Dual thermo- and light-responsive nanorods from self-assembly of the 4-propoxyazobenzene-terminated poly(N-isopropylacrylamide) in aqueous solution

Article abstract of DOI:10.1016/j.polymer.2015.07.035

Abstract Both temperature and light stimuli-responsive 4-propoxyazobenzene-terminated poly(N-isopropylacrylamide)s (PNIPAMs) were successfully synthesized by the atom transfer radical polymerization (ATRP) of NIPAM. ¹H NMR and UV-vis absorption spectra indicated rapid photoisomerization rate of 4-propoxyazobenzene moiety. Interestingly, the lower critical solution temperature (LCST) for PNIPAM aqueous solution clearly decreased after UV irradiation, and the repeated LCST difference (ΔT_maxy = 5°C) depended on both the number-average molecular weight and amount of azobenzene chromophore. Dynamic light scattering (DLS) and static light scattering (SLS) measurements showed that the PNIPAM aqueous solution could self-assemble into nano-micelles with 4-propoxyazobenzene as the hydrophobic cores and PNIPAM chains as the hydrophilic shells. UV irradiation induced the increase of particle size due to the formation of much looser cores of cis-azobenzene. TEM images showed the presence of both nanorods and spherical micelles. After UV irradiation, the unstable spherical micelles transformed to metastable nanorods, then to longer rods, and finally the longer rods began to transform to flake-like particles via horizontal inter-rod aggregation above LCST.

Full text of DOI:10.1016/j.polymer.2015.07.035

Accepted Manuscript
Dual Thermo- and Light-responsive Nanorods from Self-Assembly of the 4-
Propoxyazobenzene-Terminated Poly(N-isopropylacrylamide) in Aqueous Solution
Xiaoqiang Xue, Jing Yang, Wenyan Huang, Hongjun Yang, Bibiao Jiang, Fang Li
PII:
S0032-3861(15)30116-6
10.1016/j.polymer.2015.07.035
JPOL 17995
DOI:
Reference:
To appear in: Polymer
Received Date: 21 April 2015
Revised Date: 27 June 2015
Accepted Date: 20 July 2015
Please cite this article as: Xue X, Yang J, Huang W, Yang H, Jiang B, Li F, Dual Thermo- and
Light-responsive Nanorods from Self-Assembly of the 4-Propoxyazobenzene-Terminated Poly(N-
isopropylacrylamide) in Aqueous Solution, Polymer (2015), doi: 10.1016/j.polymer.2015.07.035.
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ACCEPTED MANUSCRIPT
Dual Thermo- and Light-responsive Nanorods from Self-Assembly
of the 4-Propoxyazobenzene-Terminated
Poly(N-isopropylacrylamide) in Aqueous Solution
Xiaoqiang Xue, Jing Yang, Wenyan Huang, Hongjun Yang, Bibiao Jiang*, Fang Li
Jiangsu Key Laboratory of Material Surface Science and Technology, School of Material
Science and Engineering, Changzhou University, Changzhou, Jiangsu, 213164, People's
Republic of China
*Corresponding author. Tel.: 8651986330328;
E-mail: jiangbibiao@cczu.edu.cn
1

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Abstract
Both
temperature
and
light
stimuli-responsive
4-propoxyazobenzene-terminated poly(N-isopropylacrylamide)s (PNIPAMs) were
successfully synthesized by the atom transfer radical polymerization (ATRP) of NIPAM.
1
H NMR and UV-vis absorption spectra indicated rapid photoisomerization rate of
4-propoxyazobenzene moiety. Interestingly, the lower critical solution temperature
(
LCST) for PNIPAM aqueous solution clearly decreased after UV irradiation, and the
o
repeated LCST difference (∆Tmaxy = 5 C) depended on both the number-average
molecular weight and amount of azobenzene chromophore. Dynamic light scattering
(
DLS) and static light scattering (SLS) measurements showed that the PNIPAM aqueous
solution could self-assemble into nano-micelles with 4-propoxyazobenzene as the
hydrophobic cores and PNIPAM chains as the hydrophilic shells. UV irradiation induced
the increase of particle size due to the formation of much looser cores of cis-azobenzene.
TEM images showed the presence of both nanorods and spherical micelles. After UV
irradiation, the unstable spherical micelles transformed to metastable nanorods, then to
longer rods, and finally the longer rods began to transform to flake-like particles via
horizontal inter-rod aggregation above LCST.
Key
words:
Azobenzene
polymer;
Self-assembly;
Stimuli-responsive.
2

