Synthesis of azobenzene-functionalized star polymers via RAFT and their photoresponsive properties

Article abstract of DOI:10.1016/j.polymer.2011.06.035

Azobenzene-functionalized star polymers were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. First, azobenzene-functionalized linear macro chain transfer agents (Macro-CTA) were synthesized by RAFT polymerization of 6-[4-(4′-Methoxyphenylazo)phenoxy] hexylmethacrylate (MAz6Mc) using 2-(2′-cyanopropyl)dithiobenzoate (CPDB) as RAFT agent in presence of AIBN as initiator in anisole. Subsequently, star azopolymers were synthesized by polymerization of a difunctional azomonomer, BMA2Az, with resultant Macro-CTA in presence of AIBN as initiator in anisole. Star azopolymers were characterized by GPC and spectroscopic methods. Thermal properties of star azopolymers were determined by DSC and TMA. Molecular weight versus conversion and molecular weight versus polymerization time attest to living polymerization characteristics. Photoisomerization behaviors of star azopolymers were studied by irradiation of both UV and visible light. Surface relief gratings were inscribed on star azopolymer films upon exposure to an interference pattern of (RCP + RCP) Ar⁺ laser. A diffraction efficiency of 20% was obtained by exposure of Star-8 K(2.6 K) polymer film to an (RCP + RCP) Ar⁺ laser for about 30 min. Surface relief grating structures were investigated by AFM and polarized optical microscopy.

Full text of DOI:10.1016/j.polymer.2011.06.035

Polymer 52 (2011) 3696e3703
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of azobenzene-functionalized star polymers via RAFT and their
photoresponsive properties
*
Md. Zahangir Alam, Akihisa Shibahara, Tomonari Ogata, Seiji Kurihara
Department of Applied Chemistry and Biochemistry, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Azobenzene-functionalized star polymers were synthesized by reversible additionefragmentation chain
transfer (RAFT) polymerization. First, azobenzene-functionalized linear macro chain transfer agents
(Macro-CTA) were synthesized by RAFT polymerization of 6-[4-(40-Methoxyphenylazo)phenoxy]hex-
ylmethacrylate (MAz6Mc) using 2-(20-cyanopropyl)dithiobenzoate (CPDB) as RAFT agent in presence of
AIBN as initiator in anisole. Subsequently, star azopolymers were synthesized by polymerization of
a difunctional azomonomer, BMA2Az, with resultant Macro-CTA in presence of AIBN as initiator in
anisole. Star azopolymers were characterized by GPC and spectroscopic methods. Thermal properties of
star azopolymers were determined by DSC and TMA. Molecular weight versus conversion and molecular
weight versus polymerization time attest to living polymerization characteristics. Photoisomerization
behaviors of star azopolymers were studied by irradiation of both UV and visible light. Surface relief
gratings were inscribed on star azopolymer films upon exposure to an interference pattern of
(RCP þ RCP) Arþ laser. A diffraction efficiency of 20% was obtained by exposure of Star-8 K(2.6 K) polymer
film to an (RCP þ RCP) Arþ laser for about 30 min. Surface relief grating structures were investigated by
AFM and polarized optical microscopy.
Received 1 April 2011
Received in revised form
17 June 2011
Accepted 19 June 2011
Available online 25 June 2011
Keywords:
Star polymer
RAFT polymerization
Azobenzene molecules
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
radical polymerisation (ATRP) [13] and reversible additione-
fragmentation chain transfer polymerisation (RAFT) [14] appear to
Star-shaped polymers are defined as polymers composed of
multiple polymer chains linked to a central core. They have attracted
great attention because of their unique properties leading to
important applications [1e3]; these include lower melting points,
lower bulk and solution viscosities and superior mechanical prop-
erties due to their more compact structure compared to their linear
analogues of similar molecular weights [4e10]. Star polymers are
also capable of possessing a higher degree of end group functionality
compared to linear polymers of similar molecular weight, enabling
their use in many specialized applications.
Schaefgen and Flory first reported on synthesis of multi-chain
polymer from e-caprolactam using tetrabasic and octabasic
carboxylic acid as a multifunctional agent [11].
