Colloidal sphere formation, H-aggregation, and photoresponsive properties of an amphiphilic random copolymer bearing branched Azo side chains

Article abstract of DOI:10.1021/ma061335p

This work studied the colloidal sphere formation, H-aggregation, and photoresponsive properties of an amphiphilic random copolymer functionalized with branched azo side chains (POAPB₆P-AC). The colloidal spheres were prepared through gradual hy

Full text of DOI:10.1021/ma061335p

6590
Macromolecules 2006, 39, 6590-6598
Colloidal Sphere Formation, H-Aggregation, and Photoresponsive
Properties of an Amphiphilic Random Copolymer Bearing Branched
Azo Side Chains
Yonghong Deng, Yaobang Li, and Xiaogong Wang*
Department of Chemical Engineering, Laboratory for AdVanced Materials, Tsinghua UniVersity,
Beijing, P. R. China 100084
ReceiVed June 15, 2006; ReVised Manuscript ReceiVed July 21, 2006
ABSTRACT: This work studied the colloidal sphere formation, H-aggregation, and photoresponsive properties
of an amphiphilic random copolymer functionalized with branched azo side chains (POAPB6P-AC). The colloidal
spheres were prepared through gradual hydrophobic association of the polymer chains in THF-H2O media, which
was induced by a continuous increase in the water content. The forming process and morphology of the colloidal
spheres were characterized by DLS, SLS, and TEM. UV-vis spectroscopy was used to study the azo chromophore
H-aggregation and structure variation in the colloid formation process by measuring the parameters such as λmax
,
the trans-to-cis photoisomerization rate, and the isomerization degree at the photostationary state. The results
indicate that the colloidal sphere formation undergoes several different stages as the water content increases. The
polymeric chains start to associate at the critical water content (CWC). CWC is related with the initial concentration
of the polymer in THF and is estimated to be 23 vol % when the initial concentration is 1.0 mg/mL. When the
water content increases beyond the CWC, more and more polymer chains change from “isolated” single chains
to clusters of associated chains. When the water content reaches 37%, almost all polymer chains are involved in
the clusters. As the water content further increases, the clusters experience a collapse caused by the strong
hydrophobic interaction, which is indicated by a dramatic size contraction detected from the DLS measurements.
When the water content reaches about 60 vol %, the azo chromophores of POAPB6P-AC start to form H-aggregates,
which is indicated by a significant blue shift (from 360 to 339 nm) of the UV-vis maximum absorption band.
The structure variation can also be detected from the photoisomerization behavior of the systems. When the
water content changes from 50 to 60 vol %, both the trans-to-cis photoisomerization rate and the isomerization
degree at the photostationary state decrease dramatically. Finally, when the water content increases from 95 to
100 vol %, the isomerization rate and degree significantly decrease again. In this stage, THF in the microphase
is almost completely replaced by H2O, and colloidal structure is “frozen” because of the free volume decrease
and chain entanglement. The above observations give insight into the occurrence of H-aggregation in colloids of
amphiphilic azo polymers for the first time. The understanding can lead to further development in expanding
functionalities of the photoresponsive colloidal spheres through manipulation of the chromophore aggregation.
1. Introduction
exhibit interesting properties such as significant second-order
optical nonlinearity without electric field poling,15-17 photo-
chromic effect and photoinduced wettability change.19,20 Photo-
deaggregatable micelles have been prepared through intermo-
lecular association of a block azo copolymer in the selective
solvent.21 To further explore those photoresponsive materials
through self-assembling approaches, a better understanding of
the different interactions existing in the systems and their
influence on the structure formation is required.
Polymers containing aromatic azo chromophores (azo poly-
mers for short) have received considerable attention in recent
years.1-4 Upon light irradiation, azo polymers can show a variety
of structure and property variations triggered by the trans-cis
photoisomerization of the azo chromophores. For instance,
depending on the polymeric architectures and condensation
states, the photoresponsive variations can behave as phase
transition,5 anisotropic chromophore orientation,6 surface relief
gratings,7,8 and film contraction or bending.9-11 Polymers with
such properties are promising in the applications such as
photoswitching, optical data-storage, sensors, light-driven reac-
tors, and artificial muscles.10,12-14 Recent research has shown
that, by incorporation of ionizable groups in the structural units
or segments, those interesting properties of azo polymers can
be further extended to new areas of material design and
preparation.15-21 The photoresponsive materials with nano/
micrometer dimensions can be prepared through methods such
as electrostatic layer-by-layer (ELBL) deposition and hydro-
philic-and-hydrophobic association in selective solvents.22,23
Electrostatic multilayer films have been prepared through
sequential deposition in solutions of an azo polyelectrolyte and
an opposite-charged polyion.15-17,19,20 The multilayer films can
Besides the electrostatic and hydrophobic interactions gener-
ally observed in ELBL multilayers and micelle-like aggregates,
there is a unique type of strong van der Waals interaction
existing between conjugated planar structures.24 The π-π
interaction can cause the organic compounds possessing such
structures to form molecular aggregates with close intermo-
lecular spacing and strong coupling. Early studies indicated that
ionic dyes could form different dye-dye aggregates and show
metachromacy in aqueous or mixed aqueous-organic solu-
tions.25,26 Because of those early stage investigations, the π-π
aggregation has been named according to the relative position
of the aggregate absorption band to the molecular absorption
band (M-band). H-aggregation denotes the aggregation showing
a blue-shifted band (hypsochromic band or H-band) to the
M-band and J-aggregation (after Jelley and Scheibe) refers to
the aggregation exhibiting a red-shifted band and resonance
* Corresponding author: e-mail wxg-dce@mail.tsinghua.edu.cn.
