Multistimuli responsive polymer nanoparticles on the basis of the amphiphilic azobenzene-contained hyperbranched poly(ether amine) (hPEA-AZO)

Article abstract of DOI:10.1021/ma1023632

We here reported the multistimuli responsive behavior of polymer nanoparticles, which was formed through direct dispersion of the amphiphilic azobenzene-contained hyperbranched poly(ether amine) (hPEA-AZO) in water. A series of hPEA-AZO were synthesized b

Full text of DOI:10.1021/ma1023632

Macromolecules 2010, 43, 10457–10465 10457
DOI: 10.1021/ma1023632
Multistimuli Responsive Polymer Nanoparticles On the basis of the
Amphiphilic Azobenzene-Contained Hyperbranched Poly(ether amine)
hPEA-AZO)
(
Bing Yu, Xuesong Jiang,* Rui Wang, and Jie Yin*
School of Chemistry & Chemical Technology, State Key Laboratory for Metal Matrix Composite Materials,
Shanghai Jiao Tong University, Shanghai, China
Received October 16, 2010; Revised Manuscript Received November 20, 2010
ABSTRACT: We here reported the multistimuli responsive behavior of polymer nanoparticles, which was
formed through direct dispersion of the amphiphilic azobenzene-contained hyperbranched poly(ether amine)
(
hPEA-AZO) in water. A series of hPEA-AZO were synthesized by introduction of azobenzene moieties into
the periphery of hyperbranched poly(ether amine) (hPEA), which was developed by our group recently. The
obtained polymer nanoparticles of hPEA-AZO which were revealed by dynamic light scattering (DLS) and
transmission electron microscopy (TEM), exhibited the sharp stimuli-response to temperature, pH, and ionic
strength with tunable cloud point (CP) from 25 to 100 °C. CP of the obtained hPEA-AZO nanoparticles in
aqueous solution decreased with the increase of azobenzene content, pH value and ionic strength, and
represented a linear relationship with the azobenzene content and ionic strength. The hPEA-AZO
nanoparticles also showed a light-controlled CP and size. After UV irradiation (365 nm), we observed lower
value of CP and larger size for hPEA-AZO nanoparticles. The CP difference of hPEA-AZO nanoparticles in
aqueous solution between before (CP
T C
, trans form) and after (CP , cis from) UV irradiation (365 nm)
increased linearly upon the azobenzene content up to 5 °C
Introduction
which were formed by directly self-assembly of amphiphilic
azobenzene-contained hyperbranched poly(ether amine)s
Because of their potential applications in separation, biosensor,
chemical storage and transport, and gene and drug delivery,
polymer nanoparticles with response to environmental stimuli have
(
hPEA-AZOs) in aqueous solution. Recently, we had developed
a novel family of amphiphilic hyperbranched poly(ether amine)
3
7
1-6
(hPEA), which can be obtained through one-pot synthesis.
attracted much attention.
The stimuli-responsive behaviors of
These hPEAs can be directly dispersed into aqueous solution to
form uniform-sized polymer nanoparticles responsive to tem-
perature, pH and ionic strength with accurately tunable cloud
point (CP). Moreover, hPEAs contain reactive amino groups in
periphery, which can be further modified with functional groups
easily. Motivated by the novel characteristics of hPEAs, we
continued to introduce azobenzene groups into the periphery of
hPEA to obtain amphiphilic azobenzene-contained hyper-
branched poly(ether amine)s (hPEA-AZOs), which are expected
to be directly dispersed in water to form polymer nanoparticles
with response of tetra-stimuli. The responsive behavior of these
resulted polymer nanoparticles was investigated in detail.
polymer nanoparticles strongly depend on the external chemical
and physical stimuli, such as ionic strength, light, electric and
7-9
magnetic field, temperature and pH.
Because of the easy-
control, light-response is becoming of special importance to poly-
1
0-12
mer nanoparticles in the application fields of drug delivery
and photo sensor.
13-15
To obtain the polymer nanopaticles re-
sponse to light, a variety of light-responsive moieties are introduced
into the polymers which construct polymer nanoparticles. Among
these light-responsive moieties, azobenzene group is most studied
due to its well-known reversible isomerization from trans- to cis-
16-21
configuration upon irradiation.