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1. Introduction
Over the past two decades, environmental stimuli-responsive polymers [1-21] to
temperature, pH, light, redox, ionic strength, or electric and magnetic fields have
attracted a great attention in the field of smart materials with their potential applications
in controlled drug delivery [1-5], smart surfaces [6-9], biomaterials [10-12], smart
sensors [13-16], and chromatographic separations [17-18]. In particular, one of the most
intriguing stimuli-responsive materials is a thermo-responsive polymer which exhibits a
readily accessible lower critical solution temperature (LCST) in aqueous solution. Such a
thermo-responsive polymer shows a reversible hydrophilic or hydrophobic property,
which can be switched by the variation of temperature. Interestingly, the LCST of
o
poly(N-isopropylacrylamide) (PNIPAM) is about 32 C, which is quite close to the
human body temperature, and therefore, PNIPAM became one of the most studied
thermo-responsive polymers [22-26]. Moreover, PNIPAM has been easily extended to
dual- or multi- stimuli responsive polymeric materials by designing complex and
well-controlled architectures [27-29], In fact, dual- or multi- stimuli responsive
polymeric materials to temperature-pH [27,30,31], temperature-ionic strength [32,33],
temperature-light [34,35], temperature-pH-light [36], temperature-pH-ionic strength
[
37-39] have been synthesized.
Recently, azobenzene compounds have gained a great deal of attention due to their
unique reversible trans-cis-trans photoisomerization [40-42]. Thus, these polymers
containing azobenzene chromophore (azobenzene polymers) are expected to be useful
3

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for applications in optical data storage,43 holographic surface relief grating (SRG)
[
44-46], liquid crystal display [47,48], optical switching [49,50] and so on. Moreover,
azobenzene polymers can also be used in the new area of light stimuli-responsive
materials [51-54] with fascinating and advantageous properties for potential uses
especially in remote activation, reversible incalculability, and in precisely controlling
wavelength, illuminated area, direction, and intensity.
There have been several reports on the temperature-light responsive polymers
containing azobenzene chromophore. Amphiphilic copolymers [55-63] with azobenzene
located on the side-chain or main-chain can be self-assembled into nanoparticles in
aqueous solution, and azobenzene groups can undergo reversible trans-cis
photoisomerization upon UV and visible light irradiation leading to significant changes
in geometry and polarity of azobenzene units. Yu and coworkers [56] reported that
amphiphilic
diblock
copolymers
were
composed
of
ethylene
oxide,
azobenzene-containing methacrylate and NIPAM units, and their results showed that
both temperature and light irradiation could induce a reversible change of hydrophobicity
of the micellar cores. Zhao and coworkers [60] successfully synthesized
poly(N-isopropylacrylamide) (PNIPAM) with an azobenzene-containing short segment
repeatedly inserted into the main chain, which self-assembled into flower micelles at
temperatures below the LCST of PNIPAM, but photoisomerization of azobenzene was
found to exert little effect on the LCST.
Another exciting azobenzene-terminated PNIPAM can also be used to control the
4

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phase transition of polymer, which was firstly published by Akiyama and Tamaoki. [64]
The LCST of these polymer solutions could also be altered reversibly by light irradiation
as well. Subsequently, Theato and coworkers [65] reported the synthesis of
poly(oligo(ethylene glycol) methyl ether methacrylate) (P(OEGMA)) polymers with
either α- or telechelic azobenzene functionalities, and there was a maximum LCST shift
o
of 4.3 C with light irradiation. Unusually, Jiang and coworkers [66] reported a unique
amphiphilic azobenzene-containing hyperbranched poly(ether amine) (hPEA-AZO),
which had a lower value of tunable cloud point (CP) after UV irradiation. The trans form
of azobenzene was more regular, but after UV irradiation, the trans form of azobenzene
changed into the cis form, and the cis form could not pack as closely as trans form in the
core of hPEA211-AZO nanoparticles resulting in larger size for hPEA-AZO
nanoparticles and lower value of the LCST. Chen et. Al. [67] extended the investigation
of the hyperbranched poly(ethylenimine) by terminating with the azobenzene groups. In
addition, they also showed that trans to cis isomerization of azobenzene units increased
the CP value at pH ≈ 7, while the opposite occurred at pH ≈ 9.
The above reports indicated that the changes (increase or decrease) in the LCST of
azobenzene-terminated polymers depended on both the isomerization of azobenzene and
topology of the polymer. To the best of our knowledge, the effect of azobenzene
structure on changes in LCST under light irradiation has not yet been investigated in
detail. Akiyama and Tamaoki [64] synthesized thermo-responsive PNIPAM with the
hydroxyethoxy (polarity substituent) azobenzene group at the chain end, which resulted
5