10.1021/ma061335p CCC: $33.50 © 2006 American Chemical Society
Published on Web 08/23/2006

Macromolecules, Vol. 39, No. 19, 2006
Amphiphilic Random Copolymers 6591
fluorescence.25,26 The large spectral shifts and distinct changes
in band shapes can be rationalized on the basis of the exciton
model and screening model.27,28 As the dye-dye aggregation
is accompanied by significant color and luminescence variations,
the aggregation and its effect have been intensively investigated
in the fields of photographic science and engineering,25,26 textile
coloring,29 biophotonics,30 and linear and nonlinear optics.31 Azo
chromophores can form π-π aggregates under different condi-
tions. As the their transition dipole moments are usually parallel
to the long axis of the molecules, azo chromophores take
H-aggregation as the typical aggregation manner.29,32-35 H-
aggregation of azo chromophores has been observed in systems
such as monolayers, bilayers, LB films, membranes, and
vesicles.32 H-aggregation of azobenzene phospholipids in bi-
layers has been studied and used to prepare photoregulatable
membrane and related materials.33 On the other hand, it has
been reported that H-aggregation cannot be observed in spherical
micelles containing azobenzene surfactants because the packing
is too loose and the chromophores cannot adopt the required
configuration in the micelles.34 For ELBL self-assembly of
bolaamphiphiles or polyelectrolytes containing azobenzene units,
H-aggregation is often observed in the dipping solutions.35
the basis of the polymer, the colloidal spheres were prepared
and the formation of the colloids was characterized by the static
light scattering (SLS), dynamic light scattering (DLS), and
transmission electron microscopy (TEM). H-aggregation of azo
chromophores occurring in the colloid formation process was
investigated by UV-vis spectroscopy. The isomerization be-
havior was studied and used as a probe to detect the microen-
vironmental variation in the process. Results indicate that as
the water content increases, the colloidal sphere formation
undergoes several different stages such as polymeric chain
association, the cluster collapse, and the structure “freeze”. In
the process, only when the water content reaches a relatively
high value and the clusters undergo a significant size contraction
do the azo chromophores of POAPB6P-AC start to form
H-aggregates. The H-aggregation is closely related with the
structure variation occurring during the colloid formation process
and shows a significant influence on the photoisomerization
behavior of the azo chromophores.
2. Experimental Section
Characterization. 1H NMR spectra were obtained on both
Bruker AM-500 (500 MHz for proton) and Varian Unity-200 (200
MHz for proton) FT-NMR spectrometers. IR spectra were deter-
mined on a Nicolet 560-IR FT-IR spectrophotometer by incorporat-
ing the powder samples in KBr disks. Elemental analyses (EA) of
C, H, and N were measured by the Heratus CHN-Rapid method.
The melting points of azo compounds and Tg of the polymer were
determined with a TA Instruments DSC 2910 at a heating rate of
10 °C/min. The UV-vis spectra of the samples were recorded by
using an Agilent 8453 UV-vis spectrophotometer. The molecular
weights and their distributions of the polymers were determined
by gel permeation chromatography (GPC) utilizing a Waters model
515 pump and a model 2410 differential refractometer with three
Styragel columns HT2, HT3, and HT4 connected in a serial fashion.
THF was used as the eluent at a flow rate of 1.0 mL/min.
Polystyrene standards with dispersity of 1.08-1.12 obtained from
Waters were employed to calibrate the instrument. TEM images
of the colloidal spheres were obtained by using a JEOL-JEM-
1200EX electron microscope with an accelerating voltage of 120
kV. The TEM samples were prepared by dropping diluted sphere
dispersions onto the copper grids coated with a thin polymer film
and then dried in a 30 °C vacuum oven for 24 h. No staining
treatment was performed for the measurements. Static light scat-
tering (SLS) and dynamic light scattering (DLS) experiments were
performed on a DLS-7000 multiangle laser photometer (OTSUKA
Instruments Corp.) equipped with a He-Ne laser (632.8 nm). The
derivative refractive index increment value (dn/dC) was measured
by an OTSUKA Rm-102 differential refractometer. The four
solutions with different concentrations used in Zimm plot processing
were obtained by dilution of a more concentrated original solution.
To ensure that SLS and DLS measurements were not affected by
dust, the stock solutions were filtered twice through membrane
filters with normal pore size of 0.45 µm, transferred to dust-free
glassware, and then diluted with the filtered solvents. All the
experiments were performed at 25 °C over a range of concentrations
and detection angles. The size and size distribution of the colloidal
spheres were also measured with a Marvern Zetasizer 3000 dynamic
light scattering instrument equipped with a multi-τ digital time
correlator and a 632.8 nm solid-state laser light source. The
scattering angle used for the measurement was 90°, and the sample
temperature was controlled to be 25 °C.
In our previous works, a series of amphiphilic azo homopoly-
mers and random copolymers were synthesized and used to
construct the uniform photoresponsive colloidal spheres.36-39
The colloidal spheres were prepared through gradual hydro-
phobic aggregation of the polymeric chains in mixed aqueous-
organic dispersion media, induced by a continuous increase in
the water content. The colloidal spheres possess some interesting
properties such as shape deformation and dichroism upon light
irradiation.36,37 The colloidal spheres can be used to prepare
two-dimensional colloidal arrays and ordered mesoporous films
through in-situ structure inversion.37,38 The photoisomerization
of azo chromophores in the colloids depends on the preparation
conditions, which influence the structure of the colloidal
spheres.39 In another separate study, a polyelectrolyte function-
alized with branched side chains bearing the pseudo-stilbene-
type azo chromophores was used as the polyanion to build up
multilayer films through ELBL adsorption by using poly-
(diallyldimethylammonium chloride) (PDAC) as the polycat-
ion.40 During this process, H-aggregation of azo chromophores
was detected to occur both in the dipping solutions and in the
corresponding multilayers. When the H-aggregation degree was
adjusted by the ratio of the organic solvent to water in the
dipping solutions, the structure and properties of the ELBL
multilayers could be significantly modified by the solution
conditions. Although it has been known that H-aggregation can
cause the intermolecular and intramolecular association in
aqueous solutions, its effect on the colloidal formation and
properties has not been studied in those early phase studies.
