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Synthesis and characterization of novel ABA-typeazobenzene-containing tri-block copolymers fromtelechelic polystyrene
a

a

b

b

Tota Rajasekhar , Mummuluri Trinadh , Rasmita Sahoo , Surajit Dhara & Annadanam V.
Sesha SainathaaPolymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology,
Tarnaka, Hyderabad 500007, Indiab

School of Physics, University of Hyderabad, Hyderabad 500046, India
Published online: 27 Oct 2014.

To cite this article: Tota Rajasekhar, Mummuluri Trinadh, Rasmita Sahoo, Surajit Dhara & Annadanam V. Sesha Sainath (2014):
Synthesis and characterization of novel ABA-type azobenzene-containing tri-block copolymers from telechelic polystyrene,
Designed Monomers and Polymers, DOI: 10.1080/15685551.2014.971396
To link to this article: http://dx.doi.org/10.1080/15685551.2014.971396

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Designed Monomers and Polymers, 2014
http://dx.doi.org/10.1080/15685551.2014.971396

Synthesis and characterization of novel ABA-type azobenzene-containing tri-block copolymers
from telechelic polystyrene
Tota Rajasekhara, Mummuluri Trinadha, Rasmita Sahoob, Surajit Dharab and Annadanam V. Sesha Sainatha*a

Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500007, India;b
School of Physics, University of Hyderabad, Hyderabad 500046, India

Downloaded by [IICT Indian Institute of Chemical Technology] at 04:08 27 October 2014

(Received 2 July 2014; accepted 19 September 2014)
The telechelic polystyrene (tPS) was used as a macro-reversible addition fragmentation chain transfer (RAFT) agent to
prepare the ABA-type tri-block copolymers (TBCs), poly{4-[4-(11-acryloyloxyundecyloxy)phenylazo]benzonitrile}-bpolystyrene-b-poly{4-[4-(11-acryloyloxyundecyloxy)phenylazo]benzonitrile} [poly(AUPAB)-b-PS-b-poly(AUPAB)] with
4-[4-(11-acryloyloxyundecyloxy)phenylazo]benzonitrile (AUPAB) as a monomer by the RAFT polymerization process.
The poly(AUPAB) was used as a reference to probe the role of polystyrene block in poly(AUPAB)-b-PS-b-poly(AUPAB)
macromolecular chain. The TBCs were characterized by spectral techniques, thermal analysis, and polarizing optical
microscopic studies and compared the results with individual blocks, poly(AUPAB), and tPS. The TBCs exhibited higher
thermal stability and liquid crystal–isotropic phase transition peak temperature (TLC-I), and glass transition temperature(Tg) values compared to the poly(AUPAB). TBC TLC-I value was dependent on the AUPAB wt% in macromolecular
chain. Furthermore, the acquired results by optical microscopy suggest that after incorporation of the polystyrene block inacrylate-based pendant azobenzene polymer chain resulted in lower domain size of focal conic texture, which ischaracteristic of smectic-A liquid crystalline phase compared to the homopolymer. The photophysical properties of the
TBCs were similar to those of the poly(AUPAB). Microphase-separated nano-segregation was observed in annealed thinﬁlms of TBCs.
Keywords: styrene-based telechelic macro-RAFT agent; azobenzene polymers; ABA-type tri-block copolymers; RAFT
process; liquid crystalline polymer