However, the synthesis of star polymer remained challenging
until the invention of living polymerization techniques. Living
radical polymerization has recently emerged as one of the most
effective synthetic routes to well-defined polymers. Among them,
nitroxide mediated polymerisation (NMP) [12], atom transfer
be the most efficient and have been successfully applied to a large
number of monomers. Very recently Matyjaszewski group reported
synthesis of star polymer by a new polymerization technique
known as activators regenerated by electron transfer (ARGET) ATRP
[15]. However, the RAFT process involves performing conventional
radical polymerisation in the presence of certain thiocarbonylthio
compounds which act as highly efficient reversible additione
fragmentation chain transfer agents and provide the polymeriza-
tion with living characteristics. Following RAFT polymerization,
most polymer chains will have attained thiocarbonylthio and R
groups as terminal groups. Synthesis of well-defined polymers of
predictable molecular weights, low polydispersity indices and
controlled architecture is possible through the control of the ratio
of chain transfer agent (CTA) and monomer as well as polymeri-
zation conditions. Moreover, RAFT polymerization is possible in
heterogeneous media. We recently reported on the synthesis of
azobenzene-functionalized 2-, 3- and 4-arm azotelomers with the
use of di-, tri- and tetra-functional CTAs [16].
The incorporation of azobenzene moiety into star polymers could
significantly expand their potential applications due to the unique
ability of azobenzene-containing polymers (azopolymers) for
reversible photoisomerization between trans- and cis- isomers of the
* Corresponding author. Tel.: þ81 96 342 3678; fax: þ81 96 342 3679.
E-mail address: kurihara@gpo.kumamoto-u.ac.jp (S. Kurihara).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.06.035

Md.Z. Alam et al. / Polymer 52 (2011) 3696e3703
3697
HO
NaNO2, HCl aq.
0 C
N+
Cl
CH3O
NH2
CH3O
N
CH3O
N
N
OH
RTfor 1 h
MAzOH
Cl(CH2)6OH, K2CO3, KI
DMF, 100 C, 48 h
CH3O
N
N
O(CH2)6OH
MAzOH
MAz6OH
CH3
CH3
H2C
C
H2C
C
,
TEA
COCl
MAz6OH
COO (CH2)6O
N
N
OCH3
THF, RT, 24 h
MAz6Mc
HO
NaNO2, HCl aq.
CH3OH, 0 C
N
N+
Cl
HO
N
N
OH
HO
NO2
HO
RTfor 4 h
4,4’-dihydroxyazobenzene (DHAB)
Cl(CH2)2OH, K2CO3, KI
HO(CH2)2O
CH3
N
N
O(CH2)2OH
DHAB
DMF, 100 C, 48 h
CH3
BAz2OH
CH3
H2C
C
H2C
C
C
CH2
,
TEA
COCl
COO (CH2)2O
N
N
O(CH2)2OOC
BAz2OH
THF, RT, 24 h
BMA2Az
Scheme 1. Synthesis of azo-monomers MAz6Mc and BMA2Az.
chromophore, causing large changes in the polymer’s size, shape, and
polarity [17,18]. These unique characteristics have lead to consider-
ation of azopolymers for application in many fields such as optical
data storage [19,20], liquid crystal displays [21], and holographic
surface relief gratings (SRGs) [22,23]. The photoinduced trans-
ecisetrans photoisomerization cycle of azo chromophores upon
irradiation by UV and visible light plays an important role in
formation of surface relief gratings on azopolymer films. It has
already been reported that azopolymers show surface relief gratings
in addition to birefringence gratings upon irradiation of two polar-
ized writing beams [24]; however, the mechanism of mass transport
induced by reversible transecis photoisomerization remains
unclarified although this phenomenon has been explored using
several linear azopolymers by a number of research groups [25,26].
Additionally, only a few star azopolymers have been reported for
inscription of surface relief gratings. Recently, Zhu et al reported on
the synthesis of a three-arm azobenzene-containing polymers by
ATRP and described the formation of surface relief gratings on their
films [27]. In this article, we report the efficient synthesis of star
azopolymers by RAFT process and describe the fabrication of
surface relief gratings on these star azopolymer films utilizing the
interference pattern of Arþ laser.
