Structure and Photoresponsive Behaviors of Multiwalled Carbon

Jul 7, 2007 - Huazhong University of Science and Technology. ‡ Xinjiang University. § City University of Hong Kong. | The University of Sydney. 112...
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J. Phys. Chem. C 2007, 111, 11231-11239

11231

Structure and Photoresponsive Behaviors of Multiwalled Carbon Nanotubes Grafted by Polyurethanes Containing Azobenzene Side Chains Yingkui Yang,† Xiaotao Wang,† Lang Liu,‡ Xiaolin Xie,*,† Zhifang Yang,† Robert Kwok Yiu Li,*,§ and Yiu-Wing Mai| Hubei Key Laboratory of Materials Chemistry and SerVice Failure, Department of Chemistry and Chemical Engineering, Huazhong UniVersity of Science and Technology, Wuhan 430074, China, Institute of Applied Chemistry, Xinjiang UniVersity, Urumqi 830046, China, Department of Physics and Materials Science, City UniVersity of Hong Kong, Tat Chee AVenue, Kowloon, Hong Kong, China, and Center for AdVanced Materials Technology, School of Aerospace, Mechanical and Mechatronic Engineering, J07, The UniVersity of Sydney, Sydney, New South Wales 2006, Australia ReceiVed: April 12, 2007; In Final Form: May 26, 2007

Two kinds of photoresponsive azobenzene polyurethane-functionalized multiwalled carbon nanotubes (AzoPUMWNTs) were successfully synthesized by in situ polycondensation of azobenzene monomer containing bishydroxyl (AzoM) with two types of diisocyanates, an aliphatic diisocyanate (HDI) as soft monomer and an aromatic diisocyanate (TDI) as rigid monomer, in the presence of MWNTs with terminated multihydroxyl groups (MWNT-OH), and their photochemical behaviors were investigated. Fourier transform infrared (FTIR), Raman, and 1H NMR spectra and transmission electron microscopy (TEM) revealed that azobenzene polyurethanes were covalently grafted onto the surfaces of MWNTs, forming core-shell structures with MWNT as hard core and polymer layer as soft shell, and the average thickness of the grafted polymers was about 7-10 nm. Thereby, evident improvements in the solubility of insoluble MWNTs and thermal stability of the azo polyurethanes were simultaneously obtained. The AzoPU-MWNTs showed reversible photoisomerism behavior. When they were compared to their parent polymers (AzoPUs), the photoisomerization rate constants of AzoPU-MWNTs decreased due to the heat sinks and steric effects of MWNTs. However, the responsive rate constant of AzoPU-MWNT could be effectively controlled by adjusting the main-chain flexibility of the grafted polyurethanes on MWNTs. This would play a key role in developing novel high-performance optic and photonic nanodevices.

Introduction Carbon nanotubes (CNTs) have been proposed for many applications due to their unique electronic, optical, mechanical, thermal, and structural properties.1 However, many applications require functionalization to make them more amenable and rational to control the final properties of the CNT-based materials. Hence, a large number of functionalization reactions for CNTs have been suggested,2 which may be divided into four categories: (a) endohedral filling or doping the inner cavity of CNTs,3 (b) exohedral inorganic functionalization of CNTs with nonmetallics4 or metals5 and metallic compounds,6 (c) exohedral organic functionalization of CNTs with organic molecules, and (d) polymerization of CNTs themselves by chemical reactions.7 The third category involves noncovalent and covalent modifications. Noncovalent methods include surfactant modification,8 π-π interaction,9 and polymer wrapping.10 Covalent functionalization can be further grouped into three types: (i) carboxyl reactions based on the CNT-bound carboxylic groups induced by oxidation, such as amidation,11 esterification,12 and ionic association;13 (ii) addition reactions, * Corresponding authors: (X.X.) tel +86-27-8754-0053, fax +86-278754-3632, e-mail [email protected]; (R.K.Y.L.) tel +852-2788-7785, e-mail [email protected]. † Huazhong University of Science and Technology. ‡ Xinjiang University. § City University of Hong Kong. | The University of Sydney.