Introduction
Design and synthesis of azobenzene [1]-containing
macromolecular architecture is one of the major current
research areas. These materials resulted in reliable reversible photoisomerization, photoalignment, and photoinduced cooperative motion for various applications such as
actuators, holographic,[2,3] and a photoswitchable
responders.[4–9] Pendant azobenzene groups in side-chain
liquid crystalline (LC) polymers inﬂuence the material
properties such as in light-driven two-dimensional movement and bending of LC elastomers and assist microphaseseparation to obtain well-ordered nanostructure assemblies
in block copolymers. It has also been reported that thevariation in volume fraction and azobenzene location[10,11] in macromolecular architecture [12–14] can tune
the material crystallization, optical transparency, LC
elastomeric nature, and photoinduced electro-opticalproperties.
In general, azobenzene polymers were obtained by the
direct polymerization of azobenzene-containing monomers
and post-functionalization [15,16] of pre-polymers. At theinitial stage of the development, pendant azobenzene
polymers had been synthesized mainly by classical free
radical polymerization (FRP) of azobenzene pendant*Corresponding author. Email: avss@iict.res.in© 2014 Taylor & Francis(meth)acrylate monomers. These homopolymers [17–20]
and copolymers [21] were used to demonstrate theirmulti-responsive reversible gel character, supramolecular
photomechanical systems,[22] optical switching, andimage storage applications.[23] Classical FRP is not talented to control the polymer molecular weights and targeted architecture of the macromolecular chains, which
are solely responsible for the polymer physical properties.
Therefore, it is necessary to control the macromoleculararchitecture by adopting controlled polymerization methods, such as ionic and controlled FRP methods. A numberof researchers synthesized polystyrene or poly(methyl
methacrylate) block possessing AB type azobenzene LC
copolymers using anionic polymerization [24] in controlled manner in quantitative yields with narrow molar
mass distribution.[25,26] Recently, well-deﬁned andcontrolled architecture of the azobenzene-containing polymers were successfully synthesized using straightforward
controlled FRP methods. In the past, side-chain azobenzene methacrylate di-block and ABA type tri-block
copolymers (TBCs) were synthesized using atom transfer
radical polymerization (ATRP) method.[27–30] On theother hand, the RAFT process competes with classical FRP
to the concerned of maintaining narrow polydispersity andDownloaded by [IICT Indian Institute of Chemical Technology] at 04:08 27 October 20142

T. Rajasekhar et al.

living nature. Using the RAFT process, the azobenzenecontaining homopolymers,[31–33] di-block copolymers,[34–36] Ag nanoparticles at the terminal end [37] of thepolymer chain, push and pull structures [38] of the azo
pendants, and tetrazole-containing [39] azo polymers weresuccessfully prepared for photoinduced birefringence andsurface-relief-gratings applications. However, before thediscovery of the ATRP and RAFT, Otsu et al. reported thatthe tetraethylthiuram disulﬁde was able to polymerize styrene by thermally (iniferter process) to produce tPS and it
was further photoinitiated to construct ABA type copolymers in living nature.[40–42] By following the similar
procedures, photoinitiated ABA type LC block copolymers [43] and ﬂuorinated AB and ABA type copolymers[44,45] were reported. This method suffers lack of molecular weight control and not amenable to a wide verity offunctional monomers. In particular, it failed to polymerizephotolabile group containing monomers. We herewith
report for the ﬁrst time the initiation of tPS (B block) byradical initiator to produce ABA type TBCs having acrylate-containing pendant azobenzene segments (A block),and characteristics of the resulted block polymers werecompared with acrylate containing pendant azobenzene
homopolymer to understand the polystyrene block role in
TBC.

Experimental
Materials
4-Aminobenzonitrile, triethylamine and azobisisobutyronitrile (AIBN) (Avra); acryloyl chloride (Hychem);
2-cyano-2-propyl dodecyltrithiocarbonate (CPDTC); 11bromoundecanolandtetraethylthiuram
disulﬁde(Sigma-Aldrich); anisole, ethanol, and methanol (Finar
Chemicals Ltd); dry 1,4-dioxane (Spectrochem);
tetrahydrofuran (THF); toluene and hydrochloric acid
(Rankem); phenol, sodiumhydroxide (NaOH), sodiumnitrite, and molecular sieves 4 Å (sd ﬁne-chem Ltd) wereused as received. Styrene (Avra) was puriﬁed by washingwith aqueous NaOH and water and dried over sodium
sulfate (sd ﬁne-chem Ltd) at cold condition. Afterward,
it was distilled at 40 °C under reduced pressure prior touse. Anisole (Sisco Research Laboratories Pvt. Ltd) wasdried by reﬂuxing over sodium metal up to 2 h anddistilled before prior to use. THF was dried over sodium
metal and benzophenone.

