Sequentially adsorbed electrostatic multilayers of branched side-chain polyelectrolytes bearing donor-acceptor type azo chromophores

Article abstract of DOI:10.1021/ma035208u

Two newly synthesized polyelectrolytes (PBANT-AC and PBACT-AC) functionalized with branched side chains bearing electron donor-acceptor type azobenzene chromophores were used as polyanions to build up multilayer films through an electrostatic sequential adsorption process by using poly(diallyldimethylammonium chloride) (PDAC) as the polycation. When dissolved in anhydrous DMF and a series of DMF-H₂O mixed solvents with different DMF to H₂O ratios, both azo polyelectrolytes could form uniform multilayer films through the layer-by-layer adsorption process. Altering the water content of the dipping solutions of both azo polyelectrolytes was found to dramatically change the thickness of the sequentially adsorbed bilayers, chromophore orientation, and surface roughness of the multilayer films. On the other hand, the solvent effect on the H-aggregation in the multilayer films was determined by the structural details of the azo polyelectrolytes. After the irradiation with a linearly polarized Ar⁺ laser beam at 488 nm, significant dichroism was induced in the PBANT-AC/PDAC multilayer films prepared from the DMF dipping solution. Upon exposure to an interference pattern of Ar⁺ laser beam at modest intensities, optically induced surface modulation on the multilayer surfaces was observed.

Full text of DOI:10.1021/ma035208u

Macromolecules 2004, 37, 135-146
135
Sequentially Adsorbed Electrostatic Multilayers of Branched Side-Chain
Polyelectrolytes Bearing Donor-Acceptor Type Azo Chromophores
Ha op en g Wa n g, Ya n in g He, Xin lin Tu o, a n d Xia ogon g Wa n g*
Department of Chemical Engineering, Tsinghua University, Beijing, P. R. China 100084
Received August 15, 2003; Revised Manuscript Received October 31, 2003
ABSTRACT: Two newly synthesized polyelectrolytes (PBANT-AC and PBACT-AC) functionalized with
branched side chains bearing electron donor-acceptor type azobenzene chromophores were used as
polyanions to build up multilayer films through an electrostatic sequential adsorption process by using
poly(diallyldimethylammonium chloride) (PDAC) as the polycation. When dissolved in anhydrous DMF
and a series of DMF-H2O mixed solvents with different DMF to H2O ratios, both azo polyelectrolytes
could form uniform multilayer films through the layer-by-layer adsorption process. Altering the water
content of the dipping solutions of both azo polyelectrolytes was found to dramatically change the thickness
of the sequentially adsorbed bilayers, chromophore orientation, and surface roughness of the multilayer
films. On the other hand, the solvent effect on the H-aggregation in the multilayer films was determined
by the structural details of the azo polyelectrolytes. After the irradiation with a linearly polarized Ar+
laser beam at 488 nm, significant dichroism was induced in the PBANT-AC/PDAC multilayer films
prepared from the DMF dipping solution. Upon exposure to an interference pattern of Ar+ laser beam at
modest intensities, optically induced surface modulation on the multilayer surfaces was observed.
In tr od u ction
properties without electric field poling.24-26 Self-as-
sembled multilayers of an azobenzene bolaamphiphile
and polycations have been prepared, and the photo-
switching function based on photoisomerization of the
multilayers has been studied.27 Alternate multilayer
films of a cationic bipolar azo amphiphile and an anionic
polyelectrolyte have been prepared by a layer-by-layer
deposition method.28 In the study, optical dichroism was
induced in the films upon linearly polarized UV irradia-
tion, which could be reversibly erased and rewritten. It
has been reported that ionenes containing rigid azoben-
zene chromophores separated by flexible spacers can
form internally ordered multilayers through the elec-
trostatic layer-by-layer deposition method.29 Photoin-
duced switching of self-assembled multilayers formed
by an alternate adsorption of azobenzene-containing
ionene polycations and anionic polyelectrolytes has been
studied.30 The ionenes used could be in the trans- or
cis-rich state of the azobenzene units. Use of cis-rich
ionene solutions was found to adsorb up to 3 times more
material at each dipping cycle as compared with the
solution of the trans polymer. A commercially available
side-chain azo polyanion and two polycations have been
used to build up multilayers.31 UV light irradiation was
found to be responsible for reversible changes in the
optical thickness of the films. A significant photochromic
effect of cis-trans isomerization of the azo chromo-
phores, accompanied by the contact angle changes of
water on the multilayer surface, has been observed for
the multilayers of a series of poly(acrylic acid)-based azo
polyelectrolytes.32 Commercially available low-molar-
mass azo dyes also present promise in creating ordered
layer-by-layer thin films.33 Surface relief gratings were
fabricated on the multilayer films of poly(diallyldi-
methylammonium chloride) and a commercial azo dye,
Congo Red. Reported in a separate article by the same
group, multilayer films formed by a poly(acrylic acid)-
based anilino-functional precursor polymer have been
postfunctionalized through azo coupling reaction and
studied for photoinduced surface modulation.34
Azobenzene derivatives are well-known for their
character to undergo the trans-cis isomerization upon
light irradiation. On the basis of the photoisomerization,
polymeric thin films containing azobenzene chromo-
phores can exhibit diverse photoresponsive properties,
which promise potential applications in areas such as
holographic information storage, photoswitching, sen-
sors, and many others.1-3 Interaction of linearly polar-
ized light with the azobenzene moieties can induce the
groups in polymeric films to align perpendicular to the
direction of light polarization and generate birefringence
and dichroism.4,5 By exploring the nature, reversible
optical storage has been demonstrated in different
ways.6,7 Photoinduced surface modulation is another
interesting effect caused by the photoisomerization,
which has attracted considerable attention and has been
intensively investigated in recent years.8,9 The photo-
induced surface structures such as surface relief grat-
ings (SRGs) can be formed on azo polymer surfaces in
a reversible way upon exposure to an interference
pattern of Ar+ laser beams at modest intensities.8-10
A
number of azobenzene-containing polymers as spin-
coated films, Langmuir-Blodgett films, or monolayers
have been studied over recent years for the photoin-
duced properties.11-18
The electrostatic layer-by-layer deposition method,
first introduced by Decher, is an innovatory way to
prepare novel types of polymeric multilayer thin films.19,20
For many applications, it is the most feasible method
to create multilayer thin films incorporating a wide
variety of functional groups.20-23 The electrostatic self-
assembled multilayer films containing azo chromophores
have been considered to be promising for photonic and
other applications.24-34 Several groups have reported
that noncentrosymmetric thin films, prepared from
electrostatic layer-by-layer deposition of azo polyelec-
trolytes, can demonstrate significant nonlinear optical
* Corresponding author.
