Influence of alkoxy tail length on the self-organization of hairy-rod polymers based on mesogen-jacketed liquid crystalline polymers

Article abstract of DOI:10.1002/pola.26194

A series of hairy-rod polymers, poly{2,5-bis[(4-alkoxyphenyl)oxycarbonyl] styrenes} (P-OCm, m = 1, 2, 4, 6, 8, 10, 12, 14, 16, and 18) were designed and successfully synthesized via free radical polymerization. The chemical structure of the monomers was confirmed by elemental analysis, ¹H NMR and ¹³C NMR. The molecular characterizations of the polymers were performed with ¹H NMR and gel permeation chromatography. The phase structures and transitions of the polymers were investigated by the combination of techniques including differential scanning calorimetry, wide-angle X-ray diffraction, polarized optical microscopy, and rheological measurement. The experimental results revealed that the self-assembly behaviors of P-OCm changed with the increase in m. First, the P-OCm (m = 1, 2) showed only a stable liquid crystalline phase above T_g. Second, with the increasing length of alkoxy tails, the P-OCm (m = 4, 6, 8) presented a re-entrant isotropic phase above T_g and a liquid crystalline phase at higher temperature. Third, the P-OCm (m = 10, 12, 14, 16, 18) exhibited an unusual re-entrant isotropic phase which was separating SmA (in low temperature) and columnar phases (in high temperature). It was the first time that mesogen-jacketed liquid crystalline polymers formed smectic phase, re-entrant isotropic phase, and columnar phases in one polymer due to the microphase separation and the driving force of the entropy.

Full text of DOI:10.1002/pola.26194

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Influence of Alkoxy Tail Length on the Self-Organization of Hairy-Rod
Polymers Based on Mesogen-Jacketed Liquid Crystalline Polymers
Sheng Chen, Changkai Jie, Helou Xie, Hailiang Zhang
Key Laboratory of Polymeric Materials and Application Technology of Hunan Province, and Universities of Hunan Province,
College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, China
Correspondence to: H. Zhang (E-mail: hailiangzhang1965@yahoo.com.cn)
Received 10 May 2012; accepted 21 May 2012; published online 00 Month 2012
DOI: 10.1002/pola.26194
ABSTRACT: A series of hairy-rod polymers, poly{2,5-bis[(4-
alkoxyphenyl)oxycarbonyl]styrenes} (P-OCm, m ¼ 1, 2, 4, 6, 8,
10, 12, 14, 16, and 18) were designed and successfully synthe-
sized via free radical polymerization. The chemical structure
of the monomers was confirmed by elemental analysis, 1H
NMR and 13C NMR. The molecular characterizations of the
polymers were performed with 1H NMR and gel permeation
chromatography. The phase structures and transitions of the
polymers were investigated by the combination of techniques
including differential scanning calorimetry, wide-angle X-ray
diffraction, polarized optical microscopy, and rheological mea-
surement. The experimental results revealed that the self-as-
sembly behaviors of P-OCm changed with the increase in m.
First, the P-OCm (m ¼ 1, 2) showed only a stable liquid crys-
talline phase above Tg. Second, with the increasing length of
alkoxy tails, the P-OCm (m ¼ 4, 6, 8) presented a re-entrant
isotropic phase above Tg and a liquid crystalline phase at
higher temperature. Third, the P-OCm (m ¼ 10, 12, 14, 16, 18)
exhibited an unusual re-entrant isotropic phase which was
separating SmA (in low temperature) and columnar phases
(in high temperature). It was the first time that mesogen-jack-
eted liquid crystalline polymers formed smectic phase, re-
entrant isotropic phase, and columnar phases in one polymer
due to the microphase separation and the driving force of the
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entropy.
2012 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 000: 000–000, 2012
KEYWORDS: hairy-rod polymers; mesogen-jacketed liquid crys-
talline polymer (LCP); phase behavior; re-entrant isotropic
phase; rheology; self-organization
INTRODUCTION Self-assembly, nanophase separation, and
confinement are widely researched in multicomponent soft
systems because soft matter is very important for functional
materials science and biosystems.1–4 So the design, synthesis,
and self-assembly behaviors of soft materials is a hot topic
of research,5–9 especially liquid crystalline polymers. In liq-
uid crystalline polymers’ systems, as well known, the rigidity
of polymers is formed not only by the rigid backbone but
also by some polymers with flexible backbone, such as meso-
gen-jacketed liquid crystalline polymers (MJLCPs) and
dendronized polymers.10–22 The MJLCP was first proposed
by Zhou et al.23,24 in the late 1980s to describe a novel type
of side-chain liquid crystal polymer (side-chain LCPs) whose
bulky mesogenic groups were laterally attached to the flexi-
ble backbone via a single or several carbon–carbon bonds.