Polymerization
Polymerization of the 4-[4-(11-acryloyloxyundecyloxy)phenylazo]benzonitrile (AUPAB)
The monomer, AUPAB, was synthesized according to thereported procedure.[27] The AUPAB was polymerized

via RAFT method.[46] A mixture of AUPAB (1 g,
2.24 mmol), AIBN (9.8 mg, 0.06 mmol) and RAFT agent,
CPDTC (31 mg, 0.09 mmol) in 10 mL of dry THF, wasdeoxygenated by three times of freeze–pump–thaw cycles.
Then, the ﬂask was sealed and heated to 80 °C understirring and allowed this condition for 30 h. After theperiod, the polymerization was terminated by the additionof 0.5 mL of methanol and exposing to air. Thepoly(AUPAB) was isolated by threefold of precipitation incold methanol. Yield: 20%.
Preparation of tPS
Tetraethylthiuram disulﬁde (4 g) and 60 mL of styrene
were dissolved in 60 mL of THF in a 500-mL round-bottom ﬂask [43,44], and the mixture was deoxygenatedthree times by freeze–pump–thaw cycles, and the ﬂask
was sealed and kept at 75 °C oil bath with constant stirring. After 5 h, the polymer was recovered by THF
evaporation on rota evaporator at room temperature. The
viscous mixture was precipitated in excess amount ofmethanol. The precipitate was puriﬁed twice by
dissolving in THF and re-precipitating in methanol.
Yield: ~20%. FTIR (KBr, Cm−1): ν 1415 (–N–C=S),[47]ν 1024 (–C=S), and ν 541 (C–S). 1H NMR (300 MHz,CDCl3): δ 6.30–7.26 (Ar–H), δ 3.60 and 4.00(–N–CH2–), δ 1.66–2.18 (–CH–, back bone), δ 1.26–1.66(–CH2–, back bone), and δ 1.08–1.26 (–CH3, terminalends). 13C NMR (125 MHz, CDCl3): δ 145.1, 127.9,
125.6, 39.8–49.7, and 11.6. In UV–vis spectrum, λmax
was at 282 nm (0.133 abs) due to N,N-diethyldithiocarbamate end groups and 253 nm (0.192 abs) due to combination of phenyl rings of the polystyrene back bone
and the dithiocarbamate end groups in THF (2 mg in
25 mL). The tPS calculated molecular weight from 1H
NMR was 2973, by considering 2 end terminal groups
per one macromolecular chain (ideal conditions, chain
terminal end groups functionality is 2). By size exclusive
chromatography (SEC) data, the tPS number averagemolecular weight (Mn) was 2634 and dispersity (Mw/Mn)
was 1.75. The macromolecular chain terminal end groups
functionality was calculated with 1H NMR spectrum and
SEC data of tPS. At ﬁrst, the number of repeating units(24) of the tPS macromolecular chain was calculated
from SEC data. Then, the phenyl rings signals intensity(24 × 5) was taken as 24 repeating units in 1H NMR
spectrum, and with that relative value, the terminal ends
were calculated. It was 1.75. In addition to that, thechain terminal end groups presence was conﬁrmed bythermogravimetry (TG) curve. The TG curve showed
two stage of degradation, ﬁrst-stage degradation [48](120–265 °C) was corresponding to terminal end groups
degradation, where weight loss was about 6.01 wt%
which was matching with the value of calculated
terminal end groups of the macromolecular chain

Designed Monomers and Polymers

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(5.47 wt%) [(macromolecular chain terminal end groups
molecular weight ÷ Mn) × 100].
Preparation of poly{4-[4-(11-acryloyloxyundecyloxy)
phenylazo]benzonitrile}-b-polystyrene-b-poly{4-[4-(11acryloyloxyundecyloxy)phenylazo]benzonitrile} [poly(AUPAB)-b-PS-b-poly(AUPAB)] TBCs
Typical polymerization procedure was as follows. A
mixture of AUPAB (750 mg, 1.68 mmol), AIBN
(8.2 mg, 0.05 mmol) and macro-RAFT agent, tPS(132 mg, 0.05 mmol) in 8 mL of dry anisole/2.5 mL dry
1,4-dioxane was deoxygenated by three times of freeze–
pump–thaw cycles. Subsequently, the ﬂask was sealed
and heated to 80 °C under stirring and allowed this condition for 20 h. Then, the polymerization was terminatedby the addition of 0.5 mL of methanol and exposed toair. The polymer was isolated by twofold precipitation incold methanol and one time in ethylacetate and n-hexane(15:85 v/v) mixture. Yield: 40%.
Thin ﬁlm preparation
The homopolymer or TBC (5 mg mL−1) was dissolvedin 1:4 (v/v) ratio of the THF and toluene mixture. A few
drops of the polymer solution were added using a chemical dropper on quartz plate, which was placed on thebase of spin coater (APEX Instruments Co. SCU 2005)
and allowed 30 s at room temperature, followed by 750,
1250, and 750 rpm rotation with each stage for 30 s.
These spin-coated ﬁlms were kept under dark conditionsfor 6 h before measuring the UV–vis absorption spectra.
After measuring the spectra, the ﬁlms were annealed at
113 and 140 °C for TBC-1 and TBC-2, respectively, for
15 h under vacuum condition and measured UV–vis
absorption spectra.
Measurements
The FTIR spectra were recorded on a Thermo Nicolet
Nexus 670 spectrometer at a resolution of 4 cm−1 using
KBr optics at room temperature, and a minimum of 32
scans were signal averaged. The NMR spectra wererecorded on AVANCE or INOVA spectrometer in CDCl3/
DMSO-d6 with tetramethylsilane as an internal standard,
and the magnetic ﬁeld frequencies were 300 or 500 MHz
for 1H NMR and 60 or 125 MHz for 13C NMR measurements. Mass spectra (ESI-MS) were recorded on FINNGAN LCQ Advance Max in methanol. The absorption
spectra were recorded on Spectro UV–vis double beam PC,
8-scanning auto cell, LABOMED INC. Melting points ofthe small molecules were obtained on apparatus VEEGO,
model: VMP-D. The SEC was performed using a Shimadzu system built in with one (300 × 7.5 mm) PLgel 5 μm
10E3Å column (VARIAN) and evaporative light-scattering

