Synthesis and photoresponsive behavior of the high-Tg azobenzene polymers via RAFT polymerization

Article abstract of DOI:10.1016/j.reactfunctpolym.2010.04.006

Well-defined photo-responsive alternating copolymers, poly(4-(N-maleimido) azobenzene-alt-styrene)s (PMSts), were successfully synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. A divinyl monomer was used in this polymerization to prepare high molecular weight azobenzene polymers. These polymers had good solubility in most organic solvents, formed films well, and had high glass transition temperatures (T_g = 174-250 °C) and were heat resistant (T_d > 320 °C). The photo-induced trans-cis isomerization of the copolymers was examined in chloroform solution. Surface-relief-gratings (SRGs) formed on the polymer films were also investigated using illumination from a linearly polarized Kr ⁺ laser beam.

Full text of DOI:10.1016/j.reactfunctpolym.2010.04.006

Reactive & Functional Polymers 70 (2010) 456–462
Contents lists available at ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Synthesis and photoresponsive behavior of the high-Tg azobenzene
polymers via RAFT polymerization
*
Xiaoqiang Xue, Jian Zhu, Zhengbiao Zhang, Nianchen Zhou, Xiulin Zhu
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science of Soochow University, Suzhou 215006, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Well-defined photo-responsive alternating copolymers, poly(4-(N-maleimido)azobenzene-alt-styrene)s
(PMSts), were successfully synthesized via reversible addition–fragmentation chain transfer (RAFT) poly-
merization. A divinyl monomer was used in this polymerization to prepare high molecular weight azo-
benzene polymers. These polymers had good solubility in most organic solvents, formed films well,
and had high glass transition temperatures (Tg = 174–250 °C) and were heat resistant (Td > 320 °C). The
photo-induced trans–cis isomerization of the copolymers was examined in chloroform solution. Sur-
face-relief-gratings (SRGs) formed on the polymer films were also investigated using illumination from
a linearly polarized Kr+ laser beam.
Received 9 January 2010
Received in revised form 11 April 2010
Accepted 13 April 2010
Available online 5 May 2010
Keywords:
Surface-relief-grating (SRG)
Reversible addition–fragmentation chain
transfer (RAFT)
Ó 2010 Elsevier Ltd. All rights reserved.
Azo polymer
Photoresponsive
Alternating copolymerization
1. Introduction
fully improved the Tg of a side-chain azobenzene polymer prepared
via free radical polymerization. They found that the SRG inscription
Azobenzene functionalized polymers have received much
attention because of their potential uses in various optical applica-
tions, including optical data storage [1,2], liquid crystal displays
[3–6], optical switching [7,8] and holographic surface-relief-grat-
ings (SRGs) [9,10] among others. Upon irradiation with polarized
light, these photoresponsive azobenzene molecules undergo un-
ique optical trans–cis–trans isomerization cycles and possibly
light-induced large-scale shape and dipole moment movement of
the rigid azobenzene chromophore [11–13], which results in
reversible surface modulation. In this way, photo-induced SRGs
on thin azobenzene polymer films can be formed by irradiation
with interfering laser beams [14,15]. These SRGs are typically sta-
ble substantially below the glass transition temperatures (Tgs) of
the azobenzene polymers and are erased by heating the polymers
to their Tgs [16]. Photo fabrication of stable SRGs on the azoben-
zene polymer films for practical applications therefore requires
polymers with good thermal stability and high Tg matrices, such
as supramolecular polymers [17] and main-chain azobenzene
polymers [18–23].
rate was affected by azobenzene functionality. However, it was dif-
ficult to control the structure of polymer by this method. In the
past decade, other techniques have been developed to synthesize
the polymers with controlled structures such as atom transfer rad-
ical polymerization (ATRP) [25–29], reversible addition–fragmen-
tation chain transfer (RAFT) polymerization [30–35] and
nitroxide-mediated radical polymerization (NMP) [36–39]. These
techniques have become powerful tools for the design of well-de-
fined polymers under mild conditions. Permitting precise control
over the polymerization process, they have provided a new set of
robust methods for pre-designed polymers. Azobenzene polymers
with well controlled molecular weights and structures have been
easily synthesized via living free radical polymerization [40–45].