As a result, the strong steric hindrance imposed by the
highly crowded bulky and rigid side groups around back-
bone forces the main-chain to be well extended, leading to
the rod-like chain, which has been confirmed by small-angle
neutron scattering technique. All those make MJLCPs exhibit
special properties similar to those of rigid backbone poly-
mers, such as high glass transition temperature, broad tem-
perature range of mesophase, long persistence length in
good solvent, and forming banded texture after mechanical
shearing in LC state. As the concept of MJLCPs was pro-
posed, rod-like MJLCP chains have tended to self-organize
into different columnar LC phases with the polymer chains
aligned parallel to one another, such as columnar nematic
(UH),25,26 hexatic columnar nematic (UHN),26–29 two-dimen-
sional (2D) hexagonal columnar phases (2D UH)30 and rec-
tangular columnar (UR) phase.31 The type of these columnar
phases depends on the molecular properties including the
shape, size, and flexibility of side chains. Recently, smectic
phases with the backbones embedded in the smectic layers
have also been reported when the length or the rigidity of
the mesogen was increased or when semifluorinated tails or
hydrogen bonding were introduced into the mesogens.32,33
As can be seen, the MJLCPs normally shows the smectic
phase or columnar phase, however, it hard forms smectic
phase and columnar phase in one polymer with the
Additional Supporting Information may be found in the online version of this article.
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temperature improved because it is difficult to simultane-
ously meet the requirements of the forming of two kinds of
liquid crystal phase.
In the rigid-rod polymers’ systems, most of polymers contain
the rigid backbone, such as aromatic polyesters,34–36 polya-
mides,37–39 polyanilines,40,41 and poly(3-alkylthiophene).42,43
However, there are some disadvantages (such as high melt-
ing points, low solubility, low compatible with thermoplastic
polymers) for the rigid backbone polymers which hinder the
further research. To overcome such defects, one way is the
attachment of flexible alkyl side chains to the polymer rigid
backbone. One thinks that the rigid polymer can dissolve in
the background of the side chains due to the (infinite)
attractive interaction between the solvent molecules and the
backbone, without loss of the rigidity of the backbone.44–47
Moreover, due to microphase separation of the aliphatic and
aromatic domains,48–51 the rigid-rod polymers with different
flexible alkyl side chains (hairy-rod polymers)52 has led to
not only the finding of the interesting structural feature in
liquid crystalline phases but also the well organization of the
polymers into highly oriented crystals. For example, the a-
helical poly(c-alkyl L-glutamates), aromatic polyesters, and
polyamides with long alkyl side chains as the type of hairy-
rod polymers can form multiple kinds of LC phaes,48,51,53–55
such as hexagonal and lamellar microphase. Subsequently,
some theoretical treatments have been presented and they
pointed that hexagonal, oblique, and lamellar microphase
separated structures as well as isotropic phase can form
because of the balance between unfavorable rod–coil inter-
action and elastic stretching of the side chains based on the
assumption about absence of rotational disordering.52 In
side-chain liquid crystalline polymer systems, the mesomor-
phic properties of side-chain LCPs are determined by the
type of backbone, spacer, mesogenic group, the length of
alkyl tail, molecular weight, and its distribution, and so
forth. Especially, when the flexible spacers and alkyl tail
length increase, side-chain LCPs are inclined to form smec-
tic phase due to microphase separation.56 From the proper-
ties of two types of polymers, the microphase separation
leads to the formation of layer structure. So, a simple ques-
tion arises as to what kind of properties are newly induced
if long alkyl groups are attached to MJLCPs and whether
MJLCPs can form smectic phase and columnar phase in one
polymer.
