Synthesis, photochemical properties, and self-assembly of diblock copolymer bearing azobenzene moieties

ARTICLE IN PRESS

JID: JTICE

[m5G;March 27, 2015;9:10]

Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10

Contents lists available at ScienceDirect

Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice

Synthesis, photochemical properties, and self-assembly of diblock copolymer bearing azobenzene moieties Po-Chih Yang a,∗, Chung-Yuan Li a, Hsin-Cheng Chen a, Ruey-Shin Juang b,∗∗ a b

Department of Chemical Engineering and Materials Science, Yuan Ze University, Chung-Li 32003, Taoyuan, Taiwan Department of Chemical and Materials Engineering, Chang Gung University, Kwei-Shan 33302, Taoyuan, Taiwan

a r t i c l e

i n f o

Article history: Received 21 October 2014 Revised 23 January 2015 Accepted 9 March 2015 Available online xxx Keywords: Diblock polymers E/Z isomerization Optical properties Azobenzene UV–vis spectroscopy

a b s t r a c t This study describes the synthesis of an azobenzene-containing diblock copolymer, poly(StO50 -b-Azo7 ), by using the macro-chain transfer agent StO macro-CTA, and employing sequential reverse addition-fragmentation transfer (RAFT) polymerization. We studied the effects of azobenzene (azo) unit on E/Z photoisomerization, the phase transition temperature, and self-assembly behavior of diblock copolymer, and evaluated the characteristic time involved in the decay process of the photoisomerization kinetics of diblock copolymer. Diblock copolymer poly(StO50 -b-Azo7 ) exhibited moderate thermal stability, with thermal decomposition temperature of 5% weight loss at approximately 340.9 °C, suggesting that the enhancement of the thermal stability was attributed to the incorporation of azo segments into block copolymer. The diblock copolymer showed lower E–Z photoisomerization rates (T1 = 68.0 s) compared with azo monomer (T1 = 10.95 s). Gradually adding water to the tetrahydrofuran (THF) solution of poly(StO50 -b-Azo7 ) produced spherical micelles. Spherical aggregates of poly(StO50 -b-Azo7 ) were obtained (mean diameter = approximately 181.4 nm) by diluting the polymer disperse in a mixture of THF/H2 O (water content = 10 vol%), and are shown in TEM images of the diblock copolymer. The results of this study contribute to research on the development of photoresponsive polymer materials. © 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Applying block polymers containing azobenzene (azo) chromophores as photosensitive materials has gained considerable academic and industrial attention in the past decade because such polymers exhibit favorable properties such as the photo induced orientation of the azo chromophores, liquid crystallize anisotropy, photomechanical bending, and surface-relief grating formations [1–4]. Ultraviolet (UV) light induced changes in configuration of polymer chains, resulting in E–Z isomerization or disorganization of chromophore fragments such as azo and stilbene, can be applied to provide a motor function for contractions in photoresponsive materials [5,6]. The self-assembly of block copolymers into various microphasesegregated nanostructures, such as spheres, cylinders, gyroids, and lamellae, has been widely researched as a novel approach to chemical synthesis, nanofabrication, and other applications in materials science [7–10]. Previous studies have prepared amphiphilic block copolymers

∗ Corresponding author at: Department of Chemical Engineering and Materials Science, Yuan Ze University, 135 Yuan-Tung Road, Chung-Li 32003, Taiwan. Tel.: +886 3 4638800; fax: + 886 3 4559373. ∗∗ Corresponding author. Tel.: +886 3 2118800; fax: +886 3 2118668. E-mail addresses: [email protected] (P.-C. Yang), [email protected] (R.-S. Juang).

by using techniques such as anionic living polymerization [11,12], atom transfer radical polymerization (ATRP) [13–18], and reversible addition–fragmentation chain transfer (RAFT) [19–22]. Living free radical polymerization techniques, such as nitroxide-mediated polymerization [23,24], ATRP [25–28], RAFT [29,30], and single-electron transfer living radical polymerization [31–33] have been widely used to generate complex macromolecular architectures with well-defined end groups of narrow polydispersity. These techniques facilitate additional control in designing advanced core-shell microspheres and functional particles. RAFT polymerization is a powerful tool for controlling the manipulation of macromolecular architectures, because it can polymerize various monomers. Furthermore, it exhibits tolerance toward a wide range of functional groups and facilitates the preparation of block copolymers exhibiting narrow polydispersity indices (PDIs) [34]. In addition, no impurity or residual reagent (e.g., ATRP metal ions, dipyridyl ligands) need be removed from the polymerization product, and the reaction temperature is relatively low (typically between 60 °C and 70 °C). The availability of RAFT components has led to an increase in reports on the synthesis of block copolymers composed of azo polymer and other segments. Recently, Zhao et al. [35] employed the RAFT technique to prepare novel sidechain liquid crystalline (LC) diblock copolymers (PAzoMA-b-PBiPMA) by using 2-(2-cyanopropyl) dithiobenzoate as the RAFT agent. The results of that study verified that the photo induced orientation of

http://dx.doi.org/10.1016/j.jtice.2015.03.008 1876-1070/© 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: P.-C. Yang et al., Synthesis, photochemical properties, and self-assembly of diblock copolymer bearing azobenzene moieties, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.008

JID: JTICE 2

ARTICLE IN PRESS

[m5G;March 27, 2015;9:10]

