Near-Infrared Responsive Liquid Crystalline Elastomers Containing

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Near-Infrared Responsive Liquid Crystalline Elastomers Containing Photothermal Conjugated Polymers Wei Liu, Ling-Xiang Guo, Bao-Ping Lin, Xue-Qin Zhang, Ying Sun, and Hong Yang* School of Chemistry and Chemical Engineering, Jiangsu Province Hi-Tech Key Laboratory for Bio-medical Research, Jiangsu Optoelectronic Functional Materials and Engineering Laboratory, Southeast University, Nanjing, 211189, China S Supporting Information *

ABSTRACT: In this work, we report the first example of uniaxial aligned conjugated polymer (CP)/liquid crystalline elastomer (LCE) composite material by doping polyaniline nanoparticles into a classical monodomain polysiloxane-based LCE matrix. A series of comparative experiments are performed to investigate the photoresponsive properties of these polyaniline/LCE samples in correspondence to the two different conjugation forms (emeralidine salt (ES) or emeralidine base (EB)) and the varied doping concentrations (0.5 or 1.0 wt %) of the incorporated polyaniline nanoparticles. Taking advantage of the excellent photothermal conversion efficiency, these novel polyaniline/LCE composite materials can lift up ca. 200 times their own weights under the NIR illumination driving force, and such a photostimulated muscle-like actuation behavior is fully reversible after NIR light is removed.

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INTRODUCTION Conjugated polymers (CPs), a kind of fascinating semiconductor materials, typically have a narrow optical bandgap and absorb in near-infrared (NIR) range. Because of the unique photonic and optoelectronic properties, CPs have been widely applied in organic photovoltaics (OPV), photodetectors (PDs), lightemitting diodes (LEDs) and field-effect transistors (FETs).1,2 Besides optoelectronic devices, CPs can also meet requirements of chemical sensors, fluorescence imaging, drug delivery and other biological applications.3,4 In particular, photothermal therapy (PPT) of tumors employing CPs that converts photon energy to heat has attracted our attention.5−7 In this work, we aim to take advantage of CPs’ excellent light-harvesting property and high photothermal heating efficiency, through incorporating CPs into liquid crystalline elastomer (LCE) matrix, to prepare a NIR-stimulated muscle-like actuating CP/LCE composite material. LCEs are anisotropic, elastic, cross-linked polymeric materials, possessing a unique shape memory function that can perform reversible shape transformations under the external stimulus (such as heat, light, electric, magnetic, pH, etc.).8−13 Recently, NIR-stimulus-responsive liquid crystal (LC) materials,14−19 in particular NIR-responsive LCEs, have become the hot spot of LC community due to the high tissue penetration ability of NIR which endows these materials prospective applications in the biotechnology field. NIR-responsive LCEs can be prepared by either doping up-conversion materials into specific LCE materials bearing azobenzene groups,20,21 or embedding into more diverse LCE matrixes inorganic or organic thermal conductive fillers which convert photons into heat energy used to trigger the liquid crystal (LC) to isotropic phase transition, © XXXX American Chemical Society

and further to force the macroscopic material to actuate under NIR irradiation. Inorganic thermal conductive fillers, such as carbon nanotubes22−29 and gold nanoparticles,30−33 have been widely applied in photoresponsive LCE composites because of their high photothermal conversion efficiency, however the poor dispersion28 of inorganic nanoparticles in organic LCE matrixes inevitably weaken materials’ mechanic properties and even the photoactuation rates.29 To solve this solubility drawback, scientists used instead organic small molecule dyes,34−36 although their NIR-absorbing ranges were very narrow and the photothermal conversion efficiencies were relatively lower. Motivated by the above dilemma, we propose an alternative compromise solution: preparing the first example of CP/LCE composite material. The mesomorphic and photoresponsive properties of this novel composite are studied and discussed in this manuscript.

