Article pubs.acs.org/Macromolecules
Mechanical Behaviors of Highly Swollen Supramolecular Hydrogels Mediated by Pseudorotaxanes Zhiqiang Li,†,§ Ying-Ming Zhang,† Huan-Yu Wang,§ Huanrong Li,§ and Yu Liu*,†,‡ †
Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, and ‡Collaborative Innovation Center of Chemical Science and Engineering, Nankai University, Tianjin 300071, P. R. China § School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, P. R. China S Supporting Information *
ABSTRACT: Supramolecular self-assemblies based on multiple noncovalent forces have emerged as an effective way to fabricate multistimuli-responsive and self-healable intelligent materials. However, the development of supramolecular nanostructures as advanced functional materials in practical fields is always subjected to their relatively weaker mechanical strength and overuse of organic components. In this work, we proposed a new strategy to construct self-supported supramolecular hydrogels with the features of tunable mechanical strength, high water content, and self-healing properties through the hierarchically organic−inorganic hybridization of Laponite matrix with cyclodextrin-based pseudopolyrotaxanes. Notably, the mechanical properties of the obtained hydrogels can be conveniently modulated by tuning the molecular weight of polymeric chains in the central pseudopolyrotaxanes, which may provide a feasible way to promote the practical application of supramolecular metamaterials in miscellaneous fields.
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INTRODUCTION
created with improved mechanical properties and new functionality.16 To further optimize the physicochemical performance of water-based soft materials and relieve the excessive burden on tedious chemical synthesis, herein we would like to report a novel strategy to construct robust and stable hydrogels with tunable mechanical strength, high transparency, high water content, and free-standing properties through the hierarchical assembly of clay nanosheets with cyclodextrin (CD)-based pseudopolyrotaxanes (PPRs, Scheme 1). The employed adhesive units are water-soluble PPRs obtained by threading per(6-guanidino-6-deoxy)-β-CD17 onto poly(propylene glycol) (PPG) chain. In our case, the guanidinium cations are evenly distributed along the axial direction of PPG chain, which makes it more conducive to realize the multivalent interactions between the molecular glue and Laponite matrix.18,19 More significantly, different from the covalently chemical synthesis, the number of CD units and the amount of guanidinium cations can be fine-tunable by alternating the molecular weight of PPG polymers, thus leading to the different types of supramolecular hydrogels with controlled mechanical performance. Thus, we can envision that the rational design of PPRinvolved hydrogels may provide a feasible way in the fabrication of ultrastrong and moldable organic−inorganic hybrid materials.
In the past decades, water-based soft materials (e.g., hydrogels) via covalent linkage have aroused considerable interest and are widely applied in food additives, separation and purification of biomacromolecules, and tissue engineering scaffolds.1,2 This is mainly due to their extraordinary mechanical properties arising from the strong covalent-type networks. Nevertheless, covalently linked hydrogels are usually brittle, poorly transparent, and lacking of desirable external responsiveness and self-healing ability.3−6 In addition, harmful organic reagents or solvents may be required during the preparation process. To overcome these shortcomings, scientists have diverted their attention to the supramolecular approach.7 Consequently, nano-supramolecular self-organization based on noncovalent driving forces has emerged as an alternative or even more powerful method to construct intelligent hydrogels with good transparency and fascinating multistimuli-responsive, self-correction, and selfhealing characteristics.8−12 An impressive progress in this area is the emergence of “aqua materials”, a type of high-watercontent moldable supramolecular hydrogels reported by Aida’s group in 2010, which can be readily available upon mixing dispersed Laponite nanosheets with dendritic molecular binders containing multiple guanidinium termini.13 To be noticed, it is found that the molecular structure of the guanidinium-rich cross-linkers is a decisive factor to affect the mechanical performance of the resultant noncovalent hydrogels.14,15 Thus, it is believed that by elaborately designing the adhesive blocks with good adaptive capacity to peripheral inorganic frameworks, more advanced organic−inorganic hybrid soft materials can be © XXXX American Chemical Society
Received: November 13, 2016 Revised: January 17, 2017
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DOI: 10.1021/acs.macromol.6b02459 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Schematic Illustration of the Molecular Structures of PPG Chain, ASAP-Exfoliated Laponite, and PPRsa
a
For PPR A, m = 4; PPR B, m = 8; PPR C, m = 17.
