Design, synthesis, and phase behaviors of a novel triphenylene-based side chain liquid crystalline diblock copolymer

Article abstract of DOI:10.1039/c8nj02568a

A novel double Tp-based liquid crystalline (LC) diblock copolymer (PMTS-b-PMT6S) composed of poly[3,6,7,10,11-pentakis (hexyloxy)-2-oxytriphenylene] (PMTS) and poly{6-[3,6,7,10,11-pentakis(hexyloxy)-2-oxytriphenylene]} (PMT6S) was designed and successfully synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. While PMTS is a rigid columnar-shaped (Φ_N) polymer, PMT6S is a stable hexagonal columnar phase (Φ_H) polymer. The phase behaviors of diblock copolymers were studied by DSC, POM and 1D WAXD. The results showed that the weight fraction of PMT6S (f_PMT6S) has a significant effect on the LC phase behaviors and phase structures of diblock copolymers. Both glass transition temperature and phase transition temperature of the diblock copolymers from LC phase to isotropic phase reduced with the weight fraction of PMT6S in the feed. When the f_PMT6S ≤ 58.1%, PMTS-b-PMT6S-1 to PMTS-b-PMT6S-3 show similar properties to PMTS, which formed a stable columnar nematic phase (Φ_N), while when the f_PMT6S ≥ 64.2%, PMTS-b-PMT6S-4 and PMTS-b-PMT6S-5 show similar properties to PMT6S, which presented a hexagonal symmetry columnar phase (Φ_H). Comparison between the diblock copolymer and homopolymer (PMTS and PMT6S) indicates that the content of the spacer was crucial to determine the LC structures. Through the study one can better understand the interrelation of microstructures and Tp DLC orders, which constitutes the key basis for various applications.

Full text of DOI:10.1039/c8nj02568a

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Design, synthesis, and phase behaviors of a novel
triphenylene-based side chain liquid crystalline
diblock copolymer
Cite this:DOI: 10.1039/c8nj02568a
a
a
a
b
Jianfeng Ban, * Lulu Pan, Bo Shi and Hailiang Zhang
A novel double Tp-based liquid crystalline (LC) diblock copolymer (PMTS-b-PMT6S) composed of
poly[3,6,7,10,11-pentakis (hexyloxy)-2-oxytriphenylene] (PMTS) and poly{6-[3,6,7,10,11-pentakis(hexyloxy)-
2
-oxytriphenylene]} (PMT6S) was designed and successfully synthesized by reversible addition–fragmentation
chain transfer (RAFT) polymerization. While PMTS is a rigid columnar-shaped (F ) polymer, PMT6S is a stable
hexagonal columnar phase (F ) polymer. The phase behaviors of diblock copolymers were studied by DSC,
N
H
POM and 1D WAXD. The results showed that the weight fraction of PMT6S (fPMT6S) has a significant effect on
the LC phase behaviors and phase structures of diblock copolymers. Both glass transition temperature and
phase transition temperature of the diblock copolymers from LC phase to isotropic phase reduced with the
weight fraction of PMT6S in the feed. When the fPMT6S r 58.1%, PMTS-b-PMT6S-1 to PMTS-b-PMT6S-3
show similar properties to PMTS, which formed a stable columnar nematic phase (F
N
), while when the
Received 24th May 2018,
Accepted 9th July 2018
f
PMT6S Z 64.2%, PMTS-b-PMT6S-4 and PMTS-b-PMT6S-5 show similar properties to PMT6S, which
H
presented a hexagonal symmetry columnar phase (F ). Comparison between the diblock copolymer and
homopolymer (PMTS and PMT6S) indicates that the content of the spacer was crucial to determine the
LC structures. Through the study one can better understand the interrelation of microstructures and Tp
DLC orders, which constitutes the key basis for various applications.
DOI: 10.1039/c8nj02568a
rsc.li/njc
of the planar aromatic cores. Thus, they can be applied in
organic semiconductors, superconducting materials, optical
Introduction
Over the last thirty years, phase structures and transitions of compensation films, one-dimensional conductors, photovoltaic
1
4–17
side chain liquid crystalline polymers (SCLCPs) have been one solar cells, etc.
