Influence of the side-chain structure and molecular weight on the re-entrant behaviors of mesogen-jacketed liquid crystalline polymers

Article abstract of DOI:10.1039/c6ra13363k

Three series of mesogen-jacketed liquid crystalline polymers (MJLCPs) containing different terminal groups (phenmethyl, diphenylmethyl and triphenylmethyl) in the side chains, abbreviated as Pv-m-Bn, Pv-m-DPM, Pv-m-Tr (m = 2, 4, 6, 8, 10, and 12, which are the number of methylene units between the terephthalate core and terminal groups in the side chains), were designed and successfully synthesized via free-radical polymerization. Molecular characterization of the polymers was performed by ¹H NMR, GPC and TG analysis. The phase structures and transitions of the polymers were investigated by a combination of techniques including DSC, POM and 1D/2D WAXD. The experimental results revealed that all the polymers exhibited excellent thermal stabilities and the re-entrant behaviors of the MJLCPs were found to be strongly dependent on the structure of the side-chain, i.e., the spacer length increased with the volume of the terminal groups when the polymers exhibited the re-entrant isotropic phase. On the other hand, a series of MJLCPs, poly{2,5-bis[(diphenylmethoxy-ethyl)oxycarbonyl]-styrenes} (Pv-2-DPMs), with different molecular weights (M_n) and narrow M_n distributions have been successfully synthesized via ATRP. The results indicated that when the M_n was below 1.73 × 10⁴ g mol^-1, only the isotropic phase was observed. When M_n was between 3.40 × 10⁴ g mol^-1 and 8.48 × 10⁴ g mol^-1, a re-entrant isotropic phase was formed at low temperatures and a columnar nematic phase at high temperatures. By further increasing the M_n to exceed 9.71 × 10⁴ g mol^-1, a stable columnar nematic phase was developed. This work provides two effective ways to design and synthesize MJLCPs with re-entrant behaviors; moreover, it is meaningful to deeply understand the structure-property relationships of MJLCPs.

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Influence of the Side-Chain Structure and Molecular Weight on
the Re-entrant Behaviors of Mesogen-Jacketed Liquid Crystalline
Polymers
Received 00th January 20xx,
Accepted 00th January 20xx
Zheng Xiang, Sheng Chen*, Yongbing Luo, Ping Li, Hailiang Zhang *
DOI: 10.1039/x0xx00000x
Three series of mesogen-jacketed liquid crystalline polymers (MJLCPs) containing different terminal groups (phenmethyl,
diphenylmethyl and triphenylmethyl) in the side chains, abbreviated named Pv-m-Bn, Pv-m-DPM, Pv-m-Tr (m =2, 4, 6, 8,
10, 12, which was the number of the methylene units between the terephthalate core and terminal groups in the side
chains), were designed and successfully synthesized via free radical polymerization. The molecular characterizations of the
polymers were performed with 1H NMR, GPC and TG analysis. The phase structures and transitions of the polymers were
investigated by the combination of techniques including DSC, POM and 1D/2D WAXD. The experimental results revealed
that all polymers exhibited excellent thermal stabilities and the re-entrant behaviors of MJLCPs were found to be strongly
dependent on the structure of the side-chain, that was, the spacer length was increased with the increasing volume of the
terminal groups when the polymers wanted to exhibit the re-entrant isotropic phase. On the other hand, a series of
MJLCPs, poly {2,5-bis[(diphenylmethoxy-ethyl)oxycarbonyl]-styrenes} (Pv-2-DPMs), with different molecular weights (Mn)
and narrow Mn distributions have been successfully synthesized via ATRP. The results indicated that when the Mn were
below 1.73 × 104 g/mol, only isotropic phase was observed. When Mn was between 3.40 × 104 g/mol and 8.48 × 104 g/mol,
a re-entrant isotropic phase in low temperature and columnar nematic phase in high temperature were formed. Further
increasing the Mn to exceed 9.71 × 104 g/mol, a stable columnar nematic phase was developed. This work provides two
effective ways to design and synthesize MJLCPs with re-entrant behaviors, meantime, it is meaningful to deeply
understand the structure–property relationships of MJLCPs.
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spacers between the side-chain mesogen and the backbone decou-
ple the motions of mesogens and main chains, facilitating the for-
1. Introduction
In the past decades, liquid crystalline polymers (LCPs) have attract-
ed considerable attention. This is a consequence not only for their
scientific interests but also from an industrial point of view due to
their potential applications in fields such as engineering plastics,
optical data storage, optic, electro-optic, nonlinear optic devices,
photomechanical and so on.1-10 Those applications strongly depend
on the phase structures and phase transitions of LCPs. Therefore,
many researchers have investigated the structure–property rela-
tionship of LCPs in order to understand the principles of structure
formation and structure manipulation.