Several research groups re-
ported the responsive polymer nanoparticles formed by self-assembly
of amphiphilic azobenzene-contained copolymers in aqueous
Experimental Section
18,22-25
solution.
Besides response to light, some of these polymer
Material. 4-Phenylazophenol (hydroxyl azobenzene, H-AZO)
was purchased from Alfa Aesar and used without further pur-
ification. 3-Chloro-1,2-epoxypropane (ECH) was purchased from
Sinopharm Chemical Reagent. Potassium carbonate (K CO )
nanoparticles exhibited the response to another stimulus such as
26,27
24
temperature
or pH. Because the behavior changes of the
responsive polymer nanoparticles are often a result of their
response to the combination of the environmental changes, poly-
mer nanoparticles which are sensitive to multiple stimuli are very
important in practical applications, therefore, become of very
2
3
was purchased from Sinopharm Chemical Reagent and grinded
into powder then dried at 110 °C for 12 h before used. Hyper-
branched poly(ether amine) (hPEA211) was synthesized according
6,19,28-36
interest.
However, to the best of our knowledge, polymer
37
to previous report.
nanoparticles responsive to the tetrastimuli of temperature, pH,
ionic strength, and light have not yet been demonstrated.
In this text, we report polymer nanoparticles with response to
the tetrastimuli of temperature, pH, ionic strength, and light,
Synthesis of 4-Phenylazophenyl Glycidyl Ether (E-AZO). The
reaction was conducted in a two-necked flask equipped with a
nitrogen inlet and a reflux condenser. The mixture of H-AZO
(
0.01 mol), ECH (0.05 mol), and K CO (0.03 mol) was refluxed
2 3
in 25 mL 2-butanone for 12 h under a nitrogen atmosphere.
After cooling to room temperature, the inorganic residue was
filtered off and washed with 2-butanone. The filtrate solution
*
Corresponding authors. Telephone: þ86-21-54743268; Fax: þ86-21-
5
4747445. E-mail: (X.J.) ponygle@sjtu.edu.cn, (J.Y.) jyin@sjtu.edu.cn..
r 2010 American Chemical Society
Published on Web 12/02/2010
pubs.acs.org/Macromolecules

1
0458 Macromolecules, Vol. 43, No. 24, 2010
Scheme 1. Total Synthesis Process of hPEA-AZO
Yu et al.
was evaporated by rotary to remove solvent and excess ECH.
The crude product was purified by recrystallization in CHCl to
Table 1. Composition Data of hPEA211-AZO
3
1
get E-AZO and the yield is about 97%. H NMR (DMSO-d
content of azobenzene (mole ratio)
6
,
a
b
c
d
ppm): δ 7.85, 7.53, 7.15 (aromatic, 9H), 4.45, 3.92 (-OCH
2
-,
polymer
in feed
in polymer
CMC (g/L)
2
H), 3.36 (CH, 1H), 2.85, 2.73 (-CH
2
-, 2H). FT-IR (KBr):
hPEA211-AZO02
hPEA211-AZO04
hPEA211-AZO06
hPEA211-AZO08
0.20
0.40
0.60
0.80
1.00
0.20
0.41
0.58
0.79
0.96
0.73
0.33
0.38
0.36
0.09
-
1
-1
-1
-1
2
973 cm , 2902 cm (C-H), 1598 cm , 1411 cm (CdC in
benzene), 1500 cm (NdN), 1249 cm (C-N), 1049 cm-1
-
1
-1
(
C-O).
Synthesis of hPEA211-AZOs. hPEA211-AZOs were synthe-
hPEA211-AZO10
a
hPEA211-AZOx, x/10 represent the proportion of the amino
groups in hPEA211 modified by azobenzene theoretically. The mole
ratio in feed is calculated as the mole ratio of E-AZO and secondary
sized according to Scheme 1. To the mixture of hPEA211 (0.005
mol, in terms of its structural units) and ethanol (20 mL),
E-AZO was added in proportion (shown in Table 1). The
reaction mixture was refluxed for 12 h with nitrogen protection.
Then most of the solvent was removed by rotary evaporation
and the mixture left was poured into n-hexane. The precipitate
was washed by n-hexane for several times and dried in vacuum
b
c
amino groups in hPEA211. Estimated from UV-visible absorption at
d
49 nm compared with pure 4-Phenylazophenol in THF. CMC was
3
measured by using Nile Red as a fluorescent probe.
polymer solution onto a copper grid coated with a thin polymer
film while the excess solution was removed by filter paper, and
then dried at room temperature for 24 h. No staining treatment
was performed for the measurement.
oven, and the product was named as hPEA211-AZO.
