Synthesis and self-assembling of diblock copolymers containing a side-on liquid crystalline block and a pseudo-stilbene azobenzene block

Article abstract of DOI:10.1080/15421406.2019.1626078

Liquid crystalline (LC) diblock copolymers containing a side-on LC block and a pseudo-stilbene azobenzene block were synthesized, and their self-assembling in water/tetrahydrofuran (THF) was investigated. Side-on LC monomer 4’’-methylacryloyloxybutyl 2,5-

Full text of DOI:10.1080/15421406.2019.1626078

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Synthesis and self-assembling of diblock
copolymers containing a side-on liquid crystalline
block and a pseudo-stilbene azobenzene block
Chenhao Zhan, Yingchuan Zhang, Hongxing Zhang, Jinyi Ma, Zhiyuan Xiong
& Renbo Wei
To cite this article: Chenhao Zhan, Yingchuan Zhang, Hongxing Zhang, Jinyi Ma, Zhiyuan Xiong &
Renbo Wei (2019) Synthesis and self-assembling of diblock copolymers containing a side-on liquid
crystalline block and a pseudo-stilbene azobenzene block, Molecular Crystals and Liquid Crystals,
680:1, 31-45, DOI: 10.1080/15421406.2019.1626078
To link to this article: https://doi.org/10.1080/15421406.2019.1626078
Published online: 20 Aug 2019.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2019, VOL. 680, NO. 1, 31–45
https://doi.org/10.1080/15421406.2019.1626078
Synthesis and self-assembling of diblock copolymers
containing a side-on liquid crystalline block and
a pseudo-stilbene azobenzene block
a
Chenhao Zhan , Yingchuan Zhang a,b, Hongxing Zhanga, Jinyi Maa, Zhiyuan
ꢀ
ꢀ
Xiongc, and Renbo Weia,c
aResearch Branch of Advanced Functional Materials, School of Materials and Energy, University of
b
Electronic Science and Technology of China, Chengdu 611731, China.; School of Automotive Studies,
c
Tongji University, Shanghai 201804, China.; Department of Chemical Engineering, Laboratory of
Advanced Materials (MOE), Tsinghua University, Beijing 100084, P. R. China.
KEYWORDS
ABSTRACT
liquid crystalline polymer;
block copolymer;
azobenzene; self-assembling
Liquid crystalline (LC) diblock copolymers containing a side-on LC
block and a pseudo-stilbene azobenzene block were synthesized,
and their self-assembling in water/tetrahydrofuran (THF) was investi-
gated. Side-on LC monomer 4’’-METHYLACRYLOYLOXYBUTYL 2,5-DI(4’-
BUTYLOXYBENZOYLOXY) BENZOATE (4-MDBB) was synthesized, and then poly-
merized into P4MDBB through reversible addition-fragmentation
chain transfer (RAFT) polymerization. A pseudo-stilbene azobenzene
monomer
6-(N-methyl-N-(4-(4’-cyanophenylazo)phenyl)amino)hexyl
methacrylate (6-NCN) was also synthesized. The diblock copolymers
P4MDBBm-b-P6NCNn were obtained through the RAFT polymeriza-
tion of 6-NCN by using P4MDBB as the chain transfer agent. The
copolymers and the intermediates were confirmed by 1H and 13C
NMR spectra, FT-IR spectra, UV-Vis spectra, mass spectra, DSC ana-
lysis, POM observations, GPC tests and contact angle measurements.
The diblock copolymers self-assembled upon adding water slowly
into their THF solutions. The self-assembling and the corresponding
assemblies were characterized by fluorescence spectra, laser light
scattering experiments as well as TEM measurements. Results dem-
onstrated that these diblock copolymers form spherical aggregates
in water/THF, but the aggregates are un-stretchable upon irradiation
with polarized light.
1. Introduction
Block copolymers have attracted plenty of attention due to their property of self-assem-
bling which can form different kinds of nanoscaled ordered structures.1–6 When a LC
segment is incorporated into the block copolymer, the LC segment forms ordered LC
sub-phase, while the isotropic block is usually highly immiscible with the LC phase. As
a result, the LC block copolymers (LCBC) self-assemble much more easily.7–10 Further,
incorporating azobenzene chromophore, which is a commonly used photo-responsive
CONTACT Renbo Wei
These authors contributed equally to this work.
ß 2019 Taylor & Francis Group, LLC
weirb10@uestc.edu.cn
ꢀ