2. Experimetal
2.1. Synthesis of 6-(4-(40-methoxyphenylazo)phenoxy]hexylmethacr-
ylate), MAz6Mc
2.1.1. 4-Methoxy-40-hydroxyazobenzene
9 g (73 mmol) of anisidine was dissolved in 300 ml of 2 M HCl;
the resulting solution was cooled to 0 ꢀC in an ice bath. 5 g
(72.5 mmol) of NaNO2 dissolved in 150 ml of water was added
drop-wise to the anisidine solution to form diazonium salt. This
solution was added to 6.9 g (73 mmol) of phenol dissolved in
200 ml of 2 M NaOH at 0 ꢀC, which formed a yellow precipitate. The
reaction mixture was then stirred at room temperature for 2 h. The
precipitate was filtered out, recrystallized from a solution of hexane
and benzene (7:2), and then dried under vacuum.
Anal. Calcd. for C13H12N2O2 (%): C, 68.41; H, 5.30; N, 12.27.
Found: C, 68.53; H, 5.39; N, 12.10.
2.1.2. 4-Methoxy-40-(6-hydroxyhexyloxy)azobenzene
To a mixture of 5 g (22.3 mmol) of 4-methoxy-40-hydrox-
yazobenzene, 2.76 g (20 mmol) of K2CO3 and a very minute amount
of KI dissolved in 100 ml of dimethylformamide (DMF), 3.05 g
S
SMgBr
H2O/H+
S
SH
S
S
CN
MgBr
CS2
AIBN
80 C
CPDB
Scheme 2. Synthesis of the RAFT agent 2-cyanoprop-2-yl-dithiobenzoate (CPDB).

3698
Md.Z. Alam et al. / Polymer 52 (2011) 3696e3703
(22.35 mmol) of 6-chlorohexanol was added. The reaction mass
was heated to 100 ꢀC and was stirred at that temperature for 24 h.
The reaction mixture was then poured into water, extracted with
CHCl3, washed with water and dried over anhydrous magnesium
sulphate. After removal of CHCl3 by evaporation, the obtained
liquid was poured into water. The precipitate was collected by
filtration and was purified by recrystallization from methanol. The
macro-CTA
2.5h
5h
20h 10h
yield was 60% as solid. 1H NMR (CDCl3,
d): 1.0 (m, 3H, methyl),
1.3e2.0 (m, 10H, methylene), 3.4 (m, 2H, CH2OH), 4.0e4.1(m, 4H,
CH2OPh), 4.4 (t, 1H, OH), 6.9e7.9 (m, 8H, aromatic). Anal. Calcd. for
C19H24N2O3 (%): C, 69.49; H, 7.36; N, 8.52. Found: C, 69.48; H, 7.40;
N, 8.51 (Scheme 1).
106
105
104
103
100
Molecular weight (g/mol)
Fig. 1. GPC traces of star azopolymers at different polymerization times.
2.1.3. 6-[4-(40-Methoxyphenylazo)phenoxy]hexylmethacrylate,
MAz6Mc
To
a solution of 3 g
(9.14 mmol) of 4-methoxy-40-(6-
filtration, purified by recrystallization from ethyl acetate, and dried
under vacuum. Yield: 24.0%, Melting point: 198 ꢀC. Anal.: Calcd. (%);
H; 6.00, C; 63.56, N; 9.27. Found(%); H; 5.95, C; 63.75, N; 8.81.
hydroxyhexyloxy) azobenzene and a minute amount of hydroqui-
none dissolved in 50 ml of THF, slowly added were 1.85 g
(18.30 mmol) of triethylamine and 1.92 g (18.30 mmol) of meth-
acryloyl chloride at 0 ꢀC with stirring. After stirring at room
temperature for 24 h, the reaction mixture was poured into water.
The precipitated product was collected and recrystallized from
2.2.3. Synthesis of BMA2Az
13.4 g (44.3 mmol) of BAz2OH was dissolved in 250 ml of
dehydrated THF and 12.3 ml (88.6 mmol) of triethylamine and
8.58 ml (88.6 mmol) of methacryloyl chloride was added to it drop-
wise using a dropping funnel at 0 ꢀC in an ice bath. The reaction
mixture was then stirred at room temperature for 24 h. After the
reaction time had lapsed, the reaction mixture was filtered, evap-
orated, dissolved in chloroform, and extracted with aqueous
sodium bicarbonate solution. The extracted product was washed
with water until the methacrylic acid was removed, was dried with
anhydrous magnesium sulphate, and was evaporated. The solid
product was purified by recrystallization from ethanol. Yield:
64.0%. Melting point: 131 ꢀC. Anal.: Calcd. (%); H; 5.98, C; 65.74, N;