SCHEME 1 Chemical structures of the polymers (P-OCm).
the MJLCPs had balanced mesogenic core and different ter-
minal flexible substituents, it exhibited more interesting LC
phases structures, like poly{2,5-bis[(4-methoxyphenyl)oxy-
carbonyl]styrene} with terminal methoxy could form colum-
nar nematic (UN) and UHN when the molecular weights
increased,26 but the polymers with long length of alkoxy tails
exhibited another the interesting phase behavior which was
quite similar to re-entrant isotropic phase, such as poly{2,5-
bis[(4-butoxyphenyl)oxycarbonyl]styrene},
poly{2,5-bis[(4-
hexyloxyphenyl)oxycarbonyl]styrene}, and poly{2,5-bis[(4-
octoxyphenyl)oxycarbonyl]styrene} with a butoxy, hexyloxy,
and octoxy, respectively.60 The author pointed that the LC
phase in high temperature was proposed to form by the
driving force of the entropy, and the whole molecular motion
was involved in the formation of the LC phase. Although the
experimental results showed that the length of alkoxy tails
had played an important role in the phase behaviors of
MJLCPs, the MJLCPs did not exhibit smectic phase and co-
lumnar phase in one polymer. However, it can be seen that
some MJLCPs can display a thermodynamically stable iso-
tropic phase at lower temperature and a columnar phase at
higher temperature. In addition, based on above literature
survey, hairy-rod polymers and side-chain LCPs with long
alkyl tail can present lamellar structure due to microphase
separation. So, combined the microphase separation nature
of hairy-rod polymers and side-chain LCPs with long alkyl
tail with the driving force of the entropy nature of MJLCPs, it
may simultaneously meet the requirements of the forming of
two kinds of liquid crystal phase in the MJLCPs’ systems.
Therefore, to systemically study the influence of the differ-
ent length of terminal flexible substituents on phase behav-
iors of MJLCPs with balanced mesogenic core, we designed
and synthesized a series of side-chain rigid-rod polymers
with different length of alkoxy tails: poly{2,5-bis[(4-alkoxy-
phenyl)oxycarbonyl]styrenes} (P-OCm, m is the number of
the carbons in the alkoxy groups, m ¼ 1, 2, 4, 6, 8, 10, 12,
14, 16, and 18). The chemical structures of the polymers
are shown in Scheme 1. More specifically, through the
investigation of P-OCm, we can obtain some knowledge
about the thermal behavior and the arrangement mode of
molecular chain of P-OCm and can get the information of
the structure variation of MJLCPs and pocking mode molec-
ular chain.
As for the phase behavior of MJLCPs with different length
alkyl tail, it has been reported before. For example, Chen
et al.57,58 reported a kind of MJLCPs with unbalanced meso-
genic core and different terminal flexible substituents. When
the carbon number for the flexible substituents were shorter
than four (butoxy groups), well-defined SA phase kept
unchanged; whereas the one reached four, well-defined SC
phase was observed. Recently, Zhang et al.59 also has
researched the properties of MJLCPs containing biphenyl
mesogen with asymmetric substitutions, poly(alkyl 40-(octy-
loxy)-2-vinylbiphenyl-4-carboxylate) (pVBP(m,8)). When m ¼
1, the polymer was not liquid crystalline. All other polymers
showed columnar liquid crystalline phase. Moreover, when
In this work, we described the synthesis of the monomers
and the corresponding homopolymers as well as the phase
behaviors of the polymers. From our experimental results,
these polymers can be divided into three types. The first
type is that polymers show the transitional MJLCP phase
behavior when they have short alkoxy terminals containing
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concentrated sulfuric acid to remove residual thiophenes, fol-
lowed by washing twice with distilled water, once with 5%
sodium carbonate solution, and again with distilled water
before being dried with anhydrous calcium chloride and
then distilled. 4-(Dimethylamino)pyridine (DMAP, 99%,
ACROS), hydroquinone (98%, Alfa Aesar) and the corre-
sponding alkyl bromides together with other reagents and
solvents were used as received without further purification.
Synthesis of Monomers and Polymers
The synthesis of P-OCm (m ¼ 1, 2, 4, 6, and 8) has been
reported by our group and the synthetic route of monomers
and the corresponding polymers are shown in Scheme 2 (m
¼ 1, 2, 4, 6, 8, 10, 12, 14, 16, and 18). The experimental
details are described as follows using 2,5-bis[(4-decyloxy-
phenyl)oxycarbonyl]styrene (M-OC10) as an example: VTA
(5.2 mmol, 1 g) was added into 30 mL of dried thionyl chlo-
ride in a 100-mL three-necked flask, then the mixture was
refluxed. After VTA was dissolved absolutely, the excess thi-
onyl chloride was removed under reduced pressure. The res-
idue was washed with petroleum ether for three times. The
petroleum ether was removed under the reduced pressure
and afforded yellow liquid. This liquid was dissolved in dried
THF to obtain solution A. P-decyloxyphenol (13.0 mmol, 3.26
g) and DMAP (26.0 mmol, 3.18 g) were dissolved in 20 ml
of dried THF to obtain solution B. Under intense stirring at 0
ꢀC, the solution A was slowly added into the solution B over
a period of 2 h. The mixture was further stirred at room
temperature for 12 h, and then, most of the THF was dis-
tilled off the system by evaporation under reduced pressure.