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Laponite under low concentration (Figure 1a).26,27 Moreover, when PPG was added to the ASAP exfoliated Laponite
RESULTS AND DISCUSSION β-CD-containing PPRs were prepared by adding different PPG chains (Mw = 1000, PPG1000; Mw = 2000, PPG2000; and Mw = 4000, PPG4000) into the saturated aqueous solution of guanidine-modified β-CD (1).20,21 The complexation behaviors of PPG with 1 were investigated by 1H NMR spectroscopy. Taking the threading of 1 with PPG2000 as an example, the number of CD units was calculated by comparing the integrated signals at 4.98 ppm (H1 of 1) and 3.95 ppm (H5 of 1) with methyl proton of PPG2000 at 1.01 ppm (Figure S1 in the Supporting Information). The value was thus obtained as 7.8, suggesting that about 8 CD units on average were located on the main chain of PPG2000 to form PPR B. It is noted that the number of 1 on PPG2000 chain was less than the typical value, and this result can be explained as follows.22 The threaded CDs could form a head-to-head channel-type structure along the polymer axis and tightly stack with each other via intermolecular hydrogen bonds;23,24 however, the unfavorable electrostatic repulsion of guanidinium pendants could break the hydrogen bonding networks between the primary faces of the neighboring 1. As a result, the intermolecular distance between two consecutive 1 dimers was eventually elongated.25 Moreover, the 1H ROESY spectrum of PPR C showed the clear correlation peaks of the methyl protons of PPG4000 with the interior protons of β-CD, further corroborating that PPG was threaded into the CD’s cavities (Figure S2).22 Similarly, in the case of PPRs A and C, the numbers of threaded CD units on PPG1000 and PPG4000 were calculated as 4.2 and 16.9, respectively. Then, pristine Laponite was homogeneously dispersed in water with assistance of sodium polyacrylate (ASAP), and no hydrogel was observed for the individual ASAP-exfoliated
Figure 1. Hydrogelation by mixing PPR B with ASAP-exfoliated Laponite in water. Pictures of supramolecular hydrogels formed by (a) 2.0/0.07 wt % (Laponite/ASAP) and (b, c) 2.0/0.07/0.075 wt % (Laponite/ASAP/PPR B, [guanidinium] = 2.8 × 10−3 mol/L).
nanosheets, no gelation was observed in the absence of 1. Interestingly, the linear PPRs were shown to rapidly form supramolecular hydrogels upon mixing with ASAP preexfoliated Laponite, and the fluidity was lost in only 3 min. In addition, the obtained hydrogels were nearly transparent and could be easily molded into self-standing objects (Figure 1b,c), which was rarely observed for synthetic supramolecular hydrogel systems. More importantly, the content of organic component of such hydrogels was ultralow (0.15%), and the B
DOI: 10.1021/acs.macromol.6b02459 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 2. (a) Frequency (ω) sweep tests at ω = 0.05−100 rad s −1 and strain (γ) = 0.5% at 25 °C ([guanidinium] = 2.8 × 10−3 mol/L; green, Laponite/ASAP/PPR A = 2.0/0.07/0.075 wt %; red, Laponite/ASAP/PPR B = 2.0/0.07/0.075 wt %; black, Laponite/ASAP/PPR C = 2.0/0.07/ 0.075 wt %; and blue, Laponite/ASAP/1 = 2.0/0.07/0.065 wt %). (b) Strain sweep tests of Laponite/ASAP/PPR B = 2.0/0.07/0.075 wt % at γ = 0.05−100% with ω = 6.28 rad s −1. (c) Continuous step strain tests of Laponite/ASAP/PPR B = 2.0/0.07/0.075 wt % at γ = 0.1 and 100%. (d) Frequency sweep tests of Laponite/ASAP/PPR B = 2.0/0.07/0.075 wt % at 25 °C (i), after self-healing (ii), and at 70 °C (iii).