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EXPERIMENTAL SECTION

General Considerations. The LC monomer MBB and cross-linker 11UB were synthesized following literature reports.37−40 All the instrumentation descriptions, detailed synthetic protocols, and NMR spectra are listed in the Supporting Information. Synthesis of Polyaniline. Aniline (18.62 g, 0.2 mol) was added to a hydrochloric acid (HCl) aqueous solution (1M, 300 mL). To the above solution, a HCl aqueous solution (1M, 200 mL) containing ammonium persulfate (11.41 g, 0.05 mol) as the oxidant was added dropwise at 4 °C over a period of 0.5 h. After the reaction mixture was stirred at 4 °C for Received: March 29, 2016 Revised: May 14, 2016

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Macromolecules another 5.5 h, the resulting dark-green polyaniline precipitate ES (6.60 g) was recovered by filtration. Polyaniline ES (3.30 g) was added into a sodium hydroxide aqueous solution (1M, 250 mL), which was then stirred at room temperature for 2 h. The deprotonated polyaniline EB was collected by filtration and redispersed in acetone (250 mL). After a second filtration and dried under vacuum, a fine EB purple powder (2.20 g) was obtained. Typical Preparation Protocol of a Polyaniline/LCE Composite Film. As shown in Figure 1, a mixture containing PMHS (36 mg, 0.6

extent between its emeralidine base (EB) form and the emeralidine salt (ES) form, which provide us not only two good photothermal conversion CP reagents but also a pair of experimental objects for comparative study. Following literature procedures,47,48 we first synthesized polyaniline as a dark-green precipitate (ES) by chemical oxidative polymerization of anilinium salts, and then deprotonated ES by using sodium hydroxide to obtain a purple powder (EB). Examined by UV−vis spectroscopy, two polyaniline samples both exhibited broad optical absorptions in vis−NIR region. As shown in Figure 2, ES

Figure 2. UV−vis absorption spectra of polyaniline ES (concentration = 9.85 × 10−2 mg/mL, dissolved in THF) and EB (concentration = 7.48 × 10−2 mg/mL, dissolved in THF).

showed an intense NIR absorption band at ca. 875 nm (conc. = 9.85 × 10−2 mg/mL in THF) while EB absorbed mainly in orange light region (λabs,max = 574 nm, concentration = 7.48 × 10−2 mg/mL in THF), which were consistent with previous reports.41,42,49,50 Regarding to the composite matrix, a classical polysiloxanebased LCE system was chosen because of its excellent mechanical property, relatively low glass phase (Tg) and LC-to-isotropic phase transition temperatures51−55 which made polysiloxane LCEs ideal study objects for photothermal heating. The detailed polyaniline/LCE composite film preparation procedure is described in the Experimental Section. As shown in Figure 1A, poly(methylhydrosiloxane) (PMHS) was mixed with LC monomer MBB, cross-linker 11UB, platinum catalyst (Pt(COD)Cl2) and organic photothermal CP reagent, polyaniline (EB or ES) with two different loading levels (0.5 and 1.0 wt % respectively). The classical two-step cross-linking process invented by Finkelmann56,57 was applied to fabricate the polyaniline-incorporated polysiloxane LCE composites. In the first cross-linking stage as illustrated in Figure 1B, the LC mixture dissolved in toluene was cast into a polytetrafluoroethylene (PTFE) rectangular mold and then heated to 60 °C to initiate a hydrosilylation reaction for mesogen grafting and elastomer cross-linking. A careful control of the heating time (2 h) could provide a partially cross-linked multidomain LCE film, which was then dried at the room temperature and uniaxially stretched with a load in an oven at 60 °C for another 48 h, to fulfill the second fully cross-linking and prepare the desired monodomain polyaniline/LCE composite film. The stretch-induced molecular alignment effect and the phasevariation-induced actuation mechanism of this LCE composite film are schematically illustrated in Figure 3. After the first crosslinking process, the partially cross-linked LCE film generated plenty of micrometer-sized small domains which although