Therefore, DLC materials have attracted
of the important topics of study in polymer chemistry and great attention from researchers. Among DLC materials, the
physics, due to which SCLCPs have been designed and synthesized phase behavior and phase structure of SCLCPs containing
1
–3
for their application potential in functional soft materials.
triphenylene (Tp) derivatives with a long flexible spacer as
Therefore, researchers have focused on investigating the structure– linkers have been widely investigated, not only because of their
property relationship of SCLCPs in order to understand the relatively easy synthesis and thermal and chemical stability, but
4–9
18–20
principles of structure formation and structure manipulation.
also because of the varieties of mesophases.
Meanwhile, our
Usually, SCLCPs are composed of four distinct structural com- group have investigated the structure–property relationship of
ponents: the backbone, the flexible spacer, the mesogenic group SCLCPs containing Tp (PBTCS) to understand the principles of
1
0
21
and the terminal substituent. Moreover, much work in the structure formation and structure manipulation. The results
literature has focused on the behavior of discotic liquid crystals indicated that PBTCS has a relatively high glass transition
(
11–13
DLCs) since the discovery of DLCs by Chandrasekhar in 1977.
DLC materials are capable of self-assembling into well- phase (F
ordered supramolecular structures because of the p–p stacking the MJLCP main chain.
On the other hand, the phase behavior of SCLCPs can be
temperature and forms a higher symmetry hexagonal columnar
H
) due to the strong ‘‘jacketing effect’’ between Tp and
tailored not only by changing the chemical structure, but also by
changing the external conditions. For example, copolymerization
represents the simplest synthetic technique, which can be used to
tailor the phase behavior of SCLCPs and broaden their application
range as high-performance materials. For example, Tang et al.
have reported that 2,5-di(n-butoxycarbonyl)styrene was employed
a
22
College of Chemical Engineering, Guangdong University of Petrochemical
Technology, Maoming, 525000, China. E-mail: banban997@sina.com
Key Laboratory of Polymeric Materials and Application Technology of Hunan
Province, Key Laboratory of Advanced Functional Polymer Materials of Colleges
and Universities of Hunan Province, College of Chemistry, Xiangtan University,
Xiangtan, Hunan Province, 411105, China
b
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to tailor the mesomorphic property of PMPCS by random
2
2
radical polymerization. Percec and Lee have systematically
studied the influence of the length of the flexible spacer on the
phase behavior of poly(x-[4-cyano-40-biphenylyl)oxy]alkylvinyl
ethers and copolymers, in which the mesogenic units were
2
3,24
identical but the spacer lengths were varied.
By using
the reversible addition–fragmentation chain transfer (RAFT)
polymerization method, Y. Zhao et al. have synthesized a
diblock polymer bearing two different mesogenic groups of
2
5
azobenzene and biphenyl. Although several double LC diblock Scheme 1 The model for PMTS-b-PMT6S.
copolymers have been reported, the precise control of the
chemical structure and composition still remains a challenge.
Therefore, the understanding of the phase behavior of such
double LC diblock copolymers at different length scales is
Experimental
Materials
rather limited, and double Tp based diblock copolymers are Anhydrous tetrahydrofuran (THF) was distilled from sodium
unknown according to our knowledge. Thus, through studying benzophenone ketyl under argon and used immediately.
double Tp based diblock copolymers, it is expected that the Triethylamine (TEA) and dichloromethane (CH Cl ) were dried
2 2
combination of microphase separation and LC can lead to new over anhydrous magnesium sulfate. 2,2-Azobisisobutyronitrile
materials with superior properties, which meet the require- (AIBN) was freshly recrystallized from methanol. Chlorobenzene
ments for practical application.
(Acros, 99%) was purified by washing with concentrated sulfuric
In our previous work, we systematically studied the phase acid to remove residual thiophenes, followed by washing first with
behavior of side-chain liquid crystalline polymers containing 5% sodium carbonate solution and then with water before it was
Tp discotic mesogen units, including the influence of the dried with anhydrous calcium chloride and distilled. All other
spacer and molecular weight on the phase behavior (PMTS and reagents and solvents were used without further purification.