10.1021/ma035208u CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/13/2003

136 Wang et al.
Macromolecules, Vol. 37, No. 1, 2004
It has been known for a quite long time that the local
structure of polymeric matrix influences both photo- and
thermochromism of azo chromophores.35,36 It has been
reported that the photochemical reactivity of the elec-
trostatic multilayers is considerably dependent on the
type of polycations used in the systems.30 A significant
influence of the polycations on the molecular orientation
of the azobenzene groups in multilayer films and on the
photoisomerization kinetics has also been observed.31
It has been shown that in most cases the polyelectro-
lytes are not stratified into well-defined layers but
are dispersed and interpenetrating in the multilayer
films.20,23,37,38 For fully charged polyelectrolytes such as
poly(styrenesulfonate) and poly(allylamine), the thick-
ness of the adsorbed layers can be fine-tuned at the
molecular level by adjusting the ionic strength of the
dipping solution.39 The organization and thickness of
the multilayers are also influenced by dielectric constant
of the deposition solvent and identity of the salt.37 The
thickness and the level of interlayer interpenetration
of sequentially adsorbed layers of weak polyelectrolytes
are extremely sensitive to dipping solution pH.40,41 For
molecules or polymers containing azo dyes, H-aggrega-
tion has been observed in the dipping solutions or
electrostatic self-assembled multilayer films.26-31 The
name H-aggregation originated in the observation of
hypsochromic or H-bands in UV-vis spectra of ionic
dyes, which mean those transitions that are blue-shifted
and thus appear at shorter wavelengths than molecular
absorption band (M-band).42 The appearance of H-
bands is rationalized in terms of face-to-face dye ag-
gregation through the formation of dimers, trimers, or
even n-mers.43-46 Azobenzene bolaamphiphile with
branched structure has been reported to be able to form
self-assembled order architecture through H-aggrega-
tion.47 Besides the direct effects on the spectral and
photoresponsive properties, H-aggregation causing the
intermolecular association and intramolecular aggrega-
tion could induce conformational change of polymer
chains in solution and consequently influence organi-
zation and thickness of multilayers. This means that,
in the process to controllably alter dipping solution
conditions of azo polyelectrolytes, the variation of the
H-aggregation degree in both the dipping solutions
and multilayer films caused by the change should be
understood. However, the study along this line is still
lacking.
dichroism and SRGs of the multilayer films were also
explored in the study.
Exp er im en ta l Section
Ch a r a cter iza tion . Infrared spectra were measured using
a Nicolet 560-IR spectrometer by incorporating the sample in
1
a KBr disk. H NMR and 13C NMR spectra were obtained on
a Varian Inova 500NB NMR spectrometer. Elemental analysis
of C, H, and N was completed through the Heratus CHN-Rapid
method. The glass transition temperatures (Tgs) of the poly-
mers were determined with TA Instrument DSC 2910 at a
heating rate of 10 °C/min. The molecular weights of the
polymers were measured by gel permeation chromatography
(GPC) (Viscotek TDA 302 GPC); THF was used as the eluting
solvent at the rate of 1.0 mL/min with the temperature of 40
°C. UV-vis spectra of the azo polyelectrolytes in solutions, as
self-assembled multilayers and spin-coated films, were re-
corded on a Perking-Elmer Lambda Bio-40 spectrometer. The
thickness of the multilayer films on silicon wafers was
determined by using an optical ellipsometer (SE400, Sentech
Instruments) with a He-Ne laser (632.8 nm) at an incident
angle of 70°. The surface images of the multilayers were
monitored using AFM (Nanoscope IIIa, tapping mode).
Ma ter ia ls. 1,4-Dioxane, tetrahydrofuran (THF), and pe-
troleum ether (bp 30-60 °C) were refluxed with cuprous
chloride for 1 h and distilled, then were refluxed with sodium
for 6 h, and distilled before use. N,N-Dimethylformamide
(DMF) was azeotropically distilled with benzene six times for
dehydrating and then distilled under vacuum before use. All
the other reagents and solvents were used as received without
further purification.
N-E t h yl-N-(2-ch lor oet h yl)a n ilin e (E CA). Phosphorus
oxychloride (2 mL, 0.021 mol) solution of THF (10 mL) was
added dropwise at room temperature into a solution of N-ethyl-
N-hydroxyethylaniline (5.00 g, 0.030 mol, from ACROS Co.)
in 10 mL of anhydrous tetrahydrofuran (THF). The mixture
was stirred at room temperature for 4 h, and then the solution
was concentrated by removing the solvent THF. The residue
was diluted with water (adjusted pH to 9) and extracted with
CH2Cl2. After dried with MgSO4, the extract was collected by
distilling off the solvent at reduced pressure and further
purified by vacuum distillation; yield 45%, bp 102-104 °C/
399Pa. IR (KBr): 2970 (s; C-H), 1600 1500 (s; benz ring), 1340
1
(s; C-N), and 720 (s; C-Cl) cm-1. H NMR (CDCl3): δ ) 1.16
(t, -CH3, 3H), 3.40 (m, N-CH2CH3, 2H), 3.60 (s, N-CH2CH2-
Cl, 4H), 6.68 (m, Ar-H, 3H, ortho and para to N), 7.22 (t, Ar-
H, 2H, meso to N). Elemental analysis: C10H14NCl (183.68)
Calcd: C, 65.40; H, 7.87; N, 7.63; Cl, 19.30. Found: C, 64.10;
H, 7.70; N, 7.81; Cl, 18.74.
3,5-Bis[2-(N-eth ylan ilin o)eth oxy]ben zyl Alcoh ol (EEBA).
A mixture of ECA (4.041 g 0.022 mol), 3,5-dihydroxybenzyl
alcohol (1.401 g, 0.01 mol), anhydrous potassium carbonate
(10 g, 0.072 mol), potassium iodide (0.5 g), and 30 mL of
dimethyl sulfoxide (DMSO) was heated to 110 °C under
nitrogen protection with vigorous stirring for 48 h. The reaction
mixture was then poured into 500 mL of cold water and
extracted with CH2Cl2. The extract was concentrated and
precipitated with petroleum ether. The precipitate was dried
at 70 °C under vacuum for 48 h. IR (KBr): 3400 (s; OH), 2970
In the present work, two azo polyelectrolytes (PBANT-
AC and PBACT-AC) with branched side chains contain-
ing donor-acceptor type azobenzene chromophores were
synthesized. The polymers can be considered as random
copolymers of acrylic acid units and acrylate units
bearing branched side chains containing donor-acceptor
type azobenzene chromophores. The molecular design
is in order to ensure that the polyelectrolytes can
contain an adequate amount of photoresponsive azo
chromophores and also enough hydrophilic COOH groups
to promote the water solubility of the polyelectrolytes.
The polyelectrolytes dissolved in dimethylformamide
(DMF) and a series of DMF-H2O mixed solvents with
different DMF to H2O ratios were used as deposition
solutions. The electrostatic layer-by-layer self-assembly
behavior of the azo polyelectrolytes in the solutions was
investigated. The effects of the DMF to H2O ratios on
the H-aggregation, self-assembled multilayer thickness,
surface roughness, and azo chromophore orientation in
the multilayers were studied in detail. Photoinduced
(s; CH), 1600 1500 1460 (s; benz ring) and 1340 (s; C-N) cm-1
.
