Preparation of Hydrophilic Encapsulated Carbon Nanotubes with

Article

Preparation of Hydrophilic Encapsulated Carbon Nanotubes with Polymer Brushes and Its Application in Composite Hydrogels Zili Li, Miao Tang, Wei Bai, and Ruke Bai Langmuir, Just Accepted Manuscript • Publication Date (Web): 27 May 2017 Downloaded from http://pubs.acs.org on May 29, 2017

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Preparation of Hydrophilic Encapsulated Carbon Nanotubes with

Polymer

Brushes

and

Its

Application in Composite Hydrogels Zili Lia, Miao Tanga, Wei Bai*,b and Ruke Bai*,a a

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and

Engineering, University of Science and Technology of China, Hefei, 230026, P. R. China b

Department of Chemistry, University of Tennessee, Knoxville, TN 37996, United States

KEYWORDS: Multiwalled carbon nanotubes, self-assembly, composite hydrogel, enhanced mechanical property, stimuli-responsiveness

ABSTRACT: Carbon nanotubes can be used as promising reinforcement materials to improve the mechanical properties of hydrogels, but their poor dispersibility in aqueous solution severely limits their application in preparation of composite hydrogels. Therefore, to develop method for modification of carbon nanotubes is still highly desired. In this paper, a facile approach for preparation of hydrophilic carbon nanotube was reported. The encapsulated multiwalled carbon nanotubes (E-CNT-PAA) with crosslinked shell structure were obatined through the selfassembly of the amphipathic azide diblock copolymers poly(acrylic acid)-b-poly(4-vinylbenzyl azide-co-styrene) (PAA-b-(PVBA-co-PS)), and the crosslinking of inside azide groups under UV irradiation. The encapsulated MWCNT was characterized by FT-IR, Raman and TEM. It was demonstrated that the dispersibility of the hydrophilic encapsulated MWCNTs was related to the


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length of the poly(acrylic acid) brushes. Subsequently, thermal-responsive composite hydrogels (PNIPAM/E-CNT-PAA) were prepared by in-situ polymerization of N-isopropylacrylamide (NIPAM) in the solution of dispersed E-CNT-PAA. The results showed that the composite hydrogels possessed high mechanical properties compared to the pure PNIPAM hydrogel. The tensile strength and elongation of the composite hydrogels were highly dependent on the content of the modified MWCNTs. The composite hydrogels with 0.46 wt% MWCNTs exhibited tensile strength of 97.7 kPa and elongation of 465%, which were at least 3.5 times higher than those of the PNIPAM hydrogel. Moreover, the composite hydrogels displayed significant and reversible stimuli-responsiveness. INTRODUCTION Carbon nanotubes (CNTs) as a novel class of nanomaterials have been undoubtedly the hottest topic due to their superb electrical, mechanical, optical and other physico-chemical properties since their discovery1-3. The remarkable mechanical properties of carbon nanotubes, such as high elastic modulus and tensile strength, make them the most ideal and promising reinforcements in substantially enhancing the mechanical properties of polymer/carbon nanotube composites4-6. CNT-based composite hydrogels which integrate the unique characters and functions of these two types of components have emerged as a new class of materials with superior properties, and have shown their potential applications in the field of healthcare7, biomedical engineering8, 9, electrical performance10, 11, and lithium ion battery12. The composite hydrogels not only possess some brand-new properties such as pH response13-15, heat and light induced phase transition13, 16, 17

, but also showed dramatic improvement in mechanical performance13-15, 18, 19, which enable

their versatile applications. Zhang et al14 reported CNT/polyethylene polyamine (PPA)


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composite hydrogels with multiple (thermal, pH and NIR light) responsive behaviors and enhanced strength due to hierarchical hydrogen bonds. In another recent study15, CNT composite hydrogels with 2.5 wt % CNTs prepared by in situ polymerization method showed high pH sensitivity and good mechanical properties. Although, some achievements in preparation of CNT-based hydrogels have been made, the poor dispersibility of CNTs in aqueous solution due to the strong van der Waals interactions severely limits their application as excellent reinforcing agents for composite hydrogels. In order to enhance the hydrophilicity of CNTs, hydrophilic groups, such as OH and COOH, can be introduced by treating CNTs with concentrated sulfuric acid and nitric acid, however, the intrinsic structure of the CNTs is usually damaged during the reaction process. On the other hand, amphiphilic polymers can be used to prepare hydrophilic CNTs by wrapping CNTs with polymers based on the noncovalent interactions between CNTs and polymers, such as hydrophilic-hydrophobic interaction and π-π stacking20, 21. Recently, micelle-encapsulated CNTs were prepared by using small molecular surfactants11, 12, 17, 22, 23, or amphiphilic polymers24-28, and could be homogenously dispersed in aqueous solution. This encapsulation method can avoid significant damage and retain the structural integrity of the CNTs, and achieve a homogenous dispersion of hydrophilic CNTs. However, the micelle-encapsulated CNTs are not stable because the dynamic self-assembled structure of these amphiphilic molecules. In order to tackle the issue, the micelles were crosslinked to form a crosslinked polymer shell around the surface of the CNTs. Compared with small molecular surfactants with relatively weak noncovalent interactions, rationally designed amphiphilic polymers could incorporate synergistic effect of more noncovalent interactions. Taton24 and Engel25 prepared micelle-encapsulated CNTs respectively, using amphiphilic polymers poly(styrene)-block-poly(acrylic acid) and polyethylene glycol-