2
6
PMT6S), the effect of different length flexible spacers on the
2
7
Instruments and measurements
phase behavior of poly(MTSx-co-MT6Sy) copolymers. From All NMR measurements were performed on a Bruker ARX400
our experimental results, the LC phase structures of PMTS were MHz spectrometer using CDCl as solvent and tetramethylsilane
found to be strongly dependent on M , but PMT6S was independent (TMS) as an internal standard at ambient temperature. The
self-organization of PMTnS, and the effect of spacer on the
2
8
3
n
of M
phase behavior of PMTnS. The content of PMT6S has significant
effects on the LC phase behavior and phase structures of dispersity index (PDI = M
poly(MTSx-co-MT6Sy). According to our experimental results, 1515) instrument with a set of HT3, HT4 and HT5. The m-styragel
n
. The length of spacer has significant effects on the LC chemical shifts were reported on the ppm scale.
The apparent number average molecular weight (M
/M ) were measured on a GPC (WATERS
n
) and poly-
w
n
ꢀ1
we have proved that PMTS formed a stable columnar nematic columns used THF as an eluent and the flow rate was 1.0 mL min
phase (F ) due to the bulky rigid side chains of Tp wrapped at 38 1C. The GPC data were calibrated with polystyrene standards.
N
around the main chain and the whole rigid columnar shaped
molecule of PMTS acts as a building block for the LC phase. heating rate of 20 1C min in a nitrogen atmosphere.
PMT6S exhibited a stable hexagonal columnar phase (F
DSC traces of the polymer were obtained using a TA Q10
TGA was performed on a TA SDT 2960 instrument at a
ꢀ
1
H
)
due to the decoupling and self-organization of Tp moieties. DSC instrument. The temperature and heat flow were calibrated
Also, the comparison between PMTS and PMT6S indicates that using standard materials (indium and zinc) at cooling and
ꢀ1
the length of the spacer was crucial to determine the LC heating rates of 10 1C min . The sample with a typical mass
structures.
In this work, a novel series of Tp-based side chain liquid
of about 5 mg was encapsulated in sealed aluminum pans.
The LC texture of the polymer was examined under a POM (Leica
crystalline diblock copolymers (PMTS-b-PMT6S) were for the DM-LM-P) equipped with a Mettler Toledo hot stage (FP82HT).
first time synthesized via reversible addition–fragmentation chain
One-dimensional wide-angle X-ray diffraction (1D WAXD)
transfer polymerization (RAFT). According to our previous work, we experiments were performed on a BRUKER AXS D8 Advance
found that PMTS-b-PMT6S was composed of the rigid columnar- diffractometer with 40 kV FL tubes as the X-ray source (Cu Ka)
shaped molecules of PMTS and PMT6S with a stable hexagonal and the LYNXEYE_XE detector. Background scattering was recorded
columnar phase (F
H
), owing to the decoupling and self-organization and subtracted from the sample patterns. The heating and cooling
ꢀ
1
of the Tp moieties (see Scheme 1). This research would help us rates in the 1D WAXD experiments were 10 1C min
.
to deeply understand the interrelation between Tp mesogenic
orders and overall morphological structures. Also, it can contribute
Synthesis of monomers
to our knowledge of the phase behaviors and provide information The precursor 2-hydroxyl-3,6,7,10,11-pentakis(hexyloxy)triphenylene
about the structure variation of the Tp-based diblock copolymers, (PHT) and the two monomers, [3,6,7,10,11-pentakis(hexyloxy)-2-
which will be useful for fundamental research and real oxytriphenylene]methacrylate (MTS) and 6-[3,6,7,10,11-pentakis-
applications.
(hexyloxy)-2-oxytriphenylene]hexyl methacrylate (MT6S), were
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(PMTS-CTA): AIBN, CPDB and MTS were added to a reaction
tube strictly in the ratio of 1/4 : 1 : m (m = 30, which is the
polymerization degree of PMTS), followed by the addition of
3
mL of THF. After being degassed with three freeze–thaw
cycles, the tube was sealed under vacuum. The polymerization
was carried out at 75 1C for 6 h. Then, the reaction solution
was diluted with dichloromethane (10 mL) and dripped into
methanol (400 mL). The polymer precipitate was further purified
by repeated dissolution in dichloromethane and precipitation in
methanol three times. The obtained PMTS-CTA was dried in a
vacuum oven at 35 1C for 20 h before use. The gel permeation
chromatography (GPC) measurement indicated that PMTS-CTA
possessed a low polydispersity index (PDI) of 1.19 and an
Scheme 2 The chemical structures of MTS and MT6S.
easy to synthesize and purify following the procedures reported
2
1
in our previous work. The chemical structures of the monomers
1
13
were confirmed by H/ C NMR and Mass Spectrometry (MS) as
in the previous work. The chemical structures of MTS and MT6S
are shown in Scheme 2.