1H NMR (CDCl3): δ ) 1.16 (t, -CH3, 6H), 3.42 (m, N-CH2-
CH3, 4H), 3.70 (t, N-CH2CH2-O, 4H), 4.00 (t, N-CH2CH2-
O, 4H), 4.58 (s, Ar-CH2-OH, 2H), 6.34 (s, Ar-H, para to
-CH2OH, 1H), 6.48 (s, Ar-H, ortho to -CH2OH, 2H), 6.68
(m, Ar-H, 6H, ortho and para to N), 7.22 (t, Ar-H, 4H, meso
to N).
P oly(a cr yloyl ch lor id e) (P AC).32 Acryloyl chloride (10
mL), dry 1,4-dioxane (10 mL), and AIBN (0.332 g) were added
into a flask. The mixture was reacted at 55 °C under the
protection of nitrogen for 14 h. The polymer was precipitated
by adding petroleum ether (60 mL) and then washed twice
with petroleum ether. The product was dried at 60 °C under
vacuum for 48 h. GPC: Mn 53 200 (estimated by using poly-

Macromolecules, Vol. 37, No. 1, 2004
Branched Side-Chain Polyelectrolytes 137
(methyl acrylate) prepared from the same batch of PAC and
an excess of methanol);48 MWD: 1.3.
spin-coated films) was investigated by using a polarized Ar+
laser beam at 488 nm with an intensity of 150 mW/cm2 as the
writing beam. The optically induced dichroism was measured
by using polarized UV-vis spectroscopy. The experimental
setup for the surface relief grating formation was similar to
that reported in the literature.8,9 A linearly polarized laser
beam at 488 nm from an Ar+ laser was used as recording light
source. The p-polarized laser beam was expanded and col-
limated. Half of the collimated beam was incident upon the
film directly. The other half of the beam was reflected onto
the film from a mirror. The intensity of the recording beam
was about 150 mW/cm2. The diffraction efficiency of the first-
order diffracted beam from the gratings in transmission mode
was probed with an unpolarized low power He-Ne laser beam
at 633 nm.
P oly{3,5-bis[2-(N-eth yla n ilin o)eth oxy]ben zyl a cr yla te-
co-a cr ylic a cid } (P BA-AC). PAC (0.3 g, 0.0033 mol), triethyl-
amine (0.56 mL, 0.0040 mol), and EEBA (whose amount was
determined by the degree of functionalization required) were
dissolved in anhydrous DMF (40 mL). The mixture was stirred
at 70 oC for 16 h under the protection of N2. Then 3 mL of
water was added into the mixture and stirred for 10 min. After
precipitated in 300 mL of water solution (adjusting pH by HCl
to 3.5-4), the precipitate collected by filtration was washed
with water and dried. The crude product was further purified
by extraction with CH2Cl2 for 24 h and dried at 60 °C under
vacuum for 48 h. IR (KBr): 3500-2500 (broad, O-H), 1700-
1750 (s; CdO), 1600 1500 (s; benz ring) cm-1. 1H NMR (DMSO-
d6, ppm): δ ) 1.04, 1.20-2.40, 3.62, 4.01, 4.90, 6.30, 6.47,6.55,
6.70, 7.14. 13C NMR (DMSO-d6, ppm): δ ) 175.6, 174.0, 159.5,
147.1, 138.2, 129.2, 115.6, 111.6, 106.2, 100.5, 65.4, 49.0, 44.6,
40.0, 34.7, 32.0, 11.7.
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion of Azo P olyelec-
tr olytes. The synthetic route of the azo polyelectrolytes
is shown in Scheme 1. N-Ethyl-N-(2-chloroethyl)aniline
(ECA) was prepared by chloridizing N-ethyl-N-hydroxy-
ethylaniline with phosphorus oxychloride (POCl3). 3,5-
Bis[2-(N-ethylanilino)ethoxy]benzyl alcohol (EEBA) was
prepared by the Williamson ether synthesis of 3,5-
dihydroxybenzyl alcohol with ECA in anhydrous DMSO
using K2CO3 and KI as HCl absorbent and catalyst,
respectively. The chemical structure of both compounds
was verified by the characterization of elemental analy-
P oly{3,5-bis[2-(N-eth yl-4-(4′-n itr op h en yla zo)a n ilin o)-
et h oxy]b en zyl a cr yla t e-co-a cr ylic a cid } (P BANT-AC).
The azo polyeletrolyte was prepared by a postpolymerization
azo coupling reaction.12 p-Nitroaniline was diazotized by a
similar method as the literature reported, and the diazonium
salt solution was added dropwise into a solution of PBA-AC
in dimethylformamide (DMF) at 0 °C. PBANT-AC was ob-
tained by precipitation of the above solution in plenty of water
1
and dried at 60 °C under vacuum for 48 h. H NMR (DMSO-
d6, ppm): δ ) 1.08, 1.20-2.40, 4.10, 4.90, 6.30-6.47, 6.80, 7.78,
8.24. 13C NMR (DMSO-d6, ppm): δ ) 175.6, 174.0, 159.4, 156.1,
151.4, 146.7, 142.8, 138.2, 126.0, 124.7, 122.3, 111.5, 106.4,
100.6, 65.5, 49.0, 45.2, 40.8, 35.0, 32.2, 11.9. UV-vis (DMF):
488 nm (ꢀ ) 3.71 × 104 L mol-1 cm-1).
1
ses, IR, and H NMR spectroscopy.
Poly(acryloyl chloride) (PAC) was prepared by the
radical polymerization of acryloyl chloride.32 As it was
difficult to measure the molecular weight and its
distribution of PAC directly due to the high reactivity
of acyl chloride groups, a GPC measurement was carried
out on a poly(methyl acrylate) sample that was prepared
by the reaction between the synthesized PAC and
excessive methanol.48 The number-average molecular
weight of the PAC estimated by the result of the poly-
(methyl acrylate) sample was 53 200 with a polydisper-
sity index of 1.3.
P oly{3,5-bis[2-(N-et h yl-4-(4′-ca r b oxyp h en yla zo)a n ili-
n o)eth oxy]ben zyl a cr yla te-co-a cr ylic a cid } (P BACT-AC).
Via a similar procedure as the synthesis of PBANT-AC.
4-Aminobenzoic acid was used instead of p-nitroaniline in the
reaction. 1H NMR (DMSO-d6, ppm): δ ) 1.05, 1.20-2.40, 4.10,
4.90, 6.30-6.47, 6.80, 7.78, 8.08. 13C NMR (DMSO-d6, ppm):
δ ) 175.4, 174.0, 166.6, 159.4, 155.1, 150.8, 142.6, 138.2, 130.8,
130.3, 125.4, 121.6, 111.4, 106.3, 100.6, 65.5, 49.0, 45.1, 40.6,
34.4, 32.2, 11.9. UV-vis (DMF): 438 nm (ꢀ ) 2.40 × 104
L
mol-1 cm-1).