However, the majority of reported side-chain azobenzene poly-
mers had low Tgs which did not favor these materials in high tem-
perature applications.
In this work, we synthesized an azobenzene functionalized
maleimide monomer, 4-(N-maleimido)azobenzene [46]. This
strongly electron accepting monomer can be copolymerized with
styrene (typical electron donation monomer) in an alternating
manner [47,48]. Using the RAFT technique, 4-(N-maleimido)azo-
benzene was copolymerized with styrene to form alternating
copolymers with steady functionalization (50%). The polymers
showed high Tg and Td after the maleimide structure was intro-
duced [49–54]. Furthermore, branched azobenzene copolymers
were successfully obtained by introducing the divinyl monomer,
However, there are few reports of the use of side-chain azoben-
zene polymers with high Tg in SRG research [24]. By introducing N-
phenylmaleimide as the co-monomer, Fukuda et al. [24] success-
* Corresponding author. Tel.: +86 512 65111278; fax: +86 512 65112796.
E-mail address: xlzhu@suda.edu.cn (X. Zhu).
1381-5148/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.reactfunctpolym.2010.04.006

X. Xue et al. / Reactive & Functional Polymers 70 (2010) 456–462
457
N,N-4,4-diphenylmethyenebismaleimide (BMI), into the polymeri-
zation system. This provided another opportunity to investigate
the effect of molecular weight on the photoproperties of azoben-
zene polymers. Several azobenzene polymers with significantly
different molecular weights were fabricated into homogeneous
thin film and their SRGs were investigated.
The purified product was found to have the following character-
istics: 1H NMR (400 MHz, CDCl3), d (TMS, ppm): 6.89 (s, 2H), 7.46–
7.59 (m, 5H), 7.90–7.97 (d, 2H), 8.00–8.06 (d, 2H). Elemental anal-
ysis: Calculated (%): C 69.31, H 4.00, N 15.15; Found (%): C 69.52, H
4.23, N 15.33.
2.3. Synthesis of alternating copolymer poly(4-(N-
maleimido)azobenzene-alt-styrene) (PMSt) via RAFT polymerization
2. Experimental
RAFT copolymerization of styrene (St) and MAB was typically
carried out by the following procedure. A dry 5 mL ampoule tube
was filled with MAB (277 mg, 1 mmol), St (104 mg, 1 mmol), CPDN
(5.4 mg, 0.02 mmol), AIBN (0.55 mg, 0.0033 mmol) and 1,4-diox-
ane (3 mL). The contents were purged with argon for 20 min to
eliminate oxygen, the ampoule was flame-sealed and the mixture
was stirred for 20 min at room temperature. The polymerization
reaction was carried out at 70 °C in an oil bath and terminated after
a predetermined time. The contents were diluted with 10 mL THF
and precipitated in 250 mL methanol. The precipitate was filtered
and dried to a constant weight at room temperature under vac-
uum. The conversion of polymerization was determined by
gravimetry.
2.1. Materials
Styrene and methyl acrylate (analytical grade; Shanghai Chemi-
cal Reagent Co. Ltd., Shanghai, China) were washed with an aqueous
solution of sodium hydroxide (5% w/v) three times and then with
deionized water until neutralization. After being dried with anhy-
drous magnesium sulfate, the monomers were distilled twice under
reduced pressure, and stored below 0 °C before use. Azobisisobuty-
ronitrile (AIBN; 99%) was recrystallized three times from ethanol
and dried under vacuum at room temperature overnight. 1,4-Diox-
ane (analytical reagent; Shanghai Wulian Chemicals Co. Ltd., Shang-
hai, China) used in polymerization was purified by vacuum
distillation over anhydrous calcium hydride (CaH2). p-Aminoazo-
benzene (98%; ACROS) and N,N-4,4-diphenylmethyenebismalei-
mide (BMI, Scheme 1, 98%; ACROS) were used as received.