content was low to 0.15 wt %, the G′ values of our hydrogels was nearly 2 orders larger in magnitude than the regular supramolecular hydrogels with high-concentration organic composition.28,29 It is also noted that the corresponding G′ value of PPR Cbased supramolecular hydrogel could reach up to 20 kPa, which was 2 times larger than the reported aqua materials.13 In addition, upon fixing strain amplitude sweep at ω = 6.28 rad s−1, the PPR B-based supramolecular hydrogels underwent a gel-to-quasi-liquid state transition at the critical strain region (γ = 18.6%, Figure 2b), which uncovered the breakdown of the hydrogel network. More interestingly, the gel-to-quasi-liquid transition was completely reversible, when treated with a large amplitude oscillatory force (γ = 100%; frequency, ω = 6.28 rad s−1). The G′ value of PPR B-based supramolecular hydrogel decreased from 9 to 0.4 kPa, thus resulting in a quasi-liquid state (tan δ ≈ 2.25). However, when the amplitude was decreased (γ = 0.1%) at the same frequency (1.0 Hz), both G′ and G″ values recovered rapidly back to the initial values within 30 s and the system returned to a gel state. To investigate the impact of PPR length on mechanical properties of the resultant supramolecular hydrogels, we continued to determine the G′ and G″ values of the hydrogels prepared with PPR A, PPR B, and PPR C. It is found that both G′ and G″ values increased as the length of PPG chains proportionally increased, with the maximum value in PPR Cbased hydrogel (Figure 2a). This is attributed to PPR C with high guanidinium distribution that is more beneficial to crosslink the neighboring clay sheets together.30−32 Although the hydrogel state could be obtained without PPG polymer, the G′ and G″ values of these PPG-free hydrogels sharply decreased
water content was quite high (98%). It is also noteworthy that supramolecular hydrogels usually underwent a gel-to-sol transition upon heating, which is mainly contributed to the nature of noncovalent interactions.13 Nevertheless, our supramolecular hydrogels showed high thermal stability, and no phase transition was observed even heating up to 70 °C (Figure S3). To clarify the complementary interactions between guanidinium and the Laponite nanosheets, an excess amount of guanidine hydrochloride (5 equiv to guanidinium of PPRs) was added as competitive molecules to presaturate the ASAP exfoliated Laponite nanosheets. As expected, no gelation behavior was found when PPRs were added (Figure S4a). These results imply that the attached guanidine hydrochloride on the Laponite nanosheets could block the further interactions between ASAP exfoliated Laponite nanosheets and PPRs, and the cooperative interactions between the PPR-involved guanidinium parts and the negatively charged Laponite nanosheets play a crucial role in the formation of supramolecular hydrogels. We next carried out rheological tests to investigate the mechanical properties of the PPR-involved supramolecular hydrogels. In order to make sure that the hydrogel network was cross-linked completely, all hydrogel samples were allowed to aging overnight. Figure 2a gave the plots of the storage moduli (G′) and loss moduli (G″) versus frequency, from which both values of supramolecular hydrogels with different PPRs showed no obvious change within the applied range of angular frequency at a fixed strain (γ) of 0.5%. Moreover, the G′ value was larger than the G″ value over a wide frequency range (ω = 0.05−100 rads−1), indicating the characteristic of a stable gel−phase material. Remarkably, even though the organic C