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polyacrylic

acid-polystyrene,

and

then

crosslinked

the

micelle

with

2,2'-

(ethylenedioxy)diethylamine as a crosslinker in the presence of catalysts. Thereafter, Shi26 used amphiphilic hyperbranched polymers (H20-MAh-PSt) with maleic anhydride modified polyester (BoltornTM H20) as a core and polystyrene as arms to prepare micelle-encapsulated CNTs, and crosslinked the double bonds using γ-irradiation. Although additional croslinkers were avoided, the dispersion of the crosslinked micelle-encapsulated CNT was poor and the CNT bundles were not separated. In 2014, Nakashima et al22 developed a different strategy for preparation of crosslinked polymer-coated CNT with the aid of small molecular surfactants, but the surfactants have to remove after shell crosslinking. Undoubtedly, these crosslinked micelle encapsulation methods provide a promising approach to achieve highly stable hydrophilic CNTs. However, these methods above have some disadvantages, such as, tedious procedures and additional reagents. Therefore, facile and efficient methods for preparation of hydrophilic CNTs are highly desired. In our previous work, we prepared the hydrophilic MWCNTs grafted polymer brushes by using the azide diblock copolymers based on the reaction of MWCNTs and azide groups. However, the modification suffered from a low efficiency due to the grafting reaction being carried out at elevated temperature in homogeneous solution19, and this is common problem for grafting of polymer brushes on nanomaterials29, 30. Herein, we present a facile and efficient approach for preparation of shell-crosslinked encapsulated MWCNT (E-CNT-PAA) with polymer brushes through photocrosslinking at room temperature in aqueous solution. In the this study, amphiphilic diblock copolymers, poly(acrylic acid)-b-poly(4-vinylbenzyl azide-costyrene) (PAA-b-(PVBA-co-PS)), with well-defined structure were designed and synthesized by RAFT polymerization, and then used to encapsulate multiwalled carbon nanotubes (MWCNTs)


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with polymer brushes. The encapsulation of the CNTs was performed in aqueous solution by adding a solution of the azide block copolymers. Since the azide groups of the block copolymers are sensitive to UV light, the encapsulated MWCNTs with crosslinked polymer shell were obtained by crosslinking of the polymer chains under UV irradiation at room temperature. Compared with previous works19, 24-26, this micelle-encapsulation method is more convenient and highly efficient for preparation of the micelle-encapsulated CNTs, in which we combined the self-assembly of the reactive amphiphilic block copolymers with photocrosslinking reaction of azide groups in confined spaces of the micelles. Due to the high hydrophilicity and the good dispersity in water, the encapsulated MWCNTs were used to fabricate composite hydrogels by in-situ polymerization of N-isopropylacrylamide in solution of E-CNT-PAA. The composite hydrogels based on E-CNT-PAA not only show high mechanical properties, but also exhibit thermal responsiveness. Experimental Section Materials. MWCNTs were provided by Tsinghua-Naine Nano-Powder Commercialization Engineering Centre (MWCNT, the average diameters is about 10-20 nm and the average length is about 30 µm). Tert-butyl acrylate (tBA) and 4-Chloromethylstyrene (CMS) were purchased from Alfa Aesar, the inhibitor having been removed by passage through an alumina column. Nisopropylacrylamide (NIPAM) was purified by recrystallization from a mixture of benzene and n-hexane (1/3, v/v) twice and dried in vacuum at 40 °C. Potassium peroxydisulfate (KPS) and 2, 2′-azobisisobutyronitrile (AIBN) were recrystallized from deionized water and methanol respectively, and then dried at room temperature under vacuum. N, N, N′, N′tetramethylethylenediamine (TEMED, Sinopharm Chemical Reagent Co., Ltd.) was distilled


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over KOH. NaN3 and CF3COOH were purchased from Shanghai Chemical Co. and used without further purification. All other chemicals were analytical-grade reagents and used as received. Synthesis of Poly(t-butyl acrylate)-b-Poly(4-chloromethylstyrene-co-styrene) (PtBA46-b(PCMS18-co-PS56)) Block Copolymers by RAFT Polymerization. The Poly(tert-butyl acrylate)46 macro chain-transfer agent (PtBA46-CTA) was synthesized through RAFT polymerization according to reports19,