Therefore, it is unclear whether H-aggregation can play an
important role to influence the structure and properties of those
photoresponsive colloids.
In the current work, a new poly(acrylic acid)-based am-
phiphilic polymer (POAPB6P-AC) was synthesized and used
to assemble photoresponsive colloidal spheres through the
gradual hydrophobic aggregation scheme. POAPB6P-AC was
designed to have branched side chains bearing the azobenzene-
type chromophores, which can form H-aggregates in mixed
aqueous-organic solvents. As the azobenzene-type chro-
mophores have a relatively stable cis form, the photoisomer-
ization can be feasibly monitored by UV-vis spectroscopy.41
The measured isomerization rate and degree are closely related
with the microenvironment around the chromophores.1,39,41 On
Materials. Analytical pure tetrahydrofuran (THF) from com-
mercial source was refluxed with cuprous chloride and distilled
for dehydration before use. Deionized water (resistivity >18 MΩ‚
cm) was obtained from a Millipore water purification system and
used for the following experiments. Other reagents and solvents
were used as received without further purification.

6592 Deng et al.
Macromolecules, Vol. 39, No. 19, 2006
Figure 1. Chemical structure of POAPB6P-AC.
Synthesis of POAPB6P-AC. The chemical structure of POAPB6P-
AC is given in Figure 1. POAPB6P-AC is a random copolymer
composed of hydrophilic ionizable carboxyl groups and hydropho-
bic branched azo side chains. POAPB6P-AC was prepared by the
Schotten-Baumann reaction between poly(acryloyl chloride) (PAC)
and 3,5-bis{6-[4-(4′-ethoxyphenylazo)phenoxy]hexyloxy}benzyl
alcohol (OAPB6P). After the reaction, the unreacted acyl chloride
groups were hydrolyzed to obtain the carboxyl groups. PAC was
prepared by the radical polymerization of acryloyl chloride as
reported in our previous paper.42 The scheme for synthesis of
OAPB6P and POAPB6P-AC is given in Scheme S1 (in the
Supporting Information). The analytical data and spectra of products
of each step reaction for OAPB6P synthesis are also given in the
Supporting Information. The reaction between PAC and OAPB6P
was carried out according to the conditions reported previously.42
As it was difficult to directly measure the molecular weight and
its distribution of PAC due to the high reactivity of acyl chloride
groups, a gel permeation chromatography (GPC) measurement was
carried out on a poly(methyl acrylate) sample that was prepared
by the reaction between the same-batch synthesized PAC and excess
methanol.42 The number-average molecular weight of PAC esti-
mated by the result of the poly(methyl acrylate) sample was 21 600
with a polydispersity index of 1.53. The degree of functionalization
(DF) of POAPB6P-AC is defined as the percentage of the structure
units bearing azo chromophores among the total structure units.
DF was controlled by selecting a suitable feed ratio between PAC
and OAPB6P. For POAPB6P-AC sample used in this work, DF
estimated by the elemental analysis was 14.5%. The weight-average
molecular mass (Mw,m) of the POAPB6P-AC sample was obtained
from the static light scattering measurement. The dn/dC value of
POAPB6P-AC in THF solution was measured to be 0.165, and Mw,m
was estimated to be 100 900 from the Zimm plot. The other
Figure 2. Scattering light intensity as a function of the water content
(vol %) in the THF-H2O dispersion media. The initial concentration
of POAPB6P-AC in THF was 1.0 mg/mL.
cm. The samples were placed at ca. 15 cm away from the lamp.
The surrounding temperature of the samples was controlled to be
about 30 °C by a cold plate. The UV-vis spectra of the samples
were measured over different irradiation time intervals by using
an Agilent 8453 UV-vis spectrophotometer. To study the effect
of the water content on photoresponsive behavior, the POAPB6P-
AC solutions or dispersions with different water contents were
irradiated with the 365 nm UV light, and the UV-vis spectra were
recorded over different time intervals until the photostationary states
were achieved. For measuring the thermal cis-to-trans isomerization,
the samples were kept in a dark oven with constant temperature
(30 ( 1 °C), and the UV-vis spectra were recorded over different
time intervals.
3. Results and Discussion
1
analytical data of POAPB6P-AC are given as follows. H NMR
The chemical structure of POAPB6P-AC is given in Figure
1. POAPB6P-AC is an amphiphilic random copolymer com-
posed of hydrophilic acrylic acid units and hydrophobic
branched azo side-chain units. The degree of functionalization
(DF) is 14.5% for the sample used in this work. Because of the
preparation method, POAPB6P-AC possesses polydispersity in
composition, sequence distribution, and molecular weight. The
polymer could be dissolved in anhydrous THF to form
homogeneous solutions with different concentrations and could
form colloidal spheres through a stepwise buildup process when
water was gradually added into the solutions. The details of
the colloid formation and its relationship with H-aggregation
and photoresponsive properties will be presented and discussed
in the following sections.