3(PL-ELS), and refractive index detectors (Polymer
Laboratories). The eluent was THF (HPLC grade) and a
ﬂow rate of 0.5 mL min−1 at 30 °C. Calibration for detector
response was gained using single narrow polystyrene standards (molecular weights: 1480; 3950; 10,680; and 31,420
from Polymer Labs). One milligram of polymer was
dissolved in 1 mL of THF. Values of Mn and Mw/Mn were
determined using LC Solution for Windows software. The
TG curves were obtained from TGA Q500 thermal analyzer at heating rate of 10 °C min−1 under nitrogen atmosphere. Thermal analyses were performed by a differential
scanning calorimeter, DSC Q100 at a heating rate of
10 °C min−1 under nitrogen atmosphere. Typical procedure
was as follows. The sample was ﬁrst equilibrated to –70 °C
and heated (ﬁrst heating) from –70 to 170 °C at
10 °C min−1, and then, it was cooled (ﬁrst cooling) from170 to –70 °C at a rate of 10 °C min−1 using TA,
Refrigerated Cooling System 90. Then, the sample wasagain heated (second heating) from –70 to 170 °C at a rateof 10 °C min−1, and then, it was cooled (second cooling)from 170 to –70 °C at a rate of 10 °C min−1. The second
heating and cooling curves were reported. The LC behaviorof the polymers was examined by polarizing optical
microscopy (POM) (Nikon, POL100 V) using a heatingstage and temperature controller (Mettler FP 90). XRD
measurement was made on non-oriented polymer sample.
The polymer was ﬁlled into a Lindemann capillary tube ofdiameter 0.5 mm in the isotropic phase, and the ends were
ﬂamed sealed. The diffraction pattern was recorded on animage plate while cooling. Wide-angle X-ray diffraction(XRD) proﬁles of the polymers were recorded using a
Bruker D-8 Advance power XRD instrument (anode: Cu –WL1: 1.5406), from 2 to 65° at a step of 0.013° and steptime of 13.6 s at room temperature. The TEM (JEOL,
200KV, Japan.) images of thin ﬁlms on TEM grids wererecorded. The samples were prepared as following procedure. 0.5 wt% polymer solution (in 1:4 THF and Toluene)
was poured onto water surface and then transferred theobtained thin ﬁlms onto TEM copper grids. These grids
were annealed for 72 h at 113 and 140 °C for TBC-1 andTBC-2, respectively. Then, a droplet of uranyl acetate (2 wt%) in ethanol was deposited onto the annealed ﬁlms. After
1 min, the excess solution was removed by a piece of ﬁlter
paper by capillary action.
Results and discussion
The poly(AUPAB)-b-PS-b-poly(AUPAB) TBCs weresynthesized by RAFT process using AIBN as an initiator, tPS as a macro-RAFT agent, and the AUPAB as amonomer. As a reference, the homopolymer of the
AUPAB was polymerized by RAFT process using
the CPDTC as a RAFT agent. The chemical structures ofthe polymers are shown in Scheme 1. The synthesized
homopolymer (HP) and TBCs with 43 (TBC-1) and4