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in the controlled LCST shift. If polarity substituent is changed into hydrophobic structure,
the obtained azobenzene-terminated homopolymers in aqueous solution may
self-assemble into nano-micelles, [69] which leads to the opposite changes after UV
irradiation. Therefore, in this paper, we successfully synthesized the
4-propoxyazobenzene-containing
initiator
(
4-propoxy-4′-(2-bromopropionyloxy)azobenzene, PAzo-Br) using the atom transfer
radical polymerization (ATRP) of NIPAM. The unique temperature-light dual responsive
behavior of the obtained 4-propoxyazobenzene-terminated polymer in water was
investigated in detail, and the obtained results are presented here.
2. Experimental part
2
.1. Materials.
-Bromoisobutyryl bromide (98%; Aldrich), p-nitrophenol (analytical reagent;
2
Shanghai Chemical Reagent Co. Ltd., Shanghai, China), 1-bromopropane (analytical
reagent; Shanghai Chemical Reagent Co. Ltd., Shanghai, China), Nile Red (≥98%;
Nanjing Oddfoni Biological Technology Co., Ltd., Nanjing, China) were used as
received. N-isopropylacrylamide (NIPAM; 98%; Aldrich) was purified thrice by
recrystallization from cyclohexane. Copper (I) chloride (CuCl; chemical pure; Shanghai
Chemical Reagent Co. Ltd., Shanghai, China) was purified via washing with acetic acid
and acetone, and then dried in vacuo. Tris[2-(dimethylamino)ethyl]amine (Me TREN)
6
was synthesized according to the previously described procedure in the literature. [69]
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Other reagents were purified using the standard procedures before use.
2.2. Characterization
Conversion of the reactants was determined using an HP-689 gas chromatography
equipped with an HP-5 column (30m×0.54mm×0.5µm). Isopropyl alcohol was used as
the internal standard. The carrier gas was hydrogen and the flow rate was set at 1 mL
-1
o
min . The column temperature was increased from a starting value of 80 C at sample
o
o
-1
injection to a maximum of 230 C at 10 C min . The peaks were identified using
chromatograms of the corresponding pure reactants. The purities of products were
determined using a Waters e2695 high performance liquid chromatography (HPLC),
comprising a Waters 2998 UV detector and X Bridge C18 column (5 µm, 4.6×250 mm).
A mixture of acetonitrile (HPLC grade) and water (deionized and filtered with 0.45 µm
membrane filter) at the gradient volume ratio (40/60~90/10~40/60) was used as the
-1
o
eluent. The flow rate was at 1.0 mL min and the column temperature was 30 C. The
molecular weights (Mw GPCs and Mn GPCs) and molecular weight distributions (M /M s) of
w
n
the polymers were determined with a Waters 1515 gel permeation chromatographer
GPC) equipped with a refractive index detector, using HR1, HR3, and HR4 column
(
-1
with molecular weights in the range of 100-500,000 g mol which were calibrated with
polystyrene (PS) standard samples. THF was used as the eluent at a flow rate of 1.0 mL
-1
o
1
min operated at 30 C. H NMR spectra of the polymers were recorded on a Bruker
ARX-500 type nuclear magnetic resonance instrument, using CDCl or D O as solvent
3
2
and tetramethyl-silane (TMS) as the internal standard. Ultraviolet visible (UV-vis)
7

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absorption spectra and the transmittance at 550 nm light for lower critical solution
-1
temperature (LCST) of polymer solution (2 mg mL ) in water were performed on an
Agilent Cary 100 equipped with a Cary dual cell peltier accessory (Varian). Dynamic
light scattering (DLS) and static light scattering (SLS) data were acquired using an
o
ALV/CGS-3 compact goniometer system at 25 C, and a He-Ne laser operating at a
wavelength of λ = 632.8 nm was used as a light source. Sample aqueous solutions for
0
-1
analysis (2 mg mL ) were poured into the sample bottle, which were placed in sample
cell filled with toluene used as the immersion liquid. Temperature control of the sample
was provided by an external thermostated circulating bath. The accessible scattering
angles range from 30 to 150°. Transmission electron microscopy (TEM) was recorded on
a JEM-2100 TEM at a 100 kV accelerating voltage. Fluorescence measurements were
recorded using a Cary eclipse fluorescence spectrophotometer equipped with Carry
-1
temperature controller (Agilent technologies) with a scanning rate of 600 nm min . The
excitation wavelength was 600 nm. The excitation and emission slits were 10 and 2.5 nm,
respectively.
2.3. Synthesis of 4-propoxy-4′-(2-bromopropionyloxy)azobenzene (PAzo-Br)
The synthetic route of PAzo-Br is shown in Scheme S1 ( see Supporting
Information), and the detailed synthesis procedure and characterization are given below.
2.3.1. 1-Nitro-4-propoxybenzene
A solution of p-nitrophenol (5.00 g, 36.0 mmol), 1-bromopropane (3.66 g, 30.0
8

