Supramolecular Networks of Hyperbranched Poly(ether amine) (hPEA

Apr 6, 2015 - School of Chemistry & Chemical Engineering, State Key Laboratory for Metal ... The hPEA-NG/CS supramolecular networks were physically ...
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Supramolecular Networks of Hyperbranched Poly(ether amine) (hPEA) Nanogel/Chitosan (CS) for the Selective Adsorption and Separation of Guest Molecules Jin Li, Zhilong Su, Hongjie Xu, Xiaodong Ma, Jie Yin, and Xuesong Jiang* School of Chemistry & Chemical Engineering, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China S Supporting Information *

ABSTRACT: Supramolecular networks with selective adsorption for guest molecules were formed via dynamic hydrogenbonding interactions between the carboxyl group-containing hyperbranched poly(ether amine) nanogels (hPEA-NG) and chitosan (CS). The hPEA-NG/CS supramolecular networks were physically cross-linked and were composed of hPEA-NGs dispersed within a crystallized CS matrix. An increasing CS content enhanced the mechanical properties of the hPEA-NG/ CS supramolecular networks. The transparent hPEA-NG supramolecular networks did not disassemble in aqueous solutions with different pH values and organic solvents and were swollen with high weight fractions of water (WH2O = 0.95) in less than 30 s. The adsorptive behavior of seven fluorescein dyes and five azobenzene (azo) dyes onto the hPEA-NG/CS supramolecular networks was investigated. These hPEA-NG/CS supramolecular networks showed rapid adsorption for Rose Bengal (RB), Erythrosin B (ETB), Eosin B (EB), 4′,5′dibromofluorescein (DBF), Ponceau S (PS) and Evans blue (EVB) dyes with high adsorption capacities (Qeq). The networks showed very low adsorption for calcein (Cal), fluorescein (FR), 4,5,6,7-tetrachlorofluorescein (TCF), and Bismarck brown (BBY) dyes. The adsorption process was found to follow pseudo-second-order kinetics. The different adsorptive behaviors for different dyes indicated the selective adsorption of the hPEA-NG/CS supramolecular networks for the azo and fluorescein dyes despite similar backbone structures and charge states. On the basis of the unique selective adsorption, the separation of mixtures of fluorescein dyes, such as RB/Cal, RB/FR, and PS/MR, was achieved using the hPEA-NG/CS supramolecular network as an adsorbent.

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INTRODUCTION Supramolecular polymer networks are cross-linked threedimensional assemblies with noncovalent and intermolecular bonds.1−4 They have gained considerable attention because of their intrinsic self-healing,5,6 shape memory,6−8 and stimuliresponse properties6 and potential applications in controlled drug delivery,9,10 and smart materials and water treatment.11,12 The supramolecular polymer networks are typically cross-linked by transient physical interactions, including hydrogen bonding,13−20 halogen bonding,14,21 ionic interactions,3,22 host− guest interactions,23,24 π−π stacking,13,25−28 and transition metal complexation.29−31 Because of the directional property and versatility, the supramolecular interaction via hydrogen bonding has been intensively studied in the construction of supramolecular polymer networks. Lehn and co-workers generated mesoscopic molecules by the assembly of bifunctional heterocyclic chiral and achiral organic compounds (as the backbone) and uracil derivatives (as side-chains) through hydrogen bonds.32 Using triuret blocks, controlled conformation and temperature-dependent hydrogen bonding, Ni and coworkers prepared supramolecular thermo-reversible polyureaurethane networks.33 In these studies, the building blocks were © XXXX American Chemical Society

usually polymers or oligomers. However, nanogels (NGs) as building blocks to fabricate supramolecular networks through dynamic hydrogen bonding have rarely been investigated. Recently, we found that nano- to macro-scale hyperbranched poly(ether amine) (hPEA) hydrogels could selectively adsorb hydrophilic dyes and had a high absorption capacity.34,35 The investigation revealed that the hydrophobic−hydrophilic interaction between hPEA-based hydrogels and hydrophilic dyes may lead to the selective adsorption property. This interaction mechanism provided an important alternative in the separation and removal of dyes in water treatment. As is wellknown, dyes are a type of organic compound used in approximately 10 000 available products. They have complex aromatic molecular structures that make them stable and difficult to biodegrade, especially synthetic dyes. The extensive uses of dyes often causes serious pollution, and for some dyes, a concentration of less than 1 ppm can color large water volumes, which may potentially be toxic or mutagenic and carcinogenic Received: December 28, 2014 Revised: March 22, 2015