31

. The copolymerization of 4-

chloromethylstyrene and styrene (molar ratio 1:3) was carried out using the PtBA homopolymer previously obtained as a RAFT macro-CTA (Scheme 1). 270 mg of PtBA46 homopolymer (macro-CTA, 0.0432 mmol), 1.4 mg of AIBN, 458 µL of CMS (3.24 mmol), 1.12 mL purified styrene (9.72 mmol) and 12 mL of dioxane, were introduced in a Schlenk tube equipped with a magnetic stirrer. The mixture was deoxygenated by three cycles of freeze-pump-thaw, the tube was sealed under vacuum and placed in an oil bath at 90 °C for 24 h. Then the reaction was quenched, and the polymer solution was precipitated three times in methanol. The product was collected and dried under vacuum for 24 h at room temperature to afford PtBA46-b-(PCMS18-coPS56) as a yellow solid. Yield: 0.830 g (37.2%). MnNMR = 14800 Da, MnGPC =14100 Da, Mw/Mn = 1.45. 1H NMR (300 MHz, CDCl3-d1, δ, ppm): 6.36-7.50 (br, styrenyl protons), 4.50 (s, 2H, CH2Cl), 2.36-2.13 (br, 1H, CHC(O) of polymer backbone), 1.00-1.95 (br, t-butyl protons, polymer backbone protons and C(S)SCH2C10H20CH3). Synthesis of Poly(t-butyl acrylate)-b-Poly(4-vinylbenzyl azide-co-styrene) (PtBA46-b(PVBA18-co-PS56)) Block Copolymers. PtBA46-(PCMS18-co-PS56) (0.771 g, 0.052 mmol) and sodium azide (0.152 g, 2.34 mmol, 2.5 equiv of 4-chloromethylstyrene) were dissolved in 15 mL of DMF. The mixture was stirred at 60 °C for 24 h under nitrogen atmosphere. After cooling to room temperature, the mixture was


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poured into excess MeOH/H2O (4:1 v/v) and the solution filtered. The final product was obtained by precipitation in MeOH/H2O twice and freeze dried. Yield: 0.756 g (98%). MnNMR = 14800 Da, MnGPC =15500 Da, Mw/Mn = 1.45. 1H NMR (300 MHz, CDCl3-d1, δ, ppm): 6.36-7.50 (br, styrenyl protons), 4.23 (s, 2H, CH2N3), 2.36-2.13 (br, 1H, CHC(O) of polymer backbone), 1.001.95 (br, t-butyl protons, polymer backbone protons and C(S)SCH2C10H20CH3). Hydrolysis of Block Copolymers PtBA-b-(PVBA-co-PS). A 50 mL flask was charged with PtBA46-b-(PVBA18-co-PS56) (0.680 g, 0.04 mmol) and 30 mL CHCl3. 1.4 mL of trifluoroacetic acid was added to the flask, and the reaction mixture was stirred for 24 h at room temperature. The final products were separated by the evaporation of the reagents under vacuum to obtain amphiphilic poly(acrylic acid)-b-poly(styrene-co-4-vinylbenzyl azide) (PAA46-b-( PVBA18-co-PS56)). Yield: 0.478 g (97.4%). 1H NMR (300 MHz, DMSO-d6, δ, ppm): 12.18 (-COOH), 6.21-7.45 (br, styrenyl protons), 4.27 (s, 2H, CH2N3), 2.36-1.05 (br, polymer backbone protons and C(S)SCH2C10H20CH3). Preparation of Encapsulated Multiwalled Carbon Nanotubes. The encapsulation of MWCNT was modified according to a previously reported strategy24. The typical procedure was described as follows. 10 mg of MWCNT was dispersed in 10 mL of DMF, and the mixture was sonicated for 30 min. Then, a solution of 10 mg of PAA46-b-(PVBA18-coPS56) dissolved in 1 mL DMF was added into the above suspension, the mixture was sonicated for another 30 min at ice-water bath. Next, 9 mL of deionized water was added at a uniform dropwise rate, with one drop added every 10-15 s. After 9 mL of water was added, the solution was continuously sonicated for another 30 min. Thereafter, the PVBA blocks of the resulting micelles on MWCNTs were irradiated crosslinked by a high pressure mercury lamp (400 W) with a wavelength of 365 nm for 10 min. Finally, the solution was transferred to a dialysis tube


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(molecular weight cutoff, 7000) and dialyzed against deionized water for 3 d. The final volume was 20 mL, resulting in a encapsulated carbon nanotube (E-CNT-PAA46) concentration of ca. 1 mg/mL. Preparation of PNIPAM/E-CNT-PAA Composite Hydrogels. In a typical synthesis process, a 1.5 mL E-CNT-PAA46 solution containing Nisopropylacrylamide monomer (NIPAM, 0.34 g, 3 mmol), N, N'-methylenebis(acrylamide) crosslinker (MAB, 20 mg/mL, 42.5 µL), N, N, N', N'-tetramethylethylenediamine (TEMED, 20 µL), and water (1.5 mL) was degased for 5min, the potassium persulfate initiator (KPS, 20 mg/mL, 170 µL) was then was added to flask with syringe. The polymerization was carried out a 25 °C for 48 h. The composite hydrogels were noted as P1-EC0.25, P1-EC0.5, P1-EC1 corresponding to concentration of PNIPAM of 1 mmol/mL and E-CNT-PAA46 of 0.25, 0.5 and 1 mg/mL respectively. As a control sample, PNIPAM hydrogel P1 was also prepared at the same condition.


