Chinese Chemical Letters  2015, Vol.26 Issue (07):857-861   PDF    
Synthesis, self-assembly and photo-responsive behavior of AB2 shaped amphiphilic azo block copolymer
Ren-Bo Wei, Xiao-Gong Wang, Ya-Ning He     
Department of Chemical Engineering, Key Laboratory of Advanced Materials (MOE), Tsinghua University, Beijing 100084, China
Abstract: In this work, we report the synthesis of AB2 shaped amphiphilic azo block copolymer by macromolecular azo coupling reaction. The product and intermediates were characterized by various methods. The selfassembly in selected solvents and photo-responsive behavior of the copolymer were studied bymeans of UV-vis spectrophotometry and TEM. Spherical aggregates can be obtained by gradually adding water into the solution of this amphiphilic azo block copolymer. Upon irradiation with polarized UV (365 nm) light, the aggregates can be elongated in the polarized direction.
Key words: Azo block copolymer     Self-assembly     Spherical aggregates     Photoinduced deformation     Macromolecular azo-coupling reaction    
1. Introduction

Azobenzene and its derivative could alter their configuration from the generally more stable trans form to the less stable cis form upon irradiation with suitable light while the cis form would go back to the trans form if another light or heat is used. The configuration changing induced by light can conspicuously influence the bulk and surface properties of the materials [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. The photoresponsive properties and possible applications of azo polymers include photoinduced dichroism and birefringence [1, 2],photoinduced phase transition [11],surface-relief-grating (SRG) [12, 13],two-dimensional surface quasi-crystal structure [14], spontaneous surface pattern [15],photo-mechanical thin film contraction and bending [16, 17, 18],light-responsive block copolymer micelles [19],photoinduced colloidal deformation [20] and many others.

The amphiphilic block copolymers,usually consisting of a hydrophobic block and a hydrophilic block,can self-assemble in selected solvents to form various polymeric aggregates such as nanotubes,vesicles,colloids,and spherical micelles,which can be used in drug delivery systems,nano-reactors and bio-diagnostics [21, 22, 23, 24, 25]. By incorporating the azo moieties into the amphiphilic block copolymers,the obtained aggregates show many interesting properties [26, 27, 28, 29]. In recent years,we have reported that polarized visible laser beam (488 nm) irradiation can induce significant deformation of the self-assembly colloids,which were prepared by using amphiphilic copolymers with electron-withdrawing azo groups [20, 30, 31, 32, 33, 34].

Compared with linear block polymers,AB2 shaped block copolymers have three building blocks linked to a single junction point,which leads to many interesting properties [35, 36]. However,report on AB2 shaped amphiphilic azo block copolymer is still lacking in the literature. In this work,an AB2 shaped amphiphilic azo block copolymer by macromolecular azo coupling reaction was reported. The obtained amphiphilic azo block copolymer showed liquid crystal properties. By gradually adding water into THF solution of this amphiphilic azo block copolymer, spherical aggregates were obtained. The self-assembly spherical aggregates can be elongated by polarized UV light (365 nm) in the polarized direction. 2. Experimental

4-Aminobenzonitrile (98%) and 2-bromoisobutyryl bromide (97%) were purchased from Alfa Aesar. 1,1,4,7,10,10-Hexamethyltriethylenetetramine (HMTETA,97%) was purchased from J&K Chemical. The ATRP initiator and PEG113-NH2 was prepared by method in literature [37, 38]. The azo monomer 6-(4-cyanoazobenzene-4'-oxide)-hexyl methacrylate was prepared by method in literature [10]. Anisole and THF were distilled from sodium with benzophenone prior to use. Deionized water (resistivity >18 MΩ cm) was prepared using the Milli-Q water purification system. All other reagents were commercially available and used as received without further purification. 1H NMR spectra were recorded on a JOEL JNM-ECA600 spectrometer using DMSO-d6 or CDCl3 as the solvent and tetramethylsilane as the internal standard. Fourier transform infrared spectrum (FT-IR) measurements were carried out on a Nicolet 560-IR spectrophotometer by incorporating the samples in the KBr tablets. UV-vis spectra were recorded by using an Agilent 8453 UV-vis spectrophotometer. The molecular weights and molecular weight distributions were measured by using gel permeation chromatography (GPC). The GPC instrument was equipped with a PLgel 5 μm mixed-D column and a refractive index (RI) detector (Wyatt Optilab rEX). The measurements were carried out at 35 °C and the molecular weights were calibrated with polystyrene standards. THF was used as the eluent with the flow rate of 1.0 mL/min. Thermal properties of the polymers were tested using TA Q2000 system with a heating rate of 10 °C/min at the nitrogen atmosphere. Polarizing microscopic (POM) observations were conducted on a Nikon LV 1000 POL microscope equipped with a Nikon DS-Fi2 CCD camera,Nikon DSU3 digital sight and a Linkam LTS420E hot stage. TEM images were obtained using a JEOL-JEM-1200EX electron microscope with an accelerating voltage of 120 kV. The TEM samples were prepared by casting diluted aggregate dispersions onto the copper grids coated with a thin polymer film and then dried in a 30 °C vacuum oven for 12 h. No staining treatment was performed for the measurement. 2.1. ATRP initiator