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mmol), Na CO (3.82 g, 36.0 mmol), and 100 mL of N,N-dimethylformamide (DMF)
2
3
was added to a 250 mL round bottom flask under vigorous stirring. The solution was
o
stirred under reflux at 140 C for 6 h. After cooling to room temperature, the mixture was
poured into 200 mL of water under vigorous stirring. The crude product was extracted
with dichloromethane (50 mL) and washed with 5% Na CO aqueous solution and
2
3
deionized water three times. The organic layer was dried with anhydrous MgSO4
overnight, filtered and evaporated under reduced pressure. The pure
1
-nitro-4-propoxybenzene (3.92 g, yield: 72.2%) was obtained as yellow liquid. The
1
purity was 93.5% (HPLC). H NMR (400MHz, CDCl
3
), δ (TMS, ppm): 8.13-8.22 (m,
O), 1.77-1.90 (m, 2H, CH ),
2
H, ArH), 6.87-6.97(m, 2H, ArH), 3.95-4.05 (t, 2H, CH
.99-1.10 (t, 3H, CH ).
.3.2. 4-Propoxyaniline
A mixture of iron powder (1.15 g, 30.0 mmol) and NH Cl salt (2.65 g, 50.0 mmol)
2
2
0
3
2
4
in 50 mL of distilled water was prepared in a 500 mL flask under vigorous stirring. Then,
a 100 mL methanolic solution (3.62 g, 20.0 mmol) was added dropwise into this mixture
over 10 min at room temperature. The resultant mixture was stirred under gentle reflux
for 5 h. After raising the room temperature, the inorganic residue was filtered, and the
methanol was evaporated under vacuum. The mixture was extracted with ethyl acetate
(
50 mL) and washed with deionized water (90 mL) three times. The organic layer was
dried with anhydrous MgSO overnight, filtered and evaporated under reduced pressure.
4
The final crude 4-propoxyaniline (2.60 g) was stored in dark condition.
9

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2.3.3. 4-Propoxy-4′-hydroxyazobenzene
The crude 4-propoxyaniline (2.60 g, about 16.0 mmol) was dissolved in
concentrated HCl (5 mL) and deionized water (15 mL). The mixture prepared by stirring
o
at 0-5 C in an ice bath, and then aqueous solution (2 mL) of NaNO (1.24 g, 18.0 mmol)
2
o
was added slowly. A yellow transparent diazonium salt solution was stirred at 0-5 C for
3
0 min. A coupling solution of phenol (1.88 g, 20.0 mmol), Na CO (3.18 g, 30.0 mmol),
2 3
o
NaHCO (2.52 g, 30.0 mmol) and deionized water (50 mL) was cooled to 0 C. Then the
3
diazonium salt solution was added dropwise to the coupling solution at a temperature
o
range of 0-5 C. The pH of the mixture was adjusted to 8~9 using a 40% NaOH solution,
o
and the mixture was further stirred for 5 h at 0-5 C. A red-orange precipitate was
collected by filtration, washed with deionized water several times, and dried under
vacuum. The crude products were recrystallized three times from ethanol to achieve pure
4
-propoxy-4′-hydroxyazobenzene as red-orange crystal (3.30 g, yield: 81.2%). The
1
purity was 95.8% (HPLC). H NMR (400MHz, CDCl ), δ (TMS, ppm): 7.76-7.89 (m,
3
4
1
H, ArH), 6.96-7.01 (d, 2H, ArH), 6.88-6.95 (d, 2H, ArH), 3.96-4.04 (t, 2H, CH O),
2
.77-1.91 (m, 2H, CH ), 1.00-1.10 (t, 3H, CH ).
2
3
2
.3.4. PAzo-Br
-Propoxy-4′-hydroxyazobenzene (3.07 g, 12.0 mmol), freshly distilled
triethylamine (1.52 g, 15.0 mmol) and freshly distilled THF (50 mL) were added to a
50 mL three-necked flask. The solution was stirred in an ice bath. 2-Bromoisobutyryl
bromide (1.85 mL, 15.0 mmol) diluted in dry THF (20 mL) was then added to the cool
4
2
1
0

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o
stirred mixture dropwise. The reaction mixture was vigorously stirred for 5 h at 0-5 C
and then at room temperature overnight. The mixture was filtered, and the filtrate was
evaporated under vacuum. The remaining yellow mixture was dissolved in ethyl acetate
and washed with 5% Na CO aqueous solution and deionized water three times, dried
2
3
with anhydrous MgSO overnight, filtered and evaporated under reduced pressure. The
4
final crude product was recrystallized three times from ethanol to yield yellow solid
1
(
3.12 g, yield: 64.2%). The purity was above 98% (HPLC). H NMR (400MHz, CDCl ),
3
δ (TMS, ppm): 7.85-8.00 (m, 4H, ArH), 7.22-7.32 (d, 2H, ArH), 6.97-7.05 (d, 2H, ArH),
3
2 3 2 2
.96-4.06 (t, 2H, CH O), 2.10 (s, 6H, C(CH ) ), 1.79-1.91 (m, 2H, CH ), 1.02-1.11 (t, 3H,
1
CH
3
) ( H NMR spectrum was provided in Supporting Information, Figure S1).
2.4.
General
procedure
for
the
4-propoxyazobenzene-terminated
poly(N-isopropylacrylamide) (PNIPAM1~PNIPAM7) using PAzo-Br
Me TREN (0.230 g, 1.00 mmol), CuCl (0.033 g, 0.33 mmol), isopropyl alcohol
6
(
3.00 g) and water (1.00 g) were added to a 25 mL Schlenk flask equipped with a stir bar.
The flask was cycled between vacuum and argon (3 times) in an ice bath, and the
solution was stirred for 30 min. PAzo-Br (0.135 g, 0.33 mmol) and NIPAM (1.13 g,
10.00 mmol) were added to the reaction vessel. After three freeze-pump-thaw cycles, the
o
flask was placed in an oil bath to start the polymerization at 25 C. After a preset reaction
time, the polymerization was stopped by adding CuBr (0.074 g, 0.33 mmol). After
2
solvent evaporation, the crude polymers were dissolved in about 10 mL THF, passed
2
+
through a small neutral Al O chromatographic column to remove Cu deactivator, and
2
3
11