mation of LC phases. Later, Zhou et al.18 had reported a special type
of side-on SCLCPs called mesogen-jacketed liquid crystalline poly-
mers (MJLCPs), which had a short spacer or with a single covalent
bond connecting the mesogen to the polymer backbone. The “jack-
eting effect” owing to steric hindrance forces the main chain to take
an extended-chain conformation, leading to the MJLCPs display
many thermotropic properties characterized by MCLCPs, although
they belong to SCLCPs chemically, such as high glass transition tem-
perature, broad LC temperature range, and forming banded texture
after mechanical shearing in LC state. Based on the concept of the
“jacketing effect”, many researchers have investigated the struc-
ture-property relationship of MJLCPs to understand the principles
of structure formation and structure manipulation.19,20 For example,
our group has researched the reentrant isotropic phase behavior of
MJLCPs.21,22
In general, LCPs have two main categories: main-chain liquid
crystalline polymers (MCLCPs) with mesogen units located in the
main chain11-13 and side-chain liquid crystalline polymers (SCLCPs)
with mesogens attached to the main chain as side groups14-16
,
which can be readily obtained by attaching anisotropic mesogens to
the backbone via a flexible spacer,17 with an excellent combination
of liquid crystalline (LC) and polymer characteristics. The flexible
Re-entrant phase is an exceptional case in LC physics, as they vio-
late the second law of thermodynamics that molecular order should
decrease with the increase of temperature,23 which was first dis-
covered by Yu et al. when they studied the lauric acid potassium
solution in 1980. Currently, the re-entrant phase behavior basically
can be divided into three categories, namely reentrant isotropic
phase, re-entrant nematic phase and re-entrant columnar phase. At
Key Laboratory of Polymeric Materials and Application Technology of Hunan Prov-
ince, Key Laboratory of Advanced Functional Polymer Materials of Colleges and
Universities of Hunan Province, College of Chemistry, Xiangtan University, Xiangtan
411105, Hunan Province, China. E-mail: huaxuechensheng@163.com Electronic
Supplementary Information (ESI) available: More detailed synthesis and characteri-
zation See DOI: 10.1039/x0xx00000x
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present, two main classes of thermotropic LC polymers with re- and the re-entrant behaviors of MJLCPs were found to be strongly
entrant phase behavior have been studied. One is end-on SCLCPs dependent on the side-chain structure and Mn. The spacer length
based on mesogenic units containing a cyano group with five or six for the re-entrant behaviors of the polymers is increased with the
atoms in the flexible spacer and a polyacrylate or poly(vinyl ether) increasing volume of the terminal groups.
backbone.24-27 These polymers exhibit a reentrant nematic phase
Meantime, a series of MJLCPs with different Mns have been pre-
due to the frustration induced by competing order parameters that
pared
by
ATRP
polymerization,
named
poly
{2,5-
favor different periodicities. The other is dendritic LCPs and MJLCPs,
which exhibits a reentrant isotropic phase.21,22,28-37 For example,
bis[(diphenylmethoxy-ethyl)oxycarbonyl]-styrenes} (Pv-2-DPMs). It
was the first time to find that the MJLCPs presented isotropic
phase→re-entrant isotropic phase→ columnar phases with the
increase of Mn. All the chemical structures of the polymers are
shown in Scheme 1.
Percec
et
al.28
have
studied
poly{4-[3,4,5-tris(n-
and
dodecanyloxy)benzoyloxy]-4-(2-vinyloxyethoxy)biphenyl},
found that as the DP was about 4 to 5, the polymer exhibited re-
entrant isotropic phase. Zhu et al.32,38 have synthesized a MJLCP
containing triphenylene (Tp) moieties in the side chains with 12
methylene units as spacers. The experiment results showed that the
low-temperature phase of the polymer was a hexagonal columnar
phase self-organized by Tp discotic mesogens. The high-
temperature phase was a nematic columnar phase with a larger
dimension developed by the rod-like supramolecular mesogen—the
MJLCP chain as a whole. A re-entrant isotropic phase was found in
the medium temperature range.Yin et al.33 and Chen et al.21 have
reported the synthesis and characterization of a series of novel
MJLCPs
with
different
length
alkyl
tail
named
poly[di(alkyl)vinylterephthalates] (P-m) and poly{2,5-bis[(4-
alkoxyphenyl)oxycarbonyl]styrenes} (P-OCm) respectively. When
m=6, 8, 10, the P-m exhibited the re-entrant isotropic phase, the P-
OCm, showed the re-entrant isotropic phase when m>4. Yu et al.30
and Zhu et al.31 also found that poly[2,5-bis(4'-
Scheme 1: Chemical structures of the polymers
alkoxycarbonylphenyl)styrene]s and poly[2,5-bis(4'-alkoxyphenyl)- 2. Experimental
styrene] both exhibited the re-entrant isotropic phase when the
2.1. Materials
alkyl tail length exceeded a minimum critical length.
Anhydrous tetrahydrofuran (THF) was distilled from sodium benzo-
Beside the structural factors, the re-entrant behavior of the
MJLCPs is also influenced by the Mn. Zhao et al.29 has reported
reentrant phase behavior of MJLCPs based on [poly-{2,5-bis[(4-
butoxyphenyl)oxycarbonyl]styrene}] (PBPCS), which were synthe-
sized by ATRP (Atom Transfer Radical Polymerization). The samples
with Mn below 2.42 × 104 g/mol were isotropic. The samples with
Mn exceed 3.36 × 104 g/mol displayed a thermodynamically stable
isotropic phase at lower temperature and a LC phase at higher tem-
perature. Our group has synthesized a series of MJLCPs via ATRP,
phenone ketyl under argon and used immediately. Triethylamine
(TEA) and dichloromethane (CH2Cl2) were dried over magnesium
sulfate anhydrous. Chlorobenzene (Acros 99 %) was purified by
washing with concentrated sulfuric acid to remove residual thio-
phenes, followed by twice washing, first with 5 % sodium carbonate
solution, and then with water before it was dried with anhydrous
calcium chloride and distilled. Cuprous bromide (CuBr) was synthe-
sized from CuBr2, purified by stirring in acetic acid, and washing
with methanol and then dried in a vacuum just before use. 2, 2-
azobisisobutyronitrile (AIBN) was freshly recrystallised from metha-
nol. N, N, N', N'', N''-Pentamethyldiethylenetriamine (PMDETA)
(99.5 %) and 2-bromoisobutyryl bromide were used as received
without further purification. All other reagents and solvents were
used as received without further purification.
poly{2,5-bis[(4-octa-decyloxyphenyl)oxycarbonyl]-styrenes}
(P-
OC18s).22 The results indicated that the polymers formed a smectic
phase at low temperature and an isotropic phase at high tempera-
ture when the Mn is lower than a critical Mn of approximately 4.6
×104 g/mol. The samples with Mn ≥ 5.2×104 g/mol displayed a reen-
trant isotropic phase.