that is, a direct attachment of organic molecules to the CNT surfaces via covalent bonding, such as hydrogenation,14 halogenation,15 radical addition,16 nucleophilic addition,17 electrophilic addition,18 carbene19 and nitrene20 cycloadditions, 1,3dipolar cycloadditions,21 Bingel cyclopropanation,22 and DielsAlder cycloadditions;23 and (iii) in situ polymerization of monomers in the presence of CNT-supported macroinitiators, such as free radical polymerization,24 atom transform radical polymerization (ATRP),25 reversible addition fragmentation chain-transfer (RAFT),26 nitroxide-mediated free-radical polymerization,27 ring-opening polymerization,28 anionic polymerization,29 and polycondensation.30 Subsequently, polymerfunctionalized CNTs represented an emerging field due to their higher solubility, dispersibility, and better manipulation and processability for the nanotube-based materials.24-32 Similar organic-inorganic hybrids have offered many possibilities for developing novel functional materials whose physicochemical properties are superior to those of the parent materials.33 Recently, photoresponsive functional materials, such as azobenzene chromophores, have attracted increasing attention due to their diverse potential applications, such as actuators,34 optical switches,35 optical data storage,36 and liquid crystal displays.37 It is well-known that azobenzene chromophores can undergo reversible photoisomerization between the stretched trans and the bent cis isomers when exposed to certain wavelengths or heating, which leads to considerable changes in their molecular shape, size, and dipole moments as well as

10.1021/jp0728510 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/07/2007

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Figure 1. Synthesis of bis-hydroxyl azobenzene monomers (AzoM).

optical properties.38 Incorporating the azobenzene chromophores into the inorganic silica network could provide good thermal stability, excellent nonlinear optical properties, and reversible photoisomerization.39 In this work, we synthesized the photoresponsive azobenzene polyurethane-modified multiwalled carbon nanotubes (AzoPUMWNTs) using a polycondensation approach. The motivation for our studies is to find interesting properties and potential applications in the stimuli-responsive materials and devices. The acceptable mechanical properties of conventional polyurethanes disappear above 80-90 °C and thermal degradation occurs above 200 °C due to their poor heat resistance,40 and the mechanical strength of their films is almost too weak to prepare real films or tubes,41 which has greatly limited their applications. Incorporating CNTs into the polyurethane matrix will effectively enhance their thermal stability and mechanical strength.42 In addition, the relatively rigid CNT backbone will prevent azobenzene aggregation to some extent and therefore provide high photoreactivity, which may be utilized as an efficient “command surface” with response to external stimulus.43 Especially, light-induced changes of the conformation of azobenzene are affected by various factors, such as polarity, viscosity, and free volume distribution of the local environment around the chromophores,44 which has been used to probe the distribution of local free volume in polymer systems.45 Recently, Thostenson and Chou46 also utilized MWNTs dispersed in the epoxy phase as distributed sensors to evaluate the onset, nature, and progression of damage in glass fiber/epoxy composites. Considering that the azobenzene polymer-grafted MWNTs act as molecular brushes with a nanotube backbone and densely grafted photoresponsive side chains, they may have many potential applications for tunable optic and photonic devices. To our best knowledge, there are no reports on azobenzene polymers to functionalize the surfaces of CNTs. Experimental Section Materials. Pristine MWNTs (purity >95%, with an average diameter of 50 nm) were purchased from Shenzhen Nanotech Port Co., Ltd., China. Hexamethylene diisocyanate (HDI) (Acros Organics, 99%+) was used without further purification. Tolylene 2,4-diisocyanate (TDI), diethanolamine (DEOA), and dimethylformamide (DMF) were purified by vacuum distillation before use. Sulfuric acid (H2SO4, 98%), nitric acid (HNO3, 67%), hydrochloric acid (HCl, 36.5%), thionyl chloride (SOCl2), phenol, 4-ethoxyaniline, potassium carbonate (K2CO3), potassium hydrogen carbonate (KHCO3), sodium nitrite (NaNO2), 1,6-dibromohexane, hydroquinone, sodium hydroxide (NaOH), dibutyltin dilaurate (DBTDL), absolute ethanol, absolute methanol, petroleum ether (30-60 °C), acetone, anhydrous magnesium sulfate (MgSO4), chloroform (CHCl3), ethyl acetate, dimethylacetamine (DMAC), dimethyl sulfoxide (DMSO), 1-methyl-2-pyrrolidinone (NMP), and other chemicals were used

