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Morphological Evolution of Self-Assembled Structures Induced by Molecular Architecture of Supra-Amphiphiles Juan Wang, Boxuan Li, Xing Wang, Fei Yang, Hong Shen, and De-Cheng Wu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03550 • Publication Date (Web): 01 Dec 2016 Downloaded from http://pubs.acs.org on December 2, 2016

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Morphological Evolution of Self-Assembled Structures Induced by Molecular Architecture of Supra-Amphiphiles Juan Wang,†,1 Boxuan Li,§ Xing Wang,*,† Fei Yang,†,‡ Hong Shen,† 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, China 1

Current Address: Department of Chemical & Life Science Engineering, Virginia

Commonwealth University, Richmond, VA 23219, USA. §

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Photochemistry,

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡

University of Chinese Academy of Sciences, Beijing 100049, P.R. China.

ABSTRACT. A series of telechelic supramolecular amphiphiles (POSS-Azo8@[β-CD-PDMAEMA]18) was accomplished by orthogonally coupling the multi-arm host polymer β-cyclodextrin-poly(dimethylaminoethyl methacrylate) (β-CD-PDMAEMA) with an octa-telechelic guest molecule azobenzene modified-polyhedral oligomeric silsesquioxanes (POSS-Azo8) under different host-guest

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ratios. These telechelic supramolecular amphiphiles possess a rigid core and flexible corona. Increasing of the multi-arm host polymer coupled onto the rigid POSS core made the molecular architecture tend to be symmetrical and spherical. POSS-Azo8@[β-CD-PDMAEMA]18 could self-assemble into diverse morphologies evolving from spherical micelles, wormlike micelles, branched aggregates, to bowl-shaped vesicles. Distinct from the traditional linear amphiphilic polymers, we discovered that the self-assembly of POSS-Azo8@[β-CD-PDMAEMA]18 was regulated dominantly by their molecular architectures instead of hydrophilicity, which has also been verified by computer simulation results.

INTRODUCTION

From the phospholipid bilayer of cell membranes to the reverse parallel coiled double helix of DNA polynucleotide chains, molecular self-assembly is ubiquitous in biological science.1-3 Along with the rising upsurge from interdiscipline of life science and chemistry, self-assembly becomes not only academically appealing but also leads to a growth in the field of biomedical application.4-10 The self-assembly of small-molecular amphiphiles and block copolymers has achieved considerable development during the last decades.11-15 Various elaborate morphologies obtained show the potential application as biomaterials.16-21 Considering the abundance and complexity of self-assembly in biological systems, it is of great significance to expand the scope of self-assembly


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precursors not only in terms of molecular structures but also the physical properties of macromolecular chains. Recently, the self-assembly of various non-linear macromolecular structures ranging from star polymers to dendrimers have been investigated.22-27 Unlike linear copolymers, factors that affect the self-assembly process of non-linear macromolecular are not limited to hydrophilicity, molecular weight, copolymer composition and so forth.28-31 For example, Shen and coworkers found that the well-defined topological structure of jellyfish-shaped amphiphilic dendrimers led to the self-assembled morphologies having extremely uniform size distributions.32 Although the self-assembly of various non-linear macromolecular structures are becoming increasing attractive, amphiphiles with complicated molecular chain conformations have little been reported as self-assembly precursors. Progress in non-covalent polymers such as host-guest supramolecules provides a strategy for the preparation of amphiphiles with structural versatility and functional modulation.33-41 In our previous work, the supra-amphiphiles with different topological structures and hydrophilicities were all prepared based on the host-guest interaction between β-cyclodextrin-poly(L-lactide) (β-CD-PLLA) and azobenzene-poly(ethylene glycol) (azo-PEG).42,43 Herein, we adopted the host-guest complexation between β-CD and azobenzene to construct a series of supra-amphiphiles, azobenzene modified-polyhedral oligomeric silsesquioxanes@β-CD-poly(dimethylaminoethyl methacrylate)


