Rational design, syntheses, characterization and solution behavior of amphiphilic azobenzene-containing linear-dendritic block copolymers

Article abstract of DOI:10.1016/j.polymer.2011.11.047

In this paper, based on the synthesis of precursor linear-dendritic block copolymers (LDBCs) with a linear poly(ethylene glycol) (PEG) block of molecular weight around 2000 and dendritic polyamidoamine (PAMAM) segments of generation G0 to G3, a series of azobenzene-containing amphiphilic LDBCs mPEG-dendr[PAMAM-(AZO)_n] (n = 2, 4, 8, 16) have been successfully prepared in moderate yields 45-60%, through complete Michael addition between the peripheral amine groups of dendritic segment and the reactive azobenzene acrylate mesogen units bearing octyloxy tails and flexible decylmethylene spacers. The purified copolymers have been characterized and confirmed by NMR, FT-IR, MALDI-TOF MS, and GPC showing narrow molecular weight distribution in the range 1.09-1.20. A generation-dependent polymeric micellar aggregation behavior of thus obtained amphiphilic LDBCs from nanofibers of uniform diameter around 20 nm to nanospheres/oval sheets, vesicles, and porous large compound micelles (LCMs) of micrometer size has been demonstrated in selective solvent mixture of dioxane/water. Furthermore, photoisomerization transformation of these azobenzene-containing LDBCs and their kinetics of reversible trans-cis-trans photochemical transitions in THF solution have also been investigated, the measured trans-cis photoisomerization rate constant 0.0240 s^-1 for G3 copolymer is twice of the 0.0127 s^-1 determined for the precursor azobenzene compound, manifesting a kind of promising photoresponsive copolymer materials.

Full text of DOI:10.1016/j.polymer.2011.11.047

Polymer 53 (2012) 359e369
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Rational design, syntheses, characterization and solution behavior of amphiphilic
azobenzene-containing linear-dendritic block copolymers
*
Zehua Shi, Huanjun Lu, Zhaocong Chen, Rongshi Cheng, Dongzhong Chen
Department of Polymer Science and Engineering, Key Laboratory of Mesoscopic Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering,
Nanjing University, Nanjing, Jiangsu 210093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, based on the synthesis of precursor linear-dendritic block copolymers (LDBCs) with a linear
poly(ethylene glycol) (PEG) block of molecular weight around 2000 and dendritic polyamidoamine
(PAMAM) segments of generation G0 to G3, a series of azobenzene-containing amphiphilic LDBCs mPEG-
dendr[PAMAM-(AZO)n] (n ¼ 2, 4, 8, 16) have been successfully prepared in moderate yields 45e60%,
through complete Michael addition between the peripheral amine groups of dendritic segment and the
reactive azobenzene acrylate mesogen units bearing octyloxy tails and flexible decylmethylene spacers.
The purified copolymers have been characterized and confirmed by NMR, FT-IR, MALDI-TOF MS, and GPC
showing narrow molecular weight distribution in the range 1.09e1.20. A generation-dependent poly-
meric micellar aggregation behavior of thus obtained amphiphilic LDBCs from nanofibers of uniform
diameter around 20 nm to nanospheres/oval sheets, vesicles, and porous large compound micelles
(LCMs) of micrometer size has been demonstrated in selective solvent mixture of dioxane/water.
Furthermore, photoisomerization transformation of these azobenzene-containing LDBCs and their
kinetics of reversible trans-cis-trans photochemical transitions in THF solution have also been investi-
gated, the measured trans-cis photoisomerization rate constant 0.0240 sꢀ1 for G3 copolymer is twice of
the 0.0127 sꢀ1 determined for the precursor azobenzene compound, manifesting a kind of promising
photoresponsive copolymer materials.
Received 2 August 2011
Received in revised form
16 November 2011
Accepted 23 November 2011
Available online 29 November 2011
Keywords:
Azobenzene-containing copolymers
Linear-dendritic block copolymers (LDBCs)
Micellar aggregates
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
which form well-ordered ultrathin LangmuireBlodgett films [4].
Meijer et al. [5] have reported that PS-dendr(PPI) can spontaneously
Linear-dendritic block copolymers (LDBCs), which combine the
highly branched architecture and multifunctionality of dendrimers
with the character of traditional linearelinear block copolymers,
represent a kind of intriguing and promising macromolecular
structure showing diverse advanced applications [1]. Fréchet and
Gitsov take the lead in proposing the concept of linear-dendritic
hybrid copolymer and developing a family of LDBCs with typi-
cally a Fréchet-type poly(benzyl ether) dendron [2]. Wiesner and
Cho et al. [3] have systematically explored self-assembled supra-
molecular structure of a series of LDBCs bearing hydrophobic alkyl
chains on the periphery of polyether-dendron and hydrophilic
polyethylene glycol (PEG) as linear coil block, which demonstrate
mesophase structure-mechanical and ionic-transport correlations
[3a]. Hammond group have synthesized amphiphilic LDBCs based
on PEG-dendr(PAMAM) tethering peripheral stearic alkyl chains
assemble into regular ordered microdomains by the peripheral
acid-functionalization of the dendritic PPI segment. We have
previously reported a kind of LDBC combining linear PEG and
dendritic aliphatic polyester with carboxylic acid groups, which as
a crystal growth modifier exhibits strong and architecture specific
influence for biomimetic inorganic crystallization [6]. Compared
with linearelinear architecture, the parameter of interfacial
curvature in LDBCs appears more important and facilely tunable by
variation of generation number of dendritic blocks [3e6].
In particular, azobenzene-containing polymers are of great
interest since azobenzene chromophores can undergo reversible
trans-cis photoisomerization [7e12], which may endow photo-
responsive properties such as the photoinduced deformation of
amphiphilic azobenzene-containing polymer (azo-polymer)
colloidal spheres reported by Wang et al. [13] In another amphi-
philic azo-polymer comprising azobenzene-containing poly-
methylacrylate and polyacrylic acid (PAzoMA-b-PAA), reversible
morphology changes of the self-assembled micellar aggregates
were observed under alternating UV and visible light irradiation as
* Corresponding author. Tel.: þ86 25 83686621; fax: þ86 25 83317761.
E-mail address: cdz@nju.edu.cn (D. Chen).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.11.047

360
Z. Shi et al. / Polymer 53 (2012) 359e369
a result of the reversible trans-cis photoisomerization of azo-
benzene mesogens [14].
standards. Fourier transform infrared spectra (FT-IR) were recorded
on a NICOLET NEXUS 870 infrared spectrometer with pressed
potassium bromide pellets. Elemental analyses (EA) were per-
formed using a Heraeus CHN-O-Rapid microanalyzer.
