Article pubs.acs.org/Langmuir
Effect of Topological Structures on the Self-Assembly Behavior of Supramolecular Amphiphiles Juan Wang,† Xing Wang,† Fei Yang,† Hong Shen,† Yezi You,‡ and Decheng Wu*,† †
Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics & Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, Anhui, P. R. China S Supporting Information *
ABSTRACT: Three types of azobenzene-based telechelic guest polymers, PEG-azo, azo-PEG-azo, and PEG-azo4, were synthesized by a facile method. Subsequently, a series supramolecular amphiphiles with three distinct topological structures (hemitelechelic, ditelechelic, and quadritelechelic) were constructed through coupling with host polymer β-cyclodextrin-poly(Llactide) (β-CD-PLLA) by combined host−guest complexation. Research on the self-assembly behavior of these amphiphiles demonstrated that the variation in self-assembly was tuned by the synergistic interaction of hydrophilicity and the curvature of the polymer chains, and very importantly, the topological structure of amphiphiles demonstrated effective control of the self-assembly behavior.
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INTRODUCTION Molecular self-assembly, which can yield a variety of structures including micelles, vesicles, tubes, and many other complex aggregates,1−3 has displayed great potential in gene and drug delivery, bioimaging, biosensors, and so forth.4−11 The structure of the aggregates plays a pivotal role in achieving targeted functions. For example, recent reports showed that the morphology and size of the aggregates have a significant influence on their in vivo behaviors as drug and gene carriers.12,13 The generation of stable self-assembled aggregates with different morphologies can be influenced by many parameters.14−18 As for linear block copolymers, tuning the hydrophilicity/hydrophobicity ratio is a very popular way to produce diverse aggregates with different morphologies and sizes.19,20 Benefitting from recent advances in polymer synthesis, amphiphilic polymers with various topological architectures, such as multiarm, linear-star, dendritic, and hyperbranched, have sprung up as precursors in self-assembly.21−28 Individual self-assembly behaviors of these precursors have been widely discussed and demonstrate unique characteristics such as the diversity of structures self-assembled from the amphiphilic hyperbranched polymers.29−36 At the same time, developing supramolecular polymers based on noncovalent bonds provides a new approach and perspective on research into building amphiphilic pseudoblock copolymers with novel topological features,37−45 which greatly enriched the scope of preparing diverse self-assembled structures.46−54 For nonlinear amphiphilic polymers, besides the hydrophilicity/hydrophobicity © 2015 American Chemical Society
ratio, the intrinsic topological structure also plays an important role in their self-assembly behavior. It is of great importance to systematically investigate the effect of topological structures on the self-assembly behavior of nonlinear amphiphiles. Recently, we adopted combined host−guest complexation to obtain linear-star supramolecular amphiphiles with customized hydrophilic fractions, and we demonstrated that the hydrophilicity/hydrophobicity ratio had an important impact on the morphological evolution of the self-assemblies.55 Motivated by the efficient host−guest complexation method, we herein further develop several groups of supramolecular polymers with similar compositions and hydrophilicity/hydrophobicity ratios but distinct topologies: hemitelechelic, ditelechelic, and