Synthesis and characterization of polytriphenylamine based graft polymers for photorefractive application

Article abstract of DOI:10.1016/j.polymer.2012.11.028

Novel graft copolymers, PTPA-g-PEAs containing poly(triphenylamine) (PTPA) backbone and poly(ethyl acrylate) (PEA) branches were synthesized by the oxidative coupling polymerization of triarylamine monomers followed by grafting of ethyl acrylate via an at

Full text of DOI:10.1016/j.polymer.2012.11.028

Polymer 54 (2013) 269e276
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and characterization of polytriphenylamine based graft polymers
for photorefractive application
*
Zhenbo Cao, Yosuke Abe, Takuo Nagahama, Kousuke Tsuchiya, Kenji Ogino
Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei-shi, Tokyo 184-8588, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel graft copolymers, PTPA-g-PEAs containing poly(triphenylamine) (PTPA) backbone and poly(ethyl
acrylate) (PEA) branches were synthesized by the oxidative coupling polymerization of triarylamine
monomers followed by grafting of ethyl acrylate via an atom transfer radical polymerization (ATRP).
Photorefractive (PR) composites based on the graft copolymers showed good static PR properties and fast
Received 18 June 2012
Received in revised form
24 October 2012
Accepted 8 November 2012
Available online 11 November 2012
response time under moderate conditions. The highest diffraction efficiency (19.7% at 45 V/
mm) was
observed with the composite containing the graft polymer with 18 mol% of graft density and 27 wt% of
PTPA. And the fastest response (8 ms at 50 V/
m
m) was achieved when PTPA content was 68 wt%.
Keywords:
Ó 2012 Elsevier Ltd. All rights reserved.
Poly(triphenylamine)
Graft copolymer
Photorefractivity
1. Introduction
polymers due to their efficient hole transporting ability, and high
photo-charge generation efficiency. As we reported previously,
various polymers containing TPA moiety in their main chains have
been prepared via an addition [17,18] or additionecondensation
[19,20] reaction using nucleophilic nature of phenyl group. We
also reported that 4-alkyltriphenylamine can be polymerized via
the chemical oxidative coupling reaction using ferric chloride as an
oxidant to afford poly(4-alkyltriphenylamime)s (PTPAs) [21,22],
and that poly(4-butyltriphenylamine) showed much higher
photoconductivity compared with PVK [22].
Although highly photoconductive PTPAs are promising candi-
dates for a PR host material as mentioned above, no attempts to
apply PTPAs to PR materials have been made as far as we know.
Generally PR host materials should show high miscibility with EO
chromophores, and plasticizers if the original polymers possess
high Tg [7,11e13]. Because of high Tg nature of PTPAs [22], it is
necessary to utilize a plasticizer for the facile reorientation of EO
chromophore. However, our preliminary studies revealed that
PTPAs show poor miscibility with a plasticizer as well as with an EO
chromophore, leading to phase separation in PR composite films. It
is considered that this fact results in no application of PTPAs to PR
materials so far.
Organic photorefractive (PR) materials have been intensively
investigated in recent years because of potential applications to
holographic optical data storage and real-time image processes
and several advantages such as higher nonlinearity, low dielectric
constant, and structural flexibility as described in several review/
feature articles [1e5]. Although it is reported more recently
that multifunctional copolymers containing carbazole and
dicyanomethylene-dihydrofuran units show outstanding PR
performance such as fast buildup and erasure times of a few tens of
milliseconds, large refractive index modulation (over 10ꢀ2) and
two-beam coupling gain (above 350 cmꢀ1) [6], generally hoste
guest systems with low glass transition temperatures (Tg), which
consist of photoconducting polymers doped with an electro-
optically (EO) active chromophore and a small amount of charge
generator or photosensitizer, have shown excellent PR properties
such as high diffraction efficiency, large net gain [7e13], and fast
response [14,15]. This suggests that the high mobility of EO active
chromophore for the orientation plays an important role for the
high performance of PR composites.
In addition to a conventional poly(N-vinylcarbazole) (PVK),
triphenylamine (TPA)-based polymers such as polyacrylate [11e14]
and polyether [16] have also attracted much attention as host
In order to increase the compatibility of EO chromophore and
lower Tg of the resulting composites, we here designed graft
copolymers consisting of PTPA main chain and polar poly(ethyl
acrylate) (PEA) side chains. This is the first attempt to apply graft
copolymers to PR materials. In general, block copolymers and graft
copolymers could provide microphase-separated morphologies
* Corresponding author. Tel./fax: þ81 42 388 7404.