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Scheme 1. (A) Synthesis of amphiphlic diblock copolymer PAA-b-(PVBA-co-PS) and (B) Schematic illustration of the encapsulation of a MWCNT using amphiphilic block copolymers and crosslinked under UV irradiation (254 nm). Characterization. All NMR spectra were recorded on a Bruker AVANCEII spectrometer (resonance frequency of 300 MHz for 1H) operated in the Fourier transform mode. CDCl3 and TMS were used as the solvent and internal standard, respectively. Fourier transform infrared (FT-IR) spectra were recorded in the spectral range from 4000 to 400 cm-1 with Thermo Nicolet 6700. Molecular weights and molecular weight distributions were determined by gel permeation chromatography (GPC) equipped with a Waters 1515 pump, a Waters414 RI detector, and Waters UV/RI detectors (set at 30 °C). Thermal gravimetric analysis (TGA) was carried out on a PE TGA-7 instrument at a heating rate of 10 °C·min-1 under nitrogen. Differential scanning


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calorimetry (DSC) measurements were carried out at a heating rate of 10 °C•min-1 under nitrogen with TA Q2000, TA Instruments. Raman spectroscopic analyses were carried out on a LABRAM-HR confocal laser micro-Raman spectrometer at room temperature. The morphologies of the dispersed MWCNTs were characterized by transmission electron microscopy (TEM) using a JEOL-2010 transmission electron microscope operated at 200 kV. The samples were prepared by placing a droplet dispersion of E-CNT-PAA onto a carbon-coated copper grid followed by air-drying. The scanning electron microscopy (SEM) images were obtained using a JSM-6700F field-emission microscope, and the samples were freeze dried for 48 h before SEM imaging. The compressive tests were performed in DMTAQ800 (TA, USA) with the crosshead speed of 0.5 N/min at room temperature, the dimension of compression testing samples was 10 mmφ×10 mm length. The tensile stress-strain tests were performed on columnar samples with a diameter of 6 mm by using a ZBC 1400−1 pendulum impact tester (Shenzhen SANS Test Machine Company, China) apparatus at a speed of 10 mm·min−1. Rheological measurements were performed on a rotational rheometer (TA-AR2000EX, TA Instruments, New Castle, DE) with a parallel plate geometry of 25 mm in diameter and a gap of 0.9 mm at constant temperatures in a nitrogen atmosphere. Dynamic frequency sweep measurements were carried out in the linear viscoelastic regime from frequency of 100 to 0.1 rad/s with strain amplitude of 1%. Swelling and Deswelling Performances. PNIPAM/E-CNT-PAA46 composite hydrogels (10 mmφ×10 mm length) were immersed in large amount of water at 25 and 50 °C respectively. The weight of hydrogels at specific time was recorded. Results and Discussion


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Synthesis and Characterization of Amphiphilic Diblock Copolymer. The amphiphlic diblock copolymer PAA46-b-(PVBA18-co-PS56) was prepared by sequential RAFT polymerization of t-butyl acrylate and styrene/4-vinylbenzyl azide using S-1-Dodecyl-S′(α, α′-dimethyl-α″-acetic acid) trithiocarbonate (DMP) as chain transfer agent, and hydrolysis with trifluoroacetic acid (Scheme 1). The structure of PtBA46-CTA was demonstrated by 1H NMR spectroscopy and polymerization degree (DP) was calculated to be 46 based on the ratio of methyl protons in the CTA at 0.80-0.90 ppm, and the methine protons in the polymer backbone at 2.36-2.13 ppm (Figure S1, Supporting Information)19. The polydispersity index (PDI) of PtBA46-CTA was measured to be 1.24 by GPC measurement (Figure S2). Then RAFT copolymerization of styrene (St) and 4-chloromethylstyrene (CMS) was performed in the presence of PtBA-CTA. The monomer conversion of CMS was calculated to be 24% by comparing the integration ratio between methylene protons (4.50 ppm) of CMS and methine protons (2.36-2.13 ppm) of PtBA from 1H NMR spectrum (Figure 1a), while the monomer conversion of St was 24.9% based on the peaks at 6.36-7.50 ppm for St and CMS. The formation of block copolymer was also demonstrated from the GPC traces (Figure S2), the molecular weight increased from 4500 to 14100 with PDI of 1.45. Azide block copolymer PtBA46-b-(PVBA18-co-PS56) was obtained by reaction of PtBA46-b(PCMS18-co-PS56) with sodium azide. The reaction was monitored by 1H NMR spectroscopy and the result was shown in Figure 1a. Disappearance of the peak at 4.5 ppm attributed to the -CH2Cl group and the appearance of the peak at 4.2 ppm corresponding to the -CH2N3 suggested the formation of PtBA46-b-(PVBA18-co-PS56). Moreover, the azide polymer was also confirmed by FT-IR spectroscopy. By comparing the FT-IR spectra of PtBA46-b-(PCMS18-co-PS56) and PtBA46-b-(PVBA18-co-PS56) in Figure 1b, noted that the stretching band of C-Cl at 675 cm-1


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vanished after reaction, whereas a peak of the azide groups at 2103 cm-1 emerged, indicating the formation of the azide diblock copolymers 19, 32.