Mu ltila yer P r ep a r a tion . Poly(diallyldimethylammonium
chloride) (PDAC, MW 20 000-35 000, 20% solution, Aldrich)
was used as polycation and diluted to a concentration of 0.1
mmol/L (repeated unit) with Milli-Q water (resistivity >18
MΩ). The azo polyelectrolytes PBANT-AC and PBACT-AC
were used as polyanion and dissolved in anhydrous DMF and
a series of DMF-H2O mixed solvents with a concentration of
0.2 mg/mL, respectively. Quartz slides (50 mm × 14 mm × 8
mm) and silicon wafers were used as adsorption substrates
and were treated as follows. The slides and wafers were
sonicated in a 98% H2SO4/30% H2O2 solution (piranha solution)
for 1 h and then sonicated in a H2O/H2O2/NH4OH (5:1:1)
solution for 1 h followed with a thorough rinse and then dried
with an air stream. A freshly treated quartz slide was first
dipped in the polycation solution for 10 min. The slide was
rinsed with Milli-Q water and blown dry with an air stream,
and then the slide was dipped in the polyanion solution for 10
min and thoroughly rinsed with the same solvent as the
polyanion solution and blown dry. This process was repeated
until the multilayers with the required number of layers were
obtained. The UV-vis absorption spectra of the multilayers
were measured after each monolayer (polycation or polyanion
layer) was formed. To monitor the thickness of the multilayers
formed, the silicon wafers were dipped and washed under the
same condition. The thickness of the multilayer films formed
on the silicon wafers was measured by ellipsometry.
PAC was reacted with EEBA to give the precursor
polymer PBA-AC through the Schotten-Baumann reac-
tion with triethylamine as the HCl absorbent.32 The
chemical structure and purity of the product were
confirmed by spectroscopic analyses. The 1H NMR
spectrum of PBA-AC is given in Figure 1. The chemical
shift of methylene protons next to the hydroxyl groups
of EEBA shifts from 4.58 to 4.90 ppm, which indicates
the formation of the ester linkages. The other proton
resonances corresponding to the branched groups teth-
ered to the polymeric chains appear at nearly the same
positions except that the peaks become broad. The board
and overlapped peaks (in the δ 1-2.5 ppm range)
corresponding to main-chain protons demonstrate a
random copolymerization sequence and atactic config-
uration. The degree of functionalization (DF) is defined
as the percentage of the structure units bearing ester
linkages among the total number of the structure units
after postfunctionalization. In this study, the DF of
PBA-AC was 17% estimated from the elemental analy-
sis.
The azo polyelectrolytes were prepared by postpoly-
merization azo coupling reactions between PBA-AC and
the diazonium salts of p-nitroaniline and 4-aminoben-
zoic acid, respectively. In the reactions, the feed ratios
of the diazonium salts to PBA-AC were determined
according to the DF mentioned above to guarantee the
anilino moieties to be reacted completely. The chemical
structure and purity of the products were confirmed by
DMF solutions of both azo polyelectrolytes were spin-coated
onto glass slides to form good optical quality thin films used
as a control. The thickness of the spin-coated films of PBANT-
AC and PBACT-AC was about 100 nm.
P h otoin d u ced An isotr op y a n d SRGs. Photoinduced an-
isotropy of the polyelectrolyte films (both as multilayers and

138 Wang et al.
Macromolecules, Vol. 37, No. 1, 2004
Sch em e 1. Syn th etic Rou te of th e Azo P olyelectr olytes P BANT-AC a n d P BACT-AC
Ta ble 1. Gla ss Tr a n sition Tem p er a tu r e (Tg) of P BA-AC,
than that of PBANT-AC (with a 20 °C difference), which
can be attributed to the hydrogen bonds of carboxyl
groups on the azobenzene groups of PBACT-AC.
P BANT-AC, a n d P BACT-AC
polymer
PBA-AC
112
PBANT-AC
139
PBACT-AC
159
H-Aggr egation in Solu tion s. Polyelectrolyte PBACT-
AC bearing ionizable COOH groups on the azo chro-
mophores exhibited high solubility in deionized water.
On the contrary, PBANT-AC could hardly be dissolved
in deionized water under similar conditions due to the
hydrophobicity of the azo chromophores. Both polyelec-
trolytes could be well dissolved in a series of DMF-H2O
mixed solvents with different DMF to H2O ratios.
However, it will be shown that the aggregate states of
azo chromophores in the solutions were found to be
different.
Tg (°C)
1
elemental analyses, IR, H NMR, and 13C NMR spec-
troscopy. The 1H NMR spectra of PBANT-AC and
PBACT-AC with the assignments are compared with
that of PBA-AC in Figure 1. For precursor polymer PBA-
AC, the chemical shifts of unreacted anilino moieties
appear at about 7.14, 6.70, and 6.55 ppm, attributed to
the protons at meta, ortho, and para positions of the
amino groups, respectively. After azo coupling reaction,
the 6.55 ppm resonance corresponding to protons at the
para positions of the amino groups in the anilino
moieties almost disappear, which indicates that the
substitutions have taken place at the para positions
with a high yield. Further, the chemical shifts of protons
ortho and meta to the amino groups shift to lower
magnetic field as the result of the introduction of the
electron-withdrawing groups and the increase of the
conjugation length. Additional resonances corresponding
to chemical shifts of the introduced benzene ring protons
can also be observed. When a slight excess of diazonium
salts (molar ratio to anilino moieties of the precursor
polymers) was used in the reactions, the resonances
corresponding to unreacted anilino moieties totally
disappeared from the 1H NMR spectra of the final
products. As the DF of PBA-AC was 17%, the molar
ratios of azobenzene chromophores to the repeated units
were estimated to be 34% both for PBANT-AC and
PBACT-AC.
Figure 2a shows the UV-vis spectra of PBANT-AC
dissolved in anhydrous DMF and a series of DMF-H2O
solvents. The maximum absorbance of PBANT-AC in
DMF appears at 488 nm, which is related to the π-π*
transition of the trans isomers of the azobenzene units.
By decreasing the ratio of DMF to H2O (v:v), the
maximum absorbance blue-shifts first gradually and
then drastically. The maximum absorbance appears at
481 nm when the ratio is 4:1 and drastically blue-shifts
about 70 to 415 nm when the ratio is 2:1. The hypso-
chromic shift is a typical spectral characteristic of the
card-packed H-aggregates formed by the azobenzene
chromophores in solutions.42-44 According to the exciton
model advanced by Kasha, the spectral changes is due
to a splitting of the excited state, in which a transition
to the higher excited-state level is allowed while to lower
energy level is forbidden.45,46 The above observation
demonstrates that the azo chromophores of PBANT-AC
exist predominately in the “isolated” state in the anhy-
drous DMF solution and transform into H-aggregates
with the increase of water content in the solutions.