E-mail address: kogino@cc.tuat.ac.jp (K. Ogino).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.11.028

270
Z. Cao et al. / Polymer 54 (2013) 269e276
such as spheres, cylinders, and lamela, dependent on the graft
density or the chemical composition [23e25]. In our previous
study, it is indicated that phase separated PR materials based on
block copolymers consisting of photoconductive and EO active
blocks show higher PR properties (105 cmꢀ1 of gain coefficient at
with water and concentrated by evaporation, followed by purifi-
cation using column chromatography (silica gel, toluene). The
white solid was obtained (43.8 g, 95%). 1H NMR (CDCl3):
d 7.29e7.00
(14H, m, eAr), 3.85 (2H, t, J ¼ 6.9 Hz, eOeCH2e), 2.82 (2H, t,
J ¼ 6.9 Hz, AreCH2e), 0.92 (9H, s, eSieteBu), 0.04 (6H, s, eSie
45 V/
m
m) compared the corresponding statistical random copol-
Me2). 13C NMR (CDCl3):
d 148.00, 145.93, 134.06, 130.05, 129.10,
ymer (15 cmꢀ1 at 45 V/
mm) [26]. Therefore, in our graft copolymer
124.63, 123.73, 122.31 (eAr), 64.55 (eOeCH2e), 38.95 (AreCH2e),
25.94 (SieCeMe3), 18.36 (eSieCeMe3), e5.38 (eSieMe2). Anal.
Calcd for C26H33NOSi: C, 77.37; H, 8.24; N, 3.47. Found: C, 77.51; H,
8.27; N, 3.35.
systems with microphase separated structure, additional
enhancement of PR performance is anticipated. In the polar and
flexible PEA rich phase, high compatibility with EO chromophore,
and its sufficient mobility for the smooth orientation through
internal space charge field are expected, which is important for
sufficient static PR performance and fast response. In addition,
photoconductive PTPA chain is expected to form such a distinctive
domain in phase separated systems, and the problem of dark
current in low Tg materials can be solved without sacrificing the
orientational mobility of dopants.
In this paper, it is reported that novel graft copolymers con-
sisting of PTPA main chain and PEA side chain were successfully
prepared via oxidative copolymerization of TPA derivatives, fol-
lowed by grafting PEA chains via atom transfer radical polymeri-
zation (ATRP) and were applied to PR host polymers. PR properties
of the composites containing the synthesized graft copolymer,
2.3. Synthesis of 4-(2-hydroxyethyl)triphenylamine (3)
To a 1000-mL flask equipped with a magnetic stirrer were added
2 (43.82 g, 0.10 mol) and 580 mL of THF, H2O, and acetic acid
mixture solution (THF/H2O/AcOH ¼ 20/20/60 vol%). After stirring at
room temperature for 4 days, 100 mL of diethyl ether was added to
the reaction mixture. The ethereal layer was washed with water
and concentrated by evaporation followed by purification using
column chromatography (silica gel, chloroform). The white solid
was obtained (23.1 g, 74%). 1H NMR (CDCl3):
Ar), 3.86 (2H, t, J ¼ 6.6 Hz, eCH2-OH), 2.83 (2H, t, J ¼ 6.6 Hz, Are
CH2e), 1.49 (1H, s, eOH). 13C NMR (CDCl3):
147.87, 146.32,
d 7.26e6.97 (14H, m, e
d
an EO chromophore (4-N,N-diethylamino-b,b-dicyanostyrene,
132.75, 129.80, 129.17, 124.48, 123.98, 122.56 (eAr), 63.66 (eCH2e
OH), 38.55 (eAreCH2e). Anal. Calcd for C20H19NO: C, 83.01; H,
6.62; N, 4.84. Found: C, 83.32; H, 6.71; N, 4.83.
DEADCST [27]) and a photosensitizer (2,4,7-trinitro-9-fluorenone,
TNF) were investigated, and relationships between the PR proper-
ties and the characteristics of graft polymer were discussed.
DEADCST is a general EO chromophore, showing moderate dipole
moment (7.8 D in chloroform [28]), and higher ionization potential
(5.7 eV) [29] than PTPAs (5.3 eV) [22,30]. Because of the higher
ionization potential, DEADCST cannot serve as a hole-trap [5]. The
relationship between the PR properties and the characteristics of
graft polymers is discussed.
2.4. Synthesis of 2-[4-(diphenylamino)phenyl]ethyl 2-
bromopropionate (TPA-I)
To a 500-mL three-neck flask equipped with a magnetic stirrer
and a nitrogen inlet were added 3 (23.09 g, 80 mmol), 2-
bromopropionic acid (16.70 g, 90 mmol), N,N0-dicyclohex-
ylcarbodiimide (DCC) (18.50 g, 90 mmol), DMAP (0.030 g,
4.0 mmol) and 400 mL of THF. The reaction mixture was stirred at
room temperature under N2 atmosphere. After the filtration, the
solution was washed with water and concentrated by evaporation,
followed by purification using column chromatography (silica gel,
toluene). The white solid was obtained (30.0 g, yield 89%). 1H NMR
2. Experimental
2.1. Materials
2-(4-Bromophenyl)ethanol,
tert-butyldimethylchlorosilane,
2,4,7-trinitro-9-fluoren- one (TNF), and 1-bromo-4-butylbenzene
were obtained commercially from Tokyo Chemical Industry Co.
(Japan). 1-Bromo-4-[2-(tert-butyldimethylsilyloxy) ethyl]benzene
(1) was synthesized according to the literature [31]. Palladium (II)
acetate was obtained from SigmaeAldrich Co. Tri-tert-butylphos-
phine was obtained from Hokko Chemical Industry Co. (Japan).