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(POSS-Azo8@β-CD-PDMAEMA). Due to the dynamic characteristics of host-guest interaction, we can easily tune the complexation ratios of host/guest moieties, which makes the molecular architectures of the supra-amphiphiles can evolve gradually from hemi-telechelic to octa-telechelic. In addition, the as prepared supra-amphiphiles also combines the unique properties of two distinct chemical moieties in one molecular structure. First, the POSS component serves as a rigid and hydrophobic backbone of the whole supra-amphiphiles. POSS is an ideal building block for the construction of star polymers containing a rigid inorganic core and eight reactive sites at the vertex of the core.44-47 Second, the flexible and hydrophilic PDMAEMA chains provide the supra-molecule with the self-assembly capability. Therefore, our supra-amphiphiles have tunable topological structures with rigid and soft conformations. It was found that a little change in the hydrophilicity of supra-amphiphiles could give rise to a big transition in self-assembled morphologies. Because of the unique nature of molecular structure and conformation, the molecular architecture, instead of hydrophilicity, took a dominant role on the regulation of self-assembly behaviors. EXPERIMENTAL SECTION

Materials. β-cyclodextrin (β-CD, TCI, >95%) and octavinyloctasilasesquioxane (POSS-ene8, Hybrid Plastics, 98%) were dried at 70 °C under vacuum for 24 h before use. Dimethylaminoethyl methacrylate (DMAEMA, Aldrich, 98%) was purified by vacuum


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distillation. Thiourea (Tianjin Jinke Chemical Research Institute, AR), 4-phenylazophenol (Alfa Aesar, 98%), 1,4-dibromobutane (Alfa Aesar, 99%) and 2,2-dimethoxy-2-phenylacephenone (DMPA, TCI, 98%) were directly used without further purification. Ethyl acetate (A.R.) was dried over P2O5 overnight and distilled before use. Chloroform and dichloromethane (DCM) were dried with CaH2 and distilled. N,N-dimethylformamide (DMF), tetrahydrofuran (THF), ethanol, diethyl ether, n-hexane, potassium hydroxide (KOH), anhydrous magnesium sulfate (MgSO4), potassium carbonate (K2CO3), sodium carbonate (Na2CO3), sodium chloride (NaCl), ceric ammonium nitrate, hydrochloric acid (HCl) and nitric acid (HNO3) were purchased from Beijing Chemical Works and used as received. Characterizations. Nuclear Magnetic Resonance Spectroscopy (NMR). 1H NMR and 13C NMR spectra for the structural analysis were obtained on a Bruker Fourier-300 (300 MHz) spectrometer or a Bruker Avance-400 III (400 MHz) spectrometer, and 2D 1H nuclear Overhauser effect spectrometry (NOESY) were recorded on a Bruker Avance-600 (600 MHz) spectrometer.

MALDI TOF Mass Spectrometry. The MALDI-TOF analysis of POSS-Azo8 was conducted on a Bruker BIFLEX III equipped with a 337 nm nitrogen laser. The sample was dissolved in CHCl3 and mixed with α-cyano-4-hydroxy cinnmaic acid/CHCl3 solution prior to dry.


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Gel Permeation Chromatography (GPC). GPC analysis was performed using a Waters 515 pump connected to a Waters 2414 refractive index detector. DMF 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.

Thermal Gravity Analysis (TGA). The thermogravimetric curves were collected by a TA Instruments, Inc., MDSC-2910. The temperature program was from 60 to 700 °C with an increasing rate of 10 °C/min in a flow of air.

Transmission Electron Microscopy (TEM). The images of 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. No inorganic staining reagents were used before the TEM observation.

Dynamic Light Scattering (DLS). The hydrodynamic sizes of assemblies were measured on a Zetasizer Nano-ZS90 (Malvern Instruments, UK). 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 instrument’s DTS software using the volume reading.

Fluorescence Spectrophotometry. The stoichiometric fluorescence spectra of supramolecules were measured on an F4600 (Hitachi) fluorescence spectrometer. The excitation wavelength was 350 nm and scanning range was from 370 to 800 nm.