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Figure 1. Preparation and characterization of hyperbranched poly(ether amine) nanogel (hPEA-NG): (a) Chemical structure of SA-hPEA-EC and strategy for fabrication of hPEA-NG; (b) Size distribution of hPEA-NG in aqueous and THF obtained by DLS at room temperature (c = 0.5 mg/ mL). (c) TEM image of hPEA-NG in aqueous solution (c = 0.5 mg/mL).

for biological diversity.36−39 As one of the most efficient approaches, selective adsorption by polymer materials provides a cost-effective and one-pot approach to simultaneously removing and separating dyes from water.40−46 In this text, we developed a novel supramolecular network of hyperbranched poly(ether amine) nanogel and chitosan (hPEA-NG/CS) for the selective adsorption and separation of dyes in water. Hyperbranched poly(ether amine) nanogel (hPEA-NG) with carboxyl groups was chosen as one of the building blocks because of its unique selective adsorption to dyes and large surfaces. As a natural polymer, CS has good comprehensive properties and a large number of amino groups on its polymer chains. It was used as another building block to construct the supramolecular network via hydrogen bonding between the carboxyl and amino groups. The resulting supramolecular network, hPEA-NG/CS, exhibited fast swelling in water. Our investigation on the interaction between hPEANG/CS and various dyes indicated that the supramolecular network could selectively adsorb dyes and could be used in the selective removal of dyes from water. To the best of our knowledge, this is one of the limited examples of a supramolecular polymer network based on nanogels for the separation and removal of dyes in water.

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solution with a total solid content of 30 mg/mL. Specified amounts of hPEA-NG were then added to the aqueous CS solutions to form homogeneous solutions at room temperature. The prepared hPEANG/CS solutions were cast onto plastic Petri dishes (d = 70 mm) and dried at 60 °C for 24 h in an oven to form supramolecular network films. After additional drying in a vacuum at 60 °C for approximately 3 days to remove the acetic acid, hPEA-NG/CS supramolecular network films were obtained.

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RESULTS AND DISCUSSION Preparation and Characterization of hPEA-NG. Hyperbranched poly(ether amine) (hPEA) possesses a large number of secondary amino groups in the periphery and a large number of hydroxyl groups on the backbone. Through the sequential functionalization of hPEA, the coumarin and carboxyl groups can be introduced into the periphery and backbone of hPEA, respectively, to obtain amphiphilic hyperbranched poly(ether amine) (SA-hPEA-EC) (Figure 1a). The detailed synthesis and characterization of SA-hPEA-EC are shown in the Supporting Information (Scheme S1 and Figures S1 and S2). Because of its amphiphilicity, SA-hPEA-EC can self-assemble into micelles when dispersed in water. These micelles are composed of hydrophilic poly(ethylene oxide) (PEO) short chains and carboxyl groups as shells and hydrophobic poly(propylene oxide) (PPO) short chains and coumarin groups as cores. Upon UV irradiation at 365 nm, the photodimerization of the coumarin moieties leads to the cross-linking of the core, resulting in the formation of the nanogel (hPEA-NG) (Figure 1a). The photodimerization of coumarin moieties was confirmed and traced by UV−vis spectra (Figure S3). The size distribution and morphology of the resulting hPEA-NG were revealed by DLS and TEM analyses, respectively. As shown in Figure 1b, the size of the hPEA-NG in water was approximately 80 nm in diameter with a low polydispersity index (PDI = 0.131). The hPEA-NG did not disassemble even in organic solvents, such as tetrahydrofuran (THF), indicating the stability of the cross-linked core and the hPEA-NG. The

EXPERIMENTAL SECTION

Preparation of Hyperbranched Poly(ether amine) Nanogel (hPEA-NG). Amphiphilic hyperbranched poly(ether amine) containing coumarin and carboxyl groups (SA-hPEA-EC) was synthesized according to previous reports34,35,47 (Figure 1a and Scheme S1 (Supporting Information). SA-hPEA-EC was directly dispersed into an aqueous solution to form a semitransparent solution at a concentration of 10 mg/mL. The solution was then irradiated by 365 nm UV light for 3 h. The coumarin units in SA-hPEA211-EC were photodimerized to form an hPEA-based nanogel (hPEA-NG) with an average diameter of approximately 80 nm in an aqueous solution. Preparation of Supramolecular Network (hPEA-NG/CS). Chitosan was dispersed into an acetic acid aqueous solution (1%) and stirred for 24 h at room temperature to form a transparent B