Other chemicals were obtained commercially from Wako Chemical
Co. (Japan) and were used without further purification except
otherwise noted. 4-Butyltriphenylamine (BTPA) was synthesized by
CeN coupling reaction of diphenylamine with 1-bromo-4-
butylbenzene as described in Ref. [22]. Xylene and chloroform
were purified by distillation from calcium hydride. 4-N,N-Dieth-
(CDCl3): d 7.24e6.95 (14H, m, eAr), 4.41e4.30 (3H, m, OOCeCHeBr,
eCH2eOeCO), 2.92 (2H, t, J ¼ 7.1 Hz, AreCH2e), 1.79 (3H, d,
J ¼ 6.9 Hz, eCH3). 13C NMR (CDCl3):
d 170.00 (eCOOe), 147.71,
146.39, 131.48, 129.70, 129.10, 124.27, 123.90, 122.51 (eAr), 66.26
(OeCH2e), 40.01 (eCHe), 34.10 (AreCH2e), 21.58 (CH3e). Anal.
Calcd for C23H22BrNO2: C, 65.10; H, 5.23; N, 3.30. Found: C, 65.20; H,
5.36; N, 3.21.
2.5. General procedure for synthesis of poly(triphenylamine)
macroinitiator (PTPA-I)
ylamino-
b
,
b
-dicyanostyrene (DEADCST) was synthesized according
To a two-necked 500-mL flask equipped with a magnetic stirrer
were added a total of 50 mmol of TPA-I and BTPA (the feed ratios
are listed on Table 1.), and 200 mL of distilled CHCl3 under nitrogen
atmosphere. After heating to 40 ꢁC, FeCl3 (32.6 g, 0.20 mol) was
added to the mixture in three portions (0 h: 0.10 mol, after 1 h:
0.05 mol, after 3 h: 0.05 mol) and the mixture was stirred at 40 ꢁC
for 6 h under nitrogen atmosphere. The reaction mixture was
poured into methanol to recover the product followed by washing
with methanol several times. Collected powder was dissolved in
THF to remove the insoluble part by filtration. The filtrate was
concentrated and reprecipitated with acetone containing small
amount of aqueous ammonia. The product was filtered and dried
to the literature [27].
2.2. Synthesis of 4-[2-(tert-butyldimethylsilyloxy)ethyl]
triphenylamine (2)
To a 500-mL three-necked flask equipped with a magnetic
stirrer, a condenser and a nitrogen inlet were added 1 (36.20 g,
0.11 mol), diphenylamine (21.40 g, 0.12 mol), palladium (II) acetate
(0.0335 g, 0.14 mmol), and t-BuONa (13.20 g, 0.13 mol). After the
addition of xylene (140 mL) and 0.1 M P(t-Bu)3 xylene solution
(1.0 mL, 0.10 mmol), the mixture was stirred at 120 ꢁC for 6 h under
nitrogen atmosphere. After cooling down to 80 ꢁC, 100 mL of water
was added to the reaction mixture. The organic layer was washed
in vacuo. 1H NMR (CDCl3):
OOCeCHeBr, eCH2eOeCO), 2.95 (t, J ¼ 6.6 Hz, AreCH2e), 2.58
d 7.47e7.09 (m, eAr), 4.41e4.33 (m,

Z. Cao et al. / Polymer 54 (2013) 269e276
271
Table 1
2.7. Characterizations
Oxidative coupling copolymerization of TPA-I with BTPAa.
Molecular weight and polydispersity (PD) were estimated by gel
permeation chromatography (GPC) equipped with JASCO 880-PU
pump, a column packed with styrene-divinylbenzene gel beads,
and a JASCO UV-970 detector. Chloroform was used as an eluent,
and the molecular weight was calibrated using polystyrene stand-
erds. 1H NMR spectra were recorded at room temperature with
a JEOL JNM-ECA500 spectrometer (1H: 500 MHz, 13C: 125 MHz). UV
spectra were obtained with a JASCO V-670 UV/VIS/NIR spectrom-
eter. The Tg was determined from differential scanning calorimetry
(DSC) thermograms recorded with a Rigaku Thermo Plus DSC8230
in the temperature range from ꢀ50 to 200 ꢁC under nitrogen flow.
The photoconductivity in the graft copolymers doped with 1 wt
Run
TPA-I content (mol%) in
Mn (gmolꢀ1) (PDI)f
Tg (ꢁC)g
Yield (%)
feed in polymere
I-1
69
41
40
22
21
68
39
41
20
18
4000 (3.0)
6000 (2.4)
5200 (1.9)
5200 (2.0)
2400 (1.6)
165
169
154
113
142
56
68
63
63
39
I-2
I-3b
I-4c
I-5d
a
Polymerization of monomers (a total of 50 mmol of TPA-I and BTPA) was carried
out using 4 equiv. of FeCl3 in 200 mL of chloroform at 40 ꢁC for 6 h.
b
Polymerized in a tenth scale.
Polymerized in a half scale.
c
d
FeCl3 was added all at once.
e
Estimated from 1H NMR data.
f
Determined by GPC using polystyrene standards.