6.39. Found (%): H; 5.75, C; 65.95, N; 6.31.
ethanol. Yield: 38% as solid. 1H NMR (CDCl3,
d): 1.0 (m, 3H, methyl),
1.4e2.0 (m, 10H, methylene), 4.0 (m, 4H, CH2OPh), 4.2 (m, 2H,
CH2OCO), 5.8 (q, 1H, CH2]CH), 6.1 (q, 1H, CH2]CH), 6.4 (q, 1H,
CH2]CH), 6.9e7.9 (m, 8H, aromatic). Anal. Calcd. for C23H28N2O4
(%): C, 69.67; H, 7.11; N, 7.06. Found: C, 69.45; H, 7.08; N, 7.20.
2.2. Synthesis of difunctional azomonomer, 4,40-methacryloylethoxy-
azobenzene, BMA2Az
2.2.1. Synthesis of 4,40-dihydroxyazobenzene
20.0 g (181 mmol) of p-aminophenol was dissolved in 200 ml
of 1 M HCl solution and was cooled to 0 ꢀC in an ice bath. Next,
12.6 g (181 mmol) of NaNO2 dissolved in 300 ml of water was
added drop-wise to a solution of p-aminophenol at 0 ꢀC. 400 ml of
pre-cooled methanol was added to this diazotized solution. Then,
17.1 g (181 mmol) of phenol dissolved in 65 ml of 3 M aqueous
sodium hydroxide was added drop-wise. This solution was stirred
at room temperature for 2 h. Methanol was removed by evapo-
ration. The reaction mass was then acidified with aqueous HCl,
and the resulting precipitate was collected by filtration and
washed with water until neutralized. The crude dihydrox-
yazobenzene was purified by recrystallization from ethanol/water
mixture and dried under vacuum. Yield: 56.1%, Melting point:
218 ꢀC.
2.3. Synthesis of RAFT initiator (2-cyanoprop-2-yl-dithiobenzoate;
CPDB)
Dithiobenzoic acid 3.8 g (0.025 mol), DMSO 1.0 mL (0.013 mol)
was added to 20 ml of ethyl acetate and was stirred under nitrogen
for 7 h. Next, AIBN 3.3 g (0.017 mol) was added to the reaction
solution, which was refluxed at 80 ꢀC for 16 h. After the reaction,
the solvent was removed by rotary evaporator, and crude CPDB was
purified by column chromatography (developing solvent; hexane:
2.2.2. Synthesis of 4,40-bis(2-hydroxyethoxyazobenzene), BAz2OH
10.0
g g
(90.5 mmol) of 4,40-dihydroxyazobenzene, 25.0
(181 mmol) of potassium carbonate, 14.57 g (181 mmol) of 2-
chloroethanol and a small amount of potassium iodide were dis-
solved in 300 ml of DMF in a flask; the reaction mass was heated to
100 ꢀC and stirred for 48 h. After the reaction time had lapsed,
a large amount of water was added; the precipitate was collected by
Table 1
Synthesis of Macro-CTA by polymerization of MAB6Mc by RAFT.
Macro-CTA
Monomer CPDB AIBN Time Conversion Mn
Mw/Mn
mmol
mmol
h
%
GPC
Macro-CTA (2.6 K)
Macro-CTA (5.1 K)
Macro-CTA (10 K)
1.3
1.3
1.3
0.05 0.034
0.05 0.034
0.05 0.034 20
1
5
13
65
95
2600 1.15
5100 1.15
10000 1.21
Fig. 2. Molecular weights of Macro-CTA as a function of monomer conversion in RAFT
polymerization of MAz6Mc.
Solvent (Anisole): 10 ml; temperature: 60 ꢀC; [M]/[CTA] ¼ 15; [CTA]/[AIBN] ¼ 1.5.

Md.Z. Alam et al. / Polymer 52 (2011) 3696e3703
3699
c
[M]/[CTA]=5
b
[M]/[CTA]=10
a
[M]/[CTA]=15
106
105
Molecular weight (g/mol)
104
1000
100
10
6
105
Molecular weight (g/mol)
104
1000
100
6
105
Molecular weight (g/mol)
104
1000
100
10
Fig. 3. GPC curves of macro-CTA2.6K (solid line) and star polymer (broken 5l.ine). (a) [M]/[CTA] ¼ 15, (b) [M]/[CTA] ¼ 10, (c) [M]/[CTA] ¼ 5.
ethyl acetate ¼ 9:1). Yield: 16%, 1H NMR (CDCl3, TMS): 8.0e8.1 ppm
(d, 2H, C6H4), 7.5e7.6 ppm (m, 1H, C6H4); 7.3e7.4 ppm (m, 2H,
C6H4), 1.9e2.0 (6H, CH3) (Scheme 2).