Figure 1. (a) 1H NMR spectra of the block copolymers in CDCl3, and (b) FT-IR spectra of the block copolymers and the amphiphilic copolymer in KBr. The amphiphilic block copolymer PAA46-b-(PVBA18-co-PS56) was prepared by hydrolysis of the tBA moieties with CF3COOH in chloroform and characterized by 1H NMR and FT-IR spectroscopy. The 1H NMR spectrum in Figure S3 revealed that the signal of tert-butyl proton at 1.4 ppm disappeared. The FT-IR spectrum of PAA46-b-(PVBA18-co-PS56) in Figure 1b showed that the carbonyl stretching band shifted from 1728 to 1712 cm-1, and the signal of the tert-butyl group at 1370 cm-1 disappeared. All the results proved the formation of amphiphilic azide copolymers. In order to compare the properties of amphiphilic block copolymers with different length of PAA blocks, PAA78-b-(PVBA25-co-PS70) and PAA141-b-(PVBA30-co-PS94) were also synthesized (Figure S4-Figure S9). Preparation and Characterization of Micelle-Encapsulated Multiwalled Carbon Nanotubes (E-CNT-PAA) with Crosslinked Shell.


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Micelle encapsulation of MWCNT was carried out as illustrated in Scheme 1B. First, the amphiphilic block copolymers PAA46-b-(PVBA18-co-PS56) were dissolved in DMF, and then the polymer solution was added into the DMF suspension of MWCNTs under ultrasonication in ice water bath. The micelle-encapsulated MWCNTs were obtained by gradually adding water into the solution. Since azide group is sensitive to UV light, they can be photochemically activated to form nitrene group, which has a high tendency to react with saturated and unsaturated bonds of organic compounds33-35. Finally, the micelle-encapsulated MWCNTs (E-CNT-PAA46) with crosslinked shell were prepared by UV photo-crosslinking under ambient conditions in the aqueous solution. We noticed that in the encapsulation process, no precipitation was observed, the encapsulated MWCNTs was obtained in a high yield of up to 96%, because the amphiphilic block copolymers first self-assembled into micelles around CNTs, the azide groups were concentrated inside the micelle and the crosslinking reaction only occurred in the confined inner shell, avoiding the crosslinking among the micelles. In contrast, in our previous work, the hydrophilic MWCNTs were obtained in a yield of only 73% by the grafting approach of polymers in homogeneous solution19. Therefore, micelle-encapsulation is a highly efficient method for preparation of hydrophilic CNTs with polymer brushes.


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Figure 2. FT-IR spectra of pristine MWCNT, E-CNT-PAA46 and PAA46-b-(PVBA18-co-PS56). Figure 2 shows the FT-IR spectra of MWCNTs, PAA46-b-(PVBA18-co-PS56) and E-CNTPAA46. Information for the encapsulation and the crosslinking reaction can be obtained by comparison of these spectra. The peaks in the regions of 3000-3100 cm-1 and 2805-2985 cm-1 are ascribed to characteristics of benzene ring and C-H stretching respectively, and a characteristic absorption band at 1710 cm-1 is assigned to the C=O stretching vibration of the carboxylic groups. Moreover, a broad shoulder band in a region of 3300-3600 cm-1 is referred to the carboxylic groups, and its intensity is greatly enhanced after micelle-encapsulation compared with MWCNTs due to attachment of PAA46-b-(PVBA18-co-PS56) onto the surface of MWCNTs. In addition, disappearance of the peak at 2103 cm-1 belonging to the azide groups, suggested that the crosslinking occurred when the micelle-encapsulated MWCNT irradiated by UV light.

Figure 3. (a) Photographs for dispersion of E-CNT-PAA46 in H2O (0.1 mg/mL, left is H2O, right is mixed solvent of H2O (upper layer) and CH2Cl2 (bottom layer)), and TEM images of crosslinked micelle-encapsulated carbon nanotubes E-CNT-PAA46 (b,c,d).


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Morphology of the encapsulated MWCNTs was characterized by transmission electron microscopy (TEM). The TEM images of micelle-encapsulated CNTs before and after UV irradiation were shown in Figure S10 and Figure 3, respectively. It can be seen that the structure of micelle-encapsulated CNTs before and after UV irradiation is similar, and the wall thickness and polymer layer thickness can be clearly observed in Figure S10. The TEM images (Figure 3b3d) displayed that the MWCNTs were individually encapsulated by the amphiphilic polymers, no bundles of MWCNT were observed, indicating that the encapsulation of the MWCNT was very successful. At the higher magnification (Figure 3b and 3d), it can be seen clearly that the tube was clothed with a polymer layer, and the end of the tube was also encapsulated by the micelle, thereby signaling the formation of micelle-encapsulated carbon nanotubes with coreshell structure. The polymer shell is uniform and the thickness of the polymer shell is about 11 nm, which is smaller than the extended length (about 15 nm) of the amphiphilic block copolymer PAA46-b-(PVBA18-co-PS56), indicating the micelle encapsulated the carbon nanotube. Actually, E-CNT-PAA46 exhibits good dispersibility in the aqueous solution (Figure 3a), which further demonstrates the micelle encapsulation of MWCNT was successful.