4
ꢀ1
apparent number-average M
n
of 1.69 ꢁ 10 g mol
.
The target diblock copolymer (PMTS-b-PMT6S) was synthe-
sized through the RAFT chain extension reaction using PMTS-
CTA. Typically, AIBN, PMTS-CTA and MT6S were added to a
reaction tube strictly in the ratio of 1/4 : 1 : n (n is the polymer-
ization degree of the PMT6S segment in block copolymers),
followed by the addition of 3 mL of THF. The polymerization
conditions and the purification procedure were the same as
that used for the synthesis of PMTS-CTA. To eliminate the
unreacted monomers completely, the purification was repeated
three times, until no peak was observed at the elution time of the
monomers in gel permeation chromatography (GPC) measurements.
After purification, the moist copolymers were dryed at 50 1C
under vacuum for 20 h. And then, the target diblock copolymers
of PMTS-b-PMT6S were obtained.
Synthesis of the chain transfer agent
The chain transfer agent 2-(2-cyanopropyl)dithiobenzoate (CPDB)
was easy to synthesize following the procedure described by
29
Winnik. The procedures and chemical structures of CPDB are
1
shown in Scheme 3. H NMR (CDCl ): 7.87–7.93 (d, 2H, –C H ),
3
6
4
7.50–7.62 (m, 1H, –C H ), 7.28–7.42 (m, 2H, –C H ), 1.91–1.93 (s,
6 4 6 4
6H, –CH).
Synthesis of the diblock copolymer
The synthetic route toward PMTS-b-PMT6S is described in
Scheme 4. To prepare the first block, macromolecular CTA
Results and discussion
Synthesis and characterization of PMTS-CTA and
PMTS-b-PMT6S
Although it has been reported that the atom-transfer radical
polymerization (ATRP) method can readily synthesize some
homopolymers, our attempts at using ATRP to synthesize the
diblock copolymers of PMTS-b-PMT6S failed. Then, we found
that the RAFT method was a versatile and facile way to prepare
the diblock.
Scheme 3 Synthetic route toward CBDP.
This is the first time that the RAFT method has been applied
to obtain Tp mesogen based diblock copolymers. The diblock
copolymers of MTS and MT6S in chlorobenzene were studied at
different monomer to monomer ratios in the feed. As shown in
Fig. 1, the GPC trace of the resultant PMTS-CTA and PMTS-b-
PMT6S is unimodal with a fairly narrow M
w
distribution (PDI r 1.4).
Table 1 summarizes the copolymerization results of PMTS-b-PMT6S.
As we predicted, with the increase in the polymerization degree of
PMT6S (n) from 10 to 100, the molecular weights of the diblock
copolymers gradually increased, which indicates that a series of
diblock copolymers PMTS-b-PMT6S with different molecular
weights were successful synthesized.
1
Fig. 2 provides the H NMR spectra of PMTS-CTA and PMTS-
b-PMT6S, wherein the assignments of resonance peaks are
included. According to the result, the signals attributed to the
PMT6S block superimpose on the spectrum of PMTS after the
Scheme 4 Synthetic route towards PMTS-b-PMT6S.
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Fig. 1 GPC traces of PMTS-CTA and the corresponding diblock copolymers
PMTS-b-PMT6S.
Fig. 3 DSC thermograms of PMTA-CTA.
Table 1 The properties of the macromolecular initiator and the diblock
copolymers
under a nitrogen atmosphere after eliminating the thermal
history. In our previous studies, we had particularly researched
a
n
(ꢁ10ꢀ4
a
b
Sample
M
)
n
PDI
fPMT6S (%)
PMTS-CTA
1.69
1.94
2.79
4.32
5.36
8.22
—
10
30
50
70
1.19
1.28
1.35
1.33
1.40
1.38
—
the phase behaviors of PMTS and confirmed that only the T
can be detected for PMTS. As can be seen in Fig. 3, PMTS-CTA
shows a single glass transition temperature, indicating that
g
PMTS-b-PMT6S-1
PMTS-b-PMT6S-2
PMTS-b-PMT6S-3
PMTS-b-PMT6S-4
PMTS-b-PMT6S-5
10.8
32.9
58.1
64.2
80.6
21
PMTS-CTA is homogeneous to PMTS, similar to mesogen-jacketed
100
30,31
liquid crystalline polymers (MJLCPs).