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Scheme 1. Synthesis Routes for (A) β-CD-PDMAEMA and (B) POSS-Azo8

Synthesis of β-CD-poly(dimethylaminoethyl methacrylate) (β-CD-PDMAEMA). The multi-arm host polymer of β-CD-PDMAEMA was synthesized as shown in Scheme 1A. β-CD (1 g), ceric ammonium nitrate (2 g) and nitric acid (65 wt%, 4 mL) were mixed in 80 mL of deionized water and degassed by nitrogen completely. The reaction mixture was then heated to 50 oC followed by the dropwise addition of DMAEMA monomer (10 mL) under a nitrogen atmosphere. After 3 h of reaction, the solution turned from initial orange to colorless. The concentrated solution was dialyzed against deionized water (Mw cut-off, 1.0 kDa) to remove the unreacted monomers and other impurities. The final product was white flake solid. Mn, NMR = 8 800, Mn, GPC = 10 500, Mw/Mn = 1.21. 400 MHz 1H NMR (δ, ppm, D2O): 4.41 (s, 98H), 3.96-3.87 (m, 21H), 3.59 (s, 98H), 3.03 (d, J = 7.2 Hz, 294H), 1.85 (d, J = 139.9 Hz, 98H), 1.37-0.69 (m, 147H).


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Synthesis of 4-(4-mercaptobutane) azobenzene (Azo-SH). Azo-SH was prepared via a two-step end-group modification of 4-phenylazophenol (Scheme 1B). Specifically, 4-phenylazophenol (1.68 g, 8.5 mmol) was first dissolved in 20 mL of acetonitrile. After 15 min of stirring at room temperature, K2CO3 (2.36 g, 17 mmol) was added. Under a nitrogen atmosphere, the reaction mixture was heated to reflux and dropwise added 1,4-dibromobutane (2.29 g, 11 mmol). The reaction was continued to reflux for another 4 h. After cooling to room temperature, the mixture was filtered. The filtrate was concentrated, re-dissolved in DCM and washed with saturated brines twice. The phase of DCM was dried over anhydrous MgSO4, filtered, and evaporated. The residue mixture obtained was further purified by silica chromatography (EA/Hex 1:8) and was evaporated to give 1.86 g (yield: 66%) of the intermediate product Azo-Br as an orange powder. 400 MHz 1H NMR (δ, ppm, CDCl3): 7.92 (dd, J = 19.1, 8.1 Hz, 4H), 7.58-7.42 (m, 3H), 7.01 (d, J = 9.0 Hz, 2H), 4.10 (t, J = 6.0 Hz, 2H), 3.51 (t, J = 6.5 Hz, 2H), 2.10 (dd, J = 13.9, 7.1 Hz, 2H), 2.01 (dd, J = 13.9, 5.7 Hz, 2H). 400 MHz

13

C NMR (δ, ppm, CDCl3):

161.57, 153.00, 147.27, 130.58, 129.25, 124.98, 122.78, 114.90, 67.42, 33.53, 29.63, 28.06. Azo-Br (1.86 g, 5.6 mmol) and thiourea (2.12 g, 28 mmol) were mixed in 60 mL of ethanol and refluxed. During the heating of reaction, the mixture completely dissolved. After 12 h, the mixture was cooled to room temperature and evaporated to obtain a pungent-smelled orange solid. KOH aqueous solution (2.3 M) was prepared using deoxygenized ultrapure water and poured into the pungent-smelled solid, forming an


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orange turbid solution. After refluxing for 4 h, the suspension was cooled and added hydrochloric acid to adjust the pH. With the change of suspension from basic to neutral, amounts of orange solid was separated out. Afterwards, the solid was extracted by DCM. The combined DCM phases were then washed by saturated brines, dried over anhydrous MgSO4, filtered, and evaporated. The desired product Azo-SH was orange powder with pungent odour (yield: 80%). 300 MHz 1H NMR (δ, ppm, CDCl3): 8.07-7.84 (m, 4H), 7.59-7.37 (m, 3H), 7.00 (dd, J = 8.9, 1.8 Hz, 2H), 4.08 (t, J = 6.0 Hz, 2H), 2.89-2.58 (m, 2H), 2.06-1.75 (m, 4H), 1.40 (t, J = 7.9 Hz, 1H). 400 MHz 13C NMR (δ, ppm, CDCl3): 161.65, 152.96, 147.16, 130.56, 129.24, 124.98, 122.77, 114.88, 67.88, 38.82, 28.16, 25.97.