Figure 4. Raman spectra of the pristine MWCNT and E-CNT-PAA46.


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As illustrated in Figure 4, the Raman spectrum of the MWCNTs exhibits a strong G band at 1584 cm-1 which is associated with the vibration of sp2-bonded carbon atoms in a graphitic layer, and the D band at 1351 cm-1 is assigned to defects in the nanotube lattice, which include sp3hybridized carbon atoms. From the ratio of ID/IG, the nature of covalent or noncovalent functionalization on MWCNTs can be identified. In fact, the ratio of ID/IG was changed from 1.12 of pristine MWCNTs to 1.13 for E-CNT-PAA46; this result indicates that very little covalent reaction occurred between the MWCNTs and the amphiphilic polymers33,

36

. Since the

amphiphilic block copolymers self-assembled into micelles, the azide groups were located inside the micelles so that they couldn’t react with the outside poly(acrylic acid) blocks. Moreover, the amphiphilic block copolymer has a long alkyl chain on the end of the polystyrene block and the polar azide groups are away from the carbon nanotubes. Therefore, it is reasonable that the crosslinking reaction of the azide groups mainly occurred among the polymer chains, instead of between the polymers and the MWCNTs.

Figure 5. TGA curves of the pristine MWCNT, E-CNT-PAA46 and block copolymers PAA46-b(PVBA18-co-PS56) under nitrogen atmosphere. Figure 5 shows TGA curves of the pristine MWCNT, E-CNT-PAA46, and the block copolymer PAA46-b-(PVBA18-co-PS56). Weight loss of the pristine MWCNTs is only 1.24 wt%


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below 600 °C. For the E-CNT-PAA46, there are two main weight-loss regions. The first one at 120∼360 °C can be designated as decomposition of carboxyl groups in the of PAA blocks37. The second region at 360∼500 °C is assigned to the decomposition of the PAA block backbone and PS and PVBA blocks. And the remaining mass was approximately 57.1 wt%. Based on the results, the weight fraction of the polymers encapsulated on MWCNTs was calculated to be about 48 wt%37, 38. It was demonstrated that the E-CNT-PAA46 could be easily dispersed in water as shown in Figure 3a, no precipitates were observed after the dispersed solution was centrifuged with 4000 rpm for 5 min. Moreover, the solution of E-CNT-PAA46 is quite stable, no precipitation occurred even if it was stood over one month (Figure S11). Unexpectedly, the micelle encapsulated MWCNTs with longer PAA blocks, the E-CNT-PAA78 and E-CNT-PAA141 showed poor dispersity, precipitation was observed after standing for a week (Figure S12). This can be attributed to the formation of hydrogen bonding among the PAA chains on the surface of MWCNTs39, 40. However, the E-CNT-PAA78 and E-CNT-PAA141 could be well dispersed in alkaline aqueous solution (Figure S13). Preparation and Characterization of MWCNT Based Composite Hydrogels. PNIPAM/E-CNT-PAA composite hydrogels were prepared by in situ polymerization of Nisopropylacrylamide in E-CNT-PAA46 solution at 25 °C using MBA as crosslinker and KPS/TEMED as redox initiator. The polymerization was performed in a glass tube as shown in Figure 6a. Uniform hydrogels were obtained and the colour of the hydrogels turned darker gradually from the left to right with increasing the amount of E-CNT-PAA46. From the FT-IR spectra (Figure S14) of the dried nanocomposite hydrogels, the characteristic bands of amide I and amide II modes at 1650 and 1548 cm-1 can be observed41. The bands at 3075, 2980, and


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2873 cm-1 are attributed respectively to the N-H stretching and asymmetric and symmetric vibration of C-H, indicating the formation of PNIPAM.

Figure 6. Photographs demonstrating the excellent mechanical behavior of the composite hydrogels PNIPAM/E-CNT-PAA46. (a) The as-prepared composite hydrogels with different CNT contents, (b) the as-prepared P1-EC1 composite hydrogel, under press, and after the press was removed and (c) the P1-EC1 composite hydrogel fixed to two clamps is stretched to 4 times of its initial length in a tensile machine. The mechanical performance of the composite hydrogel was qualitively examined as shown in Figure 6b and Figure 6c. The result exhibited that the composite hydrogel had high stretchability as it could be stretched by about 4 times the orginal length of the sample without rupture (Figure 6c). Moreover, it also possessed good elasticity and recoverability as shown in Figure 6b. The tensile stress-strain curves of the composite hydrogels with different content of E-CNT-PAA46 were shown in Figure 7a. It was found that the mechanical properties of the composite hydrogels were strongly related to the content of E-CNT-PAA46, tensile strength of the composite hydrogels increased with increasing the content of the E-CNT-PAA46. When weight ratio of the E-CNT-PAA46 and NIPAM was 0.9 wt% (about 0.46 wt% MWCNTs), fracture strength and elongation of the composite hydrogel P1-EC1 reached to 97.7 kPa and 465% respectively, both


