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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Synthesis and Properties of Azo-Based ABC Triblock Copolymers Owning Interaction and Composition Parameters That Influence Their Phase Behaviors Athmen Zenati* and Yang-Kyoo Han Functional Organic Materials Laboratory (FOML), Department of Chemistry, Faculty of Natural Sciences, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea S Supporting Information *

ABSTRACT: Reversible addition−fragmentation chain transfer polymerization is employed for the preparation of a series of novel ABC-type azo triblock copolymers (TBCs) consisting of poly(CAEMA), poly(BMA), and poly(DOPAM) using AIBN as an initiator and anisole as a solvent. The TBCs are characterized by means of 1H nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). Their mesophase, photoresponsive, and morphological behaviors are examined using differential scanning calorimetry (DSC), optical polarizing microscopy (OPM), ultraviolet−visible spectrophotometry (UV−vis), atomic force microscopy (AFM), and grazing-incidence small-angle X-ray scattering (GISAXS). Molecular weights and polydispersities (≤1.38) increase slightly with the increase of the molar ratio of monomer to macroinitiator, confirming a controlled/living radical polymerization. All TBCs demonstrated endothermic and exothermic transition peaks corresponding to the smectic-to-nematic and nematic-to-smectic phases. Thermal investigation revealed that the TBCs containing 54 and 46 wt % of azo contents displayed batonnet textures of smectic phase, while the TBC with low azo block fraction of 39 wt % showed threaded texture of nematic phase. For TBC having the lowest azo content of 31 wt %, neither liquid crystalline texture is detected. TBC-2 with 46 wt % of azo block, 28 wt % of PBMA, and 26 wt % of PDOPAM produced a lamellar compared to TBC-1 and TBC-3 which formed a mixture of cylinder and lamellar morphologies. Contrastingly, TBC-4 containing the highest PDOPAM volume ratio of 50 wt % generated hexagonal cylinder-type morphology. All TBCs exhibited a reversible trans−cis−trans photoisomerization behavior in chloroform solution and in thin film under UV and visible light irradiation (or dark storage) at varied intervals of time. highly ordered microdomains in azo BC thin film within a short period of time. Azo BCs consist of two or more immiscible blocks may also behave as thermoplastic elastomers (TPEs).28 Thus, a number of azo BCs have been prepared through living free radical polymerization (LFRP) techniques and studied extensively. Poly(n-butyl acrylate), poly(tetramethylene glycol), poly(ethylene glycol), and polyisoprene are used as amorphous blocks, whereas polyethylene and polyacrylates with long alkyl side-chain like octadecyl and stearyl groups or with side-chain liquid crystalline (SCLC) group are employed as crystalline blocks. Among the LFRP, reversible addition−fragmentation chain transfer (RAFT) polymerization is considered to be the most versatile technique in terms of monomer selection and reaction condition. Even azo-based (methyl)acrylates and Nsubstituted acrylamides which are hard to control via atom transfer radical polymerization (ATRP)22,29−33 are herein

1. INTRODUCTION Azobenzene-based block copolymers (azo BCs) composed of two or more rigid (crystalline) and soft (amorphous) segments1−11 have great potential for the control of their microphase separation between incompatible blocks as well as the formation of various well-ordered microdomain morphologies in the solid state.10−14 Likewise, azo BCs exhibit higher hierarchical structures with photoresponsive features since they have the interplay between the microphase separation and the elastic deformation of liquid crystal ordering that is known as supramolecular cooperative motion (SMCM).15−19 Such SMCM is the most effective catalyzer to control supramolecularly self-organized nanostructures including spheres, cylinders, lamellae, and gyroid.20−22 The production of perfect morphology through self-assembly is based on the selection of proper solvent, casting procedure, annealing process, and volume fractions of chemically distinct segments in azo BC.23−27 Moreover, solvent vapor annealing has recently emerged as the most inexpensive, versatile, and highly efficient technique for facilitating and controlling the formation of © XXXX American Chemical Society

Received: September 20, 2017 Revised: December 6, 2017

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DOI: 10.1021/acs.macromol.7b02012 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Lastly, we highlight the characterization of structure, molecular weight, thermal behavior, photoresponsive property, and morphology of these polymers through means of 1H NMR, GPC, DSC, OPM, UV−vis, AFM, and GISAXS. We also report the influence of mixed solvent (THF/cyclohexane) on the microphase separation of TBCs with different volume fractions of three distinct blocks. Furthermore, we compare our findings with those in recently published literature.