The glass transition temperatures (Tgs) of PBA-AC,
PBANT-AC, and PBACT-AC were determined by dif-
ferential scanning calorimetry (DSC) and are listed in
Table 1. The Tgs of the azo polyelectrolytes PBANT-AC
and PBACT-AC are considerably higher than the pre-
cursor polymer PBA-AC, which is a result of the
significant increase of the dipole moment of the side
groups. PBACT-AC exhibits a significantly higher Tg
For PBACT-AC solutions, the H-aggregation of the
azo chromophores in the DMF-H2O solutions was also
studied by the UV-vis spectroscopy. Figure 2b shows
some representative results. Although the maximum

Macromolecules, Vol. 37, No. 1, 2004
Branched Side-Chain Polyelectrolytes 139
F igu r e 1. 1H NMR spectra of (a) PBA-AC, (b) PBANT-AC, and (c) PBACT-AC.
absorbance in the absorption spectra of PBACT-AC in
the solutions also shows blue shifts associated with the
H-aggregation of the azo chromophores in the solutions,
the degree of H-aggregation and the ratio of DMF to
H2O at which the spectra vary significantly are obvi-
ously different. The spectrum of PBANT-AC in a solu-
tion of DMF:H2O ) 2:1 (v:v) mixed solvent is predomi-
nated by the H-band; on the contrary, there is only a
slight blue shift of the maximum absorbance for PBACT-
AC spectrum under the same conditions. By comparing
the structural difference between PBACT-AC and
PBANT-AC, it is believed that the ionizable carboxylic
acid groups at the 4′-positions of the azobenzene units
play an important role to enhance hydrophilicity of the
azo chromophores and inhibit the chromophores parallel
packing under the condition.
Sequ en tia lly Ad sor bed Electr osta tic Mu ltila y-
er s. Both PBANT-AC and PBACT-AC could form uni-
form multilayer films through an electrostatic layer-by-
layer adsorption process by using dipping solutions of
the polyelectrolytes in anhydrous DMF and in DMF-
H2O mixed solvents. During the process, anhydrous
DMF or DMF-H2O mixed solvents with the same DMF
to H2O ratio were used to rinse the multilayers formed
after the deposition of azo polyelectrolytes on the
substrates (both quartz slides and silicon wafers). A

140 Wang et al.
Macromolecules, Vol. 37, No. 1, 2004
F igu r e 2. UV-vis spectra of (a) PBANT-AC and (b) PBACT-
AC in solutions of anhydrous DMF and a series of DMF-H2O
mixed solvents. The concentration of azo chromophore is 0.012
mmol/L for PBANT-AC and 0.015 mmol/L for PBACT-AC.
F igu r e 3. UV-vis spectra of PBANT-AC/PDAC multilayers
changing with the deposition cycles for two types of dipping
solutions: (a) PBANT-AC in anhydrous DMF; (b) PBANT-AC
in a DMF-H2O solution (DMF:H2O ) 2:1, v:v). The concentra-
tions are 0.2 mg/mL for both.
linear increase of the maximum absorbance with the
increasing number of the bilayers was observed for these
systems, which indicated that the multilayers on the
substrates grew an equal amount after each dipping
cycle.19,20 We use the term “bilayer” in accordance with
previous publications on this type of films, which means
a pair of layers including one layer of polycation and
one layer of polyanion formed after each deposition
cycle.24-26,29,40 Although it has been reported that poly-
electrolytes can be adsorbed onto multilayer films
through an electrostatic adsorption in mixed sol-
vents,29,37 the result obtained in current work shows
that the azo polyelectrolytes can also be self-assembled
in a layer-by-layer manner by using a solution of a
nonaqueous polar organic solvent such as anhydrous
DMF. This process is favorable for the applications to a
variety of functional polyelectrolytes with poor solubility
in water.
Parts a and b of Figure 3 show UV-vis spectra of
PBANT-AC/PDAC multilayer films with the increasing
number of deposition cycles, using anhydrous DMF and
a DMF-H2O mixed solvent (DMF:H2O ) 2:1, v:v) as
solvents for the dipping solutions, respectively. A sig-
nificant blue shift of the maximum absorbance band can
be seen for the latter, which indicates a high degree of
H-aggregation existing in the multilayer films. Although
a linear increase of the maximum absorbance can be
observed as the number of bilayers increases for both
the anhydrous DMF dipping solution and the DMF-
H2O dipping solutions (Figure 4), the incremental
absorbance for each bilayer is considerably dependent
on the solvents. The solvent effect on multilayer forma-
tion is complicated. Lesser solvation of solution polymer
F igu r e 4. Relationship between the maximum absorbance of
the PBANT-AC/PDAC multilayers and the number of bilayers.
Concentrations of the azo polyelectrolytes in the dipping
solutions are all 0.2 mg/mL. The solvents of the dipping
solutions are anhydrous DMF (9), DMF:H2O ) 4:1 (v:v) (b),
and DMF:H2O ) 2:1 (v:v) ([).
segments would decrease intrapolymer segment repul-
sion, allowing more to accumulate at the surface.37
Decreasing the dielectric constant enhances the elec-
trostatic interaction between charges, which causes
charges on polymer chain to repel each other and leads
to chain expansion50 and at the same time makes the
solvation energy for the charged polyelectrolyte become
less favorable and drives polymer to the interface.37 For
polyelectrolytes bearing chromophores such as azo dyes,
the effect caused by changing solvent is even more
complicated. As discussed in above section, it may result

Macromolecules, Vol. 37, No. 1, 2004
Branched Side-Chain Polyelectrolytes 141
Ta ble 2. λm a x of (a ) P BANT-AC a n d (b) P BACT-AC in
Solu tion s, a s Mu ltila yer F ilm s a n d Sp in -Coa ted F ilm s
λmax (nm)
λmax (nm)
λmax (nm) (multilayer (spin-coated
(solution)
film)a
film)b
PBANT-AC
488
DMF
468
458
443
470
DMF:H2O ) 4:1 (v:v)
DMF:H2O ) 3:1 (v:v)
DMF:H2O ) 2.75:1 (v:v)
DMF:H2O ) 2:1 (v:v)
481
472
465
415
429
PBACT-AC
438
DMF
436
434
435
425
DMF:H2O ) 2:1 (v:v)
DMF:H2O ) 1:1 (v:v)
DMF:H2O ) 1:2 (v:v)
H2O
433
429
424
423
F igu r e 5. Relationship between the maximum absorbance of
the PBACT-AC/PDAC multilayers and the number of bilayers.
Concentrations of the azo polyelectrolytes in the dipping
solutions are all 0.2 mg/mL. The solvents of the dipping
solutions are anhydrous DMF (9) and DMF:H2O ) 2:1 (v:v)
([).
a
PBANT-AC/PDAC and PBACT-AC/PDAC multilayer films
were prepared by using the dipping solutions with the same DMF
b
to H2O ratios as listed in the first column. Spin-coated film was
prepared by using DMF solution.
thickness of the multilayer obtained by deposition in the
solution of PBANT-AC in DMF-H2O (v:v ) 2:1) mixed
solvent was about 4.2 nm, which is comparable with
those of weak polyelectrolyte multilayer films formed
in a low-pH solution.40,41 The averaged bilayer thickness
of PBACT-AC/PDAC formed from the DMF-H2O solu-
tion was also observed to be thicker than that of the
multilayer film formed from the DMF solution of the
azo polyelectrolytes. However, the difference of the
thickness caused by the solvent variation is not so
remarkable as those found in the PBANT-AC/PDAC
system. Even though this disparity cannot only be
attributed to the H-aggregation degree difference be-
tween both polyelectrolytes, by comparing with the
effect of DMF/H2O ratio on the H-aggregation degree,
it has reason to believe that the H-aggregation plays
an important role in the process. When PBANT-AC was
dissolved in DMF, a good solvent for azo chromophores,
the polymeric chains with a very low H-aggregation
extent would be expanded and form thinner layers after
adsorption. On the contrary, when PBANT-AC was
dissolved in a DMF-H2O mixture (v:v ) 2:1), as
mentioned above, a predominate amount of azo chro-
mophores formed H-aggregates. The resultant inter-
polymeric association and intrapolymeric aggregation
would result in conformation change in solution and
cause the averaged bilayer thickness of PBANT-AC/
PDAC to increase significantly. In some ways, the effect
is similar to that which causes the thickness variation
of weak polyelectrolyte multilayer films through adjust-
ing solution pH.40,41 For PBACT-AC, the solvent effect
on the layer thickness was observed to be much less
significant, as the H-aggregation degree was lower in
this system.