Synthesis of guest polymer POSS-Azo8. Via a thiol-ene click reaction, azobenzene groups were grafted onto all the eight vinyl groups of octavinyloctasilasesquioxane. Specifically, POSS-ene8 (0.13 g, 0.2 mmol), Azo-SH (0.86 g, 3 mmol) and DMPA (0.04 g, 0.16 mmol) were mixed in 5 mL of THF. The mixture was then purged with argon for 15 min and irradiated under a 365 nm UV lamp at room temperature for 5 h. After being evaporated under vacuo, the residue was re-dissolved in DCM, precipitated in ice diethyl ether until the supernate became almost colorless. Centrifuging of the precipitate yielded 0.36 g (65%) of the product as an organic solid. 400 MHz 1H NMR (δ, ppm, CDCl3): 7.89 (dd, J = 15.0, 8.2 Hz, 32H), 7.46 (dd, J = 25.4, 7.2 Hz, 24H), 6.99 (d, J = 8.9 Hz,


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16H), 4.07 (d, J = 5.5 Hz, 16H), 2.80 (t, J = 6.6 Hz, 16H), 2.40 (t, J = 6.5 Hz, 16H), 1.92 (dd, J = 16.1, 13.5 Hz, 32 H), 1.22 (t, J = 6.0 Hz, 16H). 400 MHz

13

C NMR (δ, ppm,

CDCl3): 161.34, 152.80, 147.11, 130.37, 129.03, 124.77, 122.57, 114.73, 65.83, 33.24, 29.38, 28.13, 26.24, 12.68. MALDI-TOF MS: calculated [M]+ m/z = 2924.17. Found: [M+Na+] m/z = 2947.98, [M+H+] m/z = 2925.39.

Self-assembly of supramolecular amphiphiles. Eight groups of supramolecular amphiphiles were first prepared by mixing β-CD-PDMAEMA and POSS-Azo8 in DMF with the molar ratios to be 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, and 8:1 respectively. After stirring overnight, the supramolecular solutions were set to 0.02 g/mL. The typical procedure of self-assembly was as follows. Under vigorous stirring, 2 mL of water was injected to 6 mL of supramolecular amphiphile solutions using a syringe pump at a flow rate of 4.5 μL/min. Subsequently, the solutions were stirred for another 5 h and dialyzed against water to completely remove DMF for 3 days. The obtained solution was set to 1 mg/mL for further characterization.

RESULTS AND DISCUSSION

Synthesis of host polymer (β-CD-PDMAEMA). In this work, the host polymer, β-CD-PDMAEMA was synthesized via a ceric ion initiated polymerization. The 1H NMR spectrum of β-CD-PDMAEMA was presented in Figure 1, and the characteristic signals of β-CD and PDMAEMA segments were assigned clearly. According to the 1H NMR


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analysis, the number-average degree of polymerization (DP) of the PDMAEMA segment was determined to be 49 from the integral ratio of peak c at 4.41 ppm (the methylene protons in PDMAEMA units adjacent to the oxygen moieties of ester linkages) to peak a (the 2,3,5-H protons on the glucose units of β-CD). The Mn from GPC result of β-CD-PDMAEMA in Figure 2 was 10 500 g/mol, which is consistent with that of the 1H NMR.

Figure 1. 1H NMR spectrum of β-CD-PDMAEMA in D2O.


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Figure 2. GPC trace of β-CD-PDMAEMA in DMF.