polymerized in a controlled fashion via RAFT polymerization.34−36 In general, azo diblock copolymers (DBCs) possessing an AB structure consisting of both liquid crystalline (LC) and isotropic blocks can form micro- or nanoscale structures which have an impact on the optical properties of DBCs. Unlike DBCs, ABC-type azo TBCs have more effective parameters including interactions and compositions that influence their phase behavior as well as they form multiple nanostructures through microphase separation which is a key to enable their application. For these appreciations, much effort is currently being devoted to apply these types of TBCs in diverse fields such as lithographic nanotemplates,37−39 optical data storage media,40−42 nanoporous materials,43,44 liquid crystal displays,45 nanofilters,46,47 photoswitching sensors,48 holographic surface relief gratings,49 nanotechnology,50,51 and photonic nanomaterials through the photoinduced trans−cis isomerization of the azobenzene chromophore.52−54 Elastomeric TBCs containing rigid and soft segments are known to act as TPEs. They are able to resist large deformation without breaking their chemical bonds. The processing of TPEs can be either in the melt above their order−disorder temperature or dissolved in appropriate solvents.55 Therefore, thermoplastic TBCs are among the most important research motivations. Recently, He et al.56 and Lee et al.57 reported the synthesis of series of SCLC ABC triblock copolymers composed of PEO, PS, and PMMAZO or polymethacrylate with a pendent cyanoterphenyl group by inserting the rigid PS block between the soft PEO segment and the rigid LC block using ATRP. Obviously, it is very important to design macromolecular architectures such as BCs in order to obtain great research results. In this article, we expanded our involvement in this emerging area of polymer research, especially the molecular design, synthesis, and characterization of novel azobenzene-containing hard−soft−hard TBC, since few scientific research papers are available on such a polymer. We have constructed a new series of ABC-type TBCs having azobenzene units in the side chains, narrow molecular weight distributions, and various compositions by the initiation of azo block (A block). A successful reaction is performed via RAFT polymerization of 2-[2-(4cyanoazobenzene-4′-oxy)ethyleneoxy]ethyl methacrylate (CAEMA), n-butyl methacrylate (n-BMA), and new pdodecylphenyl-N-acrylamide (DOPAM) monomer using azoisobutyronitrile (AIBN) as an initiator and anisole as a solvent. The poly(CAEMA) macroinitiator bearing azobenzene moieties is first obtained from the RAFT polymerization of CAEMA monomer using the RAFT agent 2-cyanoprop-2-yl dithiobenzoate (CPDB) and then utilized in the preparation of poly(CAEMA-b-BMA) macroinitiator. Subsequently, the acrylamide-based poly(DOPAM) is introduced into the poly(CAEMA-b-BMA) by the RAFT polymerization of DOPAM monomer to produce ABC-type TBC, poly(CAEMA-b-BMA-bDOPAM). Incorporation and insertion of soft poly(n-butyl methacrylate) (PBMA, B block) between rigid poly(p-dodecylphenyl-Nacrylamide) (PDOPAM, C block) and poly(2-[2-(4-cyanoazobenzene-4-oxy)ethyleneoxy]ethyl methacrylate) (PCAEMA, A block) would promote the phase behavior significantly, enhance the photosensitivity with additional functional performance, induce more interesting morphological structure, and give rise to new feature for TBC.

2. EXPERIMENTAL SECTION 2.1. Materials. Tetrahydrofuran (THF), chloroform, and methylene chloride are obtained from Samchun Chemical, dried over calcium hydride (CaH2), and distilled under a nitrogen atmosphere before use. 4-Aminobenzonitrile (98%), sodium nitrite (97%), potassium carbonate (97%), phenol (99%), triethylamine (99%), 2-(2-chloroethoxy)ethanol (99%), diglyme (99%), anisole (99%), p-dodecylaniline (97%), and 2-cyanoprop-2-yl dithiobenzoate (CPDB, 97%) are used as received from Aldrich. Methacryloyl chloride (97%) and acryloyl chloride (97%) are purchased from Aldrich and purified by vacuum distillation to remove inhibitor prior to use. n-Butyl methacrylate (Aldrich, 99%) is purified by passing through a neutral alumina column, drying over CaH2, and vacuum distillation to remove stabilizer. 2-[2-(4-Cyanoazobenzene-4′-oxy)ethyleneoxy]ethyl methacrylate (CAEMA) and p-dodecylphenyl-N-acrylamide (DOPAM) monomers are synthesized according to the methods reported in the Supporting Information and purified by recrystallization from methanol several times. Azoisobutyronitrile (AIBN) is received from Merck Chemicals and used after recrystallization from ethanol. Other reagents and solvents are used as received from suppliers. 2.2. Measurements. 1H nuclear magnetic resonance (NMR) spectra in deuterated chloroform are recorded on a Bruker 400 MHz spectrometer using the internal reference standard tetramethylsilane. The number-average molecular weight (Mn) and the polydispersity (Mw/Mn, PDI) are determined at 25 °C by a Waters gel permeation chromatograph (GPC) instrument equipped with four waters columns (Styragel HR 0.5, 2, 4, and 5) and a Waters 2414 refractive index detector using THF as an eluent (flow rate of 1 mL/min). Thermal behavior analyses are performed by a TA Instruments Q100 differential scanning calorimeter (DSC) at a heating rate of 10 °C/ min under a nitrogen atmosphere. Topography and morphology of spin-coated thin polymer films are examined via atomic force microscopy (AFM) in both the topography and phase mode using XE-100 system (advanced scanning probe microscopy) operating in noncontact mode utilizing a 910-NCHR cantilever (force constant: 42 N m−1; resonance frequency: 330 kHz). The LC textures of polymers are evaluated with an optical polarizing microscope (OPM, Leica, DMRXP-MPS 60) using a Linkam heating stage (THMSE600) equipped with a Linkam TMS94 stage temperature controller. The diffraction patterns of thin polymer films are recorded in grazingincidence small-angle X-ray scattering (GISAXS) equipped with monochromatized X-ray (λ = 1.1747 Å) having grazing incident angles ranging from 0.10° to 0.23° and the SCX (4300-165/2 CCD detector, Princeton Instruments). The absorption spectra of polymers in chloroform solution and in thin film are determined on a Varian Cary 50 Bio UV−vis spectrophotometer equipped with a xenon flash lamp. 2.3. Synthesis of PCAEMA-CTA Macroinitiator. The macromolecular chain-transfer agent PCAEMA-CTA is first prepared via RAFT polymerization of the monomer CAEMA58 using the starting chain transfer agent CPDB and the initiator AIBN at mole ratio of ([CAEMA]0:[CPDB]0:[AIBN]0 = 210:3:1). A mixture of CAEMA (6.0 g, 16.6 mmol), CPDB (52 mg, 0.2371 mmol), AIBN (13 mg, 0.0790 mmol), and 18 mL of anisole solvent is added into a 25 mL Schlenk flask. The solution is subsequently purged with nitrogen gas for 15 min under stirring. The flask is then sealed and placed into a preheated oil bath. The reaction is carried out at 75 °C for 24 h and then quenched by allowing the solution to cool down to room temperature. The viscous solution is rapidly diluted with 12 mL of THF and dropped into a large excess of methanol (500 mL) to B