The micellization behavior of block copolymers in mixed
solvents selective for one of the blocks has already attracted great
interest because of their broad potential applications, for instance,
as good candidates for drug delivery carriers [15] or micro/nano-
reactors [16], etc. Various micellar morphologies were formed by
self-assembling of amphiphilic block copolymers, which were
systematically investigated with linearelinear block copolymers of
PS-b-PAA and PS-b-PEO by Eisenberg et al. [17] Those multiple
variables influencing the self-assembled morphologies included
the composition and concentration of block copolymers [17aec],
the common solvent variety [17d], the type and concentration of
added ions [17e,f], and others [17g]. Moreover, among the
increasingly reported diverse architectures of block copolymers,
the introduction of dendritic structures as a building block brings
out considerable new opportunities toward novel self-assembled
structures and adjustable variable for the resulting LDBCs systems
[18e21]. Very recently, Oriol and Sánchez et al. have reported the
synthesis of cyanoazobenzene functionalized LDBCs through click
chemistry [22a], rich ordered self-organized aggregates from
cylindrical micelles to sheet-like micelles and eventually to vesicles
achieved in solution [22b]. Generally, controlling the morphology
of self-assembled aggregates of LDBCs, through the selection of the
nature and length of the different blocks, the tuning of generation
number of dendritic segment, is of great importance to their related
properties and practical applications [1a,23,24]. Furthemore, the
liquid crystalline (LC) hydrophobic blocks are responsive to various
external stimuli due to their mesomorphic properties, thus the
amphiphilic LC block copolymers may serve as promising candi-
dates for the production of smart nanomaterials in solution [25].
Herein we report the synthesis of a series of LDBCs mPEG-dendr
[PAMAM-(AZO)n] with n ¼ 2, 4, 8, 16, which consist of a defined
hydrophilic linear PEG block of molecular weight around 2000 and
hydrophobic PAMAM dendrons with photoactive azobenzene
mesogen units bearing long alkyl tails attached at the periphery.
The chemical structures and molecular weight of the synthesized
LDBCs were characterized by combination of NMR, FT-IR, MALDI-
TOF MS, and GPC. The self-assembled aggregates mophologies of
thus obtained amphiphilic LDBCs in selective solvent pair dioxane/
water were preliminarily investigated with increasing dendron
generations. Furthermore, the photoisomerization of these
azobenzene-containing LDBCs and their kinetics of reversible trans-
cis-trans photochemical transformations in tetrahydrofuran solu-
tion were also discussed.
2.2.2. Molecular weight characterization
Matrix-assisted laser desorption/ionization time-of-flight
(MALDI-TOF) mass spectrometry were carried out on an Autoflex
mass spectrometer (Bruker Daltonics) operating in reflected mode
with 2-(4-hydroxyphenylazo)-benzoic acid (HABA) as the matrix
unless otherwise specified. The multiple-layer spotting was con-
ducted as follows: first, 1
THF) was spotted on the MALDI target, then 0.5
(saturated in acetone) was applied as the second layer, thereafter
L of sample solution (in w1 mg mLꢀ1) was added on top as the
m
L of matrix solution (30e40 mg mLꢀ1 in
L of NaCl solution
m
1
m
final third layer. Gel-permeation chromatography (GPC) measure-
ments were performed at 25 ꢁC on a Waters 515 equipped with
Wyatt Technology Optilab rEX differential refractive index and
DAWN EOS laser light scattering detectors. The separation was
achieved on a set of STYRAGEL HR3, HR4 and HR5 (300 ꢂ 7.8 mm)
styrene-divinylbenzene columns from Waters. THF was used as the
eluent at flow rate 1.0 mL minꢀ1 and linear poly(ethylene glycol)
(PEG) standards were adopted for the molecular weight calculation.
THF and samples were filtered over a filter with pore size of 0.45 mm
(Nylon, Millex-HN 13 mm Syringes Filters, Millipore, US). ASTRA
software (Version 5.3.1.4) was utilized for acquisition and analysis
of data.
2.2.3. Photochemical behavior investigation
UVevis absorption measurement and photoisomerization
experiment were performed on a UV-3600 spectrometer under UV
and visible light exposure in the dark. The LDBC tetrahydrofuran
solution was prepared with concentration of 40 m
g mLꢀ1, UV irra-
diation was carried out with a 12 W ultraviolet lamp (365 nm), and
irradiation by visible light was conducted using a 9 W incandescent
lamp (w 400 nm).
2.2.4. Sample preparation for TEM investigation
Amphiphilic LDBC samples mPEG-dendr[PAMAM-(AZO)n] were
first dissolved in 1, 4-dioxane, which is a good solvent for the
copolymers, at an initial concentration of 1.0 wt% for G0 and G1, and
0.01 wt% for G2 and G3, respectively. Then, deionized water was
added dropwise to the solution with vigorous stirring until the final
water content reaching 17 wt%. Then the dispersion system was
deposited onto carbon-coated copper grids, after drying under
atmosphere at RT, and then stained with 3 wt% aqueous uranyl
acetate solution. Transmission electron microscopy (TEM) obser-
vations were performed on a JEM-2100 (JEOL) at 200 kV.
2. Experimental section
2.1. Materials
2.3. Syntheses of reactive mesogen unit azobenzene acrylate
Methoxy poly(ethylene glycol) amine (mPEG-NH2, Mn 2170 g/
mol reported by the supplier with PDI 1.02) was purchased from
JENKEM, Beijing, as a custom-made product, dried through azeo-
tropic distillation with toluene before use. 4-(dimethylamino)
pyridine (DMAP) from SigmaeAldrich, 1-bromooctane and 4-
acetamidophenol from Alfa Aesar, were used as received without
purification. Tetrahydrofuran (THF) was dried over sodium/
benzophenone. All other chemical reagents were purified accord-
ing to standard procedures before use.
The synthesis route for the reactive mesogen unit azobenzene
acrylate is illustrated in Scheme 1.
2.3.1. Synthesis of 4-octyloxyacetaniline (1)
9.27 g (48 mmol) of 1-bromooctane and 7.33 g (53 mmol) of
K2CO3 were added to 8.00
g (53 mmol, 1.1 equiv.) of 4-
acetamidophenol dissolved in 150 mL of acetone. The solution
was stirred vigorously for 24 h under reflux. The resulting mixture
was washed with excess of water, and the white precipitate was
filtered off. Then, the crude product was dried, and purified by
recrystallization from ethanol to give 1 as white solid.
2.2. Characterization methods
2.2.1. Structure characterization
1H NMR (300 MHz, CDCl3)
d
(ppm): 7.35 (d, 2 H, J ¼ 9.0 Hz, Ar-H),
1H NMR spectra were obtained on 500 MHz (Bruker DRX500)
and 300 MHz (Bruker AMX300) instruments with TMS as internal
7.06 (s, 1 H, Ar-NH), 6.86 (d, 2 H, J ¼ 9.0 Hz, Ar-H), 3.92 (t, 2H,
J ¼ 6.6 Hz, CH2O), 2.15 (s, 3 H, NHCOCH3), 1.76 (qn, 2 H, J ¼ 7.0 Hz,

Z. Shi et al. / Polymer 53 (2012) 359e369
361
O
O
n-C8H17Br
i) HCl, C2H5OH
ii) NaOH
HO
NHCCH3
C
8H17
O
NHCCH3
C8H17
O
NH2
K2CO3, Acetone
1
2
K2CO 3, KOH, KI, Acetone
Br(CH2)10OH
i)
OH
, NaOH
NaNO2, HCl
Acetone, -5~5 oC
N
N
OH
C8H17
O
ii) HCl
4
3
40% HBr
HO(CH2)10OH
Toluene
O
O
N
O(CH2)10O
N
O(CH2)10OH
Cl
C8H17
O
N
C8H17
O
N
DMAP, TEA, THF
5
Mesogen: 8-AZO-10-acr
Scheme 1. Synthesis procedure of the reactive mesogen unit azobenzene acrylate.