as received from Sinopharm Group Chemical Reagent Co., Ltd, Shanghai, China. Measurements. Fourier transform infrared (FTIR) spectra were recorded on a Bruker Equinox 55 spectrometer with a disc of KBr. 1H NMR spectra were carried out on a Bruker AV400 spectrometer with TMS as an internal standard. Raman spectra were recorded on a confocal Raman spectrometer (LabRAM HR, Jobin Yvon Co.) with He-Ne laser excitation at 632.8 nm. Thermal gravimetric analysis (TGA) was conducted on a TGA-7 Perkin-Elmer calorimeter under argon flow (20 mL/min) at 10 °C/min. Transmission electron microscopy (TEM) analysis was performed on a Tecnai G220 electron microscope at 200 kV. Scanning electron microscopy (SEM) images were recorded on a JEOL JSM- 6700F field-emission microscope. UV-vis absorption spectra were recorded at room temperature in DMF solution on a diode-array spectrophotometer (Shimadzu UV2550). The spectral region 600-270 nm was examined by using a cell path length of 10 mm. Photoisomerization experiments were carried out by using 365 nm light from an ultraviolet lamp (16w, zf-7A, Shanghai Gucun Electron Optic Instrument Factory). The optical pictures of the samples in solvents were taken with a digital camera (Olympus C-4000 ZOOM). Synthesis of Bis-hydroxyl Azobenzene Monomers (AzoM). Though the synthesis of azobenzene monomers, 1-[bis(2-hydroxyethyl)amino]-6-(4-ethoxyazobenzene-4′-oxy)hexane (AzoM), has not been published to our knowledge; some related reports47 can be used as references to synthesize AzoM, as shown in Figure 1. 4-Hydroxy-4′-ethoxyazobenzene (HEAB). A solution of sodium nitrite (15.047 g, 218 mmol) in deionized water (100 mL) was slowly added to the solution of 4-ethoxyaniline (28.411 g, 207 mmol) in hydrochloric acid (3 M, 250 mL) through a dropping funnel over a 40-min period. The solution was kept at 0 °C for 2 h with vigorous mechanical agitation. The resultant diazonium salt solution with reddish-brown color was then slowly added to the solution of phenol (20.451 g, 217 mmol) in aqueous NaOH (10%, 200 mL) at 0 °C and kept there for 2 h under stirring. The resulting yellow-brown suspension was acidified and filtered. The solid precipitate was washed with water until the pH of the filtrate reached 7. The crude product was recrystallized from the mixed solvent of ethanol and water (1/1 v/v) and dried under vacuum, giving 27.816 g of deepbrown acicular crystals. Yield 78.4%. IR (KBr, cm-1) 3324 (V O-H), 3100-3050 (V C-H, aromatic), 2980-2850 (V C-H, CH2, and CH3), 1600-1450 (V CdC, aromatic), 1383 (δ C-H, CH2, and CH3), 1250-1040 (V C-O). 1-Bromo-6-(4-ethoxyazobenzene-4′-oxy)hexane (BrEAB). HEAB (7.272 g, 30 mmol), 1,6-dibromohexane (44.017 g, 180 mmol), and anhydrous potassium carbonate (22.175 g, 161 mmol) were added to acetone (300 mL) under magnetic stirring, and the reaction mixture was heated to 75 °C and refluxed for 24 h. After the reaction was completed, the hot solution was

Photoresponsive Azobenzene Polyurethane MWNTs

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Figure 2. Synthesis of azobenzene polyurethane-grafted MWNTs (AzoPU-MWNT).