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Macromolecules Scheme 1. Whole Strategy to Fabrication hPEA-NG/CS Supramolecular Network via Hydrogen Bondinga

a

Inset image is digital-photograph of the obtained hPEA-NG/CS supramolecular network.

shifts to 3100 cm−1, indicating the formation of H-bonding with the COOH of the hPEA-NG. At increased temperature, the peak assigned to N−H (3310 cm−1) weakened and shifted to a higher wavenumber, which is typical temperature-sensitive behavior of H-bonds. The morphology of the hPEA-NG/CS supramolecular network was investigated with SEM and WAXD. The smooth surface and homogeneity of the hPEA-NG/CS-1/1 supramolecular network was revealed by SEM images of the film surface and cross-section (Figure S4). The diffraction peaks at 2θ = 8.3, 11.3, 18.0, and 22.9° attributed to crystallized CS could be observed in the hPEA-NG/CS-1/1 supramolecular network (Figure 3 and Figure S5). These peaks became less

size of the hPEA-NG in water determined by DLS was slightly larger than that in THF, which may be ascribed to the zwitterionic nature of the hPEA-NG. The large number of amino and carboxyl groups in the hydrophilic shell of the hPEA-NG resulted in its stronger swelling in water than in THF. Figure 1c shows a representative TEM image of the hPEA-NG in an aqueous solution having almost uniform particles with sizes of approximately 90 nm. The larger size determined by TEM than by DLS analysis may be due to the more flexible and flattened hPEA-NG in the TEM assays. Preparation and Characterization of Supramolecular Network (hPEA-NG/CS). The preparation of the supramolecular network comprising the hPEA-NG and chitosan (hPEA-NG/CS) is illustrated in Scheme 1. Chitosan was chosen as a building component due to its comprehensive performance and large numbers of amino groups, which provides the ability to form a cross-linked network with the hPEA-NG via H-bonding interactions between the N−H of CS and the COOH of the hPEA-NG. First, the hPEA-NG and CS were dispersed in an acetic acid aqueous solution (1%), and the mixed solution was cast onto a plastic Petri dish. After drying, the obtained hPEA-NG/CS films were transparent and flexible (see inset pictures), which suggested a good compatibility between the hPEA-NG and CS. The good compatibility may also be ascribed to the strong interaction via H-bonding between the hPEA-NG and CS, which was confirmed by the temperature-dependent FT-IR spectra (Figure 2). Compared with the free N−H stretching at 3440 cm−1, the N−H stretching vibration of CS in the supramolecular network red-

Figure 3. WAXD patterns of hPEA-NG/CS-1/1 film in dry and swollen state, respectively.

obvious with increasing hPEA-NG content, indicating that the strong interaction between the hPEA-NG and CS weakened with further CS crystallization (Figure S5). DSC curves revealed that the strong interaction between the hPEA-NG and CS increased the glass transition temperature (Tg) of the hPEA-NG phase from −8.5 °C to −2.3 °C as the CS content increased (Figure S6). On the basis of these results, the morphology of the hPEA-NG/CS was proposed in Scheme 1. The hPEA-NG was uniformly dispersed in the crystallized CS matrix because of the strong interaction between the hPEA-NG and CS via H-bonding. Because the diameter of the hPEA-NG

Figure 2. FT-IR of hPEA-NG/CS-1/1 film at different temperatures. C

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presence of both H-bonding and ionic interaction hPEA-NG and CS. Because of the large amount carboxyl groups in one hPEA-NG and protect of hydrophobic domain, there is Hbonding between hPEA-NG and CS in the swelling statement although part of H-bonding is destroyed by water molecules. At the same time, the protonated carboxyl and amino groups can lead to ionic interaction between hPEA-NG and CS, which also prevents the destruction of the supramolecular network.22 The dynamic H-bonding was expected to provide the ability of self-healing to the hPEA-NG/CS. The ability to repair microcracks was of great interest because it could prevent the cracks from developing into macro-scale failures. Generally, it is difficult for a covalently cross-linked network to self-repair without the aid of healing agents.48,49 As shown in Figure 5a,

was much less than the wavelength of visible light, the hPEANG/CS film was transparent (Scheme 1). The swelling kinetics of the hPEA-NG/CS films were studied in water. After immersion in water for 30 s, the swelling behavior of all hPEA-NG/CS films reached equilibrium. The films adsorbed more than 20 times their own weights in water (Figure 4). Compared with the water adsorption of our

Figure 5. (a) Photographs of hPEA-NG/CS-1/1 film with carve before and after self-healing; (b) Stress−strain curves of three hPEANG/CS supramolecular network films.