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Macromolecules Table 1. Experimental Conditions and Characteristics of Azo Polymers Synthesized by RAFT Polymerization polymers

[monomer]0/[CTA]0/ [AIBN]0d

solvent (anisole) (wt %)e

conv (%)f

Mn(GPC)f

Mn(calcd)h

Mw/Mn (GPC)f

PCAEMA-CTAa DBC-CTAb TBC-1c TBC-2 TBC-3 TBC-4

70:1:1/3 80:1:1/3 20:1:1/2 50:1:1/2 70:1:1/2 100:1:1/2

25 10 10 10 10 10

42 57 35 38 46 53

10600 17100 19300 23100 27300 33700

10900 17000 19500 23400 27200 34000

1.17 1.19 1.23 1.24 1.35 1.38

PDOPAM (wt %)g

PBMA (wt %)g

Azo (wt %)g

12 26 37 50

38 34 28 24 19

100 62 54 46 39 31

a

Macromolecular chain transfer agent (PCAEMA-CTA). bDiblock copolymer chain transfer agent (PCAEMA-b-PBMA-CTA). cTriblock copolymer (PCAEMA-b-PBMA-b-PDOPAM). dFeed molar ratio: [monomer]0, (CAEMA, n-BMA, or DOPAM); [CTA]0, (CPDB, PCAEMA-CTA, or DBCCTA); [AIBN]0, (azoisobutyronitrile). eCAEMA, PCAEMA-CTA, or DBC-CTA concentration versus solvent. fDetermined by GPC on the basis of polystyrene standards. gEvaluated considering molecular weights of PCAEMA-CTA, PCAEMA-b-PBMA-CTA, and TBCs obtained via GPC in THF. h Calculated according to the equation Mn(th),polymer = Mw,CTA + [monomer]0/[CTA]0 × Mw,monomer × conversion. Mw,CTA and Mw,monomer are the molecular weights of chain transfer agent and monomer, whereas [CTA]0 and [monomer]0 are the initial concentration of CTA and monomer. Mn values are also calculated from 1H NMR spectra, and they are close to the GPC and theoretical values.65,66 AIBN is used as initiator. Homo, diblock, and triblock copolymerizations are carried out at 75, 60, and 70 °C, respectively. Block copolymerization time = 48 h.

Scheme 1. Synthetic Route of Macroinitiators and Their Corresponding Triblock Copolymers

precipitate the PCAEMA-CTA. The resulting PCAEMA-CTA is collected by filtration, washed with methanol, and dried in a vacuum oven. Drying of the obtained PCAEMA-CTA under vacuum at room temperature for 24 h yielded a constant weight of 2.52 g. The CAEMA monomer conversion determined by GPC is 42%. The Mn and the Mw/Mn determined by GPC are 10 600 g/mol and 1.17, respectively. 1 H NMR (CDCl3, 400 MHz): 0.7−1.0 (3H, main chain CH3), 1.6− 2.0 (8H, CH3−CCN−CH3 and main chain CH2), 3.6−4.2 (8H, OCH2CH2O), 6.8−7.0 (2H, m-Ar H to OCH2), 7.5−7.9 (11H, o, mAr H to CN, o-Ar H to OCH2, p, o-Ar H to SCS and m-Ar H to SCS). 2.4. Synthesis of PCAEMA-b-PBMA-CTA Macroinitiator. The PCAEMA-b-PBMA-CTA is produced through the RAFT chain extension reaction using the PCAEMA-CTA macroinitiator to grow the second block PBMA. In a typical reaction, a 25 mL Schlenk flask with a magnetic stir bar is charged with PCAEMA-CTA (2.3012 g, 0.2169 mmol), n-BMA (2.4683 g, 17.352 mmol), AIBN (11.8 g, 0.0723 mmol), and anisole (20 mL). The solution is stirred until it became homogeneous. The flask is then fitted with a rubber septum, purged with nitrogen gas, and immersed in a silicone oil bath preheated to 60 °C. The polymerization is conducted for 48 h and quickly stopped by cooling down the solution to room temperature. The reaction solution is diluted with THF (10 mL) and purified by

repeated precipitation from a large volume of methanol (500 mL). Eventually, the PCAEMA-b-PBMA-CTA is collected and dried under vacuum for 24 h at room temperature. The conversion of the n-BMA monomer into PBMA block is 57% (GPC). Yield = 55% (2.62 g), Mn = 17 100 g/mol, and Mw/Mn = 1.19 (GPC). 1H NMR (CDCl3, 400 MHz): 0.7−1.0 (9H, main chain CH3 and side chain CH3), 1.3−2.0 (10H, CH3−CCN−CH3 and main chain CH2), 3.6−3.8 (4H, CH2OCH2), 3.9−4.0 (6H, side chain CH2CH2CH2OCO), 4.0−4.2 (4H, CH2O−Ar and CH2O−CO), 6.8−7.0 (2H, m-Ar H to OCH2), 7.5−7.9 (11H, o, m-Ar H to CN, o-Ar H to OCH2, p, o-Ar H to SCS and m-Ar H to SCS). 2.5. Synthesis of PCAEMA-b-BMA-b-PDOPAM Triblock Copolymer. A novel ABC-type azo triblock copolymer is synthesized by employing a typical RAFT polymerization procedure. A 10 mL Schlenk flask is filled with a mixture of PCAEMA-b-PBMA-CTA (0.406 g, 0.0237 mmol), new crystalline monomer DOPAM (0.3739 g, 1.1871 mmol), AIBN (1.9 mg, 0.0118 mmol), and anhydrous anisole (3.6 mL). The flask is tightly sealed with a rubber septum and purged with nitrogen gas. After 15 min of purge, the flask is heated to 70 °C under a nitrogen atmosphere for 48 h. The reaction is terminated by withdrawing the flask from a preheated oil bath. The solution is diluted with THF (3 mL) and dropped into 250 mL of methanol. The C