quadritelechelic (Scheme 1). The amphiphiles can be easily obtained from the host−guest interaction of β-cyclodextrinpoly(L-lactide) (β-CD-PLLA) and azobenzene-poly(ethylene glycol) (azo-PEG). Through mixing five β-CD-PLLAs (Mw,PLLA to be ca. 10, 20, 50, 70, and 100 kDa) and three azo-PEGs with different topological architectures (guest A, PEG-Azo; guest B, Azo-PEG-Azo; guest C, star PEG-Azo4; Scheme 1), we constructed 15 supramolecular amphiphiles wherein each type of telechelic topology involves 5 polymers with gradually increasing hydrophobicity. Different from our previous report,55 herein we investigated the effect of topological structures on the self-assembled morphologies and focused Received: October 14, 2015 Revised: December 3, 2015 Published: December 3, 2015 13834
DOI: 10.1021/acs.langmuir.5b03823 Langmuir 2015, 31, 13834−13841
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Instruments, U.K.). All samples were measured at 25 °C with 632.8 nm laser light set at a scattering angle of 173°. Every measurement was performed at least three times. The average hydrodynamic diameters were obtained from the DTS software of the instrument using the volume reading. Fluorescence Spectrophotometry. The fluorescence spectra were recorded on an F4600 (Hitachi) fluorescence spectrometer. The excitation wavelength was 350 nm, and the scanning range was from 370 to 800 nm. Synthesis of PEG-Azo (Guest A). Methoxypolyethylene glycol (2 g, 1 mmol) and Et3N (0.75 mL, 5 mmol) in anhydrous DCM (20 mL) were mixed and stirred at 0 °C for 0.5 h, followed by the dropwise addition of methanesulfonyl chloride (0.57 g, 5 mmol). After stirring for 6 h at room temperature, the mixture was washed with Na2CO3 solution and water three times. The organic layer was dried over MgSO4, filtered, and evaporated. The final product (PEG-OMs) was obtained in vacuo overnight (yield: 98%). Mn,GPC = 2300, Mw/Mn = 1.07. 1H NMR (300 MHz, CDCl3): δ 4.42−4.32 (m, 2H), 3.42−3.89 (m, 174H), 3.37 (s, 3H), 3.07 (s, 3H). PEG-OMs (2.01 g, 1 mmol), 4phenylazophenol (0.22 g, 1.1 mmol), and K2CO3 (0.35 g, 2.5 mmol) were mixed in acetonitrile (20 mL). The mixture was stirred under reflux for 3 days. After that, the reaction mixture was cooled to room temperature, filtered, and rinsed with acetonitrile. The filtrate (PEGAzo) was evaporated and dried in vacuum (yield: 85%). Mn,GPC = 2900, Mw/Mn = 1.11. 1H NMR (300 MHz, CDCl3): δ 7.97−7.81 (m, 4H), 7.52 (d, J = 12.9, 2H), 7.45 (t, 6.6 Hz, 1H), 7.03 (d, J = 8.8 Hz, 2H), 4.33−4.13 (m, 2H), 3.49−3.98 (m, 176H), 3.38 (s, 3H). Synthesis of Azo-PEG-Azo (Guest B). Poly(ethylene glycol) (2 g, 0.5 mmol) and Et3N (0.75 mL, 5 mmol) in anhydrous DCM (20 mL) were mixed and stirred at 0 °C for 0.5 h, followed by the dropwise addition of methanesulfonyl chloride (0.57 g, 5 mmol). After stirring for 6 h at room temperature, the mixture was washed with Na2CO3 solution and water three times. The organic layer was dried over MgSO4, filtered, and concentrated. The concentrated solution was then precipitated in diethyl ether twice. The final product (MsOPEG-OMs) was obtained under vacuum overnight (yield: 96%). Mn,GPC = 4400, Mw/Mn = 1.02. 1H NMR (400 MHz, CDCl3): δ 4.37 (dd, J = 5.3, 3.6 Hz, 4H), 3.42−3.89 (m, 360H), 3.14 (s, 3H), 3.07 (s, 3H). MsO-PEG-OMs (2.01 g, 0.5 mmol), 4-phenylazophenol (0.22 g, 1.1 mmol), and K2CO3 (0.35 g, 2.5 mmol) were mixed in acetonitrile (20 mL). The mixture was stirred under reflux for 3 days. After that, the reaction mixture was cooled to room temperature, filtered, and rinsed with acetonitrile. The filtrate (Azo-PEG-Azo) was evaporated and dried in vacuum (yield: 85%). Mn,GPC = 4600, Mw/Mn = 1.06. 