Figure 4. (a) Photographs of hPEA-NG/CS-1/1 supramolecular network film in the dry and swollen state, respectively. (b) Photographs of hPEA-NG/CS-1/1 film in different solvents, and the left is reference that CS film is dissolved in 1% HAc solution (1% HAc, pH = 3.60; 1%NaOH, pH = 11.65; 1% HCl, pH = 2.30). (c) Swelling kinetics of three hPEA-NG/CS films in pure water solution at 25 °C.

the hPEA-NG/CS-1/1 film was sliced with a razor to produce a crack of approximately 35 μm width. After equilibration at 80 °C for 2 h and subsequent cooling to room temperature, the film was examined by an optical microscope. The crack was much less obvious, indicating the self-healing ability of the films. Tensile strength tests were performed to investigate the mechanical properties of the resulting hPEA-NG/CS films. As shown in Figure 5b, both Young’s modulus and elongation at break increased as the CS content increased from hPEA-NG/ CS-3/1 to hPEA-NG/CS-1/1. This indicated that the introduction of CS could enhance the mechanical properties, which may be due to the presence of the continuous crystallized CS phase in the higher CS content hPEA-NG/CS. The Selective Adsorption of Dyes. We then studied the adsorption behavior of the obtained hPEA-NG/CS supramolecular networks for two families of hydrophilic dyes, whose structures are illustrated in Scheme 2. Five azo dyes were chosen because they are toxic and widely used in the dying industry. Seven fluorescein dyes were chosen because they share with the same backbone and charge state in aqueous solution and are therefore helpful in elucidating the adsorption mechanism. The adsorption experiments were carried out with the initial concentrations of 100 μM and 1.0 mg/mL for the

previously studied chemical cross-linked networks based on hPEA, the rapid adsorption of the hPEA-NG/CS films may be due to the dynamic physical network. Water was adsorbed into both the hPEA-NG and CS domains. After the water adsorption, the crystallized CS domain was destroyed (Figure 3). Only two broad peaks were observed at 2θ of approximately 10 o and 20 o that corresponded to the amorphous halos found in the WAXD pattern of the swelling hPEA-NG/CS-1/1 supramolecular network (Figure 3). Additionally, there was no measurable weight loss before and after swelling in pure water, which suggested strong interactions between the hPEA-NG and CS. The hPEA-NG/CS was very stable and did not disassemble after immersed it in 1% HAc (pH = 3.6), 1%NaOH (pH = 11.65) aqueous solutions or organic solvents for 48h (Figure 4b). In contrast, pure CS films completely dissolved in 1% HAc aqueous solutions after immersed it for 10h. The hPEA-NG/ CS-1/1 supramolecular network only swelled in aqueous solutions with different pH, which might be ascribed to the D

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Macromolecules Scheme 2. Structure and Abbreviation of Seven Fluorescein and Five AZO Dyes

dyes and the hPEA-NG/CS films, respectively. Taking PS and Cal as examples, it was observed that most of the PS dye was adsorbed by the hPEA-NG/CS films (Figure 6a, inside the graph); however, Cal dye still remained in the solution (Figure 6b, inside picture). UV−vis spectra were used to trace the dye solutions before and after adsorption by the hPEA-NG/CS-1/1 films for 12 h (Figure 6, parts a and b). Nearly 95% of the PS dye was adsorbed, but less than 5% of the Cal dye was absorbed. As an important parameter for adsorbents in practical applications, the saturated adsorption capacity (Qeq) can be determined by the UV−vis spectra. As shown in Figure 7, the hPEA-NG/CS supramolecular networks possessed high Qeq for a few of the fluorescein and azo dyes. For the same dye, Qeq did not significantly change with the CS content in the hPEA-NG/ CS films, which suggested that the CS matrix had no obvious effect on the dye adsorptive properties of the hPEA-NG/CS supramolecular network. For the same backbone and charge state, the hPEA-NG/CS films exhibited different adsorption behavior for different fluorescein dyes, which indicated that the adsorption of fluorescein dyes by the hPEA-NG/CS films was independent of electrostatic interactions. The hPEA-NG/CS films exhibited high Qeq for the RB, ETB, EB and DBF dyes, whose structures have hydrophobic substituents on conjoint hexatomic rings. This observation suggested that the hydrophobicity of the adsorbents may be helpful in enabling strong adsorptions by the hPEA-NG/CS films, as we had concluded in our previous report. The large difference in Qeq suggested that the hPEA-NG/CS supramolecular networks exhibited a selective interaction for the hydrophilic dyes. The adsorption kinetics of 12 dyes by the hPEA-NG/CS-1/1 film was studied to investigate the interaction mechanism of the