2.4.1. Synthesis of linear polymer by RAFT
Linear azopolymers of different molecular weights were also
synthesized by RAFT polymerization of MAz6Mc according to the
synthetic procedures described above so as to compare their
properties with star azopolymers. The characterizations of linear
azopolymers are stated in Table 3.
2.4. Synthesis of Macro-CTA
0.80 g MAB6Mc, 111 mL CPDB, and 61 mg AIBN was dissolved in
10 ml anisole. Argon gas was passed through the reaction mixture
for 1 h. It was then degassed through three freezeepumpethaw
cycles, sealed under vacuum, and polymerization was carried out at
60 ꢀC for 20 h. Macro-CTA was precipitated by pouring the polymer
solution into methanol and collected by filtration. Crude Macro-
CTA was purified by chloroform/methanol re-precipitation tech-
nique and was dried under vacuum. Macro-CTAs were termed
Macro-CTA(2.6) and Macro-CTA(10) on the basis of their respective
molecular weights of 2600 and 10,000.
2.5. Synthesis of star azopolymers by RAFT
Macro-CTA, monomer BMA2Az and AIBN was dissolved in 10 ml
anisole; Ar gas was passed through the solution for 1 h. It was then
degassed through three freezeepumpethaw cycles and sealed
under vacuum; polymerization was carried out at 60 ꢀC for a pre-
determined time. The polymer was precipitated in hexane,
collected by filtration, and dried under vacuum. The crude star
polymers were re-dissolved in THF and re-precipitated in hexane;
a
b
100
100
Linear-9.4K
80
Star-8K(2.6K)
80
60
40
20
60
40
20
0
0
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Temperature, °C
Temperature, °C
a’
b’100
50
Linear-9.4K
Star-8K(2.6K)
40
30
20
10
80
60
40
20
0
0
0
5
10
15
20
0
5
10
15
20
Load(mN)
Load(mN)
Fig. 4. TMA curves of (a,a’) linear 9.4 K and (b,b’) Star-8 K(2.6 K) azopolymer.

3700
Md.Z. Alam et al. / Polymer 52 (2011) 3696e3703
Table 2
Synthesis and characterization of star azopolymers by RAFT.
Star azopolymer
Monomer
mmol
Macro-CTA
mmol
AIBN
mmol
Time
h
Conversion
%
Mn
GPC
Mw/Mn
Tg
ꢀC
Star-8 K(2.6 K)
Star-18 K(2.6 K)
Star-48 K(10 K)
Star-73 K(10 K)
0.75
0.75
0.75
0.75
0.05
0.05
0.05
0.05
0.013
0.013
0.013
0.013
2.5
5
10
20
13
65
86
95
8000
1.15
3.82
1.87
1.75
96.1
85.2
69.6
69.0
18000
48000
73000
this was repeated until the star azopolymers were purified. Star
azopolymers of molecular weights 8000,18,000, 48,000 and 73,000
were designated as Star-8 K(2.6 K), Star-18 K(2.6 K), Star-48 K(10 K)
and Star-73 K(10 K), respectively. The numbers in the brackets
represent the molecular weights of Macro-CTAs.
Polymerization of MAz6Mc in the presence of the RAFT agent
CPDB initiated by AIBN was carried out for 2.5, 5, 10 h; conversions
of monomer determined by 1H NMR were found to be 13%, 65% and
95%, respectively. Molecular weights of the resulting Macro-CTAs
were determined by GPC and are tabulated in Table 1. It is
observed that molecular weights of Macro-CTAs increased linearly
with monomer conversion and slowly with polymerization time.
Low polydispersity of between 1.15 and 1.21 was also exhibited by
pMAz6Mc (Macro-CTAs). Thus, facile control of the polymerization
is possible.