Synthesis of guest polymer of POSS-Azo8. Octa-telechelic guest, POSS-Azo8 was synthesized via thiol-ene click reaction between POSS-ene8 and Azo-SH. Synthesis of Azo-Br and Azo-SH. 4-Phenylazophenol was first modified to have a sulfhydryl end-group via a two-step reaction. As shown in Figure 3, 1H NMR and

13

C

NMR spectra of intermediate and end products were both assigned clearly. The integral ratio

of

protons

on

the

aromatic

ring

at

7.01-7.92

ppm

to

methylenes

(-O-CH2-CH2-CH2-CH2-Br, peaks f-i) demonstrated that the intermediate product Azo-Br was synthesized successfully (Figure 3A-a). After reacting with thiourea, the end-group changed from -Br to -SH, with the adjacent methylene shifting upfield from 3.51 to 2.89 ppm (Figure 3A-b, peak i). Peak assignments on demonstrated the structure of both Azo-Br and Azo-SH.


13

C NMR spectra also

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Figure 3. (A) 1H NMR and (B) 13C NMR spectra of (a) Azo-SH and (b) Azo-Br in CDCl3.

Synthesis of octa-telechelic guest polymer of POSS-Azo8. Via the thiol-ene click reaction, octa-telechelic guest polymer was obtained. Compared with reagent POSS-ene8, peak of alkenyl (-CH=CH2) at 6.06-5.84 ppm disappeared completely, while protons of azobenzene moieties at 6.98-7.94 ppm appeared (Figure 4A). The

13

C NMR spectrum

(Figure 4B) also confirmed the structure of the guest polymer. The MALDI-TOF spectrum in Figure 4C showed a strong signal for POSS-Azo8 which is consistent with its calculated molecular weight. To further confirm the structure of POSS-Azo8, we adopted thermo-gravimetric analysis (TGA) measurement. TGA can precisely determine the inorganic POSS contents through burning out the organic segments under atmospheric conditions. Figure 4D and E showed the POSS contents estimated by TGA to be ca. 20%, 66% and 0 respectively for POSS-Azo8, POSS-ene8 and Azo-SH, which were almost


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identical with their theoretical POSS contents. The results of 1H NMR,

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13

C NMR,

MALDI-TOF and TGA validated the structure of octa-telechelic guest polymer POSS-Azo8.

Figure 4. Characterization of POSS-Azo8: (A) 1H NMR spectra of POSS-Azo8 and POSS-ene8 in CDCl3, (B) 13C NMR spectrum of POSS-Azo8 in CDCl3, (C) MALDI-TOF spectrum of POSS-Azo8, (D) TGA curves and (E) theoretical and actual POSS contents of POSS-Azo8, POSS-ene8 and Azo-SH.

Formation of supramolecular amphiphiles based on host-guest interaction. The 2D 1

H NOESY spectrum of POSS-Azo8@[β-CD-PDMAEMA]1 shown in Figure 5 provided

a direct evidence for the formation of the supramolecule via the inclusive complexation between β-CD and azobenzene groups. Cross peaks from dipolar interaction between the signals at 4.38, 4.01 and 3.31 ppm assigned to the inner protons (the 4-, 2,3,5-, and 1-H


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protons) located in the cavities of β-CD and the signals at 7.01, 7.36 and 7.76 ppm ascribed to azobenzene moieties were clearly detected, strongly indicating that azobenzene moieties have been deeply penetrated in the cavities of β-CD, and the corresponding supramolecule of POSS-Azo8@[β-CD-PDMAEMA]1 has been successfully obtained. With the increase of the β-CD-PDMAEMA, the shielding of PDMAEMA chains increased the signal noise of the 2D 1H NOESY spectra, so the coherent proton signals between of β-CD and azobenzene of other seven supramolecules of POSS-Azo8@[β-CD-PDMAEMA]28 gradually weakened to be difficultly detected.

Figure 5. 2D 1H NOESY spectrum of POSS-Azo8@[β-CD-PDMAEMA]1 in D2O.