2.2. Instruments and measurements
In this study, in order to deep understand the relationship be-
tween the chemical structure and Mn to the reentrant behavior of
the MJLCPs, series of MJLCPs have been synthesized via changing
the length of spacer and the volume of side group, named poly {2,5-
bis[(phenmethyl-alkyl)oxycarbonyl]-styrenes} Pv-m-Bn and poly
{2,5-bis[(diphenylmethoxy-alkyl)oxycarbonyl]-styrenes} Pv-m-DPM
(m=2, 4, 6, 8, 10, 12, which is the number of the methylene units
between the terephthalate core and terminal groups in the side
1H NMR spectroscopy was performed on a Bruker ARX400 MHz
spectrometer using with CDCl3 as solvent, tetramethylsilane (TMS)
as the internal standard at ambient temperature. The chemical
shifts were reported on the ppm scale.
The apparent number average molecular weight (Mn) and polydis-
persity index (PDI = Mw/Mn) were measured on a GPC (WATERS
1515) instrument with a set of HT3, HT4 and HT5. The μ-styragel
columns used THF as an eluent and the flow rate was 1.0
chains).
Combined with the results of the poly{2,5-
bis[(triphenylmethoxy-alkyl)oxycarbonyl]-styrenes} (Pv-m-Tr),39 it
revealed that all polymers exhibited excellent thermal stabilities
ml /min at 38 oC. All the GPC data were calibrated with polystyrene
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standards.
TGA was performed on a TA SDT 2960 instrument at a heating rate
of 20 oC /min in nitrogen atmosphere.
DSC traces of the polymer were obtained using a TA Q10 DSC in-
strument. The temperature and heat flow were calibrated using
standard materials (indium and zinc) at a cooling and heating rates
of 10 oC /min. The sample with a typical mass of about 5 mg was
encapsulated in sealed aluminum pans.
LC texture of the polymers was examined under POM (Leica DM-
LM-P) equipped with a Mettler Toledo hot stage (FP82HT).
Scheme 2: Synthetic route of the monomers (Mv-m-Bn) and the corresponding poly-
mers (Pv-m-Bn)
One-dimensional wide-angle X-ray diffraction (1D WAXD) experi-
ments were performed on a BRUKER AXS D8 Advance diffractome-
ter with a 40 kV FL tubes as the X-ray source (Cu Kα) and the LYNX-
EYE_XE detector. Background scattering was recorded and sub-
tracted from the sample patterns. The heating and cooling rates in
the 1D WAXD experiments were 10 oC /min.
Two-dimensional wide-angle X-ray diffraction (2D WAXD) was car-
ried out using a BRUKER AXS D8 Discover diffractometer with a 40
kV FL tubes as the X-ray source (Cu Kα) and the VANTEC 500 detec-
tor. The point-focused X-ray beam was aligned either perpendicular
or parallel to the mechanical shearing direction. For both the 1D
and 2D WAXD experiments, the background scattering was record-
ed and subtracted from the sample patterns.
2.3. Synthesis of monomers
The synthetic route of Mv-m-Bn and Mv-m-DPM are outlined in
Scheme 2 and 321, 40, 41. And the detail information about the inter-
mediate and monomers were shown in supporting information.
Scheme 3: Synthetic route of the monomers (Mv-m-DPM) and the corresponding
polymers (Pv-m-DPM)
1
2.4. Synthesis of polymers
elucidate the process. Figure 1(a) and (b) give the H NMR spectra
(CDCl3-d) of the monomer Mv-6-DPM and the polymer Pv-6-DPM,
respectively. Mv-6-DPM showed characteristic resonances of the
vinyl group at 5.39-5.76 ppm. After polymerization, the signals dis-
appeared completely. The adsorption peaks of Pv-6-DPM were
quite broad and consistent with the expected polymer structure. In
addition, the IR spectra of Mv-6-DPM and Pv-6-DPM was showed in
Figure S2. Obviously, the out-of-plane vibration of vinyl group dis-
appeared after polymerization. The apparent number-average mo-
lecular weights of the polymers determined by GPC were higher
than 1×105 g/mol, demonstrating good polymerizability of the
monomers. The molecular characterizations of the polymers were
summarized in Table 1 and Table 2, respectively.
The polymers [Pv-m-Bn and Pv-m-DPM in Scheme 2 and 3] were
synthesized by conventional solution radical polymerization. For
example, 0.30g (0.41 mmol) of Pv-6-DPM, 40 μL of THF solution of
10 mg/mL AIBN, 0.66 mL of THF and a magnetic stir bar were added
into a polymerization tube, the tube was purged with nitrogen and
subjected to four freeze-thaw cycles to remove any dissolved oxy-
gen and sealed off under vacuum. Polymerization was carried out at
70 oC for 12 h. The tube was then opened, and the reaction mixture
was diluted with 9 mL of THF. Then, the resultant polymer was
dropped slowly into the mixture of methanol/THF (4/1). The disso-
lution and precipitation were repeated three times. After drying
under vacuum, 0.21 g of polymer was obtained. Yield: 70%.
The polymers (Pv-2-DPMs) with different Mn were successfully
synthesized by ATRP. The GPC curves were shown in Figure S3 and
the results were summarized in Table S1. The Mn was gradually
increased from 1.73 × 104 to 13.65 × 104 g/mol. Furthermore, the
polydispersity of the polymers was even lower than the theoretical
value of 1.30 for the controlled “living” free radical polymerization.
In addition, the polymers Pv-2-DPMs with different Mns were
successfully synthesized by ATRP, and detailed synthetic procedures
were showed in S2.
3. Results and discussion
3.1. Synthesis and characterization of monomers and correspond-
ing polymers
3.2. Thermal and liquid-crystalline properties of the polymers
The thermal and liquid-crystalline properties of the polymers (Pv-m-
Bn, Pv-m-DPM and Pv-2-DPMs) were investigated by DSC and POM.
Before, the polymers (Pv-m-Bn and Pv-m-DPM) were investigated
by TGA. As shown in Table 1 and 2, all polymers exhibited excellent
thermal stabilities with the temperatures at 5% weight loss about
350 oC in nitrogen.