F igu r e 6. Ellipsometric thickness vs the number of bilayers
on silicon wafers prepared from different dipping solutions:
PBANT-AC dissolved in a DMF-H2O mixed solvent (DMF:
H2O ) 2:1, v:v) ([), PBANT-AC dissolved in anhydrous DMF
(9), PBACT-AC dissolved in a DMF-H2O mixed solvent (DMF:
H2O ) 1:1, v:v) (b), PBACT-AC dissolved in anhydrous DMF
(2). The concentrations of the polyelectrolytes in the solutions
are all 0.2 mg/mL.
in a significant amount of the H-aggregation in the
solutions, which will influence the multilayer formation
and organization through interpolymeric association
and intrapolymeric aggregation. This point cannot be
overlooked for polyelectrolytes such as PBANT-AC, for
which the H-aggregation degree is significantly depend-
ent on the solvents. For the PBACT-AC/PDAC system,
the incremental absorbance for each bilayer is also
increased when changing the solvent of dipping solution
from anhydrous DMF to DMF-H2O mixed solvents
(Figure 5). However, by comparing with the PBANT-
AC/PDAC system (Figure 4), the dependence on the
solvent variation is considerably less significant for the
PBACT-AC/PDAC system.
λmaxs of the multilayers obtained from different dip-
The thickness of the multilayers assembled on silicon
wafers, measured by ellipsometry, also exhibits a linear
increase with the number of adsorbed bilayers (Figure
6). The incremental rates are dependent on the solvents
of dipping solutions, which suggests that the thickness
of the bilayers containing the same azo polyelectrolyte
can be significantly adjusted by the ratio of DMF to H2O.
For PBANT-AC/PDAC multilayer formed by deposition
in the DMF solution of PBANT-AC, the average thick-
ness of each bilayer was measured to be 0.8 nm, which
is similar to that reported for the multilayers of strong
polyelectrolytes.39 On the contrary, the averaged bilayer
ping solutions are summarized in Table 2 in comparison
with those of the corresponding solutions. For PBANT-
AC, the λmax of the DMF solution is 488 nm while the
λmax of the multilayer film formed from the DMF
solution is 468 nm, which is similar to that of the spin-
coated film of PBANT-AC (470 nm). λmaxs of the multi-
layers prepared by using the PBANT-AC solutions of
DMF-H2O mixed solvents may appear at a much
shorter wavelength compared with the λmaxs of PBANT-
AC in the same solutions. For the case of DMF:H2O )
3:1 (v:v), a blue shift about 30 nm can be observed when
PBANT-AC chains transfer from the solution to the

142 Wang et al.
Macromolecules, Vol. 37, No. 1, 2004
into the PDAC water solution, a reduction of the trans
π-π* band intensity at 468 nm and a hypsochromic
shift of λmax (from 468 to 458 nm) were noticed for the
multilayers formed from the DMF dipping solution
(Figure 7a). This can be ascribed to the formation of
H-aggregates induced by the aqueous media, as it is
know that hydrophobic azobenzene groups tend to form
H-aggregates in aqueous media.42 It can also be seen
from the figure that the multilayers including more
layers show more significant blue shifts. Meanwhile, the
electrostatic interaction between the polycation (PDAC)
and polyanion (PBANT-AC) was proved to be strong
enough because no desorption was observed when the
film was rinsed with DMF, and the thickness of the
multilayer films kept increasing linearly after alter-
nately dipped in the DMF solutions. When PBANT-AC
multilayer films were formed by deposition in the
solutions of the DMF-H2O mixed solvents with a
relatively high degree of H-aggregation, no obvious
spectral variation was observed after following deposi-
tion in the PDAC water solution. Figure 7b shows the
UV-vis spectra of the PBANT-AC multilayer films
formed from a solution of the DMF-H2O (v:v ) 2:1)
mixed solvent, which do not show an obvious difference
before and after deposition in the PDAC water solution.
As it can be seen from the spectra, in this case, the
H-aggregation degree in the multilayer is predominately
high. Therefore, no further H-aggregation can be in-
duced by the aqueous medium.
The results demonstrate the different H-aggregation
behavior of the two polyelectrolytes. For PBACT-AC, the
H-aggregation degrees of azo chromophores in the
dipping solutions are relatively low and do not show
obvious variation when transferred from the dipping
solutions to multilayer films and after dipped in the
aqueous PDAC solutions. On the other hand, the H-
aggregation degrees of PBANT-AC in both solutions and
multilayer films are sensitive to the solvents and can
alter obviously during the adsorption process or even
after dipped in the aqueous PDAC solutions.
F igu r e 7. UV-vis spectra of the multilayers before and after
PDAC layer deposition: (a) the PBANT-AC layers were
adsorbed from a DMF dipping solution; (b) the PBANT-AC
layers were adsorbed from a DMF-H2O dipping solution
(DMF:H2O ) 2:1, v:v). The concentrations are 0.2 mg/mL for
both. The numbers on the figures are the sequential number
of the layers.
multilayer film through the adsorption process. The
absorption band shift of the solvatochromic effect caused
by changing surrounding medium of the chromophores
is much smeller than that.12 Such large shift of the λmax
is obviously related with the H-aggregation of the azo
chromophores. Therefore, the significant λmax blue shifts
of PBANT-AC, after transferred from the dipping solu-
tions to the multilayer films, indicate that some azoben-
zene chromophores not forming H-aggregates in solution
further aggregate during the adsorption of the PBANT-
AC. When DMF:H2O reaches 2:1 (v:v), a red shift of 14
nm can be seen after forming multilayer. As in the case
the H-aggregation degree in the solution was already
predominantly high, electrostatic adsorption caused
deaggregation to some extent. For PBACT-AC, the
maximum absorbance of the multilayer formed from the
DMF solution appears at 436 nm, which is similar to
that of the DMF solution (438 nm). When compared
with the corresponding solutions, no obvious blue shift
can be seen for the multilayer films formed by deposition
in the solutions of the DMF-H2O mixed solvents. A red
shift of 6 nm can be observed for PBACT-AC in the case
of DMF:H2O ) 1:1 (v:v).
The solvent effect can be further demonstrated by its
influence on the surface morphology. It was observed
that the surface roughness of the multilayer films also
significantly depended on the composition of the dipping
solutions in which azo polyelectrolytes were dissolved.