P.-C. Yang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10

azo mesogens in PAzoMA could propagate through the interface into microphase-segregated PBiPMA domains, thereby aligning the orientation of biphenyl mesogens in PBiPMA. Zhu et al. [36] employed the RAFT technique to synthesize polystyrene with a terminal functionalized with an azo structure, and achieved enhanced fluorescence emission after UV irradiation. Ikeda and his coworkers [8] synthesized an azo-based LC diblock copolymer (PEO-b-PM11AzCN), demonstrating that the supramolecular-ordered nanostructures that can hierarchically assemble in block copolymer films are widely used as templates or scaffolds to prepare nanomaterials according to supramolecular cooperative motions. In our previous report [37], we examined the effects of terminal groups and spacer length of low molecular weight azo chromophore structures on photoreactivity and mesomorphic properties. We found that M6 with six methylene segments as spacer and with electrondonating (methoxy) terminal group showed a steady state for Z–E isomerization in darkness after 54 h. We investigated the temperature dependence of selective light reflection of cholesteric LCs. We also used the conventional radical polymerization method to synthesize photoisomerizable LC polymers (CP3–CP5) containing cinnamoyl groups by using 2,2 -azoisobutyronitrile (AIBN) as the initiator, and performed E–Z photo stationary states within 10–20 m of UV irradiation (λirr = 300 nm). However, these LC polymers showed extremely little self-assembled aggregates in aqueous solution [38]. Based on the original investigations above, in this study, we employed a two-step RAFT polymerization technique to synthesize a macro-chain transfer agent, StO macro-CTA, and a novel azo block polymer, poly(StO50 -b-Azo7 ). The azo structures rendered the diblock copolymer photoactive, facilitating our investigation of the effects of azo on E/Z photoisomerization, as well as the self-assembly behavior of diblock copolymer at various THF/H2 O concentrations. We observed that poly(StO50 -b-Azo7 ) and Azo monomer exhibited the steady state during Z–E isomerization in darkness after 13.0 h and 52.0 h, respectively, implying that Azo monomer used a rotation mechanism with high potential energy profile to achieve a slower rate of Z–E isomerization in darkness than poly(StO50 -b-Azo7 ) did [39–41]. Meanwhile, poly(StO50 -b-Azo7 ) exhibited a high photo induced response and rapid microphase separation, indicating the practical potential of these copolymers for use in photosensitive materials. Also remarkably, poly(StO50 -b-Azo7 ) containing azo chromophores have been demonstrated to integrate the photoresponsive properties of azo polymers with the self-assembling characteristics of block copolymers through the use of RAFT techniques. 2. Experimental 2.1. Materials Synthetic routes for the target StO macro-CTAs and diblock copolymer are shown in Scheme 1. 4-Acetoxy styrene (StO) was stirred over CaH2 overnight and distilled under reduced pressure prior to use. AIBN was recrystallized from methanol twice. All organic solvents and reagents were purchased from Acros, Alfa, and Aldrich Chemical Co. and used without further purification. Toluene and THF were dried with appropriate drying agents, calcium hydride or sodium, then distilled under reduced pressure and stored over 4 A˚ molecular sieves before use. 6-(4-Methoxy-azobenzene-4 -oxy) hexyl acrylate (Azo) was synthesized following the processes reported in the literature [39] and our previous reports [37,42]. 2.2. Synthesis of polymers (Scheme 1) 2.2.1. Synthesis of macromolecular chain transfer agents (StO macro-CTAs) A Schlenk tube was charged with StO (2.0 g, 12.3 mmol), thiobenzoylmercaptoacetic acid (CTA) (0.0261 g, 0.123 mmol), AIBN

S

S OH

OH

AIBN

S

S

x

Toluene, 80oC

O

O

CTA

O C

O

CH3

StO macro-CTA O S

AIBN THF, 70oC Azo

y

x

OH

S O

O

O C

O

CH3 O

N

N

OCH3

poly(StO-b-Azo) Scheme 1. Synthetic routes of StO macro-CTA and diblock copolymer.

(0.0202 g, 0.123 mmol), toluene (20.0 mL) and a magnetic stirrer. The composition monomer/CTA/AIBN was used as 100/1/1 in molar ratio. The mixture was purged with dry nitrogen and subjected to three freeze–pump–thaw cycles to remove any dissolved oxygen. Then the tube was sealed under vacuum and immersed in an oil bath at 80 °C. The polymerization times are in the range of 4–48 h. The reaction was stopped and the tube was quickly cooled down to the room temperature with cold water. The mixture was poured into excess cold methanol. The product was purified by reprecipitating twice from chloroform to cold methanol and dried in vacuum at room temperature overnight. Monomer conversion was determined using 1 H NMR, and polystyrene-equivalent molecular weights of all polymers were determined using GPC. For kinetic studies, the same procedure was adopted as described above except aliquots were taken at predetermined intervals and the conversion and molecular weights were determined by 1 H NMR analyses. 2.2.2. Synthesis of diblock copolymer (poly(StO50 -b-Azo7 )) The typical experiment was as follows: Azo (1.0 g, 2.6 mmol), StO macro-CTA (100 mg), AIBN (10.0 mg, 0.061 mmol) and 10 mL anhydrous THF were added in a Schlenk tube. After degassing with three freeze-pump-thaw cycles, the tube was sealed under vacuum and immersed into an oil bath at 70 °C for 48 h. The mixture was diluted with THF and then dropped into diethyl ether. The product was purified by reprecipitating twice from THF to cold diethyl ether and dried in a vacuum oven at room temperature overnight. Yield: 41.6%. G 96.5 °C SA 135.6 °C I (heating). λmax = 358 nm. 1 H NMR: (acetone-d6 , δ in ppm): 6.38–7.10 (br, aromatic, Ar–H), 3.86–4.03 (br, 4H, -OCH2 –), 3.81–3.85 (br, 3H, OCH3 ), 2.30 (s, 2H, –CH2 COOH),

Please cite this article as: P.-C. Yang et al., Synthesis, photochemical properties, and self-assembly of diblock copolymer bearing azobenzene moieties, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.008

JID: JTICE

ARTICLE IN PRESS

[m5G;March 27, 2015;9:10]

P.-C. Yang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10

2.17–2.28 (br, 3H, COCH3 ), 1.26–1.82 (br, CH and CH2 ). Anal. Calcd. (%) from feed (C10 H10 O2 )(C22 H26 N2 O4 ): C, 72.59; H, 6.36; N, 2.18; O, 18.86. Found: C, 70.62; H, 6.65; N, 2.24; O, 19.84.

(a)

3

1.8

10000

8000 1.6

2.3. Measurements

2.4. Preparation of micellar aggregates A suitable quantity of poly(StO50 -b-Azo7 ) was first dissolved in anhydrous THF to form a homogeneous stock solution with a concentration of 1 mg/mL. To obtain micellar aggregates in dispersions, deionized water (1 mL) was added to the THF solution (1 mL) at a rate of 500 μL/h. The solution was prepared by ultrasonic agitation for 1 h. The solution progressively became turbid, indicating that aggregation had occurred. Finally, samples were prepared by drop-casting aqueous solution of diblock copolymer on a carbon coated copper grid or ITO substrates under ambient conditions for 3 h to evaporate THF and water to further observe spherical micelles. Then the aggregates were achieved by thermal annealing at 50 °C for 30 m. The morphologies of the micellar aggregates were investigated using SEM, AFM, and TEM.