To further confirm the structure of POSS-Azo8@[β-CD-PDMAEMA]18, we adopted the approach of fluorescent spectra to demonstrate the complex between POSS-Azo8 and β-CD-PDMAEMA. When the environment of fluorescent substance switched from polar to nonpolar, the quantum yield and fluorescence intensity would be enhanced.48-51 As the


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azobenzene groups penetrated the hydrophobic cavity of β-CD with a more nonpolar microenvironment, the density of electron cloud would increase and conjugation effect of π-electron would be strengthened, resulting in the enhancement of fluorescence intensity. As shown in Figure 6, the fluorescence intensity of POSS-Azo8@[β-CD-PDMAEMA]08 increased gradually with the increment of host polymers. When more host polymers were incorporated into the host-guest system, more azobenzene groups would be liberated from the nonemissive aggregated state.52 Continuing to increase the amount of β-CD-PDMAEMA, the fluorescence intensity remained basically unchanged. The strongest fluorescence intensities appeared at 1/8 of stoichiometric ratio (β-CD/azobenzene). The growing fluorescence intensities of POSS-Azo8@[β-CD-PDMAEMA]08 indicated the supramolecular topological evolution from hemi- to octa-telechelic.


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Figure 6. fluorescence spectra of POSS-Azo8 (110-5 M) in the absence and presence of β-CD (10-5 M): (0) 0; (A) 1; (B) 2; (C) 3; (D) 4; (E) 5; (F) 6; (G) 7; (H) 8 in DMF solution.

Self-assembly behavior of POSS-Azo8@[β-CD-PDMAEMA]18. We used TEM and DLS to analyze morphologies and sizes of the aggregates self-assembled from POSS-Azo8@[β-CD-PDMAEMA]18, as shown in Figure 7. When the host-guest ratios were 1:1, 2:1, and 3:1, the self-assembled morphologies were all spherical micelles with the hydrodynamic size increasing from 225 to 356 nm (Figure 7A-C). As the host-guest ratios became 4:1, the self-assembled morphology turned into wormlike micelle and the L/D (length/diameter) was about 2-5. With the host-guest ratio continued to increase, the morphology self-assembled from POSS-Azo8@[β-CD-PDMAEMA]5 remained wormlike micelle, while the L/D could be increased to up to 30 (Figure 7E). An obvious transition of the self-assembled morphologies and sizes occurred after the host-guest ratio reached to 6:1 and 7:1. Specifically, POSS-Azo8@[β-CD-PDMAEMA]6,7 self-assembled into branched aggregates with micron-scaled sizes. Finally, the octa-telechelic supramolecular amphiphile, POSS-Azo8@[β-CD-PDMAEMA]8 self-assembled into micro-sized bowl-like shape with the wall thickness maintaining about 116 nm.


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Figure 7. Morphologies and sizes of self-assemblies from supramolecules with different host-guest ratios: (A)1:1, (B) 2:1, (C) 3:1, (D) 4:1, (E) 5:1, (F) 6:1, (G) 7:1, and (H) 8:1.

The supramolecular amphiphiles POSS-Azo8@β-CD-PDMAEMA had hydrophobic hexahedral POSS core and hydrophilic multi-arm β-CD-PDMAEMA segments, with the latter accounted for most percent with the increasing of host-guest ratios. For instance, with the host-guest ratios increasing from 1:1 to 8:1, the hydrophilicities of corresponding supramolecular amphiphiles changed from 74% to 96%. It has been well


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known that the hydrophilicity plays an important role in self-assembly behavior of amphiphiles.53,54 According to literature reports, amphiphilic linear polymers with f hydrophilic

> 50% are expected to self-assemble to spherical micelles and with the value of f

hydrophilic

decreasing, the evolution trend of self-assembled morphologies is from spherical

micelle to wormlike micelle to vesicle.55 Interestingly, there were two aspects that were very different from the above thesis. First, although all the eight supramolecules occupied an f hydrophilic value greater than 50%, diverse self-assembled morphologies still formed including not only spherical micelles but also wormlike micelle, branched aggregate and bowl-like vesicle. Second, with f hydrophilic decreasing, the evolution trend of self-assembled morphologies just was on the contrary.