2-cyanoprop-2-yl 1-dithionaphthalate (CPDN; above 98% purity)
was synthesized as previously described [55] (Scheme 1). Other re-
agents were used without further purification.
2.4. Synthesis of branched azobenzene copolymers by RAFT
polymerization
A dry 5 mL ampoule tube was filled with MAB (277 mg,
1 mmol), St (208 mg, 2 mmol), CPDN (8.2 mg, 0.03 mmol), BMI
(35.8 mg, 0.1 mmol), AIBN (0.82 mg, 0.005 mmol) and 1,4-dioxane
(3 mL). The polymerization procedure was same as that for the
RAFT copolymerization of styrene (St) and MAB.
2.2. Synthesis of 4-(N-maleimido)azobenzene (MAB)
The azobenzene monomer MAB [46] in Scheme 1 was prepared
by a typical imidization reaction. A solution of p-aminoazobenzene
(19.2 g, 0.1 mol) in 50 mL chloroform (CHCl3) was added dropwise
to the round bottom flask containing maleic anhydride (11.8 g,
0.12 mol) and 20 mL CHCl3 cooled by ice bath. The mixture was
then stirred for 12 h. The resulting yellow crystal was removed
by filtration and dried under vacuum. The product was cyclized
with 100 mL of acetic anhydride and 10 g of sodium acetate in
100 mL tetrahydrofuran (THF) at 80 °C under N2 atmosphere for
6 h to obtain the corresponding maleimide, MAB. The resultant
reaction mixture was poured into large amount of deionized water.
Yellow solid was recovered by filtration, washed with deionized
water three times, and dried under vacuum. The crude product
was purified by recrystallization from ethanol three times to yield
the pure product (yield: 20.7 g, 75%).
2.5. Preparation of polymer film and optical characterization
Thin film for grating inscription was obtained by following pro-
cedure. A solution obtained by dissolving the alternating copoly-
mers PMSts in freshly distilled CHCl3
through a 0.2 m pore syringe filter. This solution was spin coated
(0.05 g/mL) was filtered
l
onto a clean glass slide at 3000 rpm to form a thin film. The thick-
ness of the homogeneous thin film was measured to be about
300 nm by an Ambios Technology XP-2 Stylus Profiler, and subse-
quently dried under vacuum for 24 h at room temperature. The
film was stored in a desiccator for further study.
The fabrication of surface-relief-gratings (SRGs) was as previ-
ously described (see supporting information in Fig. S1) [56]. SRGs
were measured at room temperature with a linearly polarized
Kr+ laser beam (413.1 nm, 200 mW/cm2) as the light source, and
optically inscribed on the film with a p-polarized interfering laser
beam. The surface images of the SRGs were detected using atomic
force microscopy (AFM, NT-MDT SOLVER P47-PRO). The dynamic
diffraction efficiency of the positive first order was measured using
a non-polarized low power diode laser beam (650 nm) in transmis-
sion mode.
O
O
N
O
O
N
N
CN
S
2.6. Analysis and characterization of the polymers
S
1H NMR and 13C NMR spectra were recorded on an INOVA
400 MHz nuclear magnetic resonance (NMR) instrument, using
CDCl3 or DMSO-d6 as a solvent and tetramethylsilane (TMS) as
the internal standard. Elemental analysis of C, H and N were con-
ducted with an EA1110 CHNO-S instrument. The average molecu-
lar weights (Mns) and molecular weight distributions (Mw/Mns) of
the polymers were determined with a Waters 1515 gel permeation
chromatograph (GPC) equipped with a refractive index detector,
using HR1, HR3, and HR4 columns with a molecular weight range
of 100–500,000 calibrated with PS and PMMA standard samples.