Mn

6000 1.4 4000 1.2 2000

0

0

10

20

30

40

50

60

70

1.0 80

Conversion (%)

(b)

80

1.5

Conversion (%)

60 1.0 40 0.5

ln(M o/M)

NMR (400 MHz) spectra were recorded on a Bruker AMX-400 FT–NMR, and chemical shifts were reported in ppm using tetramethylsilane (TMS) as an internal standard. Elemental analysis was carried out on a Heraeus CHN–O rapid elemental analyzer. Weightaverage molecular weight (Mw ) and PDI of polymers were measured with a gel permeation chromatograph (GPC), model CR4A from Shimadzu, using THF as an eluent and the rate of elution was 1.0 mL/min; the instrument was calibrated with a polystyrene standard. Polystyrene standard (1000–136,000 g/mol) were used for calibration. Thermal analysis was performed using a differential scanning calorimeter (Perkin Elmer DSC 7) at a scanning rate of 20 K/min under nitrogen atmosphere. Thermogravimetric analysis (TGA) was performed under nitrogen atmosphere at a heating rate of 20 K/min using a Perkin Elmer TGA-7 thermal analyzer. The LC phase transitions were investigated by an Olympus BH-2 polarized light microscope (POM) equipped with Mettler hot stage FP-82 and the temperature scanning rate were determined at a rate 10 K/min. UV–vis absorption spectra were measured using a Jasco V-670 spectrophotometer. Characterization of polymeric films by XRD measurement was recorded on a Rigaku RINT 2500 X-ray diffractometer with Ni-filtered CuKα radiation (λ = 0.154056 nm). The tube current and voltage were 30 mA and 40 kV, respectively. Dynamic light scattering (DLS) measurements were done by using a Magic Droplet Model 100SB instrument using a 4 mW He–Ne laser (λ = 632.8 nm) and equipped with a thermostated sample chamber. Scanning electron microscopy (SEM) images were recorded using a JEOL JSM-5600 SEM system equipped with an energy-dispersive X-ray (EDX) attachment (Oxford instruments ISIS 310), with an acceleration voltage of 20 kV. Transmission electron microscopy (TEM) images were obtained through a JEOL TEM-2010 electron microscopy with an accelerating voltage of 120 kV. The polymer concentration for SEM and TEM measurements were 1.0 and 0.25 mg/mL, respectively. Samples were prepared by drop-casting aqueous solution of diblock copolymer on a carbon coated copper grid or ITO substrates under ambient conditions. Atomic force microscopy (AFM) images with a polymer concentration of 0.5 mg/mL were recorded using a digital Shimadzu SPM 9500 J2 instrument operating in contact mode under ambient conditions at lowering scan rate (0.5–2.0 Hz), and using cantilevers (13 kHz resonance frequency, 0.2 N/m force constant) with conventional Al-coating silicon tips (10–15 μm nominal height). Tip was shaped like a polygon based pyramid and was located at the very end of the cantilever. Shimadzu SPM-9500 series software facilities were used to evaluate shape and quantify dimensions of the micellar aggregates.

PDI

1H

20

0

0

10

20

30

40

50

0.0

Time (h) Fig. 1. (a) Molecular weights and polydispersity indices of the StO macro-CTAs with increasing monomer conversion and (b) plot of pseudo-first-order kinetics of polymerization with increasing reaction time in toluene at 80 °C.

3. Results and discussion 3.1. Synthesis of macromolecular chain transfer agents This study investigates the effect of azo structure on E/Z photoisomerization and the self-assembly behavior of diblock copolymer. Macro-CTAs were reacted in toluene at 80 °C by using StO monomer at a constant molar ratio of [M]0 /[CTA]/[AIBN] = 100/1/1. Table 1 lists the macro-CTA polymerization results and Scheme 1 shows the synthetic route involved in the synthesis of macro-CTA and diblock copolymer. The macro-CTA structures were characterized using GPC and 1 H NMR spectroscopy. Fig. 1(a) shows the molecular weights and PDIs of the macro-CTAs with increasing StO monomer conversion. To obtain a high degree of end-group functionalization, the monomer conversion was controlled to 61.2% for StO macro-CTA. Table 1 shows that the number-average molecular weights (Mn ) of the StO macroCTA ranged from 2000 to 7600 g/mol. The table also shows that the corresponding PDIs ranged from 1.23 to 1.45 for StO macro-CTA. These results indicate that StO macro-CTAs were synthesized by employing the discussed RAFT process. The slow increase of the kinetic conversion after 24 h may be caused by the high viscosity of the solution during the polymerization reaction. Fig. 1(b) shows that a linear fit was obtained for ln([M0 ]/[M]) versus reaction time at low conversion, indicating that the polymerization occurred under pseudo-first-order app kinetic conditions. The apparent rate constant of propagation (kp ) −5 was calculated from the slope of the kinetic plot to be 1.28 × 10 s−1 . Fig. 2(a) shows the 1 H NMR spectrum of the StO macro-CTA, which was recorded in DMSO-d6 . The degree of polymerization (DP) of the StO macro-CTA was controlled by adjusting the molar ratio of

Please cite this article as: P.-C. Yang et al., Synthesis, photochemical properties, and self-assembly of diblock copolymer bearing azobenzene moieties, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.008

ARTICLE IN PRESS

JID: JTICE 4

[m5G;March 27, 2015;9:10]

P.-C. Yang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10

Fig. 2.

1

H NMR spectra of (a) StO macro-CTA and (b) diblock copolymer poly(StO50 -b-Azo7 ).

Table 1 Molecular weights and polydispersity data for StO macro-CTAsa . Entry

Conversion (%)b

Mn (g/mol)c

PDI

Mn(th) (g/mol)d

Reaction time (h)

ln(M0 /M)

kp app × 105 (s−1 )

1 2 3 4 5

9.12 29.20 37.50 50.92 61.20

2000 3600 5500 6800 7600

1.33 1.45 1.27 1.23 1.28

1700 5000 6300 8500 9000

4 8 12 24 48

0.10 0.35 0.47 0.71 0.80

1.28

a Reaction temperature was 80 °C. Reaction solvent was toluene. [M]: monomer, CTA: chain transfer agent, I: AIBN. [M]:[CTA]:[I] = 100:1:1. b Conversion = [M0 – M]/[M0 ]. Values were calculated by NMR spectra. c Mn and PDI of the polymers were determined by gel permeation chromatography using polystyrene standards in THF. d Mn(th) = [M]0 × Mm × conversion/[CTA]0 + CTAm .