Characterization. H NMR spectra in dimethyl sulfoxide-d
1
6
(
DMSO-d ) and D O were acquired with Varian Mercury Plus
6 2
4
00 MHz spectrometer equipped with a temperature control
unit, respectively. The samples in DMSO-d were measured at
The atom force microscopy (AFM) images were obtained by
using a scanning probe microscope (Nanoscope III, Digital
instruments) operated in the contacting mode. The sample was
prepared by dropping dilute polymer aqueous solution on a
mica sheet and then dried at room temperature for 24 h.
Cloud Point Measurements. The optical transmittance of the
polymer solutions were measured at 700 nm with a spectral-1.70
UV-visible spectrophotometer (GBC Cintra 100, Aus.)
equipped with a thermo cell at a heating rate of 1 °C/min. The
temperature at 90% light transmittance of the polymer solution
was defined as the cloud point (CP). The hPEA211-AZO
aqueous solutions were prepared from 0.1 M citrate buffered
aqueous solutions with 3 mg/mL copolymer concentration.
Critical Micelle Concentration (CMC) Measurements. The
critical micelle concentration (CMC) of the hPEA211-AZO
6
room temperature and the samples prepared as 1 wt % in D O
2
were measured with the proton signal of D O solvent as the
2
internal reference at 20, 40, and 60 °C, respectively.
Infrared absorption spectra (IR) measurements were carried
out with Paragon 1000 Fourier transformation infrared absorp-
tion spectrometer. The samples were prepared by dropping the
polymer solution onto a KBr film and dried below an infrared
lamp.
The UV-visible spectra of the hPEA211-AZO aqueous
solution were checked by a UV-2550 spectrophotometer
(
Shimadzu, Japan). The hPEA211-AZO aqueous solutions were
prepared from 0.1 M citrate buffered aqueous solutions with
mg/mL copolymer concentration. These samples were exposed
3
3
8
under an ultraviolet LED lamp (Uvata) at 365 and 450 nm with
was determined by using Nile Red as a fluorescent probe.
2
intensity about 8.4 mW/cm , and the UV-visible spectra were
checked every 5 min.
Solutions of hPEA211-AZO with different concentrations were
prepared by ultrapure water, and about 0.1 mg Nile Red was
added into each of the samples, respectively. The solutions were
sonicated for 30 min and equilibrated in the darkness overnight.
The emission spectra were recorded under LS-50B luminescence
The transmission electron microscopy (TEM) images were
obtained using a JEM-2100 (JEOL Ltd., Japan) microscope
operated at 200 kV. The sample was prepared by dropping the

Article
Macromolecules, Vol. 43, No. 24, 2010 10459
1
Figure 1. H NMR spectra of hPEA211, E-AZO, and hPEA211-
AZO10 in DMSO-d
6
.
spectrometer (Perkin-Elmer Company, USA). The excitation
wavelength was set at 550 nm, and the fluorescence emission
spectra were recorded between 560 and 750 nm. All fluorescence
spectra were recorded at room temperature.
Dynamic Light Scattering (DLS). The measurements were
performed in the copolymer aqueous solution using a ZS90
Zetasizer Nano ZS instrument (Malvern Instruments Ltd.,
U.K.) equipped with a multi-τ digital time correlation and a
He-Ne laser (λ = 633 nm) at an angle of 90°. The CONTIN
analysis method was used. The samples were prepared in ultrapure
water with 1 mg/mL copolymer concentration.
Results and Discussion
Synthesis and Characterization of hPEA211-AZOs. Our
previous work showed that amphiphilic hyperbranched
poly(ether amine)s (hPEAs) could be successfully synthe-
sized through nucleophilic addition/ring-opening reaction of
37
commercial diglycidyl ether and amine via one-pot synthesis.