Determined by DSC at a heating rate of 10 ꢁC/min under nitrogen.
g
% of TNF was evaluated by measuring the voltage drop by a 1 Μ
resistor resulting from the photocurrent (Iph) through the film
which was fabricated on indium tin oxide (ITO, 30 ) covered glass
U
U
(t, J ¼ 7.3 Hz, AreCH2e),1.81 (d, J ¼ 7.0 Hz, eCH3),1.66e1.55 (m, Are
CH2eCH2e), 1.42e1.32 (m, CH3eCH2e), 0.94 (t, J ¼ 7.2 Hz, eCH3).
substrate and was deposited with gold electrode [32]. Film thick-
ness was determined by a profilometer (DektakⅡ, Solan). A HeeNe
laser with an intensity of 120 mW/cm2 was used as a light source.
The photoconductive sensitivity (S) is calculated according to
2.6. General procedure for synthesis of polytriphenylamine-graft-
poly(ethyl acrylate) (PTPA-g-PEA)
Iph
L
S ¼
(1)
Macroinitiator PTPA-I (2.00 g), anisol (40 mL) and a stirring bar
were placed into 200-mL round-bottom flask and then stirred at
room temperature until the polymer totally dissolved. A stated
amount of ethyl acrylate, CuBr, and N,N,N0,N00,N00-penta- methyl-
diethylenetriamine were added to the flask which then was placed
into an oil bath at 90 ꢁC. After cooling to room temperature, the
solution was filtered with Al2O3 plug in order to remove the cata-
lyst. The concentrated solution was precipitated with methanol (or
cold methanol). After reprecipitation, filtrated product was dried in
I0AV
where L is the film thickness, I0 is the irradiated laser power
density, A is the irradiated area, V is the applied voltage.
2.8. PR sample preparation and measurements
Photorefractive (PR) devices were prepared as follows; PTPA-g-
PEA, DEADCST, and TNF were dissolved in THF or CH2Cl2 (PTPA-g-
vacuo. 1H NMR (CDCl3):
d
7.41e7.15 (m), 4.18e3.98 (br m), 2.92e
PEA/DEADCST/TNF
through a 0.5 m filter, concentrated to around one-third the
original volume and dropped on indium tin oxide (ITO, 30
¼
90/9/1 wt%). The solution was filtered
2.89 (br m), 2.59e2.56 (br m), 2.31e2.27 (br m), 1.92e1.84 (br m),
1.66e1.63 (br m), 1.41e1.36 (br m), 1.22e1.16 (br m), 0.95 (t,
J ¼ 7.1 Hz).
m
U
)
covered glass. After the evaporation of the solvent, the sample was
H
N
N
N
t-
BuSi(CH3)2Cl
pyridine
DMAP
t-
NaO Bu
TBDMS : tert-butyldimethylsilyl
Pd(CH3COO)2
CH2
OH
TBDMS
CH2
t-
O
CH2
CH3COOH
P( Bu)3
CH2
CH2
CH2
H2O, THF
r.t., 4 days
Br
Xylene
120oC, 6 h
CH2Cl2
r.t., 24 h
Br
CH2
CH2
1
O
TBDMS
OH
2
3
N
NH
N
2-bromopropionic acid
t-
NaO Bu
DCC
Pd(CH3COO)2
DMAP
CH2
CH2
CH2
t-
CH2
P( Bu)3
CH3
H2C
O
THF
r.t., 24 h
Br
Xylene
120 oC, 6 h
C
O
CH
Br
CH2
CH2
CH3
CH2
TPA-I
CH3
BTPA
CH
C
O
CH2
N
N
N
N
O
n
m
n
m
CH2 CH3
FeCl3
CH2
CH2
CH2
CH2
CH2
m BTPA
TPA-I
n
+
CH2
CH2
CH2
CH2
CH2
CuBr, Ligand
CHCl3
40oC, 6h
O
C
O
CH CH2 CH Br
x
CH3
O
C
O
CH Br
anisole
90oC
CH3
C
O
O
CH3
CH3
CH2 CH3
PTPA-I
PTPA-g-PEA
Scheme 1. Synthetic route of PTPA-g-PEA.

272
Z. Cao et al. / Polymer 54 (2013) 269e276
Fig. 1. 1H NMR spectrum of PTPA-I (I-1 in Table 1) measured in CDCl3.
heated on a hot plate and the surface was covered with another ITO
glass to form a thin film sandwiched by two transparent electrodes.
moderate total yields (TPA-I: 56.7%, BTPA: 94.4%). An initiating unit,
2-bromopropionyl group, for ATRP was introduced to TPA-I so that
ethyl acrylate can be propagated as a graft chain.
The sample thickness was approximately 100
mm which was
determined by the Teflon spacer used.
Macroinitiators (PTPA-Is) with different TPA-I and BTPA
compositions were synthesized utilizing oxidative coupling with
FeCl3 as an oxidant. 1H NMR spectrum of PTPA-I is shown in Fig. 1.