Please cite this article as: P.-C. Yang et al., Synthesis, photochemical properties, and self-assembly of diblock copolymer bearing azobenzene moieties, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.008

ARTICLE IN PRESS

JID: JTICE

[m5G;March 27, 2015;9:10]

P.-C. Yang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10

5

Table 2 Polymerization results of StO macro-CTA and diblock copolymera . Polymer

Mn b

PDIb

Td (°C)c

Heating cycle (°C)d

x:ye

StO macro-CTA Poly(StO50 -b-azo7 )

7600 11,500

1.28 1.26

312.0 340.9

G 120.6 I G 96.5 SA 135.6 I

50:0 50:7

b

a Reaction temperature was 70 °C and reaction time was 48 h. Reaction solvent was THF. b Mn and PDI of the polymers were determined by gel permeation chromatography. c Decomposition temperature of 5% weight loss was measured by TGA. d Phase transition temperature were measured by DSC. G, glassy; SA , smectic A; I, isotropic phase. e Ratio of block length in copolymer was calculated by 1 H NMR spectroscopy.

monomer to CTA, and its value was calculated based on the integral ratio of methylene protons Ha in the CTA agent (at 2.35 ppm) to phenyl protons Hb (at approximately 6.31–7.08 ppm) derived from the StO monomer. The DP value of macro-CTA was approximately 50, indicating that the moiety of the RAFT agent was attached to the ends of the macro-CTAs. The Mn(NMR) of the StO macro-CTA calculated from Fig. 2(a) was 8300 g/mol by applying the equation below:

Mn(NMR) = [(2Hb − 5Ha )/4Ha ] × Mn,StO + Mn,CTA

Fig. 3. Thermogravimetric curves of StO macro-CTA and poly(StO50 -b-Azo7 ) at a heating rate of 20 °C/min in nitrogen.

(1)

where Ha : integral of methylene protons, Hb : integral of phenyl protons, Mn,StO : molecular weight of StO, Mn,CTA : molecular weight of CTA. The Mn determined by GPC was 7600 g/mol. From these data, it can also be estimated the degree of end-functionality by Mn(GPC) /Mn(NMR) . Consequently, the result indicated that 91.6% (7600/8300) chains of the StO macro-CTA were end-functionalized with the CTA group. 3.2. Synthesis and thermotropic phase properties of diblock copolymer The diblock copolymer poly(StO50 -b-Azo7 ) were prepared by applying the second-step RAFT polymerization method, using StO macro-CTA (i.e., entry 5 in Table 1) as the macro-RAFT agent, and AIBN was as the initiator in the THF at 70 °C. The chemical structures and constitutional composition of the poly(StO50 -b-Azo7 ) were confirmed using elemental analysis and NMR spectroscopies. Fig. 2(b) shows the 1 H NMR spectrum of the poly(StO50 -b-Azo7 ) in acetoned6 . The molar ratio of the diblock length of poly(StO50 -b-Azo7 ) was calculated at 50:7 by comparing the integrated peak areas of acetoxy protons (Ha : 2.17–2.28 ppm) to those of the phenyl protons (Hb : 6.38–7.10 ppm) in the StO and Azo (Hb ). The molecular structure of the diblock copolymer was also supported by performing elemental analysis. Table 2 lists the Mn of poly(StO50 -b-Azo7 ) as 11,500 g/mol and lists the corresponding PDI, determined by GPC, as 1.26. The agreement between Mn(GPC) and Mn(NMR) (11,000) was consistently good. The diblock copolymer exhibited moderate thermal stability at thermal decomposition temperatures (Td ; at 5% weight losses) approximately 340.9 °C in a nitrogen atmosphere (Fig. 3). These results indicate that azo groups can be incorporated into block copolymers to enhance the thermal stability of polymers. Fig. 4(a) shows the DSC thermograms of the StO macro-CTA and poly(StO50 -b-Azo7 ) at a heating rate of 20 K/min. The diblock copolymer, comprising six methylene segments as spacers and methoxy as terminal group in the azo unit, exhibited a monotropic smectic LC phase (SA ; fan-shaped texture) at 96.5–135.6 °C. We attribute the formation of the smectic phase in the diblock copolymer to the formation of the lateral molecular interaction between the side chains. In addition, poly(StO50 -b-Azo7 ) showed a smectic LC phase (࢞T = 39.1 °C) during the heating cycle, indicating a successful incorporation of mesogenic azo groups in poly(StO50 -b-Azo7 ). The result suggests that the intermolecular interaction, dipole-induced dipole interactions might play vital roles in determining the type of mesophase textures and influence the physical properties of the polymers. The

Fig. 4. (a) DSC traces of StO macro-CTA and poly(StO50 -b-Azo7 ) at a heating rate of 20 °C/min in nitrogen and (b) XRD pattern of poly(StO50 -b-Azo7 ).

POM texture of poly(StO50 -b-Azo7 ) at 120.4 °C under a heating cycle is shown in Fig. S1. To further elucidate the structures of the mesophases, X-ray diffraction measurements of polymer were carried out. Samples were heated to the temperature ranges of the mesophases and then quenched. Fig. 4(b) shows the X-ray diffraction curve of the poly(StO50 -b-Azo7 ) peaked at 2θ = 3.98o , corresponding to the layer d-spacing value of 22.2 A˚ at 110.5 °C under the heating cycle.

Please cite this article as: P.-C. Yang et al., Synthesis, photochemical properties, and self-assembly of diblock copolymer bearing azobenzene moieties, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.008

ARTICLE IN PRESS

JID: JTICE 6

[m5G;March 27, 2015;9:10]

P.-C. Yang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10 Table 3 Parameters of the photoisomerization kinetics (E–Z-form) obtained from the curve fitting for the diblock copolymer and azo monomer in CHCl3 solution. No.