1H NMR (300 MHz, CDCl3): δ 7.96−7.82 (m, 8H), 7.46 (d, J = 12.9, 4H), 7.45 (t, 6.6 Hz, 2H), 6.99 (d, J = 8.8 Hz, 4H), 4.24−4.20 (m, 4H), 3.39−3.91 (m, 360H). Synthesis of Star PEG-Azo4 (Guest C). Four-arm poly(ethylene glycol) (2.5 g, 0.25 mmol) and Et3N (0.75 mL, 5 mmol) in anhydrous DCM (20 mL) were mixed and stirred at 0 °C for 0.5 h, followed by the dropwise addition of methanesulfonyl chloride (0.57 g, 5 mmol). After stirring for 6 h at room temperature, the mixture was dialyzed against water for 3 days. The final product (star PEG-OMs4) was obtained after removing water and was dried in vacuum overnight (yield: 80%). Mn,GPC = 10 000, Mw/Mn = 1.03. 1H NMR (300 MHz, CDCl3): δ 4.44−4.32 (m, 8H), 3.33−3.92 (m, 896H), 3.07 (s, 12H). Star PEG-OMs4 (2.58 g, 0.25 mmol), 4-phenylazophenol (0.22 g, 1.1 mmol), and K2CO3 (0.35 g, 2.5 mmol) were mixed in acetonitrile (20 mL). The mixture was stirred under reflux for 3 days. After that, the reaction mixture was cooled to room temperature, filtered, and rinsed with acetonitrile. The filtrate (star PEG-Azo4) was evaporated and dried under vacuum (yield: 85%). Mn,GPC = 10100, Mw/Mn = 1.07. 1H NMR (400 MHz, CDCl3): δ 7.97−7.79 (m, 16H), 7.46 (d, J = 23.6, 8H), 7.45 (t, 6.1 Hz, 4H), 6.99 (d, J = 8.9 Hz, 8H), 4.26−4.17 (m, 8H), 3.44−3.90 (m, 898H).
Scheme 1. Construction of Supramolecular Amphiphiles with Hemitelechelic (Supra A), Ditelechelic (Supra B), and Quadritelechelic (Supra C) Structures
especially on the regulation of self-assembly behaviors when topological structures changed from hemitelechelic to ditelechelic to quadritelechelic.
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EXPERIMENTAL SECTION
Materials. 4-Phenylazophenol (98%), methoxypolyethylene glycol (mPEG, Mn = 2 kDa), and poly(ethylene glycol) (PEG, Mn = 4 kDa) were purchased from Energy Chemical and used directly without further purification. Four-arm polyethylene glycol (4-arm-PEG, Mn = 10 kDa) was received from Xiamen SINOPEG Biotech Co., Ltd. β-CD (TCI, >95%) was used after being dried at 70 °C under vacuum for 24 h. L-Lactide (PURAC, Netherlands) was purified by recrystallization from ethyl acetate twice. Stannous octoate (Sn(Oct)2, Sigma, analytical reagent (AR)) was used as received. Ethyl acetate (AR) were dried over P2O5 overnight and distilled before use. Dichloromethane (DCM) and chloroform were dried with CaH2 and distilled. Other reagents were purchased from Beijing Chemical Works and used as received. Characterization. Nuclear Magnetic Resonance (NMR) Spectroscopy. 1H NMR spectra were recorded with a Bruker Fourier 300 (300 MHz) spectrometer or a Bruker Avance 400 III (400 MHz) spectrometer with CDCl3 as deuterated solvents. 2D NOESY (nuclear Overhauser enhancement spectroscopy) was performed on a Bruker Avance 600 (600 MHz) spectrometer with D2O as the deuterated solvent. Gel Permeation Chromatography (GPC). GPC analyses of host and guest polymers were performed on a GPCmax VE-2001 (Viscotek) equipped with a Viscotek TriSEC model 302 triple detector array (refractive index detector, viscometer detector, and laser light scattering detector (7 and 90°)) using two I-3078 polar organic columns. THF was used as the eluent at a flow rate of 1.0 mL/min. Calibration of the molecular weight of the polymer was based on polystyrene standards. Transmission Electron Microscopy (TEM). The visualized images of assemblies were measured with a JEM-2200FS microscope at an acceleration voltage of 120 kV, and the samples were prepared by drop-coating the aqueous solution on a carbon-coated copper grid. Scanning Electron Microscopy (SEM). The morphologies of the assemblies were measured with a JSM-6700F microscope at an acceleration voltage of 5 kV. The samples were prepared by dropcoating the aqueous solution on a silica plate and then coating with gold−palladium in argon gas using a sputter coater (E-1010, Hitachi, Ltd., Tokyo, Japan). Dynamic Light Scattering (DLS). The hydrodynamic sizes of assemblies were measured on a Zetasizer Nano-ZS90 (Malvern