From the spectrum, it can be clearly seen that the signals appear at
1.8 (CHBrCH3) and 4.3 ppm (CHBrCH3 and CH2OCO) confirming the
The PR properties were studied by the two-beam coupling (2BC)
and four-wave mixing (FWM) techniques using the experimental
setup described elsewhere [26]. A holographic grating was written
using coherent beams from an NEC GLS-5410 HeeNe laser operated
at 633 nm. All experiments were carried out at room temperature.
In the 2BC experiments, the incoming laser beam (p-polarized) was
split into two writing beams with the same intensity (130 mW/
cm2). The two writing beams at an angle of 20ꢁ were directed onto
the sample, which was tilted at an angle of 50ꢁ with respect to the
bisector of the two writing beams to achieve a nonzero projection
of the EO coefficients. The transmitted beam intensities were
monitored with photodiodes (Hamamatsu Photonics, S2281). In the
FWM experiments, two s-polarized beams of equal intensity
(130 mW/cm2) were used to write a grating, which was probed by
ATRP initiating groups on the backbone of PTPA-I. Table
1
summarizes the polymerization conditions and characteristics of
PTPA-I. Compositions of TPA-I were calculated by the integral ratio
(0.94 ppm for CH2CH2CH2CH3 vs. 4.41e4.33 ppm for CHBrCH3 and
CH2OCO) in 1H NMR spectrum and the results showed that the
polymer compositions are roughly the same as the initial TPA-I feed
ratios (Table 1). The number-average molecular weight (Mn) and Tg
of PTPA-I are about 2400e6000 (PDI ¼ 2.0e3.0) and 113e169 ꢁC,
respectively (Table 1).
a
much weaker p-polarized beam (0.8 mW/cm2) counter-
3.2. Graft copolymers
propagating to one of the writing beams.
Graft polymerizations were conducted using PTPA-Is with
various compositions as macroinitiators. Graft copolymer (PTPA-g-
PEA) was prepared by grafting PEA via ATRP from the initiating sites
on the backbone of PTPA-I. The polymerization conditions and
characteristics of PTPA-g-PEA are listed in Table 2. Fig. 2 shows 1H
NMR spectrum of PTPA-g-PEA (G-4). New signals appeared in the
region from 1.3 to 2.3 ppm compared with the spectrum of
3. Results and discussion
3.1. Synthesis of monomers and macroinitiator
All the synthetic routes are shown in Scheme 1. Two triphe-
nylamine (TPA) monomers were successfully synthesized in
Table 2
Conditions for graft polymerization and characteristics of PTPA-g-PEAa.
d
Run
PTPA-I
EA/PTPA-I feed ratioc
Time (h)
Grafted PEA Mn (gmolꢀ1
)
Tg (ꢁC)e
PTPA ratio (wt%)d (GD (mol%))d
Yield (%)
G-1
G-2
G-3
G-4
G-5b
G-6
G-7
G-8
G-9
I-1
I-1
I-1
I-3
I-2
I-3
I-5
I-4
I-4
10
5
3
30
34
5
100
35
25
21
20
20
20
24
20
12
21
12
870
450
270
1800
1300
260
4800
1900
800
ꢀ6
39 (68)
56 (68)
68 (68)
33 (39)
40 (41)
78 (39)
27 (18)
46 (20)
67 (20)
49
69
68
41
30
50
36
29
35
ꢀ4
25, 120
ꢀ14
ꢀ9
21, 139
ꢀ17
ꢀ15
55, 98
a
Polymerization was carried out in anisole in the presence of CuBr and PMDETA with a mole ratio of [Initiator]0/[CuBr]0/[PMDETA]0 ¼ 1/1/1.2.
Polymerized in a tenth scale.
b
c
Feed ratio of ethyl acrylate (EA) to ATRP initiating units of PTPA-I.
d
e
Calculated from 1H NMR.
Determined by DSC at a heating rate of 10 ꢁC/min under nitrogen.

Z. Cao et al. / Polymer 54 (2013) 269e276
273
Fig. 2. 1H NMR spectrum of PTPA-g-PEA (G-4 in Table 2) measured in CDCl3.
macroinitiator, which are attributed to grafted PEA units. Since PEA
homopolymer is soluble in methanol, and can be removed from the
product in the purification process, it was confirmed that the PEA
chains were successfully grafted onto the PTPA main chain. The
degree of polymerization of PEA side chains was determined by the
intensity ratio of the signals at 3.98e4.18 to those at 2.82e2.92 ppm
in 1H NMR spectrum of PTPA-g-PEA. Weight ratio of PTPA moieties
was calculated using the following equation;
prohibits the phase separation observed in G-3 and G-6. These
results suggested that PTPA-g-PEAs can provide different micro-
domains by their compositions.
3.3. Two-beam coupling (2BC) and photoconductivity
To verify the photorefractivity in composites, 2BC experiments
were performed on the sample films with a thickness of 100 mm.
nMTPAꢀI þ mMBTPA
All the devices were uniform and transparent films except for high
PTPA content polymers (G-3, G-6) which afforded turbid films
resulting from the crystallization of dopant. Although PBTPA shows
almost no compatibility with DEADCST, the introduction of high
polar and soft grafted PEA chain significantly improved compati-
bility of the chromophore in the composite. The high optical quality
of the PR devices was maintained even after one month of sample
preparation at room temperature.