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Macromolecules because free CD derivatives 1 may prefer squatting on an individual Laponite plate rather than bridging several clay nanosheets together (Figure 2a and Figure S5). In one word, the PPR-based hydrogels possess both improved mechanical strength and rapid thixotropic response capability. These features are arising from not only the mechanical toughness of Laponite33,34 but also the high guanidinium density in the polymer chain. There were 7 guanidinium cations on one CD unit, and the average number of guanidinium cations on PPR A, PPR B, and PPR C were accordingly calculated as 29, 55, and 118, respectively, according to the number of threaded CD units on PPGs. Meanwhile, the confinement of CD by PPG further facilitates the guanidinium cations evenly distributed on the polymer chains, and the sliding and rotation of the CD molecules around the axial PPG can also make a close contact with peripheral Laponite, which are believed to be jointly ascribed to the enhanced mechanical properties.35,36 Furthermore, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) gave the structural information on the hydrogels. SEM images showed that the dried supramolecular hydrogels existed as a highly cross-linked porous network (Figure. 3a). Accordingly, TEM images gave
Figure 4. Self-healing experiments of the organic−inorganic hybrid hydrogels ([guanidinium] = 2.8 × 10−3 mol/L, Laponite/ASAP/PPR B = 2.0/0.07/0.075 wt %). Pictures of (a) original hydrogel and the freshly cut surfaces (b) before and (c) after adhesion at 25 °C. Picture of (d) TCPPNa-doped, (e) pyranine-doped supramolecular hydrogels, and (f) adhesion of two different dye-load hydrogels under UV light irradiation at 365 nm ([TCPPNa] = [pyranine] = 2.0 × 10−5 mol/L). The diameter of the Petri dish is 9 cm.
recovered, and the crack completely disappeared after 60 min (Figure 4c). Moreover, we determined the rheological properties of supramolecular hydrogels after self-healing and heating treatment up to 70 °C (Figure 2d), and no obvious change was observed in both G′ and G″ values as compared to the ones at room temperature, indicating the satisfactory thermostability and self-healing behaviors of the obtained pseudorotaxane-based hydrogels. Although numbers of supramolecular hydrogels based on macroscopic self-assembly have been reported,39 including redox-responsive self-healing materials40 and photoswitchable gels,41,42 these materials were generally constructed from the host−guest complexation between macrocyclic host and guest molecules. Nevertheless, it is still highly desirable to explore new strategies in preparing supramolecular hydrogels with improved mechanical toughness and self-repairable behaviors. Alternatively, organic−inorganic hybrid macroscopic self-assemblies illustrate a new and elegant strategy to reduce the organic consumption and enhance the mechanical properties in the meantime. As investigated by morphological and rheological tests, the PPR-based supramolecular hydrogels are not only environmentally friendly and self-healable but also have high stability and great mechanical strength. Owing to the three-dimensional (3D) micronetworks in the hydrogels, we wonder whether the supramolecular hydrogels can act as scaffold to carry some functional molecules, such as organic dyes. Therefore, two πconjugated aromatic chromophores, i.e., tetrakis(4-sodium benzoate)porphyrin (TCPPNa) and pyranine, were used to study the loading capacity of the hydrogel architectures. As expected, luminescent supramolecular hydrogels with bright red and green emission were obtained in TCPPNa- and pyraninedoped hydrogels, respectively. Generally, the fluorescence of aromatic chromophores was seriously quenched at high concentrations because of the self-aggregation via hydrophobic interactions and π−π stackings.43−45 In contrast, the characteristic fluorescence of both dyes was still observed in our PPRbased supramolecular hydrogels, which may be originated from the rigidified 3D gel network that can inhibit the intrachromophoric interaction in the self-aggregation.19 Moreover,
Figure 3. (a) SEM and (b) TEM images of supramolecular hydrogels ([guanidinium] = 2.8 × 10−3 mol/L, Laponite/ASAP/PPR B = 2.0/ 0.07/0.075 wt %, and the scale bars in SEM and TEM images are 10 and 1 μm, respectively).
