Self-structured surface patterns on epoxy-based azo polymer films induced by laser light irradiation

Article abstract of DOI:10.1021/ma200832b

In this study, two series of epoxy-based azo polymers with high chromophore density were synthesized and self-structured surface pattern formation on the polymer films was studied by laser light irradiation under different conditions. To synthesize the azo polymers, two epoxy-based precursor polymers (PEP-AN and PEP-35AN) were prepared by step polymerizations of N,N-di(epoxypropyl)aniline and N,N-di(epoxypropyl)-3,5-dimethylaniline with aniline and 3,5-dimethylaniline, respectively. The azo polymers were obtained through postpolymerization azo-coupling reactions between the precursor polymers and diazonium salts of 4-chloroaniline, 4-aminobenzonitrile, 4-nitroaniline, and 2-methyl-4-nitroaniline. The epoxy-based precursor polymers and azo polymers were characterized by using ¹H NMR, FT-IR, UV-vis, and DSC analyses. The self-structured surface pattern formation on films of the two series of azo polymers was studied by irradiating the polymer films with a normal-incident laser beam at two different wavelengths (488 and 532 nm). The results show that the photoinduced surface pattern formation is closely related with the structure of azo chormophores, excitation wavelength, and light polarization condition. The efficient excitation wavelength is closely related with the absorption band position, which is mainly determined by the electron-withdrawing groups on the azo chromophores. The methyl substituents on the azo chromophores can enhance the photoinduced surface pattern formation ability in some cases. For comparison, the surface-relief-grating (SRG) formation was studied by irradiating the polymer films with interfering laser light. The SRG formation rate is also dependent on the azo chromophore structure and wavelength of the incident laser light. The self-structured surface pattern formation needs a higher energy input and shows stricter wavelength requirement compared with those of the SRG formation. These observations could lead to the deeper understanding of the mechanism of the self-structured surface pattern formation and development of materials with better performance.

Full text of DOI:10.1021/ma200832b

ARTICLE
pubs.acs.org/Macromolecules
Self-Structured Surface Patterns on Epoxy-Based Azo Polymer Films
Induced by Laser Light Irradiation
Xiaolin Wang, Jianjun Yin, and Xiaogong Wang*
Department of Chemical Engineering, Laboratory for Advanced Materials, Tsinghua University, Beijing 100084, P. R. China
S Supporting Information
b
ABSTRACT: In this study, two series of epoxy-based azo
polymers with high chromophore density were synthesized
and self-structured surface pattern formation on the polymer
films was studied by laser light irradiation under different
conditions. To synthesize the azo polymers, two epoxy-based
precursor polymers (PEP-AN and PEP-35AN) were prepared
by step polymerizations of N,N-di(epoxypropyl)aniline and N,
N-di(epoxypropyl)-3,5-dimethylaniline with aniline and 3,5-
dimethylaniline, respectively. The azo polymers were obtained
through postpolymerization azo-coupling reactions between the precursor polymers and diazonium salts of 4-chloroaniline,
4-aminobenzonitrile, 4-nitroaniline, and 2-methyl-4-nitroaniline. The epoxy-based precursor polymers and azo polymers were
characterized by using 1H NMR, FT-IR, UVÀvis, and DSC analyses. The self-structured surface pattern formation on films of the
two series of azo polymers was studied by irradiating the polymer films with a normal-incident laser beam at two different
wavelengths (488 and 532 nm). The results show that the photoinduced surface pattern formation is closely related with the
structure of azo chormophores, excitation wavelength, and light polarization condition. The efficient excitation wavelength is closely
related with the absorption band position, which is mainly determined by the electron-withdrawing groups on the azo
chromophores. The methyl substituents on the azo chromophores can enhance the photoinduced surface pattern formation
ability in some cases. For comparison, the surface-relief-grating (SRG) formation was studied by irradiating the polymer films with
interfering laser light. The SRG formation rate is also dependent on the azo chromophore structure and wavelength of the incident
laser light. The self-structured surface pattern formation needs a higher energy input and shows stricter wavelength requirement
compared with those of the SRG formation. These observations could lead to the deeper understanding of the mechanism of the
self-structured surface pattern formation and development of materials with better performance.
’ INTRODUCTION
The SRGs are stable below the Tgs of the polymers and can be
erased by heating samples to a temperature above their Tgs or by
irradiation with a uniform circularly polarized laser beam below
the Tgs. Photoinduced SRG formation has been observed for hostÀ
guest systems, side-chain azo polymers, main-chain azo polymers,
and hyperbranched azo polymers among others.5,6,10,11,16,17 The
surface modulation has been attributed to the mass-transport
induced by the light field as a result of the transÀcis isomeriza-
tion of azo chromophores. Although different models and
theories have been proposed to explain this interesting photo-
responsive property, the mechanism of SRG formation has not
been fully understood until now.18À26 This unique function of
azo polymers has been used for optical device fabrication and
surface modulations.27À32
Azobenzene and its derivatives have been widely used as
dyestuffs and pigments for a long time owing to their colorful
nature.1,2 In recent years, these chromophoric structures have
been intensively explored as functional groups for developing
photoresponsive polymers, whose structure and property varia-
tions can be triggered by the photoinduced transÀcis isomeriza-
tion of the azo groups.3,4 Polymers containing azobenzene and its
derivatives (azo polymer for short) have been synthesized
through various molecular engineering approaches and investi-
gated for the unique properties.5À7 Azo polymers can show some
very interesting photoresponsive properties such as phase
transition,8 chromophore orientation,9 surface-relief-grating
(SRG) formation,10,11 and deformation of thin films.12À15 Poly-
mers with such properties are currently being expected for the
applications in areas such as data-storage, optical switching,
sensors and actuators.
Recent research uncovers another unusual photoresponsive
behavior of azo polymer films when irradiated with a uniform
laser beam.33,34 The observations reveal that submicrometer
hexagonal patterns on the azo polymer films can be induced by
Photoinduced SRG formation on azo polymer films has
attracted considerable attention since it was first reported in
1995.10,11 Upon irradiation with interfering laser beams, SRGs
can be formed on azo polymer films at a temperature well below
the glass transition temperatures (Tgs) of the polymers.5,6,10,11
Received: April 11, 2011
Revised:
May 20, 2011
Published: August 10, 2011
r
2011 American Chemical Society
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Scheme 1. Synthetic Route of the Epoxy-Based Azo Polymers
irradiation with the uniform laser beam at normal incidence.33
The patterns are characterized by regularly spaced pillar-like
structures with saturated height about 100 nm. This effect has
been known as the spontaneous surface pattern formation
through light irradiation.33 The self-structured characteristics of
the pattern formation makes a clear distinction between this
effect and the formation of parallel ripple structures on polymer
surfaces induced by irradiation with a single beam of excimer
laser or Nd:YAG pulsed laser.35À39 The latter has been known as
laser-induced periodic surface structure (LIPSS) and explained
by the models based on electromagnetic field theory such as the
surface-scattered wave model.40,41 In order to understand the
self-structured pattern formation on azo polymer film, the
influences of laser intensity, irradiation time, and wavelength
have been studied in detail.34,42 The possible application of this
effect in high efficient data-storage has been demonstrated.43,44
However, in contrast to the extensive study of SRGs for different
azo polymers, the self-structured surface pattern formation has
been observed only for a poly(methyl methacrylate)-based
copolymer (DR1MA/MMA 35/65) among few others.32,43,44
To our knowledge, a deep understanding of the relationship
between polymer structures and this unique photoresponsive
behavior of azo polymers has not been reached at this moment.