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RESULTS AND DISCUSSION Polymer Synthesis and Characterization. Host polymers, β-CD-PLLAs with multiarm PLLA segments, are the 13835
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hydrophilicity. The 1H NMR spectra from the reagents to products (Figure 1) proved that each hydroxyl group of every PEG reagent was completely substituted with a azobenzene group to obtain the desired guest polymers. Formation of Supramolecular Amphiphilic Polymers. In mixing of guests A, B, and C with β-CD-PLLAs in molar ratios of 1:1, 1:2, and 1:4 in N,N-dimethylformamide (DMF), supramolecular amphiphiles including hemitelechelic (Supra A), ditelechelic (Supra B), and quadritelechelic (Supra C) were formed in situ. The partial 1H NMR spectra (Figure S3), stoichiometric fluorescence spectra (Figure S4), and 2D 1H NOESY spectra (Figure 2) provided evidence for the construction of the supramolecular amphiphiles via host− guest complexation. As shown in Figure 2, protons (7.84−7.09 ppm) on the azobenzene groups of all three guest polymers had obvious cross peaks with protons (4.24−3.36 ppm) on the cavity of β-CD. Because each arm of guests A−C possess similar hydrophilicity, the corresponding three supramolecular amphiphiles under the same β-CD-PLLA have almost the same theoretical hydrophilic fractions (Table 1). Self-Assembly of Supramolecular Amphiphilic Polymers. The self-assembly morphologies of these 15 of amphiphiles were characterized by TEM, SEM, and DLS. With the molecular weight increment of β-CD-PLLA, hydrophilic fractions of the amphiphiles decreased gradually, resulting in the evolution of the self-assembled structures. As for Supra A with a hemitelechelic topological structure, when the molecular weight of β-CD-PLLA changed from 10, 20, and 50 kDa to more than 70 kDa, the corresponding self-assembled morphologies underwent a variation process from a carambola micelle (Dh = 1.8 μm) to a shuttlelike film (Dh = 1.3 μm), to a mixed stage with fibers and a shuttlelike film, and finally to intertwined long fibers (Figure 3). As a common process throughout nature and technology, molecular self-assembly involves many kinds of situations.56−58 Hierarchical selfassembly plays crucial roles in the construction of supra-
Scheme 2. Synthesis Routes of Guest Polymers with Different Topological Structuresa
a
(A) Hemitelechelic PEG-Azo, (B) ditelechelic Azo-PEG-Azo, and (C) quadritelechelic star PEG-Azo4.
same as in our previous report.55 Guests A−C were synthesized via a two-step modification of methoxypoly(ethylene glycol) (2 kDa), poly(ethylene glycol) (4 kDa), and 4-arm poly(ethylene glycol) (10 kDa) to make sure that each arm has similar
Figure 1. 1H NMR spectra of guest polymers in CDCl3: (A) guest A ((a) PEG-Azo, (b) PEG-OMs, and (c) PEG-OH (Mn = 2K)); (B) guest B ((a) Azo-PEG-Azo, (b) MsO-PEG-OMs, and (c) HO-PEG-OH (Mn = 4K)); (C) guest C ((a) PEG-Azo4, (b) PEG-OMs4, and (c) PEG-OH4 (Mn = 10K)). 13836
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Figure 2. 2D 1H NOESY spectra of supramolecular amphiphiles (A) β-CD-PLLA-10 kDa@PEG-Azo, (B) β-CD-PLLA-10 kDa@Azo-PEG-Azo, and (C) β-CD-PLLA-10 kDa@PEG-Azo4 in D2O.
