Article pubs.acs.org/Langmuir
Self-Assembly Behavior of a Linear-Star Supramolecular Amphiphile Based on Host−Guest Complexation 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 and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Chinese Academy of Sciences (CAS) Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *
ABSTRACT: A star polymer, β-cyclodextrin-poly(L-lactide) (βCD−PLLA), and a linear polymer, azobenzene−poly(ethylene glycol) (Azo−PEG), could self-assemble into a supramolecular amphiphilic copolymer (β-CD−PLLA@Azo−PEG) based on the host−guest interaction between β-CD and azobenzene moieties. This linear-star supramolecular amphiphilic copolymer further selfassembled into a variety of morphologies, including sphere-like micelle, carambola-like micelle, naan-like micelle, shuttle-like lamellae, tube-like fiber, and random curled-up lamellae, by tuning the length of hydrophilic or hydrophobic chains. The variation of morphology was closely related to the topological structure and block ratio of the supramolecular amphiphiles. These selfassembly structures could disassemble upon an ultraviolet (UV) light irradiation.
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INTRODUCTION Self-assembly of block copolymers has attracted considerable attention for decades because it can generate diverse structures, including micelle, tube, fiber, lamellae, and vesicle, and has extensive applications in drug and gene delivery, bioimaging, biomimic, etc.1−5 Generally, most of the reported precursors used in self-assembly are linear amphiphiles connected by covalent bonds. Recently, a series of nonlinear amphiphilies, such as multi-armed and dendritic polymers, has been adopted for the self-assembly study.6−11 These nonlinear structures can generate new opportunities for yielding unique self-assembly behavior and novel morphologies. Yan and co-workers have investigated the self-assembly of amphiphilic hyperbranched polymers (HBPs) and found that HBPs demonstrated individual characteristics in self-assembly behavior, including structural diversities, special self-assembly mechanism, facile functionalization, and intelligent responses.12−16 Shen and coworkers synthesized jellyfish-shaped amphiphilic dendrimers and found that these amphiphilic dendrimers could selfassemble into different morphologies with extremely uniform size distributions because of the well-defined topological structure of the dendrimers.17 Allcock and co-workers reported the self-assembly of palm-tree-like pseudo-block organophosphazene copolymers prepared by host−guest complexation between the adamantyl group and β-cyclodextrin (β-CD) group and found that the existence of the morphologies with multi-core micelle structures resulted from intermicellar aggregations.18 Supramolecular polymers, which have attracted increasing attention in various fields, are formed on the basis of non© 2014 American Chemical Society
covalent bonds, such as hydrogen bonding, ligand−metal coordination, π−π stacking, and host−guest interaction.19−24 Much of the recent research focus has been on the host−guest interaction for designing supramolecular polymers with different topological structures, including amphiphilic pseudo-block copolymers.25−29 Benefiting from their dynamic nature, the block ratio of supramolecular copolymers can be easily altered by changing the combination of host or guest building segments.30 The reversibility of the non-covalent connection also endows supramolecular polymers with unique selfassembly characteristic and responsiveness to external stimuli.31−35 β-CD, which possesses a hydrophilic exterior surface and hydrophobic interior cavity, has long been widely recognized as a host for many guest molecules.36−38 It is known that azobenzene (Azo), which has two isomers (trans and cis), can be recognized by β-CD reversibly upon light irradiation.39 In this work, we synthesized hydrophobic star host polymers of β-CD−poly(L-lactide) (β-CD−PLLA) with five different molecular weights and hydrophilic linear guest polymers of azobenzene−poly(ethylene glycol) (Azo−PEG) with three different molecular weights. By combining the hydrophobic and hydrophilic polymers based on the host−guest interaction between β-CD and azobenzene, 15 linear-star supramolecular polymers with different block ratios were prepared easily. The linear-star amphiphiles were adopted as self-assembly preReceived: August 18, 2014 Revised: October 8, 2014 Published: October 13, 2014 13014