32
C. ZHAN ET AL.
group, into the LC block copolymers endows the obtained assembles with photo-
manipulative properties.11–14
Azobenzene can be divided into aminoazobenzene, azobenzene and pseudo-stilbene
branches basing on their isomerization behaviors and spectral features.15,16 For azoben-
zene branch, it usually shows LC feature due to the mesogenic property of itself.17,18
When this type of azobenzene is introduced into the LCBC, the obtained copolymers
can combine the specific characteristics of azopolymer and diblock copolymer. After
self-assembling, this obtained azobenzene LCBC assemblies can demonstrate interesting
photo-responsive properties in the nanoscaled ordered structures.19–21
The pseudo-stilbene azobenzene, which has electron-withdrawing/donating groups at
its 4 and 4’ sites, is also known as push-pull azobenzene.22 Resulting from the p-p
ꢀ
transition of the azobenzene, it exhibits intense absorptions in visible light.23 In add-
ition, this kind of azobenzene shows rapid trans-cis-trans isomerization, resulting plenty
of photo-responsive properties.24–28 Their promising applications in sensors, holo-
graphic gratings, modulators, etc. have been reported.29–31
Unfortunately, LC phase is rarely observed from pseudo-stilbene azobenzene resulting
from its nonlinear structure as well as intense dipole.32 At this point, a probable method
in developing LCBCs containing this kind of azobenzene would be covalently linking
this pseudo-stilbene azopolymer with another LC polymer. Theoretically, different kinds
of pseudo-stilbene azobenzene containing LCBCs would be prepared. However, this
kind of pseudo-stilbene azopolymer cannot be prepared by polymerizing the corre-
sponding monomer through the traditional polymerization method directly.26 As a
result, there is few report concerning on the investigation of LCBC containing this kind
of azobenzene.14
In this study, LC diblock polymers containing a side-on LC block and a pseudo-stil-
bene azobenzene block were synthesized, and the self-assembling of them was studied.
Firstly, a side-on LC polymer was prepared though RAFT polymerization. Then, a series
of LC diblock copolymers with different lengths of push-pull azobenzene block was
obtained through RAFT polymerization of a push-pull azo monomer by using the LC
polymer as macromolecular chain transfer agent. The self-assembling of the diblock pol-
ymers was investigated by adding water into their THF solutions.
2. Experimental Section
2.1. materials
Butoxybenzoic acid, 2,5-dihydroxybenzoic acid, 4-aminobenzonitrile and 6-chloro-1-
hexanol were bought from Alfa Aesar. 1,4-Dibromobutane and dicyclohexylcarbodii-
mide (DCC) were purchased from Sinopharm Chemical Reagent. Pyrrolidinopyridine
was obtained from Adamas-beta. N-methylaniline (98%) was purchased from TCI.
Methacryloyl chloride (98%) was purchased from J&K. 4-Cyano-4-(phenylcarbono-
thioylthio) pentanoic acid (CPPA) was supplied by Sigma Aldrich. Methacrylic acid was
obtained from Tianjin Bodi Chemical reagent. All other reagents were commercially
available products and used as received without further purification.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
33
2.2. Synthesis of the diblock copolymers
4’’-Methylacryloyloxybutyl 2,5-di(4’-butyloxybenzoyloxy) benzoate (4-MDBB). 4-
MDBB was synthesized according to the synthetic route shown in Figure S1.33 The
detail of the synthesis of 4-MDBB and the intermediates were also descripted in the
1
supporting information (SI). H NMR (600 MHz, d6-DMSO) d (ppm): 8.09 (m, 4H),
7.84 (d, 1H), 7.65 (m, 1H), 7.47 (d, 1H), 7.12 (m, 4H), 5.97 (s, 1H), 5.63 (s, 1H), 4.15
(t, 2H), 4.10 (t, 4H), 3.94 (t, 2H), 1.84 (s, 3H), 1.75 (m, 4H), 1.56 (m, 2H), 1.47 (m,
6H), 0.95 (t, 6H). 13C NMR (150 MHz, d6-DMSO) d (ppm): 166.9, 164.8, 164.6, 164.1,
163.9, 163.8, 148.6, 148.0, 136.4, 132.7, 132.7, 128.3, 126.0, 125.0, 124.9, 121.2, 120.9,
115.2, 115.1, 68.3, 68.3, 65.4, 64.3, 31.1, 25.2, 25.1, 19.2, 18.4 and 14.2. IR (KBr, cmꢁ1):
3074, 2957, 2933, 2872, 1731, 1634, 1608, 1580, 1512, 1475, 1389, 1252, 1182, 1166 and
1073. MS (ESI): calcd. for C37H42O10 [M þ Na]þ: m/z ¼ 669.27; found: 669.27.
6-(N-methyl-N-(4-(4’-cyanophenylazo)phenyl)amino)hexyl methacrylate (6-NCN).
6-NCN WAS SYNTHESIZED ACCORDING TO THE SYNTHETIC ROUTE AS SHOWN IN FIGURE S4.14 THE
DETAIL OF THE SYNTHESIS OF 6-NCN AND THE INTERMEDIATES WERE ALSO DESCRIPTED IN THE SI.
1H NMR (600 MHz, CDCl3) d (ppm): 7.85 (d, 4H), 7.71 (d, 2H), 6.71 (d, 2H), 6.08 (s,
1H), 5.54 (s, 1H), 4.14 (t, 2H), 3.42 (t, 2H), 3.06 (s, 3H), 1.93 (s, 3H), 1.67 (m, 4H),
1.40 (m, 4H). 13C NMR (150 MHz, CDCl3) d (ppm): 167.6, 155.6, 152.4, 143.5, 136.5,
133.1, 126.1, 125.4, 122.7, 119.1, 111.8, 111.4, 64.6, 52.6, 38.8, 28.7, 27.1, 26.8, 26.0 and
18.4. IR (KBr, cmꢁ1): 2941, 2226, 1713, 1597, 1517, 1381 and 1142. MS (ESI): calcd. for
C24H28N4O2 [M þ H]þ: m/z ¼ 405.22; found: 405.43.
P4MDBBm (P4MDBBm-CTA). 4-MDBB (1.6 g, 2.5 mmol), AIBN (0.5 mL, 1.64 mg/
mL in anisole), and 4-cyano-4-(phenylcarbonothioylthio)pentanoic acid (14 mg,
0.05 mmol) were added into a 50 mL Schlenk flask, followed by adding of 1.5 mL of ani-
sole. The mixture was degassed by three freeze-pump-thaw cycles and sealed under vac-
uum. After that, the flask was placed in an oil bath at 75 ꢂC for 72 h. Then, the reaction
was halted by immersing in ice water. The reaction solution was diluted with THF and
precipitated into petroleum ether to obtain the crude product. A few precipitate was
used to calculate the monomer conversion by NMR. The other crude product was puri-
fied by washing it with hot ethanol at least for 5 times, and then dried in a vacuum
oven overnight at room temperature to yield a pink powder (80%). The monomer con-
version estimated by NMR is 84%. Mn (GPC) ¼1.39 ꢃ 104, Mw/Mn (GPC) ¼ 1.16, Mn
1
(NMR) ¼2.71 ꢃ 104. H NMR (300 MHz, CDCl3) d (ppm): 8.01, 7.77, 7.32, 7.13, 6.84,
4.03, 3.93, 3.66, 1.72-0.71. IR (KBr, cmꢁ1): 2958, 2873, 1731, 1605, 1579, 1511, 1475,
1391, 1250, 1162, 1060.
P6NCNn. The azo homopolymer P6NCNn was prepared for comparison with the
diblock copolymers. It was synthesized by using a similar procedure as the synthesis of
P4MDBBm-CTA. Mn (GPC) ¼6.88 ꢃ 103, Mw/Mn (GPC) ¼ 1.15, Mn (NMR)
1
¼1.01 ꢃ 104. H NMR (300 MHz, CDCl3) d (ppm): 7.79, 7.64, 6.61, 3.89, 3.27, 2.93,
1.89-0.88. IR (KBr, cmꢁ1): 2930, 2873, 2222, 1724, 1596, 1517, 1378, 1250, 1135.
P4MDBB42-b-P6NCN9. In the diblock copolymers of P4MDBBm-b-P6NCNn, m and
n is the numbers of polymerization degrees of the blocks. The block copolymers were
synthesized through RAFT polymerization of 6-NCN by using the obtained P4MDBBm-
CTA (m ¼ 42) as macro-CTA and AIBN as the initiator. Four diblock copolymers
P4MDBBm-b-P6NCNn possessing a nematic liquid crystalline block and strong push-