immediately filtered and the inorganic residues were washed thoroughly with hot acetone. The filtrates were collected and distilled under vacuum to remove acetone, and the remaining concentrated extracts were poured into cooled petroleum ether (30-60 °C). The precipitated product was filtered off and recrystallized twice with hot filtration from ethyl acetate to yield 6.423 g of yellow crystals. Yield 52.9%. IR (KBr, cm-1) 31003050 (V C-H, aromatic), 2980-2850 (V C-H, CH2, and CH3), 1600-1450 (V CdC, aromatic), 1394 (δ C-H, CH2, and CH3), 1250-1040 (V C-O). 1-[Bis(2-hydroxyethyl)amino]-6-(4-ethoxyazobenzene-4′oxy)hexane (AzoM). BrEAB (6.107 g, 15 mmol) and DEOA (4.725 g, 45 mmol) were dissolved in 150 mL of acetone, and the reaction mixture was stirred at 75 °C for 24 h. After the reaction, the mixed solution was evaporated under vacuum. The residues were dissolved in chloroform. The resulting chloroform solution and distilled water were added to a separating funnel and shaken vigorously to remove any excess of DEOA and hydrogen bromide (HBr) generated during the reaction until the water phase became neutral. The organic chloroform layer was dried over anhydrous magnesium sulfate to remove any water remaining in the solution. After removal of the solvent by reduced pressure distillation, the crude product was recrystallized twice from ethyl acetate and dried under vacuum to get 3.582 g of yellow acicular crystals. Yield 55.7%. IR (KBr, cm-1) 3450 (V O-H), 3100-3050 (V C-H, aromatic), 2980-2850 (V C-H, CH2, and CH3), 1600-1450 (V CdC, aromatic), 1395 (δ C-H, CH2, and CH3), 1250-1040 (V C-O). 1H NMR (400 MHz, CDCl3, δ, ppm) 7.70-7.73 (4H, Ar-H), 6.80-6.85 (4H, ArH), 3.86-4.00 (10H, -OCH2- and -OH), 3.25-3.30 (6H, -NCH2-), 1.65-1.80 (5H, -CH2CH2O- and -OCH2CH3), 1.25-1.50 [6H, -(CH2)3-]. Synthesis of Azobenzene Polyurethanes-Grafted MWNTs (AzoPU-MWNTs). Two kinds of AzoPU-MWNTs involving HDI and TDI, denoted as HAzoPUNT and TAzoPUNT, were synthesized by reaction of AzoM with two types of diisocyanate, an aliphatic diisocyanate (HDI) and an aromatic diisocyanate (TDI) in the presence of MWNT-OH. Their synthesis and chemical structure are shown in Figure 2. It should be noted that the hydroxyl groups were induced on MWNTs by reaction

Figure 3. FTIR spectra of (a) pristine MWNTs, (b) MWNT-OH, (c) HAzoPUNT, and (d) TAzoPUNT.

of the acid-oxidized MWNTs (MWNT-COOH) with excess thionyl chloride and then excess DEOA as described before.32a Typically for HAzoPUNT, 60 mg of MWNT-OH, 0.508 g (3 mmol) of HDI, and 10 mg of DBTDL as a catalyst were dispersed in 20 mL of DMF under nitrogen, and then the mixture was stirred at 80 °C for 8 h. Then the temperature was reduced to 0 °C, and the solution of AzoM (1.285 g, 3 mmol) in DMF (20 mL) was added dropwise to the mixture of MWNT-OH and HDI, followed by holding at 0 °C for 16 h and at 80 °C for 24 h, wherein the solution viscosity increased dramatically. After the reaction, the mixture was diluted with hot DMF (50 °C). The final products were achieved by sequential filtering and washing with hot DMF until the filtrate became colorless and no turbid mass was observed in methanol upon addition of the filtrate, generating darkish HAzoPUNT. The polymers (HAzoPU) collected from the filtrate were obtained by precipitation and extraction with methanol. 1H NMR (400 MHz, DMSO-d6, δ, ppm) 7.80 (Ar-H), 7.06 (Ar-H), 5.71 (N-H), 4.11-4.01 (-OCH2), 3.06-2.67 (N-CH2), 1.97-1.71 (-OCH2CH2, -OCH2CH3), 1.35-1.20 (-CH2-). The TAzoPUNT was prepared by the same procedure as HAzoPUNT except TDI was used instead of HDI without any catalyst. The free polymers (TAzoPUs) were also collected from

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Figure 4. 1H NMR spectra of polyurethane segments within (A) HAzoPUNT and (B) TAzoPUNT.