T. Rajasekhar et al.

87 wt% (TBC-2) of azobenzene content in copolymer
showed monomodal SEC curves, and the molar masses
are summarized in Table 1. The molecular weights of the
HP and TBCs were calculated from 1H NMR spectrausing Equations (1) and (2).
HP molecular weight ¼ AUPABðf w Þ  ½ðI 4:03  I 3:32 Þ 2 þ CPDTCðf w Þ

Downloaded by [IICT Indian Institute of Chemical Technology] at 04:08 27 October 2014(1)
where AUPAB(fw): AUPAB monomer molecular weight,
I4.03 and I3.32: intensities in 1H NMR spectra at δ 4.03
and 3.32 ppm, and CPDTC(fw): RAFT agent molecularweight.
TBC molecular weight ¼ tPSðMNMR Þ þ AUPABðf w Þ ½ðazo I7:688:02  6Þ ðtPS I6:286:78  54Þ(2)
where tPS(MNMR): molecular weight of tPS from 1H
NMR, azoI7.68–8.02 and tPSI6.28–6.78: intensities in 1H
NMR spectra for azo block at δ 7.68–8.02 ppm and fortPS block δ 6.28–6.78 ppm.
The calculated molecular weights from the 1H NMR
spectra and SEC values of the HP and TBC-1 had good
agreement with each other, whereas the TBC-2 molecularweight was conﬂicted with the SEC value. A similar
observation was made for azobenzene block copolymers
by Zhao et al.[35] The FTIR spectra of the HP and
TBCs showed characteristic bands around 2227, 1733,
and 1468 cm−1 due to the stretchings of cyano, ester carbonyl, and azobenzene groups, respectively. The ester
carbonyl stretchings of the HP and TBCs were shifted
about 14 cm−1 higher wavenumber compared to themonomer ester carbonyl. The HP and TBCs were readilysoluble in polar aprotic solvents such as dimethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone andTable 1.

RAFT polymerization experimental conditions.a

Polymer code

Solvent

Homopolymer
HP
THF
Tri-block copolymer
TBC-1
AnisoleTBC-2
Dioxanea

THF and in chlorinated solvents, for instance, chloroform, 1,2-dichloroethane and dichloromethane.
The 1H NMR spectrum of the TBC-1 (Figure 1(a))
showed broad and coalesced signals centered at δ 7.90,
7.76, and 6.97 ppm due to the azo block benzene protons, and δ 7.10 and 6.59 ppm signals were due to thebenzene protons of polystyrene block. The TBC signalsof the –O–CH2– methylene (centered at δ 4.05 ppm),
pendant methylene spacer, polymer backbone methylene
and methine protons, and terminal ends of the polymer
chain (δ 0.8–2.7 ppm) were observed similar to thehomopolymer. The signals at δ 3.2–3.7 ppm indicatedthe living nature of the macromolecular chain terminal
ends (–N–CH2–) of the TBC-1. In the case of TBC of
higher molecular weight, from Mw: 12,385 onwards, theterminal end –N–CH2– signals were not observed in 1H
NMR spectrum. It indicates that the living nature of themacromolecular chain disappeared. The 13C NMR
signals of the TBC were in good agreement with theassigned macromolecular structure as shown in
Figure 1(b).
The HP and TBCs had three stages of thermal degradation, whereas tPS exhibited two-stage degradation
under nitrogen atmosphere as shown in Figure 2. The
TBCs onset decomposition temperatures were higher
about 40 °C compared to the HP. The temperatures of
5% (Td5) and 10% (Td10) weight loss of the HP, tPS, and
TBCs are presented in Table 2. The Td5 and Td10 values
of TBCs were higher compared to the HP. Hence, we
deduce that this could be due to the incorporation ofpolystyrene block in the middle of the acrylate-based
azobenzene polymer chain. Higher molecular weightTBC-2 exhibited about 8 °C higher thermal stability at
Td5 compared to their respective low molecular counterpart (TBC-1). This is due to the vanishing of chain
terminal end functional groups for higher molecularweight TBC-2 in the process of the polymerization. We
observed that the chain terminal end –N–CH2– signals
were not noticed in 1H NMR spectrum for the TBC-2.