To a CH2Cl2 (20 mL) solution of a mixture of N,N-dihydroxyethylaniline (1.81 g,10 mmol) and triethylamine (7 mL) under ice bath was added 2-bromoisobutyryl bromide (6.9 g,30 mmol) in 20 mL CH2Cl2. The solution was stirred at room temperature overnight. To the resulting solution was added CH2Cl2 and washed with water,dried over anhydrous MgSO4. After solvent removal,column chromatography (SiO2,ethyl acetate:petroleum ether = 1:6) was undertaken to afford the product. Yield: 95%. 1H NMR (600 MHz,DMSO-d6): δ 1.88 (s,12H,-CH3),3.70 (t,4H,-CH2-N),4.29 (t,4H,-CH2-O),6.64 (t,1H,ArH),6.80 (d,2H,ArH),7.17 (d,2H,ArH). 2.2. Azo polymer with anilino functionality (2PAzo8) CuBr (57.4 mg,0.4 mmol) was added to a Schlenk flask. Then it was degassed and back-filled with argon three times. Following this step,deoxygenated ATRP initiator (0.05 g, 0.1 mmol),1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) (109 μL,0.4 mmol),and 6-(4-cyanoazobenzene-4'-oxide)-hexyl methacrylate (0.63 g,1.6 mmol) were added via gas-tight syringes which had been previously purged with argon. After degassing by three freeze-pump-thaw cycles,the flask was immersed in an oil bath preheated to 80 °C. After the polymerization for 72 h,the reaction mixture was diluted with THF and passed through an alumina column to remove catalyst. The filtrate was concentrated and poured into an excess amount of petroleum ether. The precipitate was collected by filtration,washed with petroleum ether and then dried in a vacuum oven for 24 h. The conversion of the monomers can be calculated by Eq. (1) on the basis of the integration areas of the peaks at 6.10 ppm (I6.10) and 6.90 ppm (I6.90),corresponding to the vinyl proton of the methacrylate group of the remaining monomer and the protons of the phenyl group at the ortho-position to the ether group:

The conversion estimated by this method was approximately 100%,and the numbers of the repeat unit of 2PAzo was 8 according to the feeding ratio. GPC: Mn = 1.1 × 104,and PDI = 1.27. 1H NMR (600 MHz,CDCl3): δ 7.82 (m,ArH),7.68 (m,ArH),6.90 (m,ArH), 3.94 (m,-CH2),2.03-0.91 (m,-CH2,-CH3). IR (KBr; cm-1): v 2941, 2862 (C-H; s),2225 (C≡N; s),1728 (C≡N; s),1599,1581,1500 (Benz. ring,s),1255 (C-O-C,s). DSC: Tg = 51.2 °C,TLC-I = 136.7 °C (heating). UV-vis: λmax = 363 nm (chloroform). 2.3. Amphiphilic azo block copolymer(PEG113-b-2PAzo8)