Table 1. Theoretical Hydrophilic Fractions of 15 Supramolecular Amphiphiles molecular weight of β-CD-PLLA (g/mol) f hydrophilic (%)a
10 kDa
20 kDa
50 kDa
70 kDa
100 kDa
Supra A Supra B Supra C
22.0 22.0 27.0
11.0 11.0 13.5
4.4 4.4 5.4
3.1 3.1 3.9
2.2 2.2 2.7
a
f hydrophilic refers to the hydrophilic fraction, and its theoretical value is calculated approximately on the basis of the molecular weight of host and guest polymers.
molecular structures in nature.59 We put forward a possible selfassembly mechanism to illustrate the self-assembly process of microsized aggregates, as shown in Figure 4. In our proposed mechanism, supramolecular amphiphiles underwent two successive self-assembly processes: primary and secondary self-assembly. Similar to the self-assembly of linear block copolymers, the primary process was mainly tuned by the hydrophilicity of amphiphiles. With the increase in molecular weight of β-CD-PLLA, the primary self-assembled micelles gradually changed from spheres to cylinders. Because of the linear-star topological architecture, in the primary self-
Figure 4. Schematic illustrating the evolution of the self-assembled morphologies of Supra A.
assembled micelles, there were fewer coronal PEG chains than PLLA chains aggregated in the core, making the primary
Figure 3. (A) TEM and (B) SEM images of the morphologies self-assembled from Supra A (Mw, β‑CD‑PLLA = 10, 20, 50, 70, and 100 kDa for labels a, b, c, d, and e, respectively). 13837
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Figure 5. (A) TEM and (B) SEM images of the morphologies self-assembled from Supra B (Mw, β‑CD‑PLLA = 10, 20, 50, 70, and 100 kDa for labels a, b, c, d and e respectively).
Figure 6. (A) TEM and (B) SEM images of the morphologies self-assembled from Supra C (Mw, β‑CD‑PLLA = 10, 20, 50, 70, and 100 kDa for labels a, b, c, d, and e respectively).
and Supra C showed some differences. When the molecular weight of β-CD-PLLA changed from 10 to 20 kDa to more than 50 kDa, the corresponding self-assembled morphologies from Supra B underwent a variation process from a flower-like micelle (Dh = 1.1 μm), to a nest-like shape (Dh = 1.5 μm) communicated by fine fibrils, to intertwined fibers (Figure 5). As for Supra C with a quadritelechelic topological structure, the self-assembled structures transformed from a naan-like micelle (Dh = 1.7 μm) to fibers as soon as the molecular weight of β-CD-PLLA was more than 20 kDa (Figure 6). The differences in self-assembly behavior among Supra A, Supra B, and Supra C were mainly manifested in two ways. First, keeping the molecular weight of β-CD-PLLA unchanged at 10 kDa, the self-assemblies gained from Supra A, Supra B, and Supra C were a carambola-like micelle, flower-like micelle, and naan-like
micelles not stable enough in water. In this regard, primary selfassembled micelles tended to go through a secondary selfassembly to obtain stable self-assembled structures driven by hydrophobic interaction. Magnified TEM images of the carambola, shuttle, and fiber clearly showed the primary structures (Figure S5). Although the microsized carambola and shuttle structures were both composed of primary spherical micelles, the sizes of spheres increased from about 100 to 300 nm (Figure S5A,D). As shown in Figure S5F, a large number of fibrils (d = 30−50 nm) composed the secondary fiber (d = 400−500 nm, PEG-Azo@β-CD-PLLA-100 kDa) in the form of irregular orientation arrangement. The TEM evidence confirmed the proposed self-assembly mechanism. Under the same evolution of the molecular weight of β-CDPLLA, the self-assembled morphological variation of Supra B 13838
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amphiphiles caused the threshold occurred earlier and earlier from Supra A to Supra B to Supra C (Figure 7B). In addition, from Figures 3d,e, 5c−e, and 6b−e, it was obvious that under the same molecular weight of β-CD-PLLA, the length of the fibers shortened and the entanglement of the fibers became more compact from Supra A to Supra B to Supra C in turn. This was because restricted topological structures also reduced the stacking accuracy of the polymer chains, decreasing the corresponding length and regularity of entanglement of the selfassembled fibers. It is well known that the hydrophilicity of amphiphilic polymers plays an important role in the transition of the assembled morphologies.62 Tuning hydrophilic or hydrophobic segments is a very popular and direct approach to changing the assembled morphology of amphiphiles. This study vividly demonstrated that topological structure could influence the motion of the molecular chains and afterward had an impact on the final self-assembled morphologies. Different topological structures certainly possess distinct inherent curvatures of polymer chains. During the self-assembly process in water, the hydrophilic chains need to extend to the outside to maintain the stability of the self-assemblies. However, a restricted polymer chain does not favor outer stacking. In our system, because the restriction of PEG chains in Supra A, Supra B, and Supra C increases successively, the difficulty of PEG chains being exposed outside increases accordingly. Hence, a change in topological structure should also be an effective and powerful route to regulating self-assembly behavior, resulting in a variation of the assembled morphology, but the related study has received little attention.