In this work, two series of epoxy-based azo polymers bearing
pushÀpull type azo chromophores with high density were
synthesized. Epoxy-based azo polymers have been synthesized
and studied for the nonlinear optical properties and SRG
formation for years.11,45À47 However, no self-structured pattern
formation has been observed for those epoxy-based azo polymers
under the single laser beam irradiation. In order to enhance the
light irradiation effect, the epoxy-based azo polymers developed
in this study were prepared to bear high density of azo
chromophores with different structures. The photoinduced sur-
face pattern formation was studied by irradiating the azo polymer
films with a single laser beam at 488 and 532 nm. For comparison,
the SRG formation on the azo polymer films were studied by
irradiation with interfering laser beams at the wavelengths. The
correlation of the surface pattern formation with polymer
structures was investigated for the processes. The experimental
methods and Results and Discussion will be presented in the
following sections in detail.
’ EXPERIMENTAL SECTION
Materials. Aniline, 3,5-dimethylaniline, 4-chloroaniline, 4-amino-
benzonitrile, 4-nitroaniline, and 2-methyl-4-nitroaniline were purchased
from Alfa Aesar. The other materials, reagents and solvents were
commercial products and used as received without further purification.
Characterization. Infrared spectra were determined using a Nico-
let 560-IR FT-IR spectrophotometer by incorporating samples in KBr
1
disks. H NMR spectra were recorded using a JEOL JNM-ECA600
NMR spectrometer. Mass spectra were determined by Agilent 6300 Ion
Trap LC/MS Systems. Molecular weights and molecular weight dis-
tributions were determined by using a gel permeation chromatography
(GPC) apparatus at room temperature with THF as the eluent (1 mL
minÀ1). The instrument was equipped with a refractive index (RI)
detector (Wyatt Optilab rEX) and fitted with a PLgel 5 μm mixed-D
column. The molecular weights and distributions were obtained by
calibration with linear polystyrene standards. Thermal phase transitions
of the polymers were determined using TA Instruments DSC 2920 with
a heating rate of 10 °C/min in nitrogen atmosphere. UVÀvis absorption
spectra were recorded on a Perkin-Elmer Lamba Bio-40 spectrophot-
ometer. The surface images of the photoinduced surface patterns and
surface-relief-gratings were monitored using an atomic force microscope
(AFM, Nanoscope IIIa) in the tapping mode.
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Table 1. Tgs of the Epoxy-Based Azo Polymers
Tgs (°C) of azo polymers
X
PEP-AZ-X
PEP-35AZ-X
ÀCl
160
--
150
150
152
149
ÀCN
ÀNT
ÀmNT
--
156
N,N-Di(epoxypropyl)aniline. Silica gel catalyzed ring-opening
method was used to prepare this compound.48 It was carried out by
adding silica gel (0.93 g, 100À200 mesh) into a homogeneous mixture of
aniline (9.3 g, 0.1 mol) and 2-(chloromethyl)oxirane (37 g, 0.4 mol),
and reacting at 40 °C for 5 h. After the reaction, the mixture was filtrated
and added dropwise into petroleum ether to precipitate the white
powder. The dried powder without purification was dissolved in a
solution of THF (100 mL) and KOH (15 g). The solution was stirred at
room temperature for 3 h and filtrated. The THF was removed by rotary
evaporation and the residue was purified by column chromatography
using a mixture of ethyl acetate and petroleum ether (v/v = 1:10) as
eluent. The final product was obtained as a yellow liquid. Yield: 54%.
MS: m/z [M]+, 204.97; calcd, 205.11. 1H NMR (DMSO-d6), δ (ppm):
2.57 (m, 2H), 2.73 (m, 2H), 3.11 (m, 2H), 3.38 (m, 2H), 3.66 (m, 2H),
6.65 (d, 1H), 6.83 (d, 2H), 7.17 (m, 2H).
N,N-Di(epoxypropyl)-3,5-dimethylaniline. It was synthesized
by the reaction between 3,5-dimethylaniline (12.1 g, 0.1 mol) and
2-(chloromethyl)oxirane (37 g, 0.4 mol) via a procedure similar to that
used for preparing N,N-di(epoxypropyl)aniline. The final product was
obtained as a yellow liquid. Yield: 47%. MS: m/z [M]+, 233.04; calcd,
1
233.14. H NMR (DMSO-d6), δ (ppm): 2.19 (s, 6H), 2.56 (m, 2H),
2.72 (m, 2H), 3.09 (m, 2H), 3.39 (m, 2H), 3.61 (m, 2H), 6.30 (s, 1H),
6.45 (s, 2H).
PEP-AN. Aniline (1.86 g, 20 mmol) and N,N-di(epoxypropyl)aniline
(4.1 g, 20 mmol) were homogeneously mixed and stirred at 110 °C for
10 h under nitrogen protection. The crude product as a slightly yellow
solid was dissolved in THF and added dropwise into a mixture of ethanol
and petroleum ether (v/v = 1:3) to obtain a sticky precipitate. The
precipitate was collected by removing the liquid and was dried in a
vacuum oven at 60 °C for 48 h to get product. Yield: 31%. DSC: Tg
87 °C. GPC: Mn = 5000; MWD = 1.56. 1H NMR (DMSO-d6), δ (ppm):
3.25 (1H), 3.40 (1H), 3.45 (1H), 3.60 (1H), 4.03 (1H), 5.28 (1H), 6.49
(1H), 6.60 (2H), 6.99 (2H). IR (KBr, cmÀ1): 3307, 2870, 1599, 1504,
1346, 1232, 1194, 1117, 1034, 991, 858, 746, 692, 509.
Figure 1. UVÀvis spectra of the epoxy-based azo polymers as spin-
coated films: (a) azo polymers obtained from PEP-AN; (b) azo
polymers obtained from PEP-35AN.
Table 2. λmax of the Epoxy-Based Azo Polymers
λmax (nm) (DMF solutions/spin-coated films)
X
PEP-AZ-X
PEP-35AZ-X
PEP-35AN. PEP-35AN was synthesized from the reaction between
3,5-dimethylaniline (2.42 g, 20 mmol) and N,N-di(epoxypropyl)-3,5-
dimethylaniline (4.66 g, 20 mmol) through a similar procedure for
preparing PEP-AN. The polymerization was carried out at 110 °C for 3 h.