34
C. ZHAN ET AL.
pull azo chromophore blocks with different lengths were polymerized. Different azo
block lengths were obtained by changing the molar ratio of 6-NCN and P4MDBB-CTA
under the same reaction time and temperature. The synthesis of P4MDBB42-b-P6NCN9
is given as a typical example. The other three copolymers were prepared through a simi-
lar procedure by changing the amount of 6-NCN. 6-NCN (0.033 g, 0.075 mmol), AIBN
(0.1 mL, 1.64 mg/mL of anisole solution), P4MDBBm-CTA (0.13 g, 0.005 mmol) and ani-
sole (0.3 mL) were placed in a 50 mL Schlenk flask. The mixture was degassed by three
freeze-pump-thaw cycles, and then sealed under vacuum. The polymerization was car-
ried out in an oil bath at 75 ꢂC for 24 h. After cooling down to room temperature, the
mixture was diluted with THF. The diluent was dropped in petroleum ether obtaining
the crude product. Finally, the crude product was purified by washing it with hot etha-
nol at least for 5 times. It was dried in a vacuum oven overnight to yield a red powder
1
(60%). Mn (GPC) ¼ 1.55 ꢃ 104, Mw/Mn (GPC) ¼ 1.19, Mn (NMR) ¼ 3.01 ꢃ 104. H
NMR (300 MHz, CDCl3) d (ppm): 8.01, 7.78, 7.64, 7.32, 7.13, 6.84, 6.61, 4.03, 3.92, 3.66,
3.27, 2.93, 1.89-0.71. IR (KBr, cmꢁ1): 2958, 2873, 2224, 1728, 1603, 1580, 1511, 1473,
1391, 1250, 1162, 1135, 1060.
Other P4MDBBm-b-P6NCNn block copolymers were prepared by using the similar
procedure and condition. The detailed amounts of 6-NCN used in the polymerizations
were as following:
P4MDBB42-b-P6NCN25. 6-NCN (0.061 g, 0.15 mmol). Mn (GPC) ¼ 1.76 ꢃ 104, Mw/Mn
(GPC) ¼ 1.23, Mn (NMR) ¼ 3.73 ꢃ 104. 1H NMR (300 MHz, CDCl3) d (ppm): 8.01, 7.78,
7.64, 7.32, 7.13, 6.84, 6.61, 4.03, 3.92, 3.66, 3.27, 2.93, 1.89-0.71. IR (KBr, cmꢁ1): 2958,
2873, 2224, 1728, 1603, 1580, 1511, 1473, 1391, 1250, 1162, 1135, 1060.
P4MDBB42-b-P6NCN40. 6-NCN (0.109 g, 0.25 mmol). Mn (GPC) ¼ 1.93 ꢃ 104, Mw/Mn
(GPC) ¼ 1.25, Mn (NMR) ¼ 4.33 ꢃ 104. 1H NMR (300 MHz, CDCl3) d (ppm): 8.01, 7.78,
7.64, 7.32, 7.13, 6.84, 6.61, 4.03, 3.92, 3.66, 3.27, 2.93, 1.89-0.71. IR (KBr, cmꢁ1): 2958,
2873, 2224, 1728, 1603, 1580, 1511, 1473, 1391, 1250, 1162, 1135, 1060.
P4MDBB42-b-P6NCN72. 6-NCN (0.174 g, 0.4 mmol). Mn (GPC) ¼ 2.28 ꢃ 104, Mw/
1
Mn (GPC) ¼ 1.27, Mn (NMR) ¼ 5.62 ꢃ 104. H NMR (300 MHz, CDCl3) d (ppm):
8.01, 7.78, 7.64, 7.32, 7.13, 6.84, 6.61, 4.03, 3.92, 3.66, 3.27, 2.93, 1.89-0.71. IR (KBr,
cmꢁ1): 2958, 2873, 2224, 1728, 1603, 1580, 1511, 1473, 1391, 1250, 1162, 1135, 1060.
2.3. Self-assembling of the block copolymers
Suitable amount of the block copolymers was dissolved in THF (0.2 mg/mL), which is a
good solvent for the block polymers. Then, the solutions were placed in a shaking incu-
bator (constant temperature 25 ꢂC) for 24 h. To obtain stable self-assembly colloidal dis-
persions, 1 mL of deionized water was slowly added (7.2 mL/h) into the stirred THF
solutions (1 mL) via a syringe pump. After that, the solutions were quenched with
excess of deionized water (9 mL). The obtained suspensions were dialyzed against deion-
ized water for 72 h to remove THF.
The aggregates of diblock copolymers assembled in the solvents were characterized by
TEM and DSL. For TEM observation, some drops of diluted colloid dispersions were
added onto a copper grid coated with carbon film, and then dried under vacuum at
30 ꢂC for 24 h. No staining treatment was required for the TEM observations. The