3.1. Critical Water Content (CWC) for Chain Association.
When water was gradually added into the THF solutions,
POAPB6P-AC chains started to associate with each other in the
solutions due to the hydrophobic interaction, which was reflected
in an abrupt increase in the scattered light intensity. Like the
study of amphiphilic block copolymers,43 the critical water
content (CWC) is defined as the water content at which the
polymer chains start to associate. CWC was obtained from the
turning-up point on the plots of the light scattering intensity vs
the water content in the medium. Figure 2 presents a typical
plot, where the initial concentration of POAPB6P-AC in THF
is 1.0 mg/mL. The diagram shows that a jump in scattered light
intensity occurs at the water content of 23 vol %, which
corresponds to the critical water content of the POAPB6P-AC
solution.
(DMSO-d6), δ: 12.40, 7.80, 7.00, 6.60, 6.45, 4.97, 4.32, 3.99, 2.25,
1.81, 1.35. IR (KBr, cm-1): 3600-3200 (COOH, br), 2921 and
2861 (CH2, str), 1725 (CdO, str), 1600 1580 1500 (benzene ring,
str), 1250 (C-O, str). EA: 4.42 (N), 65.33 (C); 6.55 (H); Tg: 154
°C; UV-vis (THF): λmax ) 360 nm.
Sample Preparation. Suitable amounts of POAPB6P-AC were
dissolved in THF to obtain solutions with different initial concentra-
tions (0.03-1.0 mg/mL). The initial concentrations mentioned in
the following text refer to the initial concentrations of POAPB6P-
AC in THF. An initial concentration of 1.0 mg/mL was used for
most experiments in this work, except those specifically designed
to study the effect of the initial concentration. The solutions were
kept stirring for 20 min and then put aside for 72 h. To obtain
solutions or dispersions of POAPB6P-AC in THF-H2O media with
different water contents, the required amounts of water were added
into the stirred THF solutions at a rate of 5 µL/s. After the water
addition was completed, the solutions or dispersions were left to
equilibrate for at least 24 h. The light scattering measurement was
preformed on the solutions or dispersions to determine the
parameters such as the critical water content (CWC) and the
hydrodynamic radius (Rh). For preparation of the stable colloidal
dispersions, water was added into the THF solution in a similar
manner as mentioned above until the water content reached 70 vol
%, and then the dispersion was “quenched” by adding excess water
and dialyzed against water for 72 h to remove THF.
Photoisomerization Study. The photoisomerization of the azo
chromophores was induced by irradiation with UV light, which
was from a high-intensity 365 nm UV lamp equipped with 12.7
cm diameter filter (Cole-Parmer L-97600-05 long wave UV lamp,
U-09819-23 filter). The light intensity of the lamp was 7000 mW/
cm2 at a distance of 38 cm and 21 000 mW/cm2 at a distance of 5

Macromolecules, Vol. 39, No. 19, 2006
Amphiphilic Random Copolymers 6593
Figure 3. Plot of the critical water content (CWC) vs logarithm of
the initial concentration (C0) of POAPB6P-AC in THF.
Figure 4. Fraction of the associated chains as a function of the water
increment beyond CWC. Arrow indicates the water content where
99.5% chains are involved in the clusters.
For amphiphilic block copolymers, CWC depends on both
the initial polymer concentration and the molecular weight of
the polymer.43a The higher the initial polymer concentration or
the molecular weight, the lower is the CWC value. A similar
tendency has been observed for amphiphilic random azo
copolymers.37,39 For POAPB6P-AC, the same tendency was also
observed (Figure 3). A linear inverse proportion relationship
exists between CWC and the logarithm of the initial polymer
concentration (C0).
3.2. Influence of the Water Content on Chain Association
Fraction. When the water content increases beyond CWC, more
and more polymer chains associate to form clusters, and
consequently the concentration of “isolated” single chains in
the solution decreases gradually. The change of the fraction of
the associated chains as a function of the water content can be
estimated by using the method proposed by Eisenberg et al. to
calculate the micelle fraction of amphiphilic block copolymers.43a
As shown in Figure 3, the relationship between CWC and
the initial polymer concentration C0 can be expressed as
Figure 5. Relationship between the average hydrodynamic radius (Rh)
of the structures formed in the H2O-THF media and the water content.
The initial concentration of POAPB6P-AC in THF was 1.0 mg/mL.
The constant A, whose value depends on specific polymer
and organic solvent, can be obtained from the slope of the plot
of CWC vs log C0 (Figure 3). By using eq 4, the change of
(C0 - Cunass)/C0 as a function of the H2O increment can be
calculated. A plot of (C0 - Cunass)/C0 against the H2O increment
is shown in Figure 4, where the initial polymer concentration
is 1.0 mg/mL and CWC is 23 vol %. When ∆H2O is 14 vol %
(i.e., the water content is 37%), the percentage of polymer chains
involved in the clusters of the associated chains is calculated
to be 99.5%. Therefore, over the water content, the amount of
the “isolated” single chains in the solution is negligible.
Comparing with amphiphilic block copolymer,43a ∆H2O is much
larger, which indicates that as the water content increases, the
transition of POAPB6P-AC molecules from “isolated” single-
chains to associated chains is a gradual process.