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Figure 1. 1H NMR spectrum of (A) PCAEMA-CTA, (B) PCAEMA-b-PBMA-CTA, and (C) PCAEMA-b-PBMA-b-PDOPAM (TBC-2) in chloroform solvent (CDCl3). (13H, CH3−CCN−CH3, main chain CH2 and CHCON), 2.2−2.7 (2H, CH2−Ar), 3.6−3.8 (4H, CH2OCH2), 3.9−4.0 (6H, side chain CH2CH2CH2OCO), 4.0−4.2 (4H, CH2O−Ar and CH2O−CO), 6.8− 7.0 (7H, m-Ar H to OCH2, m, o-Ar H to NH and Ar-NHCO), 7.5− 7.9 (11H, o, m-Ar H to CN, o-Ar H to OCH2, p, o-Ar H to SCS and m-Ar H to SCS).

precipitated TBC is purified by repeating the precipitation twice and dried in a vacuum oven for 24 h at 40 °C. The conversion of the DOPAM monomer into the PDOPAM block is 38% after 48 h of polymerization. Yield = 36% (0.28 g), Mn = 23 100 g/mol, and Mw/Mn = 1.24 (GPC). 1H NMR (CDCl3, 400 MHz): 0.7−1.0 (12H, main chain CH3 and side chains CH3), 1.0−1.3 (20H, (CH2)10), 1.3−2.0 D


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Figure 2. Evolution of molecular weight during RAFT polymerization in anisole: (A) GPC curves of macroinitiators and their corresponding TBCs, (B) dependence of molecular weight (Mn) and polydispersity (Mw/Mn) on molar ratio ([monomer]/[CTA]), and (C) evolution of molecular weights (GPC, 1H NMR, theoretical value) and polydispersity with molar ratio ([monomer]/[CTA]) (see Table 1). Other ABC-type TBCs with various compositions (Table 1) are prepared by varying the molar ratio of the DOPAM monomer to the PCAEMA-b-PBMA-CTA macroinitiator using a similar procedure. 2.6. Thin Film Preparation. A 1.0 wt % solution of azo TBC with varied compositions in THF is spin-coated onto silicon wafer at 3000 rpm for 1 min.29,59 The spin-coated film is kept under ambient conditions for 24 h to let THF solvent evaporates slowly. The thin film is then vapor-annealed for 24 h under the mixed solvent of THF and cyclohexane (70/30, v/v) in a covered round Petri dish (diameter, 10 cm) placed into a desiccator containing anhydrous calcium sulfate.60,61 The microphase-separated structure of the annealed thin film is investigated at room temperature by noncontact mode AFM and GISAXS.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Macroinitiators and Triblock Copolymers. ABC-type azobenzene-containing TBCs are successfully prepared by a powerful technique called RAFT62 process using DOPAM as a new acrylamide monomer, PCAEMA-b-PBMA-CTA as a macroinitiator, AIBN as an initiator, and anisole as a solvent. The reaction details are reported in Table 1. The PCAEMA-CTA macroinitiator is first obtained by adjusting the feed concentration of 2-[2-(4cyanoazobenzene-4′-oxy)ethyleneoxy]ethyl methacrylate monomer, 2-cyanoprop-2-yl-1-dithionaphthalate RAFT agent, and azoisobutyronitrile radical initiator in anisole at 75 °C for 24 h ([CAEMA]0:[CPDB]0:[AIBN]0 = 210:3:1) (Scheme 1A). The PCAEMA-CTA with both precisely controlled molecular E


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Macromolecules Table 2. Thermal Characteristicsa of the Prepared Azo Polymers Determined by Thermal Analysis (DSC) phase transition temp (T, °C), 1st cooling polymers (DOPAM/PBMA/Azo)/%

Tg

PCAEMA-CTA (0/0/100) DBC-CTA (0/38/62) TBC-1 (12/34/54) TBC-2 (26/28/46) TBC-3 (37/24/39) TBC-4 (50/19/31)

41 30 29 29 29 30

Tlc 91 104 110 112 113 115

(N−S) (N−S) (N−S) (N−S) (N--S) (N−S)

phase transition temp (T, °C), 2nd heating

Tm

Tg

213 215 216 218

42 47 48 49 51 52

Tlc 99 106 114 116 117 119

(S−N) (S−N) (S−N) (S−N) (S−N) (S−N)

enthalpy change (ΔH, J g−1), 2nd heating

Tm

ΔHlc

ΔHTm

233 235 237 239

2.62 1.64 1.46 0.64 0.42 0.22

3.23 5.31 6.13 8.22

Phase transition temperatures and their corresponding enthalpies during heating and cooling cycles are determined by DSC at a rate of 10 °C/min. Tg = glass transition temperature; Tlc = liquid crystal temperature; Tm = melting temperature; N−S = nematic-to-smectic phase; S−N = smectic-tonematic phase; ΔHlc and ΔHTm are enthalpies of mesophase transition and melting transition, respectively. a