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
35
sample for DSL measurement was obtained by diluting the colloid suspension with
deionized water. Through the DSL measurement, average hydrodynamic radius of the
aggregates was obtained.
2.4. Characterization
1
The diblock copolymers and the intermediates were confirmed by H and 13C NMR
spectra, Fourier transform infrared spectroscopic (FT-IR) spectra, UV-Vis spectra, mass
spectra (MS), differential scanning calorimeter (DSC) analysis, polarizing microscopy
(POM) observations, gel permeation chromatography (GPC) and contact angles (CAs)
tests. The self-assembling and the corresponding assemblies were characterized by fluor-
escence spectrum (FL), laser light scattering experiments and transmission electron
microscope (TEM) observations. The details of the characterizations can be found in
the supporting information
3. Results and discussion
The diblock copolymers P4MDBBm-b-P6NCNn are polymerized through RAFT polymer-
ization, as shown in Figure 1. Firstly, the P4MDBBm block is obtained through RAFT poly-
merization of the liquid crystal monomer 4-MDBB, under the presence of chain transfer
agent CPPA and initiator AIBN. Then, the diblock copolymers P4MDBBm-b-P6NCNn
with different azo block lengths are synthesized through RAFT polymerization of 6-NCN
by using P4MDBB as chain transfer agent and AIBN as initiator.
Synthetic procedure used to prepare 4-MDBB is shown in Figure S1.33 Firstly, 1,4-dibro-
mobutane reacts with 2,5-dihydroxybenzoic acid forming 4’-bromobutyl 2,5-dihydroxy-
benzoate. After that, 4’-methylacryloyloxybutyl 2,5-dihydroxybenzoate is prepared under a
similar condition. Finally, 4-ADMB is fabricated through esterification between the
obtained intermediate with butyloxybenzoic acid. The synthetic details of 4’-bromobutyl
2,5-dihydroxybenzoate and 4’-methylacryloyloxybutyl 2,5-dihydroxybenzoate are shown
1
in the supporting information. Figure S2 and S3 show H and 13C NMR spectra of these
intermediates. Figure 2 shows the COSY spectrum of the LC monomer 4-MDBB.
1
Comparing with H NMR spectrum of 4’-methylacryloyloxybutyl 2,5-dihydroxybenzoate,
the resonance peaks located at 9.0-10.0 ppm deriving from hydroxy groups disappear, indi-
cating that they are consumed during the reaction. After esterification, two new multiple
peaks coming from the benzene ring protons are observed. The other new multiple peaks
Figure 1. Synthetic route for the diblock copolymers P4MDBBm-b-P6NCNn.

36
C. ZHAN ET AL.
Figure 2. COSY spectrum 4-MDBB.
appeared in the high-filed are assigned to the protons of methylene groups from the
butoxy groups. According to the positions and integrated areas of the protons in the COSY
spectrum, it can be confirmed that the 4-MDBB has been successfully prepared.
The synthetic route for the azo monomer 6-NCN is shown in Figure S4, which also
includes three reaction steps.14 Firstly, 6-(N-methyl-N-phenyl amino)hexan-1-ol is synthe-
sized through the nucleophilic substitution reaction between the N-methylaniline and 6-
chloro-1-hexanol with the existence of K2CO3 and KI. Then, 6-(N-methyl-N-(4-(4’-cyano-
phenylazo)phenyl)amino)hexan-1-ol is prepared through azo coupling at a temperature
lower than 5 ꢂC. Finally, the monomer 6-NCN is obtained through the esterification
between the obtained intermediate and methacryloyl chloride. The synthetic details of 6-
1
NCN and the intermediates are shown in supporting information. Figure S5-S7 show H
and 13C NMR spectra of them, which indicate the successful synthesis of 6-NCN.
The LC polymer P4MDBBm is polymerized through RAFT polymerization. For RAFT
polymerization, the molecular weight can be obtained by calculating the monomer con-
version basing on the fact that molecular weight of the obtained polymer is proportional
to the monomer conversion. In addition, the conversion of the monomer can be