the filtrate. 1H NMR (400 MHz, DMSO-d6, δ, ppm) 8.92, 8.73 (N-H), 7.95-7.07 (Ar-H), 4.11-4.01 (-OCH2), 2.90-2.73 (N-CH2), 2.12 (Ar-CH3), 1.97-1.70 (-OCH2CH2, -OCH2CH3), 1.41-1.20 (-CH2-). Results and Discussion Synthesis and Structure of AzoPU-MWNTs. The azobenzene monomer (AzoM) is synthesized by a diazo coupling reaction between 4-ethoxyaniline and phenol in the presence of sodium nitrite and hydrochloric acid, followed by a substitution reaction with 1,6-dibromohexane and diethanolamine (DEOA) (Figure 1). The 1H NMR and FTIR spectra are consistent with the proposed chemical structures (see Supporting Information). The synthesis of azobenzene polyurethane-grafted multiwalled carbon nanotubes (AzoPU-MWNTs) is shown in Figure 2. First, the acid-oxidized MWNTs (MWNT-COOH) react with excess thionyl chloride and then excess DEOA, which induces bishydroxyl groups onto the surfaces of MWNTs (MWNT-OH). The AzoPUs are anchored to MWNTs by in situ polycondensation of diisocyanate and AzoM with bis-hydroxyl groups in the presence of MWNT-OH. Usually, the synthesis is started at a higher temperature (80 °C) to speed up the reaction of diisocyanate and hydroxyl groups on the MWNT surfaces, forming the NCO-group-functionalized MWNT (MWNTNCO) to further increase the reactivity of MWNTs.32a This temperature is kept for 8 h and subsequently reduced to 0 °C, and then AzoM is slowly added. After 16 h, this low temperature is raised to 80 °C and held for 24 h with an increased viscosity. It is noticed that the reactivity of the NCO group in TDI with an aromatic ring is much faster than that of NCO in HDI with aliphatic structure.48 Thus, the synthesis involving TDI is fast and does not need any catalyst, whereas it is necessary to use DBTDL as a catalyst to activate the reaction of HDI. The adsorbed free polymers are efficiently removed from the resulting products by filtering and washing as mentioned for other polymer-functionalized MWNTs.24a,32a The resulting HAzoPUNT and TAzoPUNT are readily soluble in DMF,

Figure 5. Raman spectra excited at 632.8 nm of (a) oxidized MWNTs (MWNT-COOH), (b) HAzoPUNT, (c) TAzoPUNT, (d) a mixture of MWNT-OH and 65% neat HAzoPU, and (e) HAzoPU.

DMSO, and THF as compared with crude MWNTs, which will provide good processing and manipulation ability for MWNTs. In the FTIR spectrum of pristine MWNTs, a weak peak at 1580 cm-1 corresponds to the CdC stretch of the nanotube backbones (Figure 3a). The strong O-H and CdO stretching (amide I) vibrations generated by the reaction between the acylated MWNTs (MWNT-COCl) and diethanolamine appear at 3429 and 1632 cm-1, respectively (Figure 3b). The FTIR spectrum of HAzoPUNT (Figure 3c) is very similar to that of TAzoPUNT (Figure 3d) due to their similar characteristic vibration bands in the molecular structures (in KBr: V N-H at 3500-3200, V Ar-H at 3070, V C-H at 2950-2850, V CdO at 1720-1650, aromatic V CdC at 1600 and 1500, amide II band at 1530, δ C-H at 1450-1300, amide III band at 1260, and V C-O at 1250-1040 cm-1). However, the proton signals of the grafted polyurethane chains in HAzoPUNT and TAzoPUNT have an obvious difference in 1H NMR spectra (Figure 4). For TAzoPUNT, two N-H proton signals of the urethane linkage are observed at 8.92 and 8.73 ppm (Figure 4B), and the methyl group in TDI causes nonequivalency of the N-H protons.49 In

Photoresponsive Azobenzene Polyurethane MWNTs

Figure 6. TGA curves of (a) pristine MWNTs, (b) HAzoPUNT, (c) TAzoPUNT, (d) HAzoPU, and (e) TAzoPU under argon (10 °C/min).