N
N
O
O
CPDN
MAB
BMI
Scheme 1. Chemical structures of 2-cyanoprop-2-yl 1-dithionaphthalate (CPDN),
4-(N-maleimido)azobenzene (MAB) and N,N-4,4-diphenylmethyenebismaleimide
(BMI).

458
X. Xue et al. / Reactive & Functional Polymers 70 (2010) 456–462
THF was used as the eluent at a flow rate of 1.0 mL minꢀ1 at 30 °C.
UV–vis spectra were recorded with a Hitachi U-3900 spectropho-
tometer at room temperature. Differential scanning calorimetry
(DSC) was performed using a TA instruments DSC2010 with a heat-
ing/cooling rate of 10 °C minꢀ1 under a continuous nitrogen flow.
5
4
3
2
1
14000
12000
10000
8000
6000
4000
2000
0
M
M
n(th)
n(GPC)
M
/M
n
w
FT-IR spectra were recorded on
a
Nicolette-6700 FT-IR
spectrometer.
3. Results and discussion
3.1. RAFT copolymerization of MAB and styrene
Maleimide and styrene are one of the most extensively studied
co-monomer pairs to spontaneously copolymerize in an alternat-
ing manner [57]. The combination of the polarity of the double
bonds in the strongly electron-accepting maleimide monomer
and the electron-donating styrene monomer produce an alternat-
ing copolymer. Recently, several research groups reported the syn-
thesis of well-defined alternating copolymers of maleimide and
styrene with controlled molecular weights (Mns) and narrow
molecular weight distribution (Mw/Mn) via LRP [58–60].
0
10
20
30
40
50
60
70
80
Conversion (%)
Fig. 2. Dependence of the molecular weights and the molecular weight distribu-
tions on the monomer conversions for the RAFT copolymerization of MAB and St.
The polymerization conditions are same as in Fig. 1.
3.2. Branched azobenzene copolymers by RAFT copolymerization
The highest molecular weight of copolymers synthesized in a
controlled manner was 12,300 g/mol (Table S1). Most of the poly-
mers showed similar molecular weights even using different molar
ratios of [monomer]0: [CPDN]0 for polymerization. Therefore, in or-
der to prepare high molecular weight copolymers with good solu-
bility in common organic solvents, a divinyl monomer, N,N-4,4-
diphenylmethyenebismaleimide (BMI), was introduced into the
polymerization system. The RAFT copolymerization of MAB and
St with BMI was carried out using AIBN as an initiator and CPDN
as the RAFT agent in 1,4-dioxane solution at 70 °C. The results
are shown in supporting information (Table S1). No gel was ob-
served during the polymerization process and polymers obtained
were soluble in common organic solvents. The molecular weights
(Mn(GPC)) of these branched azobenzene copolymers significantly
increased when the molar ratio of [BMI]0:[MAB]0 was increased
from 1:20 to 3:10. Samples obtained from different concentrations
of BMI with same polymerization time (40 h) had two peaks in
their GPC profile (Fig. S2; P2, P4, P6 and P8) which became signif-
icantly more separated with increasing concentrations of BMI in
polymerization system. These results indicated the formation of
microgelation or branched polymer structure, in agreement with
previous reports which had observed these structures following
the addition of divinyl monomer to living free controlled polymer-
ization systems [64–66].
This paper investigated the RAFT copolymerization behavior of
4-(N-maleimido)azobenzene (MAB) with styrene using AIBN as the
initiator and 2-cyanoprop-2-yl 1-dithionaphthalate (CPDN) as the
RAFT agent in 1,4-dioxane solution. The chemical structure of
alternating copolymers PMSt is shown in supporting information
(Scheme S1). The polymerization showed first-order kinetics at
70 °C (Fig. 1). The molecular weights measured by GPC (Mn(GPC)s)
increased linearly with monomer conversions, and the molecular
weight distribution was relatively narrow (Mw/Mn < 1.50) (Fig. 2).