It illustrated that the
a
Obtained from Waters 1515 instrument, linear PS as standards. liquid crystalline properties of PMTS-CTA were maintained after
Calculated from eqn (1) according to the peaks I and I of H NMR.
c f
b
1
introducing a disulfide ester in the end group.
Polarized optical microscopy (POM) experiments further
confirmed the DSC results. Thin films of the copolymers were
prepared by the CH Cl solution-cast method on clean cover glass,
2 2
followed by drying slowly at room temperature. To maintain
consistency with the DSC results, the samples were heated to
the same high temperature (190 1C), and then slowly cooled to
room temperature. The POM experimental results also show
2
1,26–28
that PMTS-CTA is similar to PMTS,
and forms a colorful
LC texture at room temperature. When slowly heated to 80 1C,
PMTS-CTA became soft and maintained the colorful LC texture
(Fig. 4). During the heating procedure, the samples maintained
zero birefringence until over 117 1C. During cooling from
1
Fig. 2 H NMR spectra of PMTS-CTA (a) and PMTS-b-PMT6S-4 (b).
1
17 1C, no LC textures were observed again at low temperature.
RAFT chain extension reaction. On the basis of the spectrum of
the diblock copolymers, the weight fraction of the PMT6S block
( fPMT6S) can be estimated according to
This phenomenon was attributed to the fact that the backbone
was compelled to stretch to a great extent, in order to minimize
the torque arising from the rigid side chains of Tp DLCs. Thus
the bulky side chain will be wrapped around the main chain.
9
13 ꢁ Ic
fMT6S ¼
(1)
9
13 ꢁ Ic þ 813 ꢁ If
where 813 and 913 is the molecular weight of the repeating
units MTS and MT6S respectively. I represents the integral
c
intensities of PMTS and PMT6S that belong to the Tp mesogen,
and the signals at 8.13 ppm (peak a) and 7.79–7.91 ppm (peak
b) are shown in Fig. 2. I represents the integral intensities of
f
PMTS and PMT6S that belong to the methoxy group, and the
signals at 4.22 ppm (peak c) and 1.57–1.91 ppm (peaks d, e) are
shown in Fig. 2. The calculation results are shown in Table 1.
Phase transitions and phase structures of PMTS-CTA
Fig. 3 depicts the DSC thermograms of PMTS-CTA recorded
ꢀ1
during cooling and subsequent heating at a rate of 10 1C min
Fig. 4 POM images of the texture of PMTS-CTA at 95 1C (200ꢁ).
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Fig. 7 DSC curves of PMTS-b-PMT6S during the second heating process.
Fig. 5 1D WAXD patterns of PMTS-CTA during the second heating.
We further performed the 1D WAXD thermal experiment of compositions of PMTS-b-PMT6S was studied by DSC experiments.
PMTS-CTA to verify the LC phase transitions, and the results Fig. 7 shows the second heating DSC curves of PMTS-b-PMT6S at a
ꢀ
1
are plotted in Fig. 5. 1D WAXD experimental results showed rate of 10 1C min under a nitrogen atmosphere after eliminating
that at low temperature, for example, at 30 1C, only one sharp the thermal history. In our previous studies, the phase behaviors of
peak was in the low-angle region, 2y = 4.761 (d = 1.86 nm), PMTS and PMT6S had been particularly researched, and the
indicating the existence of an ordered structure, similar to experimental results showed that only Tg can be detected for
PMTS, which can be identified as a F
N
g i
phase developed by PMTS. Meanwhile, both T and T can be detected from PMT6S.
2
6–28
the main chain and Tp as a whole.
Meanwhile, in the wide- The values of transition temperatures of PMTS-b-PMT6S are
angle region, two amorphous halos were found at 2y values of summarized in Table 2. As can be seen in Fig. 7, each diblock
B20.41 (d = 0.44 nm) and B25.41 (d = 0.35 nm) which represent copolymer showed a single glass transition temperature,
the characteristic dimensions of the amorphous packing of the indicating that the PMTS-b-PMT6S copolymers were homogeneous.
3
2–34
alkyl chains and the p–p stacking in a discotic LC column.