PTPA wt% ¼
(2)
nMTPAꢀI þ mMBTPA þ nMnðPEAÞ
where n and m are the unit numbers of TPA-I and BTPA in PTPA-g-
PEA, MTPA-I and MBTPA are molecular mass of TPA-I and BTPA units,
and Mn(PEA) is the number-average molecular weight of grafted PEA,
respectively.
The thermal behaviors of PTPA-g-PEA were investigated by DSC,
and complicated results were obtained as reported in the literature
[33]. DSC thermograms are shown in Fig. 3, and Table 2 shows Tg of
each polymer. When the PTPA content is lower than 60 wt%, one
glass transition was observed near Tg of PEA, suggesting that glass
transition is predominantly governed by PEA side chain regardless
of the graft density. Similar phenomenon was also observed in
polythiophene derivatives containing polystyrene side arms [34].
Whereas PTPA-g-PEAs with higher PTPA content (G-3, G-6, and G-
9) show two distinct glass transitions, indicating that two domains
exist, one PTPA rich and the other PEA rich phases. For G-9, two Tg
values seem to be more a averaged, which is resulted from the
cooperative contribution of both PTPA backbone and PEA side arms.
It is considered that the relatively low graft density nature of G-9
Asymmetric energy transfer was observed for devices based on
PTPA-g-PEAs in 2BC experiments. The 2BC gain coefficient
G
was
calculated using the following equation;
ꢀ
ꢁ
1
L
gb
G
¼
ln
(3)
b
þ 1 ꢀ
g
where L is the optical path,
beams, and
¼ I/I0 the beam coupling ratio where I and I0 are the
detected signal intensities with and without the pumping beam,
respectively. The applied field dependence of gain coefficients (
b the intensity ratio of the two incident
g
G
)
measured for devices D-1 and D-2 based on the same graft density
(68 mol%, see Table 3) of PTPA-g-PEAs are shown in Fig. 4 (left). The
coupling gains for both devices monotonically increased with the
Fig. 3. DSC thermograms for synthesized graft copolymers: graft density is approximately 68% (a), 40% (b), and 20% c). The chemical composition of each polymer is listed on
Table 2.

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Z. Cao et al. / Polymer 54 (2013) 269e276
Table 3
Photorefractive properties for the devices based on PTPA-g-PEA.
b
c
Run
Polymer
PTPA ratio (wt%) (GD (mol%))
Tg (ꢁC)a
a
(cmꢀ1
)
G
(cmꢀ1
)
h
(%)d
s
(ms)e
D-1
D-2
D-3
D-4
D-5
D-6
D-7
G-1
G-2
G-4
G-5
G-7
G-8
G-9
39 (68)
56 (68)
33 (39)
40 (41)
27 (18)
46 (20)
67 (20)
ꢀ6
ꢀ4
24
28
14
24
7
76 (35)f
30
42
66
21
10.8
1.6
17.8
0.3
19.7
5.2
2.8
33 (30)f
28
35
14
63
ꢀ14
ꢀ9
ꢀ17
ꢀ15
98
9
19
24
18
15
8(50)f
a
Glass transition temperatures of PTPA-g-PEAs.
b
c
d
e
f
Absorption coefficients at 633 nm determined by UV spectra.
Gain coefficient at 55 V/
Diffraction efficiency at 45 V/
Response time constant at 40 V/
The values in parentheses represent the applied electric field (V/
m
m determined by 2BC.
m determined by FWM.
m.
m
m
m
m).
applied electric field. The values of gain coefficient (
G
) at 55 V/
m
m
D-1 (using G-1 as a host), shows the highest gain value. From the
practical point of view for optical amplification, gain coefficients (
must exceed the absorption loss of a photorefractive sample
> 0). For all the samples the net gain was obtained.
Fig. 6 shows the electric field dependencies of the photocon-
are listed in Table 3 for all devices examined. PR devices with low
PTPA ratios tend to show high coupling gains. Graft density is also
a significant factor affecting on coupling gains. The denser grafting
G)
(G
ꢀ
a
of PEA side chains becomes, the higher
obtained.
G of the PR composite is
ductive sensitivity in PTPA-g-PEA graft copolymers (G-1, G-4, and
G-7). For all copolymers examined (G-1, G-4, and G-7), the sensi-
tivity increased with the applied electric field. For each sample, the
photoconductive sensitivity was at least an order of magnitude
higher than that of reported PVK-based systems at a given applied
electric field [6]. Notice that the values in these samples were
comparable to that of bis-triarylamine side-chain polymer based
Absorption coefficients (a) were determined in UVevis spectra
of the PR composite films with a 100
mm thickness from absorption
tails resulting from charge transfer (CT) interaction between PTPA
chain and TNF. These PR composites have the absorption coeffi-
cients from 7 to 28 cmꢀ1 at 633 nm (Table 3). The values of
absorption coefficient increase as the graft density as well as PTPA
ratio of polymer increase in spite of the same sensitizer content. It is
speculated that the absorption coefficient was dependent on the
local inhomogeneity in the composite. Fig. 5 shows the illustration
of the graft density dependence of the local structures in the
composite (e.g. D-1 and D-6). For high graft density, PTPA main
chain is almost isolated in the PEA, and PTPA moiety is uniformly
located. In such a structure, TNF molecules are easily accessible to
PTPA chains, and interact with them to generate CT complexes,
which consequently provide a high absorption coefficient. On the
other hand, PTPA chains can aggregate to some extent in the case of
low graft density, although only one Tg was observed as discussed
above. The PTPA rich domains in the composite become relatively
larger than that in the composite with high graft density assuming
that TNF molecules are mainly located in a PEA rich phase. Thus the
possibility for PTPA chain to encounter dopants decreases resulting
in a low absorption coefficient.