Figure 1. (A) Molecular structures of LC monomer MBB, cross-linker 11UB, PMHS, Pt(COD)Cl2, and polyaniline (ES or EB). (B) Schematic illustration of CP/LCE composite film preparation protocol. mmol Si−H groups), MBB (150 mg, 0.5 mmol), 11UB (21 mg, 0.05 mmol) and polyaniline ES (1.1 mg) was dissolved in 2 mL of toluene. After an ultrasonication process for about 2 min to ensure a homogeneous dispersion, 40 μL of preprepared Pt−catalyst solution (25 mg of dichloro(1,5-cyclooctadiene) platinum(II) dissolved in 2 mL of dichloromethane and then diluted with 20 mL of toluene) was added into the reaction mixture solution. The solution was cast into a polytetrafluoroethylene (PTFE) rectangular mold with dimensions of 2 cm ×2 cm ×2 cm. The mold was first ultrasonicated for 5 min and then heated in an oven at 60 °C for 120 min. After this pre-cross-linking step, the mold was cooled to room temperature and the composite film was released. After naturally evaporating off the inside toluene solvent, the resulting film was uniaxially stretched with a load and meanwhile heated in an oven at 60 °C for another 48 h to complete the cross-linking reaction to provide the desired ES-0.5%/LCE composite film. Following the above procedure, we also prepared ES-1%/LCE, EB0.5%/LCE, EB-1%/LCE and pure LCE film samples, respectively. Herein, −x% represents the weight percentage of polyaniline doped in composite samples.

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RESULTS AND DISCUSSION Preparation and Physical Properties of Polyaniline/LCE Composite Films. Polyanline, as a well-known electroactive CP material, has been widely used in photothermal therapy,41−46 due to its excellent biocompatibility and broad optical absorption in vis-NIR region. Most interestingly, the maximum opticalabsorbance peak of polyaniline can be tuned by protonation B

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Figure 3. Schematic illustration of the stretch-induced molecular alignment effect and the phase-variation-induced actuation mechanism of this LCE composite film.

individually disciplined the molecular directors of the inside mesogens in a unidirectional way, could not unify the molecular arrangements of all the domains along one orientation, so that the mesogens on average were randomly oriented. Stretching the precross-linked LCE film uniaxially, could force all the mesogens to change the multidomain molecular packing into a monodomain alignment, which was eventually fixed by the following complete cross-linking process. When heat or light was applied to trigger the LC-to-isotropic phase transition, all the mesogens behaved in a random arrangement manner, which was similar to the multidomain molecular packing from a macroscopic view. Thus, the whole LCE film shrinked and recovered the original shape of precross-linked LCE film. For comparison purpose, we prepared five LCE films including ES-0.5%/LCE, ES-1%/LCE, EB-0.5%/LCE, EB-1%/LCE, and a pure LCE film sample without any CP dopant. Interestingly, ES-0.5%/LCE, EB-0.5%/LCE and pure LCE film could be easily stretched to achieve ca. 50% elongation of the original lengths, while ES-1%/LCE and EB-1%/LCE composite films with a doubled CP-doping concentration became more brittle and could bear relatively smaller extensional strains (ca. 25%). Polarized optical microscope (POM) was used to examine the parallel-alignment effect of these polyaniline/LCE composite films. As indicated in Figure 4A,B, the pure LCE sample showed the highest transmittance when the polarizer tilted at an angle of 45 deg to the film stretching direction, while rotation of the LCE film to parallel or perpendicular to the analyzer would minimize the transmittance. Doping LCE samples with polyaniline (either ES or EB) nanoparticles (Figure 4C−F), although apparently lowered the film birefringences, still preserved the phenomenon that light transmittance varied along with rotation angles, which demonstrated that the homogeneous alignments of polyaniline/ LCE composite films have been achieved in good quality. Scanning electron microscopy (SEM) technique was further applied to investigate the morphology of polyaniline nanoparticles and their dispersion in the LCE matrix. As shown in Figure 5A,B, typical fused granular structures of polyaniline nanoparticles were clearly observed, which always happened in acidic precipitation polymerization, according to literatures.45,49,58 SEM images of polyaniline/LCE composite films revealed moderate-dispersion of polyaniline nanoparticles in LCE matrixes, as presented in Figure 5C−F. Although the aggregation of nanoparticles was inevitable, polyaniline as an organic compound, compared with some inorganic nanoparticles,29,33 was obviously advantageous for dispersion in organic LCE systems. Thermal Properties of Polyaniline/LCE Composite Films. The thermal properties of polyaniline/LCE composite films were first investigated by differential scanning calorimetry

Figure 4. POM images of (A, B) pure LCE, (C, D) ES-0.5%/LCE, and (E, F) EB-0.5%/LCE composite films recorded at room temperature.