Moreover, there were reactive amino groups in periphery of the
obtained hPEAs. Because nucleophilic addition/ring-opening
reaction between amine and epoxy group possesses the
characteristics of “click-chemistry”, hPEA can be further func-
tionalized by molecules with epoxy groups. As a result, amphi-
philic azobenzene-contained hPEAs (hPEA-AZOs) can be
easily obtained by introducing azobenzene moieties into the
periphery of hPEAs. The process for synthesis of hPEA-AZO is
shown in Scheme 1. As azobenzene is a relative hydrophobic
compound, then a relative hydrophilic hPEA (hPEA211) is
chosen to ensure that the hPEA-AZO produced can be dis-
persed directly in water at room temperature. In the structure of
hPEA211, the mole ratio of poly(propylene oxide) (PPO) chains
and poly(ethylene oxide) (PEO) chains is 1:1. The successful
functionalization of hPEAs with azobenzene groups was con-
Figure 2. (a) Plot of the maximum fluorescence emission intensity of
the Nile Red vs concentration of hPEA211-AZO06. Inset: fluorescence
emission spectra of Nile Red solution with different hPEA211-AZO06
concentration. (b) Size distribution of nanoparticles formed by
hPEA211-AZO10, hPEA211-AZO06, and hPEA211-AZO04 deter-
mined by DLS at 25 °C in aqueous solution. Polymer concentration
is 1 mg/mL. (c) TEM image of the nanoparticles formed by hPEA211-
AZO10 at room temperature.
1
by H NMR spectra, and the attribution of each signal is also
signed in Figure 1. In comparison with hPEA211, the existence
of azobenzene groups in hPEA211-AZO10 is proved by signals
1
of aromatic compound at ∼7.15, ∼7.55, and ∼7.85 ppm in H
1
firmed by FT-IR (Figure S1, Supporting Infomation) and H
NMR (Figure 1). Compared with hPEA211, new peak at 1580
NMR spectrum of hPEA211-AZO, and a new signal appears
at ∼4.0 ppm, which also indicates the occurrence of the
reaction between epoxy groups in E-AZO and amino groups
in hPEA211. In comparison with E-AZO, the disappearance
-
1
cm assigned to phenyl ring and new peak at 1498 cm
-1
assigned to azo groups appeared in FT-IR spectrum of
hPEA211-AZO10, indicating introduction of azobenzene to
hPEA211. The structure of hPEA211-AZO10 is also supported
1
of the signals at ∼2.7 and ∼2.85 ppm in H NMR spectrum
of hPEA211-AZO10 proves that all the E-AZO are reacted,

1
0460 Macromolecules, Vol. 43, No. 24, 2010
Yu et al.
Table 2. Characterization of hPEA211-AZO Nanoparticles
a
b
cloud point (°C)
average diameter (nm)
nanoparticles
CP
T
CP
C
ΔCP
trans
cis
hPEA211-AZO02 72.4
hPEA211-AZO04 60.3
hPEA211-AZO06 49.6
hPEA211-AZO08 41.8
hPEA211-AZO10 30.2
a
Cloud point was determined by a UV-visible spectrophotometer
equipped with a thermo cell. All the samples were prepared at pH 7.4
71.7
58.5
46.8
37.8
24.9
0.7
1.8
2.8
4.0
5.3
10.4 ( 1.5
11.9 ( 1.6
15.0 ( 2.7
18.3 ( 3.3
12.5 ( 1.9
12.5 ( 3.3
16.5 ( 1.7
18.8 ( 0.8
21.2 ( 2.3
16.3 ( 2.2
with 3 mg/mL polymer concentration. CP
samples were prepared, while CP was tested after UV irradiation
365 nm) for 40 min. ΔCP is calculated as CP
T
was tested as soon as the
C
b
(
T
- CP
C
The average
diameters of the nanoparticles based on hPEA211-AZO were determined by
DLS spectra. All the samples were prepared using ultrapure water with
1
mg/mL polymer concentration and tested at 25 °C. The diameter with
azobenzene moieties in trans form was tested as soon as the samples were
prepared while the diameter with azobenzene moieties in cis form was tested
after UV irradiation (365 nm) for 40 min. The error range was calculated as
the standard deviation of five times of measurement.
indicating that the proportion of amino groups of hPEA211
modified by azobenzene is tunable by the amount of E-AZO in
feed during the synthesis process.