CH2CH2CH2O), 1.28e1.44 (m, 10 H, CH2), 0.89 (t, 3 H, J ¼ 6.6 Hz,
CH3CH2). FT-IR (cmꢀ1): 3325, 2926, 2857, 1658, 1605, 1552, 1510,
1470, 1240. Anal. Calcd. (%) for C16H25O2N: C 73.00, H 9.51, N 5.32;
Found: C 73.11, H 9.50, N 5.50.
ambient temperature, then the organic layer was separated by
extraction and dried over anhydrous MgSO4, after filtration and
removing solvents by rotary evaporation, a crude product in yellow
oil was obtained, then further dissolved in toluene and the solution
was cooled to around 0 ꢁC for the residual diol to separate out in
crystalline solid, after filtration and completely removing solvent, 4
was obtained as an almost colorless liquid. Yield: 74.5% (30.37 g).
2.3.2. Synthesis of 4-octyloxyaniline (2)
Thus synthesized 1 was dissolved in 140 mL of ethanol, and
35 mL of concentrated hydrochloric acid was added dropwise. The
reaction solution was stirred for 24 h under reflux. The resulting
claret-red mixture was cooled to ambient temperature and then
poured over 500 g of ice, white precipitate was generated. After the
solution was neutralized to pH 7e8 with NaOH solution, the
precipitate was filtered off, and washed with water, after dried in
vacuum oven, 2 was obtained as a pale yellow solid. Yield: 65.8%
(6.73 g).
1H NMR (300 MHz, CDCl3)
d
(ppm): 3.65 (t, 2 H, J ¼ 6.6 Hz,
CH2OH), 3.41 (t, 2 H, J ¼ 6.9 Hz, CH2Br), 1.85 (qn, 2H, J ¼ 7.1 Hz,
CH2CH2CH2Br), 1.57 (qn, 2 H, J ¼ 6.4 Hz, CH2CH2CH2OH), 1.29e1.44
(m, 12 H, CH2). FT-IR (cmꢀ1): 3370, 2928, 2854, 1465, 1257. Anal.
Calcd. (%) for C10H21OBr: C 50.63, H 8.86; Found: C 50.87, H 8.97.
2.3.5. Synthesis of 4-(10-hydroxydecyloxy)-40-octyloxyazobenzene
(5)
1H NMR (300 MHz, CDCl3)
d
(ppm): 6.84 (d, 2 H, J ¼ 8.7 Hz, Ar-
2.76 g (20 mmol) of K2CO3, 0.28 g (5 mmol) of KOH and 0.17 g
(1 mmol) of KI were added to 6.50 g (20 mmol) of 3 dissolved in
150 mL of acetone. To this mixture, 5.69 g (24 mmol, 1.2 equiv.) of 4
was added dropwise, the reaction mixture was refluxed for 24 h
under vigorous stirring. The resulting mixture was cooled to
ambient temperature and filtered. The precipitate was washed with
water, ethanol/water (1:1, V/V) respectively. Then, the crude
product was dried, and purified by recrystallization from methanol
to give 5 as a yellow solid. Yield: 89.0% (8.58 g).
H), 6.86 (d, 2 H, J ¼ 8.7 Hz, Ar-H), 3.89 (t, 2H, J ¼ 6.6 Hz, CH2O), 1.75
(qn, 2 H, J ¼ 7.0 Hz, CH2CH2CH2O), 1.28e1.43 (m, 10 H, CH2), 0.88 (t,
3 H, J ¼ 6.9 Hz, CH3CH2). FT-IR (cmꢀ1): 2922, 2847, 1635, 1510, 1474,
1230. Anal. Calcd. (%) for C14H23ON: C 76.02, H 10.41, N 6.33; Found:
C 75.94, H 10.34, N 6.40.
2.3.3. Synthesis of 4-hydroxy-40-octyloxyazobenzene (3)
2.10 g (30.4 mmol) of sodium nitrite in a mixture of water
(20 mL) and ice (20 mL), was slowly added dropwise into the
container including 6.72 g (30.4 mmol) of 2 dissolved in a mixture
of concentrated hydrochloric acid (10 mL), ice (110 g) and acetone
(180 mL), and the reaction mixture was stirred for 30 min at
ꢀ5 w 5 ꢁC. To the cooled mixture, 2.86 g (30.4 mmol) of phenol in
2 mol/L sodium hydroxide solution was added. The reaction
mixture was maintained at pH 9e10, and stirred for 2 h at room
temperature. After neutralization with HCl solution, the resulting
mixture was filtered off and washed with water. Then, the crude
product was dried, and purified by recrystallization from ethanol to
give 3 as red-brown solid. Yield: 65.5% (6.50 g).
1H NMR (300 MHz, CDCl3)
d
(ppm): 7.88 (d, 4 H, J ¼ 9.0 Hz, Ar-H),
7.00 (d, 4 H, J ¼ 9.3 Hz, Ar-H), 4.03 (t, 4 H, J ¼ 6.6 Hz, ArOCH2), 3.64
(t, 2 H, J ¼ 6.6 Hz, CH2OH), 1.82 (qn, 4 H, J ¼ 6.9 Hz, ArOCH2CH2),
1.25e1.39 (m, 24 H, CH2), 0.89 (t, 3 H, J ¼ 6.6 Hz, CH3CH2). FT-IR
(cmꢀ1): 3310, 2921, 2853, 1603, 1582, 1499, 1474, 1249. Anal.
Calcd. (%) for C30H46O3N2: C 74.69, H 9.54, N 5.81; Found: C 74.50, H
9.31, N 5.68.
2.3.6. Synthesis of 4-[10-(acryloyloxy)decyloxy]-40-
octyloxyazobenzene (8-AZO-10-acr)
4.93 g (10.2 mmol) of 5, and 3.0 mL of triethylamine (TEA) were
dissolved in 25 mL of freshly distilled dry THF below 5 ꢁC, to which
1.85 g (20.4 mmol, 2 equiv.) of acryloyl chloride in 25 mL of dry THF
was added dropwise for 30 min. After the solution was allowed to
warm to ambient temperature, a catalytic amount of DMAP was
added, and the reaction solution was stirred for another 12 h. The
resulting mixture was concentrated by rotary evaporation, then
washed with acetone/water (1:10). The crude product was further
purified through silica gel column chromatography using CH2Cl2/
hexane (1:1, V/V) as the eluent, and the product 4-[10-(acryloyloxy)
decyloxy]-40-octyloxyazobenzene, noted as 8-AZO-10-acr, was
obtained as yellow solid. Yield: 49.6% (2.71 g).