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a Bruker Fourier 300 (300 MHz) spectrometer or Bruker Avance 400 III (400 MHz) spectrometer, and the two-dimensional (2D) 1H nuclear Overhauser effect spectrometry (NOESY) spectrum was recorded on a Bruker Avance 600 (600 MHz) spectrometer. Gel Permeation Chromatography (GPC). GPC analyses of polymers were performed on 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. Tetrahydrofuran (THF) was used as the eluent at a flow rate of 1.0 mL/min. Calibration of the molecular weight of polymer was based on polystyrene standards. Transmission Electron Microscopy (TEM). The images of the assemblies were obtained from a JEM-2200FS microscope, and the samples were prepared by drop-coating the aqueous solution on a carbon-coated copper grid. Scanning Electron Microscopy (SEM). The morphologies of assemblies were obtained from a JSM-6700F microscope with an accelerating voltage of 5 kV and a working distance of 8 mm. The samples were prepared by drop-coating the aqueous solution on a silica plate and then coated 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 Zetasizer Nano-ZS90 (Malvern Instruments, U.K.). All samples were measured at 25 °C with 632.8 nm laser light set at a scattering angle of 173°. The average diameter was obtained from the DTS software of the instrument using the volume reading. UV−Vis Spectroscopy. The transmittance of assembly solutions were measured on the TU 1901 spectrophotometer at λ = 600 nm. Synthesis of β-CD−PLLA (10, 20, 50, 70, and 100 kDa). As illustrated in Scheme 2a, the star β-CD polymer was synthesized
cursors for two reasons: (1) in comparison to linear amphiphiles with the same molecular weight, linear-star copolymers have shorter molecular chains, resulting in less chain entanglement and smaller hydrodynamic radii, and (2) the multi-arm topological structure of linear-star copolymers may affect the packing of molecular chains. These two characteristics pave the way for developing diverse selfassembly morphologies and a unique mechanism. These supramolecular polymers could self-assemble to different morphologies when the ratio of hydrophilic/hydrophobic changed (Scheme 1). The self-assembly behavior and variation of morphologies were studied systematically. In addition, the ultraviolet (UV)-responsive disassembly behavior was also investigated. Scheme 1. Illustration of Various Assembled Morphologies of the Supramolecular Polymers
Scheme 2. Synthetic Routes of (a) β-CD−PLLA and (b) Azo−PEG
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EXPERIMENTAL SECTION
Materials. β-CD (TCI, >95%) was dried at 70 °C under vacuum for 24 h before use. L-Lactide (PURAC, Netherlands) was purified by recrystallization from ethyl acetate twice. Stannous octoate [Sn(Oct)2, Sigma, analytical reagent (AR)], 4-phenylazophenol (Alfa Aesar, 98%), and polyethylene glycol monomethyl ether (PEG, Alfa Aesar) were directly used without further purification. Ethyl acetate (AR) was dried over P2O5 overnight and distilled before use. Chloroform and dichloromethane (DCM) were dried with CaH2 and distilled. N,Ndimethylformamide (DMF), tetrahydrofuran (THF), ethanol, NaOH, MgSO4, K2CO3, triethylamine (Et3N), and methanesulfonyl chloride were purchased from Beijing Chemical Works and used as received. Characterizations. Nuclear Magnetic Resonance (NMR) Spectroscopy. 1H NMR spectra for the structural analysis were obtained on