However, the Mn(GPC)s were slightly higher than the theoretical
values in the early stage of the polymerization, which could be
due to the incomplete usage of RAFT agent [61]. The molecular
weights were lower than the theoretical values later in the poly-
merization, possibly due to the PS standards used for GPC calibra-
tion [62]. The theoretical molecular weight of the copolymers can
be obtained with the following the following formula:
MnðthÞ ¼ ½mꢁ0=½CPDNꢁ0 ꢂ Mm ꢂ Conversion þ MCPDN
ð1Þ
where [m]0 and [CPDN]0 are the starting concentrations of the
monomer and the RAFT agent, CPDN, respectively; Mm and MCPDN
are the molar mass of the monomer (Mm = (MMAB + MSt)/2) and
CPDN, respectively [63].
Meanwhile, we also studied RAFT copolymerization behavior
with a 30% molar ratio of [BMI]0:[MAB]0 over different polymeriza-
tion times. All of the obtained azobenzene polymers showed good
solubility in common solvents such as CHCl3 and THF, and had
good film forming ability, even at molecular weights greater than
50,000 g/mol. Thus, the introduction of BMI into the St–MAB copo-
lymerization system significantly improved the molecular weight
of obtained polymer.
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
3.3. Chain extension
The structures of the copolymers, PMSts, were characterized by
1H NMR, 13C NMR, UV–vis absorption and FT-IR spectra (see sup-
porting information). In addition, in order to demonstrate the pres-
ence of CPDN moieties at the polymer end and ‘living” nature of the
polymer, the chain-extension experiments were carried out using
0
5
10
15
20
t (h)
25
30
35
poly(4-(N-maleimido)azobenzene-alt-styrene)
Mn(GPC) = 3600 g/mol, Mw/Mn = 1.12) as the macro-RAFT agent and
methyl acrylate (MA) as the second monomer ([MA]0: [macro-
(PMSt,
Fig. 1. Kinetic plot of the copolymerization of MAB and St in 1,4-dioxane with CPDN
as the RAFT agent and AIBN as the initiator at 70 °C ([Monomer]0: [CPDN]0:
[AIBN]0 = 300: 3: 0.5, MAB: St = 1: 1).

X. Xue et al. / Reactive & Functional Polymers 70 (2010) 456–462
459
PMSt2, which had about 5% weight loss at 319 °C under nitrogen
atmosphere and 40% weight loss at 408 °C.
PMSt-b-PMA-b-PMSt
PMSt-b-PMA
PMSt
3.5. Photoisomerization behavior
Azobenzene polymers undergo photoisomerization, converting
from trans- to cis-forms under ultraviolet irradiation at 365 nm.
The UV–vis absorption spectra of these polymers were recorded
at the different time intervals until a photo-stationary state was
reached.
The
UV–vis
absorption
spectra
of
PMSt
(Mn(GPC) = 5400 g/mol, Mw/Mn = 1.22) showed an absorption at
about 329 nm, before UV exposure which was due to the character-
istic intense
p-
pꢃ transition of azobenzene (trans-form) (Fig. 4).
17
18
19
20
21
22
23
24
25
26
After irradiation with UV light at 365 nm, the trans-form of the
azobenzene changed to the cis-form with an absorption peak at
440 nm (Fig. 4). The absorption of trans-form azobenzene at
329 nm rapidly decreased, and that of cis-form azobenzene at
440 nm increased slightly with the irradiation time. There was still
59.2% trans- form azobenzene in the photo-stationary state
([trans]% = A1/A0 ꢂ 100%, A1, A0 are absorbance at 329 nm at infi-
nite, time zero), indicating that the photoisomerization of the poly-
mer was incomplete.