micelle, respectively (Figures 3a, 5a, and 6a). Second, with the increasing molecular weight of β-CD-PLLA, all three types of supramolecular amphiphiles finally self-assembled into fibers; however, the initial formation of the fibers was under a different molecular weight of β-CD-PLLA, with Supra A being 70 kDa, Supra B being 50 kDa, and Supra C being 20 kDa (Figures 3d, 5c, and 6b). Kikuchi and co-workers reported that a gradually increased graft density would decrease the flexibility of the main chains.60 Kim and co-workers reported that the self-assembly morphologies and structures of miktoarm star polymer can differ significantly because of the molecular configuration and structural ordering.61 Similarly, the transition of topological structures from hemitelechelic to ditelechelic to quadritelechelic actually changed the curvature of hydrophilic PEG chains and the authentic hydrophilicity of Supra B and Supra C. Specifically, the flexibility of PEG chains gradually decreased with the topological structure change from Supra A to Supra B to Supra C. The restriction of PEG chains inevitably led to the reduction of their contribution to the hydrophilicity, so the sequence of authentic hydrophilicity should be Supra A > Supra B > Supra C. Accordingly, we explained the two abovementioned aspects, as shown in Figure 7. With the molecular
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CONCLUSIONS
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ASSOCIATED CONTENT
We adopted simple and effective combinational host−guest complexation to obtain supramolecular amphiphiles with five different hydrophibilicity/hydrophobicity ratios and three types of topological structures (hemitelechelic, ditelechelic, and quadritelechelic). The linear-star topological architecture of supramolecular amphiphiles led to a hierarchical self-assembly process. The transition of topological structures from hemitelechelic to ditelechelic to quadritelechelic changed the curvature of the polymer chains and the authentic hydrophilicity. Both the primary and secondary self-assembly processes were affected by the synergistic interaction of hydrophilicity and the curvature of the polymer chains and finally yielded various morphologies involving carambola-like micelles, flower-like micelles, naan-like micelles, shuttle-like films, and a nest-like shape communicated by fine fibrils and fibers. It is demonstrated that changes in the topological structures of amphiphiles can play an important role in tuning self-assembled morphologies to transfer into the intended target, opening an effective avenue in the field of controlled self-assembly.
Figure 7. Schematic for the different self-assembly behaviors of Supra A, Supra B, and Supra C tuned by topological structures.
weight of β-CD-PLLA unchanged at 10 kDa, because of the increase in the curvature of hydrophilic PEG chains and the decrease in hydrophilicity of supramolecular amphiphiles, the primary self-assembled spherical micelles became larger, the core of micelles became looser, and the corona became sparser (Figure 7A). Magnified TEM images of the carambola, flower, and naan obviously revealed that sizes of their respective primary micelles increased from 100 to 200 nm to 600−900 nm (Figure S5A−C). During the secondary self-assembly, primary self-assembled spherical micelles could be taken as amphiphilic supramolecules. Considering the structural difference of primary micelles, the hydrophilicity of them would decrease from Supra A to Supra B to Supra C, which consequently led to the difference in the secondary self-assembly structures (Figure 7A). Driving by the hydrophobic interaction, from Supra A to Supra B to Supra C, primary micelles tended to stack in more and more well-organized ways to obtain enough hydrophilic domains exposed in water. The transition from sphere to cylinder of primary selfassembled micelles played a crucial role in the formation of fiber structure. The increase of curvature of hydrophilic PEG chains and decrease of hydrophilicity of supramolecular
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03823. Additional 1H NMR spectra, GPC traces, characterization, and discussion of the host−guest interaction and magnified TEM (PDF) 13839
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge MOST (2014CB932200 and 2014BAI11B04), the Young Thousand Talents Program, and the NSFC (21174147 and 21474115) for financial support.
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