3.3. Influence of the Water Content on Colloid Formation.
When the water content further increases, the POAPB6P-AC
chains can finally form colloidal spheres in the dispersions. The
colloid formation through chain association is a complicated
process. In the process, the polymer chains undergo transition
from dilute polymer solution, through semidilute state, to the
densely packed state before colloid formation. Although it is
difficult to specify the structural details in the colloid formation
process, light scattering study can give some useful information
about the transition process. For consistency, the initial polymer
concentration of 1.0 mg/mL is used as a typical condition for
all the following discussions. Figure 5 shows the hydrodynamic
radius (Rh) of the structures forming in the dispersions with
different water contents, which was measured by dynamic light
scattering (DLS). When the water content increases in the range
from 23 to 37 vol %, Rh correspondingly increases with the
water content increase because more and more chains associate
to form the clusters. When the water content increases from 37
CWC ) -A log C0 + B
(1)
(2)
C0 ) exp[2.303(B - CWC)/A]
where A and B are two constants and need to be determined for
a specific polymer system. At the critical water content, the
polymer concentration represents essentially the critical polymer
concentration (CPC) for colloid formation, which depends on
the molecular weight of the polymer and the water content. For
POAPB6P-AC with a specific molecular weight, the higher the
water content, the lower is CPC. CPC mentioned here also
corresponds to the concentration of the unassociated polymer
chains in the solution (Cunass). When the added water increases
gradually, more and more polymer chains form clusters of
associated chains. However, the concentration of the unasso-
ciated polymer chains still obeys a relationship similar to eq 1.
Therefore, a similar equation as eq 2 can be obtained
Cunass ) exp[2.303(B - Cw)/A]
(3)
where Cw represents the water content (vol %) in the solution.
The fraction of the associated polymeric chains among the
total can be calculated by (C0 - Cunass)/C0. By combining eqs
2 and 3, the following relationship can be obtained:
(C0 - Cunass)/C0 ) 1 - exp[2.303(CWC - Cw)/A]
) 1 - exp(-2.303∆H2O/A)
(4)
where ∆H2O represents the increment of added water above
CWC, which is also given as the volume percentage.

6594 Deng et al.
Macromolecules, Vol. 39, No. 19, 2006
Figure 7. Distribution of the hydrodynamic radius (Rh) of the
POAPB6P-AC colloidal spheres in the aqueous dispersion. The initial
polymer concentration in THF was 1.0 mg/mL.
Figure 6. Typical TEM image of the colloidal spheres obtained from
the aqueous dispersion. The initial concentration of POAPB6P-AC in
THF was 1.0 mg/mL.
to 60 vol %, Rh decreases dramatically, which corresponds to
the cluster collapse induced by hydrophobic interaction. When
the water content increase over 60 vol %, Rh starts to decrease
slowly as the water content further increases. After this stage,
SEM and TEM observations indicate that uniform colloidal
spheres can be obtained by “quenching” the structures in
dispersions with excess water. The 60% water content is a
turning point. Before this point, the structures formed in the
dispersions are believed to be loose clusters having irregular
shape and fracton dimensionality. Therefore, when the water
content is in the range from 23 to 60 vol %, the Rh estimated
from DLS by using the Stokes-Einstein relation can only be
treated as semiquantitative values. The spectroscopic study given
in the following sections can further confirm that the structures
forming in the water content range have characteristics of loose
clusters.
Figure 8. Static Zimm plot of the POAPB6P-AC colloid spheres in
the water dispersion. The spheres were prepared from a THF solution
with an initial polymer concentration of 1.0 mg/mL. The four
concentrations of the POAPB6P-AC colloidal dispersions were C1 )
4 × 10-6 g/mL, C2 ) 8 × 10-6 g/mL, C3 ) 1.2 × 10-5 g/mL, and
C4 ) 1.6 × 10-5 g/mL.
3.4. Characterization of Colloidal Spheres. When the water
contents are higher than 60%, stable aqueous dispersions of
POAPB6P-AC colloids can be obtained by adding an excessive
amount of water to “quench” the structures formed in the
dispersions and then dialyzing against water to remove THF.
Figure 6 shows a typical TEM image of the colloids separated
from the stable colloid dispersion. It shows that uniform colloidal
spheres form under the condition. The average size of the
spheres was estimated to be 120 nm, obtained from the statistics
of 100 contiguous spheres in the TEM images.
The DLS was used to determine the average hydrodynamic
radius (Rh) and polydispersity of colloidal spheres in the aqueous
dispersion medium. For estimating those parameters, the cu-
mulant method was used to describe logarithm of the total
autocorrelation function as a series expansion, where the first
cumulant (Γ) yields the z-averaged diffusion coefficient and the
second cumulant (µ2) is a measure of polydispersity.44a Rh was
obtained from the particle diffusion coefficient based on the
Stokes-Einstein relation.44b Figure 7 shows the size distribution
curve obtained from the DLS measurement. The z-averaged
hydrodynamic radius (Rh) is 65.6 nm with the polydispersity
(µ2/Γ2) of 0.025.
Table 1. DLS and SLS Experimental Results for the Colloidal
Spheres Dispersed in Watera
Rh
Rg
Mw,p
(g/mol)
Fc
Nagg (g/cm3)
b
(nm) µ2/Γ2 (nm) Rg/Rh
A2
65.6 0.025 50.8 0.774 5.05 × 10-5 8.62 × 107 854
0.121
a Here µ2/Γ2 is the polydispersity index of the spheres dispersed in the
water, measured by DLS. The dn/dC value of the colloidal dispersion was
b
measured to be 0.255. Nagg ) Mw,p/Mw,m
.
c F ) Mw,p/(NA4π Rh 3/3).
where K is the contrast factor, C is the concentration, θ is the
angle at which scattering light is measured, R(θ) is the Rayleigh
ratio at the angle of measurement, n is the refractive index of
the solvent, λ0 is the wavelength of the incident light, and A2 is
the second virial coefficient. The parameters were estimated
through the Zimm plot analysis (Figure 8). The extrapolations
regarding to C and θ yield Mw,p from the intercept on the
2
ordinate as well as A2 and Rg from the slope of the
corresponding curves. Mw,p, Rg, and A2 obtained from Figure 8
are 8.62 × 107 g/mol, 50.8 nm, and 5.05 × 10-5, respectively.