through ring-opening polymerization (ROP). Typically, L-lactide, βCD, and Sn(Oct)2 were added to a polymerization tube. The tube was degassed by three freeze−pump−thaw cycles, sealed under vacuum, and then immersed into an oil bath at 140 °C for 20 h. The mixture was then exposed to air to terminate the polymerization and cooled to room temperature. The resultant was dissolved in chloroform, precipitated in ethanol, and then dried in vacuum for 48 h. Synthesis of Azo−PEG (Mn,PEG = 750, 1900, and 5000). Azo− PEG was prepared via two-step reactions, as shown in Scheme 2b. 13015
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hydroxyl signals of β-CD disappeared, demonstrating that the final product of β-CD−PLLA has 21 arms. The multi-arm βCD−PLLA polymers were further measured by GPC (Table 1). As shown in Figure S3 of the Supporting Information, no
Polyethylene glycol monomethyl ether (10 mmol) and Et3N (7.5 mL, 50 mmol) in anhydrous DCM (100 mL) were mixed and stirred at 0 °C for 30 min. Methanesulfonyl chloride was then added dropwise to the mixture. After stirring for 6 h at room temperature, the mixture was washed by Na2CO3 solution and water 3 times. The layer of DCM was dried over MgSO4, filtered, and evaporated. The final product of PEG−OMs was obtained in vacuo. PEG−OMs (10 mmol), 4phenylazophenol (2.23 g, 11 mmol), and K2CO3 (3.46 g, 25 mmol) were mixed in acetonitrile (150 mL) and refluxed at 85 °C for 3 days. The mixture was cooled to room temperature, filtered, and rinsed with acetonitrile. The filtrate was evaporated and dried under vacuum to give the final product as an orange powder. Formation and Self-Assembly of Supramolecular Amphiphilic Polymers. The linear-star supramolecular amphiphilic polymers formed after mixing of Azo−PEG and β-CD−PLLA in N,N-dimethylformamide (DMF) (a common solvent for both hydrophilic PEG chains and hydrophobic PLLA chains). Specifically, equimolar quantities of β-CD−PLLA and Azo−PEG were dissolved in DMF (5 mL) for about 5 h under stirring. Water (1.5 mL) was added to the supramolecular polymer solution under vigorous stirring using a syringe pump with a rate of 4.5 μL/min. Subsequently, the solution was stirred for another 5 h and dialyzed against water to completely remove DMF for 4 days. The obtained solution was set to 1 mg/mL.
Table 1. GPC Results of β-CD−PLLA A B C D E
Mn,theory
Mn,GPC
PDI
10000 20000 50000 70000 100000
9600 19300 49800 70600 109000
1.33 1.23 1.20 1.40 1.34
traces of linear homopolymers were detected, also indicating that the multi-arm polymers (β-CD−PLLA) were obtained successfully. Synthesis of Azo−PEG. Polymers of Azo−PEG with three different molecular weights were synthesized via two-step reactions (Scheme 2b). The 1H NMR spectra of Azo−PEG and PEG−OMs with a PEG molecular weight of 750 were presented in Figure 2, and the characteristic signals of the
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RESULTS AND DISCUSSION Polymer Synthesis and Characterizations. Synthesis of β-CD−PLLA. Host polymers of β-CD−PLLA with five different molecular weights were obtained through anionic ring-opening polymerization using β-CD as a multi-armed initiator (Scheme 2a). On the basis of the literature reports, all 7 primary and 14 secondary hydroxyl groups of β-CD have the ability to initiate the polymerization of L-lactide.40 Figure 1 showed that all of the
Figure 2. 1H NMR spectra of (a) Azo−PEG and (b) PEG−OMs (Mn,PEG = 750) in CDCl3.
azobenzene group, methyl sulfonic group, and PEG moieties were assigned clearly. The 1H NMR spectra of Azo−PEG and PEG−OMs with PEG molecular weights of 1900 and 5000 were presented in Figures S1 and S2 of the Supporting Information.
Figure 1. 1H NMR spectra of (a) β-CD−PLLA (Mn = 20 kDa) and (b) β-CD in dimethyl sulfoxide (DMSO)-d6. 13016
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Figure 3. TEM images of the morphologies self-assembled from Azo−PEG@β-CD−PLLA (left labels and top labels refer to the molecular weights of Azo−PEG and β-CD−PLLA, respectively).