After water was added into the residue, the crude solid
product was first collected by filtration and dried under vac-
uum, and then purified using column chromatography on
silica gel with chloroform as eluent. The product was further
purified by recrystallization from chloroform/methanol
(yield 71%). The characterization data of these monomers
were as follows: 1H NMR (CDCl3): d ¼ 0.88–1.84 (m, 38H,
alkoxy H), 3.96–4.00 (t, 4H, AOCH2A), 5.48–5.87 (2d, 2H,
¼¼CH2), 6.95–7.17 (m, 8H, Ar-H), 7.53–7.60 (q, 1H, ACHA),
8.17–8.45 (m, 3H, Ar-H). 13C NMR (CDCl3): d ¼ 14.11
(ACH3), 22.69 (ACH2A), 26.07 (ACH2A), 29.31–29.60
(ACH2A), 31.92 (ACH2A), 68.51 (AOCH2A), 115.21 (¼¼CH2),
118.35–122.26 (aromatic C ortho to CAO), 128.63 (aromatic
C), 129.03 (aromatic C), 130.91 (aromatic C), 132.04 (aro-
matic C), 133.23 (aromatic C), 134.72 (AHC¼¼CH2), 140.49
(aromatic CAHC¼¼CH2), 144.00–144.10 (aromatic CAO),
157.14 (aromatic CAO), 164.59–165.39 (C¼¼O). (C42H56O6)
(656.41): Calcd. C 76.79, H 8.59; Found C 76.50, H 8.60.
SCHEME 2 Synthetic route of the monomers (M-OCm) and the
corresponding polymers (P-OCm).
one and two carbon atoms. The second type is that the
P-OCm (m ¼ 4, 6, and 8) presents a re-entrant isotropic
phase and a LC phase. In MJLCPs, it is difficult to observe
clearing transition because most of the MJLCPs decomposes
before reaching its transition temperature from liquid crystal
phase to isotropic phase (‘‘clearing point’’), so the occurrence
of a re-entrant isotropic phase of MJLCPs has based on the
assumption that it can observe one isotropic phase in high
temperature. They are considered to be the first kind of
MJLCPs with a re-entrant phase. The third type is that the P-
OCm (m ¼ 10, 12, 14, 16, and 18) exhibits a re-entrant iso-
tropic phase which is separating SmA and columnar phases.
They are considered to be the second kind of MJLCPs with a
re-entrant phase. To our best of our knowledge, the MJLCPs
can form the smectic phase and the columnar phase with
the increase in temperature, which is first reported in the
MJLCPs system. Compared to the re-entrant isotropic behav-
ior of MJLCPs in the past, this article reported that the re-
entrant isotropic phase was separating the smectic phase
and the columnar phases. Moreover, due to the competing
between the smectic phase and the columnar phases over a
specific temperature range, the appearance and disappear-
ance of the re-entrant isotropic phase are influenced by it. In
all, the length of alkoxy tails greatly affect the packing behav-
iors of MJLCPs.
All the other monomers were synthesized and characterized
similarly. The characterization data of M-OCm (m ¼ 12, 14,
16, and 18) were as follows:
EXPERIMENTAL
2,5-Bis[(4-dodecyloxyphenyl)oxycarbonyl]styrene (M-OC12
)
Materials
1H NMR (CDCl3): d ¼ 0.89–1.84 (m, 46H, alkoxy H), 3.98–
4.00 (t, 4H, AOCH2A), 5.48–5.88 (2d, 2H, ¼¼CH2), 6.95–7.17
(m, 8H, Ar-H), 7.53–7.60 (q, 1H, ACH¼¼), 8.17–8.45 (m, 3H,
Ar-H). 13C NMR (CDCl3): d ¼ 14.11 (ACH3), 22.70 (ACH2A),
26.07 (ACH2A), 29.31–29.68 (ACH2A), 31.94 (ACH2A),
The precursor 2-vinylterephthalic acid (VTA) was synthe-
sized according to previous paper.61,62 The p-(n-alkoxyl)phe-
nols were prepared as described by Klarmann et al.63 Chlor-
obenzene (Acros, 99%) was purified by washing with
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68.53 (AOCH2A), 115.23 (¼¼CH2), 118.36–122.26 (aromatic
C ortho to CAO), 128.64 (aromatic C), 129.05 (aromatic C),
130.92 (aromatic C), 132.06 (aromatic C), 133.24 (aromatic
C), 134.72 (AHC¼¼CH2), 140.50 (aromatic CAHC¼¼CH2),
144.01–144.11 (aromatic CAO), 157.16 (aromatic CAO),
164.62–165.42 (C¼¼O). (C46H64O6) (712.47): Calcd. C 77.49,
H 9.05; Found C 76.60, H 9.13.