Table 1. The Hydrophilic Fraction, Self-assembled Morphologies and Sizes of POSS-Azo8@[β-CD-PDMAEMA]18

Size f

a hydrophilic

Supramolecule

Morphology

TEM

DLS

(nm)

(nm)

(%)

POSS-Azo8@[β-CD-PDMAEMA]1

71

sphere

114

225


85

sphere

124

267


90

sphere

151

356


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92

worm

334

483


94

worm

478

598


95

branch

863

1324


95

branch

1180

1853


96

bowl

1803

2606

a

f hydrophilic refers to hydrophilic fraction calculated from the ratio of

Mn,β-CD-PDMAEMA/Mn,POSS-Azo8@β-CD-PDMAEMA. With the host-guest ratios increasing from 1:1 to 8:1, topological structures of the corresponding supramolecular amphiphiles evolved gradually from hemi-telechelic to octa-telechalic. Compared with linear amphiphilic polymers, the self-assembly behavior of supramolecular amphiphiles in this work was undoubtedly more complex. As more and more non-linear amphiphilic polymers were used as precursor of self-assembly, it was found that the influence of molecular architecture on self-assembly process could not be ignored. Yan and co-workers have reported that small changes in the molecular architecture of hyperbranched block copolymers could lead to a significant change in the self-assembled morphology, which was called amplification effect.8 We first assumed that the topological structure and hydrophilicity had complementary influences on the self-assembly behavior of POSS-Azo8@[β-CD-PDMAEMA]18. That is to say, due to the particular molecular architecture of POSS-Azo8@[β-CD-PDMAEMA]18, the influence of hydrophilicity on the morphological evolution should be different with


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traditional linear polymers, and it is possible that diverse morphologies appeared under the value of f hydrophilic only changes from 74% to 96%. However, this view could not explain why the evolution trend of self-assembled morphologies was on the contrary with traditional linear polymers.

As the f hydrophilic values of POSS-Azo8@[β-CD-PDMAEMA]18 were all greater than 70%, and especially after the host-guest ratios increased to 4:1, the f hydrophilic values had little changes that could be ignored. Based on these, we put forward a hypothesis that the topological structure of supramolecules played an absolute dominant role on the self-assemble morphological evolution in this work.

Initially, both the POSS core and the PDMAEMA chains stretched in their common solvent of DMF. When the self-assembly procedure was carried out, driving by the hydrophobic interaction, the original outstretched state of molecular chains had to make corresponding adjustment to adapt for the new aqueous environment. During the rearrangement, the supramolecules needed to find an appropriate way to keep the hydrophobic POSS portion away from aqueous phase and be surrounded by the hydrophilic PDMAEMA segments. The complex adjustment process finally led to the appearance of diverse self-assembled morphologies. As the central POSS element was rigid and nonflexible, the adjustment process depended mainly on the movement and rearrangement of PDMAEMA chains. As for POSS-Azo8@[β-CD-PDMAEMA]13, the


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hydrophilic host polymers enclosed on the guest arms were not enough to protect the whole hydrophobic POSS core, they tended to restacked into spherical micelles with the hydrophobic moieties aggregated in the core and the hydrophilic moieties stretched in the corona, as shown in Figure 8A-C. It was worth noting that with the host-guest ratios increased from 1:1 to 3:1, the increment of host polymer chains surrounded in the guest raised the repulsion force among the adjacent POSS cores during the formation of spherical micelles, therefore, the sizes of micelles increased gradually. As the host-guest ratios continued to increase to 4:1, the repulsion force among POSS cores was so strong that the spherical stacking state could not be implemented, the wormlike micelle emerged at this point (Figure 7D). When the host-guest ratios reached at 5:1, more PDMAEMA chains surrounded at the POSS core put forward the wormlike micelle display a large L/D value under the powerful repulsion force (Figure 8E). As for supramolecules POSS-Azo8@[β-CD-PDMAEMA]68, due to the host polymers complexed on the POSS core achieved a considerable level, some defective supramolecular structure which did not reach the required host-guest ratio inevitably coexisted. Based on the above discussion, although most of POSS-Azo8@[β-CD-PDMAEMA]67 could bear their stability in water, they still adopted a liberal stacking way to help those defective supramolecular structures to enclose hydrophobic segments, leading to the branched aggregates occur (Figure 8F, G). As for the octa-telechelic POSS-Azo8@[β-CD-PDMAEMA]8, the hydrophobic core was completely protected by