As shown in Scheme 2 and 3, all the monomers were synthesized by
only two steps. The structures of the monomers were confirmed
through 1H NMR, IR (see S3) and mass spectrometry measurements.
All the monomers could be easily polymerized via free radical
polymerization. Herein, we use Pv-6-DPM as an example to
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Figure S5(a) and (b) showed the second heating and the first
cooling DSC curves of Pv-2-DPM with different Mns at a rate of 10 oC
/min respectively. A single and obvious Tg was observed under heat-
ing-cooling cycles, and the Tg increased slightly from 44.4 oC to 45.8
oC with the increase of the Mn. This implied when the Mn of poly-
mer exceed 1.73×104 g/mol, the Mn of polymer has no effect on the
Tg of the polymer.
Polarized optical microscopy (POM) was used to investigate the
liquid crystallinity of the polymers. To be consistent with the DSC
result (the second heating), the testing condition was the same as
DSC. For the series of Pv-m-Bn, the results showed that the poly-
mers can be divided into two types. The first one was the Pv-m-Bn
(m=2, 10 and 12). No birefringence was detected by POM, implying
the absence of the thermotropic mesophase. The second one was
the Pv-m-Bn (m=4, 6 and 8). On heating process, the sample be-
came soft above the Tg, but there was no birefringence to be seen,
indicating the formation of isotropic phase. Then, the granular LC
texture was observed when the temperature reached about 160,
139, and 237 oC, respectively. Figure 4 showed the POM images of
the representational sample Pv-6-Bn at low temperature [Figure
4(a)] and high temperature [Figure 4(b)]. With furthering heating,
no visible change of birefringence was observed before decomposi-
o
tion (onset temperature > 300 C). On the subsequent cooling, the
original scene was reestablished, that was, the birefringence disap-
peared. Therefore, based on our previous research,21 the former
polymers were amorphous polymers, but the latter were the first
kind of MJLCPs with a re-entrant phase.29, 31
Figure 1: 1H NMR spectrums of the monomer Mv-6-DPM (a) and the polymer Pv-6-DPM
(b) in CDCl3
In order to observe the phase transition temperature intuitively,
we performed reflection light intensity (birefringence) measure-
ments. While the phase transition process from isotropic phase to
LC phase, it could not be detected by DSC but reflection light inten-
sity measurements, because the thermal enthalpy change was ex-
tremely small. Figure 5 showed the recorded changes in reflection
light intensity for Pv-4-Bn, Pv-6-Bn and Pv-8-Bn by POM at a rate of
The phase-transition behaviors of the polymers were investigated
by DSC. The samples firstly were heated from ambient temperature
to 300 oC at a rate of 20 oC /min under nitrogen atmosphere to
eliminate the thermal history. Figure 2, Figure 3 and Figure S5
showed the DSC curves of Pv-m-Bn, Pv-m-DPM and Pv-2-DPMs at a
rate of 10 oC /min respectively.
o
10 C /min. As can been seen, an obvious transition peak appeared
at 160 oC, 139 oC and 237 oC (T1) respectively, in which the reflection
light intensity sharply increased to 56% from 11%, showing a transi-
tion from isotropic state to ordered state.42 When cooling, the
change of intensity was slower than heating process, indicating
after the polymer formed columnar phase, the polymer could keep
the order structure in some degree to reduce the influence of driv-
ing force of the entropy (see Figure S4) .
For the series of Pv-m-Bn (m =2, 4, 6, 8, 10, 12), Figure 2(a) and (b)
showed the second heating and the first cooling DSC curves of Pv-
m-Bn at a rate of 10 oC /min respectively. The polymer Pv-2-Bn
showed a clear glass transition (Tg') temperature during the first
cooling at -8.0 oC, however, no peaks was detected when the se-
cond heating. For the polymers Pv-4-Bn and Pv-6-Bn, they showed
Tg whatever heating or cooling. For Pv-8-Bn, we could not find any
peak, but the polymers Pv-10-Bn and Pv-12-Bn showed a significant
transition peak during the heating and cooling processes at -46.4 oC
and -31.9 oC, possibly as a result of the melting and crystallization of
the alkyl chain. The results are shown in the Table 1.
For the series of Pv-m-DPM, the results showed that the poly-
mers can be divided into three types. The first one was the Pv-m-
DPM (m=2, 4 and 6). They exhibited strong birefringence that re-
mained invariably even when heated to 300 oC or subsequently
cooled to room temperature. All these polymers showed the stable
needle texture and the POM image of polymer Pv-6-DPM was
shown in Figure 6(a), suggesting the formation of columnar phase.
Second one was Pv-8-DPM in which phase transition was the same
as the Pv-m-Bn (m=4, 6 and 8), and the transition temperature from
isotropic phase to the LC phase was about 126 oC. Figure 6(b) and
Figure 6(c) showed the POM image of polymer at isotropic phase
and LC phase respectively. The last one was the Pv-m-DPM (m=10
For the series of Pv-m-DPM (m=2, 4, 6, 8, 10, 12), Figure 3(a) and
(b) showed the second heating and the first cooling DSC curves of
o
Pv-m-DPM at a rate of 10 C /min respectively. All samples exhibit-
ed a single and obvious Tg when heating or cooling. As expected, Tg
decreases from 45.2 oC to -33.0 oC as the spacer length increases
owing to the strong internal plasticization effect of the longer alkyl
spacers and the results are listed in Table 2. No other transition
peak was observed except Tg and this phenomenon was found in
the other MJLCPs.