systems (0.2e1.5 ꢂ 10ꢀ9 cm/
UW at 40 V/mm) [3], although TPA unit
contents in our polymer are lower than that of side-chain polymer.
This fact could be attributed to the more effective conjugation of
the polymeric triarylamine main chains [22].
Photoconductivity is also dependent on the graft density or the
micro morphology of graft copolymer. Bycomparison of sensitivities
for the samples, G-1, G-4 and G-7 (relatively lower PTPA content
with different graft density), G-1 with the highest graft density
showed the highest sensitivity. It seems that the accessibility of TNF
molecules to PTPA backbone mainly governs the photoconductive
sensitivity since these polymers possess similar PTPA contents.
Since photoconductivity is affected by a variety of factors such as
drift mobility, trap densityand so on, further studies are necessary to
elucidate the process in heterogenous samples presented here.
3.4. Four wave mixing (FWM)
This morphologic difference influenced on the coupling gain
because the complex between PTPA and TNF is a site to generate
charges which related with photoconductivity as described below.
Consequently, the device using PTPA-g-PEA with high graft density,
The electric field dependencies of the steady-state diffraction
efficiency (h) for the devices were investigated, and the values of h
at 45 V/ m are listed in Table 3. The diffraction efficiency was
m
Fig. 4. Electric field dependencies of the gain coefficients (left) and the diffraction efficiencies (right) using PTPA-g-PEA with the same graft density.

Z. Cao et al. / Polymer 54 (2013) 269e276
275
Fig. 7. Electric field dependence on the response times in D-7 and a build-up transient
at 50 V/ m (inset).
m
3.5. Response time
To determine response time (s), time-resolved FWM experi-
ments were performed. This parameter is very important for real
applications such as dynamic holography and real-data processing.
The response time of PR grating formation was measured as fol-
lowed. One writing beam and the reading beam were illuminated
on the sample and an electric field was applied. After 60 s, the
second writing beam was illuminated on the sample. The inset in
Fig. 7 shows an example of a buildup transient for the device
Fig. 5. Schematic illustrations of (a); the structures of PTPA-g-PEAs with the similar
PTPA ratio, (b); plausible morphology in the composite, and (c); the accessibility of
dopants (TNF).
determined by the ratio of the intensity of diffracted signal to that
of incident reading beam. PR devices with low PTPA contents show
high diffraction efficiencies. In the devices containing high PEA
contents, it is possible that EO chromophore maintains high
mobility to respond the incident beams. Fig. 4 (right) shows electric
field dependencies of diffraction efficiency for D-1 and D-2, which
are similar to those of gain coefficient. As shown in Table 3, there is
denoted as D-7 and curve fitting using the following equation [1];
ꢂ ꢃ
h
t
¼
h0½1 ꢀ expðꢀt=
s
Þꢃ2
(4)
where h0 is the steady-state diffraction efficiency.
Fig. 7 shows electric field dependence of
D-7. Table 3 lists response time constants at 40 V/
s
estimated for sample
m for each
inconsistency between
in D-1 and D-5, respectively. This is probably due to the difference
of absorption coefficient. Since values are determined from the
G and h. The highest G and h were observed
m
device. PR composites showed response times of the order of 10 ms.
The composites with higher PTPA contents generally afforded faster
responses, and the shortest response time was observed when the
PTPA contents was 67 wt% with the graft density of 20 mol% (D-7).
Moon et al. reported that the PR composites based on the polymer
containing dicarbazole moiety in the main chain showed high
PR performance [12]. However the response time was ca. 1 s at
h
ratio of the intensity of the diffracted beam to that of the incident
reading beam, the diffraction efficiency is underestimated for the
devices with high absorption.
50 V/mm. In general, it is well-known that the response time is
related with the photoconductivity of the material. As discussed
above, it is considered that relatively fast response is attributed to
high photoconductive nature of PTPA main chain.