Precursor azo polymer with anilino functionality (2PAzo8) (0.44 g,0.04 mmol) was dissolved in DMF (50 mL) at 0 °C. A diazonium salt of PEG113-NH2 was prepared by adding an aqueous solution of sodium nitrite (14 mg,0.2 mmol in 0.6 mL of water) into a mixture of PEG113-NH2 (0.21 g,0.1 mmol,Mn = 5000),HCl (36%,0.07 mL) and H2O 1.0 mL. The mixture was stirred at 0 °C for 15 min and then was added dropwise into above DMF solution. The solution was stirred at 0 °C for 72 h and then poured into plenty of water. The precipitate was collected by filtration,washed with water and then dried in a vacuum oven for 24 h. Yield: 95%. GPC: Mn = 1.4 × 104,and PDI = 1.25. 1H NMR (600 MHz,CDCl3): δ 7.82 (m,ArH),7.68 (m,ArH),6.90 (m,ArH),3.94 (m,-CH2),3.63 (m,O- CH2-CH2-O),2.03-0.91 (m,-CH2,-CH3). IR (KBr,cm-1): v 2941, 2862 (C-H; s),2225 (C≡N; s),1728 (C=O; s),1599,1581,1500 (Benz. ring,s),1255 (C-O-C,s). DSC: TLC-I = 137.5 °C,TC = 54.3 °C (heating). UV-vis: λmax = 363 nm (chloroform). 2.4. Spherical aggregates preparation

A suitable amount of PEG113-b-2PAzo8 was first dissolved in anhydrous THF to form a homogeneous stock solution with the concentration of 0.5 mg/mL. To obtain the aggregates in dispersions,the deionized water (2 mL) was added dropwise into the THF solution (2 mL) via a syringe pump with a predetermined water addition rate of 7.2 mL/h. Then,excessive water was slowly added into the polymer dispersion to dilute it. The aggregates were finally obtained by slow evaporation of THF,which was carried out under the dust-free ambient condition at room temperature. 2.5. Photo-induced shape deformation

For photo-induced deformation of aggregate structure,sample was prepared by casting diluted aggregate dispersion onto the copper grids coated with a thin polymer film and then dried in a 30 °C vacuum oven for 12 h. The sample was irradiated by polarized light with the wavelength of 365 nm (5 h,LED lamp, 60 mW/cm2). The linearly polarized light was incident perpendicularly to the grid surface. The experiment was carried out at room temperature under an ambient condition. After the sample was irradiated,the TEM observation was performed to detect the shape deformation. 3. Results and discussion

The synthetic route of the amphiphilic azo block copolymer is outlined in Scheme 1. Firstly,an ATRP initiator was synthesized through the esterification between N,N-dihydroxyethylaniline and 2-bromoisobutyryl bromide. Then,the precursor azo polymer with anilino functionality (2PAzo8) was synthesized through ATRP by using the above initiator. Finally,the target block copolymer was obtained through the macromolecular azo-coupling reaction between 2PAzo8 and diazonium salt of PEG113-NH2.

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Scheme 1.The synthetic route of the amphiphilic azo block copolymer.

As anilines and its derivatives can be coupled with aromatic diazonium efficiently to give the azo compounds with high yield, we recently developed a new synthetic strategy to prepare diblock copolymers by azo coupling reaction between a macromolecular diazonium salt and a polymer with terminal anilino functionality[37, 38]. An ATRP initiator with a moiety (N-Ph) was designed and synthesized. The azo monomer used for the polymerization,6-(4- cyano-azobenzene-4'-oxide)-hexyl methacrylate,was prepared by using the literature method [10]. Then,the functionalized azo polymer 2PAzo8 with the moiety suitable for the azo coupling reaction was synthesized by ATRP. The PEG113-NH2 was synthesized by using the literature method [37, 38]. The diazonium salt of PEG113-NH2 was prepared by diazotization of PEG113-NH2 in acidic aqueous solution using NaNO2. Finally,the macromolecular azo coupling reaction between 2PAzo8 and the diazonium salt of PEG113-NH2 was carried out in a polar organic solvent such as DMF under mild conditions.

Fig. 1 shows the 1H NMR spectra of the azo homopolymer 2PAzo8 and the amphiphilic azo block copolymer PEG113-b-2PAzo8 in CDCl3. Comparing with 2PAzo8,the 1H NMR spectrum of PEG113- b-2PAzo8 shows the similar resonances signals of the protons on the azo block of the polymer. On the other hand,the new resonance signal appeared at δ 3.63 on the 1H NMR spectrum of PEG113-b- 2PAzo8 isfrom thePEG block,whichconfirms the successofthe azo coupling reaction between 2PAzo8 and the diazonium salt of PEG113-NH2.