Minutes
Fig. 3. GPC curves of the original PMSt (Mn = 3600 g/mol, Mw/Mn = 1.12), block
copolymer PMSt-b-PMA (Mn = 6700 g/mol, Mw/Mn = 1.23) and PMSt-b-PMA-b-PMSt
(Mn = 9400 g/mol, Mw/Mn = 1.50).
RAFT]0: [AIBN]0 = 300:3:0.5, T = 70 °C, solvent: 1,4-dioxane, t = 4 h,
conversion% = 31.6%). The obtained block copolymers, poly((4-(N-
maleimido)azobenzene-alt-styrene)-b-methyl acrylate) (PMSt-b-
PMA), were used as the macro-RAFT agent once again to prepare
tri-block copolymers, poly((4-(N-maleimido)azobenzene-alt-sty-
rene)-b-methyl acrylate -b-(4-(N-maleimido)azobenzene-alt-sty-
rene)) (PMSt-b-PMA-b-PMSt), with MAB and St as the second co-
monomer ([Monomer]0: [macro-RAFT]0: [AIBN]0 = 300: 3: 0.5,
MAB: St = 1: 1, T = 70 °C, solvent: 1,4-dioxane, t = 12 h, conversion
% = 26.7%). The GPC curves of the original PMSt, block copolymers
PMSt-b-PMA and PMSt-b-PMA-b-PMSt showed an obvious peak
shift from the macro-RAFT agent PMSt to PMSt-b-PMA with the
molecular weight increasing from 3600 to 6700 g/mol, and then
to 9400 g/mol for PMSt-b-PMA-b-PMSt (Fig. 3). However, the
monomodel molecular weight distributions of the chain-extended
block copolymers (Mw/Mn = 1.23 and 1.50) were broader than that
of the original polymer (Mw/Mn = 1.14), possibly due to the pres-
ence of ‘‘dead” polymer chains in the original polymer [62]. The
successful RAFT block copolymerization demonstrated the pres-
ence of CPDN moieties in the end of the polymer chains and ‘living”
nature of the polymers.
0 S
2.0
1.6
1.2
0.8
0.4
0.0
1560 S
250
300
350
400
450
500
550
Wave Length (nm)
Fig. 4. Changes in the UV–vis absorption spectra of PMSt (Mn(GPC) = 5400 g/mol,
Mw/Mn = 1.22). Polymer solution (5.0 ꢂ 10ꢀ6 M) was irradiated with 365 nm UV
light in CHCl3 at room temperature for different time intervals.
3.4. Thermal properties
It is well-known that alternating copolymers of maleimide and
styrene have a high glass transition temperature (Tg) even with low
molecular weight. Thermal properties of the alternating copoly-
mers, PMSts, were determined by differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA). The polymers were
heated or cooled at a heating/cooling rate of 10 °C minꢀ1 under a
continuous nitrogen flow. The Tgs of PMSt1, PMSt2, PMSt3, and
PMSt4 were 174 °C, 178 °C 197 °C, and 201 °C, respectively
(Table S3). The branched azobenzene copolymers (P001–P007)
had substantially higher Tgs, in the range of 214–239 °C
(Table S2), which may have been due to their high molecular
weight [67]. For example, the Tg of P007 (Mn = 52,000 g/mol) was
239 °C, about 39 °C higher than that of PMSt4 (Mn = 11,000 g/
mol). The Tg of block copolymer PMSt-b-PMA decreased from
178 °C to 104 °C due to the introduction of the flexible PMA chain.
However, after the readdition of chain-extended block copolymers
of MAB and St, the tri-block copolymer PMSt-b-PMA-b-PMSt had a
higher Tg (147 °C) than that of the original block copolymer PMSt-
b-PMA. Thus, the alternating copolymers exhibited high Tg (174 °C)
even with low molecular weight (Mn = 2400 g/mol). The good ther-
mal stability of alternating copolymers [68] was demonstrated by
2.4
0
2.2
-1
2.0
-2
1.8
-3
1.6
-4
-5
1.4
1.2
0
300
600
900 1200 1500 1800 2100
Time (s)
Fig. 5. First-order plot (j) for trans–cis photoisomerization of PMSt (Mn(GPC)
5400 g/mol, Mw/Mn = 1.22) and changes (h) of absorbance at kmax (329 nm) in the
=
UV–vis absorption spectra in CHCl3 at room temperature.