Yield: 27%. DSC: Tg 80 °C. GPC: Mn = 2400; MWD = 1.48. 1H NMR
(DMSO-d6), δ (ppm): 1.96 (6H), 3.18 (1H), 3.37 (1H), 3.39 (1H), 3.60
(1H), 4.03 (1H), 5.30 (1H), 6.12 (2H), 6.17 (1H). IR (KBr, cmÀ1): 3334,
2916, 1601, 1481, 1360, 1196, 1124, 1038, 989, 816, 692.
PEP-AZ-Cl. 4-Chloroaniline (0.64 g, 5 mmol) was dissolved in a
homogeneous mixture of glacial acetic acid (6 mL), propionic acid
(3 mL) and sulfuric acid (98%, 0.7 mL). Diazonium salt was prepared by
adding an aqueous solution of sodium nitrite (0.38 g, 5.5 mmol in 1 mL
of water) dropwise into the 4-chloroaniline solution. The mixture was
stirred at 5 °C for 5 min and then added dropwise into a solution of PEP-
AN (0.6 g, 4 mmol) in DMF (100 mL) at 0 °C. The solution was stirred
at 0 °C for 12 h and then added dropwise into plenty of water. The
precipitate collected by filtration was dried and then dissolved in THF,
filtrated and precipitated with abundant petroleum ether. The product
was collected by filtration and dried in a vacuum oven at 60 °C for 48 h.
ÀCl
427/401
454/440
488/459
485/453
416/394
440/424
471/445
472/448
ÀCN
ÀNT
ÀmNT
1
Yield: 85%. DSC: Tg 160 °C. H NMR (DMSO-d6), δ (ppm): 3.56
(2H), 3.78 (2H), 4.21 (1H), 5.46 (1H), 6.91 (2H), 7.47 (2H), 7.67
(4H). IR (KBr, cmÀ1): 3313, 2902, 1599, 1510, 1477, 1390, 1360, 1313,
1232, 1155, 1142, 1086, 1009, 835, 542.
Other azo polymers were synthesized via a similar procedure as the
PEP-AZ-Cl synthesis. PEP-AZ-CN, PEP-AZ-NT, and PEP-AZ-mNT
were synthesized by azo-coupling reactions between PEP-AN and
diazonium salts of 4-aminobenzonitrile, 4-nitroaniline, and 2-methyl-
4-nitroaniline, respectively. PEP-35AZ-Cl, PEP-35AZ-CN, PEP-35AZ-
NT, and PEP-35AZ-mNT were synthesized through the azo-coupling
reactions between PEP-35AN and diazonium salt of 4-chloroaniline,
4-aminobenzonitrile, 4-nitroaniline, and 2-methyl-4-nitroaniline, re-
spectively. Analytical results of these polymers are given below.
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Figure 2. Typical AFM images (10 μm  10 μm) and their two-dimensional Fourier transform (2D-FT) images of the photoinduced surface patterns
formed on PEP-AZ-Cl films upon irradiation with incident laser light at 488 nm and different polarizations: (a) linearly polarized, 2D-view; (b) linearly
polarized, 3D-view; (c) linearly polarized, 2D-FT image; (d) circularly polarized, 2D-view; (e) circularly polarized, 3D-view; (f) circularly polarized,
2D-FT image. The intensity of the laser light was 200 mW/cm2 and the irradiation time was 30 min.
2225, 1597, 1510, 1443, 1419, 1387, 1313, 1243, 1159, 1138, 845,
823, 559.
Table 3. Spatial Periods of the Self-Structured Surface Pat-
terns Formed by Irradiation with 488 and 532 nm Laser Lighta
PEP-AZ-NT. Yield: 81%. DSC: no obvious glass transition. 1H NMR
(DMSO-d6), δ (ppm): 3.60 (2H), 3.82 (2H), 4.24 (1H), 5.51 (1H),
6.96 (2H), 7.78 (4H), 8.20 (2H). IR (KBr, cmÀ1): 3342, 2914, 1601,
1514, 1443, 1421, 1389, 1336, 1234, 1140, 1103, 858, 823, 754, 688, 538.
PEP-AZ-mNT. Yield: 81%. DSC: Tg 156 °C. 1H NMR (DMSO-d6),
δ (ppm): 2.51 (3H), 3.62 (2H), 3.80 (2H), 4.24 (1H), 5.50 (1H), 6.94
(2H), 7.45 (1H), 7.72 (2H), 7.96 (1H), 8.04 (1H). IR (KBr, cmÀ1):
3346, 2920, 1599, 1514, 1443, 1419, 1389, 1338, 1259, 1236, 1146, 1117,
1084, 931, 823, 796, 752.
period (nm) (488 nm/532 nm)
X
PEP-AZ-X
440 ( 10/--
PEP-35AZ-X
ÀCl
ÀCN
ÀmNT
450 ( 15/495 ( 15
405 ( 15/500 ( 15
--/--
400 ( 10/495 ( 15
--/460 ( 15
a . The AFM images of the patterns are given in Figures 2À6.
PEP-35AZ-Cl. Yield: 83%. DSC: Tg 150 °C. 1H NMR (DMSO-d6),
δ (ppm): 2.34 (6H), 3.43 (1H), 3.56 (1H), 3.75 (2H), 4.18 (1H), 5.47
(1H), 6.50 (2H), 7.46 (2H), 7.62 (2H). IR (KBr, cmÀ1): 3311, 2956,
2916, 1599, 1481, 1356, 1271, 1169, 1111, 1088, 1009, 980, 833, 519.
PEP-AZ-CN. Yield: 84%. DSC: no obvious glass transition. 1H NMR
(DMSO-d6), δ (ppm): 3.60 (2H), 3.81 (2H), 4.22 (1H), 5.49 (1H),
6.96 (2H), 7.74 (4H), 7.86 (2H). IR (KBr, cmÀ1): 3363, 2904,
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laser (532 nm) was used as the light source. The laser beam with proper
intensity was obtained after being spatially filtered, expanded, and
collimated. For inducing the self-structured surface patterns, the laser
beam (typically with intensity of 200 mW/cm2) was incident perpendi-
cularly to the film surfaces. Surface-relief-grating (SRG) inscription was
carried out by the experimental setup similar to those reported
before.10,11 A linearly polarized laser beam with different wavelengths
(488 nm, 532 nm) was used as the light source. The p-polarized laser
beam, obtained after being spatially filtered, expanded, and collimated, was
adjusted to the intensity of 100 mW/cm2. The writing beam was split by a
mirror in order that one-half of the beam reflected onto the film surface was
coincident with the other half beam which is directly incident onto the film
to form an interference pattern. The formation of SRGs was probed with a
low power HeÀNe laser beam at 632.8 nm by measuring the diffraction
efficiency of the first-order diffracted beam in a real time mode.