From the transitional phenomenon of the diblock copolymers,
On heating to 130 1C, two amorphous halos were formed PMTS-b-PMT6S could be divided into two types. The first type
located in the low-angle region near 51 and the wide-angle included PMTS-b-PMT6S-1 to PMTS-b-PMT6S-3, whose molar
region around 221, indicating that PMTS-CTA entered an amorphous content of PMT6S ( fPMT6S) was below 64.2%. They were similar
state, which identified with the DSC and POM result.
to PMTS, and showed only a glass-transition step but no
endothermic peak correlating to a phase transition. Meanwhile,
the second type included PMTS-b-PMT6S-4 and PMTS-b-PMT6S-5,
Thermal and liquid crystalline properties of PMTS-b-PMT6S
The thermal stability and liquid crystalline properties of PMTS- whose molar content of PMT6S was above 64.2%. They were
b-PMT6S were investigated by TGA, DSC and POM. The thermal similar to PMT6S, where a glass-transition step and an endother-
stability is an important property for potential applications. mic peak were observed. From the combined POM and 1D WAXD
As can be seen in Fig. 6, the TGA test results revealed that results, this endothermic peak was attributed to transitions from
3
5
PMTS-CTA and PMTS-b-PMT6S showed good thermal stability, LC phase to isotropic phase. Meanwhile, with the increase in
with almost no weight loss at temperatures as high as 375 1C. the molar content of PMT6S, the T decreased from 81 1C to
The dependence of the thermal transition temperatures on the 55 1C (see Table 2), indicating that the more flexible spacer
PMT6S) contained, the easier the formation of the’’ more stable
g
(
hexagonal columnar phase.
Table 2 LC of PMTA-CTA and PMTS-b-PMT6S
a
b
c
d
b
Sample
T
g
(1C)
T
i
(1C)
T
(1C)
LC
PMTS-CTA
117
81
70
64
56
55
186
116
94
91
88
378
379
377
380
379
382
Yes
Yes
Yes
Yes
Yes
Yes
PMTS-b-PMT6S-1
PMTS-b-PMT6S-2
PMTS-b-PMT6S-3
PMTS-b-PMT6S-4
PMTS-b-PMT6S-5
87
a
Evaluated by DSC during the second heating process at a rate of
ꢀ
1
b
1
0 1C min under a nitrogen atmosphere. Evaluated by POM at a
ꢀ
1 c
heating and cooling rate of 10 1C min
weight loss of the samples under nitrogen [T
TGA heating experiments at a rate of 20 1C min
.
The temperatures at 5%
(N )] were measured by
d
2
ꢀ
1
Fig. 6 TGA thermograms of PMTA-CTA and PMTS-b-PMT6S.
.
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Fig. 8 Representative POM images of the texture of PMTS-b-PMT6S-3 (a)
and PMTS-b-PMT6S-5 (b) (200ꢁ).
Fig. 9 The transition temperatures of PMTS-b-PMT6S.
Polarized optical microscopy (POM) experiments further
confirmed the phase structures of PMTS-b-PMT6S and the DSC
results. To be consistent with the DSC result (the second heating
process), all the samples were heated to the same high temperature
as in the DSC test, and then slowly cooled to room temperature. The
POM test shows similar results to DSC in that PMTS-b-PMT6S could
be divided into two different groups.
The first group comprised PMTS-b-PMT6S-1 to PMTS-b-
PMT6S-3, with fPMT6S r 58.1%, taking PMTS-b-PMT6S-3 as an
example. As can be seen in Fig. 8a, PMTS-b-PMT6S-3 formed a
colorful LC texture at room temperature. During the heating
procedure, the diblock copolymer maintained zero birefringence
until over 91 1C. During cooling from 91 1C to room temperature,
no LC textures were observed again. This phenomenon was
attributed to the following mechanism: when the molar content
of PMT6S was below 58.1%, the diblock copolymer acted as PMTS
forming a columnar nematic phase due to the strong coupling
effect.