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precipitated by dropwise addition to cold hexane (200 mL). The precipitates were
filtrated. The obtained polymers were redissolved in THF, isolated by precipitation into
hexane, and dried to a constant weight at room temperature in vacuum. The other
polymers were prepared using similar procedures. All the polymers were dissolved in
THF and filtered by PTFE film for GPC analysis. The samples were dissolved in CDCl3,
1
and measured by H NMR spectroscopy.
3. Results and discussion
3.1. Synthesis of the 4-propoxyazobenzene-terminated poly(N-isopropylacrylamide)
(PNIPAMs) via the atom transfer radical polymerization (ATRP)
The synthesis route is shown in Scheme 1, and the detailed discussion of PNIPAMs
were provided in Supporting Information.
1
2

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Scheme 1. Synthetic Route of the 4-propoxyazobenzene-terminated poly(N-isopropylacrylamide)
(
PNIPAM).
3.2. Photoisomerization behavior of 4-propoxyazobenzene-terminated PNIPAM
Azobenzene compounds exhibited photoisomerization behavior, which undergoes
reversible conversion between trans- and cis- form under the irradiation of UV and
1
visible light. Closer inspection of the H NMR spectra of PNIPAM4 (Figure 1) could
provide a powerful line of evidence about trans-cis form variation of azobenzene at the
end of the polymer after UV irradiation. As shown in Figure 1, the weak signals at
around 7.82-7.98 ppm (a in Figure 1) due to phenyl protons of the trans-form
azobenzene group were mostly shifted upfield resulting in two peaks at 6.85-6.98 ppm
and 6.71-6.81 ppm (a’ in Figure 1) relative to the cis- form under only 5 min UV
irradiation (365 nm). Meanwhile, a new resonance signal at 3.92 ppm (b in Figure 1)
1
3

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appeared and should be assigned to methylene protons from the terminal
4-propoxyazobenzene, which were overlapped by the methenyl protons of the NIPAM
units before UV irradiation. However, after a prolonged UV irradiation (15 and 30 min),
the trans-form of the azobenzene did not completely disappear, which indicated the
incomplete photoisomerization, and content of trans-form azobenzene (xtrans) was still
only 25% according to the integrals calculated by eq 1.
I
7.82 - 7.98
xtrans
=
×100%
(1)
I
7.82 - 7.98 + I6.85 - 6.98 + I6.71 - 6.81
Where, I7.82-7.98, I6.85-6.98 and I6.71-6.81 were the integrals of the signals at 7.82-7.98 ppm,
6.85-6.98 ppm and 6.71-6.81 ppm in Figure 1 (B), respectively.
1
4

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1
-1
Figure 1. H NMR spectra of the PNIPAM4 (M
= 5300 g mol , M /M = 1.16) in CDCl under
n GPC
w
n
3
irradiation of 365 nm UV light for 0 min (A); 5 min (B); 15 min (C); 30 min (D).
3
In addition, the UV-vis absorption spectra of PNIPAM4 in CHCl solution were
recorded at different time intervals of UV irradiation until photostationary state was
reached (Figure 2 (A)). Upon UV irradiation (365 nm), the maximum absorption at about
1
5

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350 nm corresponding to the π-π* transition of trans azobenzene in polymer decreased
7
0
rapidly. Meanwhile, the weak n-π* transition band of the cis form azobenzene at about
440 nm slightly increased with the irradiation time. The trans-form of the azobenzene
did not completely disappear as there was about 27% trans-form of azobenzene, which
1
was consistent with the results determined by H NMR spectra of PNIPAM4.
Interestingly, there were only 30 s to reach photostationary state, and the trans-cis
isomerization rate of PNIPAM4 was very fast. On the other hand, cis-trans isomerization
of the irradiated PNIPAM4 could also be achieved by visible light irradiation. As shown
in Figure 2 (B), the absorption of trans azobenzene at 350 nm was restored to the initial
state of PNIPAM4 in 1000 s as same as the absorption of cis form, which showed the
equilibrium between cis and trans forms was completely established. This result
confirmed that the rapid photoisomerization of azobenzene in the end of PNIPAM was
reversible between the UV and visible light irradiation, which could have an appealing
application in light stimuli-responsive material.
1
6