1H NMR (300 MHz, CDCl3)
d
(ppm): 7.89 (d, 2 H, J ¼ 9.0 Hz, Ar-H),
7.86 (d, 2 H, J ¼ 9.0 Hz, Ar-H), 7.00 (d, 2H, J ¼ 9.0 Hz, Ar-H), 6.96 (d,
2 H, J ¼ 9.0 Hz, Ar-H), 4.03 (t, 2 H, J ¼ 6.6 Hz, CH2O), 1.82 (qn, 2 H,
J ¼ 7.1 Hz, CH2CH2CH2O), 1.28e1.39 (m, 10 H, CH2), 0.89 (t, 3 H,
J ¼ 6.9 Hz, CH3CH2). FT-IR (cmꢀ1): 3468, 2921, 2855, 1600, 1583,
1500, 1474, 1249. Anal. Calcd. (%) for C20H26O2N2: C 73.62, H 7.98, N
8.59; Found: C 73.45, H 8.10, N 8.42.
2.3.4. Synthesis of 10-bromodecyl-1-ol (4) [26]
To a mixture of decane-1,10-diol (30.0 g, 0.172 mol) and toluene
(600 mL) was added 27.7 mL (0.194 mol, 1.1 equiv.) of 40% hydro-
bromic acid. The heterogeneous mixture was stirred and heated at
reflux for 36 h. The resulting mixture was allowed to cool to
1H NMR (300 MHz, CDCl3)
7.00 (d, 4 H, J ¼ 9.0 Hz, Ar-H), 6.43 (dd, 1 H, J ¼ 17.4 Hz, cis-
d
(ppm): 7.89 (d, 4 H, J ¼ 9.0 Hz, Ar-H),

362
Z. Shi et al. / Polymer 53 (2012) 359e369
CH2 ¼ CH-), 6.17 (dd, 1 H, J ¼ 10.5 Hz, eCH2 ¼ CH-), 5.83 (dd, 1 H,
10.2 Hz, trans-CH2 CH-), 4.15 (t, H, 6.6 Hz,
a vacuum oven at 30 ꢁC, 0.48 g product was obtained in white solid.
Yield: 67.6%.
J
¼
¼
2
J
¼
CH2OCOCH ¼ CH2), 4.03 (t, 4 H, J ¼ 6.6 Hz, ArOCH2), 1.67 (qn, 2 H,
J ¼ 6.6 Hz, CH2CH2OCOCH¼CH2), 1.82 (qn, 4 H, J ¼ 6.9 Hz,
ArOCH2CH2), 1.26e1.50 (m, 22 H, CH2), 0.89 (t, 3 H, J ¼ 6.6 Hz,
CH3CH2). FT-IR (cmꢀ1): 2924, 2851, 1729, 1603, 1581, 1501, 1472,
1247. Anal. Calcd. (%) for C33H48O4N2: C 73.90, H 8.96, N 5.22;
Found: C 74.01, H 8.95, N 5.24.
1H NMR (500 MHz, DMSO-d6)
d (ppm): 3.57 (s, -COOCH3), 3.51
(b, CH2CH2O), 3.24 (s, CH3O-), 3.06 (m, -CONHCH2-), 2.68 (m,
>NCH2CH2COOCH3, >NCH2CH2CONH- and eCONHCH2CH2N<),
2.39 (m, -CH2COOCH3), 2.17 (m, -CH2CONH-). FT-IR (cmꢀ1): 2885,
1736, 1648, 1552, 1467, 1342, 1106.
2.4.2. General procedure for syntheses of full-generation mPEG-
dendr(PAMAM) (mPEG-G3.0-(NH2)8 as an example)
2.4. Syntheses of precursor LDBCs with peripheral amine groups
0.42 g (0.13 mmol) of mPEG-G2.5-(CO2CH3)8 was dissolved in
10 mL of methanol and dripped into 14.0 mL (0.21 mol, 200 equiv.)
of freshly distilled ethylenediamine. Methanol and unreacted eth-
ylenediamine were removed under vacuum after reaction for 72 h
at 50 ꢁC, then 10 mL of toluene/methanol (9:1) was added to
remove any residual ethylenediamine through azeotropic evapo-
ration under reduced pressure. The residue was washed with
250 mL of dry ethyl ether to remove any residual impurities, after
filtration and completely drying under vacuum at 30 ꢁC, 0.30 g
white solid product was obtained in yield 66.2%.
The synthesis of precursor LDBCs mPEG-dendr(PAMAM) from
the starting linear PEG initiator core with amine end group of
mPEG-NH2, follows the well-established divergent protocol as
skilfully adopted by Hammond group [4], including two iterative
reactions of Michael addition and amidation to construct the
dendritic segment generation by generation (Scheme 2).
2.4.1. General procedure for syntheses of half generation mPEG-
dendr(PAMAM) (mPEG-G2.5-(CO2CH3)8 as an example)
0.57 g (0.20 mmol) of mPEG-G2-(NH2)4 was dissolved in 5 mL of
methanol and dripped slowly into 29.0 mL (0.32 mol, 200 equiv.) of
purified methyl acrylate kept at 35 ꢁC. The solvent methanol and
unreacted methyl acrylate were removed under vacuum after
reaction lasting for 72 h and then a viscous white residue left.
The residue was washed with 250 mL of dry ethyl ether to remove
any residual impurities, after filtration and completely drying in
1H NMR (500 MHz, DMSO-d6)
d (ppm): 3.51 (b, CH2CH2O),
3.24(s, CH3Oe), 3.07 (m, eCONHCH2e), 2.64 (m, >NCH2CH2CONH-,
eCONHCH2CH2NH2 and eCONHCH2CH2N<), 2.19 (m, -CH2CONH-).
FT-IR (cmꢀ1): 2885, 1648, 1552, 1467, 1342, 1106.
According to the above general procedure, other half- and full-
generation mPEG-dendr(PAMAM) have been synthesized with the
yields and characterization results as follows.
O
NH2
O
N
H
O
O
i)
ii)
H3C
O
PEG
N
m
PEG
N
m
O
NH2
H
n
N
NH2
O
O
iii)
mPEG-G0-(AZO)2
mPEG-G1-(NH2)2
mPEG-G0.5-(CO2CH3)2
mPEG-G0-NH2
iii)
i)
mPEG-G1-(AZO)4
mPEG-G1.5-(CO2CH3)4
Conditions and Notes :
O
i)
, CH3OH, 35oC
ii)
O
2 , CH3OH, 50oC
NH
ii)
H N
2
iii)
mPEG-G2-(NH2)4
mPEG-G2-(AZO)8
8-AZO-10-acr, THF, reflux
iii)
R=
i)
N
OC H
8 17
R
O(CH
) O
2
10
N
NH
2
N
R
O
O
O
NH
R
mPEG-G2.5-(CO2CH3)8
NH
H
NH
2
H
N
N
N
R
R
N
N
ii)
O
O
O
O
O
NH
NH
2
O
NH
N
R
R
N
N
H
H
H
O
N
H
N
O
N
N
H
N
H
N
N
N
N
H
N
H
NH
2
N
O
O
O
O
R
iii)
O
O
O
N
O
N
R
H C
3
O
H C
3
O
O
O
n
n
H
H
N
NH
N
2
N
N
N
H
R
H
N
N
N
N
N
H
N
H
O
O
H
N
H
N
R
N
NH
2
NH
O
NH
O
O
O
R
O
O
R
N
N
N
H
N
N
NH
2
H
R
mPEG-G3-(NH2)8
NH
mPEG-G3-(AZO)16
NH
O
O
N
R
NH
2
R
Scheme 2. Synthesis pathway of the amphiphilic liquid crystalline LDBCs mPEG-dendr[PAMAM-(AZO)n] with dendritic segment of generation G0 to G3.