Fig. 1 illustrates typical GPC chromatograms of star azopolymers
synthesized by RAFT polymerization of BMA2Az initiated by AIBN
in 2.5, 5,10 and 20 h polymerization. GPC peaks of the resulting star
azopolymers clearly shift to shorter retention time (higher molec-
ular weight) with increasing polymerization time. 1H NMR spectra
of the polymer sample after 2.5, 5, 10 and 20 h were taken and were
used to calculate the conversion of monomer according to the
following equation:
2.6. Characterization
Molecular weights and molecular weight distributions of star
azopolymers synthesized in this study were determined by gel
permeation chromatography (GPC) equipped with JASCO RI 930
Intelligent refractive index (RI) detector and JASCO 870 UV Intelli-
gent UV/VIS detector. The GPC was operated using Shodex KF-804F
column at a flow rate of 1 ml/min THF at 40 ꢀC and polystyrene as
standard. 1H NMR spectra of monomers and polymers were taken
with a 400 MHz NMR spectrophotometer. Deuterated chloroform
(Cambridge Isotope Laboratories) was used as solvent. IR spectra of
star azopolymers were taken by FTIR spectrophotometer. The
UVeVis spectra of samples in THF and their films were recorded
using a Shimadzu spectrophotometer. Thermal properties of star-
shaped azopolymers were determined by differential scanning
calorimetry (DSC) at a heating rate of 10 ꢀC per minute, thermo-
mechanical analysis (TMA), and polarized optical microscopy using
Olympus BHSP polarizing microscope equipped with a Mettler
FP80 and FP82 hot stage and controller. TMA experiments were
carried out with a SEIKO TMA SS 100 instrument. Linear and star
azopolymer films were used for TMA experiment. Reversible trans-
cis photoisomerization of star-shaped azopolymers was studied by
irradiating UV (366 nm) and visible light (435 nm) on their solution
in THF and spin-coated films. Arþ laser of (488 nm) at modest
intensity was used for inscription of SRG and HeeNe laser (633 nm)
was used as probe light. SRG structures were investigated by AFM
(Agilent 5500, Tapping Mode) and polarized optical microscopy.
The scan rate and frequency of AFM was 0.5 inch/sec and 350 kHz,
respectively. The tip used in AFM was 1e10 Ohm-cm Phosphorous
(n) doped Si.
À
Áꢀ
Conversion ¼ ½Mꢁ0ꢂ½Mꢁt ½Mꢁ0
where [M]0 is the initial concentration of monomer corresponding
to the summation of integral value of vinyl group protons and
benzene ring protons, and [M]t is the concentration of monomer at
time t, represented by the integral value of vinyl group protons.
Using the above equation and 1H NMR spectra of polymer samples,
monomer conversion was calculated to be 16%, 65%, 86% and 95% at
2.5, 5, 10 and 20 h, respectively. Fig. 2 shows the dependence of
molecular weights of star azopolymers on monomer conversion. It
is clear that molecular weight of star azopolymers synthesized by
RAFT polymerization increased linearly with monomer conversion,
thus proving the livingness of the process.
Theoretical molecular weights of Star azopolymers can be
calculated according to the following equation [29]:
Mn
¼ ½Mꢁ=½CTAꢁ ꢃ conversion ꢃ MWmonomer þ MWCTA
ðtheoÞ
where [M] is the molar concentration of monomer, [CTA] is the
molar concentration of CTA, MWmonomer is the molecular weight of
monomer, and MWCTA is the molecular weight of CTA. Theoretical
molecular weights of tar azopolymers calculated by the above
equation and molecular weights determined by GPC did not show
good agreement. This may be due to the use of linear polystyrene as
the standard in GPC measurement. The physical characteristics of
star-shaped polymers and linear polystyrene differ in areas such as
hydrodynamic volume, in which star polymers exhibit lower
values.
3. Results and discussion
3.1. Synthesis of Macro-CTA by RAFT polymerization of MAz6Mc
with CPDB initiated by AIBN
Azobenzene-functionalized monomer MAz6Mc was synthe-
sized in good yield and purity. Linear Macro-CTA (pMAz6Mc) was
synthesized by RAFT polymerization of MAz6Mc in the presence of
a RAFT agent (CPDB) and AIBN initiator in anisole. CPDB was chosen
as the RAFT agent because Rizzardo et al. had previously reported
CPDB to be an efficient transfer agent in the polymerization of
methyl methacrylate, methyl acrylate and styrene [28]. We also
investigated RAFT polymerization using other RAFT agents, but no
polymer with controlled molecular weight was obtained. In RAFT
polymerization, which is also known as living or controlled radical
polymerization, the molecular weights of the resulting polymers
increase steadily with reaction time and linearly with monomer
conversion.