UV–vis λmax (nm)a

A0 b

A1 b

T1 (s)c

χ 2d

(A0 – At )/A0 e

K × 102 (s)f

Poly(StO50 -b-azo7 ) Azo

358 359

0.28 0.32

1.31 0.73

68.00 10.95

1.05 × 103 2.89 × 103

0.89 0.96

1.31 8.77

a b c d e f

Measured in chloroform solution (1 × 10−5 M). At = A0 + A1 exp(–t/T1 ), At is the absorbance after the irradiation for different time periods. A0 and A1 are the constants. T1 is the characteristic time of the decay process. χ 2 is the mean squared error of the fitting. Obtained in E–Z photostationary state. K = [(A0 – At )/A0 ]/T1 .

Table 4 Parameters of the thermal isomerization kinetics (Z–E-form) in darkness obtained from the curve fitting for the diblock copolymer and azo monomer in CHCl3 solution. No.

A0 a

A1 a

T1 (h)b

χ 2c

(A0 – At )/A0 d

K (h)e

Poly(StO50 -b-Azo7 ) Azo

27.48 2.40

26.48 –1.62

11.89 15.43

8.56 × 102 1.81 × 101

–18.06 –24.42

–1.52 –1.58

a At = A0 + A1 exp(–t/T1 ), At is the absorbance after the thermal isomerization for different time periods. A0 and A1 are the constants. b T1 is the characteristic time of the decay process. c χ 2 is the mean squared error of the fitting. d Obtained in a steady state during Z–E isomerization. e K = [(A0 – At )/A0 ]/T1 .

2.5

2.0

(the XRD patterns showed that the layer d-spacing value was approxi˚ d/l = 0.78). Therefore, we inferred that a layer structure mately 22.2 A; of poly(StO50 -b-Azo7 ) might exhibit intercalated packing of the side chains. The results show that the unapparent peaks may be ascribed to the shorter azo mesogenic block (at approximately 12.3%) in the diblock copolymer and narrow LC temperature range (T = 39.1 °C) of polymer, thereby causing the irregular structure.

1.5

0.8 0.6 0.4 0.2 0.0 0

20

40

60

80

UV Irradiation Time (s)

1.0

0.5

3.3. Optical properties

0.0

300

400

500

600

700

800

Wavelength (nm) 0 0.5h 1h 1.5h 2h 3h 5h 7h 9h 10h 11h 13h

2.5

2.0

1.5

1.0

-20

-15

(A 0-At)/A0

(b)

Abs Intensity

The kinetic rate of photoisomerization of the polymer was studied using UV absorption spectroscopy in a chloroform solution. The E–Z photoisomerization of polymer in solution was obtained, while irradiating the sample with UV light (365 nm, 2 mW/cm). Tables 3 and 4 show the parameters of the E–Z photoisomerization and Z–E thermal isomerization kinetics of poly(StO50 -b-Azo7 ), respectively. Fig. 5 showed the variation of the UV–vis spectra of poly(StO50 -bAzo7 ), at various 365 nm light irradiation times, and the stability of the UV–vis spectra in the dark. The diblock copolymer exhibited strong UV–vis absorption bands at 358 nm, which was attributed to a π –π ∗ transition in the E-isomer of the chromophores. Weak absorption bands at approximately 440–450 nm resulted from an n–π ∗ transition in the E-isomer. UV irradiation induced a decrease in the absorption intensity of the absorption maxima due to the bent-like structures of Z-isomer (cis isomer) of the chromophores, and an increase in absorption at approximately 450 nm during irradiation. Isosbestic points of poly(StO50 -b-Azo7 ) were observed at 320 and 429 nm, corresponding to E–Z photoisomerization [43]. Additionally, the π –π ∗ transition shifted to shorter wavelength, and the n–π ∗ absorption intensity increased. The spectral variations in absorption might result from the geometric change from E- to Zisomer of the azo compounds. The degree of photoisomerization at the photostationary state was evaluated from the relative absorbance,

1.0

0s 10s 20s 30s 40s 50s 60s 70s 80s

(A 0-At)/A0

(a)

Abs Intensity

Furthermore, a fan-shaped texture, which is a characteristic texture of the smectic A phase, was observed using the polarized optical microscopy (POM). According to the molecular modeling calculation, which was performed using CS Chem3DPro (based on MM2 energy parameters), the estimated all-trans molecular length l of the most extended conformation of the azo monomer was approximately 28.5 A˚

-10

-5

0 0

2

4

6

8

10

12

14

Time in darkness (h)

0.5

0.0

300

400

500

600

700

800

Wavelength (nm) Fig. 5. (a) Variation of the UV–vis spectra of poly(StO50 -b-Azo7 ) with irradiation times and (b) its stability of the UV–vis spectra in the dark. Inset: plot of the relative absorbance ((A0 – At )/A0 ) of polymer in CHCl3 solution versus the irradiated time or dark period.

Please cite this article as: P.-C. Yang et al., Synthesis, photochemical properties, and self-assembly of diblock copolymer bearing azobenzene moieties, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.008

ARTICLE IN PRESS

JID: JTICE

[m5G;March 27, 2015;9:10]

P.-C. Yang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10

7

THF: H2O= 9: 1

Poly(StO50-b-Azo7)

ultrasonic agitation for 1hr

ITO or Cu grid drying thermal annealing Scheme 2. Schematic representation of spherical micelles formed from poly(StO50 -b-Azo7 ) in THF/H2 O solution.

expressed as (A0 – At )/A0 (where A0 and At are the absorbance intensity of azo chromophores at absorption maxima (λabs = 358 nm) before and after UV irradiation, respectively), and its variations after time t were employed to characterize the relative amount of Eisomers at time t, as well as the kinetics of the photoisomerization process. The insets of Fig. 5 show the experimental data and fitting curves, whereas those of azo monomer (Azo) were showed in Fig. S2. A linear fitting of the experimental data (Fig. 5(a), inset) is valid for the individual stage of the reaction within 50 s of irradiation times, indicating first-order kinetic rates of E–Z photoisomerization within 50 s, and a deviation from the straight line occurred. As shown in Fig. 5(a), an E–Z photostationary state was obtained for poly(StO50 -bAzo7 ) within 80 s of irradiation and the photoisomerization process with a conversion in Z-isomer of 0.89 was achieved. It should be noted that the photoisomerization process lasted approximately 60 s for Azo monomer with a conversion in Z-isomer of 0.96 (Fig. S2). These results suggest that the segmental mobility of the polymer was restrained in its surroundings and the azo units of the diblock copolymer tended to associate with each other, attributing to the covalent bond of polymer chain and the π –π interaction of azo units. The six methylene segment as a spacer between the main chain and azo units enhanced the ordered arrangement of the azo mesogens. This indicates that the E–Z isomerization rates and times of the photostationary states under UV irradiation were influenced by various potential energy profiles between the E/Z isomers, the local environments of the chromophores, and the chromophore phase itself. However, poly(StO50 -b-Azo7 ) and Azo monomer exhibited the steady state during Z–E thermal isomerization in darkness after 13.0 h and 52.0 h, respectively, implying that Azo monomer used a rotation mechanism with high potential energy profile to achieve a slower rate of Z–E thermal isomerization in darkness than poly(StO50 -b-Azo7 ) did [39–41]. The best fit of the experimental points for poly(StO50 -b-Azo7 ) and Azo in solution was obtained by simulating the absorbance (A) data with the following first-order exponential decay function:

  t At = A0 + A1 exp − T1

(2)

where T1 is the characteristic time of the decay process, and A0 and A1 are constants. The results obtained from the best fit are listed in Tables 3 and 4. It is worth noting that the time constant (T1 = 68.0 s) of the diblock copolymer poly(StO50 -b-Azo7 ) was considerably larger than that of the azo monomer (T1 = 10.95 s). Based on the results, the slow isomerization rate of the diblock copolymer was ascribed to the

Fig. 6. SEM image of spherical micelle formed from poly(StO50 -b-Azo7 ). The THF/H2 O ratio was 9. The polymer concentration for SEM measurement was 1.0 mg/mL.

entanglement of the polymer chain and the limit of free volume for the E–Z photoisomerization of azo chromophores within the diblock copolymers. These results also suggest that introducing the azo block into poly(StO50 -b-Azo7 ) reduced the E–Z photoisomerization rate of the azo chromophores. The subsequent thermal isomerization of the chromophores showed an almost complete recovery to the original state. The experimental data points were also fitted to the first-order exponential decay function (2). 3.4. Self-assembly of diblock copolymer The micellar aggregates were prepared by gradually adding water to dilute the homogeneous solution of the diblock copolymer. When the volume of THF/H2 O was 9/1 (i.e., water content Cw = 10 vol%), the mean diameter of the spherical micelles of poly(StO50 -b-Azo7 ) was in the range of approximately 150–250 nm, by using SEM observation, which was attributed to the relatively short hydrophobic azo block (at approximately 12.3%) in the diblock copolymer. Scheme 2 shows the schematic representation of spherical micelles formed from poly(StO50 -b-Azo7 ) in THF/H2 O solution. Fig. 6 shows the SEM image of micellar aggregates in the mixture of Cw of 10 vol% with a polymer concentration of 1 mg/mL. However, the spherical micelles were approximately dissolved in the pure THF solution. These results indicate that the THF was an appropriate solvent for both copolymer blocks, the diblock copolymer could be adequately dissolved in the THF without aggregation. As water was gradually added to the

Please cite this article as: P.-C. Yang et al., Synthesis, photochemical properties, and self-assembly of diblock copolymer bearing azobenzene moieties, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.008

JID: JTICE 8

ARTICLE IN PRESS

[m5G;March 27, 2015;9:10]

P.-C. Yang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10

Fig. 7. (a) 2D and (b) 3D AFM images of spherical micelle formed from poly(StO50 -b-Azo7 ) in the solution of THF/H2 O = 9/1. The polymer concentration was 0.5 mg/mL.

poly(StO50 -b-Azo7 ) solution in the THF, the solubility of solvent was exacerbated for the azo block; after adding a critical amount of water, the copolymer solutions exhibited microphase separation. Subsequently, the azo chains began to self-assemble into aggregates. It also means that the solubility of the solvent to the hydrophobic blocks decreased and the Flory–Huggins interaction parameter (x) increased as the water content increased [44]. We also found that polymer concentrations (e.g., 0.01–1 mg/mL) exert only a minimal effect on the diameter of the spherical micelles prepared using poly(StO50 -b-Azo7 ). Fig. S3 shows the DLS distribution of spherical micelles formed from poly(StO50 -b-Azo7 ) in the THF/H2 O mixture of Cw of 10 vol%.

The average diameter of the spherical micelles of diblock copolymer was approximately 205.7 nm, which was determined based on the analyzed wavelengths of 94.9, 189.3, 238.3, and 377.7 nm in DLS measurements. These findings also show that morphological aggregates of the diblock copolymer depend on both block copolymer composition and preparation conditions, such as the block length, block ratio, the hydrophobic/hydrophilic properties of the block, and solvent properties [45]. Both AFM and TEM were conducted further to confirm this observation (i.e., to demonstrate that spherical micelles had formed). Fig. 7 shows the 2D and 3D AFM views of the micelles (2 μm × 2 μm

Please cite this article as: P.-C. Yang et al., Synthesis, photochemical properties, and self-assembly of diblock copolymer bearing azobenzene moieties, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.008

JID: JTICE

ARTICLE IN PRESS

[m5G;March 27, 2015;9:10]

P.-C. Yang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10

9

hydrophobic azo block (at approximately 12.3%) in diblock copolymer in a THF–H2 O mixture. Increasing the water content from 10 vol% to 50 vol% caused the mean diameter to increase from 181.4 to 616.5 nm. The diblock copolymer underwent E–Z isomerization following UV irradiation, reducing the hydrophobic properties of the azo block and disrupting the micelles because of the high dipole moment of the Z-isomer chromophores. These characteristics indicate that the diblock copolymer micelles are suitable for use in UV light controllable devices that exhibit photoresponsive anisotropic and tunable encapsulation and release properties [17,46–48]. Acknowledgments The authors are thankful for the Ministry of Science and Technology, Taiwan for its financial aid through projects NSC 100-2113-M115-001-MY2 and MOST 103-2221-E-155-072. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2015.03.008. Fig. 8. TEM images of spherical micelles formed from poly(StO50 -b-Azo7 ) (a) before and (b) after UV irradiation for 10 min. The ratio of THF/H2 O solution was 9. The polymer concentration was 0.25 mg/mL.