Z. Shi et al. / Polymer 53 (2012) 359e369
363
2.4.2.1. mPEG-G0.5-(CO2CH3)2. Yield: 87.9% (3.22 g). 1H NMR
(500 MHz, DMSO-d6) (ppm): 3.57 (s, -COOCH3), 3.51 (b,
CH2CH2O), 3.24 (s, CH3Oe), 2.71 (t, >NCH2CH2 COOCH3), 2.39 (t,
eCH2COOCH3). FT-IR (cmꢀ1): 2889, 1736, 1637, 1467, 1343, 1114.
>NCH2CH2COO), 1.80 (b, ArOCH2CH2), 1.61 (b, CH2CH2OCOCH2),
1.26e1.50 (m, CH2), 0.88 (b, CH3CH2). FT-IR (cmꢀ1): 2886, 1732,
1651, 1603, 1556, 1502, 1468, 1343, 1250, 1109.
d
3. Results and discussion
2.4.2.2. mPEG-G1.0-(NH2)2. Yield 88.9%. 1H NMR (500 MHz,
DMSO-d6)
d
(ppm): 3.51 (b, CH2CH2O), 3.24 (s, CH3O-),
3.1. Rational structure design of block copolymer
3.06 (m, eCONHCH2e), 2.65 (m, >NCH2CH2CONH- and
eCONHCH2CH2NH2), 2.20 (m, eCH2CONH-). FT-IR (cmꢀ1):
2888, 1651, 1468, 1342, 1111.
Compared with other LC segments containing block copoly-
mers [25,27,28], the adoption of a dendritic scaffold provides the
main advantage of well-defined structure with tunable molecular
weight of variant generations, as well as the curvature effect
introduced by the dendritic architecture and rich possibility for
tailoring the surface functional groups [1e6]. The rational design,
synthesis and careful purification of a set of well-defined LDBCs
functionalized with mesogen units are a necessary prerequisite for
the preparation of novel liquid crystalline polymeric materials
with complex superstructures for potential optical storage appli-
cation. In this paper, a series of liquid crystalline LDBCs mPEG-
dendr[PAMAM-(AZO)n] with n ¼ 2, 4, 8, 16, composed of a linear
PEG coil with molecular weight around 2000 and PAMAM den-
dron block of generations 0 to 3 (noted as G0, G1, G2, G3) func-
tionalized with azobenzene mesogenic units were synthesized
(see illustration in Scheme 2). Thus synthesized LDBCs were also
of amphiphilic character with the hydrophilic PEG and hydro-
phobic azobenzene-containing dendron block, which exhibited
rich self-assembled structures and reversible photoisomerization
transformation in solution as demonstrated in the following
sections.
2.4.2.3. mPEG-G1.5e(CO2CH3)4. Yield: 88.0% (3.00 g). 1H NMR
(500 MHz, DMSO-d6)
d (ppm): 3.57 (s, -COOCH3), 3.51 (b,
CH2CH2O), 3.24 (s, CH3O-), 3.04 (m, eCONHCH2e), 2.68 (m,
>NCH2CH2COOCH3, >NCH2CH2CONH- and eCONHCH2CH2N<),
2.39 (m, eCH2COOCH3), 2.17 (m, eCH2CONH-). FT-IR (cmꢀ1): 2885,
1736, 1654, 1467, 1345, 1111.
2.4.2.4. mPEG-G2.0-(NH2)4. Yield 95.9%. 1H NMR (500 MHz, DMSO-
d6)
d (ppm): 3.51 (b, CH2CH2O), 3.24 (s, CH3O-), 3.07 (m,
eCONHCH2e), 2.64 (m, >NCH2CH2CONHe, eCONHCH2CH2NH2
and eCONHCH2CH2N<), 2.19 (m, eCH2CONH-). FT-IR (cmꢀ1): 2887,
1651, 1556, 1467, 1343, 1109.
2.5. Syntheses of LDBCs functionalized with azobenzene mesogenic
units
2.5.1. General procedure for syntheses of mPEG-dendr[PAMAM-
(AZO)n] (mPEG-G3-(AZO)16 as an example)
Michael addition between amine and azobenzene acrylate was
employed for the attachment of mesogenic units. A mixture solu-
tion of mPEG-G3-(NH2)8 (0.20 g, 0.06 mmol) and reactive mesogen
unit 8-AZO-10-acr (1.03 g, 1.92 mmol) in 8 mL of THF was stirred at
65 ꢁC for a week. Removing the solvent under reduced pressure,
then the residue was washed several times with 250 mL of ethyl
ether, after filtration and exhaustive drying under vacuum at 30 ꢁC,
0.33 g yellow solid powder product was harvested in yield 45.6%.
3.2. Synthesis of 8-AZO-10-acr
For the synthesis of reactive mesogenic compound azobenzene
acrylate 8-AZO-10-acr, the diazo coupling reaction was adopted and
followed by reaction with acryloyl chloride in the presence of
DMAP and triethylamine (Scheme 1). Instead of 4-aminophenol as
used in the literature [29], we chose 4-acetamidophenol as the
starting material avoiding some complication resulted from oxi-
dization of 4-aminophenol. The terminal octyloxy chain introduced
in the mesogenic unit is of crucial importance for enhancing the
incompatibility and microphase segregation between linear and
dendritic blocks and inducing explicit suborganization of the
PAMAM dendritic segment and mesogenic units, which will be
detailedly clarified in representing the unprecendented hierar-
chical structures formation in their liquid crystalline states in bulk
in a forthcoming paper. After purification through column chro-
matography, high quality air-stable yellow solid product was
obtained.
1H NMR (500 MHz, CDCl3)
d (ppm): 7.85 (b, Ar-H), 6.98 (b, Ar-H),
4.02(b, CH2OCOCH2 andArOCH2), 3.65(b, CH2CH2O), 3.38(s, CH3O-),
3.28 (b, -CONHCH2-), 2.73 (b, >NCH2CH2COO), 2.55 (b, CH2CH2N<),
2.41 (b, >NCH2CH2COO), 1.80 (b, ArOCH2CH2), 1.61 (b, CH2CH2O-
COCH2), 1.26e1.50 (m, CH2), 0.88 (b, CH3CH2). FT-IR (cmꢀ1): 2922,
2857, 1733, 1659, 1545, 1467, 1348, 1249, 1106 .