The AFM images for the PBANT-AC/PDAC multilayers
(20 bilayers) on quartz slides recorded in the tapping
mode are given in a 3-D view (Figure 8a,b). As it can be
seen from Figure 8a, the film surface of PBANT-AC/
PDAC obtained by deposition in the DMF solution of
PBANT-AC is relatively smooth with a small-scale
fluctuation on the surface. On the contrary, for the
multilayer obtained by deposition in the DMF-H2O
solution (DMF:H2O ) 2:1, v:v) of PBANT-AC, the
surface is rough and exhibits a “mount-and-valley”
morphology.
Azo Ch r om op h or e Or ien ta tion in Mu ltila yer s.
Orientation of the azobenzene chromophores in PBANT-
AC/PDAC and PBACT-AC/PDAC multilayer films was
estimated from the polarized UV-vis spectra obtained
at a 45° incident angle. The ratio Ap/As was used to
characterize the orientation, where Ap represents the
maximum absorbance of the multilayer film for p-
polarized light and As for s-polarized light.51,52 p-
Polarized light with the electric vector parallel to the
plane of incidence is sensitive to the out-of-plane
orientation, while s-polarized light with the electric
To further investigate the solvent effect on the H-
aggregation, the UV-vis spectra of the multilayers after
deposition in the PDAC water solutions were also
measured in each dipping cycle to compare with those
measured before the deposition. Figure 7 shows the
difference between the two groups of spectra. After the
substrate covered by the multilayer had been dipped

Macromolecules, Vol. 37, No. 1, 2004
Branched Side-Chain Polyelectrolytes 143
F igu r e 8. Tapping mode AFM images (2.0 µm × 2.0 µm) of the PBANT-AC/PDAC multilayer films on quartz slides (20 bilayers),
prepared by using dipping solutions with two different solvents: (a) anhydrous DMF, (b) DMF-H2O mixed solvent (DMF:H2O )
2:1, v:v).
Ta ble 3. Ap /As Va lu es of th e Azoben zen e Ch r om op h or es
multilayer films, chromophore orientation, and solvent
effect for the polyelectrolytes with branched side chains
bearing electron donor-acceptor type azobenzene chro-
in P BANT-AC/P DAC a n d P BACT-AC/P DAC Mu ltila yer
F ilm sa
multilayer films
solventsb
Ap/As
mophores. The most closely related results appeared in
previous publications are the study on the azo am-
phiphile and ionenes.26,27,29,49 It has been reported that
the spacer length of the main-chain azobenezene ionenes
and the chain length of the amphiphile influence the
aggregate state of azobenzene in aqueous solutions.29,49
The H-aggregate state of the azobenzene ionene in the
electrostatic multilayer films is dependent on the charge
densities of the counter-charged polyanions.26 The H-
aggregation degree of the azobenzene bolaamphiphile
in the electrostatic multilayer films is dramatically
affected by the charge densities of the counter-charged
polycations, in which poly(allylamine hydrochloride)
(PAH) with a high concentration of charged groups
causes a significantly higher degree of the H-aggrega-
tion than PDAC.27 The azobenzenes adopt a more tilted
alignment in the direction to the surface with increasing
the spacer length of main-chain azobenzene ionene.29
The current study demonstrates that the aggregate
state of the branched side-chain azo polyelectolytes in
solutions considerably depends on the terminal substi-
tutes on the azobenzenes and the type of solvent. Even
the same polycation (PDAC) is used in the process, the
H-aggregation degree of the azo polyelectrolytes in the
multilayer films is closely related with the substitutes
on the azobenzenes and can be significantly altered by
changing the solvents of the dipping solutions. The
orientation of the azobenzene chromophores in the
multilayer films can also be adjusted by the variation
of solvent for the dipping solutions. This understanding
can be used to controllably alter the H-aggregation
PBANT-AC/PDAC
DMF
0.77
0.86
0.95
0.80
0.89
0.95
DMF:H2O ) 4:1 (v:v)
DMF:H2O ) 2:1 (v:v)
DMF
DMF:H2O ) 2:1 (v:v)
DMF:H2O ) 1:1 (v:v)
PBACT-AC/PDAC
a
Ap is the maximum absorbance of the film for p-polarized light,
and As is the maximum absorbance of the film for s-polarized light.
b
In which the azo polyelectrolytes were dissolved with concentra-
tions of 0.2 mg/mL.
vector perpendicular to the plane of incidence is rela-
tively inert to it.51 Ap/As ) 1 represents an azimuthally
isotropic orientational distribution of the azobenzene
units, while Ap/As < 1 or Ap/As > 1 means that the azo
chromophores tend to bear in-plane or out-of-plane
orientation.52 In Table 3, the ratios of Ap/As of some
representative samples are listed. For PBANT-AC/
PDAC multilayer prepared by adsorbing in the DMF
solution, the ratio Ap/As is 0.77 and indicates the
tendency of chromophore in-plane orientation. While for
the films formed by using DMF-H2O dipping solutions,
the Ap/As ratio increases as the amount of H2O increases
and reaches a value of 0.95 at DMF:H2O ) 2:1 (v:v),
which shows isotropic chromophore orientation in the
multilayer films. A similar tendency can be seen for the
PBACT-AC/PDAC multilayers (Table 3).
Com p a r ison w ith Oth er Electr osta tic Mu ltila yer
Film s Con tain in g Azo Ch r om oph or es. To our knowl-
edge, no directly comparable results have been pub-
lished about the H-aggregate state in both solutions and

144 Wang et al.
Macromolecules, Vol. 37, No. 1, 2004
F igu r e 9. UV-vis spectra of PBANT-AC/PDAC multilayer
films (20 bilayers): curve a, perpendicular to the polarization
direction of laser beam; curve b, parallel to the polarization
direction of laser beam. The samples were prepared by using
an anhydrous DMF dipping solution of PBANT-AC and
measured after irradiation with linearly polarized Ar+ laser
beam for 20 min.
Ta ble 4. Or ien ta tion Or d er P a r a m eter (S) of th e
Azoben zen e Ch r om op h or es in P BANT-AC/P DAC a n d
P BACT-AC/P DAC Mu ltila yer F ilm s a fter Ir r a d ia tion w ith
a P ola r ized Ar + La ser Bea m for 20 m in
samples
S
PBANT-AC/PDAC multilayer filma
PBANT spin-coated filmb
0.09
0.11
0.03
0.09
PBACT-AC/PDAC multilayer filma
PBACT spin-coated filmb
a
The azo polyelectrolytes were dissolved in anhydrous DMF as
b
the dipping solutions with concentrations of 0.2 mg/mL. Spin-
coated films with thickness about 100 nm were prepared by using
DMF solutions.
F igu r e 10. AFM image of the surface modulation: (a) a
PBACT-AC/PDAC multilayer film (20 bilayers) prepared by
deposition in an anhydrous DMF dipping solution; (b) a spin-
coated 1 µm thick film of PBACT-AC.
degree, chromophore orientation, and even surface
morphology of the azo polyelectrolyte multilayer films
through adjusting polymeric architecture and solution
conditions.
of azo chromophores and the cationic group in PDAC,
which can inhibit the azo chromophores from free
rotation.