’ RESULTS AND DISCUSSION
The two series of epoxy-based polymers with high chromo-
phore density were synthesized through the postpolymerization
azo-coupling reactions of the two precursor polymers
(Scheme 1). In order to obtain the azo polymers with the high
chromophore density, the precursor polymers were prepared to
contain aniline moieties in the high density. The postfunctiona-
lization scheme was adopted to yield the polymers with the high
degree of functionalization and avoid possible side-reactions of
the azo chromophores in the polymerization process.45À47 The
degree of polymerization (DP) of the azo polymers in each series
was the same because they were obtained from the same-batch of
the precursor polymers, which could avoid possible influence of
DP on the properties in the following study. The self-structured
surface patterns and SRGs were induced by irradiating the spin-
coated solid films of the polymers with the laser light at two
different wavelengths. The surface patterns formed in the
processes were characterized by AFM and the results were
obtained to understand the influences of the chromophoric
structures and light irradiation conditions on the pattern forma-
tion behavior.
Figure 3. AFM images (10 μm  10 μm) of the photoinduced surface
patterns formed on PEP-35AZ-Cl film upon irradiation at (a) 488 and
(b) 532 nm. The intensity of the laser light was 200 mW/cm2, and the
irradiation time was 30 min.
1. Polymer Synthesis and Characterization. The precursor
polymers were synthesized through step polymerization of the
monomers as shown in Scheme 1, where PEP-AN and PEP-
35AN were obtained by reactions of N, N-di(epoxypropyl)-
aniline (2a) with aniline and N,N-di(epoxypropyl) À3,5-di-
methylaniline (2b) with 3,5-dimethylaniline, respectively. The
precursor polymers possess the high density of aniline moieties
compared with the precursor polymer obtained from the reaction
between aniline and bisphenol A diglycidyl ether (BADGE),
which was typically used in previous study.11,45,47 The polymer-
ization temperature was controlled to be 110 °C to avoid the
possible side reaction between the hydroxyl groups generated by
ring-opening reaction and the unreacted epoxide rings.45 Low
molecular weight oligomers, which were mainly cyclic oligomers,
were removed by dissolving and precipitation.49 The number-
average molecular weights (Mn) of PEP-AN and PEP-35AN are
5000 and 2400 with the polydispersity indexes of 1.56 and 1.48,
which were determined by GPC. The polymers show good
solubility in polar solvents such as tetrahydrofuran (THF), N,
N-dimethylformamide (DMF), and N,N-dimethylacetamide
(DMAc). The precursor polymers were functionalized by intro-
ducing four types of azo chromophores through the postpoly-
merization azo-coupling reactions in DMF. The chemical
structure and abbreviated names of the azo polymers are shown
in Scheme 1. The first two parts of the names specify the azo
PEP-35AZ-CN. Yield: 81%. DSC: Tg 150 °C. 1H NMR (DMSO-d6),
δ (ppm): 2.38 (6H), 3.50 (2H), 3.80 (2H), 4.20 (1H), 5.48 (1H), 6.52
(2H), 7.74 (2H), 7.84 (2H). IR (KBr, cmÀ1): 3446, 2956, 2918, 2224,
1655, 1597, 1493, 1431, 1356, 1269, 1174, 1144, 1111, 978, 843, 553.
PEP-35AZ-NT. Yield: 78%. DSC: Tg 152 °C. 1H NMR (DMSO-d6),
δ (ppm): 2.40 (6H), 3.59 (2H), 3.85 (2H), 4.24 (1H), 5.50 (1H), 6.55
(2H), 7.79 (2H), 8.26 (2H). IR (KBr, cmÀ1): 3417, 2956, 2918, 1601,
1514, 1495, 1431, 1336, 1269, 1171, 1144, 1103, 980, 858, 756, 688.
PEP-35AZ-mNT. Yield: 80%. DSC: Tg 149 °C. 1H NMR (DMSO-
d6), δ (ppm): 2.30À2.57 (9H), 3.60 (2H), 3.82 (2H), 4.22 (1H), 5.51
(1H), 6.49 (2H), 7.42 (1H), 7.96 (2H). IR (KBr, cmÀ1): 3419, 2956,
2918, 1601, 1516, 1495, 1423, 1336, 1259, 1190, 1163, 1113, 1086, 980,
926, 835, 808, 754.
Film Preparation. Solid films of the azo polymers with smooth
surfaces were prepared by spin-coating. The homogeneous solutions of
the polymers were obtained by dissolving a suitable amount of the
polymers in N,N-dimethylformamide (DMF). The solutions filtered
through 0.45 μm membranes were spin-coated onto clean glass slides.
The film thicknesses were controlled to be in a range from 400 to 600 nm
by adjusting the solution concentrations and the spinning speed. The
films obtained from the spin-coating were dried at 60 °C under vacuum
for 48 h before use.
Laser Irradiation. A linearly or circularly polarized beam from an
Ar+ laser (488 nm) and diode-pumped frequency doubled solid state
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Figure 4. AFM images (10 μm  10 μm) of the photoinduced surface patterns formed on PEP-AZ-CN and PEP-35AZ-CN films upon irradiation at
488 and 532 nm: (a) PEP-AZ-CN, 488 nm; (b) PEP-AZ-CN, 532 nm; (c) PEP-35AZ-CN, 488 nm; (d) PEP-35AZ-CN, 532 nm. The intensity of the
laser light was 200 mW/cm2. The irradiation time with the 488 nm laser was 30 min and the irradiation time with the 532 nm laser was 15 min.
polymers obtained from the precursor polymers PEP-AN or
PEP-35AN and the last part makes the distinctions of the
substituents on the azo chromophores.
amino group in PEP-35AN) shift to relative lower magnetic field.
These spectral changes are attributed to the introduction of the
electron-withdrawing groups and the increase of the conjugation
length after forming the azo bonds. Moreover, additional reso-
nance signals corresponding to the newly introduced benzenoid
protons appear at the even lower magnetic field. The high DFs
can also be confirmed by the ratios of the integration areas for the
protons on the azo chromophores. The high efficiency of the
postpolymerization azo-coupling reactions is consistent with
those reported before.45À47 From the 1H NMR and other
analytical results, it can be concluded that the epoxy-based azo
polymers with the high chromophore density were successfully
synthesized.