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
37
Figure 3. 1H NMR spectrum of the crude product after RAFT polymerization of 4-MDBB.
1
calculated by measuring the H NMR spectrum of the crude product after polymeriza-
tion.34 For the preparation of P4MDBBm, the conversion of the monomer can be calcu-
lated by Equation 1:
ð Þ
Conv % ¼ ð1 ꢁ 4 ꢃ I5:97=I6:84Þ ꢃ 100%
(1)
where I5.97 and I6.84 are the integration areas of resonance peaks at 5.97 and 6.84 ppm
1
on the H NMR spectrum of the crude product after polymerization. The resonance
peaks at 5.97 and 6.84 ppm are corresponding to the double bond protons from residual
monomer and protons from phenyl on o-position of ether in the obtained polymer.
1
According to the H NMR spectrum of the crude product (Figure 3), the 4-MDBB
monomer conversion is calculated to be 84%. In the experiment section, the feed ratio
([4-MDBB]/[CTA]) was controlled to be 50, herein, the corresponding polymerization
degree of P4MDBBm is m ¼ 50 ꢃ 84% ¼ 42, and thus the average molecular weight of
block P4MDBB is calculated to be 2.71 ꢃ 104. The azo homopolymer P6NCNn is synthe-
sized through the similar procedures with that of P4MDBB42. The corresponding poly-
merization degree of P6NCNn is calculated to be 25, and thus the average molecular
weight of P6NCN25 is 1.01 ꢃ 104. The crude products were purified by washing with
1
hot alcohol up to 4-6 times. The curve 1 in Figure 4(a) is the H NMR spectrum of
P4MDBB42 after purification. It is notable that the resonance peaks of double bond dis-
appear completely when compared with those in Figure 3. The cure 6 in Figure 4(a) is
1
the H NMR spectrum of P6NCN25 after purification.
The synthesis of the diblock copolymers P4MDBBm-b-P6NCNn is similar to that of
1
P6NCN25 except for the changing of CPPA into P4MDBB42. Figure 5 shows the H
NMR spectrum of P4MDBB42-b-P6NCN40 with the assignment of the resonance signals.
1
Comparing with the H NMR spectrum of P4MDBB42, the specific resonances from
1
P6NCN can be clearly identified. Figure 4(a) gives the H NMR spectra of the diblock
copolymers and homopolymers. It can be seen that the integrated areas of the resonance
peaks from the protons of the P6NCN block increase with the increasing length of
P6NCN block in the diblock copolymer. The P6NCN length in the diblock copolymer
can be obtained according to Equation 2:
À
n ¼ 4 ꢃ m ꢃ IhÞ=ð3 ꢃ IkÞ
(2)

38
C. ZHAN ET AL.
Figure 4. 1H NMR spectra (a), GPC traces (b), FT-IR spectra (c) and UV-vis spectra (d) of the polymers.
1: P4MDBB42, 2: P4MDBB42-b-P6NCN9, 3: P4MDBB42-b-P6NCN25, 4: P4MDBB42-b-P6NCN40, 5: P4MDBB42-
b-P6NCN72, 6: P6NCN25.
where m ¼ 42 is the polymerization degree of the P4MDBB42, Ih and Ik are the inte-
1
grated areas of the resonance peaks h and k on the H NMR spectrum of the diblock
copolymer, as typically shown in Figure 5. The calculated length of the P6NCN block
and the corresponding molecular weight of P4MDBBm-b-P6NCNn are summarized in
Table 1.
Figure 4(b) gives GPC traces of P4MDBB42, P6NCN25, and P4MDBBm-b-P6NCNn.
Mono-peak of curve 1 (P4MDBB42) and curve 6 (P6NCN25) demonstrate that the
residual monomer has been removed completely after washing with the hot alcohol.
Besides, the small polydispersity index (PDI < 1.3) of the samples also manifests that
the polymerization of the monomers is well controlled by RAFT polymerization. Curves
2-5 are the GPC traces of the diblock copolymers. The peaks of the traces elute earlier
than the homopolymers indicating the preparation of the diblock copolymers (Table 1).
Figure 4(c) shows the FT-IR spectra of P6NCN25 (curve 6), P4MDBB42 (curve 1) and
the diblock copolymers (curve 2-5) respectively. The absorption at 1060 cmꢁ1 is result-
ing from vibration of ether in P4MDBB block. The absorption at 2222 cmꢁ1 is coming
from vibration of cyano in P6NCN block.35 It can be observed that intensity of the
absorption at 2222 cmꢁ1 becomes stronger with the increasing length of P6NCN block.
Figure 4(d) presents the UV-vis spectra of 0.04 mg/mL THF solution of the polymers.
As illustrated in the figure, the maximum absorption peak of P4MDBBm-b-P6NCNn