(DSC). As demonstrated in Figure 6, the DSC curves of ES0.5%/LCE, ES-1%/LCE, EB-0.5%/LCE, EB-1%/LCE, and pure LCE films all presented an enantiotropic nematic mesophase in a temperature range of −6−60 °C. Because of the very low doping ratios (0.5−1.0 wt %) of polyaniline, the critical LC-to-isotropic phase transition temperatures of four polyaniline/LCE composite films were almost the same as the one of pure LCE sample with only a small deviation of ca. 0−2 degrees. Most of monodomain LCE samples possess a reversible shrinking/expanding behavior stimulated by temperature which varies between LC phases and the isotropic phase, to change the mesogenic orders of the cross-linked LCE network so that a macroscopic shape transformation along the molecular director can be visualized.8,9,12,13,59−61 In order to study the thermalactuation properties of polyaniline/LCE composite films, we first cut the films along the stretching direction into ribbons (ES0.5%/LCE and EB-0.5%/LCE, ca. 2.85 cm in length and 0.50 cm in width; ES-1%/LCE and EB-1%/LCE, ca. 2.25 cm in length and 0.50 cm in width), placed the LCE film ribbons on a hot stage, and investigated the variations of ribbon lengths depending on the temperature. As demonstrated in Figure 7A-B, four polyaniline/LCE composite films and the referenced pure LCE film contracted dramatically when heated from 20 °C up to 100 °C with a constant heating rate of 10 oC/min, and fully expanded to their original lengths after cooling back to 20 °C with a constant cooling rate of −10 oC/min. Such reversible shrink/ expansion phenomena were observed repeatedly during multiple successive heating/cooling cycles. The detailed shape transformation behaviors of four polyaniline/LCE composite films and the referenced pure LCE film are analyzed by plotting the film one-dimensional thermal expansion (L/Liso) versus temperature curves as shown in Figure 8, where L is the length of the film along the stretching direction at any assigned temperature, and Liso is the minimum length of the C

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Figure 5. SEM images of (A) polyaniline ES, (B) polyaniline EB, (C) ES-0.5%/LCE composite, (D) EB-0.5%/LCE, (E) ES-1%/LCE composite, and (F) EB-1%/LCE composite samples.

power: 8W, Center wavelength: 808 ± 3 nm). First, we set up a series of intuitionistic comparison experiments as shown in Figure 9 and Videos S1−S4. Each of four polyaniline/LCE composite film ribbons accompanied by a pure LCE ribbon used as a contrast, were hung on a horizontal stick. A small binder clip was then attached at the bottom of every LCE film. The weight of such a binder clip was ca. 1.30 g which was roughly 21−29 times the weight of one LCE film ribbon (ES-0.5%/LCE, ca. 45.6 mg; EB-0.5%/LCE, ca. 46.2 mg; ES-1%/LCE, ca. 60.5 mg; EB-1%/ LCE, ca. 56.2 mg). Under NIR irradiation, pure LCE films remained stationary while all the other four polyaniline/LCE composite film ribbons performed a rapid shrinking and could each easily lift up the binder clip. After NIR light was removed, the four polyaniline/LCE composite ribbons quickly elongated to achieve a full recovery of their original shapes. Such reversible photoactuation phenomena were observed repeatedly during multiple successive NIR-light on/off cycles. In order to further investigate the photothermal conversion efficiencies of these polyaniline/LCE composite materials, a comparative heat generation experiment was undertaken by using a delicate thermal imager (FLUKE Ti90) equipment to monitor the surface temperature variations of four polyaniline/ LCE composite films and one pure LCE film in the same period