4. Conclusions
Here we have prepared novel graft polymers containing a pol-
y(triphenylamine) (PTPA) backbone and poly(ethyl acrylate) (PEA)
branches by the combination of oxidative coupling polymerization
and ATRP techniques. PR composites based on these polymers
show good static PR properties and fast response times under
moderate conditions. In 2BC experiment, the higher is the graft
density, the higher gain coefficient is obtained. In FWM experi-
ment, with the increase of PEA content, diffraction efficiency
increased and the highest diffraction efficiency (19.7% at 45 V/mm)
was observed for PTPA-g-PEA (graft density: 18 mol%, PTPA ratio:
27 wt%) composite. PTPA-g-PEA (graft density: 20 mol%, PTPA ratio:
68 wt%) composite showed response time of 8 ms at an applied
Fig. 6. Electric field dependencies of the photoconductive sensitivity (PC) in PTPA-g-
PEA graft copolymers (G-1, G-4, and G-7) sensitized with 1 wt% of TNF at wavelength of
633 nm.
field of 50 V/mm. It is also found that local morphology of the
composite plays an important role to determine the PR

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Z. Cao et al. / Polymer 54 (2013) 269e276
[14] Ogino K, Nomura T, Shichi T, Park SH, Sato H, Aoyama T, et al. Chem Mater
1997;9:2768e75.
[15] Hofmann U, Schreiber A, Haarer D, Zilker SJ, Bacher A, Bradley DDC, et al.
Chem Phys Lett 1999;311:41e6.
[16] Ostrauskaite J, Karickal HR, Leopold A, Haarer D, Thelakkat DM. J Mater Chem
2002;12:58e64.
[17] Wang X, Chen Z, Ogino K, Sato H, Miyata S, Tan H. Polym J 2000;32:778e83.
[18] Wang X, Nakao M, Ogino K, Sato H, Tan H. Macromol Chem Phys 2001;202:
117e25.
[19] Son J-M, Nakao M, Ogino K, Sato H. Macromol Chem Phys 1999;200:65e70.
[20] Son J-M, Mori T, Ogino K, Sato H. Macromolecules 1999;32:4849e54.
[21] Ogino K, Kanegae A, Yamaguchi R, Sato H, Kurjata J. Macromol Rapid Commun
1999;20:103e6.
[22] Takahashi C, Moriya S, Fugono N, Lee HC, Sato H. Synth Met 2002;129:123e8.
[23] Olvera de la Cruz M, Sanchez IC. Macromolecules 1986;19:2501e8.
[24] Eisenbach CD, Heinemann T. Macromolecules 1995;28:4815e21.
[25] Miyata T, Takagi T, Uragami T. Macromolecules 1996;29:7787e94.
[26] Ogino K, Goma T, Kagayama D, Sato H, Yonezawa N. J Photopolym Sci Technol
2006;19:419e24.
[27] Díaz-García MA, Wright D, Casperson JD, Smith B, Glazer E, Moerner WE, et al.
Chem Mater 1999;11:1784e91.
[28] Ogino K, Park S-H, Sato H. Appl Phys Lett 1999;74:3936e8.
[29] Jeong J-M, Abe K, Sato H, Pretula J, Kaluzynski K, Ogino K. Synth Met 2003;
139:11e9.
performance. In spite of fast response, there is a room to improve
gain coefficient and diffraction efficiency in the composite based on
graft copolymers presented here. Optimization of the composite
parameters, such as the type and the content of a chromophore and
a sensitizer, and the composite morphology is still ongoing.
References
[1] Moerner WE, Silence SM. Chem Rev 1994;94:127e55.
[2] Ostroverkhova O, Moerner WE. Chem Rev 2004;104:3267e314.
[3] Thomas J, Norwood RA, Peyghambarian N. J Mater Chem 2009;19:7476e89.
[4] Thomas J, Christenson CW, Blanche P-A, Yamamoto M, Norwood RA,
Peyghambarian N. Chem Mater 2011;23:416e29.
[5] Köber S, Salvador M, Meerholz K. Adv Mater 2011;23:4725e63.
[6] Gallego-Gómez F, Álvarez-Santos JC, Rodríguez-Redondo JL, Font-Sanchis E,
Villalvilla JM, Sastre-Santos Á, et al. J Mater Chem 2012;22:12220e8.
[7] MeerholzK,VolodinBL,KippelenB,PeyghambarianN.Nature1994;371:497e500.
[8] Chun H, Moon IK, Shin DH, Kim N. Chem Mater 2001;13:2813e7.
[9] Moon IK, Choi CS, Kim N. Polymer 2007;48:3461e7.
[10] Moon IK, Choi CS, Kim N. J Photochem Photobio A Chem 2009;202:57e62.
[11] Tay S, Blanche P-A, Voorakaranam R, Tunc AV, Lin W, Rokutanda, et al. Nature
2008;451:694e8.
[12] Thomas J, Hernandez CF, Yamamoto M, Camack K, Matsumoto K, Walker GA,
et al. Adv Mater 2004;16:2032e6.
[30] Tan Y, Gu Z, Tsuchiya K, Ogino K. Polymer 2012;53:1444e52.
[31] Cao Z, Tsuchiya K, Ogino K. Chem Lett 2012;41:1541e3.
[32] Park S-H, Ogino K, Sato H. Polym Adv Technol 2000;11:349e58.
[33] HongSC,PakulaT,MatyjaszewskiK.MacromolChemPhys2001;202:3392e402.
[34] Shen J, Tsuchiya K, Ogino K. J Polym Sci Part A Polym Chem 2008;46:1003e13.
[13] Blanche P-A, Bablumian A, Voorakaranam R, Christenson C, Lin W, Gu T, et al.
Nature 2010;468:80e3.