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-1
Figure 2. Evolution of UV-vis absorption spectra of the polymer PNIPAM4 (Mn GPC = 5300 g mol ,
M /M = 1.16) aqueous solution prepared under different time intervals in CHCl (The
w
n
3
-1
concentration of solution is 2 mg mL ) at room temperature with UV light of 365 nm (A) and
further visible light (B) irradiation.
3.3. The response of 4-propoxyazobenzene-terminated PNIPAMs to temperature and light
It is well-known that PNIPAM is a widely investigated thermo-responsive polymer,
which exhibits reversible drastic phase-transition at its lower critical solution
o
temperature (LCST) around 32 C. The introduction of the hydrophilic or hydrophobic
moiety into the PNIPAM chain would appreciably increase or decrease the LCST of the
corresponding polymer in a characteristic manner. On the basis of this valuable behavior
of PNIPAM, we successfully introduced a dual-stimuli responsive PNIPAM terminated
with the hydrophobic 4-propoxyazobenzene moiety, and then studied temperature
dependence of the 550 nm optical transmittance to measure the changes of LCSTs of the
-1
PNIPAM aqueous solution (2 mg mL ) before and after UV (365 nm) and further visible
light irradiation. These results are presented in Figure 3 (A). The transmittance of
o
PNIPAM4 solution remained almost constant below 29 � C, and the sharp decrease of
o
transmittance above 29 � C means phase separation without precipitation. The LCSTs of
PNIPAMs ended with 4-propoxyazobenzene as listed in Table S1 (see the Supporting
o
Information) were lower than the normal value (about � 32 � C), which was due to the
hydrophobicity of azobenzene structure [64]. Moreover, this process under the
o
o
alternating temperature of 25 C and 35 C was reversible and repeatable at least 5 cycles
1
7

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without any change of transmittance, as shown in Figure S4 (see the Supporting
Information). By the incorporation of 4-propoxyazobenzene in the end of PNIPAM, the
LCST would be influenced by the photoisomerization of azobenzene [64,65].
Surprisingly, when the PNIPAM4 solution was exposed to UV light at 365 nm for 5 min,
the trans-cis photoisomerization reaction had completed a photostationary state, and
o
o
there was a significant decrease (~5 C) of the LCST value (~24 C) compared with that
o
of the initial state (~29 C). This result is different from the previously reports about
linear PNIPAM containing azobenzene group [64,65]. Moreover, the transmittance
decreased thereafter with accompanied by expanding temperature range of the transition,
which was consistent with the previously reports due to inhomogeneity in the polymer
[
71]. The LCST downshift (∆T) of the irradiated PNIPAM4 solution could be also
restored to the initial temperature under the further exposure of visible light (Figure
(A)), and the LCST differences between irradiated and nonirradiated solution were
3
affected by amount of azobenzene chromophore [63-65] of PNIPAM. It is interesting to
note that in Figure 3 (B), and the LCST downshifts increased, as expected, from 1.9 to
o
-1
5
.0 � C with the increase of Mn GPC from 3100 to 5300 g mol , following decrease with
the decreasing amount of azobenzene chromophore. The results thus showed that the
LCST downshifts of 4-propoxyazobenzene-terminated PNIPAM depended on both the
Mn GPC and amount of azobenzene chromophore. Interestingly, this material might be
well used for the light-responsive trigger switch of the phase transition with the large
temperature range. Figure 4 showed the changes in transmittance of PNIPAM4 solution
1
8

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-1
o
(
2 mg mL ) irradiated with alternating UV and visible light at 28 C. The transmittance
between 99% and 12% was reversibly switched by the tuning of external conditions of
UV light and visible light irradiation, which indicated that the phase transition of the
PNIPAM ended with 4-propoxyazobenzene could be reversibly controlled by light.
-1
Figure 3. (A) Temperature dependence of the transmittance at 550 nm light through a 2 mg mL
-1
w n
PNIPAM4 (Mn GPC = 5300 g mol , M /M = 1.16) aqueous solution; (B) Lower critical solution
temperature (LCST) downshifts (∆T) of PNIPAMs plotted as a function of the molecular weights (Mn
GPCs) of PNIPAMs.
1
9

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Figure 4. Reversible changes of transmittance at 550 nm for the PNIPAM4 (Mn GPC = 5300 g/mol,
-1
o
M /M = 1.16) aqueous solution (2 mg mL ) at 28 C under alternating irradiation of UV and
w
n
visible light.
Generally speaking, the LCST of PNIPAM functionalized with azobenzene units
could shift in polymer water solution response upon exposure to UV light [63-65,71],
which was due to the higher polarity of cis-form than trans-form azobenzene. However,
when the terminated azobenzene in PNIPAM was designed to hydrophobic
4-propoxyazobenzene,
the
results
were
the
opposite.
This
4-propoxyazobenzene-terminated PNIPAM aqueous solution could self-assemble into
nano-micelles [68], and UV irradiation induced changes to the aggregate micelles with
the result of the LCST downshift [66]. Therefore, the transition temperature of
PNIPAM4 solution by measuring the transmittance could not accurately reflect the LCST
of the isolated polymer chains, and it displayed the solution of the aggregates. The
2
0