contrast, the singlet N-H proton peak of HAzoPUNT is clearly detected at 5.71 ppm due to the symmetric molecular structure of HDI (Figure 4A). The aromatic protons (Figure 4B, peaks c, e, and f) in TDI are also seen at 7.95, 7.51, and 7.22 ppm as a singlet resonance. The other proton peaks of azobenzene polyurethane segments in HAzoPUNT and TAzoPUNT are clearly observed in 1H NMR spectra due to the high-density hairy polymers grown from MWNTs. The Raman spectra of HAzoPUNT (Figure 5b) and TAzoPUNT (Figure 5c) show characteristic disorder-band peaks at ca. 1325 (D-band) and 1610 cm-1 (D′-band) and tangentialmode peaks at ca. 1570 cm-1 (G-band),50 similar to those of the oxidized MWNTs (MWNT- COOH) (Figure 5a) except that their relative intensity changes due to the existence of the grafted polymer. The Raman signals of the grafted polymer on the MWNT surfaces are not observed for HAzoPUNT and TAzoPUNT, though the Raman signals of neat polymer appear very strong (Figure 5e). However, for the mixture of MWNTOH and HAzoPU with comparable polymer content (65%) relative to HAzoPUNT, both the polymer and nanotubes can be detected in the Raman signals, respectively (Figure 5d), which is very different from AzoPU-MWNT. This indicates that the grafted-polymer layer might form a discontinuous phase

J. Phys. Chem. C, Vol. 111, No. 30, 2007 11235 owing to the random distribution of defects on the nanotube surfaces, which makes it difficult or even impossible to totally absorb and reflect all the excited energy for the polymer phase. Hence, only the nanotube signals in Raman spectrum can be detected when the polymer is covalently grafted to the MWNT surfaces. Conversely, the mixture of polymer and nanotubes forms two macroscopic phases as a result of two separated Raman signals of both components. This result is in agreement with other polymer-grafted MWNTs.30 The amount of grafted polyurethanes covalently attached in HAzoPUNT and TAzoPUNT can be determined by thermal decomposition. Figure 6 shows the TGA curves of pristine MWNTs, HAzoPUNT, TAzoPUNT, HAzoPU, and TAzoPU under argon (10 °C/min). Only 1.3% weight loss for the pristine MWNTs is detected at 700 °C (Figure 6a), and the neat polyurethanes (Figure 6d,e) give full weight loss at 700 °C. Therefore, almost all of the weight loss during fragmentation is due to the pyrolysis of the grafted polyurethane segments, which corresponds to 65.2% and 70.3% polymer content for HAzoPUNT (Figure 6b) and TAzoPUNT (Figure 6c), respectively. For HAzoPUNT, three weight-loss regions, 230400, 400-500, and 500-700 °C, are due to the decomposition of azobenzene double bond, the residual side chains, and the polyurethane backbone on the MWNT surfaces, respectively. Similar decomposition is observed for TAzoPUNT, HAzoPU, and TAzoPU. The onset of decomposition temperature of the grafted polyurethane moieties in AzoPU-MWNTs is about 30 °C higher than that of neat polyurethanes, which is due to the constraint effect of MWNTs on the grafted polyurethane moieties and the high thermal conductivity of MWNTs themselves. The nanostructures of the MWNT samples are observed by TEM and SEM. As shown in Figure 7, the surfaces of MWNTOH (Figure 7B) seem to be smooth and clear due to the attachment of small molecular diethanolamine, similar to MWNT-COOH (Figure 7A). However, the polyurethanegrafted MWNTs show evident core-shell structures with MWNT as the hard core and the polymer layer as the soft shell, and the average thickness of the grafted polymers in HAzoPUNT (Figure 7C,D) and TAzoPUNT (Figure 7E,F) is in the range of

Figure 7. TEM images of (A) MWNT-COOH, (B) MWNT-OH, (C, D) HAzoPUNT, and (E, F) TAzoPUNT.

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Figure 8. SEM images of (A) pristine MWNT, (B) HAzoPUNT, and (C) TAzoPUNT.

Figure 10. First-order plots for trans-cis isomerization of (O) HAzoPU, (]) TAzoPU, (b) HAzoPUNT, and ([) TAzoPUNT.