Formation and Morphology of the hPEA211-AZO Nano-
particles. Just as amphiphilic hPEA211, hPEA211-AZOs
were expected to self-assemble directly into nanoparticles
in water, which are comprised of hydrophobic PPO chains
and azobenzene moieties as core and hydrophilic PEO chains
as shell. As a strong evidence for self-assembly behavior of
amphiphilic polymer into nanoparticles, critical micelle con-
centration (CMC) value for hPEA211-AZO was determined
by a fluorescence spectroscopy using Nile Red as a fluores-
3
8
cence probe. Figure 2a shows the maximum fluorescence
intensity as a function of the logarithm of hPEA211-AZO06
concentration ranging from 0.002 g/L to 3 g/L, from which a
sudden increase of the maximum fluorescence intensity is
observed at the concentration of about 0.38 g/L, indicating
the formation of the micelles. The CMCs of other hPEA211-
AZO at room temperature are measured by the same meth-
od, and the results are summarized in Table 1.
The formation of nanoparticles from hPEA211-AZOs was
further investigated by dynamic light scatting (DLS) experi-
ments, a series of 1 mg/mL hPEA211-AZO aqueous solu-
tions were measured at 25 °C. The polymer concentration is
higher than CMC to ensure the formation of nanoparticles.
Taking hPEA211-AZO10 as example, the size distribution
of hPEA211-AZO10 nanoparticle is shown in Figure 2b,
which indicated the formation of uniform-sized nanoparticle
with diameter of about 12.53 ( 1.90 nm and polydispersity
index (PDI) of 0.327. The size of all hPEA211-AZO nano-
particles determined by DLS is summarized in Table 2.
The morphology of hPEA211-AZO10 nanoparticles was
revealed by TEM (Figure 2c). It should be noted that the
diameter of the nanoparticles determined by TEM is bigger
than that of DLS analysis. This might be ascribed that the
nanoparticles were somewhat flattened on the copper grids
Figure 3. (a) Temperature dependence of optical transmittance at
7
3
00 nm for hPEA211-AZO aqueous solution at pH 7.4. (b) CP of
mg/mL hPEA211-AZO aqueous solutions at pH 7.4 as a function of
the proportion of amino groups in hPEA211 modified by azobenzene.
c) Reversible change of transmittance at 700 nm for hPEA211-AZO10
(
aqueous solution at pH 7.4 between 25 and 35 °C.
hydrophilic, leading to the aggregation of hPEA211-AZO
nanoparticles and the turbidity of the solutions. The thermal
responsive aggregation behaviors of hPEA211-AZO nano-
particles under different conditions were investigated by
measuring their cloud point (CP). As can be seen in Figure 3a,
the transmittances of all hPEA211-AZO aqueous solution
exhibit a sharp decrease above their CPs. With the increase of
the proportion of amino groups in hPEA211 modified
by azobenzene from 0.2 to 1.0, CP decreases from 72.4 to
30.2 °C, indicating that the proportion of amino groups
modified by azobenzene has obvious effect on CP. Because
of the higher hydrophobicity of azobenzene moieties in
comparison with terminal amino groups, increasing of the
azobenzene proportion can make hPEA211-AZO more
3
9-42
in TEM experiments.
The Behavior of hPEA211-AZO Nanoparticles Responsive
to Temperature, pH, and Ionic Strength. hPEA211-AZO
nanoparticles were designed to be responsive to temperature,
pH, and ionic strength. hPEA211-AZO nanoparticles are
comprised of hydrophobic azobenzene moieties and PPO
short chains as core and hydrophilic PEO short chains as
outer shell, which prevents the nanoparticles from aggregat-
ing further. With the increase of temperature, hydrogen
bonds between water molecules and hydrophilic part of the
nanoparticles are destroyed. The outer shell becomes less

Article
Macromolecules, Vol. 43, No. 24, 2010 10461
1
Figure 5. H NMR spectra of 1 wt % hPEA211-AZO06 in D
2
O at
different temperatures.