ACCEPTED MANUSCRIPT
change in the transmission was caused by the aggregation of the self-assembled
1
structures. The LCST of PNIPAM4 solution could be determined by H NMR spectra in
D O (Micro-THF as the internal standard) with the different temperature before and after
2
UV irradiation. As shown in Figure 5 (A), the weak signals at around 7.82-7.98 ppm (b)
due to phenyl protons of the trans-form azobenzene group before UV irradiation
decreased rapidly as the temperature increased, which was due to the closer cores of
trans-azobenzene from the hydrophobic conversion of the PNIPAM chains. When the
o
temperature was up to 31 C, phenyl proton signals disappeared, which demonstrated
that the isolated polymer chains got together via the hydrophobic conversion. After UV
irradiation, the phenyl proton signals of the trans-form azobenzene were mostly shifted
upfield at 6.57-7.15 ppm relative to the cis-form. The most important point was that
o
phenyl proton signals disappeared at the lower temperature (29 C), and the LCST of
PNIPAM solution decreased after UV irradiation. In order to determine the LCST of
PNIPAM4 solution, the relative integrals (THF integrals as the internal standard at
3.70-3.80 ppm ) of the signals at 7.82-7.98 ppm (a) had been traced on line at the
different temperature in Figure 5, and the results was shown in Figure 6. It is also clear
o
from Figure 6 that the LCST of PNIPAM4 solution was about 25 C before UV
o
irradiation, and 28 C after UV irradiation, respectively, and these results accorded with
the experiments by measuring the transmittance.
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1

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1
-1
-1
Figure 5. H NMR spectra of the 2 mg mL PNIPAM4 (M
= 5300 g mol , M /M = 1.16)
w n
n GPC
solution in D O (Micro-THF as the internal standard) with the different temperature. A:
2
Before UV irradiation; B: After UV irradiation for 30 min.
-1
Figure 6. Temperature dependence of I3.90-4.10/I3.70-3.80 through a 2 mg mL PNIPAM4 (M
= 5300
n GPC
-1
g mol , M
w
/M
n
= 1.16) aqueous solution, I3.90-4.10 and I3.70-3.80 were the integrals of the
1
signals at 3.90-4.10 ppm and 3.70-3.80 ppm in H NMR spectra of the PNIPAM4 (Figure
5
).
In order to understand why the photoisomerization of azobenzene could downshift
the LCST of the 4-propoxyazobenzene-terminated PNIPAM aqueous solution, we
conducted detailed experiments with the light-responsive self-assembly behavior of
polymer solution by dynamic light scattering (DLS) and static light scattering (SLS)
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2

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measurements simultaneously as shown in Figure 7 (A and B). Figure 7 (A) showed
intensity-weighted size distributions of R for the PNIPAM4 solution before UV
h
o
o
irradiation (23 and 25 C) and after UV irradiation (23, 23.6 and 24 C). DLS
measurements indicated that sample PNIPAM4 formed a rather broad size distribution of
micelles with an average intensity-weighted size of about 17 nm, which remained stable
o
at 23 and 25 C. These results showed that the particles were composed of the
hydrophobic core (terminated 4-propoxyazobenzene) and the hydrophilic shell (PNIPAM
chains) in amphiphilic homopolymer (PNIPAM4) [68]. Due to both tran- and cis-
azobenzene in the initial state, these micelles contained some larger particles, which
were in exist with complex morphology as revealed by the tailing peak. After UV
h
irradiation, the R of the PNIPAM4 solution increased to 29 nm, and the size distribution
developed into bimodal distribution with much wider, which indicated the PNIPAM4
further self-assembled to generate the larger micelles. The hydrophilic PNIPAM chains
transformed to hydrophobic chains by heating. The collapsed micelles became unstable
and assemble together as multiple larger particles. Moreover, R increased significantly
h
o
to 70 and 106 nm with raising temperature (23.6 and 24 C), and the bimodal distribution
was rapidly changed into trimodal and multimodal type, which suggests that UV
irradiation induced changes to the aggregate micelles with a faster thermo-response.
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3

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h
Figure 7. (A) Intensity-weighted size distributions of the hydrodynamic radius (R ) obtained by
-1
-1
dynamic light scattering (DLS) for a 2 mg mL PNIPAM4 (M
= 5300 g mol , M /M = 1.16)
w n
n GPC
aqueous solution measured before UV irradiation; (B) Temperature dependence of average radius of
gyration (<R >) and average hydrodynamic radius (<R >) of the PNIPAM4 aqueous solution.
g
h
On the other hand, SLS measurements at several angles were extrapolated in a Zimm
plot, and self-assembly properties were further calculated in Figure 7 (B). Figure 7 (B)
shows temperature dependence of average radius of gyration (<R >) and average
g
hydrodynamic radius (<R >) of the PNIPAM4 solution obtained from SLS and DLS
h
o
measurements, respectively. When the solution was heated to 27.0 C, there was a stably
g h
aggregate size (<R > = 32 nm,), but <R > dropped slowly due to regional shrinkage of
the PNIPAM chains. An abrupt increase of the PNIPAM4 solution was observed above
o
2
7.0 C, which verified the LCST behavior and also displayed narrow gaps with optical
o
transmittance tests (29.0 C) due to instrumental errors. These results further confirmed
the micellar aggregation. Both of <R > and <R > unobtrusively changed before and after
g
h
o
irradiation below 22.5 C. When the temperature would have continued rising, the
2
4