Figure 9. UV-vis spectra of (A) neat TAzoPU and (B) TAzoPUNT in DMF upon irradiation with 365 nm UV light at room temperature. Arrows indicate changes upon irradiation.

7-10 nm. From the SEM images of HAzoPUNT (Figure 8B) and TAzoPUNT (Figure 8C), they appear relatively thicker compared to the crude MWNTs (Figure 8A) caused by the grafted polyurethanes on the MWNT surfaces. These results further confirm the successful functionalization of MWNTs. Photochemical Behaviors. Since the polyurethanes containing large numbers of azobenzene units in their side chains are covalently grafted onto the MWNT surfaces, the photoresponsive properties of azobenzene should be seen in AzoPUMWNTs. All samples were irradiated with 365 nm unpolarized UV light until they reached the photostationary state. The changes in the UV-vis spectra with different irradiation time are shown for TAzoPU and TAzoPUNT solutions in DMF in Figure 9. The absorption spectra of trans-azobenzene moieties show a high-intensity π-π* transition at 360 nm and a lowintensity π-π* and n-π* transition of cis isomer at 310 and 460 nm, respectively. After UV irradiation, the absorbance at 360 nm decreases significantly while the absorbance at 310 and 460 nm increases slightly for trans-cis photoisomerization. Especially, the absorbance change of the TAzoPUNT solution appears negligible in the n-π* transition at 460 nm (Figure 9B). Similar phenomena can be observed for the HAzoPU and

Figure 11. Conformational changes of azobenzene moieties in AzoPUMWNT.

HAzoPUNT solutions. The corresponding first-order rate constants of π-π* transition (trans to cis isomer, ktc) are determined by fitting the experimental data to51

ln

A ∞ - At ) -ktct A∞ - A 0

where At, A0, and A∞ are absorbance at 360 nm at time t, time zero, and infinite time, respectively. The first-order plots according to this equation for trans-cis isomerization of azobenzene moieties are shown in Figure 10. The slope corresponds to the value of ktc. Therefore, ktc of the TAzoPU solution (k1tc, 0.0582 ( 0.0011 min-1, R ) 0.9989) is fractionally smaller than that of the HAzoPU solution (k2tc, 0.0634 ( 0.0022 min-1, R ) 0.9964), which indicates a slower photoisomerization rate due to a relatively more rigid polymer backbone involving the use of TDI instead of HDI. It should be mentioned that when TAzoPU are grafted to MWNTs, the photoisomerization rate constant of the TAzoPUNT solution (k3tc, 0.0075 ( 0.0008 min-1, R ) 0.9731) is further reduced by the incorporation of the rigid segments of MWNTs. Interestingly, when the soft main-chain polyurethanes (HAzoPU) are

Photoresponsive Azobenzene Polyurethane MWNTs

Figure 12. Optical photographs of (A) neat HAzoPU and (C) HAzoPUNT in DMF solutions before irradiation and of (B) HAzoPU and (D) HAzoPUNT after UV irradiation for 24 h at 365 nm.

grafted on the MWNTs surfaces, an increased rate constant for the HAzoPUNT solution (k4tc, 0.0120 ( 0.0023 min-1, R ) 0.9327) is observed as compared with k3tc. These results imply that the responsive rate constant of isomerization of AzoPUMWNT can be effectively controlled by adjusting the mainchain flexibility of the grafted polyurethanes on MWNTs. As discussed above, the photoisomerization rates of polyurethane-grafted MWNTs are clearly slower than those of corresponding parent polyurethanes. The results can be explained by the following mechanism:51b,52 AI