Table 1 summarizes the Rh, Rg, Rg/Rh, Mw,p, Nagg, and F values
of the POAPB6P-AC colloidal spheres. The Rg/Rh value can be
used to characterize the shape of the colloids.46 Rg/Rh estimated
for the above sample is around 0.774, which confirms that the
POAPB6P-AC colloids are spherical particles in the suspension.
The average aggregate number in each colloidal particle is
estimated to be 854, based on the equation
The static light scattering (SLS) was used to determine the
weight-average mass of the colloidal particles (Mw,p), the radius
of gyration (Rg), and the second virial coefficient (A2).45
According to the basic SLS theory, the particles scatter light
according to the following relation:
Nagg ) Mw,p/Mw,m
(6)
θ
2
16π2n2 sin2 Rg
2
( )
KC
R(θ)
1
Mw,p
)
1 +
+ 2A2C
(5)
where Mw,p and Mw,m are the weight-average mass of the
(
2
)
colloidal particles and the polymeric molecules, respectively.47,48
3λ0

Macromolecules, Vol. 39, No. 19, 2006
Amphiphilic Random Copolymers 6595
Figure 9. (a) UV-vis spectrum of POAPB6P-AC in THF solution (5
mg/L). (b) UV-vis spectrum of POAPB6P-AC colloids in the aqueous
dispersion, which was prepared from the dispersion with the water
content of 70 vol % through the “quenching” and dialyzing procedure,
and the initial polymer concentration in THF was 1.0 mg/mL.
Figure 10. UV-vis spectra of POAPB6P-AC colloids in the dispersion
(0.04 mg/mL) measured for different storage periods: (a) right after
water addition, (b) after 2 days, and (c) after 7 days. The colloidal
dispersion was prepared by dropping water into the THF solution of
POAPB6P-AC until the water content reached 60 vol %; the initial
polymer concentration in THF was 1.0 mg/mL.
The average density of the colloidal particles (F) is calculated
to be 0.121 g/cm3 by applying the following equation:49
F ) Mw,p/(NA4π Rh 3/3)
(7)
The above results all indicate that the structures formed in
the aqueous dispersion are uniform colloidal spheres. The
colloidal spheres of POAPB6P-AC are stable in the dispersion
medium because of the repulsive electrostatic interaction of the
surface charges. No obvious variation was observed from the
DLS measurement and the TEM observation after storing the
colloidal dispersions at room temperature for months.
3.5. H-Aggregation of Azo Chromophores. The H-aggrega-
tion of azo chromophores of POAPB6P-AC is closely related
with the colloidal formation process, which can be monitored
by spectroscopic analysis. Figure 9 gives UV-vis spectra of
both POAPB6P-AC dissolved in THF and POAPB6P-AC
colloids dispersed in water. POAPB6P-AC in THF shows a
maximum absorbance at 360 nm, corresponding to the π-π*
transition of the “isolated” azo chromophores. For the aqueous
dispersion of the colloidal spheres, the π-π* transition band
shows a significant blue shift and appears at 334 nm. The blue
shift is attributed to H-aggregation involving “card-packed”
stacking of the azo chromophores.25-27 The blue shift induced
by H-aggregation can be distinguished from the band shifts
caused by the solvatochromic effect. λmax of POAPB6P-AC in
dioxane, THF, and DMF appears at 358, 360, and 362 nm,
which indicates that the solvatochromic effect causes a red shift
when polarity of the solvent increases, and the band shift is
usually in a range of a few nanometers.
The transition from the “isolated” azo chromophores in THF
solution to H-aggregates in the colloids is a process related with
the water content increase in the system. UV-vis spectra
measured in the process can be used to indicate the transition
point of the H-aggregate formation. When the water content is
low, UV-vis spectra of POAPB6P-AC solutions or dispersions
show the maximum absorption band at 360 ( 1 nm, which is
almost the same as that of POAPB6P-AC in THF. There is no
significant band shift when the water content increases from
0% to a value below 60 vol %, which indicates that no
significant H-aggregation occurs in the systems. When the water
content reaches 60 vol %, the UV-vis spectrum of the
dispersion changes significantly as the storage time increases.
Figure 10 shows the UV-vis spectra of the colloidal dispersion
with the water content of 60 vol % measured after different
storage periods. For the spectrum measured right after the water
addition, the spectral character is the same as that of the THF
Figure 11. λmax of the π-π* transition band of POAPB6P-AC as a
function of the water content in the mixed THF-H2O media. The initial
polymer concentration in THF was 1.0 mg/mL.
solution, where the maximum absorption band appears at 360
nm. When storing for 2 days, the UV-vis spectrum of the
dispersion exhibits a broad absorption band with the peak at
340 nm and a shoulder at 360 nm, which indicates the
coexistence of the “isolated” azo chromophores and H-ag-
gregates of the chromophores. After storing for 7 days, the
spectrum shows the maximum absorption band at 339 nm, which
suggests that most of azo chromophores of POAPB6P-AC form
H-aggregates. When the water content further increases, λmax
of the POAPB6P-AC dispersions changes from 360 to 339 nm
in a shorter time period after the water addition. When the water
content reaches 80%, the UV-vis spectrum of the dispersion
shows the characteristics of H-aggregates immediately after the
water addition. This indicates that under this condition the
H-aggregates can be formed in an extremely short time period.