Self-Assembly of Supramolecular Amphiphilic Polymers. It is well-known that the ratio of hydrophilic/ hydrophobic segments plays an important role in the selfassembly morphologies of amphiphilic polymers. Generally, amphiphilic linear block copolymers can generate structures ranging from spherical micelles, worm- or rod-like micelles, vesicles, to inverted microstructures with the hydrophilic ratio decreasing. Transition of the morphology is based on the rearrangement of hydrophilic and hydrophobic segments driven by the hydrophobic interaction. Different from the linear block copolymers, the as-prepared supramolecular polymers were linear-star amphiphiles containing a hydrophilic PEG chain and 21 hydrophobic PLLA arms. Besides, the hydrophilic and hydrophobic segments were jointed through a host−guest interaction between β-CD and azobenzene. These structural characteristics made the arrangement of hydrophilic and hydrophobic segments different from the linear polymers, resulting in a distinct transition of the morphology when the ratio of hydrophilic to hydrophobic chains changed. The morphologies self-assembled from all of the 15 supramolecular amphiphiles with different block ratios were detected by TEM, as shown in Figure 3. Driven by hydrophobic interaction and benefiting from the linear-star topological structure, diverse morphologies have been obtained. To understand variation of the morphology, the self-assembly mechanism and morphology change were explained in details as follows. With the same length of star PLLA chains (Mn,β‑CD−PLLA = 10 kDa), the micelle morphologies changed from sphere (Figures 3a3 and 4a3) to carambola-like shape (Figures 3a2 and 4a2) and then to naan-like shape (Figures 3a1 and 4a1) with the molecular weight of PEG changing from 5000 to 1900 and then to 750. When water was added to the solution of the supramolecular amphiphilic polymer (Azo−PEG-5000@βCD−PLLA-10 kDa), the hydrophobic PLLA segments aggregated in the core, while the hydrophilic PEG segments extended in the shell to afford the micelle solubility in water. The spherical micelle is very basic self-assembly morphology
Figure 4. SEM images of the morphologies self-assembled from Azo− PEG@β-CD−PLLA (left labels and top labels refer to the molecular weights of Azo−PEG and β-CD−PLLA, respectively).
with monolayered arrangement of copolymer chains. When the length of PEG decreased, the sphere-like micelle transferred to carambola-like and naan-like micelles to increase the area of the hydrophilic shell to ensure stability of the assembled structures in water. Accordingly, the stacking of PLLA chains in the 13017
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hydrophobic domain underwent a transition from random to refined or from thick to thin, which had been illustrated in panels a−c of Scheme 1. When the molecular weight of β-CD−PLLA is kept fixed at 50 kDa, a morphological transition was also observed from shuttle-like lamellae (Figures 3c3 and 4b3) to an interim stage having both micelle and tube-like fiber (Figures 3c2 and 4b2) and then to random curled-up lamellae (Figures 3c1 and 4b1), with the molecular weight of PEG changing from 5000 to 1900 and then to 750. Interestingly, the supramolecular amphiphilic polymer Azo−PEG@β-CD−PLLA-50 kDa with longer hydrophobic arms than Azo−PEG@β-CD−PLLA-10 kDa selfassembled to the lamellae structure. That is because, as the length of PLLA chains increased, the hydrophilicity of the supramolecular amphiphilic polymer decreased and more hydrophilic chains needed to be exposed in the outside of the assembled structures. The micellar structure with a monolayered arrangement of hydrophilic and hydrophobic chains was unable to stabilize the self-assemblies. Thus, the lamellae structure occurred with the formation of the bilayers, in which the hydrophobic segments stacked inside and hydrophilic segments stacked outside (panels d−f of Scheme 1). The morphological transition from shuttle-like lamellae to tube-like fiber and random curled-up lamellae can be explained as follows: with