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Fig. 1.1H NMR spectra (in CDCl3) of 2PAzo8 (line 1) and PEG113-b-2PAzo8 (line 2).

Both the precursor polymer 2PAzo8 and the final block polymer PEG113-b-2PAzo8 show liquid crystal properties. Fig. 2 shows the DSC curves of 2PAzo8 and PEG113-b-2PAzo8 on the second heating scan. 2PAzo8 shows a glass transition temperature (Tg) around 51.2 °C and an endothermic transition around 136.7 °C on the heating scan. Comparing with 2PAzo8 (Tg = 51.2 °C,TLCI = 136.7 °C),the new crystallization peak at 54.3 °C on PEG113- b-2PAzo8 curve is attributed to the PEG block. The POM micrographs of 2PAzo8 and PEG113-b-2PAzo8 (Fig. 3) confirm the liquid crystal properties.

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Fig. 2.DSC curves of 2PAzo8 (solid line) and PEG113-b-2PAzo8 (dash line), the second heating scan.

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Fig. 3.Polarizing optical micrographs of 2PAzo8 (A) and PEG113-b-2PAzo8 (B).

The UV-vis spectra of PEG113-b-2PAzo8 in solution irradiated with 365 nm light (0.8 mW/cm2) at varied time intervals is shown in Fig. 4(a). It can be seen that the max absorption of the copolymer is around 363 nm (π-π* absorption band). Upon irradiating,the intensity at 363 nm decreased while the intensity at 448 nm (n-π* absorption band) increased gradually. After about 70 s irradiation with 365 nm light,the solution reaches the photostationary state (PSS). Fig. 4(b) shows the UV-vis spectra of PEG113-b-2PAzo8 PSS solution irradiated with 450 nm light (2.4 mW/cm2) at varied time intervals. After about 100 s irradiation with 450 nm light,the UV- vis spectrum can recover its original state.

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Fig. 4.The UV–vis spectra of the solution of PEG113-b-2PAzo8 measured after the irradiation (a) with UV light at 365 nm for different time periods, then (b) with visible light 450 nm for different time periods, the inset is the relative absorbance at λmax and the corresponding fitted curve.

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Fig. 5.TEM images of the aggregates of PEG113-b-2PAzo8. (A) Original aggregates and (B) irradiated with polarized 365 nm light.

The self-assembly of PEG113-b-2PAzo8 was studied by adding water into its THF solution. First,a suitable sample was dissolved in anhydrous THF to form a homogeneous stock solution with the concentration of 0.5 mg/mL. To obtain the aggregates in dispersions,the deionized water (2 mL) was added dropwise into the THF solution (2 mL) via a syringe pump with a predetermined water addition rate 7.2 mL/h. Then,excessive of H2O was slowly added into the polymer dispersion to dilute it. Spherical aggregates were finally obtained by slow evaporation of THF,which was carried out under the dust-free ambient condition at room temperature. In order to determine the structures of the aggregates by TEM, samples were prepared by casting diluted aggregate dispersions onto the copper grids coated with a thin polymer film and then dried in a 30 °C vacuum oven for 12 h. Fig. 5A shows the image of the aggregates for PEG113-b-2PAzo8. The average diameter of the spherical aggregates is about 380 nm. To observe the photoinduced deformation of aggregate structure,the samples were irradiated perpendicularly by polarized light with the wavelength of 365 nm. After irradiation,the aggregates were significantly elongated along the polarization direction of the polarizer (Fig. 5B). The average major-to-minor axis ratio (l/d) of the aggregates are 2.37 ± 0.28. 4. Conclusion

In conclusion,an AB2 amphiphilic azobenzene containing block copolymer has been synthesized by macromolecular azo coupling reaction. The products and intermediates were characterized by various methods. By irradiating the solution of the sample with UV and visible light,the photo-induced reversible isomerization behavior of the azo block copolymer was observed. Spherical aggregates with the average diameter of about 380 nm were obtained by adding water into the THF solution of the block copolymer. When irradiated by polarized lights with the wavelength of 365 nm,the aggregates could be elongated along the polarization direction of the light with axis ratio (l/d) of 2.37 ± 0.28.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (No. 21474056).

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