Figure 11 shows that λmax as a function of the water content in
the solutions or dispersions, which were all measured after
storing for 7 days. A sudden fall of λmax in the diagram can be
seen at the water content about 60 vol %. Over this water
content, most of azo chromophores in the system form H-
aggregates. In other words, azo chromophores of POAPB6P-
AC exist as “isolated” chromophores when water content is
below 60 vol % and form H-aggregates when the water content
is over this value. Comparing with the results given in section
3.3, it can be seen that H-aggregation occurs after the clusters
undergo a significant size contraction. When water content
reaches about 95 vol %, the λmax again shows a blue shift from
339 to 334 nm. This transition is related with the structure
“freeze”, which will be discussed in the following section.
The above result can be rationalized on the basis of the
following considerations. As the π-π attraction is a short-range

6596 Deng et al.
Macromolecules, Vol. 39, No. 19, 2006
interaction, H-aggregation can occur only when the chro-
mophores can approach each other in space.24-28 For low
molecular weight compounds in solutions, the molecules can
come close to each other through Brownian motion. In many
cases, an equilibrium between “isolated” molecules and H-
aggregates exists, which can be described by the mass action
law.25,26 In the current case, the azo chromophores are tethered
on the polymeric chain, and their movement through Brownian
motion is restricted by the backbone. Therefore, the azo
chromophores can approach each other only when polymer
chains are closely packed through the chain association and
cluster collapse. This condition can be satisfied only when the
water content in the system is relatively high, such as 60 vol %
for the above case. Only in this high water content range can
H-aggregation be observed in a reasonably short period of time.
It has been reported that for ionic dyes H-aggregation can be
directly promoted by the hydrophobic interactions.26 For
polymeric system discussed in this paper, the hydrophobic
interaction shows a more complicated influence on the H-
aggregation, which is closely related with the polymer confor-
mation and association state in the dispersion media.
3.6. Photoisomerization Behavior. As the photoisomeriza-
tion of azobenzene units is sensitive to the local environment
surrounding the units, information about the local structure and
its variation can be obtained by measuring the isomerization
rate and the isomerization degree at the photostationary state.1,39,42
In this work, the photoisomerization of the azobenzene units
was investigated and used as a tool to probe the structure
variations of POAPB6P-AC in the different dispersion media.
Figure 12a-c shows the spectra corresponding to the pho-
toisomerization of POAPB6P-AC, which is dissolved in anhy-
drous THF, dispersed in the THF-H2O medium with the water
content of 60 vol %, and dispersed in the aqueous medium,
respectively. The samples were irradiated with 365 nm UV light
for different time periods, and the UV-vis spectra of the
samples were recorded until the photostationary states were
achieved. For POAPB6P-AC dissolved in anhydrous THF
(Figure 12a), the absorbance of the π-π* transition band at
360 nm gradually decreases, and the absorbance of the n-π*
transition band at 450 nm gradually increases upon the light
irradiation. The spectral variations evidence the trans-to-cis
isomerization of the azobenzene-type chromophores.1,41 When
the sample was kept in a dark oven, the spectra gradually
recovered the original curve, where the azo chromophores
underwent a slow thermal cis-to-trans isomerization. For the
POAPB6P-AC dispersion with the water content of 60 vol %
(Figure 12b) and the aqueous colloidal dispersion (Figure 12c),
Figure 12. Variation of the UV-vis spectra of POAPB6P-AC in
different media induced by the UV light irradiation. Inset: the relative
absorbance (At/Aorigin) varying with the irradiation time and the fitting
curves. (a) POAPB6P-AC dissolved in THF solution (0.033 mg/mL);
(b) POAPB6P-AC dispersed in the mixed THF-H2O media (0.035 mg/
mL) with the water content of 60 vol %; (c) POAPB6P-AC dispersed
in water (0.035 mg/mL). The initial polymer concentrations in THF
were all 1.0 mg/mL.
Figure 12a-c. T1 obtained from the curve fitting is 5.4, 27.3,
and 33.7 s for parts a, b, and c of Figure 12. Comparing with
the THF solution, the trans-to-cis isomerization rate declines
significantly for the dispersion given in Figure 12b. For the
aqueous colloidal dispersion (Figure 12c), the isomerization rate
shows an even further decrease.
λ
max before light irradiation appears at 339 and 334 nm because
of H-aggregation and the “frozen” structure. Upon the light
irradiation, those systems can also show obvious trans-to-cis
isomerization. However, it can be seen that the photoinduced
spectral variations show different characteristics. From Figure
12a-c, the absorbance at λmax before the light irradiation (Aorigin
)
For the other solutions or dispersions of POAPB6P-AC in
THF-H2O media with different water contents, the photo-
isomerization processes were studied in the same way. Figure
13a shows the relative absorbance (At/Aorigin) of the samples
varied with the UV light irradiation time period. For all the
samples, the variations of At/Aorigin with the irradiation time can
be best fitted by the first-order exponential decay function. Both
the experimental data and fitting curves are shown in Figure
13a, and the parameters obtained from the fitting are given in
Table 2. The results indicate that the trans-to-cis isomerization
rate is closely related with the water content in the system. When
the water content changes from 0% (THF solution) to 23 vol
and the absorbance at the same wavelength after the irradiation
for different time periods (At) can be obtained. The relative
absorbance (At/Aorigin) of the samples can be used to indicate
the relative amount of the trans isomers remaining at t time.