2.5.2. Characterization results for other lower generation
functionalized LDBCs
2.5.2.1. mPEG-G0-(AZO)2. Yield 53.1%. 1H NMR (500 MHz, CDCl3)
d
(ppm): 7.86 (b, Ar-H), 6.98 (b, Ar-H), 4.03 (b, CH2OCOCH2 and
ArOCH2), 3.65 (b, CH2CH2O), 3.38 (s, CH3O-), 2.82 (b,
>NCH2CH2COO), 2.45 (b, >NCH2CH2COO), 1.81 (b, ArOCH2CH2),
1.62 (b, CH2CH2OCOCH2), 1.26e1.50 (m, CH2), 0.89 (b, CH3CH2). FT-
IR (cmꢀ1): 2888, 1733, 1604, 1499, 1467, 1343, 1243, 1109.
Fig. 1A shows the 1H NMR spectrum of 8-AZO-10-acr. Compared
with that of precursor compound 5, the methylene peak at h
(3.64 ppm) obviously shifted downfield to 4.15 ppm after esterifi-
cation and three groups of double doublets (dd) at k (6.43), i (6.17),
and j (5.83) featuring the vinyl protons of acrylate [30] newly
appeared. Elemental analyses (see Experimental Section) and
characteristic FT-IR spectrum (Fig. 2a) further confirmed the
successful preparation of the reactive azobenzene mesogenic
chromophore.
2.5.2.2. mPEG-G1-(AZO)4. Yield 60.7%. 1H NMR (500 MHz, CDCl3)
d
(ppm): 7.85 (b, Ar-H), 6.97 (b, Ar-H), 4.02 (b, CH2OCOCH2 and
ArOCH2), 3.64 (b, CH2CH2O), 3.38 (s, CH3O-), 3.28 (b, -CONHCH2-),
2.77 (b, >NCH2CH2COO), 2.56 (b, CH2CH2N<), 2.42 (b,
>NCH2CH2COO), 1.81 (b, ArOCH2CH2), 1.61 (b, CH2CH2OCOCH2),
1.26e1.50 (m, CH2), 0.89 (b, CH3CH2). FT-IR (cmꢀ1): 2886, 1732,
1652, 1603, 1499, 1467, 1343, 1249, 1110.
3.3. Synthesis of mPEG-dendr(PAMAM)e(NH2)n
The synthesis route of mPEG-dendr(PAMAM) from a mono-
methyl ether terminated PEG with amine end group initiator core
(mPEG-NH2), follows the divergent approach well established by
Tomalia et al. [31] and developed by Hammond group [4] involving
two iterative steps to build up the dendritic segment generation by
2.5.2.3. mPEG-G2-(AZO)8. Yield 46.8%. 1H NMR (500 MHz, CDCl3)
d
(ppm): 7.85 (b, Ar-H), 6.96 (b, Ar-H), 4.01 (b, CH2OCOCH2 and
ArOCH2), 3.64 (b, CH2CH2O), 3.37 (s, CH3O-), 3.28 (b, -CONHCH2-),
2.75 (b, >NCH2CH2COO), 2.55 (b, CH2CH2N<), 2.41 (b,

364
Z. Shi et al. / Polymer 53 (2012) 359e369
amine terminal groups [4a,c]. After removal of excess reactant and
methanol under high vacuum, additionally, for all full generations,
a mixture of toluene/methanol (9:1) was added to eliminate any
residual ethylenediamine by azeotropic distillation [4c]. The
selection of PEG as the linear coil block offered tremendous
convenience for the synthesis and purification of block copolymers
due to its good solubility in water and many organic solvents, the
pure copolymer products were obtained with a simple solvent
extraction and precipitation procedure as reported by Hammond
group [4a]. The relatively short hydrophilic PEG coil block of
molecular weight around 2000 was selected considering both the
hydrophilic/hydrophobic balance matched with the lower genera-
tion hydrophobic dendritic segments in the solution self-assembly
and their relative symmetry of two incompatible blocks in the bulk
organization. The chemical composition and structure of thus
prepared copolymers were characterized and affirmed by FT-IR, 1H
NMR (as described in detail in the Experimental Section) and
MALDI-TOF MS. 1H NMR spectrum of mPEG-G2.5-(CO2CH3)8 is
representatively shown in Fig. 1B. FT-IR characteristic absorptions
of C¼O stretching vibration at 1736 cmꢀ1 and the single peak at
3.57 ppm in 1H NMR (Fig. 1B, h), ascribed to terminal methyl esters
of half generation, were monitored after each step growth of
PAMAM. Fig. 3 shows the MALDI-TOF mass spectra of mPEG-
dendr(PAMAM) series, particularly for all half generation LDBCs
together with the initial mPEG-NH2 for comparison. One can clearly
see that tethering different generation dendrons to PEG accordingly
shifts the whole mass distribution curves of the MALDI-TOF MS
patterns to increased molecular weights while the shape of the
curves keeps unchanged (the mass difference between each adja-
cent oligomeric ion peak is 44 Da corresponding to the mass of PEG
repeat unit eOCH2CH2e). Moreover, for the copolymer of the
highest dendron generation investigated in this work as shown in
Fig. 3d, a second set of peaks of lower intensity, which is 22 Da
smaller than the nearest peak value of the main set, are observed
attributed to Hþ adduct besides the main peak set of Naþ adduct,
the expanded ion peaks showing in detail the bimodal series peaks
of Naþ and Hþ adducts of mPEG-G2.5-(CO2CH3)8, respectively, and
the determination of the copolymer composition and the calcula-
tion of PEG repeat unit n in a representative oligomer are provided
in the Electronic Supplementary Material. This kind of bimodal
even multiplet peaks have often been observed in many other cases
of MALDI-TOF patterns of synthetic polymers [2c,32] and it is
reasonable in view of the reported results that PEG was strongly
inclined to form alkaline cation adduct [2c,33] and nitrogen-
containing PAMAM might also generate molecular ion peaks or
protonated species [34]. Polydispersity index (PDI, Mw/Mn) calcu-
lated from the MALDI-TOF MS are all around 1.01, indicative of the
dendritic segment with perfect structure. The experimental
molecular weight (MW) is in good agreement with the theoretical
value calculated by adding the MW increment of growing dendron
generation and the starting MW of mPEG-NH2. The theoretical and
experimental molecular weights and PDI, together with the
calculated composition weight percentage of different segments
are summarized in Table 1.
3.4. Synthesis of mPEG-dendr(PAMAM)-(AZO)n
Fig. 1. 1H NMR spectra of (A) 8-AZO-10-acr (300 MHz, CDCl3); (B) mPEG-G2.5-
(CO2CH3)8 (500 MHz, DMSO-d6); and (C) mPEG-G3-(AZO)16 (500 MHz, CDCl3).
