the composition of the associating polymer chain. This promising
Figure 3. Rheological response of hydrogels prepared from PS27-bPNIPAAm300-tpy copolymers and Ni(II) ions, in the presence of various HCl concentrations.
that medium-to-high acidic conditions open an enhanced dissociation pathway for ligand exchange, via the protonation of pyridine units. As a general rule, first-order dependencies on the acid concentration were predicted and observed, independently of the nature of the metal ions. In particular, a roughly 100× increase has been reported for the dissociation rate of terpyridine bis-complexes when increasing the concentration of acid from 0.001 to 0.1 M.35,36 These independent kinetic determinations correlate very nicely with the 2 orders of magnitude increase in the low frequency relaxation rate (1/τ2) of the network estimated from the crossover frequencies in Figure 3. On the other hand, our rheological characterization demonstrates no significant impact on the high frequency response of the hydrogels. In particular, the fast relaxation of the network remains unchanged when varying the HCl concentration (Figure 3). This observation attests the specificity of the stimulus and thus paves the way toward orthogonal control over material dynamics. In addition, the invariance of the high frequency elastic plateau modulus provides clear evidence that the network structure, including connectivity and architecture, remains almost unaffected. In other words, the structure is essentially identical in presence or absence of stimuli, and only the dynamics of the self-assembled material is affected by the stimuli. A second series of gel samples with increasing amounts, 1, 5, and 10% v/v, of DMF was subsequently prepared, while keeping pH around neutral and polymer concentration constant. As shown in Figure 4, the presence of the plasticizing solvent essentially accelerates the fast relaxation of the gels (τ1),
Figure 4. Rheological response of hydrogels prepared from PS27-bPNIPAAm300-tpy copolymers and Ni(II) ions, in the presence of various DMF concentrations. 1366
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F.R.I.A. for a Ph.D. thesis grant. Christian Bailly and Evelyne Van Ruymbeke are acknowledged for providing access to rheological facilities.
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(1) Appel, E. A.; del Barrio, J.; Loh, X. J.; Scherman, O. A. Chem. Soc. Rev. 2012, 41, 6195−6214. (2) Noro, A.; Hayashi, M.; Matsushita, Y. Soft Matter 2012, 8, 2416− 2429. (3) Noro, A.; Hayashi, M.; Ohshika, A.; Matsushita, Y. Soft Matter 2011, 7, 1667−1670. (4) Nair, K. P.; Breedveld, V.; Weck, M. Macromolecules 2008, 41, 3429−3438. (5) Brassinne, J.; Fustin, C.-A.; Gohy, J.-F. J. Inorg. Organomet. Polym. Mater. 2013, 23, 24−40. (6) Li, S.; Lu, H.-Y.; Shen, Y.; Chen, C.-F. Macromol. Chem. Phys. 2013, 214, 1596−1601. (7) Liu, D.; Wang, D.; Wang, M.; Zheng, Y.; Koynov, K.; Auernhammer, G. K.; Butt, H.-J.; Ikeda, T. Macromolecules 2013, 46, 4617−4625. (8) Zhang, M.; Xu, D.; Yan, X.; Chen, J.; Dong, S.; Zheng, B.; Huang, F. Angew. Chem., Int. Ed. 2012, 51, 7011−7015. (9) Chassenieux, C.; Nicolai, T.; Benyahia, L. Curr. Opin. Colloid Interface Sci. 2011, 16, 18−26. (10) Hietala, S.; Strandman, S.; Jarvi, P.; Torkkeli, M.; Jankova, K.; Hvilsted, S.; Tenhu, H. Macromolecules 2009, 42, 1726−1732. (11) Suzuki, S.; Uneyama, T.; Watanabe, H. Macromolecules 2013, 46, 3497−3504. (12) Zhou, C.; Hillmyer, M. A.; Lodge, T. P. J. Am. Chem. Soc. 2012, 134, 10365−10368. (13) Reinicke, S.; Schmelz, J.; Lapp, A.; Karg, M.; Hellweg, T.; Schmalz, H. Soft Matter 2009, 5, 2648−2657. (14) Hunt, J. N.; Feldman, K. E.; Lynd, N. A.; Deek, J.; Campos, L. M.; Spruell, J. M.; Hernandez, B. M.; Kramer, E. J.; Hawker, C. J. Adv. Mater. 