supramolecular network (Figure 8). For the dyes with high Qeq, such as EB, RB, ETB, DBF, MO, and EVB, the adsorption capacity initially increased rapidly and continued to increase at a relatively slow rate. In contrast, the adsorption rate and capacity of the Cal, TCF, FR, BBY, and MR dyes were much lower, regardless of their charge states, which indicated that the electrostatic interaction between the dyes and the hybrid hydrogels had no obvious effect on the adsorption kinetics. A pseudo-second-order equation was used to analyze the adsorption kinetics to investigate the adsorption mechanism of dyes onto the hPEA-NG/CS-1/1 film (Figure S7). The pseudo-second-order adsorption rate constant k, the calculated and experimental adsorption capacities Qeq,cal and Qeq,exp, and the correlation coefficient R2 are summarized in Table S1. On the basis of the very high correlation coefficient (nearly 1) and the approximately similar values of Qeq,cal and Qeq,exp, the pseudo-second-order model well predicted the adsorption of dyes by hPEA-NG/CS. The different k and Qeq,exp values also indicated that the hPEA-NG/CS-1/1 film possessed unique selective adsorption properties for different dyes. To understand how the supramolecular network and different hydrophilic dyes interacted and to evaluate the characteristics of the adsorption system, the MR, PS, RB, and TCF dyes were used in an isotherm study because of the large differences in their k and Qeq values (Figure S8 and Table S2). On the basis of the equilibrium adsorption data, it was found that the Langmuir model was suitable for describing the adsorption equilibrium by the supramolecular network of MR and TCF, while the Freundlich model was appropriate for that of PS and RB. For the same backbone structures and charge states, but different substituents of the RB and TCF dyes, the different isotherm adsorption models indicated that the E

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Figure 7. Saturated adsorption capacities of hPEA-NG/CS-1/1, 2/1, 3/1 for seven fluorescein dyes (a) and five azo dyes (b) at 25 °C.

Figure 6. UV−vis spectra of PS (a) and Cal (b) before and after adsorption by hPEA-NG/CS films for 12 h. Inset are photographs of solution of PS and Cal solution before and after adsorption.

hydrophobicity of the adsorbate may dominate the interaction between the hPEA-NG/CS supramolecular network and the dyes. Separation of Mixture of Dyes. Motivated by its unique selective adsorption, physical cross-linking and high stability in different solvents, we studied practical applications of the hPEA-NG/CS supramolecular network. The hPEA-NG/CS supramolecular network may have the potential to simultaneously remove and separate dye mixtures in water because of its selective adsorption for the dyes. Dyes with high Qeq and k can be adsorbed, while dyes with low Qeq and k are expected to be left in solution. By using the hPEA NG/CS-1/1 film as an adsorbent, two fluorescein mixtures of FR-RB and TCF-RB were separated. The initial concentration ratios of FR to RB ([FR]0/[RB]0) and TCF to RB ([TCF]0/[RB]0) were 1.0. After the addition of hPEA-NG/CS-1/1 to the mixed solutions of FR-RB and TCF-RB, the color of the hPEA-NG/CS-1/1 film turned red (indicative of RB adsorption) with time (Figures 9a and S9). UV−vis spectra were used to trace the separation process to measure the dye concentrations in solution. The concentration of FR and TCF remained approximately constant, while the concentration of RB decreased, indicating the successful separation of RB from the mixtures of dyes. The same method was also used to separate an azo mixture of MR-PS dyes. The initial concentration ratio of MR and PS ([MR]0/[PS]0) was 1.0. After immersing the hPEA-NG/CS-1/ 1 film into the solution of the MR-PS mixture, the color of the solution changed from orange to light pink, indicative of MR,