Table 3
Molecular weights and phase transition behaviors of linear polymers.
Sample
Mn
2600
9400
31000
43000
Mw/Mn
G/S
S/N
N/I
Linear 2.6 K
Linear 9.4 K
Linear 31 K
Linear 43 K
1.23
1.52
3.29
1.49
43
70
72
47
88
96
83
130
136
145
116
120

Md.Z. Alam et al. / Polymer 52 (2011) 3696e3703
3701
A
1
0.8
0.6
0.4
0.2
0
1
Vis
UV
0.8
0s
1s
0s
2s
0.6
0.4
0.2
0
2s
5s
3s
10s
25s
50s
5s
10s
300
350
400 450 500 550
Wavelength(nm)
600
300 350
400 450 500 550 600
Wavelength(nm)
B
1
0.8
0.6
0.4
0.2
0
1
0s
1s
UV
Vis
0.8
0s
1s
3s
0.6
0.4
0.2
0
3s
8s
30s
60s
15s
30s
300 350
400 450 500 550
Wavelength(nm)
600
300 350
400 450 500 550 600
Wavelength(nm)
Fig. 5. Photoisomerization behaviors of Star-8 K(2.6 K) azopolymer in toluene (A), and film(B).
3.2. Effect of molar ratio of monomer to CTA ([M]/[CTA])
[M]/[CTA] of 5, 10, 15 and 50. Fig. 3 shows GPC chromatograms of
the resulting Macro-CTA synthesized by RAFT process. Molecular
weights of Macro-CTAs were found to increase with an increase in
the molar ratio of monomer to CTA because of the higher conver-
sion of monomers into polymer. However, gelation was observed at
To study the effect of the molar ratio of monomer to CTA i.e.,
concentration of monomer RAFT polymerization was carried out
under several conditions. The polymerization was carried out for
Ar+ laser
Mirror
Probe light;
He-Ne laser
Polarizer
Lens
ND filter
Pinhole
Sample
Lens
Mirror
Polarizer
Detector;
Photo diode
Plate
Mirror
Beam splitter
Mirror
PC
Fig. 6. Schemtic diagram of experimental set-up for inscription of SRG on azopolymer films.

3702
Md.Z. Alam et al. / Polymer 52 (2011) 3696e3703
described in Tables 2 and 3. Linear azopolymers showed liquid
crystalline properties. Phase transition temperatures and iso-
tropization temperatures of linear azopolymers increased with an
increase in molecular weight. All linear polymers showed
G / S / N / I phase transitions, where G, S, N and I represents
glass transition, smectic, nematic and isotropic phases, respec-
tively. On the other hand star azopolymers did not show such
sharp liquid crystalline phases other than a clear glass transition
temperature and isotropization temperature. As described above,
the linear polymers displayed liquid crystalline properties more
clearly than star polymers because the linear polymers can orient
themselves in different liquid crystalline phases.
3.5. Photoresponsive properties
3.5.1. Reversible photoisomerization of star azopolymers
Photoisomerization behaviors of star azopolymers were studied
in solution and film. Star azopolymer films were prepared by spin-
coating of their solutions in THF onto cleaned glass substrate. The
spin-coated films were air dried at room temperature. Film thick-
ness was controlled by adjusting the concentration of solution and
speed of spin-coating. Photoisomerization of the star azopolymer
solution and films were carried out by UV and visible light
irradiation.
Fig. 7. Diffraction efficiencies of star azopolymers as a function of irradiation time.
the molar ratio of monomer to CTA of 50 due to the very high
concentration of monomer. Similar trends were observed in other
cases of synthesizing star azopolymers.