2011, 23, 2327−2331. (15) Hisamatsu, Y.; Banerjee, S.; Avinash, M. B.; Govindaraju, T.; Schmuck, C. Angew. Chem., Int. Ed. 2013, 52, 12550−12554. (16) Loveless, D. M.; Jeon, S. L.; Craig, S. L. Macromolecules 2005, 38, 10171−10177. (17) Yount, W. C.; Loveless, D. M.; Craig, S. L. J. Am. Chem. Soc. 2005, 127, 14488−14496. (18) Hackelbusch, S.; Rossow, T.; van Assenbergh, P.; Seiffert, S. Macromolecules 2013, 46, 6273−6286. (19) Nair, K. P.; Breedveld, V.; Weck, M. Macromolecules 2011, 44, 3346−3357. (20) Tian, Y.-K.; Chen, L.; Tian, Y.-J.; Wang, X.-Y.; Wang, F. Polym. Chem. 2013, 4, 453−457. (21) Weng, W.; Fang, X.; Zhang, H.; Peng, H.; Lin, Y.; Chen, Y. Eur. Polym. J. 2013, 49, 4062−4071. (22) Yan, X.; Xu, D.; Chen, J.; Zhang, M.; Hu, B.; Yu, Y.; Huang, F. Polym. Chem. 2013, 4, 3312−3322. (23) Hackelbusch, S.; Rossow, T.; Becker, H.; Seiffert, S. Macromolecules 2014, 47, 4028−4036. (24) Zeng, F.; Han, Y.; Yan, Z.-C.; Liu, C.-Y.; Chen, C.-F. Polymer 2013, 54, 6929−6935. (25) Weng, W. G.; Beck, J. B.; Jamieson, A. M.; Rowan, S. J. J. Am. Chem. Soc. 2006, 128, 11663−11672. (26) Chen, L.; Tian, Y. K.; Ding, Y.; Tian, Y. J.; Wang, F. Macromolecules 2012, 45, 8412−8419. (27) Guillet, P.; Mugemana, C.; Stadler, F. J.; Schubert, U. S.; Fustin, C.-A.; Bailly, C.; Gohy, J.-F. Soft Matter 2009, 5, 3409−3411. (28) Brassinne, J.; Mugemana, C.; Guillet, P.; Bertrand, O.; Auhl, D.; Bailly, C.; Fustin, C.-A.; Gohy, J.-F. Soft Matter 2012, 8, 4499−4506. (29) Jochum, F. D.; Brassinne, J.; Fustin, C.-A.; Gohy, J.-F. Soft Matter 2013, 9, 2314−2320. (30) Brassinne, J.; Stevens, A. M.; Van Ruymbeke, E.; Gohy, J.-F.; Fustin, C.-A. Macromolecules 2013, 46, 9134−9143.
Figure 5. Rheological response of hydrogels prepared from PS27-bPNIPAAm300-tpy copolymers and Ni(II) ions in the presence of a given concentration in DMF and/or HCl.
candidate is thus a source of interest to develop, for example, modular sensory systems or an advanced fluid damper that utilize a combination of orthogonal supramolecular interactions.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00831. Experimental details (PDF).
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Charles-André Fustin: 0000-0002-3021-5438 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to Marie Curie ITN 2013 “Supolen”, No. 607937, and to the P2M Programme from the ESF. C.-A.F. is a Senior Research Associate of the FRS-FNRS. J.B. thanks 1367
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ACS Macro Letters (31) Brassinne, J.; Gohy, J.-F.; Fustin, C.-A. Macromolecules 2014, 47, 4514−4524. (32) Brassinne, J.; Bourgeois, J.-P.; Fustin, C.-A.; Gohy, J.-F. Soft Matter 2014, 10, 3086−3092. (33) Lohmeijer, B. G. G.; Schubert, U. S. Macromol. Chem. Phys. 2003, 204, 1072−1078. (34) Tsitsilianis, C.; Aubry, T.; Iliopoulos, I.; Norvez, S. Macromolecules 2010, 43, 7779−7784. (35) Farina, R. D.; Hogg, R.; Wilkins, R. G. Inorg. Chem. 1968, 7, 170−172. (36) Holyer, R. H.; Hubbard, C. D.; Kettle, S. F. A.; Wilkins, R. G. Inorg. Chem. 1966, 5, 622−625. (37) Nicolai, T.; Colombani, O.; Chassenieux, C. Soft Matter 2010, 6, 3111−3118. (38) Hara, M.; Jar, P.; Sauer, J. Polymer 1991, 32, 1380−1383.
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