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Figure 2: DSC curves of polymers Pv-m-Bn during the second heating scan (a) and the first cooling scan (b) at a rate of 10 oC /min under nitrogen atmosphere
Figure 3: DSC curves of polymers Pv-m-DPM during the second heating scan (a) and the first cooling scan (b) at a rate of 10 oC /min under nitrogen atmosphere
and 12). No birefringence was detected by POM, indicating the the LC birefringence unchanged even when heated to 300 oC. While
o
absence of the thermotropic mesophase. In general, Pv-m-DPMs cooling to room temperature from 300 C, the birefringence of the
can be classified into three types. The first type of the polymers sample remained unchanged, showing that the ordered structure
(m=2, 4 and 6) were transitional MJLCPs, while the second one kept unchanged upon cooling. So the polymers were amorphous
(m=8) was the first kind of MJLCPs with a re-entrant phase,29, 31 and when Mn below 1.73 × 104 g/mol. The polymers belonged to the
the last one (m=10 and 12) was amorphous polymer.
first kind of MJLCPs with a re-entrant phase when Mn between 3.40
× 104 g/mol and 8.48 × 104 g/mol.29, 31 Lastly, the polymers were
transitional MJLCPs when Mn exceeds 9.71 × 104 g/mol.
Figure 7 shows the recorded changes in reflection light intensity
o
for Pv-6-DPM and Pv-8-DPM by POM at a rate of 10 C /min. From
the curves, there was little change for the intensity of Pv-6-DPM
Figure 9 shows the recorded changes in reflection light intensity
o
and the reflection light intensity kept at 59%. However, for Pv-8- for P1-P6 by POM at a rate of 10 C /min. For the P1 and P6, there
DPM, an obvious transition peak appeared at 126 oC (T1) in which was little change for the intensity and the reflection light intensity
the reflection light intensity sharply increased to 55% from 11%, kept at 7% and 72%, respectively. However, P2-P5 showed obvious
o
o
o
o
showing a transition from isotropic state to ordered state.42 When transition peak appeared at 224 C, 162 C, 143 C and 137 C (T1)
cooling, the reflection light intensity returned to the original state in which the reflection light intensity sharply increased to 64% from
(see Figure S4).
7%, showing a transition from isotropic state to ordered state,42 and
the transition temperature was decreased with the increasing of Mn.
For the series of Pv-2-DPMs, the POM results showed that the
polymers can also be divided into three types. The first one was P1. 3.3. Phase structure identification of the polymers
No birefringence was detected by POM. The second kind was P2-P5.
To further characterize the mesomorphic structure of Pv-m-Bn, Pv-
The observation results were similar with the Pv-m-Bn (m=4, 6 and
m-DPM and Pv-2-DPMs, temperature-dependent powder 1D WAXD
8) and Pv-8-DPM. Taking P3 as the example, the POM pictures were
was used. About 60 mg of the polymer was added into an aluminum
shown in Figure 8(a) and Figure 8(b). And the transition tempera-
foil substrate. The testing conditions were consistent with the DSC
ture from isotropic phase to the LC phase was about 162 oC. The
and
POM
measurements.
third type was P6-P7. The samples exhibited the stable texture and
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Table 1: Molecular characteristics and properties of the series of Pv-m-Bn
Sample
Pv-2-Bn
Pv-4-Bn
Pv-6-Bn
Pv-8-Bn
Pv-10-Bn
Pv-12-Bn
Mn (×10-5)a
PDIa
1.87
1.83
1.91
1.83
1.81
1.80
Td (oC)b
Tm (oC)c
Tg (oC)c
Tg' (oC)c
-8.0
T1 (oC)d
Liquid crystallinityd
1.85
365
No
Yes
Yes
Yes
No
No
1.05
350
-32.4
-44.0
-36.5
-46.4
160
139
237
1.14
359
1.53
369
1.24
344
-46.4
-31.9
1.01
358
Table 2: Molecular characteristics and properties of the series of Pv-m-DPM
Sample
Mn (×10-5)a
PDIa
1.95
2.16
2.01
1.81
1.75
1.85
Td (oC)b
Tg (oC)c
T1(oC)d
Liquid crystallinityd
Pv-2-DPM
Pv-4-DPM
Pv-6-DPM
Pv-8-DPM
Pv-10-DPM
Pv-12-DPM
1.32
361
355
349
358
358
346
45.2
10.8
-5.0
Yes
Yes
Yes
Yes
No
1.21
1.93
1.27
-19.2
-27.5
-33.0
126
1.42
1.13
No
a The apparent number-average molecular weight (Mn) and polydispersityindex (PDI) were measured by GPC using PS standards.
b The temperatures of 5% weight loss under nitrogen were measured by TGA heating experiments at a rate of 20 oC /min.
c The melting temperatures (Tm) and the glass transition temperatures (Tg) during the second heating process and the glass transition temperatures (Tg') during the first cooling
process were all measured by DSC at a rate of 10 oC /min under a nitrogen atmosphere.
d Evaluated by POM at a heating and cooling rate of 10 oC /min.
For the series of Pv-m-Bn and Pv-m-DPM, 1D WAXD experiment patterns of Pv-8-DPM. When the temperature was below 100 oC,
results were consistent with the POM results. Figure 10(a) and 10(b) the low-angle diffraction peak was diffuse and weak. When tem-
illustrated the temperature variable 1D WAXD patterns of Pv-6- peratures were above 100 oC, a sharp and intense peak at 2θ =
DPM during the second heating (30-250oC) and subsequent cooling 3.81o (d = 2.32 nm) appeared. With the further increasing tempera-
(250-30oC). And the test region of 2θ was from 1.5o to 35o. In the ture, the intensity of the peak increases. The peak became weak
low angle region, one narrow reflection peak was observed at 2θ = and disappeared at 100 °C on the following cooling scan [see Figure
4.22o (d = 2.09 nm), demonstrating that the ordered structures at 11 (b)]. The distinct discontinuous intensities corresponded with the
the nanometer scale formed. The intensity of the halo basically kept endothermal transition of POM at about 130 °C during the second
the same with the temperature elevated, i.e., 250 oC. When cooling heating. The discontinuous observation during cooling procedure
down, the diffraction peak intensity unchanged, suggesting that clearly indicated that the LC phase formed at higher temperature
polymer always kept the ordered structures in the whole tempera- disappeared during cooling. Those implied that the Pv-8-DPM
ture. In the high angle region, only an amorphous halo around 20o formed the re-entrant phase and similar results were obtained from
could be recognized. This reflected that no long range ordered Pv-m-Bn (m =4, 6, 8).
structure formed via molecular packing was detected over the en-
However, for the Pv-m-Bn (m=2, 10, 12) and Pv-m-DPM (m = 10,
tire temperature region studied. In addition, no noticeable change 12), no low angle scattering peaks were observed in the 1D WAXD
could be observed during the second heating and cooling processes patterns during the heating and cooling processes, suggesting that
in 1D WAXD experiments, implying that there was no isotropic they were amorphous throughout the temperature region.
phase for Pv-6-DPM in the temperature region from 30 to 250 oC.