et al. for azobenzene-containing block copolymers.65,66 The characteristics including compositions, molecular weights, and polydispersities are listed in Table 1. In order to investigate the kinetic study and molecular weight evolution during RAFT polymerization of DOPAM monomer with PCAEMA-b-PBMA-CTA macroinitiator, the dependency of Mn, Mw/Mn, and conversion on the feed ratio of monomer (DOPAM) to macroinitiator (PCAEMA-b-PBMA-CTA) is determined by GPC data evaluation of the prepared TBCs. Figure 2 displays typical GPC chromatograms and kinetics study of the obtained polymers, including macroinitiators and their TBCs. As observed in Figure 2A, all the GPC curves of the resulting TBCs are separated from the curve of PCAEMA-bPBMA-CTA macroinitiator and shifted linearly toward the highest molecular weight (33 700 g/mol) as the molar ratio of DOPAM monomer to PCAEMA-b-PBMA-CTA macroinitiator increases from 20 to 100 mol, proving that the obtained PCAEMA-b-PBMA-CTA with the dithiobenzoate end group has good reactivity to initiation and polymerization of the monomer DOPAM to produce PCAEMA-b-PBMA-b-PDOPAM in anisole.67 All the obtained TBCs exhibited a monomodal peak possessing relatively low PDI values ranging between 1.23 and 1.38, indicating that the PCAEMA-b-PBMACTA is converted to TBCs with no significant amount of free polymer as a side reaction. The behavior of GPC traces of the constructed TBCs signifies a well-controlled/living characteristic of RAFT polymerization, which means that the preparation of the TBC (PCAEMA-b-PBMA-b-PDOPAM) is a controllable process by varying the feed molar ratio of DOPAM monomer to PCAEMA-b-PBMA-CTA macroinitiator. In addition, the conversion of DOPAM into a block (PDOPMA) rose linearly with increasing DOPAM molar ratio as demonstrated in Figure 2B. This is accordance with a controlled/living RAFT polymerization. The molecular masses and polydispersities of the TBCs are dependent on the molar concentration of DOPAM monomer versus PCAEMA-b-PBMA-CTA macroinitiator (Figure 2C). However, the Mn(1H NMR)s are slightly elevated than the Mn(GPC)s and Mn(th)s, mainly due to the broad values of peaks integrals between different protons of different segments. 3.2. Thermal Properties and Liquid Crystalline Textures. Thermal and LC properties are investigated by thermal analysis (DSC) and OPM equipped with heating stage and temperature controller. Table 2 collects the phase transition temperatures and their corresponding enthalpies of all the obtained TBCs and their related macroinitiators (PCAEAMA-CTA and PCAEMA-b-PBMA-CTA). Typical first cooling and second heating DSC scans for all the TBCs

weight (Mn = 10 600 g/mol) and narrow polydispersity (Mw/ Mn = 1.17) is then used as a macromolecular chain-transfer agent to extend further the chain by addition of the second monomer (n-BMA) to form the PCAEMA-b-PBMA-CTA macroinitiator ([n-BMA] 0 :[PCAEMA-CTA] 0 :[AIBN] 0 = 240:3:1) (Scheme 1B). The PCAEMA-b-PBMA-CTA with well-controlled Mn (17 100 g/mol) and low Mw/Mn (1.19) is subsequently employed as a macroinitiator for the polymerization of the third monomer (DOPAM) to form the corresponding TBCs (Scheme 1C). The RAFT polymerization has provided a superior performance compared to other LFRP techniques due to its simplicity and applicability to monomers without use of catalyst.63,64 A series of azo-based TBCs with different compositions are synthesized by varying the feed molar ratio of [DOPAM]0/ [PCAEMA-b-PBMA-CTA]0 in the present of initiator AIBN at a fixed temperature (70 °C) for 48 h to examine the influence of molecular weight on phase transition properties. Both monomer conversion and molecular weight of the obtained TBCs increased linearly with increasing the molar ratio of DOPAM monomer to PCAEMA-b-PBMA-CTA macroinitiator, confirming the controlled/living manner of RAFT polymerization. The molecular structures of macroinitiators and their related TBCs are confirmed by 1H NMR spectroscopy. As a result, signals of all the protons in NMR spectra are in accordance with structures of polymers. Figure 1A−C shows 1H NMR spectra for macroinitiators (PCAEMA-CTA, PCAEMA-b-PBMA-CTA) and one of the triblock copolymers (TBC-2) as an example. As seen in Figure 1A, the signal at 1.0 ppm is assigned to protons of the methyl group (main chain CH3) in the repeating unit of the LC block, signifying the conversion of CAEMA monomer into PCAEMACTA. After the addition polymerization of n-BMA and DOPAM monomers, new signals corresponding to protons [side chain CH3, (CH2)10, Ar−CH2, and OCOCH2CH2CH2] related to the alkyl side chains of PBMA segment and PDOPAM block are observed respectively at 0.8, 1.3, 2.4, and 3.9 ppm (Figure 1B,C). This indicates the formation of triblock copolymer with three distinct blocks (PCAEMA, PBMA, and PDOPAM). The Mn values and compositions of the resulting polymers, including macroinitiators and their TBCs are estimated by GPC and confirmed by calculations using either the ratio of integrations of characteristic peaks in the 1H NMR spectra or the equation mentioned in the footnote at the bottom of Table 1. Obviously, the Mn(GPC)s have good agreement with the Mn(1H NMR)s and the theoretical molecular weights (Mn(th)s). Similar observations are made by Zhao et al. and Rajasekhar F


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Figure 3. DSC thermograms of homopolymer, macroinitiators and triblock copolymers with different azo contents at a scan rate of 10 °C/min: (A) first cooling scan and (B) second heating scan.

TBCs demonstrated a Tg of 30 °C, a TN−S in the vicinity of 112 °C, and a Tm in the range from 213 and 218 °C. On heating, PCAEAMA-CTA displayed a Tg around 42 °C and two mesophase temperatures at 99 and 179 °C. The sharp peak at 99 °C (2.6 J/g) corresponds to the transition temperature from smectic-to-nematic phase (TS−N), while on the contrary the weak peak around 179 °C is designated as the nematic-toisotropic transition temperature (TN−I). These phase transition behaviors are similar to those observed for azobenzene-based polymers in the literature.69 For PCAEMA-b-PBMA-CTA, only Tg and one mesophase temperature (TS−N) are observed at 47 and 106 °C. This is due mainly to the additional noncrystalline PBMA block that structurally different from the crystalline PCAEMA block within PCAEMA-b-PBMA-CTA. Contrast-

with their related poly(DOPAM) homopolymer as well as PCAEMA-CTA and PCAEMA-b-PBMA-CTA macroinitiators are highlighted in Figure 3. All the prepared azo polymers showed monotropic liquid crystallinity during heating and cooling scans, whereas poly(DOPAM) homopolymer displayed a melting temperature (Tm). Regarding DSC thermograms of the macroinitiators (Figure 3A), PCAEMA-CTA and PCAEMA-b-PBMA-CTA revealed a single glass transition temperature (Tg) at 41 and 30 °C, respectively, and they exhibited a strong and sharp exothermic peak at 91 and 104 °C during cooling cycle.68 The exothermic peak is assigned to the nematic-to-smectic transition temperature (TN−S). In the case of the poly(DOPAM) homopolymer, only a Tm at around 224 °C is observed during cooling scan. On the other hand, all G