2. Thermal and Spectral Properties. The epoxy-based pre-
cursor polymers and most of the azo polymers show typical phase
transition behavior of an amorphous polymer. The glass transi-
tion temperatures (Tgs) of PEP-AN and PEP-35AN are 87 and
80 °C determined by DSC (Figure S7 and S8, in the Supporting
Information). Because of the existence of the azo chromophores,
the Tgs of the azo polymers are significantly higher than those of
the corresponding precursor polymers, which were determined
by DSC and are given in Table 1. However, no obvious glass
transition can be observed for PEP-AZ-CN and PEP-AZ-NT,
which is different from the other azo polymers. This observation
can be rationalized by considering the high chromophore density
in the polymers and strong dipoleÀdipole interaction for these
1
The precursor and azo polymers were characterized by H
NMR and other spectral analyses. The results are given in the
Experimental Section and the Supporting Information (Figure
S1ÀS6). The 1H NMR spectrum of precursor polymer PEP-AN
shows the resonance signals of the aniline moieties at 6.49, 6.99,
and 6.60 ppm (Figure S1, Supporting Information), which are
attributed to the protons at para, meta, and ortho positions of the
1
amino group. In the H NMR spectrum of precursor polymer
PEP-35AN (Figure S1, Supporting Information), the resonance
signals of the 3,5-dimethylaniline moieties appear at 6.17 and
6.12 ppm attributed to the protons at para and ortho positions of
the amino group, and at 1.96 ppm attributed to the protons of the
methyl groups. After the postpolymerization azo-coupling reac-
tions, where excess amount of diazonium salts (molar ratios to
1
aniline moieties of the precursor polymers) was used, the H
NMR spectra show the changes described below. The resonance
signals corresponding to the protons at para positions of the
aniline moieties (6.49 ppm in PEP-AN and 6.17 ppm in PEP-
35AN) completely disappear (Figure S2 and S3, Supporting
Information). It indicates that the azo-coupling reaction occurs at
the para positions with the degree of functionalization (DF)
close to 100%. The signals of other protons in aniline residues
(ortho and meta to the amino group in PEP-AN; ortho to the
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Figure 1 gives the UVÀvis absorption spectra of spin-coated
films of the azo polymers. The spectra show typical character-
istics of the pseudostilbene type azo chromophores, where the
absorption bands corresponding to the πÀπ* transition appear
in visible spectrum region. The λmaxs of the azo polymers in
DMF solutions and the spin-coated films are compared in
Table 2. The λmaxs of the azo polymers are strongly affected by
the electron-withdrawing groups on the azo chromophores. As
the electron-withdrawing ability increases, the λmaxs show
significant red-shift from the top to bottom in the table. The
λmaxs of the azo polymers in DMF solutions show red-shifts
compared with those of spin-coated films, which can be attrib-
uted to the strong solvatochromic effect of DMF. When com-
pared between the two series of polymers with same electron-
withdrawing groups, the λmax of the azo polymers obtained from
PEP-35AN show blue-shift compared with those of the corre-
sponding polymers obtained from PEP-AN. It can be reasonably
attributed to the steric hindrance of the 3,5-methyl substituents,
where the planar configuration is crowded for the groups in the
excited states.
3. Photoinduced Self-Structured Surface Pattern Forma-
tion. The self-structured surface pattern formation was investi-
gated by irradiating the spin-coated films of the azo polymers
with the uniform laser single beam at 488 and 532 nm under the
normal incidence condition. Figure 2 shows typical AFM images
of the photoinduced surface patterns formed on the PEP-AZ-Cl
films after the irradiation with the laser light at 488 nm with the
intensity of 200 mW/cm2 for 30 min. After the irradiation, the
film surfaces show regularly spaced pillars with periods correlated
with the light wavelengths (Table 3), and localized hexagonal
arrangement of the pillars can be recognized. Obvious differences
can be seen for the surface patterns formed after irradiation with
linearly and circularly polarized laser lights with the same
irradiation time and light intensity. When irradiated with the
linearly polarized light, the alignment of the localized hexagonal
pattern shows correlation with the polarization direction, which
can be seen from the two-dimensional Fourier transform
(2D-FT) images (Figure 2c). The period of the regularly spaced
pillars is 440 ( 10 nm, which were obtained along (60° with
respect to the polarization direction. The pattern formed after
the circularly polarized light irradiation does not show the
orientational correlation (Figure 2, parts d and e). The 2D-FT
analysis indicates the 2D isotropic alignment of the pillars
(Figure 2f). The saturated amplitude of the surface patterns is
56 ( 8 nm for irradiation with the linearly polarized light. The
saturated amplitude of the surface patterns induced by the circularly
polarized light is 71 ( 14 nm. The above observations of the
influence of light polarization on self-structured surface pattern
formation is consistent with those reported for DR1MA/MMA 35/
65 and molecular azo glass.33,34,50 When irradiated with 532 nm laser
light under similar conditions, no self-structured surface pattern
formation was observed for PEP-AZ-Cl film.
Figure 5. AFM images (10 μm  10 μm) of the surface morphology of
four azo polymers containing chromophoric nitro groups upon irradia-
tion at 532 nm: (a) PEP-AZ-NT; (b) PEP-AZ-mNT; (c) PEP-35AZ-
NT; (d) PEP-35AZ-mNT. The incident laser light was linearly polar-
ized. The intensity of the laser light was 300 mW/cm2 and the irradiation
time was 40 min.
two type azo chromophores. The strong interaction between the
chromophores can inhibit the thermal-induced movement of the
segments. Compared with cyano and nitro groups, chloro is a
relatively weaker electron-withdrawing group and weakly con-
jugated with the aromatic ring. PEP-AZ-mNT bears a methyl
group on the azo chromophore, which will prevents the chro-
mophores to aggregate to each other. Therefore, the intermole-
cular interaction is weaker in PEP-AZ-Cl and PEP-AZ-mNT,
which show the obvious glass transition. For the series of azo
polymers obtained from PEP-35AN, all of them show typical
glass transition behavior. It can be attributed to the 3,5-dimethyl
substituents on the azo chromophores, which also barricade the
tight packing of the chromophores. Compared with the epoxy-
based azo polymers reported before,45À47 the azo polymers with
high chromophore density prepared in this work show higher Tgs
because of the high chromophore density.
Figure 3 shows the patterns formed on the films of PEP-35AZ-
Cl after irradiated with linearly polarized laser light at 488 and
532 nm with the intensity of 200 mW/cm2 for 30 min. Similar
surface pattern formation as PEP-AZ-Cl can be observed. The
saturated amplitudes of the pillar-like structures for the PEP-
35AZ-Cl films are 36 ( 8 nm and 58 ( 8 nm for the 488 and
532 nm light irradiation. When irradiated with 488 nm light,
PEP-35AZ-Cl shows the smaller surface modulation compared
with that of PEP-AZ-Cl, which could be attributed to the steric
hindrance of the bulky 3,5-dimethyl groups and its inhibition to
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Figure 6. AFM images (10 μm  10 μm) of the photoinduced surface patterns on PEP-35AZ-CN films upon the irradiation with 532 nm laser light for
different time. The intensity of the laser light was 200 mW/cm2. The irradiation time was (a) 5 min, (b) 10 min, (c) 15 min, (d) 20 min, (e) 30 min, (f)
40 min.
irradiation with laser light at both 488 and 532 nm with the
intensity of 200 mW/cm2. The irradiation time was 30 and 15
min for the 488 and 532 nm illuminations, respectively. The
surface patterns show similar characteristics as described above
for PEP-AZ-Cl and PEP-35AZ-Cl, where the surface patterns
comprise regularly spaced pillars with the period determined by
the light wavelength. The space periods for PEP-AZ-CN are
400 ( 10 and 495 ( 15 nm when irradiated with 488 and 532 nm
laser light (Figure 4a and 4b); correspondingly the periods are
405 ( 15 and 500 ( 15 nm for PEP-35AZ-CN (Figure 4, parts c
and d. The saturated amplitude of the surface structures is also
closely related with the light wavelength for both polymers. After
the irradiation at 488 nm, the saturated heights of the pillars are
19 ( 3 nm and 15 ( 3 for PEP-AZ-CN and PEP-35AZ-CN films
(Figure 4, parts a and c). After the irradiation with 532 nm laser
light with the same intensity and time, the saturated heights of
the pillars can reach 55 ( 10 and 47 ( 9 nm for PEP-AZ-CN and
Figure 7. Amplitude of the surface patterns formed on PEP-35AZ-CN
PEP-35AZ-CN (Figure 4, parts b and d).