Figure 12 (a) and 12(b) depict two sets of 1D WAXD patterns for
1D WAXD patterns of Pv-2-DPM and Pv-4-DPM were similar to
Pv-m-Bn and Pv-m-DPM (m = 2, 4, 6, 8, 10, 12) samples obtained at
250 oC, and the data of all polymers at 250 oC are summarized in
those of Pv-6-DPM. Therefore, Pv-m-DPM (m=2, 4 and 6) probably
forms a stable columnar phase.
Table 3 and Table 4. The results showed that the d-spacing value
Figure 11(a) and 11(b) showed the structurally sensitive 1D WAXD
increases
with
the
increase
of
m.
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cooling. Those suggested that the P4 formed the re-entrant phase,
and the polymers (P2, P3, and P5) showed the similar results. The
transition temperatures detected by 1D WAXD was decreased with
the increasing of Mn, which were consistent with the POM results.
Figure 4: Representative POM images of the texture of the Pv-6-Bn maintained at 100
oC(a) and 180 oC(b) (Magnification: ×200).
Figure 7: The change of reflection light intensity for Pv-6-DPM and Pv-8-DPM at a
heating rate 10 oC/min in POM
Figure 5: The change of reflection light intensity for Pv-4-Bn, Pv-6-Bn and Pv-8-Bn at a
heating rate 10 oC/min in POM
Figure 8: Representative POM images of the texture of the P3 maintained at 100 oC(a)
and 180 oC(b) (Magnification: ×200).
Figure 6: Representative POM images of the texture of the Pv-6-DPM maintained at
180 oC(a), and representative POM images of the texture of the Pv-8-DPM maintained
at 100 oC(b) and 180 oC(c) (Magnification: ×200).
1D WAXD experiment results for the Pv-2-DPMs showed that the
samples could also be divided into three types. The first type was P1,
no low angle scattering peaks were seen in the 1D WAXD patterns
during the heating and cooling processes [as shown in Figure S6(a)
and S6(b)]. The second one was P2-P5, taking the P4 as the example,
the observation of 1D WAXD was shown in Figure 13. From Figure
Figure 9: The change of reflection light intensity for P1-P6 at a heating rate 10 oC/min in
POM
The third one was P6-P7, taking the P6 as the example, Figure 14
(a) and 14(b) showed the structurally sensitive 1D WAXD patterns
of P6 from 30 to 180 oC and from 180 to 30 oC during the second
heating and subsequent cooling. The result was similar to the Pv-6-
DPM [see Figure 10], just one narrow diffraction peak was observed
at the low angle and an amorphous halo at the high angle through-
out the heating and cooling processes, illuminated that P6 present-
ed a stable columnar phase. The 1D WAXD patterns for P1-P7 ob-
tained at 250 oC were showed in Figure 15. As can been, the
o
13(a), below 130 C, the low-angle diffraction peak was diffuse and
o
weak. When temperatures were above 130 C, a sharp and intense
peak at 2θ = 4.42o (d = 2.00 nm) appears. This peak becomes weak
and disappears at 100 °C on the following cooling scan [see Figure
13(b)], which was similar to Pv-8-DPM [see Figure 11], indicated
that the LC phase formed at higher temperature disappeared during
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Figure 10: 1D WAXD patterns of Pv-6-DPM during the second heating (a) and subsequent cooling (b)
Figure 11: 1D WAXD patterns of Pv-8-DPM during the second heating (a) and subsequent cooling (b)
Figure 12: 1D WAXD patterns of Pv-m-Bn (a) and Pv-m-DPM (b) (m = 2, 4, 6, 8, 10 and 12) at 250 oC
2θ value of the narrow reflection peak for P2-P7 at the low angle indicating that the ordered structures had developed along the
kept the same, implied that rod diameter of Pv-2-DPMs was not direction perpendicular to the fiber axis on the nanometer scale.
changed with the increasing of the Mn.
When the X-ray incident beam was parallel to the shear direction, a
2D WAXD was used to further identify the phase structures. All ring pattern at 2θ = 4.30o (d = 2.05 nm) was detected, which exhib-
fiber samples were drawn with a pair of tweezers at above glass- ited an isotropic intensity distribution [Figure 16(b)]. Meanwhile, a
transition temperature. Figure 16 showed the 2D WAXD patterns of diffusion scattering halos in the high 2θ angle were observed, sug-
the polymer Pv-6-DPM at room temperature with X-ray incident gesting only the short-range order exists along the fiber direction.
beam perpendicular and parallel to the fiber direction. In the Figure Considering the similar X-ray results reported previously,20,43 we
16(a), the fiber axis was perpendicular to the meridian direction. A proposed the polymer Pv-6-DPM exhibited a columnar nematic
pair of strong diffraction arcs could be seen on the equator at 2θ = phase. The polymers Pv-m-DPM (m=2 and 4) and P6-P7 showed
4.22o (d = 2.09 nm), which was consistent with the 1D WAXD results, similar
2D
WAXD
experiment
results,
which
were
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Figure 13: 1D WAXD patterns of P4 during the second heating (a) and subsequent cooling (b)
Figure 14: 1D patterns of P6 during the second heating (a) and subsequent cooling (b).