P h otoin d u ced An isotr op y a n d Su r fa ce Relief
Gr a tin gs. As the multilayer films prepared from the
DMF dipping solutions had relatively smooth surfaces
and showed little H-aggregation, the films were used
for the photoresponsive tests. To study the photoinduced
anisotropy, the self-assembled multilayer films were
irradiated with linearly polarized Ar+ laser beam at 488
nm. In-plane dichroism was induced in the film as
indicated by the polarized UV-vis spectroscopy (Figure
9). The film was exposed to the laser beam for 20 min
to guarantee that the photoinduced chromophore ori-
entation had been saturated. Figure 9 shows that the
absorbance in the direction of the polarization of the
laser beam is smaller than that in the direction per-
pendicular to the polarization of the laser beam. From
the dichroic ratio, an orientation order parameter of ca.
0.09 can be estimated.53 The orientation order param-
eters induced by the linearly polarized light for different
multilayer films and spin-coated films (as the control
samples) are listed in Table 4. For PBANT-AC/PDAC
multilayer, the photoinduced orientation order of the
film is comparable with that of the spin-coated film,
while for PBACT-AC/PDAC film the photoinduced ori-
entation order is much less than that of the spin-coated
film. It is believed that the decreased capability for the
photoinduced orientation of PBACT-AC/PDAC multi-
layer film is related with the electrostatic interaction
between the ionized carboxyl group (COO-) at the end
Figure 10a shows an AFM image of the 20-bilayer
PBACT-AC/PDAC multilayer film surface after exposing
the sample to an interference pattern of Ar+ laser beams
at modest intensities (150 mW/cm2). Under similar
conditions, surface modifications were also observed for
the PBANT-AC/PDAC multilayer films. Figure 10b
shows a typical surface-relief-grating (SRG) image for
a spin-coated film of PBACT-AC with a modulation
depth of 85 nm. The sinusoidally modulated surface
structure on the azo polyelectrolyte film was observed
after Ar+ laser beam irradiation with the same intensity
and irradiation time as the multilayer film. Compared
with the AFM image of SRG on the spin-coated film of
PBACT-AC, the surface modulation on the multilayer
surface is relatively small and overlapped with the
random structures on the surface. Photofabrication of
SRGs has been extensively investigated for spin-coated
azo polymer films.8-18 The earlier attempt to fabricate
SRGs on electrostatic layer-by-layer multilayer films of
polyelectrolytes containing azo chromophores was not
very successful.12 Since the electrostatic attraction
between layers exists in the multilayer films, the light-
driven mass transport process is much more difficult
to be achieved.33,34 Improved results have been pre-
sented for electrostatic multilayer films containing low-
molecular-weight azo dye and electrostatic multilayers
obtained through layer-by-layer post-azo-coupling reac-
tion.33,34 Current result shows another improved ex-

Macromolecules, Vol. 37, No. 1, 2004
Branched Side-Chain Polyelectrolytes 145
ample for direct photofabrication of SRGs on sequen-
tially adsorbed multilayer films of azo polyelectrolytes.
Even the quality of SRGs is still not as good as spin-
coated films, the electrostatic layer-by-layer self-as-
sembly method can offer an easy way to control the
thickness of thin film and a possible new route to
explore the mechanism of SRG formation.
of the same azo polyelectrolyte. Optically induced
surface modulation on the multilayer surfaces of both
polyelectrolytes was observed upon exposure to an
interference pattern of Ar+ laser beam at modest
intensities.
Results in this work show that it is possible to prepare
photoresponsive multilayers with controllable thickness,
organization, and required properties through a proper
molecular design and by selecting suitable solution
conditions. It is believed that the incorporation of
donor-acceptor type azo chromophores into electrostatic
multilayers through the controllable manner can finally
extend the sequentially adsorbed technology to some
new optical applications.
Con clu sion s
Two polyelectrolytes (PBANT-AC and PBACT-AC),
functionalized with branched side chains bearing elec-
tron donor-acceptor type azobenzene chromophores,
were synthesized and characterized by using spectro-
scopic methods and thermal analysis. H-aggregation
extent of the azo chromophores of PBANT-AC and
PBACT-AC in solutions of anhydrous DMF and DMF-
H2O mixed solvents was related with the solvent
composition and chromophore structure. Adjusting the
ratio of DMF to H2O could considerably alter the
H-aggregation degree of PBANT-AC. When the ratio
reached 2:1, the absorption band was predominated by
the absorption of the H-aggregates. The ionizable car-
boxylic acid groups on the azo chromophores of PBACT-
AC hindered the azo chromophores from forming card-
packed H-aggregates, which caused a significantly lower
H-aggregation degree than those observed for PBANT-
AC under the same conditions.
Ack n ow led gm en t. The financial support from the
NSFC under Project 59925309 is gratefully acknowl-
edged.
Su p p or tin g In for m a tion Ava ila ble: 13C NMR spectra of
the precursor polymer PBA-AC and the azo polyelectrolytes
PBANT-AC and PBACT-AC. This material is available free
of charge via the Internet at http://pubs.acs.org.
Refer en ces a n d Notes
(1) Xie, S.; Natansohn, A.; Rochon, P. Chem. Mater. 1993, 5,
403-411.
(2) Delaire, J . A.; Nakatani, K. Chem. Rev. 2000, 100, 1817-
1845.
Both azo polyelectrolytes could form uniform multi-
layer films through a layer-by-layer adsorption process
by deposition in solutions of anhydrous DMF or DMF-
H2O mixed solvents together with an alternate deposi-
tion in a PDAC aqueous solution. Although the thick-
ness of the multilayer films all increased linearly with
the dipping cycles, the incremental rates were found to
be considerably dependent on the water content in the
dipping solutions. The multilayer films of PBANT-AC/
PDAC formed from the DMF-H2O solutions showed
significant azo chromophore H-aggregation. The H-
aggregate state of PBANT-AC in the multilayer films
was closely related with the H-aggregation extent in the
dipping solutions, which could be adjusted by the change
of water content in the solutions. Except for a dipping
solution with a high water content such as DMF:H2O
) 2:1 (v:v), the H-aggregation was enhanced when the
polyelectrolyte molecules were transferred from the
dipping solutions to the multilayer films through ad-
sorption process. For PBANT-AC/PDAC multilayers
formed from a DMF solution, the H-aggregation was
also be enhanced when the multilayer was dipped in
the aqueous PDAC solution. For PBACT-AC, the H-
aggregation degrees of azo chromophores in the multi-
layer films were relatively low. No obvious variation was
observed after the adsorption of PBACT-AC from aque-
ous dipping solutions or sequentially dipped in the
aqueous PDAC solutions. The orientations of azo chro-
mophores in the multilayer films and the surface
roughness of the multilayer films of both polyelectro-
lytes were also related with the ratio of DMF to H2O in
the dipping solutions of the azo polyelectrolytes. Bire-
fringence and dichroism, optically induced under the
irradiation of a polarized Ar+ laser beam at 488 nm,
were observed for the multilayer films of both polyelec-
trolytes. The resulting dichroism detected from UV-
vis spectra indicated that a significant chromophore
orientation was photoinduced in the PBANT-AC/PDAC
multilayer films formed from the DMF dipping solution,
which is comparable with that of the spin-coated film
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