3.3. Thermomechanical analysis of linear and star polymers
Fig. 5 shows the changes in absorption spectra of Star-8 K(2.6 K)
azopolymer in toluene (A) and in film (B) by UV and visible light
irradiation. The UV/visible absorption spectra are characterized by
Thermomechanical analysis shows specimen deformation
occurrence under very low load as a function of scanned temper-
ature. Thermomechanical analysis of linear 9.4 K polymer and Star-
8 K(2.6 K) polymer films were studied to investigate changes of
dimension or mechanical properties of samples subjected to
a predefined temperature program and applied load. Fig. 4 shows
the changes in deformation of linear 9.4 K polymer and Star-
8 K(2.6 K) polymer as a function of temperature at constant applied
load. Linear 9.4 K polymer showed about 40% deformation at 8 mN
load, while Star-8 K(2.6 K) polymer showed 100% deformation at
a similar load. Linear polymer has the structure of a polymer chain,
which may be easily entangled and thus is rigid in nature.
Conversely, star polymers, due to their unique characteristics, show
less entanglement between molecules; also, their molecular chains
are short branches that are considered to provide star polymers
with high flexibility due to their low degree of freedom. In the case
of linear 9.4 K polymer, no significant deformation was observed up
to 65 ꢀC (Tg); a large deformation of linear polymer film was
observed at 90 ꢀC, the phase transition temperature of smectic to
nematic phase. The Tg of Star-8 K(2.6 K) polymer was 90 ꢀC, and
a large deformation was observed at that temperature.
a strong
pep
* transition of trans-azobenzene moiety at 359 nm
and a weak absorption at 450 nm, which originates from ne
p
*
transition. Upon UV (366 nm; 58 mW/cm2) irradiation, absorbance
at 359 nm decreased and absorbance at 450 nm increased slightly
due to trans / cis photoisomerization. Conversely, visible light
irradiation (436 nm, 58 mW/cm2) caused a reverse situation to be
observed. Thus, UV irradiation generates a photostationary state
with a high amount of cis-isomer, while visible irradiation gener-
ates a trans-isomer rich photostationary state. The subsequent
cis / trans photoisomerization by visible light irradiation rebuilt
the initial state completely, which demonstrates the reversibility of
the reaction. In the case of film, photoisomerization was slower
compared to that in solution. It is assumed that, because azopol-
ymers associate or aggregate in solid film [30], the rate of photo-
isomerization decreased slightly.
3.5.2. Fabrication of surface relief gratings on star azopolymer films
Photofabrication of SRGs was performed on spin-coated thin
films of star azopolymers. Good quality film was obtained by taking
proper care in spin-coating. Polymer film thickness was controlled
by changing the concentrations and speeds of coating. The spin-
coated azopolymer films were kept at room temperature for 48 h or
3.4. Differential scanning calorimetric analysis
The thermal properties of linear and star azopolymers were
also studied by differential scanning calorimetry (DSC) and are
more to remove solvent. The thickness of the film was about 1 mm.
An interference pattern of (CRP þ CRP) Arþ laser was used to
Fig. 8. AFM images of SRG on Star-8 K(2.6 K) azopolymer film by exposure to interference pattern of Arþ laser for 30 min.

Md.Z. Alam et al. / Polymer 52 (2011) 3696e3703
3703
inscribe SRG. Relatively low light intensity of Arþ laser (5 mW) was
used to write the gratings in order to avoid sample damage and
other possible side effects caused by high-intensity laser irradia-
tion. Fig. 6 illustrates the optical set-up for the formation of SRGs on
POM and TMA. Synthesized star-shaped azopolymers showed
reversible tran-cis photoisomerization in solution and film upon
irradiation of UV and visible light, respectively. A diffraction effi-
ciency as high as 20% was obtained upon exposure of Star-8 K(2.6 K)
azopolymer film to interference pattern of Arþ laser for 30 min. The
formation of SRG on Star-8 K(2.6 K) azopolymer film occured more
rapidly than on other polymers due to its size, molecular weight
and lower viscosity. The modulation depths of SRGs were about
150 nm and were found to be stable at room temperature. Thus, the
star azopolymers synthesized in this study by RAFT polymerization
may have potential applications in photonics and the emerging
area of nanotechnology.
azopolymer films. A fringe spacing of 4 mm was maintained. Using
suitable wave plates, two orthogonally polarized or p-polarized
beams were used to produce the interference pattern on the
polymer films. The rates of grating formation on azopolymer films
was probed by measuring first-order diffraction efficiency with
a HeeNe laser (100
mW), which is defined as the ratio of the
intensity of the first-order diffraction beam (ID) to that of the
transmitted beam (IT) through the film:
Diffraction efficiency;
h
ð%Þ ¼ ðID=ITÞ ꢃ 100
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linear polymers with similar molecular weights.
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