scan) when the Cw was at 10 vol%. The pattern in the AFM image was consistent with the SEM observation. When the Cw was 30 vol%, the mean diameter was 616.5 nm. When the Cw was increased to 50 vol%, the average diameter of the aggregates decreased considerably to 240.8 nm. After increasing the Cw (i.e., 50 vol%), no obvious micelles were observed in the poly(StO50 -b-Azo7 ) solution. Therefore, these results indicate that increasing the Cw led to the disruption and dissociation of micellar aggregates into small micelles because of the insoluble states of these two blocks at high Cw values, resulting in the collapse of the micellar aggregates. Fig. 8 shows the TEM images of the micelles of poly(StO50 -b-Azo7 ) when the Cw was at 10 vol%. The spherical micelles with an average diameter of approximately 181.4 nm were observed for poly(StO50 b-Azo7 ), as shown in Fig. 8(a). The average diameter estimated from TEM images was inconsistent with the DLS observations. This is probably due to the fact that the diameter estimated from TEM results is a number-average value, whereas the diameter in DLS data is an intensity-average value. The concentration of these spherical micelles decreased when exposed to continuous UV irradiation for 10 min, and the micelles gradually formed rod-like aggregates. After irradiating the aggregates with UV light for 1 h, no spherical micelles were observed on the TEM grid, indicating that the polymeric aggregates underwent photoisomerization from E- to Z-isomers, thereby causing the azo chromophores to isomerize. Consequently, UV irradiation increased the overall hydrophilicity of the polymer with short lengths of the azo block, attributing to the polarity enhancement of the hydrophobic azo block in the Z form, and the micelles aggregates were collapsed when irradiated. 4. Conclusions We employed the RAFT polymerization technique to synthesize and characterize an azo-based diblock copolymer (poly(StO50 b-Azo7 )). The diblock copolymer exhibited favorable thermal stability at thermal decomposition temperatures greater than 340.9 °C. Poly(StO50 -b-Azo7 ) exhibited a smectic A mesophase at temperatures ranging from 96.5 °C to 135.6 °C, indicating that introducing polar acetoxy groups to the polymer chains induced lateral molecular interaction, leading to the generation of the mesomorphic smectic A phase. The sphere sizes were closely related to the relative short

References [1] Hamley IW, Castellertto V, Lu Z, Imrie C, Itoh T, Al-Hussein M. Interplay between smectic ordering and microphase separation in a series of side-group liquidcrystal block copolymers. Macromolecules 2004;37:4798–807. [2] Häckel M, Kador L, Kropp D, Frenz C, Schmidt H. Holographic gratings in diblock copolymers with azobenzene and mesogenic side groups in the photoaddressable dispersed phase. Adv Funct Mater 2005;15:1722–7. [3] Breiner T, Kreger K, Hagen R, Häckel M, Kador L, Müller AHE, et al. Blends of poly(methacrylate) block copolymers with photoaddressable segments. Macromolecules 2007;40:2100–8. [4] del Barrio J, Oriol L, Alcalá R, Sánchez C. Photoresponsive poly(methyl methacrylate)-b-azodendron block copolymers prepared by ATRP and click chemistry. J Polym Sci Part A: Polym Chem 2010;48:1538–50. [5] Tamai N, Miyasaka H. Ultrafast dynamics of photochromic systems. Chem Rev 2000;100:1875–90. [6] Yang PC, Wang YH, Wu H. Synthesis, characterization, and photochemical properties of chromophores containing photoresponsive azobenzene and stilbene. J Appl Polym Sci 2012;124:4193–205. [7] Kadota S, Aoki K, Nagano S, Seki T. Photocontrolled microphase separation of block copolymers in two dimensions. J Am Chem Soc 2005;127:8266–7. [8] Yu H, Iyoda T, Ikeda T. Photoinduced alignment of nanocylinders by supramolecular cooperative motions. J Am Chem Soc 2006;128:11010–11. [9] Zhao Y. Light-responsive block copolymer micelles. Macromolecules 2012;45:3647–57. [10] Tang X, Gao L, Fan X, Liang X, Zhou Q. Self-assembly and photoresponsivity property of amphiphilic ABA triblock copolymers containing azobenzene moieties in dilute solution. Macromol Chem Phys 2009;210:1556–62. [11] Hruska Z, Hurtrez G, Walter S, Riess G. An improved technique of the cumylpotassium preparation: application in the synthesis of spectroscopically pure polystyrene–poly(ethylene oxide) diblock copolymers. Polymer 1992;33:2447–9. [12] Gohy JF, Lohmeijer BGG, Varshney SK, Schubert US. Covalent vs metallosupramolecular block copolymer micelles. Macromolecules 2002;35:7427–35. [13] Matyjaszewski K, Xia J. Atom transfer radical polymerization. Chem Rev 2001;101:2921–90. [14] Mei Y, Beers KL, Byrd MHC, VanderHart DL, Washburn NR. Solid-phase ATRP synthesis of peptide−polymer hybrids. J Am Chem Soc 2004;126:3472–6. [15] Chen H, Liu D, Chen L, Qu R. Use of 1-alkyl-3-methylimidazolium l-lactates as both ligand and reaction media for AGET ATRP of acrylonitrile. Mater Chem Phys 2011;128:331–5. [16] Tian Y, Watanabe K, Kong X, Abe J, Iyoda T. Synthesis, nanostructures, and functionality of amphiphilic liquid crystalline block copolymers with azobenzene moieties. Macromolecules 2002;35:3739–47. [17] Wang G, Tong X, Zhao Y. Preparation of azobenzene-containing amphiphilic diblock copolymers for light-responsive micellar aggregates. Macromolecules 2004;37:8911–17. [18] Tang X, Gao L, Fan X, Zhou Q. ABA-type amphiphilic triblock copolymers containing p-ethoxy azobenzene via atom transfer radical polymerization: Synthesis, characterization, and properties. J Polym Sci Part A: Polym Chem 2007;45:2225– 34. [19] Perrier S, Takolpuckdee P. Macromolecular design via reversible addition– fragmentation chain transfer (RAFT)/xanthates (MADIX) polymerization. J Polym Sci Part A: Polym Chem 2005;43:5347–93. [20] Lokitz BS, Convertine AJ, Ezell RG, Heidenreich A, Li Y, McCormick CL. Responsive nanoassemblies via interpolyelectrolyte complexation of amphiphilic block copolymer micelles. Macromolecules 2006;39:8594–602.