The second group comprised PMTS-b-PMT6S-4 and PMTS-b-
PMT6S-5, with the fPMT6S Z 64.2%, and showed similar results to
PMT6S. Taking PMTS-b-PMT6S-5 as an example, as seen in Fig. 8b,
PMTS-b-PMT6S-5 formed typical sand-shaped textures during slow
heating, suggesting the formation of a columnar phase. Once the
temperature reached 89 1C, the textures disappeared and the field of
vision became dark, indicating that PMTS-b-PMT6S-5 entered into
the isotropic state and a corresponding exothermic peak appeared
in the DSC curve (Fig. 7). During cooling from 89 1C, the sand-
shaped textures were observed again at low temperature, and it was
considered that the phase transitions of PMTS-b-PMT6S-5 followed
the sequence of columnar phase 2 isotropic phase. We attributed
this phenomenon to the following: when the molar content of
PMT6S was above 64.2%, the diblock copolymers showed the same
properties as PMT6S, i.e., formed a hexagonal columnar phase
owing to the decoupling and self-organization of the Tp moieties.
The transition temperatures from LC phase to isotropic phase
of the polymer, which was added into an aluminum foil substrate.
To be consistent with the DSC and POM results, the sample was
heated to the same high temperature as in the DSC and POM test,
and then slowly cooled to room temperature. Based on the 1D
WAXD observations, the diblock copolymers could be divided into
two different phase structures, consistent with the POM and DSC
results.
The first group comprised PMTS-b-PMT6S-1 to PMTS-b-PMT6S-3,
which to some extent were similar to PMTS, which formed a stable
36
F phase just like many MJLCPS. Taking PMTS-b-PMT6S-3 as an
N
example, the 1D WAXD powder patterns of PMTS-b-PMT6S-3 during
the second heating step are shown in Fig. 10a. It shows only one
peak in the low-angle region, 2y = 4.861 (d = 1.82 nm), indicating the
existence of an ordered structure, which can be identified as a
N
F phase developed by the main chain and Tp as a whole.
However, in the wide-angle region, at room temperature, two
amorphous halos were found at 2y values of B201 (d = 0.44 nm)
and B251 (d = 0.36 nm), which represent the characteristic
dimensions of the amorphous packing of the alkyl chains and
the p–p stacking in a discotic LC column, respectively. The
samples for PMTS-b-PMT6S-1 and PMTS-b-PMT6S-2 showed
similar results to PMTS-b-PMT6S-3.
The second group included PMTS-b-PMT6S-4 and PMTS-
b-PMT6S-5. When the weight fraction of the PMT6S block
increased to 64.2%, the molecular packing model was changed.
Our previous work has proved that PMT6S formed stable
FH phases, and thus, we will not discuss the experimental
3
3
(measured by POM and DSC) are listed in Table 2. As can be seen
in Fig. 9, the T and the T of the diblock copolymers decreased with
i
g
the increase in the molar content of PMT6S.
Phase structure identification of the diblock copolymers
Temperature dependent 1D WAXD experiments were conducted
to study the mesomorphic structure of the diblock copolymers.
Fig. 10 1D WAXD patterns of PMTS-b-PMT6S-3 (a) and PMTS-b-PMT6S-5
In 1D WAXD experiments, the sample consisted of about 50 mg (b) during the second heating process.
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results again. The 1D WAXD powder patterns of PMTS-b-PMT6S-5
during the second heating step are shown in Fig. 10b. As can be
seen in Fig. 10b, only one peak at 2y = 4.731 (d = 1.87 nm) is seen
in the low-angle region below 50 1C, indicating the existence of an
ordered structure, and was identified as a FN phase. After the
temperature increased over 60 1C, two diffraction peaks in
the low-angle region at 2y = 4.731 (d = 1.87 nm) and 8.141
(
d = 1.09 nm) were detected. The d-spacing values derived from
1
/2
these two peaks have the typical ratio of the q values of 1 : 3
and the two peaks can be assigned to (100) and (110) diffractions,
similar to PMT6S and demonstrating a long-range ordered
hexagonal lattice (F ) with a = b = 2.18 nm and g = 1201. Due
H
to the thermal expansion, these two diffraction peak positions
slightly shift to a low angle with the increase in temperature.
Meanwhile, as with the first group, two peaks are found in the
wide-angle region, because the two peaks belong to amorphous
packing of the alkyl chains (2y = 201) and p–p stacking of Tp
Fig. 11 Simulated results of PMTS-b-PMT6S.