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
39
Figure 5. 1H NMR spectrum of P4MDBB42-b-P6NCN40 with assignment of the resonance peaks.
Table 1. Molecular weights and molecular weight distributions of the polymers.
Sample
P6NCN (wt %)
Mn (GPC)
Mw/Mn (GPC)
Mn (NMR)
P4MDBB42
0
16.3
35.0
46.3
60.8
100
1.4 ꢃ 104
1.6 ꢃ 104
1.8 ꢃ 104
1.9 ꢃ 104
2.3 ꢃ 104
6.9 ꢃ 103
1.16
1.19
1.23
1.25
1.27
1.15
1.9 ꢃ 104
2.2 ꢃ 104
2.9 ꢃ 104
3.5 ꢃ 104
4.8 ꢃ 104
3.6 ꢃ 104
P4MDBB42-b-P6NCN9
P4MDBB42-b-P6NCN25
P4MDBB42-b-P6NCN40
P4MDBB42-b-P6NCN72
P6NCN25
locates at 452 nm, corresponding to p-p transition of azobenzene.30 Moreover, the
intensity of maximum absorption peak of the samples shows a prominent increment
with the increasing length of the P6NCN block. All these results confirm the successful
preparation of the diblock copolymers.
ꢀ
The liquid crystalline performance of the diblock copolymers is further studied after
the synthesis of them. The P4MDBB42 block in the diblock copolymers P4MDBBm-b-
P6NCNn can form liquid crystalline phase.36 However, the P6NCN block cannot form
liquid crystalline due to the nonlinear structure and the intense dipole.14 Figure S8
depicts DSC curves of the side-on liquid crystalline monomer 4-MDBB during the 1st
and 2nd heating/cooling scans. 4-MDBB exhibits a melting point at 83.4 ꢂC during the
first heating scan. POM observation demonstrates 4-MDBB enters nematic phase after
cooling from its melt and the isotropic-nematic phase transition temperature is around
84 ꢂC. Figure 6(a) and Figure 6(b) show the typical POM images of 4-MDBB at 83 ꢂC
and 42 ꢂC, respectively. Combining the DSC and POM results, it can be confirmed that
the phase transition point (83.4 ꢂC) on the DSC curve of 4-MDBB during the first heat-
ing scan corresponds to the TNI of it, while no crystallization peak is detected by DSC.
Figure 7 depicts DSC results of P4MDBB42 and P4MDBBm-b-P6NCNn from their
2nd scans. After polymerizing, P4MDBB42 shows a liquid crystalline-isotropic phase

40
C. ZHAN ET AL.
Figure 6. POM micrographs of 4-MDBB at 83 ꢂC (a) and 42 ꢂC (b); P4MDBB42 at 85 ꢂC (c); P4MDBB42-
b-P6NCN9 at 70 ꢂC (d).
Figure 7. DSC curves of the polymers during the second heating and second cooling scans. 1:
P4MDBB42, 2: P4MDBB42-b-P6NCN9, 3: P4MDBB42-b-P6NCN25, 4: P4MDBB42-b-P6NCN40, 5: P4MDBB42-
b-P6NCN72.
transition temperature (TLC-I) of 85.3 ꢂC and a glass transition temperature (Tg) of
52.3 ꢂC (curve 1 in Figure 7) on the 2nd heating scanning. Cooling from the isotropic
state, P4MDBB42 shows a POM texture as shown in Figure 6(c). As it is difficult to
identify the texture from the POM image, XRD measurement is employed to character-
ize P4MDBB42. The result shows that there is no diffraction peak in the small angle
region, indicating that P4MDBB42 forms a nematic phase when cooled from its isotropic
phase.33 After introducing of P6NCN block, the phase transition temperatures of the
diblock copolymers decreases. For block copolymer P4MDBB42-b-P6NCN9, its liquid
crystalline-isotropic phase transition temperature (TLC-I) is 79.8 ꢂC (curve 2 in Figure 7
and Figure 6d). Table 2 listed the values of Tg and TLC-I of the samples.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
41
Table 2. Glass transition temperatures and phase transition temperatures of the polymers obtained
from the second heating and cooling scans.
Second Heating
Second cooling
Sample
Tg (oC)
TLC-I (oC)
Tg (oC)
TLC-I (oC)
P4MDBB42
51.2
50.7
54.5
56.2
57.3
58.1
85.3
79.8
77.8
76.4
75.5
–
49.4
49.0
50.2
51.7
53.7
56.2
84.3
77.3
73.4
71.2
69.7
–
P4MDBB42-b-P6NCN9
P4MDBB42-b-P6NCN25
P4MDBB42-b-P6NCN40
P4MDBB42-b-P6NCN72
P6NCN25
All kinds of aggregates including nanotubes, nano-spheres and vesicles have been
obtained through self-assembling of amphiphilic block copolymers.37 Azobenzene-con-
taining amphiphilic polymers can assemble into nano-spheres easily.37,38 Moreover, the
nano-spheres can be stretched upon illumination by linear light.37–40 The hydrophilicity
of the blocks in P4MDBBm-b-P6NCNn is firstly investigated by measuring water contact
angles on spin-coated film of P4MDBB42 and P6NCN25. As shown in Figure S9, contact
angles are 109.8ꢂ and 99.8ꢂ for P4MDBB42 and P6NCN25, respectively. The results show
that both of P4MDBB42 and P6NCN25 are hydrophobic polymers, and thus the diblock
copolymers P4MDBBm-b-P6NCNn are none amphiphilic polymers.
Recently, it has been reported that the none amphiphilic polymers bearing strong
push-pull azobenzene form uniform colloidal spheres upon self-assembling.41 As the
diblock copolymers P4MDBBm-b-P6NCNn also contain the strong push-pull azobenzene
chromophores, the self-assembling of these copolymers is studied according to the
method proposed by Adi Eisenberg.42 Purified water is slowly added into the THF solu-
tions of P4MDBBm-b-P6NCNn to drive the self-assembling. The diblock copolymers
self-assemble due to their decreased solubility in the blended solvent caused by the add-
ing of water. Fluorescence spectrophotometer is employed to monitor the formation of
aggregates. The wavelength of the incident light is controlled to be 620 nm, and the cor-
responding scattered light spectrum in the range of 610-630 nm is collected. Figure 8
shows the maximum light intensity on the spectrum of the scattered light with the
changing of water contents in the block copolymer dispersions. As the diblock copoly-
mers P4MDBBm-b-P6NCNn are none fluorescence materials, the light intensity is almost
zero before addition of water. When a critical value of water content (15 vol% for
P4MDBB42-b-P6NCN9, Figure 8) is reached, the scattered light intensity increases dra-
matically resulting from aggregation of polymer chains. Critical water content (CWC) is
usually called according to the critical value.37 In this study, the CWC is 12-15 vol% at
the initial content of 0.2 mg/mL of the block copolymers, and it decreases with the
increasing length of P6NCN block. According to the literature,37 the intensity of scat-
tered light will increase rapidly and finally level off when the water content further
increases. However, the water content is only recorded to around 25 vol% due to the
limitation of the of fluorescence spectrophotometer. Finally, plenty of water is used to
“freeze” the formed aggregates.
TEM is employed to observe the morphology of the obtained aggregates. The aggre-
gates for the TEM observation are obtained at the initial concentration of the block
copolymers of 0.2 mg/mL and 7.2 mL/h water adding rate until 50 vol% water is added.
As shown in Figure 9(a), uniform spheres are observed from aggregates of P4MDBB42-