Please cite this article as: P.-C. Yang et al., Synthesis, photochemical properties, and self-assembly of diblock copolymer bearing azobenzene moieties, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.008

JID: JTICE 10

ARTICLE IN PRESS

[m5G;March 27, 2015;9:10]

P.-C. Yang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10

[21] Lowe AB, McCormick CL. Reversible addition–fragmentation chain transfer (RAFT) radical polymerization and the synthesis of water-soluble (co)polymers under homogeneous conditions in organic and aqueous media. Prog Polym Sci 2007;32:283–351. [22] Boyer C, Bulmus V, Liu J, Davis TP, Stenzel MH, Barner-Kowollik C. Well-defined protein-polymer conjugates via in situ RAFT polymerization. J Am Chem Soc 2007;129:7145–54. [23] Hawker CJ, Bosman AW, Harth E. New polymer synthesis by nitroxide mediated living radical polymerizations. Chem Rev 2001;101:3661–88. [24] Becker ML, Liu J, Wooley KL. Chemical communications. Cambridge; 2003. p. 180– 1. [25] Braunecker WA, Matyjaszewski K. Controlled/living radical polymerization: features, developments, and perspectives. Prog Polym Sci 2007;32:93–146. [26] Kamigaito M, Ando T, Sawamoto M. Metal-catalyzed living radical polymerization. Chem Rev 2001;101:3689–746. [27] Gromadzki D, Štˇepánek P, Makuška R. Synthesis of densely grafted copolymers with tert-butyl methacrylate/2-(dimethylamino ethyl) methacrylate side chains as precursors for brush polyelectrolytes and polyampholytes. Mater Chem Phys 2013;137:709–15. [28] Tang X, Gao L, Fan X, Zhou Q. Controlled grafting of ethyl cellulose with azobenzene-containing polymethacrylates via atom transfer radical polymerization. J Polym Sci Part A: Polym Chem 2007;45:1653–60. [29] Sordi MLT, Riegel IC, Ceschi MA, Müller AHE, Petzhold CL. Synthesis of block copolymers based on poly(2,3-epithiopropylmethacrylate) via RAFT polymerization and preliminary investigations on thin film formation. Eur Polym J 2010;46:336–44. [30] Moad G, Rizzardo E, Thang SH. Living radical polymerization by the RAFT process— a first update. Aust J Chem 2006;59:669–92. [31] Percec V, Guliashvili T, Ladislaw JS, Wistrand A, Stjerndahl A, Sienkowska MJ, et al. Ultrafast synthesis of ultrahigh molar mass polymers by metal-catalyzed living radical polymerization of acrylates, methacrylates, and vinyl chloride mediated by SET at 25 °C. J Am Chem Soc 2006;128:14156–65. [32] Tang X, Liang X, Yang Q, Fan X, Shen Z, Zhou Q. AB2 -type amphiphilic block copolymers composed of poly(ethylene glycol) and poly(N-isopropylacrylamide) via single-electron transfer living radical polymerization: Synthesis and characterization. J Polym Sci Part A: Polym Chem 2009;47:4420–7. [33] Zhang N, Samanta SR, Rosen BM, Percec V. Single electron transfer in radical ion and radical-mediated organic, materials and polymer synthesis. Chem Rev 2014;114:5848–958.

[34] Boyer C, Bulmus V, Davis TP, Ladmiral V, Liu J, Perrier S. Bioapplications of RAFT polymerization. Chem Rev 2009;109:5402–36. [35] Zhao Y, Qi B, Tong X, Zhao Y. Synthesis of double side-chain liquid crystalline block copolymers using RAFT polymerization and the orientational cooperative effect. Macromolecules 2008;41:3823–31. [36] Ma F, Zhou N, Zhu J, Zhang W, Fan L, Zhu X. Light-driven fluorescence enhancement of phenylazo indazole-terminated polystyrene. Eur Polym J 2009;45:2131–7. [37] Yang PC, Li CY, Wu H, Chiang JC. Synthesis and mesomorphic properties of photoresponsive azobenzene-containing chromophores with various terminal groups. J Taiwan Inst Chem Eng 2012;43:480–90. [38] Yang PC, Liu JH. Synthesis and characterization of novel photoisomerizable liquid crystalline polymers containing cinnamoyl groups. J Polym Sci Part A: Polym Chem 2008;46:1289–303. [39] Ho CH, Yang KN, Lee SN. Mechanistic study of transcis isomerization of the substituted azobenzene moiety bound on a liquid-crystalline polymer. J Polym Sci Part A: Polym Chem 2001;39:2296–307. [40] Rau H. Photoisomerization of azobenzenes. In: Rabek JF, editor. Photochemistry and photophysics, vol. II. Boca Raton, FL, USA: CRC Press; 1990 [chapter 4]. [41] Sin SL, Gan LH, Hu X, Tam KC, Gan YY. Photochemical and thermal isomerizations of azobenzene-containing amphiphilic diblock copolymers in aqueous micellar aggregates and in film. Macromolecules 2005;38:3943–8. [42] Yang PC, Wu MZ, Liu JH. Synthesis and characterization of liquid crystalline copolymers with dual photochromic pendant groups. Polymer 2008;49:2845–56. [43] Kumar GS, Neckers DC. Photochemistry of azobenzene-containing polymers. Chem Rev 1989;89:1915–25. [44] Zhang L, Shen H, Eisenberg A. Phase separation behavior and crew-cut micelle formation of polystyrene-b-poly(acrylic acid) copolymers in solutions. Macromolecules 1997;30:1001–11. [45] Shen HW, Eisenberg A. Block length dependence of morphological phase diagrams of the ternary system of PS-b-PAA/dioxane/H2 O. Macromolecules 2000;33:2561– 72. [46] Jiang J, Tong X, Zhao Y. A new design for light-breakable polymer micelles. J Am Chem Soc 2005;127:8290–1. [47] Tang X, Liang X, Gao L, Fan X, Zhou Q. Water-soluble triply-responsive homopolymers of N,N-dimethylaminoethyl methacrylate with a terminal azobenzene moiety. J Polym Sci Part A: Polym Chem 2010;48:2564–70. [48] Guo W, Wang T, Tang X, Zhang Q, Yu F, Pei M. Triple stimuli-responsive amphiphilic glycopolymer. J Polym Sci Part A: Polym Chem 2014;52:2131–8.

Please cite this article as: P.-C. Yang et al., Synthesis, photochemical properties, and self-assembly of diblock copolymer bearing azobenzene moieties, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.008