Because salt has influence on the hydrogen bond between
PEO chains and water molecules, hPEA211-AZO nanopar-
ticles in aqueous solution are also expected to be responsive
to ionic strength. Figure 4b shows transmittance versus
temperature for hPEA211-AZO06 at different NaCl con-
centrations, which indicates that CP decreases with the
increase of NaCl concentration. Figure 4c shows CP re-
corded for hPEA211-AZO06 in the presence of increasing
amount of NaCl. CP decreases very obviously from 49.0 to
Figure 4. (a) pH dependence of optical transmittance at 700 nm for
hPEA211-AZO06 aqueous solution. (b) NaCl concentration depen-
dence of optical transmittance at 700 nm for hPEA211-AZO06 aqueous
solution at pH 7.4. (c) the effect of NaCl concentration on the CP of
3
4.5 °C with the increase of NaCl concentration from 0 to 9
mg/mL, indicating that hPEA211-AZO nanoparticles are
very sensitive to ionic strength. It is interesting that a linear
relationship (y = 49.2 - 1.66x) can be obtained between the
CP and NaCl concentration. These results can be explained
3
mg/mL hPEA211-AZO06 aqueous solution at pH 7.4.
hydrophobic, resulting in a lower CP. Figure 3b shows the
relationship between CP and azobenzene proportion at pH
7
4
3,44
.4. A good linear relationship (y = 81.61 - 51.4x) was
by salt-out effect.
The presence of NaCl might lead to
obtained between the CP and azobenzene proportion, sug-
gesting well-tunable CP of hPEA211-AZO aqueous solu-
tion. It should be noted that the thermal-responsive behavior
of hPEA211-AZO nanoparticles is completely reversible,
which can be confirmed by Figure 3c. After several circles
of heating to cooling, hPEA211-AZO aqueous solution is
still transparent.
Because amino groups in the backbone and periphery of
hPEA211-AZO can be protonated and deprotonated at
different pH, hPEA211-AZO aqueous solution also exhibits
a response to pH. Figure 4a shows a typical transmittance
versus temperature at different pH for hPEA211-AZO06
aqueous solution, which indicates that CP of hPEA211-
AZO06 decrease from 57.2 to 37.1 °C with increasing of
pH value from 6.6 to 8.0. This can be explained by the fact
that deprotonation of amino groups at a higher pH reduces
the hydrophilicity of the outer shell of nanoparticles, result-
ing in aggregation at a lower temperature.
partial dehydration of PEO chains, resulting in a decrease of
the CP.
The responsive aggregation behavior of hPEA211-AZO
nanoparticles in aqueous solution is further investigated by
H NMR spectra (Figure 5). As PPO chains are not hydro-
1
phobic completely, the signals of methyl can be observed at
20 °C, while azobenzene moieties are more hydrophobic, and
the signals of azobenzene cannot be obviously observed at
20 °C. With the increase of the temperature above CP, PPO
segments become more hydrophobic and some of which
associate together to form hydrophobic cores; at the same
time, the hydrophilic shell layer formed by PEO chains are
dehydrated and shrink. The compression of PEO shell at
high temperature might cause that part of hydrophobic core
appear outside. As a result, the signals of azobenzene can be
also obviously observed at 60 °C, which is similar to cou-
marin-contained linear poly(ether amine) in our previous
4
5
study. A possible process for responsive aggregation of

1
0462 Macromolecules, Vol. 43, No. 24, 2010
Yu et al.
hPEA211-AZO in water is proposed in Scheme 2. hPEA211-
AZOs are dispersed directly as nanoparticles in aqueous
solution, which are composed of PPO chains and azobenzene
moieties as hydrophobic cores while PEO chains and amino
moieties as hydrophilic shells. With increase of the tempera-
ture, the hydrogen bonds between PEO chains and water are
destroyed, which decreased the hydrophilicity of PEO shell,
resulting in aggregation of these nanoparticles into large
particles.
The Behavior of hPEA211-AZO Nanoparticles Responsive
to Light. As hPEA211-AZO nanoparticles are comprised of
photoswitchable azobenzene as cores, the light-responsive
behavior was investigated. We traced the isomerization of
azobenzene under irradiation of 365 and 450 nm wavelength
by UV-vis spectra (Figure 6). There are two major absorption
bands, one is at ∼343 nm attributed to the π-π*transitionofthe
azobenzene, the other is at ∼435 nm attributed to the n-π*
transition. Under the irradiation of 365 nm, π-π* transition
adsorption of azobenzene at 343 nm decreased with the increase
of irradiation time, while n-π* transition at 435 nm increased,
suggesting the isomerization of azobenzene from trans to cis
form (Figure 6a). It should be noted that the process of
isomerization from trans to cis form lasts for about 40 min,
which is much slower than some other azobenzene-contained
17,25-27
polymer reported.