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o
particles size began to grow after UV irradiation, and <R > was 35.8 nm at 22 C, 40 nm
g
o
o
at 22.5 C, 44 nm at 23 C, and so on. Therefore, UV irradiation induced not only
o
o
increase of particle size above 22.5 C, but also led to a decrease of the LCST (22.5 C).
The hydrophobic core of the nanostructured micelles was 4-propoxyazobenzene attached
to the end of PNIPAM. Before UV irradiation, trans form of azobenzene had a regular
structure, and PNIPAM4 solution formed the tight cores of trans-azobenzene in an
ordered array. When PNIPAM4 solution was irradiated by UV light, the cis form packed
to the loose cores with the disordered array in the PNIPAM4 nanoparticles, resulting in
the increase of the size of the micelles [66]. With a further increase in temperature, the
loose cores from the cis form azobenzene became unstable and formed the much larger
micelles due to the coiled PNIPAM chains.
2
5

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Figure 8. Ratio of average radius of gyration (<R >) and average hydrodynamic radius (<R >) for a 2
g
h
-1
-1
mg mL PNIPAM4 (M
= 5300 g mol , M /M = 1.16) aqueous solution at different
w n
n GPC
temperatures.
To get an insight into the morphology of the micelles, the ratio (<R >/<R >) of
g
h
average radius of gyration (<R
characterized by the aggregate particle shape and chain density distribution. Ordinarily,
>/<R > value could be attributed to spheres (<R >/<R > < 1) [72], vesicles
<R >/<R > > 1) [74]. Figure 8 shows the temperature
dependence of the <R >/<R > value for PNIPAM4 aqueous solution before and after UV
g h
>) and average hydrodynamic radius (<R >) could be
<R
g
h
g
h
7
3
(
g
h g h
> ≈ 1) and rods (<R >/<R
g
h
irradiation. All the <R >/<R > values in the interval 1.3-2.1 were attributed to the
g
h
2
6

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rod-like micelles. Before UV irradiation, the <R >/<R > values of the micelles gradually
g
h
decreased from 2.1 to 1.4 with raising temperature below LCST, while both the <R > and
g
<
R > decreased a little as shown in Figure 7 (B), because of intrachain contraction of
h
7
5
thermo-responsive PNIPAM chains. The lower value of <R >/<R > further confirmed
g
h
that the collapsed PNIPAM chain became the denser shell in the core-shell micelle. After
UV irradiation, the <R >/<R > values rapidly decreased from 2.0 to 1.3 in the narrow
g
h
o
temperature range of 22-24 C, further indicating the much faster temperature sensitivity
of the rod-like micelles compared with the unirradiated solution. When the temperature
o
of PNIPAM4 aqueous solution reached LCST (T = 24 C), the <R
g
>/<R
h
> value was
close to 2, which indicated the conformation change of aggregate PNIPAM particles.
Figure 9. Transmission electron micrographs (TEMs) of aggregates formed in the PNIPAM4 (Mn GPC
-1
-1
=
5300 g mol , M /M = 1.16) aqueous solution (2 mg mL ) at different temperature before (A:
w n
o
o
o
o
o
2
3 C; B :24 C) and after irradiation with UV light (C: 23 C; D: 24 C and E: 26 C). F: Images
of the PNIPAM4 solution before (left) and after irradiation with UV light (right).
2
7

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To further gain direct information about the rod-like micelles, typical transmission
electron micrographs (TEMs) in Figure 9 could directly exhibit the overall shape of the
particles. Before UV irradiation, Figure 9 (A) and (B) revealed the presence of both rods
(
about 10-80 nm in length) and spherical micelles (about 25-100 nm in diameter) with
rather broad size distribution, which is close to the R of particles in solution obtained by
h
DLS analysis. Therefore, the <R >/<R > (above 1) of the rods was an average value, and
g
h
o
the particles with rod-like morphology were predominant in PNIPAM4 solution at 24 C
Figure 9 (B)). During UV irradiation, spherical micelles disappeared with the whole
(
range of temperature (Figure 9 (C)), and the rods continued to grow longitudinally with
increasing temperature (Figure 9 (D)). It is likely that the hydrophobic trans-form of the
azobenzene transformed into the strong polar cis-form, which then induced conversion
from the unstable spheres to metastable rod-like micelles, along with the formation of
much looser cores from cis-azobenzene. Moreover, heating made the hydrophilic
PNIPAM shells shrink to rods with half width, and the two collapsed rods gather together
as the longer rods keeping the constant width (Scheme 2), which also confirmed the
collapsed PNIPAM chain became a denser shell in the core-shell micelle. When the
o
temperature increased to 26 C (higher than LCST), the longer rods began to transform
as the flake particles via horizontal inter-rod aggregation (Figure 9 (E)), which could be
attributed to the formed hydrophobic PNIPAM chains. Therefore, micelles
self-assembled by the 4-propoxyazobenzene-terminated PNIPAMs could be triggered to
2
8