42
C. ZHAN ET AL.
Figure 8. The maximum light intensity on the spectrum of the scattered light versus water content
of the block copolymers solutions and dispersions.
Figure 9. TEM micro-images of aggregates of P4MDBB42-b-P6NCN72 before (a) and after (b) illumin-
ation with laser beam.
b-P6NCN72. The average diameter of these sphere is statistical to be 550 nm. For the
other diblock copolymers, uniform dispersed spherical aggregates are also observed. The
aggregates are also confirmed through dynamic light scattering (DLS). Figure 10 depicts
size distribution curves of the diblock polymers detected by DLS. Hydrodynamic radius
Rh of P4MDBB42-b-P6NCN72 spheres detected by DLS is 270 nm, which is consistent
with TEM result. In addition, it can be seen that the size distribution curve of the
hydrodynamic radius decreases with the increasing length of P6NCN block in the
diblock copolymers. For P4MDBB42-b-P6NCN9, the obtained aggregates show two peaks
on its size distribution curve. It might be due to the poor self-assembling ability result-
ing from the too short length of P6NCN block.
The assembled spheres obtained from azobenzene containing polymers can be
stretched upon illumination by linear polarized light.37–40 Photoinduced stretching of
the obtained spheres is studied by irradiating with polarized light at 450 nm (100 mW/

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
43
Figure 10. The distribution function of the hydrodynamic radius of the spheres in the aqueous dis-
persions obtained from the block copolymers.
cm2) and 532 nm (100 mW/cm2). After dropping the corresponding dispersions on cop-
per grids and drying in vacuum, the spheres for the photoinduced stretching investiga-
tion are obtained. Figure 9(b) shows TEM micrograph of P4MDBB42-b-P6NCN72
spheres after being irradiated with 450 nm polarized light for 9 h. The result demon-
strates that the spheres can be stretched tiny or even not be stretched under this condi-
tion. What’s more, light-induced deformation of the spheres is not observed when
irradiating with 532 nm light for 9 hours. For the other diblock block copolymers, the
results are similar under the same conditions. Comparing with the colloidal spheres
obtained from azo homopolymers which can be stretched to average axial ratios (l/d)
beyond 1.5 upon irradiation with laser beam (532 nm, 100 mW/cm2) in 1 h,41 the aggre-
gates obtained from the block copolymers in this work are consider as un-stretchable.
This might be resulted from the restriction of the liquid crystalline sub-phase formed
from the P4MDBB42 block during the self-assembling process.
4. Conclusions
A series of diblock copolymers (P4MDBBm-b-P6NCNn) with well-defined structure was
synthesized and then self-assembled into colloidal spheres in selected solvents. LC poly-
mer P4MDBB42 was polymerized through RAFT polymerization of the newly synthe-
sized LC monomer 4-MDBB. The degree of polymerization was calculated to be 42
1
according to the H NMR result and the feed ratio. POM, DSC and XRD results sug-
gested that P4MDBB42 can form nematic phase. Diblock copolymers P4MDBBm-b-
P6NCNn (m ¼ 42, n ¼ 9, 25, 40 and 72) were prepared through RAFT polymerization of
azo monomer 6-NCN by using P4MDBB42 as the macromolecular chain transfer agent.
1H NMR, FT-IR, POM and DSC were employed to characterize the diblock copolymers.
When water was slowly added into the THF solutions of P4MDBBm-b-P6NCNn, nano-
spheres aggregates were obtained. TEM observation revealed that the obtained spherical