Then the hPEA211-AZO10 aqueous
solution after UV irradiation is exposed under visible light of
450 nm, and the changes of UV-vis spectra are shown in
Figure 6b, the process of back-conversion from cis to trans form
25,27
only lasts for 10 min,
which is more rapid than the isomer-
ization from trans to cis form. This might be ascribed to the close
packing of azobenzene in the core of nanoparticles. Because the
trans form of azobenzene is easier to pack closely than the cis
form, the isomerization from trans to cis form in the core of
nanoparticles is limited, which consequently lead to the slow
conversion. This can be further verified by the following DLS
and AFM experiments, and will be discussed later.
Because the isomerization of azobenzene is accompanied
by the change of dipole moment of the molecular structure,
the CP of nanoparticles comprised of azobenzene core is
expected to be shifted upon irradiation of UV-light. We
measured the CP of hPEA211-AZO nanoparticles in aque-
ous solution again after irradiation of UV-light (365 nm) for
4
0 min. As an example, the temperature dependence of
optical transmittance for hPEA211-AZO08 and hPEA211-
AZO06 aqueous solution before and after UV irradiation is
illustrated in Figure 7. It is really surprised that lower CPs for
all hPEA211-AZO aqueous solutions were observed after
UV irradiation (365 nm), and the differences of CP between
nonirradiated (trans) and irradiated (cis) hPEA211-AZO
nanoparticles in aqueous solution increased linearly upon
the azobenzene content up to 5.3 °C (Table 2 and Figure S2
(
Supporting Information)). For most of the responsive
2
3,26
polymers containing azobenzene reported, however,
the lowest critical soluble temperature (LCST) became higher
Figure 6. (a) Dependence ofUV-vis spectraofhPEA-AZO10 aqueous
solution at pH 7.4 on UV irradiation (365 nm) time. (b) Dependence of
UV-vis spectra of hPEA-AZO10 aqueous solution at pH 7.4 on visible
light irradiation (450 nm) time. Polymer concentration is 3 mg/mL,
diluted into 0.1 mg/mL when it is tested.
Figure 7. Temperature dependence of optical transmittance at 700 nm
for hPEA211-AZO aqueous solution at pH 7.4 before and after UV
irradiation (365 nm) for 40 min,.
Scheme 2. Proposed Mechanism for Phase Transition of hPEA211-AZO Aqueous Solution

Article
Macromolecules, Vol. 43, No. 24, 2010 10463
after irradiation of 365 nm light, which was explained by an
increase in dipole moment of the cis form and thus an
increased local polarity of the polymer chain. The reason
why CP decreases after irradiation of 365 nm light might be
attributed that hPEA211-AZOs are dispersed as nanoparti-
cles instead of single polymer chain in water.
determined by AFM is larger than that of DLS analysis,
which might be resulted from that the nanoparticles were
somewhat flattened on the mica sheet in AFM experiments,
To understand what happened to hPEA211-AZO nano-
particles after UV irradiation (365 nm), DLS experiments
were conducted. The size distribution of hPEA211-AZO10
aqueous solution before and after UV irradiation is shown in
Figure 8, which indicates that the average diameter of the
hPEA211-AZO10 nanoparticles increases from 12.5 ( 1.9
nm to 16.3 ( 2.2 nm after UV irradiation. Similar results are
also obtained by DLS measurements of the other hPEA211-
AZO nanoparticles, which are summarized in Table 2. The
increase of the size of hPEA211-AZO nanoparticles is
further confirmed by the AFM images of hPEA211-
AZO10 in aqueous solution. As shown in Figure 9, the size
of hPEA211-AZO10 nanoparticles increases from ∼25 nm
to ∼35 nm after UV irradiation of 365 nm. The diameter
Figure 10. (a) Dependence of optical transmittance at 700 nm for
hPEA211-AZO10 aqueous solution on irradiation time at 365 nm at
pH 7.4/28 °C and photograph before and after phase transition.
Polymer concentration is 3 mg/mL. (b) Size distribution of hPEA211-
AZO10 aqueous solution at pH 7.4/25 °C before and after UV
irradiation (365 nm) for 40 min.
Figure 8. Size distribution of hPEA211-AZO10 aqueous solution at
pH 7.4/25 °C before and after UV irradiation (365 nm) for 40 min.