film as a function of irradiation time, irradiated with linearly polarized
The four polymers containing nitro group as the electron-
laser light. The wavelength of the laser light was 532 nm and the intensity
withdrawing group were studied by the similar method. When
irradiated with linearly or circularly polarized laser light at
488 nm, no surface pattern formation can be seen for films of
PEP-AZ-NT, PEP-35AZ-NT, PEP-AZ-mNT, and PEP-35AZ-
mNT even with the higher light intensity and longer irradiation
time (300 mW/cm2, 40 min). When irradiated with the laser
light at 532 nm, the self-structured surface pattern formation can
only be observed for film of PEP-AZ-mNT. Figure 5 shows AFM
images of the surface morphology of the azo polymer films after
irradiation with laser light at 532 nm (300 mW/cm2, 40 min).
The surface pattern is only observed on the film of PEP-AZ-mNT
(Figure 5b) with the saturated amplitude of 69 ( 14 nm and
of the laser light was 200 mW/cm2.
the transÀcisÀtrans isomerization of the adjacent azo bond.
However, when irradiated with 532 nm laser light, the self-
structured surface patterns can be observed for PEP-35AZ-Cl
film, which is contrast to the case of PEP-AZ-Cl. The periods of
the ordered structures show obvious correlation with the light
wavelengths, which are 450 ( 15 nm and 495 ( 15 nm for the
irradiation with the 488 and 532 nm laser light.
Figure 4 shows typical AFM images of the self-structured
patterns formed on PEP-AZ-CN and PEP-35AZ-CN films upon
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Figure 8. Typical AFM images (10 μm  10 μm) of the surface relief grating formed on PEP-35AZ-CN and PEP-AZ-NT films with irradiation of the
488 nm laser light: (a) PEP-35AZ-CN, 2D-view; (b) PEP-35AZ-CN, 3D-view; (c) PEP-AZ-NT, 2D-view; (d) PEP-AZ-NT, 3D-view; The intensity of
the incident laser light was 100 mW/cm2 and the irradiation time was 600 s.
space period of 460 ( 15 nm. In this case, the methyl substitution
on the benzene ring bearing the nitro group plays a role to
promote the pattern formation.
kinds of azo polymers. Figure 8 shows some representative AFM
images of the SRG formed on the films of PEP-35AZ-CN and
PEP-AZ-NT when irradiated with the incident laser light at
488 nm for 600 s. The SRGs formed on azo polymer films appear
as regularly spaced sinusoidal surface structures, which are similar
to many previous reports.10,11,18À25 The first order diffraction
efficiency of the SRGs was used to describe the inscription rate of
the SRG formation. Figure 9 gives the first order diffraction
efficiency of the SRG as a function of time in the process of SRG
inscription on the films of the azo polymers. Parts a and b of
Figure 9 show the curves for the azo polymers obtained from
PEP-AN when irradiated with 488 and 532 nm laser light. Parts c
and d of Figure 9 give similar results for azo polymers obtained
from PEP-35AN.
The SRG formation rates show a correlation with the type of
electron-withdrawing groups, methyl substitution and excitation
wavelength. PEP-AZ-Cl and PEP-35AZ-Cl, bearing chloro group
as the electron-withdrawing group, show relative fast SRG
formation rate when irradiated with 488 nm light (Figure 9,
parts a and c). However, when irradiated with 532 nm light, the
SRG formation rates become very low (Figure 9, parts b and d),
where the excitation wavelength is near the tail of the absorption
bands of the polymers (Figure 1). In this case, PEP-AZ-CN and
PEP-35AZ-CN, bearing cyano as the electron-withdrawing
group, show the fastest SRG formation rates in the series
(Figure 9, parts b and d). The azo polymers bearing nitro as
Figure 6 shows the pattern formation on PEP-35AZ-CN film
at different times when irradiated with linearly polarized 532 nm
laser light. Figure 7 gives the corresponding relationship between
the modulation amplitude and the irradiation time. It can be seen
that the self-structured pattern formation undergoes a gradually
developing process and is saturated in about 20 min. Careful
observation can reveal another interesting characteristic of the
self-structured surface pattern related with the light polarization
conditions. After the irradiation with linearly polarized laser light
for long time (such as longer than 20 min), the pillars obviously
deformed in two directions that are about (60° to the polariza-
tion direction. This characteristic can also be recognized for other
azo polymers (Figure 2À5). However, no such deformation is
observed after the irradiation with circularly polarized laser light,
where the deformation seems to be around some incomplete
circles (Figure 2d).
4. Photoinduced Surface-Relief-Gratings. For comparison,
the surface-relief-grating (SRG) formation on the azo polymer
films was also studied by irradiating the spin-coated films with the
488 and 532 nm laser light. SGRs were inscribed by exposing the
films to interference pattern formed by two p-polarized laser
beams with the intensity of 100 mW/cm2. Upon the light
irradiation, SRG formations can be observed on the films of all
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Figure 9. First order diffraction efficiency of the SRG as a function of time in the course of SRG inscription on the films of the epoxy-based azo polymers
with incident laser light at 488 and 532 nm: (a) azo polymers obtained from PEP-AN, 488 nm; (b) azo polymers obtained from PEP-AN, 532 nm; (c) azo
polymers obtained from PEP-35AN, 488 nm; (d) azo polymers obtained from PEP-35AN, 532 nm.
the electron-withdrawing group (PEP-AZ-NT, PEP-AZ-mNT,
PEP-35AZ-NT, and PEP-35AZ-mNT) show relatively low SRG
formation rate. The 3,5-methyl substituents at the benzene ring
show a significant effect to inhibit or retard the SRG formation
for most of the azo polymers. However, as exceptions, the 3,5-
methyl substitution almost show no effect on the SRG formation
rate for azo polymers bearing cyano as the electron-withdrawing
group (PEP-AZ-CN and PEP-35AZ-CN) no matter irradiated
with 488 or 532 nm light. PEP-AZ-mNT shows a lower SRG
formation rate compared with PEP-AZ-NT, owning to the steric
hindrance of methyl group ortho to the azo bond.
5. Discussion. Above results indicates some unique character-
istics related with the self-structured surface pattern formation.
First, for the epoxy-based azo polymers, the self-structured
surface pattern formation can only be observed for those with
high chromophore density as prepared in this study. In order to
confirm this point, the light irradiation experiment was also
carried out on the epoxy-based azo polymers with lower chro-
mophore densities, which were synthesized from a precursor
polymer obtained through the step polymerization of bisphenol
A diglycidyl ether (BADGE) and aniline.47 The polymers contain
the same types of azo chromophores as reported in this article.