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31 wt %, while the melting transition enthalpies are raised from 3.23 to 8.22 J/g with the increase of PDOPAM content from 12 to 50 wt % in the TBC systems, as evident in Figure 4B and Table 2. This suggests that the obtained results from the thermal analysis (DSC) are consistent with our expectation and they reveal similar trends to those reported in the literature.70,71 The mesophase structures of macroinitiators and their related TBCs are studied by OPM during the first cooling run as depicted in Figure 5. The TBCs showed significant differences as opposed to PCAEMA-CTA and PCAEMA-bPAMA-CTA macroinitiator. The macroinitiator (PCAEMACTA) displayed a homogeneous nematic phase with a typical Schlieren texture of four brushes at 108 °C upon cooling the sample from the isotropic melt (180 °C) without thermal annealing (Figure 5A).72 Subsequently, cooling to 100 °C and annealing for 5 min revealed a significant increase in the number of four brushes within the Schlieren texture (image not shown). Upon further cooling to the lower temperature of 45 °C and annealing for 2 h, the Schlieren texture is converted to the smectic structure with a typical rod-shaped texture (Figure 5B). By contrast, the PCAEMA-b-PAMA-CTA exhibited a smectic structure with a model fan-shaped focal conic texture at 92 °C by cooling from the isotropic phase after 1 h annealing at 160 °C (Figure 5C). This result is correspondent with that described previously by He et al.73 Unlike the PCAEMA-CTA and PCAEMA-b-PAMA-CTA macroinitiators, the TBCs with various azo contents are annealed for a few hours to develop their mesophase structures. In addition, the TBCs (TBC-3 and TBC-4) with low volume ratios (39 and 31 wt %) of the azo block had longer annealing times than those (TBC-1 and -2) with high LC contents of 54 and 46 wt %, respectively. For the DBC-1 and DBC-2 with 54 and 46 wt % of LC block, the batonnet texture of a smectic phase is obtained after annealing the samples at 240 °C for 2 h and then slow cooling to low temperature ranging from 110 to 49 °C. Subsequently, annealing the TBC-1 at 49 °C and TBC-2 at 57 °C for 1 h transformed the batonnet texture into more ordered smectic phase, suggesting the smectic C phase (Figure 5D,E). This remarkable result indicates a wide range of temperatures for the formation of mesophase. With regard to the DBC-3 containing low azo block fraction of 39 wt %, a nematic phase with a poorly threaded texture is observed by annealing the polymer at 240 °C for 4 h in order to develop its LC texture and then slow cooling to low temperature of 120 °C (Figure 5F). Upon further cooling to lower temperatures, a highly threaded texture is obtained (OPM image not shown). However, neither the smectic texture nor the nematic texture is observed in the case of the TBC-4, probably due to the low fraction of azo block (31 wt %) within this TBC. All observations under crossed polarizers on an optical microscope are in consonance with DSC measurements (Figure 3), and they are slightly different from those reported in the literature.74 3.3. Self-Assembled Structures in Triblock Copolymer Thin Films. It is common for the annealed block copolymer thin films to generate microphase-separated nanostructures (spheres, cylinders, lamellae, and gyroids), depending on several parameters such as the volume fractions of different segments and the extent of incompatibility between blocks within block copolymers.75 By incorporation of microphase separation of block copolymer and supramolecular corporation motion caused by the orientation of liquid crystal domains, the self-assembly system of block copolymer can be easily

ingly, the poly(DOPAM) homopolymer showed only a single Tm at 224 °C. The TBCs exhibited a Tg at the vicinity of 50 °C, a TS−N around 115 °C, and a Tm in the range between 233 and 239 °C, which is related to the PDOPAM block within TBCs (Figure 3B and Figure S10).60 The first two TBCs (TBC-1 and TBC-2) with high azo contents of 54 and 46 wt % had relatively sharp endothermic transition peaks (114 and 116 °C) corresponding to TS−N. On the contrary, the TBCs (TBC-3 and TBC-4) having low azo ratios of 39 and 31 wt % possessed broad mesophase transition peaks (TS−N) at 117 and 119 °C, respectively. Besides, the TBC-1 and TBC-2 displayed weak and broad peaks (Tm) in the range of 233−235 °C, whereas the TBC-3 and TBC-4 showed strong and broad Tm between 237 and 239 °C, which is partly due to the increase in the molecular weight (volume fraction) of PDOPAM block in the TBCs. As marked in Figure 4A, the thermal transitions (Tg and Tm) of

Figure 4. Evolution of thermal transitions and their enthalpies triblock copolymers: (A) dependence of transition temperature molecular weight (Mn); (B) dependence of transition enthalpy molecular weight of azo block and PDOPAM segment (see Table

for on on 2).