the decrease of the hydrophilic chains, more regular arrangements in the bilayers were involved, leading to reduction of the lamellae thickness and increase of the lamellae size. The lamellae with a large size tended to curl to a tube-like fiber or a random morphology to maintain stability of the structures, as shown in panels c2 and c1 of Figure 3, respectively. Keeping the guest polymer remaining in the same molecular weight of 1900, when the length of PLLA chains changed from 10 to 100 kDa, different assembled morphologies were also detected by TEM (panels a2−e2 of Figure 3). When the molecular weight of β-CD−PLLA increased from 20 to 50 kDa, the self-assemblies showed a qualitative change from micelle to fiber, with the arrangement of copolymer chains changing from monolayer to bilayer. As the molecular weight of β-CD−PLLA continued to increase from 50 to 100 kDa, the length of the fiber increased concomitantly. On the basis of the above explanation, the transition from carambola-like micelle to tubelike fiber was quite reasonable. It is worth noted that the change of the PLLA length caused a transition of the assembled structure from micelle to lamellae, while the change of the PEG length could only cause transition of the morphology under the same structure either micelle or lamellae. The star character of the host β-CD−PLLA polymer may account for why changes of the morphologies resulting from PLLA lengths were faster than those from PEG. The above results indicated that the self-assembly morphology underwent a variation, including sphere-like micelle, carambola-like micelle, naan-like micelle, shuttle-like lamellae, tube-like fiber, and random curled-up lamellae, in turn, which were summarized in Figure 5. We attributed the morphological diversities to the linear-star topological architecture and the easily changed block ratio. All of the morphologies were summed up to two self-assembly structures: micelle and lamellae. Supramolecular amphiphilic polymers with the molecular weight of β-CD−PLLA as 10 and 20 kDa selfassembled to micelle structures, while those with the molecular weight of β-CD−PLLA equal to or greater than 50 kDa selfassembled to lamellae structures. The structural transition from
Figure 5. Phase diagram of the assemblies from Azo−PEG@β-CD− PLLA as functions of molecular weights of PEG and PLLA.
micelle to lamellae corresponded to the change of molecular rearrangement from monolayer to bilayer (Scheme 1). With the same molecular weight of β-CD−PLLA, the change of Azo− PEG could only cause a change of the morphology under the same structures of either micelle or lamellae. Transition of the morphology was ascribed to the stacking degree of polymer chains in the hydrophilic and hydrophobic domains. UV Stimuli−Responsive Behavior. Upon irradiation of UV light (365 nm), the Azo group can undergo light-triggered isomerization from trans to cis form. As presented in Figure S7 of the Supporting Information, after irradiation with UV light (365 nm, 100 W) for 10 min, the peaks at 7.01, 7.49, and 7.88 ppm that arise from the aromatic protons of the trans-Azo group shifted to 6.77, 6.87, and 7.28 ppm, which can be assigned to the cis-Azo group. Because only trans-Azo can form host−guest complexation with β-CD, the UV irradiation will lead to disassembly of the self-aggregate. As shown in panels a and c of Figure 6, the solution of Azo−PEG-1900@β-CD− PLLA-10 kDa transformed from turbid to transparent after irradiation of UV light (365 nm, 100 W) for 10 min. Meanwhile, some white precipitates were observed, which were demonstrated to be the water-insoluble β-CD−PLLA (see Figure S9 of the Supporting Information). The TEM results proved the UV-triggered disassembly more directly. The micelles were very stable for at least 2 months without any external stimuli (Figure 6b), but upon 10 min of UV irradiation, they completely disassembled into small fragments (about dozens of nanometers). This UV responsiveness of the selfassemblies endowed them with potential applications in the field of controlled release.