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are at least three times higher than that of the PNIPAM hydrogels. Enhancement of mechanical properties of the composite hydrogels is related to the entanglements between the polymer chains on the surface of the E-CNT-PAA and the networks of the PNIPAM19, 42-44. On the other hand, the hydrogen bonds between carboxylic groups of the PAA blocks and the amide groups of PNIPAM play a role on the enhancement of mechanical properties14, 45. The synergistic effect of entanglements and hydrogen bonding interactions is responsible for the excellent mechanical properties. However, hydrogen-bonding interactions exist between the PNIPAM chains in the dried state, therefore, it is difficult to identified hydrogen bonding between the PAA chains and the PNIPAM chains (Figure S14)46. When the composite hydrogel was stretched, breaking of the hydrogen bonds and slide of the randomly entangled polymer chains in the hydrogel network could spread the load over many chains and effectively dissipated the on-loading crack energy43, 47

. As a result, the process with large deformation was accompanied by dissipation of a large

amount of energy, which was also proved from the obvious hysteresis loop in the loadingunloading cycles (Figure 7c).


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Figure 7. Mechanical properties of the composite hydrogels PNIPAM/E-CNT-PAA46. (a) Tensile stress-strain curves of the composite hydrogels, (b) true stress-true strain curves of the composite hydrogels, (c) tensile stress-strain curves of the composite hydrogel P1-EC1 during loading-unloading cycles, (d) typical compressive stress-strain curves of the composite hydrogels. For calculation of the true stress and the strain, the data of engineering stress versus engineering strain obtained from tensile tests were then converted. The true stress (σtrue) can be expressed by eq 1 as follows48, 49 1

(1)

where σE is the engineering stress, εE is the engineering strain. The true strain (εH) is related to εE by eq 2.

ln 1

(2)


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Figure 7b shows the true stress-true strain curves for the composite hydrogels. The stress response is almost linear, when the deformation is lower than 0.5, but the true stress abruptly increases without yielding after the deformation becomes larger than 0.5. For the composite hydrogels with high content of E-CNT-PAA46, the stress upturn in the true stress-strain curve shifts towards a lower deformation (smaller εH) due to the increase in the cross-linking density by the entanglements and hydrogen-bonding interactions. The similar phenomenon was also observed from the as-prepared laponite composite hydrogels48 and the double network hydrogels50. The stress-strain curves of P1-EC1 in Figure 7c indicated that the composite hydrogel could effectively dissipate energy effectively during loading-unloading cycles, as illustrated by the pronounced hysteresis loop43, 47. Actually, the hydrogel P1-EC1 exhibited negligible hysteresis loop at low elongation of 50%, in contrast, pronounced hysteresis was observed at an elongation of 200%. However, the permanent deformation after unloading was significantly smaller at either 50% or 200% elongation. The fine hysteresis loops and small permanent deformations of the P1EC1 hydrogel were further illuminated from the stress-strain measurements at different critical stretch during loading-unloading cycles in Figure 7c and Figure S15. This phenomenon is related to the hydrogen bonding interactions and entanglements of the polymer chains in the 3D network structure of the composite hydrogels. Furthermore, composite hydrogels also exhibit enhanced stiffness due to the incorporation of E-CNT-PAA (Figure 7d and Figure S16). Figure 7d shows the typical compressive stress-strain curves of the composite hydrogels. Compressive strength of the composite hydrogel P1-EC1 was measured to be 0.46 MPa, which is more than 4 times that of the pure hydrogel P1. These results indicate that the mechanical properties of the composite hydrogels were remarkably enhanced by the incorporation of E-CNT-PAA46.


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To examine the influence of E-CNT-PAA46 on the dynamical modulus, the rheological properties of the composite hydrogels with different content of E-CNT-PAA46 were investigated. The results shown in Figure 8 revealed that incorporation of E-CNT-PAA46 had obvious influence on the dynamical modulus of the hydrogels. In comparison with G′, G′′ is strongly dependent on the frequency (0.1-100 rad/s). Furthermore, the G′ value remains larger than the G′′ value over a wide frequency range, suggesting that the composite hydrogels were predominantly elastic14, 19. Moreover, the G′ of composite hydrogel P1-EC1 is about 3.5 times more than that of the pure hydrogel, indicating the formation of the strong and rigid hydrogels. In addition, increases of the E-CNT-PAA46 content in the composite hydrogels leads to an increase in G′.

Figure 8. Frequency dependence of the storage modulus (G′, filled symbols) and loss modulus (G′′, unfilled symbols) for PNIPAM/E-CNT-PAA46 composite hydrogels at 25 °C. Morphology of the freeze-dried hydrogel P1-EC1 and P1 was characterized by electron microscopy (SEM) and the images were shown in Figure 9 and Figure S17. Comparing with the morphology of P1, the composite hydrogel has a uniform sponge-like structure. The SEM images revealed that the composite hydrogels had 3D network structure and the diameters of the pores with smooth surface were tens of micrometers, indicating the uniform dispersion of the MWCNTs in the polymer matrix.