TBCs are slightly moved to high temperature of 52 and 239 °C, respectively, as the composition of PDOPAM segment rises from 12 to 50 wt %. The mesophase transitions (TS−N) of TBCs are also slightly shifted toward higher temperature in the vicinity of 119 °C with the decrease in their peaks intensities as the volume ratio of LC block decreases from 54 to 31 wt %. The mesophase transition enthalpies are decreased from 1.46 to 0.22 J/g with reducing the azo side-chain segments from 54 to H


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Figure 5. Mesophase textures of the macroinitiators and the related triblock copolymers: (A) PCAEMA-CTA-108 °C, (B) PCAEMA-CTAannealed-45 °C for 2 h, (C) PCAEMA-b-PBMA-CTA-92 °C, (D) TBC-1-annealed-49 °C for 1 h, (E) TBC-2-annealed-57 °C for 1 h, and (F) TBC3-120 °C. Neither LC texture is detected for the TBC-4. Magnification: ×100.

controlled in the nanometric scale with excellent producibility. Moreover, the insertion of soft segment between two hard blocks enables the block copolymer to self-assemble into complex nanostructures with new properties.76−78 The azobenzene-containing triblock copolymers are superb candidates for this research study. Thin films are prepared by spincoating the triblock polymers onto silicon wafers using THF solvent. The spin-coated films are annealed under the vapor of the mixed solvent of THF/cyclohexane (70/30, v/v) for 24 h. The thin films possessed thickness in the range between 2.8 and 3.3 nm, which is measured by AFM using EXI-V1.7.6 image processing software program developed by Park Systems. The self-assemblies of TBC thin films with different compositions are generated by the orientation and lateral ordering of TBC domains caused by the degree of swelling using the mixed solvent (THF/cyclohexane). The selfassembled nanostructures of TBCs in thin films are studied using tapping-mode AFM. The morphologies of TBC thin films are dependent on several physical factors including casting process (spin coating), annealing procedure (solvent annealing treatment), and volume fractions of three different components. The TBC-1 with higher volume fractions of the azo side chains (54 wt %) and the PBMA segment (34 wt %) versus lower fraction of PDOPAM block (12 wt %) exhibited a morphology consists of a mixture of cylinders and lamellae (Figure 6A), indicating that the TBC-1 is trapped in metastable morphology.58 In contrast, the TBC-2 possessed a micophaseseparated lamellar morphology, as expected from the ratios of the three different blocks (LC block: 46 wt %; PBMA: 28 wt %; and PDOPAM: 26 wt %) (Figure 6B). The domain spacing (d spacing) of the lamellar phase determined by AFM is 34 nm, which is relatively in good agreement with GISAXS data. The formation of the lamellar structure is most likely due to identical length of distinct blocks (PCAEMA, PBMA, and PDOPAM). This result is in conformity with the result found by Zhu et al.79 In the case of the TBC-3 containing 39 wt % of LC block, 24 wt % of PBMA, and 37 wt % of PDOPAM, a similar morphology like in the TBC-1 is found (AFM image not shown). On the other hand, the TBC-4 with higher

Figure 6. Typical tapping-mode 2D and 3D AFM images (scan size = 2 × 2 μm and scale bar = 400 nm) of tiblock copolymer thin films with different compositions: (A) TBC-1-THF-SC (3000 rpm)-solventannealing (THF-70%/cyclohexane-30%); (B) TBC-2-THF-SC (3000 rpm)-solvent-annealing (THF-70%/cyclohexane-30%); (C) TBC-4THF-SC (3000 rpm)-solvent-annealing (THF-70%/cyclohexane30%); (D) TBC-4-THF-SC (3000 rpm)-solvent-annealing (THF70%/cyclohexane-30%). Inserts are two-dimensional fast Fouriertransform (FFT) images.

concentration of PDOPAM block (50 wt %) versus lower concentrations of PBMA segment (19 wt %) and azo side chains (31 wt %) revealed hexagonally ordered cylinder morphology that is consistent with GISAXS patterns. The cylinder d spacing and pore size obtained from AFM are 36 and 32 nm, respectively (Figure 6C). The hexagonally packed cylinders of the minority components (PCAEMA, A block and PBMA, B block) are enclosed in the majority component (PDOPAM, C block) that formed the matrix phase.74,80 Figure 6D depicts the 3D-AFM topography image of the TBC-4 thin I


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Macromolecules film as an example to define the film thickness. The morphologies of TBCs in thin films are also supported by the clear fast Fourier transform patterns. These results indicate that the morphologies of the prepared TBCs varied significantly, depending on volume fractions of three different blocks and their chemical nature. Therefore, these TBCs appear to be superior to those for conventional azo-based TBCs, and they can promise to find their applications in the field of nanolithography. 3.4. Nanostructure Identification by GISAXS. To study self-assembly (microphase separation) of TBCs in thin films, GISAXS [monochromatized X-ray (λ = 1.1747 Å): 10°−0.23°; the SCX: 4300-165/2 CCD detector] experiments are conducted on the 3C1 beamline at the Pohang Accelerator Laboratory (S. Korean). All GISAXS measurements are performed at room temperature. Figure 7 demonstrates in-

%-PBMA, and 26 wt %-PDOPAM suggests lamellar nanostructure which is consistent with the characteristic peak ratios of q/ q* = √1 = 1, √4 = 2, and √9 = 3 (Figure 7A), where q is defined as the length of scattering vector and q* is denoted the position of the first diffraction peak. On the contrary, the semilogarithmic profile of TBC-4 (Figure 7B) displays scattering vectors of √1 = 1, √3, and √7 times relative to the first-order reflection, suggesting hexagonally packed cylinder morphology. The morphologies of TBCs in thin films are also supported by 2D scattering images obtained from GISAXS scattering patterns. The d spacings of lamellar (34.8 nm) and hexagonally ordered cylinder (36.9 nm) are calculated from GISAXS profiles using the following equation: d = 2π/q. 3.5. Photoresponsive Behavior of Azo Dyes in Triblock Copolymer. In general, azobenzene-based systems undergo photoisomerization from the trans isomer to the cis isomer upon UV light irradiation with appropriate wavelength or when chiral azobenzene molecules are doped in cholesteric liquid crystals. However, the cis isomer can return to the trans isomer by visible light irradiation or in dark storage. The photoisomerization behavior of azo chromophores in triblock copolymer (TBC-2) is investigated through UV−vis absorption spectrophotometer in chloroform solution (Figure 8) and in thin film (Figure 9). Figure 8A reveals a slight decrease in the absorption band intensity corresponding to the π−π* transition