the homogeneously dispersed flaky nanostructures (Figure 3b). The schematic representation of the hydrogelation process of PPRs with Laponite is shown in Scheme 1. The dried and ground xerogels were also characterized by powder X-ray diffraction (XRD) experiments, and the broad diffraction pattern of pristine Laponite at approximately 2θ = 6.96° implies that the interlayer space was approximately 1.3 nm in pristine Laponite (Figure S6a), which was in good agree with the reported value.37 The addition of ASAP could not affect the interlayer space of Laponite nanosheets (Figure S6b). These results confirmed that ASAP was specifically wrapped at the positively charged edge, but not the interlayer of Laponite. However, the XRD peak was shifted to 2θ = 4.08° in the presence of PPRs, revealing a 2.1 nm interlayer space (Figure S6c). The increased Laponite interlayer space in supramolecular xerogels demonstrated PPRs were mainly located in the interlayer spaces of Laponite.38 Considering the obtained supramolecular hydrogels were driven by noncovalent interactions, we then investigated their self-healing capability. As shown in Figure 4, a piece of diskshaped hydrogel composed of 2.0% Laponite, 0.07% ASAP, and 0.075% PPR B was cut in half, and then the halves were put together immediately. After standing for only 1 min, the two pieces merged into one piece, and the reproduced hydrogel was strong enough to lift up without breakup (details for selfhealing experiments could be found in Video S1 of the Supporting Information). This hydrogel piece was totally D
DOI: 10.1021/acs.macromol.6b02459 Macromolecules XXXX, XXX, XXX−XXX
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the introduction of chromophores could not affect the selfhealing behaviors of the supramolecular hydrogels (Figure 4). Since the desired luminescent properties of resultant hybrids can be easily tailored by an appropriate choice of different chromophores, this strategy is believed to be a new choice for the design and preparation of flexible optical devices and selfrepairable luminescent devices.
Article
AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (Y.L.). ORCID
Yu Liu: 0000-0001-8723-1896 Notes
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The authors declare no competing financial interest.
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CONCLUSION In conclusion, through the hierarchically organic−inorganic hybridization of Laponite matrix with CD-based pseudopolyrotaxanes, we successfully constructed a series of supramolecular hydrogels with tunable mechanical strength, high water content, and self-reparable ability. The mechanical properties of the obtained hydrogels could be conveniently modulated by PPG polymer with different molecular weights. Significantly, the introduction of PPR structures as cross-linkers can not only largely avoid the complicated synthetic process but also endow the supramolecular hydrogel with controlled mechanical performance. Meanwhile, the PPR-based hydrogel still retained its original framework and shape even after loading fluorescent dyes in its 3D hydrogel networks. The PPR-based hydrogels in the present study further emphasize the power of supramolecular cooperativity in regulating the physicochemical properties of nanoassemblies, thus providing us with promising opportunities to construct more advanced multifunctional and adaptable water-based materials.
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21432004, 21472100, 91527301, and 21502039) and the Natural Science Foundation of Hebei Province (No. B2016202149 and B2016202147).