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Figure 9. SEM images of the freeze-dried composite hydrogel P1-EC1. Lower critical solution temperature (LCST) of the composite hydrogels was measured by DSC technique16. It was found that the endothermic peaks appeared at around 33.5 °C for both the PNIPAM/E-CNT-PAA46 and the PNIPAM gels (Figure S18). It is clear that the incorporation of MWCNTs into the composite hydrogel has no influence on the thermo-responsivity of the composite hydrogels. Then deswelling property of the composite hydrogels as a function of temperature was investigated. Compared to the pure PNIPAM hydrogel, incorporation of ECNT-PAA has some influence on deswelling of the composite hydrogels as shown in Figure 10a. Reversibility between swelling and deswelling could be tuned by alternating temperature as shown in Figure 10b. The composite hydrogels exhibit reversible temperature-sensitive swelling/deswelling behaviors, with no significant decay in swelling/deswelling ratio.


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Figure 10. (A) Deswelling ratio (W/W0) of PNIPAM/E-CNT-PAA46 and PNIPAM hydrogels as function of temperature. Insert are the photo images of the composite hydrogel P1-EC1 before and after deswelling. (B) The swelling and deswelling behaviors of PNIPAM/E-CNT-PAA46 and PNIPAM hydrogels as a function of time with modulation of temperature. W0 is the weight of the hydrogel at 25 °C, and W is the weight of the hydrogel at specific temperature or time in the experiments. Conclusion In conclusion, we developed a facile and efficient approach for preparation of hydrophilic encapsulated multiwalled carbon nanotubes with crosslinked polymer shells. First, amphipathic azide diblock copolymers were synthesized by RAFT polymerization and used for the micelle encapsulation of MWCNT in aqueous solution. Then the encapsulated MWCNT with crosslinked PAA shell was obtained under UV irradiation. It was demonstrated that the hydrophilic encapsulated MWCNT has very good dispersibility in water with high stability. Moreover, hydrophilic encapsulated MWCNT was used to prepare composite hydrogels by insitu polymerization of N-isopropylacrylamide. The results demonstrated that the incorporation of the hydrophilic encapsulated MWCNT can effectively improve the mechanical properties of the composite hydrogels through the synergistic effect of entanglements and hydrogen-bond interactions, and the composite hydrogels exhibit high mechanical strength and high elasticity. In addition, the composite hydrogels also display excellent reversible stimuli-responsiveness. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. 1H NMR spectra and GPC traces of polymers with different block ratio; dispersibility of the


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hydrophilic micelle-encapsulated MWCNTs; FT-IR spectra, DSC curves, and mechanical properties of composite hydrogels; SEM image of the PNIPAM hydrogel. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (R.K. Bai) * E-mail: [email protected] (W.B.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors are grateful for financial support from the National Natural Science Foundation (NNSF) of China (NO. 21674101 and NO. 21474093). REFERENCES 1. De Volder, M. F.; Tawfick, S. H.; Baughman, R. H.; Hart, A. J., Carbon nanotubes: present and future commercial applications. Science 2013, 339, (6119), 535-539. 2. Saha, A.; Jiang, C.; Martí, A. A., Carbon nanotube networks on different platforms. Carbon 2014, 79, 1-18. 3. Wang, X.; Kalali, E. N.; Wan, J.-T.; Wang, D.-Y., Carbon-family materials for flame retardant polymeric materials. Prog. Polym. Sci. 2017 doi: 10.1016/j.progpolymsci.2017.02.001. 4. Sahoo, N. G.; Rana, S.; Cho, J. W.; Li, L.; Chan, S. H., Polymer nanocomposites based on functionalized carbon nanotubes. Prog. Polym. Sci. 2010, 35, (7), 837-867. 5. Li, Q.; Liu, L.; Liang, S.; Dong, Q.; Jin, B.; Bai, R., Preparation and characterization of composite membranes with ionic liquid polymer-functionalized multiwalled carbon nanotubes for alkaline fuel cells. RSC Advances 2013, 3, (32), 13477-13485. 6. Liu, Y.; Kumar, S., Polymer/carbon nanotube nano composite fibers–a review. ACS Appl. Mater. Interfaces 2014, 6, (9), 6069-6087. 7. Kumar, S.; Rani, R.; Dilbaghi, N.; Tankeshwar, K.; Kim, K.-H., Carbon nanotubes: a novel material for multifaceted applications in human healthcare. Chem. Soc. Rev. 2017, 46, (1), 158196. 8. Shin, S. R.; Shin, C.; Memic, A.; Shadmehr, S.; Miscuglio, M.; Jung, H. Y.; Jung, S. M.; Bae, H.; Khademhosseini, A.; Tang, X. S., Aligned Carbon Nanotube–Based Flexible Gel Substrates for Engineering Biohybrid Tissue Actuators. Adv. Funct. Mater. 2015, 25, (28), 4486-4495.


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Table of Contents

Micelle-encapsulated multiwalled carbon nanotubes (E-CNT-PAA) with polymer brushes were prepared and used for fabrication of mechanically robust, stimuli-responsive composite hydrogels by in situ polymerization of N-isopropylacrylamide.


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Preparation of Hydrophilic Encapsulated Carbon Nanotubes with

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