Figure 7. GISAXS diffraction patterns and their corresponding twodimensional images for triblock copolymers with different compositions: (A) TBC-2; (B) TBC-4. The d values are calculated according to the following equation: d = 2π/q.

plane intensity profiles of GISAXS patterns as a function of scattering vector for the PCAEMA-b-PBMA-b-PDOPAM triblock copolymer with two different molecular weights and compositions. The microdomain structures and d spacings of TBCs in thin films are determined from scattering peaks in GISAXS diffraction patterns (Figure 7A,B). The GISAXS scattering profile of TBC-2 containing 46 wt %-azo block, 28 wt

Figure 8. UV−vis absorption spectra of azo triblock copolymer (TBC2) in chloroform solution: (A) upon irradiation with UV light at 365 nm for (a) 0, (b) 25, (c) 50, (d) 75, (e) 100, and (f) 125 s; (B) in dark storage for (f) 0, (e) 5, (d) 10, (c) 15, (b) 20, and (a) 25 min for stability. J


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4. CONCLUSION Azo-based ABC triblock copolymers with precise control over their molecular weights, compositions, and architectures are successfully prepared by RAFT polymerization. The TBCs are made up of poly(CAEMA) and poly(DOPAM) as hard blocks and central poly(BMA) as soft block. Molecular weights and polydispersities of TBCs increased linearly with conversion and feed ratio of monomer to macroinitiator, indicating a wellcontrolled/living fashion of RAFT polymerization. The TBCs possessed low polydispersities which varied in the range of 1.23−1.38, and they revealed different mesophase textures depending on ratios of azo dye contents. The careful investigations of phase transitions and LC textures of TBCs indicated that the TBC-1 and TBC-2 with high azo contents of 54 and 46 wt % exhibited sharp mesophase transition peaks and smectic textures compared to the TBC-3 possessing low azo part (39 wt %) which showed broad LC phase transition peaks and nematic texture as well as the TBC-4 having the lowest fraction of azo block (31 wt %) which revealed no mesophase texture. The TBCs self-organized into varied nanostructures depending on volume fractions of three different components. The TBC-1 and TBC-3 generated a mixture of cylinder and lamella morphologies, whereas the TBC-2 having 46 wt % of azo side chaims, 28 wt % of PBMA, and 26 wt % of PDOPAM produced lamellar nanostructure as evidenced by both AFM and GISAXS measurements. By contrast, the TBC-4 with the highest fraction of PDOPAM (50 wt %) formed hexagonal cylinder-type morphology. Furthermore, all the prepared azo TBCs exhibited a reversible trans−cis−trans photoisomerization behavior driven by UV and visible light irradiation (or dark storage) at different intervals of time. On the basis of these results, we believe that the designed TBCs can potentially promise for optics, display technology, light-responsive materials, and nanolithography.

Figure 9. UV−vis absorption spectra of azo triblock copolymer thin films possessing varied azo dye contents: (A) the irradiation of TBC-2 thin film with UV light at 350 nm for (a) 0, (b) 2, (c) 4, and (d) 8 min and with visible light at 460 nm for (a′) 1, (b′) 2, and (c′) 4 min; (B) absorption of UV and visible radiation in TBC thin films containing (a) azo-54%, (b) azo-46%, (c) azo-39%, and (d) azo-31%.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02012. Details about the synthesis and characterization of azobased monomer (CAEMA), new acrylamide monomer (DOPAM) bearing long alkyl side chain, as well as poly(DOPAM) homopolymer; Schemes S1 and S2 describing the synthetic routes to CAEMA and DOPAM; Figures S1−S5 and Figurse S7−S10 representing 1H NMR spectra and DSC thermograms of the prepared azo-phenol compound, mesogenic alcohol, CAEMA, DOPAM, and poly(DOPAM); besides, Figure S6 demonstrating mesophase textures of mesogenic alcohol and CAEMA (PDF)

(λmax = 361 nm) and a small increase in the n−π* transition absorption (λmax = 450 nm) as a function of irradiation time with UV light for 25, 50, 75, 100, and 125 s. In contrast, Figure 8B displays that the reversion from cis isomer to trans isomer is induced in dark storage after saturated UV light irradiation for 125 s. Remarkably, the peak intensity of the absorption band π−π* is significantly increased, while the absorption band n−π* is slightly decreased in its peak intensity, confirming equilibrium achievement of azo dyes in the triblock copolymer. In case of the TBC-2 thin film prepared by spin-coating the 0.5 wt % solution of TBC-2 in chloroform at 500 rpm for 0.5 min under UV and visible light irradiation, Figure 9A shows exactly similar observations to those in chloroform solution under UV light exposure and dark storage. This reversible trans−cis−trans photoisomerization enables azo-based TBC to change its polarity, surface tension and configurational structure. Moreover, Figure 9B demonstrates the UV−vis absorption spectra of TBC thin films (thickness: 0.29−0.34 μm) with different azo dye ratios. As recorded, both strong absorption band π−π* at 361 nm and weak absorption band n−π* around 450 nm are clearly decreased in intensity with the decrease of azo content within TBCs.81 This shows consistency with DSC measurements.

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AUTHOR INFORMATION

Corresponding Author

*(A.Z.) E-mail [email protected], athmen2004@ hotmail.com; Tel +821083288685. ORCID

Athmen Zenati: 0000-0002-0775-6295 Notes

The authors declare no competing financial interest. K


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ACKNOWLEDGMENTS This work was supported by the Korea Science and Engineering Foundation (KOSEF), World Premier Materials (WPM) Program and Hanyang University Scholarship (HUS) Program. A.Z. expresses his profound gratitude to Hanyang University for providing an excellent atmosphere to do this research, Prof. Yang-Kyoo Han for his guidance, and Dr. JeGwon Lee for sharing his knowledge, discussions, and assistance during experiments.

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