SEC[AUPAB]0b[RAFT agent]0b × 102

Molecular weight(1H NMR)

Mw g mol−1

Mw/Mn

AUPAB contentc/wt%

0.22

0.89

5497d

5089

1.26

94.07

0.21
0.67

0.63
2.01

5178e
23311e6699
13,394

1.60
1.32

43
87

[AUPAB]:[CPDTC]:[AIBN] = 75:3:2 (HP), [AUPAB]:[tPS]:[AIBN] = 100:3:3 (TBC-1 and 2), polymerization temperature: 80 °C, and time: 30 h.
In feed.
c
Calculated from 1H NMR spectrum.
d
Obtained from Equation (1).
eObtained from Equation (2).
b

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Designed Monomers and Polymers

Figure 1.

5

NMR spectra of poly(AUPAB)-b-PS-b-poly(AUPAB) of (a) 1H and (b) 13C.

The DSC thermogram of the HP exhibited Tg value
at 17 °C, and an endothermic peak centered at 100 °C,
which is assigned to a LC–isotropic phase transition temperature (TLC-I) on heating cycle, and the crystal transition temperature (Tc) was at 98 °C as shown in Figure 3.
In the case of the TBC-1 with 43 wt% pendant azobenzene content, the thermogram revealed two Tg points.
The ﬁrst Tg was assigned to segmental mobility of poly
(AUPAB) blocks, at about 29 °C, and the second Tg wasdue to the polystyrene block, at about 85 °C, while theneat tPS showed Tg at 81 °C. This indicates that the
TBC-1 exhibited phase separation [43,49,50] behavior.
On the other hand, having 87 wt% of azobenzene
content, TBC-2 exhibited single Tg point at 30 °C. The

TLC-I in the TBCs increased with increasing the pendantazobenzene content (121 °C for 43 wt% azobenzene content (TBC-1) and 148 °C for 87 wt% azobenzene content(TBC-2)) as shown in Table 2. The increase in Tg and
TLC-I values of the TBC compared to the HP may be
due to the differences in polymer chain terminal end
groups and azobenzene aggregations by phase separation[35] causes the changes in crystal packing of themacromolecular chains.
The HP and TBCs LC behaviors were examined,
and the representative polarized optical micrographs areshown in Figure 4. A typical focal conic texture,
characteristic of smectic-A (SmA) phase was observed inboth heating and cooling cycles of the polymers.

Downloaded by [IICT Indian Institute of Chemical Technology] at 04:08 27 October 2014

6

Figure 2.
TBCs.

T. Rajasekhar et al.

TG curves of synthesized homopolymer, tPS, and

The incorporation of polystyrene block in acrylatebased pendant azobenzene polymer chain exhibited
SmA liquid crystal phase with lower focal conic domain
size compared to the homopolymer.[43] The XRD
pattern of TBC (TBC-2) at 115 °C is shown in Figure 5.
In the small angle region, it showed only one small peakcorresponding to a d spacing of 21.1 Å (2θ = 4.1°) and abroad diffuse wide-angle reﬂection at about 4.1 Å(2θ = 21°) indicating a liquid-like in-plane order. As thisis a side-chain liquid crystalline polymer and exhibited asmectic type texture, at the very small angle, that is2θ < 4.1°, a sharp peak corresponding to spacing ofabout 42.2 Å is expected, which was beyond the layer
accessible range of our instrument. The ratio of expected
and observed (2θ = 4.1°) layer spacing is 1: ½.
The approximate calculated length of the side-chain of
the macromolecule (considering the bond length) is
about 38 Å, which is closer to the expected value(42.2 Å). As both the values are comparable, we
conjecture that this is the ﬁrst-order signal of a SmA1
mesophase. Isotropic phase transition temperatures (Tiso)of the HP (Tiso: 140 °C), TBC-1 (Tiso: 150 °C), and
TBC-2 (Tiso: 168 °C) were observed using opticalpolarized microscope on heating. The polarized optical
micrographs of the polymers around Tiso are given insupporting information.

Table 2.

Figure 3. DSC curves of homopolymer, tPS, and TBCs (a)heating and (b) cooling cycles.