The amphiphilic liquid crystalline LDBCs were prepared by
further attaching mesogenic groups on the periphery of mPEG-
dendr(PAMAM) through exhaustive Michael addition of reactive
azobenzene acrylate mesogenic units 8-AZO-10-acr onto the
terminal amine groups. Two different adducts of secondary or
tertiary amino product can be obtained from the Michael addition
of a primary amine depending on the reaction conditions [35]. In
order to realize complete double Michael addition of amine groups
generation (Scheme 2). Half generation PAMAM with methyl ester
terminal groups is synthesized by thorough Michael addition of
methyl acrylate to the primary amine terminal groups of PEG core,
followed by amidation with 200 times molar excess ethylenedi-
amine to produce full integral generation, regenerating the primary

Z. Shi et al. / Polymer 53 (2012) 359e369
365
Fig. 2. FT-IR spectra of (a) 8-AZO-10-acr, (b) mPEG-G3-(NH2)8, and (c) mPEG-G3-(AZO)16
.
by reactive mesogenic units, 2 equiv. molar excess of 8-AZO-10-acr
and one week of reaction time is applied to assure both hydrogens
of primary amine end groups completely transformed. The progress
of amine alkylation can be monitored by 1H NMR as reported by
Yonetake et al. [35], with the appearance of peaks around 2.6 and
2.8 ppm featuring the single hydrogen addition, while the disap-
pearance of these signals and the appearance of signals at 2.4 and
2.7 ppm indicating complete addition. As shown in Fig. 1C, the
characteristic proton signals at 2.41 (g) and 2.73 ppm (f) after
exhaustive Michael addition confirmed the complete reaction of
the peripheral amine groups and the disappearance of three dd
peaks corresponding to vinyl protons of acrylate at 5.8e6.4 ppm
evidenced no residual reactive mesogenic reagents.
For the liquid crystalline copolymers, the MALDI-TOF MS signals
were dissatisfactory with no peak resolution after tethering
mesogenic units, although many different matrices such as 2,5-
dihydroxybenzoic acid (DHBA),
a-cyano-4-hydroxycinnamic acid
(a
-CHCA), 2-(4-hydroxyphenul-azo)-benzoic acid (HABA), and
dithranol ever tried. Therefore, GPC was utilized to determine the
molecular weight and polydispersity of the liquid crystalline LDBCs
using linear PEG standards. As shown in Fig. 4, monomodal GPC
curves shifting toward lower elution volume as the dendron
generation and mesogen units increasing were obtained with
narrow PDI in the range of 1.09e1.20 (Table 1). It should be noted
that for higher generation samples with the increase of the number
Table 1
Theoretical and experimental composition and molecular weight of LDBCs before
and after functionalized with azobenzene mesongenic groups.
Sample code
Mn
Mn
PDI
Composition wt%b
(calcd.) (exptl.) (Mw/Mn)
mPEG dendr(PAMAM) AZO
mPEG-NH2
mPEG-G0.5
mPEG-G1.5
mPEG-G2.5
2170a 2344c 1.02c
2342b 2531c 1.01c
2742b 2836c 1.01c
3542b 3758c 1.01c
3242b 4800d 1.09d
4542b 5200d 1.15d
7142b 6200d 1.18d
NAe
NA
NA
NA
NA
NA
NA
0
5.0
9.6
12.9
NA
NA
NA
NA
33.1
47.2
60.0
69.5
NA
mPEG-G0-(AZO)2
mPEG-G1-(AZO)4
mPEG-G2-(AZO)8
66.9
47.8
30.4
17.6
mPEG-G3-(AZO)16 12342b 8800d 1.20d
a
Provided by the supplier.
Calculated from designed structure.
From MALDI-TOF-MS.
From GPC based on PEG standards.
b
c
Fig. 3. MALDI-TOF mass spectra of Naþ adducts of (a) mPEG-NH2, (b) mPEG-G0.5-
(CO2CH3)2, (c) mPEG-G1.5-(CO2CH3)4, (d) mPEG-G2.5-(CO2CH3)8 (also with Hþ adduct as
the increase of the nitrogen-containing PAMAM segment).
d
e
Not applicable.

366
Z. Shi et al. / Polymer 53 (2012) 359e369
TEM images of mPEG-G0-(AZO)2 (hydrophilic/hydrophobic weight
ratio, 67/33) exhibited long cylindrical micellar morphology e
nanofibers as representatively shown in Fig. 5A, with dark regions
corresponding to PEG domains and lighter areas corresponding to
dendritic azobenzene blocks due to the selective staining [20d].
The nanofibers were of high uniformity in size with an estimated
external diameter of 20 nm from TEM (inset in Fig. 5A), and the
length of the fibers was beyond 3 mm according to the accessible
visible region in the TEM image with the lowest magnification.
Considering the layer spacing d ¼ 13 nm of mPEG-G0-(AZO)2 in
bulk (will be discussed in detail in another paper focusing on the
bulk superstructures of liquid crystalline LDBCs), we speculated
that mPEG-G0-(AZO)2 self-assembled into cylindrical micelles
consisting of a PEG corona and a core composed of dendritic
segments in which the azobenzene mesogenic units were asso-
ciated in a head-to-head manner (H-aggregation) [22], which also
evidenced by the remarkable blue shift in UVevis spectra of the
micellar dispersion compared with the maximum absorption at
360 nm in THF solution (data not shown). For mPEG-G1-(AZO)4
with hydrophilic/hydrophobic weight ratio 48/52, TEM micro-
graphs revealed uniform spherical micelles with an average
diameter of 24 nm (Fig. 5B) in coexistence with two dimensional
micelles e oval sheets (arrow marked in Fig. 5B and those in
Fig. 5C). Fig. 5C was from another area of the same TEM sample,
the thickness of the sheets was of the similar size of diameter of
the nanospheres. Very interestingly, owing to their specific linear-
dendritic architecture, the relative balance in hydrophilic/hydro-
phobic composition and the inclusion of mesogenic segments,
rich morphologies such as helices, platelets and many other
transitional structures have been observed for the lower genera-
tion liquid crystalline LDBCs such as mPEG-G1-(AZO)4 in dioxane/
water mixture solution under variant conditions, which seem to
lie between the relative simple solution self-assembly driven by
the stretching entropy and core-corona interfacial energy of
coilecoil diblock copolymer [17,37] and crystallization-driven
living self-assembly of polyferrocenylsilane (PFS) contained coil-
crystalline diblock copolymers [38], thus offer an attractive
opportunity to explore the transitions between variant particular
self-assembled morphologies and will be reported soon in
a forthcoming paper in detail. For mPEG-G2-(AZO)8 with hydro-
philic/hydrophobic weight ratio 30/70, uniform vesicular struc-
tures with external diameter of about 430 nm and wall thickness
around 80 nm were formed (Fig. 5D). The highest generation LDBC
investigated in this work, mPEG-G3-(AZO)16, with much dimin-
ished hydrophilic content of hydrophilic/hydrophobic weight
ratio 18/82, appeared to self-organize into large compound
micelles (LCMs, Fig. 5E) of diameter several micrometers con-
sisting of many inverted sub-micelles surrounded by a hydrophilic
surface [17a]. Interestingly, a closer inspection of the high reso-
lution TEM image revealed a kind of porous channel structure on
the surface of all the LCMs (Fig. 5F). The channels were probably
generated by the trapped solvents’ escaping through drying
process of LCM aggregates.