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REFERENCES
(1) Lee, K. Y.; Mooney, D. J. Hydrogels for tissue engineering. Chem. Rev. 2001, 101, 1869−1880. (2) Zhang, X.; Malhotra, S.; Molina, M.; Haag, R. Micro-and nanogels with labile crosslinks−from synthesis to biomedical applications. Chem. Soc. Rev. 2015, 44, 1948−1973. (3) 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. Tunable, high modulus hydrogels driven by ionic coacervation. Adv. Mater. 2011, 23, 2327−2331. (4) Peppas, N.; Huang, Y.; Torres-Lugo, M.; Ward, J. H.; Zhang, J. Physicochemical foundations and structural design of hydrogels in medicine and biology. Annu. Rev. Biomed. Eng. 2000, 2, 9−29. (5) Raeburn, J.; Cardoso, A. Z.; Adams, D. J. The importance of the self-assembly process to control mechanical properties of low molecular weight hydrogels. Chem. Soc. Rev. 2013, 42, 5143−5156. (6) Seiffert, S.; Sprakel, J. Physical chemistry of supramolecular polymer networks. Chem. Soc. Rev. 2012, 41, 909−930. (7) Avestro, A.-J.; Belowich, M. E.; Stoddart, J. F. Cooperative selfassembly: producing synthetic polymers with precise and concise primary structures. Chem. Soc. Rev. 2012, 41, 5881−5895. (8) Dimitrov, I.; Trzebicka, B.; Müller, A. H.; Dworak, A.; Tsvetanov, C. B. Thermosensitive water-soluble copolymers with doubly responsive reversibly interacting entities. Prog. Polym. Sci. 2007, 32, 1275−1343. (9) Segarra-Maset, M. D.; Nebot, V. J.; Miravet, J. F.; Escuder, B. Control of molecular gelation by chemical stimuli. Chem. Soc. Rev. 2013, 42, 7086−7098. (10) Aida, T.; Meijer, E.; Stupp, S. Functional supramolecular polymers. Science 2012, 335, 813−817. (11) Adler-Abramovich, L.; Gazit, E. The physical properties of supramolecular peptide assemblies: from building block association to technological applications. Chem. Soc. Rev. 2014, 43, 6881−6893. (12) Buerkle, L. E.; Rowan, S. J. Supramolecular gels formed from multi-component low molecular weight species. Chem. Soc. Rev. 2012, 41, 6089−6102. (13) Wang, Q.; Mynar, J. L.; Yoshida, M.; Lee, E.; Lee, M.; Okuro, K.; Kinbara, K.; Aida, T. High-water-content mouldable hydrogels by mixing clay and a dendritic molecular binder. Nature 2010, 463, 339− 343. (14) Okuro, K.; Kinbara, K.; Tsumoto, K.; Ishii, N.; Aida, T. Molecular glues carrying multiple guanidinium ion pendants via an oligoether spacer: stabilization of microtubules against depolymerization. J. Am. Chem. Soc. 2009, 131, 1626−1627. (15) Kostiainen, M. A.; Kasyutich, O.; Cornelissen, J. J.; Nolte, R. J. Self-assembly and optically triggered disassembly of hierarchical dendron−virus complexes. Nat. Chem. 2010, 2, 394−399. (16) Tamesue, S.; Ohtani, M.; Yamada, K.; Ishida, Y.; Spruell, J. M.; Lynd, N. A.; Hawker, C. J.; Aida, T. Linear versus dendritic molecular binders for hydrogel network formation with clay nanosheets: studies with ABA triblock copolyethers carrying guanidinium ion pendants. J. Am. Chem. Soc. 2013, 135, 15650−15655.