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the circumjacent eight host molecules, the supramolecule itself could be seen as a unimolecular micelle. Theoretically, the unimolecular micelle was able to stable in water individually. However, there inevitably existed those supramolecules as POSS-Azo8@[β-CD-PDMAEMA]17 whose host-guest ratios did not reach 8:1. This fact made the final self-assembled morphology may not present as the ideal unimolecular micelle. In addition, when the host-guest ratio reached 8:1, the steric hindrance reached its maximum. The rearrangement during the self-assembly was most possibly conducted with the whole supra-amphiphile as unit. Specially, the supramolecules with the anticipated value of host-guest ratio to be 8:1 tended to be firstly stacked into a membrane with a thickness of ~100 nm including several molecular layers to adequately protect the hydrophobic POSS core away from water. The membrane afterwards curved and bended into a bowl-shaped morphology, as shown in Figure 8H. The self-assembly structure transition from branched to vesicle was a milestone, which further confirmed that the small change in the molecular architecture can cause a big transition in the self-assembly structure. When the host-guest ratio was increased to 12:1, we observed the formation of nano-sized micelles (~15 nm) as shown in Figure S1. The excess stoichiometry of host-guest complex led to the occurrence of ideal unimolecular micelle. This result confirmed our assumption on the self-assembly mechanism as shown in Figure 8.


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Figure 8. Schematic illustration on morphological evolution of self-assembled from supramolecular POSS-Azo8@[β-CD-PDMAEMA]18.

The experimental results and above discussion revealed an interesting and meaningful phenomenon that the evolution of self-assembled morphologies of supramolecular POSS-Azo8@[β-CD-PDMAEMA]18 was tuned dominantly by their molecular architectures. To verify the hypothesis, a dissipative particle dynamics (DPD) simulation was performed to explore the self-assembly process. The theoretical basis of DPD simulations was discussed in supporting information. Figure 9 displayed the snapshots of self-assemblies through the DPD simulation. The results confirmed the hypothesis of the morphological evolution. The initially POSS-Azo8@[β-CD-PDMAEMA]1 aggregated to


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spherical micelles with the POSS-azo8 as the core and β-CD-PDMAEMA as the coronal (Figure 9A). The aggregates gradually became larger to make sure there were enough PDMAEMA chains surrounded the POSS core (Figure 9B-C). The large aggregates changed into worm-like shape and then to branched shape shown in Figure 9D to 9G. Finally, an incomplete vesicle formed from POSS-Azo8@[β-CD-PDMAEMA]8 and Figure 9H showed the hollow structure of the vesicle.

Figure 9. DPD simulations on the self-assembly of supramolecular POSS-Azo8@[β-CD-PDMAEMA]18 with different host-guest ratios: (A)1:1, (B) 2:1, (C)


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3:1, (D) 4:1, (E) 5:1, (F) 6:1, (G) 7:1, and (H) 8:1.

CONCLUSION

In summary, we synthesized a multi-arm host polymer, β-CD-PDMAEMA, and an octa-telechelic guest molecule, POSS-Azo8. Based on the host-guest coupling between β-CD and azobenzene groups, eight supramolecular amphiphiles, POSS-Azo8@[β-CD-PDMAEMA]18 with different host-guest ratios could be easily obtained. With the host-guest ratios increasing, the supramolecular amphiphiles further self-assembled into diverse morphologies evolving from spherical micelles, wormlike micelles, branched aggregates, to bowl-shaped vesicles. The self-assembly of POSS-Azo8@[β-CD-PDMAEMA]18 was regulated dominantly by their molecular architectures. The computer simulation results verified and deepened our understanding of this self-assembly process and mechanism. We believe that this study will cast a new light on the self-assembly of supramolecular amphiphiles with more complex topological structures.

ASSOCIATED CONTENT

Supporting Information. The theoretical basis of DPD simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge MOST (2014CB932200 and 2014BAI11B04), the Young Thousand Talents Program and NSFC (21504096, 51573195, 21174147 and 21474115) for financial support.

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for Table of Contents use only

Morphological evolution of self-assembled structures from the “rigid and flexible” supra-amphiphiles POSS-Azo8@[β-CD-PDMAEMA]18.


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