The variations of At/Aorigin with t represent the kinetics of the
photoisomerization. For Figure 12a-c, the variations can be
best fitted by the first-order exponential decay function
At/Aorigin ) A0 + A1 exp(-t/T1)
(8)
where T1 is the characteristic time of the decay process. Both
experimental data and fitting curves are shown in the insets of

Macromolecules, Vol. 39, No. 19, 2006
Amphiphilic Random Copolymers 6597
tion. The isomerization degree at the photostationary state is
estimated from
isomerization degree ) (Aorigin - As)/Aorigin
(9)
where Aorigin is the absorbance at λmax before the light irradiation
and As is the absorbance at the same wavelength measured at
the photostationary state. Figure 13b shows the isomerization
degree of POAPB6P-AC solutions or dispersions varying with
the water content. There is a sudden drop of the isomerization
degree when the water content increases from 50 to 60 vol %.
When THF in the microphase is almost completely replaced
by H2O, the isomerization degree shows another sudden drop.
The result obtained from the photoisomerization study is
consistent with the results discussed in the other parts of this
paper. It strongly suggests that the colloid formation undergoes
several different stages in the stepwise buildup process as the
water content increases. For the system discussed above, when
the water content reaches 23%, the POAPB6P-AC chains start
to associate, which is accompanied by the slight decrease of
the photoisomerization rate and the isomerization degree. When
the water content increases from 23 to 37 vol %, more and more
polymer chains transfer from the “isolated” single-chains to the
associated chains, which is indicated by the cluster size increase
(Figure 5). At this stage, the structures formed in the dispersions
are loose and can be disrupted by the UV light irradiation, which
is a result of the volume expansion and hydrophilicity increase
caused by the trans-to-cis isomerization.1,41 As a result, the
observed photoisomerization rate and isomerization degree do
not show an obvious variation. When the water content increases
from 37 to 60 vol %, the loose clusters of associated chains
undergo a collapse, which is indicated by the significant cluster
size contraction. When the water content varies in a range from
50 to 60 vol %, the photoisomerization rate and isomerization
degree significantly decrease because of the hindrance of the
closely packed polymer chains. Another important factor causing
such variation is the formation of H-aggregates when the water
content reaches 60 vol %. The H-aggregation can also strongly
hinder the isomerization, as pointed out by many authors
previously.25,26,32 Finally, when THF in the microphase is almost
completely replaced by H2O, the colloidal structure is completely
“frozen” due to the free volume decrease and chain entangle-
ment, which again causes the obvious decrease in the isomer-
ization rate and degree. The spectral blue shift appearing in this
stage (Figure 11) is also related with the structure variation in
the microphase.
Figure 13. (a) Plot of the relative absorbance (At/Aorigin) of POAPB6P-
AC in various media vs the irradiation time period. (b) Plot of the
photoisomerization degree at the photostationary state vs the water
content in the dispersion media.
Table 2. Parameters of the Photoisomerization Kinetics Obtained
from the Curve Fitting for POAPB6P-AC Dissolved or Dispersed in
THF-H2O Media with Different Water Contentsa
sample
(water content, %)
A0
A1
T1 (s)
ø2
0
23
37
40
50
60
70
80
100
0.12
0.14
0.15
0.17
0.20
0.41
0.43
0.46
0.63
0.88
0.88
0.87
0.84
0.79
0.58
0.58
0.54
0.38
5.45
6.29
6.83
8.64
8.98
27.32
29.70
30.06
33.72
2.87 × 10-4
4.29 × 10-4
4.13 × 10-4
3.06 × 10-4
1.81 × 10-4
1.20 × 10-4
9.45 × 10-5
5.30 × 10-5
1.25 × 10-4
a Here ø2 is the mean-squared error of the fitting.
The above results clearly show that H-aggregation can be
formed only when polymer chains are closely packed after the
cluster collapse, but at the same time, the chromophore motion
has not been completely “frozen” by the free volume decrease
and chain entanglement. The specific water contents for
H-aggregate formation and other microphase transitions should
depend on the factors such as chemical structure of the chain
units, degree of functionalization, molecular weight and distri-
bution, and initial concentration of the polymer in the organic
solvent, among others. Although this paper has no attempt to
give a full picture of those correlations, it is believed that the
results obtained in this paper can supply some basic understand-
ing of colloidal sphere formation and its relationship with
H-aggregation in the system. On the basis of the understanding,
the inner structure of the colloidal spheres can be controlled by
adjusting the preparation conditions.
%, the trans-to-cis photoisomerization rate shows a slight
decrease. When the water content varies in a range from 23 to
37 vol %, the trans-to-cis photoisomerization rate does not show
a detectable variation. When the water content increases from
37 to 50 vol %, the trans-to-cis isomerization rate decreases
gradually, which results from the cluster collapse due to the
hydrophobic interaction. When the water content increases from
50 to 60 vol %, a dramatic decrease of the photoisomerization
rate can be observed. As indicated in section 3.5, H-aggregation
occurs at the water content of 60 vol %. When the water content
further increases to 80%, the trans-to-cis isomerization rate
shows a slight decrease. Finally, when the water content
approaches about 100% (i.e., THF in the microphase is almost
completely replaced by H2O), the isomerization rate significantly
decreases again due to the structure “freeze” and chain entangle-
ment.
4. Summary
The study on the trans-to-cis isomerization degree at the
photostationary state gives the same structure variation informa-
An amphiphilic random copolymer functionalized with
branched azo side chains, POAPB6P-AC, has been synthesized

6598 Deng et al.
Macromolecules, Vol. 39, No. 19, 2006
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Acknowledgment. The financial support from the NSFC
under Projects 50533040 and 20374033 is gratefully acknowl-
edged.
Supporting Information Available: Scheme for synthesis of
OAPB6P and POAPB6P-AC; analytical data and spectra of products
of each step reaction for OAPB6P synthesis. This material is
available free of charge via the Internet at http://pubs.acs.org.
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