The chemical structure of the two azo polymers (BP-AZ-CN and
BP-AZ-NT) is given in Figure S9 (in the Supporting Information).
Figure S10 (Supporting Information) shows AFM images of the
epoxy-based azo polymers after the laser light irradiation under
the typical pattern-formation conditions. No self-structured
patterns can be observed on the films of BP-AZ-CN and
BP-AZ-NT. On the other hand, although the SRG formation
rate is dependent on the azo chromophore density, SRG forma-
tion can be observed for epoxy-based azo polymers with a very
low chromophore density.47 Second, the self-structured surface
pattern formation shows reliance on the excitation wavelength,
which is similar to SRG formation, but the exact correlation with
the wavelength is different in some ways. The SRG formation
rates of PEP-AZ-Cl and PEP-35AZ-Cl are much larger when
irradiated with 488 nm light compared with 532 nm light
(Figure 9). It is consistent with previous reports that when the
excitation wavelength is between the λmax and the band tail at
longer wavelength side, the SRG formation rate is the largest.47,51
On the contrary, PEP-35AZ-Cl shows ability to form self-
structured surface pattern in a efficient way when irradiated with
532 nm laser light (Figure 3). For self-structured surface pattern
formation, the heights of self-structured surface structures for
PEP-AZ-CN and PEP-35AZ-CN are much larger when irra-
diated with 532 nm light compared with the cases irradiated with
488 nm light (Figure 4). No obvious dependence on the two
wavelengths has been observed for SRG formation of these two
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polymers. Third, in most cases, the methyl substitution shows
effect to inhibit or retard the SRG formation. However, for the
self-structured surface pattern formation, the promotion effect of
the methyl substitution can be observed for PEP-35AZ-Cl and
PEP-AZ-mNT when irradiated with 532 nm laser light.
formation also shows reliance on the high chromophore density
of the epoxy-based azo polymers, which are distinct from the
epoxy-based azo polymers reported before.
’ ASSOCIATED CONTENT
The exact mechanism of the self-structured surface pattern
formation is still unclear to us at this stage. The laser-induced
periodic surface structure (LIPSS), i.e., the formation of parallel
ripple structures on polymer surfaces induced by irradiation with
a single beam of excimer laser or Nd:YAG pulsed laser, has been
partially explained by the models based on electromagnetic field
theory.40,41 The models suggest that one or several Fourier
components of the random surface disturbance scatter light from
the incident beam nearly along the surface. The interference of
the diffracted wave with the incident beam then gives rise optical
interference fringes that reinforce the initial disturbance in a
positive feedback way to cause the temperature distribution on
the surface. The well-organized structures of the surface patterns
reported here are quite distinct from parallel ripple structures of
LIPSS. The light intensity used here is much lower than those
typically used to produce LIPSS. Moreover, one common
characteristic of LIPSS is that when the laser beam is normally
incident, the period of ripples is nearly equal to the incident
wavelength.40,41 The periods of the self-structured surface pat-
terns also show some correlations with the incident wavelengths.
But the period is obviously smaller than the incident wavelength.
Especially, when there is strong absorbance for the incident light,
the period is further deviated to the smaller values, such as PEP-
AZ-CN and PEP35-AZ-CN irradiated with the laser light at
488 nm, PEP-AZ-mNT at 532 nm (Table 3). Very recently, a
model based on the Fick diffusion was proposed for the
molecular motion induced by the light in the pattern formation
process.42 However, unique characteristics of the self-structured
surface pattern formation, reported in this article and other
previous literature, cannot be explained by the theories.
1
S
Supporting Information. Synthesis of the anilines, H
b
NMR spectra of epoxide monomers, 1H NMR spectra, IR
spectra, and DSC diagrams of epoxy-based precursor polymers
and azo polymers, and the chemical structure and AFM images of
the two epoxy-based azo polymers (BP-AZ-CN and BP-AZ-
NT). This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: wxg-dce@mail.tsinghua.edu.cn.
’ ACKNOWLEDGMENT
Financial support from the NSFC under Projects 91027024
and 50873054 is gratefully acknowledged.
’ REFERENCES
(1) Zollinger, H. Azo and diazo chemistry: aliphatic and aromatic
compounds; Interscience Publishers: New York, 1961.
(2) Zollinger, H. Color chemistry: synthesis, properties, and applications
of organic dyes and pigments, 2nd rev. ed.; VCH: Weinheim, Germany,
1991.
(3) Kumar, G. S.; Neckers, D. C. Chem. Rev. 1989, 89 (8), 1915–
1925.
(4) Xie, S.; Natansohn, A.; Rochon, P. Chem. Mater. 1993, 5 (4),
403–411.
(5) Delaire, J. A.; Nakatani, K. Chem. Rev. 2000, 100 (5), 1817–1845.
(6) Natansohn, A.; Rochon, P. Chem. Rev 2002, 102 (11),
4139–4175.
Previous reports indicate that the SRG formation is closely
related with the photoisomerization of azo chromophores.18À25
The results obtained in this study strongly suggest that the self-
structured surface pattern formation also has a close correlation
with the excitation manner and photoisomerization pathway of
the azo chromophores. The observations reported here can lead
to better understanding of this interesting photoinduced phe-
nomenon in its correlation with the chromophoric structures.
However, to elucidate its mechanism at molecular level will
require further theoretical and experimental investigations in the
future.
(7) Ikeda, T.; Mamiya, J.; Yu, Y. L. Angew. Chem. Int. Ed. 2007, 46
(4), 506–528.
(8) Ikeda, T.; Horiuchi, S.; Karanjit, D. B.; Kurihara, S.; Tazuke, S. P.
Macromolecules 1990, 23 (1), 42–48.
(9) Todorov, T.; Nikolova, L.; Tomova, N. Appl. Opt. 1984, 23 (23),
4309–4312.
(10) Rochon, P.; Batalla, E.; Natansohn, A. Appl. Phys. Lett. 1995, 66
(2), 136–138.
(11) Kim, D. Y.; Tripathy, S. K.; Li, L.; Kumar, J. Appl. Phys. Lett.
1995, 66 (10), 1166–1168.
(12) Finkelmann, H.; Nishikawa, E.; Pereira, G. G.; Warner, M. Phys.
Rev. Lett. 2001, 8701 (1), 015501(4pp).
’ CONCLUSIONS
(13) Li, M. H.; Keller, P.; Li, B.; Wang, X. G.; Brunet, M. Adv. Mater.
2003, 15 (7À8), 569–572.