The HP and TBCs exhibited λmax at about 365 nm
due to π–π* and 436 nm due to n–π* electronic transitions in THF solution (0.5 mg/25 mL) (Figure 6(a)). The
spin-coated HP and TBC-2 ﬁlms exhibited λmax at about
335 nm corresponding to H-type aggregation [51,52] andalong with a sudden jump in absorbance (more or less inall spectra) toward higher wavelengths, at 360 nm(Figure 6(b) and (c)) corresponding to likely related withthe lamp change in instrument and combined with the
non-associated azobenzene groups present in the ﬁlm.
The HP and TBCs ﬁlms spectra were broad in naturecompared to the spectra obtained in THF solution.[51]After annealing the spin-coated ﬁlms in LC phase, theThermal characteristics of the synthesized polymers.
TGAa

DSCb

Polymer code

Td5/°C

Td10/°C

Tg/°C

TLC-Ic/°C (ΔH/J g−1)

Tc/°C (ΔH/J g−1)

HP
tPS
TBC-1
TBC-2

262
248
289
297

282
318
314
319

17
81
29, 8530

100 (4.48)–d121 (1.17)
148 (4.1)

98 (5.56)–
116 (1.67)
142 (6.0)aTemperatures of 5 and 10% weight of the sample decomposition.
Obtained from DSC second heating curve.
Endotherm peak temperature.
d
Not observed.
bc

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Designed Monomers and Polymers

Figure 4.
60 °C.

7

Photomicrograph of (a) HP and (b) TBC-2 at

Figure 6. UV–vis absorption spectra of (a) poly(AUPAB) andpoly(AUPAB)-b-PS-b-poly(AUPAB) in THF and as cast andannealed ﬁlms of (b) poly(AUPAB) and (c) poly(AUPAB)-bPS-b-poly(AUPAB) at room temperature.

Figure 5. X-ray intensity as a function of angle 2θ of TBC-2
at 115 °C.

λmax was shifted to about 32 nm higher wavelength andthe absorbance decreased by about 65% for HP and
TBCs.[53,54]
The pendant azobenzene group E–Z conﬁgurations
were calculated with ambient light and conventional

ﬂuorescent tube light exposure, while preparing the sample without UV irradiation at room temperature in CDCl3
solution by 1H NMR spectra. The monomer, AUPAB,
E-conﬁguration aromatic ring chemical shift values in 1H
NMR spectrum were centered at δ 7.93, 7.78, and7.01 ppm, whereas Z-conﬁguration showed the values in
upﬁeld,[34,55] such as, at δ 7.59, 6.92, and 6.75 ppm,
respectively, as shown in Figure 7. A similar behavior wasobserved in the HP with broad signals, whereas the TBCs
spectrum showed δ 7.59 ppm broad and overlapped signal
with the E-conﬁguration protons. The pendant azobenzene
group E:Z ratios were 96:4 for monomer, 94:6 for HP, and89:11 for TBC. The TBC higher Z-conﬁguration value
could be due to the overlapping of the E-conﬁguration
protons in 1H NMR spectrum.

T. Rajasekhar et al.

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8

Figure 7. Magniﬁed 1H NMR spectra of (a) AUPAB, (b) poly(AUPAB), and (c) poly(AUPAB)-b-PS-b-poly(AUPAB) to calculate
E- and Z-conﬁgurations ratio.

Downloaded by [IICT Indian Institute of Chemical Technology] at 04:08 27 October 2014

Designed Monomers and Polymers
The spectra of spin-coated ﬁlms (Spin NXG-p2) ofthe HP, TBC-1, and TBC-2 upon 365 nm UV light exposure (BLX-365, made in EEC, serial no: 0410512) withvarious radiation energies are shown in Figure 8. The
HP and TBC-2 ﬁlms exhibited the π–π* absorbancedecrease upon UV light exposure due to trans/cis isomerization until 128 mJ/cm2 UV light energy, and then,
π–π* absorbance gradually increased upon increasing
UV light energy up to establishing of photostationary
state. The similar trend was observed by several researchgroups using annealed ﬁlms.[27,56] In our case, in the
spin-coated ﬁlms, some extent of the azobenzene units
were already in the upright orientation to the substrateplane, which were randomized upon UV light exposureand caused to increase in the π–π* absorbance. On theother hand, the TBC-1 ﬁlms exhibited slight and randomchanges in π–π* absorbance intensities upon UV light
exposure. This anomalous behavior may be due tothe >50 wt% polystyrene content presence in the TBC,

Figure 8. UV–visible absorption spectra of thin ﬁlms of (a)HP, (b) TBC-1, and (c) TBC-2 upon exposure to 365 nm light.