(2y = 251). According to the previous work, we found PMT6S with
a = b = 2.21 nm; however, the column diameter of the second
group which includes PMTS-b-PMT6S-4 (a = b = 2.17 nm) and
PMTS-b-PMT6S-5 (a = b = 2.18 nm) is smaller. This phenomenon
was attributed to the fact that the activity of the PMT6S chain was
limited after copolymerization, and thus, the Tp unit in the
diblock copolymers was not as free as in PMT6S. Consequently,
when the order structures reduced, the intensity of the diffraction
peaks of the p–p stacking of Tp weakened, compared with
PMT6S. Also, the 1D WAXD patterns (at room temperature) for
diblock copolymers were used to calculate the diameter of the
column, and the results are listed in Table 3. As can be seen in
the Tp DLC side chains wrapped around the main chain to form
stable F . When the weight fraction of the PMT6S block
increased to 64.2%, PMTS-b-PMT6S-4 and PMTS-b-PMT6S-5
were similar to PMT6S, and formed stable F based on the
self-organization of Tp, as shown in Fig. 11b. Meanwhile, the
column diameter was smaller than that of PMT6S, which was
identified from all the test results. The reason for this phenomenon
is unknown at the moment, but we suspect that it is attributed to
the both intramolecular and intermolecular interactions.
N
H
Table 3, with the increase in the molar content of PMT6S the Conclusions
diameter of the column increased from 1.76 nm to 1.84 nm.
This is the first time that a Tp based liquid crystalline diblock
From the DSC, POM and 1D WAXD test results, based on the
study of the liquid crystal restricted behavior of PMTS-b-PMT6S,
we found that the diblock copolymers exhibit two different
stabilized phase structures. As shown in Fig. 11, the diblock
copolymers showed similar properties to PMTS; when the
weight fraction of the PMT6S block (fPMT6S) was below 58.1%,
PMTS-b-PMT6S-1 to PMTS-b-PMT6S-3 showed similar properties
to PMTS. They had only a single glass transition temperature
according to the DSC test. From this result, we considered that
the movement of the PMT6S chain was limited by the rigid
PMTS, when the diblock copolymers had a low weight fraction
of the PMT6S block. Thus, the diblock copolymers acted as
PMTS; the packing model of the columns and molecular
dynamics simulation results of the diblock copolymers are
shown in Fig. 11a. The polymethylacrylic acid main chain and
copolymer PMTS-b-PMT6S was designed and successfully
synthesized by reversible addition–fragmentation chain transfer
(RAFT) polymerization. The phase behaviors of the PMTS-CTA and
PMTS-b-PMT6S were studied by TGA, DSC, POM and 1D WAXD.
The thermal analysis showed that all diblock copolymers had only
a single glass transition temperature, indicating a homogeneous
microstructure of the main chain. The phase behaviors of diblock
copolymers were dependent on the weight fraction of PMT6S in
diblock copolymers. When the weight fraction of PMT6S was
below 58.1%, the diblock copolymers formed stable F
to PMTS. However, the diblock copolymers with a weight fraction
of PMT6S above 64.2% were similar to PMT6S, presenting F
N
, similar
H
symmetry. It was considered that the formation mechanism of the
two different phase behaviors depended on the quantity of the
spacer. Meanwhile, the column diameter of PMTS-b-PMT6S-4 and
PMTS-b-PMT6S-5 was smaller than that of PMT6S, suggesting that
the diblock copolymers can provide new functional materials with
fine-tuned ordered structures. The study of the phase behaviors
of these novel Tp DLC diblock copolymers will be useful for
fundamental research and real applications.
Table 3 The 2y d-spacing values of PMTS-b-PMT6S
a
a
Sample
2y (1)
d100 (nm)
PMTS-b-PMT6S-1
PMTS-b-PMT6S-2
PMTS-b-PMT6S-3
PMTS-b-PMT6S-4
PMTS-b-PMT6S-5
4.99
4.94
4.86
4.80
4.73
1.76
1.78
1.82
1.84
1.87
Conflicts of interest
a
Obtained from one-dimensional WAXD experiments.
There are no conflicts to declare.
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Acknowledgements
L. Emmanuelle, K. David, M. Fabrice and A. Andr ´e -Jean,
Macromolecules, 2014, 47, 1715–1731.
7 T. Emiliano, C. Rube n´ , S. E. Gerardo, L. F. Ce ´s ar, O. Josu,
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This research was financially supported by the National Natural
Science Foundation of China (51503044), Guangdong Province
Rubber/plastic Materials Preparation & Processing Engineering
Technology Development Centre (2015B090903083), and Scientific
Research Fund of Guangdong University of Petrochemical
Technology (518045).
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