considered to form columnar nematic phase as well as each cylin- Table 3: The2θ d-spacing values and calculated length of the side-chain of Pv-m-Bn
der is attributed to a single polymer chain with the side groups
tightly jacketing the backbone.
d-spacing
(nm)a
Calculated length of the
side-chain (nm)b
Sample
2θ (o)a
Pv-2-Bn
Pv-4-Bn
Pv-6-Bn
Pv-8-Bn
Pv-10-Bn
Pv-12-Bn
—
4.65
4.34
4.05
—
—
1.90
2.04
2.18
—
2.58
3.08
3.59
4.12
4.54
5.16
For the polymers Pv-m-Bn (m=4, 6, 8), Pv-8-DPM and P2-P5, we
could obtain 2D WAXD patterns through the sample heated to 250
oC, then quickly put into the liquid nitrogen to keep the LC phase
structure. The 2D WAXD patterns of Pv-6-Bn and Pv-8-DPM were
shown in Figure 17. From the patterns, a ring pattern at low 2θ
angle and a diffusion scattering halos in the high 2θ angle were
presented when the X-ray incident beam was perpendicular to the
fiber direction, however, they couldn’t be orientated because of the
extremely low Tg of these polymers. Combined with the results of
POM and 1D WAXD, it was considered for these polymers to form
columnar nematic phase at high temperature.
—
—
Table 4: The2θ d-spacing values and calculated length of the side-chain of Pv-m-DPM
2θ
d-spacing
(nm)a
Calculated length of the
side-chain (nm)b
Sample
(o)a
3.4. Influence of the side-chain structure on the phase behaviors
Pv-2-DPM
Pv-4-DPM
Pv-6-DPM
Pv-8-DPM
Pv-10-DPM
Pv-12-DPM
4.51
4.20
4.02
3.81
—
1.96
2.10
2.20
2.32
—
2.59
3.10
3.60
4.10
4.52
5.14
Based on the results of DSC, POM, 1D WAXD and 2D WAXD experi-
ments, the phase behaviors of Pv-m-Bn, Pv-m-DPM and Pv-m-Tr
(m=2, 4, 6, 8, 10, 12) were summarized in Figure 18. For the series 1
(Pv-m-Bn) and series 2 (Pv-m-DPM), the polymers show isotropic
phase when m is 10 or 12. However, the Pv-10-Tr exhibits a stable
columnar nematic phase and the Pv-12-Tr forms a re-entrant phase.
On the other hand, the Pv-2-Bn cannot form the LC phase while the
Pv-2-DPM and Pv-2-Tr both can present the stable LC phase. Those
results can be explained by the steric effect of the side group. The
strength of steric effect is determined by the “static
—
—
a
b
Obtained from the one-dimensional WAXD experiments.
assuming the n-alkyl spacer in the side chains have an all-trans conformation.
steric effect” and “dynamic steric effect”. For the strength of “static
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Previously, our group has researched the influence of alkyl tail
length on the phase behavior and phase structure of poly{2,5-
bis[(4-alkoxyphenyl)oxycarbonyl]styrenes} (P-OCm, m=1, 2, 4, 6, 8,
10, 12, 14, 16 and 18). It found that the polymers could show the
re-entrant isotropic phase when tail length exceeded a critical value.
According to the results of this article, thus, the alkyl spacer length
and alkyl tail length play a very important role in the formation of
re-entrant phase for the MJLCPs.
Figure 15: 1D WAXD patterns of P1-P7 at 250 oC
Figure 16: 2D WAXD fiber patterns of Pv-6-DPM. The X-ray incident beam was perpen-
dicular (a) and parallel (b) to the fiber axis.
Figure 18: Schematic drawing of the thermotropic phase behavior of the Pv-m-Bn, Pv-
m-DPM and Pv-m-Tr (m=2, 4, 6, 8, 10, 12)
3.5. Influence of the molecular weight on the phase behaviors
The evolution of the phase behaviors with increasing the Mn in this
series of Pv-2-DPMs can be seen in Figure 19. The phase transitions
of the Pv-2-DPMs with middle Mn follow the sequence of reentrant
isotropic phase ↔ columnar nematic phase ↔ isotropic phase (it
is not true). In the past, Zhao et al.29 synthesized a series of PBPCS
with different Mns by ATRP and found that phase behavior depend-
ed on the Mn. When Mn below 2.42 × 104 g/mol, the polymers pre-
sented isotropic phase, with the increasing of Mn, the polymer ex-
hibited re-entrant isotropic phase in low temperature and hexago-
nal columnar phase in high temperature. However, the PBPCS
didn’t display a stable columnar phase even the Mn was 35.6 × 104
g/mol, the Pv-2-DPMs showed a stable columnar phase when
Figure 17: 2D WAXD patterns of Pv-6-Bn (a) and Pv-8-DPM (b) with the X-ray incident
beam perpendicular to the fiber direction.
steric effect”, it relies on the side-chain structure occupation vol-
ume at low temperature, which is related to the rigid, size and
shape of side chain. Yet, for the strength of “dynamic steric effect”,
it relies on the dynamic volumetric of side-chain at high tempera-
ture, which is determined by the chemical structure of side-chain
and the temperature. As for the Pv-m-Bn, Pv-m-DPM and Pv-m-Tr,
the change of terminal group structure from phenmethyl to triphe-
nylmethyl means that the static space volume of side-chain increas-
es. Therefore, it is easy to understand that the Pv-2-Bn cannot dis-
play the LC phase due to the weak static steric effect. On the other
hand, with increasing benzene ring of terminal group, the critical
value of flexible spacer length (m) increases from 4 to 12 when the
polymer displays the re-entrant phase (Pv-m-Bn, Pv-m-DPM and Pv-
m-Tr). It is caused by the dynamic steric effect. When the spacer (m)
increases, the static steric effect becomes weak, leading to that the
polymer forms isotropic phase in the low temperature. However,
the dynamic steric effect becomes strong in the high temperature,
resulting in the formation of LC phase. Furtherly increasing the
spacer length, the dynamic steric effect also becomes weak.