sample beyond its clearing point. As illustrated in Figure 8A, three 50% elongated LCE films (ES-0.5%/LCE, EB-0.5%/LCE and pure LCE) all presented an abrupt change of the L/Liso values jumping from 150% to 100% at the temperature range of ca. 52− 75 °C during the heating process, while the cooling recoveries of these three samples showed a temperature relaxation and the jump discontinuity of thermal expansion (L/Liso) happened at the temperature range of ca. 65−40 °C. These data match well with DSC results as described in Figure 6. In particular, two polyaniline/LCE composites containing 0.5 wt % polyanilines (ES or EB), compared with pure LCE sample, both required more thermal energy to achieve the same level of shrinkage during the heating process, since their L/Liso curves slightly shifted to the right about 2−3 °C of the black line which represents the pure LCE sample. The other two polyaniline/LCE composites with a doubled CP-doping concentration (1.0 wt %) although had weaker stretching capabilities, did show the exactly same reversible shrinking/expanding scenarios as investigated in Figure 8B. Photoresponsive Behavior of Polyaniline/LCE Composite Films. NIR-stimulated photoactuation behaviors of four polyaniline/LCE composite films and the referenced pure LCE film were investigated by using a NIR light source (Output D

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Figure 8. Shape deformation L(T)/Liso of (A) ES-0.5%/LCE, EB0.5%/LCE, and pure LCE films and (B) ES-1%/LCE and EB-1%/LCE composite films along the stretching direction during heating and cooling circles (heating rate =10 °C/min).

Figure 6. DSC curves of (A) ES-0.5%/LCE, EB-0.5%/LCE, and pure LCE films and (B) ES-1%/LCE, EB-1%/LCE, and pure LCE films during the first cooling scan and the second heating scan at a rate of 10 °C/min under nitrogen atmosphere.

Doping higher concentration polyaniline nanoparticles will dramatically improve both the heat generation capability and the photothermal conversion rate of the corresponding composite film. (3) Compared with its EB analogue, polyaniline ES dopant obviously has much more advantageous photothermal efficiency, which can be rationally explained by their optical absorption performances that ES and EB absorb mainly in NIR region and orange light region respectively (Figure 2). Further NIR-triggered photoactuation experiments have been executed on four polyaniline/LCE composite film ribbons individually, as recorded in Videos S5−S8. The NIR irradiation time was exactly 30 s and the light-off time was also set as 30 s for comparison. The polyaniline/LCE composite films’ uniaxial expansions (L/Liso) versus irradiation time are plotted in Figure 11. As indicated in Figure 11A, the photoresponsive rate of ES0.5%/LCE film was apparently much faster than its EB analogue, which gradually achieved the maximum shrinkage (ca. 50%) in 26 s, which was 6 s longer in comparison with ES-0.5%/LCE sample. When the NIR light was turned off, both samples apace recovered their original shapes in 12−14 s. On the contrary, ES1%/LCE and EB-1%/LCE films (Figure 11B) with a doubled polyaniline doping concentration showed two similar photoresponsive rates which both shortened the time required for reaching the maximum shrinkage (ca. 25%) to 16 s, although the NIR-off relaxation time still needed 12−14 s. Moreover, these polyaniline/LCE composite materials possess not only fast photoresponsibilities but also good heavy-lift capabilities. For example, as shown in Figure 12A and Video S9, under NIR irradiation, a ES-0.5%/LCE film ribbon (ca. 42.2 mg) could lift up a big binder clip (ca. 8.60 g) which was, in total, 204 times its own weight, while EB-1%/LCE film ribbon (ca. 56.2

Figure 7. Photo images of thermal actuation behaviors of (A) ES-0.5%/ LCE, EB-0.5%/LCE, and pure LCE films, (B) ES-1%/LCE, EB-1%/ LCE, and pure LCE films heated on a hot plate.

of NIR irradiation time. As shown in Figure 10, the heat generation capability (defined as the maximum increase in temperature over a specific time period) and the photothermal conversion rate (defined as the gradient of the temperature versus NIR illumination time curve) were both highest in ES1%/LCE composite film, followed by ES-0.5%/LCE, EB-1%/ LCE, EB-0.5%/LCE, and the least pure LCE sample. Overall, these data provide us three conclusion points: (1) the photothermal effect of the embedded polyaniline nanoparticles is the fundamental driving force behind the photoactuation scenarios, because the surface temperatures of four polyaniline/ LCE composites could easily jump over the LC-to-isotropic transition temperature (ca. 60−61 oC) within 21 s while the highest temperature that the pure LCE sample reached in a much longer illumination time period was even lower than 40 °C. (2) E

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Figure 9. Photo images of (A) ES-0.5%/LCE and pure LCE films, (B) EB-0.5%/LCE and pure LCE films, (C) ES-1%/LCE and pure LCE films, and (D) EB-1%/LCE and pure LCE films under NIR (808 nm) illumination.