44
C. ZHAN ET AL.
aggregates are quite uninform, and the average diameter is around 550 nm for
P4MDBB42-b-P6NCN72. Hydrodynamic radius Rh of P4MDBB42-b-P6NCN72 nano-
spheres detected by DLS is 270 nm. The hydrodynamic radius decreases with the
increasing length of P6NCN block in the diblock copolymers. The colloidal spheres of
the block copolymers cannot be stretched upon irradiation with polarized light.
Acknowledgements
The financial support from National Natural Science Foundation of China (Project No.
51603029) is gratefully acknowledged.
Disclosure statement
No potential conflict of interest was reported by the authors.
References
[1] Derry, M. J., Fielding, L. A., Armes, S. P. (2016). Prog. Polym. Sci., 52, 1.
[2] Cabral, H., Miyata, K., & Kataoka, K. (2018). Chem. Rev., 118, 6844.
[3] Keddie, D. J. (2014). Chem. Soc. Rev., 43, 496.
[4] Smart, T., Lomas, H., & Battaglia, G. (2008). Nano Today, 3, 38.
[5] Bang, J., Jeong, U., & Hawker, C. J. (2009). Adv. Mater., 21, 4769.
[6] He, Y. N., He, W., & Wang, X. G. (2013). Polym. Chem., 4, 402.
[7] Yu, H. F. (2014). Prog. Polym. Sci., 39, 781.
[8] Anthamatten, M., Zheng, W., Hammond, P. T. (1999). Macromolecules, 32, 4838.
[9] Shi, L. Y., Hsieh, I. F., & Shen, Z. H. (2012). Macromolecules, 45, 9719.
[10] Chen, X. F., Shen, Z. H., & Zhou, Q. F. (2010). Chem. Soc. Rev., 39, 3072.
[11] Ren, H., Chen, D., & Yang, W. (2018). Macromolecules, 51, 3290.
[12] Wang, T. J., Li, X., & Dong, Z. J. (2017). ACS Appl. Mater. Inter., 9, 24846.
[13] Wei, R. B., He, Y. N., Wang, X. G. (2014). RSC Adv., 4, 58386.
[14] Zhu, Y., Zhou, Y. Q., & Chen, Z. (2012). Polymer, 53, 3566.
[15] Rau H. Boca Raton (FL): CRC Press, 1990.
[16] Ho, M. S., Natansohn, A., & Barrett, C. (1995). Can. J. Chem., 73, 1773.
[17] Wei, R. B., Wang, X. G., He, Y. N. (2015). Chinese Chem. Lett., 26, 857.
[18] Beppu, K., Nagashima, Y., & Hara, M. (2017). Macromol. Rapid. Commun., 38, 160.
[19] Zhang, Y. D., Ping, J., & Wu, Q. W. (2017). Polym. Chem., 8, 1689.
[20] Nagano, S. (2018). Langmuir, DOI: 10.1021/acs.langmuir.8b01824.
[21] Wei, R. B., Ma, J. Y., & Zhang, H. X. (2016). J. Appl. Polym. Sci., 133, 43695.
[22] Liu, B. Y., Zhou, Y. Q., & Zhou, X. R. (2017). Macromol. Chem. Phys., 218, 1700020.
[23] Zhu, Y., Wang, X. G. (2013). Dyes Pigments., 97, 222.
[24] Zhou, X. R., Du, Y., Wang, X. G. (2016). ACS Macro Lett., 5, 234.
[25] Zhou, Y. Q., Tang, B., Wang, X. G. (2015). Polymer, 60, 292.
[26] Zhu, Y., Zhou, Y. Q., Wang, X. G. (2013). Dyes Pigments., 99, 209.
[27] Weis, P., Tian, W., Wu, S. (2018). Chem. Eur. J., 24, 6494.
[28] Zong, C., Zhao, Y., & Ji, H. (2016). Angew. Chem. Int. Edit., 55, 3931.
[29] Wei, R. B., Xu, Z. D., Wang, X. G. (2015). Opt. Mater. Express., 5, 1348.
[30] Wei, R. B., Xu, Z. D., & Liu, X. Y. (2015). J. Mater. Chem. C., 3, 10925.
[31] Li, Y. B., He, Y. N., & Tong, X. L. (2005). J. Am. Chem. Soc., 127, 2402.
[32] Ye, G., Wang, D. R., & He, Y. N. (2010). J. Mater. Chem., 20, 10680.
[33] Wei, R. B., Zhou, L. Y., & He, Y. N. (2013). Polymer, 54, 5321.
[34] Lutz, J. F., Neugebauer, D., Matyjaszewski, K. (2003). J. Am. Chem. Soc., 125, 6986.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
45
[35] Wei, R. B., Tu, L., & You, Y. (2019). Polymer, 161, 162.
[36] Wei, R. B., Ma, J. Y., & Zhang, H. X. (2016). Liq. Cryst., 43, 1682.
[37] Wang, D. R., Wang, X. G. (2013). Prog. Polym. Sci., 38, 271.
[38] Li, N., Ye, G., & He, Y. N. (2011). Chem. Commun., 47, 4757.
[39] Liu, J. P., He, Y. N., Wang, X. G. (2010). Polymer, 51, 2879.
[40] Wang, Y. Y., Lin, S. L., & Zang, M. H. (2012). Soft Matter, 8, 3131.
[41] Zhou, Y. Q., Liu, B. Y., Wang, X. G. (2016). Polymer, 97, 1.
[42] Zhang, L. F., Eisenberg, A. (1995). Science, 268, 1728.