Figure 19: The relationship between the transition temperature and the Mn of Pv-2-
DPMs. (I: isotropic phase, ΦN: columnar nematic phase).
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the Mn exceed to 9.71 × 104 g/mol. Therefore, the entropy effect of
the side-chain depends strongly on the Mn. The transition tempera-
ture of the isotropic phase to the LC phase decreases with the fur-
ther increasing of the Mn and is even lower than the room tempera-
ture.
4
5
M. Wang, J. Wang, Hong Yang, B. P. Lin, E. Q. Chen, P. Keller,
X. Q. Zhang and Y. Sun, Chem. Commun., 2016, 52, 4313-
4316.
S. F. Zhang, C. X. Zhang, J. C. Wang, F. Hong, X. T. Hao, A.
Zhang, Y. F. Wang, H. Wu, W. Y. Zhang and J. L. Pu, Polym.
Chem., 2016,
S. Hassan, R. Anandakathir, M. J. Sobkowicz and B. M.
Budhlall, Polym. Chem., 2016, , 1452-1460.
7, 3013-3025.
6
7
8
9
7
4. Conclusions
Z. Y. Zhang, Q. K. Zhang, Z. H. Shen, J. P. Yu, Y. X. Wu and X. H.
Fan, Macromolecules, 2016, 49, 475–482.
S. Q. Yang, W. Qu, H. B. Pan, Y. D. Zhang, S. J. Zheng, X. H.
Fan and Z. H. Shen, Polymer, 2016, 84, 355-364.
S. Bronnikov, S. Kostromin, V. Musteaţa and V. Cozan, J.
Polym. Res., 2016, 23, 1-8.
In summary, we have successfully synthesized two series of MJLCPs
(Pv-m-Bn and Pv-m-DPM, m=2, 4, 6, 8, 10, 12) with different spacer
lengths and terminal groups by free radical polymerization, and a
series of MJLCPs (Pv-2-DPMs) with different Mns by ATRP. The
phase structures and transitions of these polymers were investigat-
ed by the combined technologies of DSC, POM, and 1D/2D WAXD.
10 H. N. Gayathri, B. Kumar, K. A. Suresh, H. K. Bisoyi and S.
Kumar, Phys. Chem. Chem. Phys., 2016, 18, 12101-12107.
11 M. Tokita, S. Okuda, S. Yoshihara, C. Takahashi, S. Kang, K.
Sakajiri and J. Watanabe, Polymer, 2012, 53, 5596-5599.
12 T. S. Jo, A. K. Nedeltchev, B. Biswas, H. Han and P. K.
Bhowmik, Polymer, 2012, 53, 1063-1071.
For the series of Pv-m-Bn and Pv-m-DPM, the Tg was decreased
with the increasing of m due to the alkyl plastification. The poly-
mers Pv-m-Bn (m=2, 10 and 12) and Pv-m-DPM (m=10 and 12) pre-
sented the isotropic phase because of the poor jacketing effect.44
13 C. Ruan, L. Chen, R. Yang, H. Y. Zhong and Y. Z. Wang, RSC
Adv., 2015, 5, 48541-48550.
Pv-m-DPM (m=2, 4 and 6) showed stable columnar nematic phase 14 B. Geng, L. X. Guo, B. P. Lin, P. Keller, X. Q. Zhang, Y. Sun and
H. Yang, Polym. Chem., 2015,
15 Y. S. Xu, D. Shi, J. Gu, Z. Lei, H. L. Xie, T. P. Zhao, S. Yang and E.
Q. Chen, Polym. Chem., 2016, , 462-473.
16 X. Q. Liu, J. Wang, S. Yang and E. Q. Chen, ACS Macro Lett.,
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6, 5281-5287.
because of the strong static steric effect. Pv-m-Bn (m=4, 6, 8) and
Pv-8-DPM showed the isotropic phase in low temperature and co-
lumnar nematic phase in high temperature, because the backbone
adopts a somewhat extended conformation due to the consecutive
7
3
tempestuously motions of side-chain around the main-chain,45 lead- 17 H. Finkelmann, H. Ringsdorf and J. H. Wendorff, Makromol.
Chem., 1978, 179, 273.
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ing to produce the strong dynamic steric effect with the tempera-
ture increased. Combined with the results of the Pv-m-Tr (m=2, 4, 6,
8, 10 and 12), we understand that if the polymers wants to form the
re-entrant phase, it needs the longer flexible spacer when the pol-
ymers have larger terminal groups.
,
20 L. Y. Zhang, H. L Wu, Z. H. Shen, X. H. Fan and Q. F. Zhou, J.
Polym. Sci. Part A: Polym. Chem., 2011, 49, 3207-3217.
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For the series of Pv-2-DPMs, when Mn below 1.73 × 104 g/mol,
only the amorphous state was observed. When Mn between 3.40 ×
104 g/mol and 8.48 × 104 g/mol, a re-entrant isotropic phase in low
temperature and columnar nematic phase in high temperature
formed. Further increasing the Mn to exceed 9.71 × 104 g/mol, a
stable columnar nematic phase was developed.
In all, this work provided two facile ways to discover and synthe-
size the polymers with re-entrant behaviors.
W. H. D. Jeu, Eur. Phys. J. E., 2001, 6, 277-285.
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Acknowledgements
This research was financially supported by the National Nat-
ural Science Foundation of China (21504075) and the Research
Foundation of Education College of Hunan Province, China
(Grant No. 14C1090).
28 V. Percec, M. Lee. J. Heck, H. E. Blackwell, G. Ungar and A.
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RSC Advances
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DOI: 10.1039/C6RA13363K
Graphical Abstract:
The alkyl spacer length and the terminal group volume influence on the phase behavior and
structure of mesogen-jacketed liquid-crystalline polymers