Figure 8. Adsorption capacity Qt versus time for the adsorption of fluorescein dyes (a) and azo dyes (b). F

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Figure 9. One-pot separation of dye mixture of TCF-RB and MR-PS by hPEA-NG/CS-1/1 film in aqueous solution: (a) Dye concentration of of [TCF] and [RB] and dye concentration ratio in solution deterimed by UV−vis spectra vs time, inset picture is photographs of TCF-RB before and after separation for 30 h; (b) Dye concentration of of [MR] and [PS] and dye concentration ratio in solution deterimed by UV−vis spectra vs time, inset picture is photographs of MR-PS before and after separation for 30 h.

while the hPEA-NG/CS-1/1 film turned red (indicative of PS adsorption). As shown in Figure 9b, the concentration of MR remained constant, but the concentration of PS decreased from 21 μM to 6 μM, which indicated that PS was selectively removed from the MR-PS mixture. Finally, the important regenerative capacity of the hPEA-NG/CS films was demonstrated. Using PS adsorption as an example, the hPEA-NG/CS1/1 film was immersed in an aqueous solution of PS until adsorption equilibrium was reached. The PS-adsorbed film was then put into a 0.1 mol/L NaOH aqueous solution and equilibrated for 1 h. PS was found to be removed from the hPEA-NG/CS films and a pure, PS-free hPEA-NG/CS-1/1 film was obtained (Figure S10). This process of regeneration could be repeated more than five times for similar Qeq at more than 100 μM/g. The shape of the hPEA-NG/CS-1/1 film remained nearly unchanged after five regeneration cycles, which indicated the high stability of the hPEA-NG/CS supramolecular network.

Hydrophobic interactions (rather than electrostatic interactions) dominated the interaction between the hPEA-NG/CS supramolecular networks and the dyes. On the basis of the unique selective adsorption properties, the separations of several mixtures, such as FR-RB, TCF-RB and MR-PS, were achieved using the hPEA-NG/CS supramolecular network as an adsorbent. This indicated the applicability of a one-pot process to remove and separate toxic components in water treatment. Additionally, the hPEA-NG/CS supramolecular networks were easily regenerated and retained their shapes during separation-regeneration tests. We propose that the design of polymer networks could benefit from the simple and cost-effective use of the dynamic, H-bonded, cross-linked supramolecular network of hPEA-NG/CS.

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ASSOCIATED CONTENT

S Supporting Information *

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Experimental details, preparation and characterization of hPEANG, structure and performance of hPEA-NG/CS supramolecular network, selective adsorption and separation of dyes, and adsorption isotherm. This material is available free of charge via the Internet at http://pubs.acs.org.

CONCLUSIONS We described the preparation of supramolecular networks comprising carboxy-containing hyperbranched poly(ether amine) nanogel (hPEA-NG) and chitosan (CS) and demonstrated that these supramolecular networks could be used in the separation of dyes due to their selective adsorption of the dyes. The hPEA-NG dispersed in a crystallized CS matrix to form physically cross-linked networks via hydrogen bonding between the carboxyl and amino groups. The introduction of CS improved the mechanical properties of the hPEA-NG/CS supramolecular networks. The network was transparent and did not disassemble in various aqueous solutions with different pH values and organic solvents. The hPEA-NG/CS supramolecular networks could be swollen with a high weight fraction of water (WH2O = 0.95) in less than 30 s. The detailed investigation on the adsorptive behavior of the hPEA-NG/CS supramolecular networks for two families of dyes (seven fluorescein and five azo dyes) revealed that the hPEA-NG/CS supramolecular networks selectively adsorbed the dyes, especially fluorescein dyes having the same backbone structure and charge states.

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AUTHOR INFORMATION

Corresponding Author

*(X.J.) Telephone: +86-21-54743268. Fax: +86-21-54747445. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Basic Research Program (2013CB834506), National Nature Science Foundation of China (21174085, 21274088, 51373098), and Education Commission of Shanghai Municipal Government (12ZZ020) for their financial support. X.J. is supported by the NCET-123050 Project. G

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DOI: 10.1021/ma502607p Macromolecules XXXX, XXX, XXX−XXX