tion was carried out at 70 ꢀC for 24 h. The tube was then
opened, and the reaction mixture was diluted with 10 mL of
THF. The resultant polymer was precipitated and washed
with methanol. To eliminate the unreacted monomers com-
pletely, the purification was repeated, until no peak was
observed at the elution time of monomer in gel permeation
chromatography (GPC) measurement.
2,5-Bis[(4-tetradecyloxyphenyl)oxycarbonyl]styrene
Instruments and Measurements
(M-OC14
)
Elemental analysis was performed on an Elementar Vario EL
instrument. All NMR measurements were performed on a
Bruker ARX400 MHz spectrometer using CDCl3 as solvent,
tetramethylsilane as the internal standard at room
temperature.
1H NMR (CDCl3): d ¼ 0.91–1.81 (m, 54H, alkoxy H), 3.96–
4.00 (t, 4H, AOCH2A), 5.48–5.88 (2d, 2H, ¼¼CH2), 6.95–7.17
(m, 8H, Ar-H), 7.53–7.60 (q, 1H, ACH¼¼), 8.19–8.45 (m, 3H,
Ar-H). 13C NMR (CDCl3): d ¼ 14.11 (ACH3), 22.70 (ACH2A),
26.07 (ACH2A), 29.31–29.68 (ACH2A), 31.95 (ACH2A),
68.53 (AOCH2A), 115.23 (¼¼CH2), 118.36–122.26 (aromatic
C ortho to CAO), 128.64 (aromatic C), 129.05 (aromatic C),
130.92 (aromatic C), 132.06 (aromatic C), 133.24 (aromatic
C), 134.72 (AHC¼¼CH2), 140.50 (aromatic CAHC¼¼CH2),
144.01–144.11 (aromatic CAO), 157.16 (aromatic CAO),
164.62–165.41 (C¼¼O). (C50H72O6) (768.53): Calcd. C 78.08,
H 9.44; Found C 77.83, H 9.62.
The apparent number-average molecular weight (Mn) and
polydispersity index (PDI ¼ Mw/Mn) were measured on a
GPC (WATERS 1515) instrument with a set of HT3, HT4, and
HT5. The l-styragel columns used THF as an eluent and the
flow rate was 1.0 mL/min at 38 ꢀC. All the GPC data were
calibrated with polystyrene standards.
TGA was performed on a TA-SDT 2960 instrument at a heat-
ꢀ
ing rate of 20 C/min in nitrogen atmosphere.
2,5-Bis[(4-hexadecyloxyphenyl)oxycarbonyl]styrene
(M-OC16
)
DSC traces of all the polymers were obtained using a TA-Q10
DSC instrument. The temperature and heat flow were cali-
brated using standard materials (indium and zinc) at a cooling
and heating rates of 10 ꢀC/min. Samples with a typical mass of
about 5 mg were encapsulated in sealed aluminum pans.
1H NMR (CDCl3): d ¼ 0.87–1.84 (m, 62H, alkoxy H), 3.97–
4.00 (t, 4H, AOCH2A), 5.48–5.87 (2d, 2H, ¼¼CH2), 6.95–7.17
(m, 8H, Ar-H), 7.53–7.60 (q, 1H, ACH¼¼), 8.19–8.45 (m, 3H,
Ar-H). 13C NMR (CDCl3): d ¼ 14.11 (ACH3), 22.70 (ACH2A),
26.07 (ACH2A), 29.30–29.71 (ACH2A), 31.95 (ACH2A),
68.74 (AOCH2A), 115.23 (¼¼CH2), 118.36–122.26 (aromatic
C ortho to CAO), 128.65 (aromatic C), 129.05 (aromatic C),
130.92 (aromatic C), 132.06 (aromatic C),133.24 (aromatic
C), 134.73 (AHC¼¼CH2), 140.50 (aromatic CAHC¼¼CH2),
144.01–144.11 (aromatic CAO), 157.16 (aromatic CAO),
164.62–165.42 (CAO). (C54H80O6) (824.60): Calcd. C 78.60,
H 9.77; Found C 78.31, H 10.19.