Two series of epoxy-based azo polymers with high chromo-
phore density were prepared by postpolymerization azo-cou-
pling reaction. The laser light irradiation study shows that the
photoinduced self-structured surface patterns can be formed on
the polymer films, which shows closely correlation with the type
of azo chormophore and light irradiation condition. Upon the
irradiation of the normally incident 488 nm laser light (200 mW/
cm2 for 30 min), the self-structured surface patterns can be
formed on films of PEP-AZ-Cl, PEP-35AZ-Cl, PEP-AZ-CN, and
PEP-35AZ-CN. When irradiated with the 532 nm laser light
under similar condition, the self-structured surface patterns can
be formed on films of PEP-35AZ-Cl, PEP-AZ-CN, PEP-35AZ-
CN and PEP-AZ-mNT. The photoinduced surface pattern
(14) Yu, Y. L.; Nakano, M.; Ikeda, T. Nature 2003, 425 (6954),
145–145.
(15) Camacho-Lopez, M.; Finkelmann, H.; Palffy-Muhoray, P.;
Shelley, M. Nat. Mater. 2004, 3 (5), 307–310.
(16) Viswanathan, N. K.; Kim, D. Y.; Bian, S. P.; Williams, J.; Liu, W.;
Li, L.; Samuelson, L.; Kumar, J.; Tripathy, S. K. J. Mater. Chem. 1999, 9
(9), 1941–1955.
(17) Che, P. C.; He, Y. N.; Wang, X. G. Macromolecules 2005, 38
(21), 8657–8663.
(18) Barrett, C. J.; Natansohn, A. L.; Rochon, P. L. J. Phys. Chem.
1996, 100 (21), 8836–8842.
(19) Barrett, C. J.; Rochon, P. L.; Natansohn, A. L. J. Chem. Phys.
1998, 109 (4), 1505–1516.
6866
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Macromolecules
ARTICLE
(20) Kumar, J.; Li, L.; Jiang, X. L.; Kim, D. Y.; Lee, T. S.; Tripathy, S.
Appl. Phys. Lett. 1998, 72 (17), 2096–2098.
(21) Bian, S. P.; Williams, J. M.; Kim, D. Y.; Li, L.; Balasubramanian,
S.; Kumar, J.; Tripathy, S. K. J. Appl. Phys. 1999, 86 (8), 4498–4508.
(22) Lefin, P.; Fiorini, C.; Nunzi, J. M. Pure Appl. Opt. 1998, 7 (1),
71–82.
(23) Pedersen, T. G.; Johansen, P. M.; Holme, N. C. R.; Ramanujam,
P. S.; Hvilsted, S. Phys. Rev. Lett. 1998, 80 (1), 89–92.
(24) Tanchak, O. M.; Barrett, C. J. Macromolecules 2005, 38 (25),
10566–10570.
(25) Yager, K. G.; Tanchak, O. M.; Godbout, C.; Fritzsche, H.;
Barrett, C. J. Macromolecules 2006, 39 (26), 9311–9319.
(26) Yager, K. G.; Barrett, C. J. Macromolecules 2006, 39 (26),
9320–9326.
(27) Perschke, A.; Fuhrmann, T. Adv. Mater. 2002, 14 (11),
841–843.
(28) Kim, M. J.; Kumar, J.; Kim, D. Y. Adv. Mater. 2003, 15 (23),
2005–2008.
(29) Li, Y. B.; He, Y. N.; Tong, X. L.; Wang, X. G. J. Am. Chem. Soc.
2005, 127 (8), 2402–2403.
(30) Liu, B.; He, Y. N.; Wang, X. G. Langmuir 2006, 22 (24),
10233–10237.
(31) Guo, M. C.; Xu, Z. D.; Wang, X. G. Langmuir 2008, 24 (6),
2740–2745.
(32) Goldenberg, L. M.; Gritsai, Y.; Kulikovska, O.; Stumpe, J. Opt.
Lett. 2008, 33 (12), 1309–1311.
(33) Hubert, C.; Fiorini-Debuisschert, C.; Maurin, I.; Nunzi, J. M.;
Raimond, P. Adv. Mater. 2002, 14 (10), 729–732.
(34) Hubert, C.; Fiorini-Debuisschert, C.; Rocha, L.; Raimond, P.;
Nunzi, J. M. J. Opt. Soc. Am. B-Opt. Phys. 2007, 24 (8), 1839–1846.
(35) Bolle, M.; Lazare, S.; Leblanc, M.; Wilmes, A. Appl. Phys. Lett.
1992, 60 (6), 674–676.
(36) Bolle, M.; Lazare, S. J. Appl. Phys. 1993, 73 (7), 3516–3524.
(37) Hiraoka, H.; Sendova, M. Appl. Phys. Lett. 1994, 64 (5),
563–565.
(38) Ivanov, M.; Rochon, P. Appl. Phys. Lett. 2004, 84 (22),
4511–4513.
(39) Tsutsumi, N.; Fujihara, A. Appl. Phys. Lett. 2004, 85 (20),
4582–4584.
(40) Sipe, J. E.; Young, J. F.; Preston, J. S.; Vandriel, H. M. Phys. Rev.
B 1983, 27 (2), 1141–1154.
(41) Zhou, G. S.; Fauchet, P. N.; Siegman, A. E. Phys. Rev. B 1982, 26
(10), 5366–5382.
(42) Leblond, H.; Barille, R.; Ahmadi-Kandjani, S.; Nunzi, J. M.;
Ortyl, E.; Kucharski, S. J. Phys. B. At. Mol. Opt. Phys. 2009, 42, 205401.
(43) Ahmadi-Kandjani, S.; Barille, R.; Dabos-Seignon, S.; Nunzi,
J. M.; Ortyl, E.; Kucharski, S. Opt. Lett. 2005, 30 (15), 1986–1988.
(44) Ahmadi-Kandjani, S.; Barille, R.; Dabos-Seignon, S.; Nunzi,
J. M.; Ortyl, E.; Kucharski, S. Mol. Cryst. Liq. Cryst. 2006, 446, 99–109.
(45) Wang, X. G.; Kumar, J.; Tripathy, S. K.; Li, L.; Chen, J. I.;
Marturunkakul, S. E. Macromolecules 1997, 30 (2), 219–225.
(46) Wang, X. G.; Balasubramanian, S.; Kumar, J.; Tripathy, S. K.; Li,
L. Chem. Mater. 1998, 10 (6), 1546–1553.
(47) He, Y. N.; Wang, X. G.; Zhou, Q. X. Polymer 2002, 43 (26),
7325–7333.
(48) Chakraborti, A. K.; Rudrawar, S.; Kondaskar, A. Org. Biomol.
Chem. 2004, 2 (9), 1277–1280.
(49) Klee, J. E.; Flammersheim, H. J. Macromol. Chem. Phys. 2002,
203 (1), 100–108.
(50) Yin, J. J.; Ye, G.; Wang, X. G. Langmuir 2010, 26 (9),
6755–6761.
(51) Kim, M. J.; Lee, J. D.; Chun, C.; Kim, D. Y.; Higuchi, S.;
Nakayama, T. Macromol. Chem. Phys. 2007, 208 (16), 1753–1763.
6867
dx.doi.org/10.1021/ma200832b |Macromolecules 2011, 44, 6856–6867