Figure 10. NIR illumination time vs temperature diagrams of ES-0.5%/ LCE, EB-0.5%/LCE, ES-1%/LCE, EB-1%/LCE, and pure LCE films.

mg) could lift up a small binder clip (ca. 1.30 g) as well as a 10.0 g counterpoise (ca. 201 times its own weight) as demonstrated in Figure 12B and Video S10.

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CONCLUSION In summary, we synthesized the first example of uniaxially oriented monodomain conjugated polymer/liquid crystalline elastomer composite material by doping polyaniline nanoparticles into polysiloxane-based LCE matrix. Through a series of comparative experiments studying the conjugation form-properties and dopant concentration-properties correlations of polyaniline nanoparticles with respect to the NIR-stimulated photoactuation performances of CP/LCE composite materials, polyaniline emeralidine salt (ES) with a 0.5 wt % doping concentration was found to be an optimized photothermal conversion reagent for CP/LCE composite materials to embrace not only good stretchability but also a satisfying photoresponsive speed. This ES-0.5%/LCE composite material can easily lift up ca. 200 times its own weight under the NIR illumination driving force, and such a photoactuation behavior is fully reversible after NIR light source is removed. Materials based upon these CP/ LCE systems are of potential utility as NIR-responsive shape memory materials. Further development of novel CP/LCE composites containing other CP molecules (such as poly(3,4-

Figure 11. NIR illumination time vs the shape deformation L/Liso of (A) ES-0.5%/LCE and EB-0.5%/LCE composite films and (B) ES-1%/ LCE and EB-1%/LCE composite films along the alignment direction.

Figure 12. Photo images of (A) ES-0.5%/LCE film ribbon loaded with a weight of ca. 8.60 g and (B) EB-1%/LCE film ribbon loaded with a weight of ca. 11.30 g under NIR (808 nm) illumination.

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ethylenedioxythiophene), polypyrrole, etc.) are under investigation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00640. Materials and instrumentation, detailed synthetic procedures, NMR and FT-IR spectra (PDF) Video S1: A comparison experiment of ES-0.5%/LCE film and pure LCE film under NIR (808 nm) illumination (AVI) Video S2: A comparison experiment of EB-0.5%/LCE film and pure LCE film under NIR (808 nm) illumination(AVI) Video S3: A comparison experiment of ES-1%/LCE film and pure LCE film under NIR (808 nm) illumination (AVI) Video S4: A comparison experiment of EB-1%/LCE film and pure LCE film under NIR (808 nm) illumination (AVI) Video S5: A NIR illumination experiment of ES-0.5%/ LCE film in correspondence to Figure 11A (AVI) Video S6: A NIR illumination experiment of EB-0.5%/ LCE film in correspondence to Figure 11A (AVI) Video S7: A NIR illumination experiment of ES-1%/LCE film in correspondence to Figure 11B (AVI) Video S8: A NIR illumination experiment of EB-1%/LCE film in correspondence to Figure 11B (AVI) Video S9: A NIR illumination experiment of ES-0.5%/ LCE film loaded with a weight of ca. 8.60 g (AVI) Video S10: A NIR illumination experiment of EB-1%/ LCEfilm loaded with a weight of ca. 11.30 g (AVI)

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AUTHOR INFORMATION

Corresponding Author

*(H.Y.) Telephone: 86 25 52090620. Fax: 86 25 52090616. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by National Natural Science Foundation of China (Grant No. 21374016) and Priority Academic Program Development of Jiangsu Higher Education Institutions.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b00640 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b00640 Macromolecules XXXX, XXX, XXX−XXX