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CONCLUSION A series of amphiphilic supramolecular polymers was synthesized by assembling linear guest polymers of Azo−PEG and multi-arm host polymers of β-CD−PLLA. Different morphologies, including sphere-like micelle, carambola-like micelle, naan-like micelle, shuttle-like lamellae, tube-like fiber, and random curled-up lamellae, were obtained through selfassembly of these supramolecular polymers. The morphological diversities were attributed to the linear-star topological architecture and the easily changed block ratio. The transition from micelle to lamellae was realized by changing the length of PLLA chains. With the same PLLA chains, the change of the PEG length could only lead to the change of morphologies 13018
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Figure 6. UV-responsive disassembly of Azo−PEG-1900@β-CD−PLLA-10 kDa of (a) just assembled, (b) after 2 months, and (c) after UV irradiation. alternative to fabricate drug-delivery platforms for cancer therapy. Angew. Chem., Int. Ed. 2011, 50, 9162−9166. (7) Pang, X. C.; Zhao, L.; Akinc, M.; Kim, J. K.; Lin, Z. Q. Novel amphiphilic multi-arm, star-like block copolymers as unimolecular micelles. Macromolecules 2011, 44, 3746−3752. (8) Park, W.; Kim, D.; Kang, H. C.; Bae, Y. H.; Na, K. Multi-arm histidine copolymer for controlled release of insulin from poly(lactideco-glycolide) microsphere. Biomaterials 2012, 33, 8848−8857. (9) Percec, V.; Imam, M. R.; Peterca, M.; Leowanawat, P. Selforganizable vesicular columns assembled from polymers dendronized with semifluorinated Janus dendrimers act as reverse thermal actuators. J. Am. Chem. Soc. 2012, 134, 4408−4420. (10) Percec, V.; Wilson, D. A.; Leowanawat, P.; Wilson, C. J.; Hughes, A. D.; Kaucher, M. S.; Hammer, D. A.; Levine, D. H.; Kim, A. J.; Bates, F. S.; Davis, K. P.; Lodge, T. P.; Klein, M. L.; DeVane, R. H.; Aqad, E.; Rosen, B. M.; Argintaru, A. O.; Sienkowska, M. J.; Rissanen, K.; Nummelin, S.; Ropponen, J. Self-assembly of Janus dendrimers into uniform dendrimersomes and other complex architectures. Science 2010, 328, 1009−1014. (11) Zhou, Y. F.; Yan, D. Y. Supramolecular self-assembly of amphiphilic hyperbranched polymers at all scales and dimensions: Progress, characteristics and perspectives. Chem. Commun. 2009, 1172−1188. (12) Jin, H. B.; Huang, W.; Zhu, X. Y.; Zhou, Y. F.; Yan, D. Y. Biocompatible or biodegradable hyperbranched polymers: From selfassembly to cytomimetic applications. Chem. Soc. Rev. 2012, 41, 5986− 5997. (13) Jin, H. B.; Zhou, Y. F.; Huang, W.; Yan, D. Y. Polymerizationlike multilevel hierarchical self-assembly of polymer vesicles into macroscopic superstructures with controlled complexity. Langmuir 2010, 26, 14512−14519. (14) Liu, Y.; Yu, C. Y.; Jin, H. B.; Jiang, B. B.; Zhu, X. Y.; Zhou, Y. F.; Lu, Z. Y.; Yan, D. Y. A supramolecular Janus hyperbranched polymer and its photoresponsive self-assembly of vesicles with narrow size distribution. J. Am. Chem. Soc. 2013, 135, 4765−4770. (15) Tao, W.; Liu, Y.; Jiang, B. B.; Yu, S. R.; Huang, W.; Zhou, Y. F.; Yan, D. Y. A linear-hyperbranched supramolecular amphiphile and its self-assembly into vesicles with great ductility. J. Am. Chem. Soc. 2012, 134, 762−764. (16) Yan, D. Y.; Zhou, Y. F.; Hou, J. Supramolecular self-assembly of macroscopic tubes. Science 2004, 303, 65−67. (17) Shao, S. Q.; Si, J. X.; Tang, J. B.; Sui, M. H.; Shen, Y. Q. Jellyfishshaped amphiphilic dendrimers: Synthesis and formation of extremely uniform aggregates. Macromolecules 2014, 47, 916−921. (18) Tian, Z. C.; Chen, C.; Allcock, H. R. Synthesis and assembly of novel poly(organophosphazene) structures based on noncovalent
under the same structures of either micelle or lamellae. The assemblies were UV-responsive, giving them potential applications in the field of controlled release, which are currently under investigation.
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ASSOCIATED CONTENT
S Supporting Information *
Additional 1H NMR spectra, GPC traces, 2D 1H NOESY spectrum of 4-phenylazophenol and β-CD, UV−vis results, and DLS data. This material is available free of charge via the Internet at http://pubs.acs.org.
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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 The authors gratefully acknowledge the Ministry of Science and Technology (MOST) (2014CB932200), the “Young Thousand Talents Program”, and the National Natural Science Foundation of China (NSFC) (21174147 and 51325304) for financial support.
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