9

which causes the geometrical restriction to azobenzene
units for isomerization.
The features of the π–π* absorbance at 331 nm
spectral changes in the back process upon visible lightirradiation (F8T5/BLB, Radium) by varying the exposure
time for HP and TBC ﬁlms after reaching photostationary state with 365 nm light are summarized in Figure 9.
For the HP ﬁlm, the absorbance increased with increasing irradiation time of 2 min and then decreased uponfurther irradiation and reached stable state. The earlier
stage reﬂects the proceeding of cis/trans isomerization,and the following stages, decrease and reaching stablestate, could indicate the contribution of upright orientation of the azobenzene units to the substrate plane.[56]The TBC-1 and TBC-2 ﬁlms followed opposite trend to
HP at initial stage (<2 min exposure). The magnitude of
the absorbance decrease at initial stage was higher in
TBC-2 compared to the TBC-1. This could be due to thecontribution of more number of azobenzene units’
upright orientation on the plane of measuring light direction upon light exposure, and further, light exposure azobenzene units achieved stable state.[57]The nano-segregations were observed in TBC annealed
thin ﬁlms by TEM analyses, and typical TEM images of
the TBC-1 and TBC-2 are presented in Figure 10. The
TBC-1 exhibited LC azobenzene blocks as non-uniform
spheres (relatively black regions) with a width/diameter
about 30–92 nm dispersed into the PS matrix, whereas in
TBC-2 thin ﬁlms, PS block randomly aggregated as particles (relatively white regions) with a width/diameter about
2 nm dispersed into the LC azobenzene blocks matrix. It isalso reported that a block copolymer with 85 wt% of LC
fraction and 15 wt% of PS exhibited PS domains as dispersed segregations into bilayer LC matrix.[27,56,58] Phase
separation behavior dependence with the volume ratio of
PS and poly(AUPAB) blocks in TBC needs further investigation. Hence, we expect to use these macromoleculararchitectures design for developing nanotemplate applications in the future.

Figure 9. Changes in the absorbance at 332 nm in UV–visible
spectra by varying visible light irradiation time for HP, TBC-1,
and TBC-2.

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T. Rajasekhar et al.

Figure 10.

TEM images of TBC thin ﬁlms of (a) TBC-1 and (b) TBC-2.

Scheme 1.

Preparation of (a) homopolymer and (b) TBC by the RAFT process.

Conclusions
Azobenzene TBCs, poly(AUPAB)-b-PS-b-poly(AUPAB),
composed of a central polystyrene block, and two lateralLC azobenzene polyacrylate blocks were for the ﬁrst
time successfully synthesized using telechelic polystyrene as a macro-RAFT agent by the reversible addition
fragmentation chain transfer process. The synthesized
TBCs exhibited higher thermal stability and higher Tg

and TLC-I values compared to the homopolymer, poly
(AUPAB). The TBC having 43 wt% azobenzene content
showed phase separation by exhibiting two Tg values at
29 and 85 °C compared to the higher azobenzene content(87 wt%) TBC, which had single Tg value at 30 °C. The
TLC-I values of TBCs were dependent on the AUPAB wt% in the polymer chain. The incorporation of the polystyrene block in acrylate-based pendant azobenzene polymer chain showed smectic-A liquid crystalline phaseDesigned Monomers and Polymers
with lower focal conic domain size compared to the poly
(AUPAB). The TBCs exhibited microphase-separated
nano-segregation in thin ﬁlms. Introducing polystyrene
block into the LC polymer chain could render better ﬁlm
forming ability and expected as a potential material forcoating applications.

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Acknowledgments
TR and MT acknowledge the University Grants Commission
and Council of Scientiﬁc and Industrial Research, New Delhi,
respectively, for providing Senior Research Fellowships. AVSSis grateful to TAPSUN grant (NWP-0054). We thank Ms K. N.
Vasudha, Raman Research Institute, Bangalore, for help in the
X-ray measurements. Authors are thankful to Mr A.
Harikrishna and Dr Marshal Dhayal, CSIR-CCMB, Hyderabad,for their TEM analysis.

Supplementary data
Supplementary information for this article is accessed here
http://dx.doi.org/10.1080/15685551.2014.971396.

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