Materials and Methods. All chemicals were commercially available unless noted otherwise. The layered clay (Laponite XLG) was purchased from Rockwood Additives Ltd. and was used as received without further purification. Rheological tests of hydrogels were carried out by using an Anton Paar model MCR-301 rheometer, with a 25 mm diameter parallel plate attached to a transducer. The gap was set at 1.0 mm. TEM experiments were performed using a Tecnai 20 high-resolution transmission electron microscope operating at an accelerating voltage of 200 keV. The sample for TEM measurements was prepared by smearing the hydrogel onto a copper grid. The grid was then air-dried and imaged. SEM images were recorded on a FEI Nova Nano SEM450 scanning electron microscope. The sample for SEM measurements was prepared as follow: the dried xerogel was first grounded to powder, and then the powder was dispersed onto a coverslip. Preparation of Supramolecular Hydrogels. Taking the preparation of PPR B-based supramolecular hydrogel as an example, 100 mg of Laponite XLG was suspended in 3.5 mL of water and stirred at 25 °C for 10 min, and then ASAP (3.5 mg) in 1.0 mL of water was added to the to a stirred suspension. Then 0.5 mL of an aqueous solution of PPR B (3.85 mg) was added while stirring. The mixture was immediately stirred and lost its fluidity, thus forming a transparent, self-standing supramolecular hydrogels. Other hydrogels were prepared according to a similar method.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02459. Experimental procedures and characterization data (PDF) Video S1 (AVI) E
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Macromolecules (17) Mourtzis, N.; Eliadou, K.; Aggelidou, C.; Sophianopoulou, V.; Mavridis, I. M.; Yannakopoulou, K. Per (6-guanidino-6-deoxy) cyclodextrins: synthesis, characterisation and binding behaviour toward selected small molecules and DNA. Org. Biomol. Chem. 2007, 5, 125−131. (18) Liao, X.; Chen, G.; Jiang, M. Pseudopolyrotaxanes on inorganic nanoplatelets and their supramolecular hydrogels. Langmuir 2011, 27, 12650−12656. (19) Li, Z. Q.; Zhang, Y. M.; Guo, D. S.; Chen, H. Z.; Liu, Y. Supramolecular Assembly with Multiple Preorganized π-Electronic Cages. Chem. - Eur. J. 2013, 19, 96−100. (20) Tamura, M.; Gao, D.; Ueno, A. A series of polyrotaxanes containing α-cyclodextrin and naphthalene-modified α-cyclodextrin and solvent effects on the fluorescence quenching by terminal units. J. Chem. Soc., Perkin Trans 2 2001, 2012−2021. (21) Liu, Y.; Yu, L.; Chen, Y.; Zhao, Y.-L.; Yang, H. Construction and DNA Condensation of Cyclodextrin-Based Polypseudorotaxanes with Anthryl Grafts. J. Am. Chem. Soc. 2007, 129, 10656−10657. (22) Liu, Y.; Ke, C.-F.; Zhang, H.-Y.; Wu, W.-J.; Shi, J. Reversible 2D Pseudopolyrotaxanes Based on Cyclodextrins and Cucurbit[6]uril. J. Org. Chem. 2007, 72, 280−283. (23) Miyauchi, M.; Hoshino, T.; Yamaguchi, H.; Kamitori, S.; Harada, A. A [2] rotaxane capped by a cyclodextrin and a guest: Formation of supramolecular [2] rotaxane polymer. J. Am. Chem. Soc. 2005, 127, 2034−2035. (24) Okumura, H.; Okada, M.; Kawaguchi, Y.; Harada, A. Complex Formation between Poly (dimethylsiloxane) and Cyclodextrins: New P seudo-Polyrotaxanes Containing Inorganic Polymers. Macromolecules 2000, 33, 4297−4298. (25) Araki, J.; Ito, K. Recent advances in the preparation of cyclodextrin-based polyrotaxanes and their applications to soft materials. Soft Matter 2007, 3, 1456−1473. (26) Baghdadi, H. A.; Jensen, E. C.; Easwar, N.; Bhatia, S. R. Evidence for re-entrant behavior in laponite−PEO systems. Rheol. Acta 2008, 47, 121−127. (27) Baghdadi, H. A.; Parrella, J.; Bhatia, S. R. Long-term aging effects on the rheology of neat Laponite and Laponite-PEO dispersions. Rheol. Acta 2008, 47, 349−357. (28) Wang, K.