460
X. Xue et al. / Reactive & Functional Polymers 70 (2010) 456–462
Fig. 6. AFM images of the SRGs formed on PMSt (Mn(GPC) = 5400 g/mol, Mw/Mn = 1.22) films: (A) plane view of the SRG and (B) three-dimensional view of the SRG.
maintained the SRG profile well when the thin film was heated to
the temperature lower than Tg.
The rate of trans–cis photoisomerization was analyzed from the
UV absorption at 329 nm. The photoisomerization kinetics of PMSt
confirmed the first-order kinetic plot for the trans–cis photoiso-
merization of PMSt (Fig. 5). The first-order rate constants were
determined by the following formula:
The availability of azobenzene polymers of different molecular
weight allowed the investigation of the effect of azobenzene poly-
mer molecular weights on their SRG properties. Three films were
prepared for SRG fabrication using three polymers with different
molecular weights: Mn(GPC) = 5400 g/mol, 11,000 g/mol and
52,000 g/mol. The diffraction efficiency of the SRGs decreased with
the molecular weights of the copolymers (Fig. 7, Table 1) which im-
plied that the SRG formed more easily on low molecular weight
than high molecular weight film [70,71]. The results also indicated
that the diffraction efficiency of the PMSt film was proportional to
the thickness of the film, in a manner similar to that previously re-
ported [72].
ꢀ
ꢁ
A
1 ꢀ At
Ln
¼ ꢀKet
ð2Þ
A
1 ꢀ A0
where A1, A0 and At are absorbance at 329 nm at time infinite, time
zero and time t, respectively. The photoisomerization rate constant,
ke, was calculated to be 0.0027 sꢀ1. The low ke of polymer PMSt was
most likely due to the sterically hindering effect of the main-chain
maleimide configuration in the copolymer.
3.6. Photo-induced surface-relief-gratings
4.0
Surface-relief-grating (SRG) formation was investigated using a
spin-coated thin film of alternating copolymer PMSt
(Mn(GPC) = 5400 g/mol, Mw/Mn = 1.22) with a high glass transition
temperature Tg (197.04 °C). Observation of the film using atomic
force microscopy (AFM) indicated that surface relief structures
with regular spaces were fabricated on the film surfaces of alter-
nating copolymer PMSt with a thickness of about 300 nm. Fig. 6
shows atomic force microscopy (AFM) plane (A) and three-dimen-
sional (B) images of the alternate surface relief structures with reg-
ular spaces fabricated on the polymer film surfaces following
irradiation with a Kr+ laser for 1000 s at room temperature. AFM
image analysis showed that the surface modulation depth was
P1:Mn = 5400 g/mol
3.5
3.0
P2:Mn = 11000 g/mol
2.5
2.0
1.5
P3:Mn = 52000 g/mol
1.0
0.5
0.0
about 138 nm, and the grating spacing was about 1.5
lm (Fig. 6).
0
200
400
600
Time (s)
800
1000
The space period of the SRG was determined by the wavelength
K
k of the writing beam and the angle h between the two interfering
beams, calculated by the following formula:
Fig. 7. Diffraction efficiency as a function of the irradiation time for the similar
thickness thin film of three sample PMSts (P1: Mn(GPC) = 5400 g/mol, 300 nm; P2:
Mn(GPC) = 11,000 g/mol, 306 nm; P3: Mn(GPC) = 52,000 g/mol, 335 nm).