kd

A y\ z A* 98 B k r

ktc ) A I kd/(kd + kr) where A is the trans isomer, A* is the excitation state, B is the cis isomer, I is beam intensity, and kr and kd are rate constants of A* reversing to A and conversing to B, respectively. Under the experimental conditions, I is a constant and A (molar absorption coefficient) can also be regarded as a constant for similar azobenzene segments. The values of kr and kd are related to the properties and chemical environment of azobenzene in molecules.51,52 Notice that the nanotubes in AzoPU-MWNTs can act as nanometric heat sinks,53 which will dissipate energy from UV irradiation, resulting in delay to the excited state (A*) after photon absorption. Also, photoisomerization of azobenzene moieties in AzoPU-MWNTs is restricted due to the relative large-scale MWNTs, although the azobenzene moieties are located in the side chains. Especially, the azobenzene moieties show a large change in the molecular length, in which the distance between 4- and 4′-carbons reduces from trans to cis isomer upon UV irradiation.54 Thus, with increasing irradiation time, the distance between 4- and 4′-carbons in azobenzenes is gradually decreased (Figure 11), and the restriction effect caused by the rigid rodlike nanotubes further increases. This increase is due to a combination of the cooperative motion of neighboring units and steric effects. Hence, first-order plots corresponding to k3tc (TAzoPUNT) and k4tc (HAzoPUNT) were slightly curved (Figure 10), indicating a further decrease in trans-cis isomerization rate. A distinct deviation from the first-order kinetics has also been observed in the azobenzene-containing silica hybrids55 and glass-state azobenzene-containing polymers.56 After irradiation by UV light, the colors of the HAzoPU and HAzoPUNT solutions in DMF were significantly changed as compared with their original states but still appeared optically transparent (Figure 12), which is due to trans-cis photochemical isomerization. When the UV-irradiated solutions of HAzoPU and HAzoPUNT were kept in the dark for several days, the original color of solutions was gradually recovered and showed the same UV-vis spectra as those before irradiation. This is

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Figure 13. 1H NMR spectra of HAzoPUNT in DMSO-d6 (A) without UV irradiation, (B) after UV irradiation for 24 h at 365 nm, and (C) kept in the dark for 144 h.

caused by a thermal cis-trans back isomerization since the trans form corresponds to an energy lower by about 48 kJ/mol.57 This indicates that the photoisomerization behavior of the azobenzene moieties in AzoPU-MWNT is reversible, similar to those of azobenzene units attached on a quartz plate.58 The reversible trans-cis-trans isomerization is further demonstrated by the 1H NMR spectra, as shown in Figure 13. When the HAzoPUNT dissolved in DMSO-d6 was irradiated at 365 nm UV light for 24 h, the 1H NMR spectrum (Figure 13B) drastically changed, especially in the aromatic region, as compared with its original state (Figure 13A). However, after the UV-irradiated solutions were kept in the dark for 144 h, the 1H NMR spectrum (Figure 13C) was restored. This is because of the reversible expandability and contractibility of trans-cis-trans isomers, resulting in disturbance of the protons environment surrounding the azobenzene chromophores (Figure 11).59 Conclusions Two kinds of photoresponsive azobenzene polyurethanegrafted multiwalled carbon nanotubes (AzoPU-MWNTs) have been successfully synthesized by reaction of azobenzene monomer containing bis-hydroxyl (AzoM) with two types of diisocyanate, an aliphatic diisocyanate (HDI) as soft monomer, and an aromatic diisocyanate (TDI) as rigid monomer in the presence of MWNT-OH. AzoPU-MWNTs show evident coreshell structures with MWNT as the hard core and the polymer layer as the soft shell, and the average thickness of the grafted polymers is about 7-10 nm. Thus, obvious improvements in solubility of the insoluble MWNTs and thermal stability of the azo polymers are obtained at the same time. AzoPU-MWNTs show reversible photoisomerism behavior. Compared with neat AzoPU, the photoisomerization rate constants of AzoPUMWNTs decrease due to the heat sinks and the steric effects of MWNTs. However, the responsive rate constant of AzoPUMWNTs can be effectively controlled by adjusting the mainchain flexibility of grafted polyurethanes on MWNTs. This, therefore, provides an effective way to develop novel highperformance optic and photonic nanodevices. Acknowledgment. We are grateful for the financial support of the National Natural Science Foundation of China (20474021), National Basic Research Program of China (61337), Program for New Century Excellent Talents in Universities of China (NCET-05-0640), Open Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University), and the Australian Research Council (ARC) for their continuing support of the project on “Polymer Nanocomposites”. Y.-W.M. acknowledges the award of an Australian Federation Fellowship by the ARC tenable at the University of Sydney.

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