-P.; Chen, Y.; Liu, Y. A Polycation-induced Secondary Assembly of Amphiphilc Calixarene and Its Multi-stimuli Responsive Gelation Behavior. Chem. Commun. 2015, 51, 1647−1649. (29) Chen, H.; Ma, X.; Wu, S.; Tian, H. A Rapidly Self-Healing Supramolecular Polymer Hydrogel with Photostimulated RoomTemperature Phosphorescence Responsiveness. Angew. Chem., Int. Ed. 2014, 53, 14149−14152. (30) Haraguchi, K.; Takehisa, T. Nanocomposite hydrogels: a unique organic-inorganic network structure with extraordinary mechanical, optical, and swelling/de-swelling properties. Adv. Mater. 2002, 14, 1120−1124. (31) Okay, O.; Oppermann, W. Polyacrylamide-clay nanocomposite hydrogels: Rheological and light scattering characterization. Macromolecules 2007, 40, 3378−3387. (32) Haraguchi, K.; Farnworth, R.; Ohbayashi, A.; Takehisa, T. Compositional effects on mechanical properties of nanocomposite hydrogels composed of poly (N, N-dimethylacrylamide) and clay. Macromolecules 2003, 36, 5732−5741. (33) (a) Bergaya, F.; Lagaly, G. Handbook of Clay Science; Newnes: 2013; Vol. 5. (34) Vaia, R. A.; Vasudevan, S.; Krawiec, W.; Scanlon, L. G.; Giannelis, E. P. New polymer electrolyte nanocomposites: Melt intercalation of poly (ethylene oxide) in mica-type silicates. Adv. Mater. 1995, 7, 154−156. (35) Kato, K.; Ito, K. Dynamic transition between rubber and sliding states attributed to slidable cross-links. Soft Matter 2011, 7, 8737− 8740. (36) Mayumi, K.; Ito, K. Structure and dynamics of polyrotaxane and slide-ring materials. Polymer 2010, 51, 959−967.
(37) Lezhnina, M. M.; Grewe, T.; Stoehr, H.; Kynast, U. Laponite Blue: Dissolving the Insoluble. Angew. Chem., Int. Ed. 2012, 51, 10652−10655. (38) Li, Z.; Hou, Z.; Fan, H.; Li, H. Organic−Inorganic Hierarchical Self-Assembly into Robust Luminescent Supramolecular Hydrogel. Adv. Funct. Mater. 2017, 27, 1604379. (39) Harada, A.; Kobayashi, R.; Takashima, Y.; Hashidzume, A.; Yamaguchi, H. Macroscopic self-assembly through molecular recognition. Nat. Chem. 2011, 3, 34−37. (40) Ji, X.; Shi, B.; Wang, H.; Xia, D.; Jie, K.; Wu, Z.; Huang, F. Supramolecular Construction of Multifluorescent Gels: Interfacial Assembly of Discrete Fluorescent Gels through Multiple Hydrogen Bonding. Adv. Mater. 2015, 27, 8062−8066. (41) Kai, D.; Low, Z. W.; Liow, S. S.; Abdul Karim, A.; Ye, H.; Jin, G.; Loh, X. J. Development of lignin supramolecular hydrogels with mechanically responsive and self-healing properties. ACS Sustainable Chem. Eng. 2015, 3, 2160−2169. (42) Li, Z. Q.; Zhang, Y. M.; Chen, H. Z.; Zhao, J.; Liu, Y. Hierarchical Organization of Spherical Assembly with Reversibly Photocontrollable Cross-Links. J. Org. Chem. 2013, 78, 5110−5114. (43) Helmich, F.; Lee, C. C.; Nieuwenhuizen, M. M.; Gielen, J. C.; Christianen, P.; Larsen, A.; Fytas, G.; Leclère, P. E.; Schenning, A. P.; Meijer, E. Dilution-Induced Self-Assembly of Porphyrin Aggregates: A Consequence of Coupled Equilibria. Angew. Chem., Int. Ed. 2010, 49, 3939−3942. (44) Li, Z.; Hou, Z.; Ha, D.; Li, H. A Ratiometric Luminescent Thermometer Co-doped with Lanthanide and Transition Metals. Chem. - Asian J. 2015, 10, 2720−2724. (45) Li, Z. Q.; Zhang, Y. M.; Chen, Y.; Liu, Y. A Supramolecular Tubular Nanoreactor. Chem. - Eur. J. 2014, 20, 8566−8570.
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