Figure 9. AFM images of hPEA211-AZO10 in aqueous solution: (a) before UV irradiation; (b) after UV irradiation (365 nm) for 40 min.

1
0464 Macromolecules, Vol. 43, No. 24, 2010
Scheme 3. Proposed Mechanism of the Light Response of hPEA211-AZO Aqueous Solution
Yu et al.
and this phenomenon is also proved by the height profiles of
the AFM images in Figure 9. The increase of the size of
hPEA211-AZO10 nanoparticles after UV irradiation should
be caused by the isomerization of azobenzene from trans to
cis form. Because the trans form of azobenzene is more
regular, the cis form can not pack as closely as trans form
in the core of hPEA211-AZO10 nanoparticles, resulting in
the increase of the size of the core. Because of the looser
packing of the cis form in the core, the isomerization from cis
to trans form should be faster under visible light irradiation
irradiation (365 nm) is believed to be the key factor to cause
the CP shift to the lower temperature.
Accordingly, light-induced aggregation of hPEA211-
AZO nanoparticles was possible within the temperature
range between CP
CP (trans form, before irradiation). Taking hPEA211-
(cis form, after UV irradiation) and
C
T
AZO10 as an example, Figure 10a illustrated the depen-
dence of optical transmittance for hPEA211-AZO10 aque-
ous solution on UV irradiation time with inset of the
photographs. hPEA211-AZO10 can be dispersed in water
at 28 °C to form an orange transparent aqueous solution,
and its optical transmittance at 700 nm is about 100%. After
UV irradiation, the optical transmittance of the hPEA211-
AZO10 aqueous solution decreases sharply to ∼0% at about
400 s, and the solution turns into orange opaque suspension.
This phenomenon can also be supported by DLS experi-
ments, and the results are shown in Figure 10b. After UV
irradiation, only one peak with an average diameter ∼1800
nm was observed in DLS experiment, suggesting the light-
induced aggregation of the nanoparticles. As a result, the
hPEA211-AZO10 aqueous solution turned opaque. On the
basis of these results, the proposed mechanism for the light-
induced change of the morphology of hPEA211-AZO nano-
particles in aqueous solution was illustrated in Scheme 3. UV
irradiation (365 nm) can make hPEA211-AZO nanoparticles
(450 nm) than that from trans to cis form, which explains the
results from Figure 6.
Taking the structure of hPEA211-AZO nanoparticles into
consideration, the polarity of azobenzene has little effect on
CP of hPEA211-AZO nanoparticles as most of the azoben-
zene moieties are in the core, which have no interact with
water molecules. At the same time, the nanoparticles are
stabilized by the hydrophilic PEO shell which prevents them
from aggregating together in aqueous solution. During UV
irradiation, the size of hPEA211-AZO nanoparticles be-
comes larger slowly. When the core of the nanoparticles
becomes large enough and the hydrophilic PEO shell may
not be sufficient to stabilize the nanoparticles, these nano-
micelles will aggregate at lower temperature. Therefore, the
larger size of hPEA211-AZO nanoparticles induced by UV

Article
Macromolecules, Vol. 43, No. 24, 2010 10465
become larger nanoparticles or aggregate into microparti-
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Conclusion
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4
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1
0-18 nm in aqueous solution, which exhibited very sharp
response to temperature, pH, ionic strength and light with well-
tunable CP from 25 to 90 °C. After UV irradiation (365 nm), the
lower value of CP was observed, which is believed to be caused by
the larger size of hPEA211-AZO nanoparticles. The CP differ-
ence of hPEA211-AZO nanoparticles in aqueous solution be-
tween before (trans form) and after (cis form) UV irradiation
increases linearly upon the azobenzene content up to 5 °C.
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8
Acknowledgment. We thank the National Nature Science
Foundation of China (No: 50803036) and Science & Technology
Commission of Shanghai Municipal Government (No: 08520704700)
for their financial support.
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2
Supporting Information Available: Figures showing FT-IR
spectra of hPEA211, E-AZO, and hPEA211-AZO10 and a plot
of CP and ΔCP of 3 mg/mL hPEA211-AZO aqueous solutions
at pH 7.4 during irradiation as a function of azobenzene content
in hPEA211-AZO. This material is available free of charge via
the Internet at http://pubs.acs.org.
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