K
¼ k=2 sinðh=2Þ
ð3Þ
When the polymer film was exposed to the writing beam for 1000 s
at room temperature, the rates of gratings formation can be probed
by measuring the first-order diffraction efficiency of the SRG. This
increased rapidly with Kr+ laser irradiation time (Fig. 7). The diffrac-
tion efficiency was about 3.4% with a saturation level at 1000 s and
a thickness of about 300 nm. The diffraction efficiency of SRG is
dependent on the surface modulation depth, the grating spacing
and the space period [69]. Moreover, gratings can be erased by heat-
ing the film to Tg [15,18–23]. The high-Tg azobenzene polymer PMSt
Table 1
Photoresponsive characterization of the azobenzene polymers.
Sample
Mn
(g/mol)
Film
thickness
(nm)
Surface
modulation
depth (nm)
Grating
spacing
(lm)
Diffraction
efficiency
(%)
P1
P2
P3
5400
11,000
52,000
300
306
335
138
142
98
1.5
1.4
1.4
3.4
2.5
1.0

X. Xue et al. / Reactive & Functional Polymers 70 (2010) 456–462
461
Fig. 8. AFM plane images of the SRGs formed on PMSt (Mn(GPC) = 5400 g/mol, Mw/Mn = 1.22) film (d = 227 nm) at various temperatures: (A) 150 °C and (B) 200 °C.
To confirm the SRGs based on the polymer PMSt (Mn(GPC)
=
and 20974071), the Specialized Research Fund for the Doctoral
5400 g/mol, Mw/Mn = 1.22) had the desired temperature stability,
SRGs formed on PMSt film (227 nm) at various temperatures were
investigated. The SRG did not change when the temperature was
increased from room temperature to 150 °C and maintained for
30 min (Fig. 8A). When the sample was heated to 200 °C for
30 min, which was higher than the Tg of the polymer, the SRGs
were successfully erased (Fig. 8). The stability of the SRG formed
from PMSt at temperatures as high as 150 °C should favor future
practical applications.
Program of Higher Education contract grant (No. 200802850005),
the Program of Innovative Research Team of Soochow University
and the Qing Lan Project are gratefully acknowledged.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.reactfunctpolym.2010.04.006.
4. Conclusion
References
Well-defined photo-responsive alternating copolymers contain-
ing the azobenzene chromophore, poly(4-(N-maleimido)azoben-
zene-alt-styrene) (PMSt) was successfully synthesized from the
copolymerization of 4-(N-maleimido)azobenzene (MAB) and sty-
rene (St) via reversible addition–fragmentation chain transfer
(RAFT) polymerization using CPDN as the RAFT agent and AIBN
as an initiator in 1,4-dioxane solution. The copolymerization was
well controlled, resulting in polymers with controlled molecular
weight and narrow molecular weight distribution. Block copoly-
mers using PMSt as the macro-RAFT agent and MA as the monomer
was successfully prepared. Furthermore, highly branched azoben-
zene copolymers of high molecular weight were successfully ob-
tained (Mn is higher than 50,000) after the addition of N,N-4,4-
diphenylmethyenebismaleimide (BMI) into the polymerization
system. The copolymers showed high glass transition tempera-
tures (Tg) and decomposition temperatures (Td). On irradiation
with a linearly polarized Kr+ laser beam (413.1 nm), the diffraction
efficiency from SRGs on PMSt film (Mn(GPC) = 5400 g/mol, Mw/
Mn = 1.22) was about 3.4% with saturation level at 1000 s. The dif-
fraction efficiency of SRG decreased with increasing molecular
weight of polymer film; diffraction efficiencies at Mn(GPC)s of
5400 g/mol, 11,000 g/mol and 52,000 g/mol were 3.4%, 2.5% and
1.0%, respectively. SRG diffraction efficiency of these polymers in-
creased with increasing thickness of the film. The SRG formed on
PMSt1 film was stable even at 150 °C due to the high Tg of PMSt1,
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