Fig. 4. GPC traces of (a) mPEG-NH2, (b) mPEG-G0-(AZO)2, (c) mPEG-G1-(AZO)4,
(d) mPEG-G2-(AZO)8, (e) mPEG-G3-(AZO)16
.
of azobenzene units, the GPC profiles exhibit tailing as shown in
Fig. 4(d) and (e) which may imply the eluting LDBCs start to interact
with the styrene-divinylbenzene column filling materials. This
might somewhat obscure the trace impurities such as the incom-
pletely reacted precursor LDBCs although a thorough washing
purification with different organic solvent based on their solubility
difference has been implemented. The precise MW measurement
of dendritic segment-containing macromolecules in itself is still
a challenge, although the GPC data provide valuable information
about the changing tendency of this sample series, the molecular
weights for these products based on linear PEG standards differ
remarkably from the theoretical values, this obvious derivation has
also been observed in other linear-dendritic copolymer series
[6,36], which may mainly result from the globular shape adopted
by the dendron segments and the remarkable solution property
difference of azobenzene-containing copolymer compared with
flexible PEG coil standard.
3.5. Generation-dependent self-assembly and polymeric micellar
aggregation in solution
The self-assembly and polymeric micellar aggregation behavior
of thus obtained amphiphilic LDBCs in aqueous solution has been
preliminarily investigated. According to the well-established
induction by selective solvent protocol [17], variant generation
LDBCs, mPEG-dendr[PAMAM-(AZO)n] (n ¼ 2, 4, 8, 16), were first
dissolved in dioxane, a good solvent for the copolymers, to make
a stock solution. Subsequently, deionized water was added drop-
wise into the copolymer solution until the content of water
reaching 17 wt%, to induce the assembly and formation of aggre-
gation. TEM observation indicated that rich and ordered self-
assembled aggregates were obtained showing generation-depen-
dent, as shown in Fig. 5.
Therefore, the formation of polymeric micellar self-assemblies
in dioxane/water (weight ratio 83/17) mixture solution has been
preliminarily investigated and a generation-dependent aggregation
behavior from nanofibers to nanospheres/oval sheets, vesicles, and
porous LCMs with hydrophilic/hydrophobic weight ratio from 67/
33 to 18/82 has been demonstrated for the amphiphilic LDBCs
mPEG-dendr[PAMAM-(AZO)n]. The driving forces responsible for
the self-assembly process and morphology evolution were
presumably minimization of the interfacial energy and the
p-p
stacking between azobenzene mesogenic units thanks to the flex-
ible dendritic segment and longer spacers as compared with those
reported by Oriol et al. [22b].
The morphology of the polymeric micellar self-assemblies
were investigated by TEM with uranyl acetate stained samples.

Z. Shi et al. / Polymer 53 (2012) 359e369
367
Fig. 5. TEM images of the aggregates self-assembled by variant generation LDBCs in dioxane/H2O at 17 wt% water: (A) mPEG-G0-(AZO)2, nanofibers; (B, C) mPEG-G1-(AZO)4,
nanospheres in coexistence with oval nanosheets; (D) mPEG-G2-(AZO)8, vesicles; (E) mPEG-G3-(AZO)16, large compound micelles (LCMs) and magnification of the porous surface of
an LCM (F).
3.6. Reversible photochemical transformation in solution
where A0, At and AN, are the absorbance at lmax ¼ 360 nm at initial
zero time, t and infinite, respectively. As shown in the inset of
Fig. 6A, they are in accordance with the first order kinetics and the
slope is the photoisomerization rate constant kp. The kp of mPEG-
G3-(AZO)16 in THF was measured to be 0.0240 sꢀ1, which was twice
larger than that of the low molecular weight precursor mesogen
compound 8-AZO-10-acr (kp, precursor ¼ 0.0127 sꢀ1, data not shown)
in the corresponding solution. These results indicated that the
dendritic structure even promoted the photoisomerization rate in
solution, probably due to the presence of the flexible decyl-
methylene spacer and the synergetic effect between closely teth-
ered azobenzene groups [12].
The kinetics of the photo back-isomerization was also studied.
Irradiating the above mPEG-G3-(AZO)16 solution at the photosta-
tionary state in the dark by visible light, induced cis to trans
isomerization process (see Fig. 6B). The experimental results also
can be fitted by the following Equation [12]:
The trans-cis and cis-trans photoisomerization transformation
and photochemical reaction kinetics of this transformation of
azobenzene-containing LDBCs (taking the highest generation
mPEG-G3-(AZO)16 as an example) were studied with UVevis spec-
troscopy in dilute solution at 25 ꢁC.
Fig. 6 shows the UVevis spectra of mPEG-G3-(AZO)16 in THF
solution after exposure to different conditions of ultraviolet light
irradiation at 365 nm (Fig. 6A) or visible light (Fig. 6B). The spectra
were recorded at different irradiation time intervals until spectral
absorption change was no longer evident. The UVevis spectra
exhibited a strong absorption band at around 360 nm characteristic
of
p
-
p
* transition of the trans isomer and the weak absorption near
* transition [11]. As shown in Fig. 6A,
450 nm attributed to the nꢀ
p
exposure to 365 nm UV irradiation caused the trans-cis isomeri-
zation, resulting in a dramatic decrease in the optical density of the
360 nm absorption band and a slight increase in absorbance at
450 nm, and reaching a photostationary state after about 180 s
irradiation at which there were no further significant changes in
the spectra. The experimental results were fitted by Equation [12]:
ꢀ
ꢁ
Ae ꢀ AN
Ae ꢀ At
A0 ꢀ AN
Ae ꢀ AN
ln
¼
*kt*t
(2)
where Ae is the absorbance of the equilibrium state of photo back-
isomerization, kt is rate constant of the cis-trans isomerization
reaction. The photoinduced cis-trans transition process obeyed the
first order kinetics as well, the rate constant kt measured from the
ꢀ
ꢁ
A0 ꢀ AN
At ꢀ AN
ln
¼ kp*t
(1)

368
Z. Shi et al. / Polymer 53 (2012) 359e369
dependent polymeric micellar aggregation behavior of thus
synthesized amphiphilic liquid crystalline LDBCs, from nano-
fibers to nanospheres/oval sheets, vesicles, and porous LCMs, has
been observed in selective solvent combination of dioxane/H2O
in weight ratio 83/17. Furthermore, photoisomerization trans-
formation of these azobenzene-containing LDBCs and their
reaction kinetics of reversible trans-cis/cis-trans photochemical
transitions in THF solution have also been discussed, which
exhibit highly reversible character and even promoted trans-
formation rate compared with the precursor azobenzene
compound presumably ascribed to the presence of the longer
spacer and the synergetic effect of the azobenzene groups closely
tethered on the dendritic surface.
Acknowledgments
We gratefully acknowledge financial support of this work from
the National Natural Science Foundation of China (No. 20874044)
and Fundamental Research Plan Project (Natural Science Founda-
tion) of Jiangsu Province (BK2010244).
Appendix. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.polymer.2011.11.047.
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