LC texture of the polymers was examined under polarized
optical microscopy (POM) (Leica DM-LM-P) equipped with a
Mettler-Toledo hot stage (FP82HT).
One-dimensional (1D) wide-angle X-ray diffraction (WAXD)
experiments were performed on a Philips X’Pert Pro diffrac-
tometer with a 3-kW ceramic tube as the X-ray source (Cu
Ka) and an X’celerator detector. The sample stage was set
horizontally. The reflection peak positions were calibrated
with silicon powder (2h > 15ꢀ) and silver behenate (2h <
10ꢀ). A temperature control unit (Paar Physica TCU 100) in
conjunction with the diffractometer was used to study the
structure evolutions as a function of temperature. To keep
the equilibrium of systems, we kept a testing temperature
for 40 min when heating or cooling the samples.
2,5-Bis[(4-octadecanalkoxylphenyl)oxycarbonyl]styrene
(M-OC18
)
1H NMR (CDCl3): d ¼ 0.86–1.86 (m, 70H, alkoxy H), 3.97–
4.00 (t, 4H, AOCH2A), 5.48–5.87 (2d, 2H, ¼¼CH2), 6.95–7.17
(m, 8H, Ar-H), 7.53–7.60 (q, 1H, ACH¼¼), 8.18–8.44 (m, 3H,
Ar-H). 13C NMR (CDCl3): d ¼ 14.11 (ACH3), 22.70 (ACH2A),
26.08 (ACH2A), 29.31–29.712 (ACH2A), 31.95 (ACH2A),
68.75 (AOCH2A), 115.23 (¼¼CH2), 118.36–122.26 (aromatic
C ortho to CAO), 128.65 (aromatic C), 129.05 (aromatic C),
130.92 (aromatic C), 132.07 (aromatic C), 133.25 (aromatic
C), 134.73 (AHC¼¼CH2), 140.51 (aromatic CAHC¼¼CH2),
144.02–144.12 (aromatic CAO), 157.17 (aromatic CAO),
164.62–165.42 (C¼¼O). (C58H88O6) (880.66): Calcd. C 79.04,
H 10.06; Found C 79.08, H 10.55.
A Rheometrics ARES rheometer (TA ARES rheometer) was
applied to measure the viscoelastic properties of the samples
based on oscillatory shear. The experimental temperature was
controlled using forced N2 gas convection. Isochronal temper-
ature scans were performed using a 20-mm diameter paral-
lel-plate geometry with a frequency of 10 rad/s and a small
strain amplitude. Isothermal frequency sweeps were made
using the same setup at frequencies from 0.1 to 100 rad/s.
All the polymers were synthesized by conventional solution
radical polymerization. For example, 0.656 g (1 mmol) of M-
OC10, 100 lL of chlorobenzene solution of 0.01 mmol AIBN,
2 mL of chlorobenzene, and a magnetic stir bar were added
into a polymerization tube, the tube was purged with nitro-
gen and subjected to four freeze–thaw cycles to remove any
dissolved oxygen and sealed off under vacuum. Polymeriza-
RESULTS AND DISCUSSION
Synthesis and Characterization of Monomers and
Polymers
All the monomers could be easily polymerized via free radi-
cal polymerization method. Herein, we use P-OC10 as an
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M-OCm (m ¼ 1, 2, 4, 6, 8, 10, 12, 14, 16, and 18) were meas-
ured to be 111.ꢀ1, 121.5, 107.7, 66.0, 79.2, 79.4, 87.5, 94.9,
95.5, and 100.3 C, respectively.
Figure 2 shows the first cooling and the second heating DSC
curves of P-OCm (m ¼ 1, 2, 4, 6, 8, 10, 12, 14, 16, and 18) at
a rate of 10 ꢀC/min under nitrogen atmosphere after elimi-
nating the thermal history. On the basis of transitional phe-
nomenon of the polymers, the curves in Figure 2(a) can be
divided into three types. The first one, there is no transition
peak except a glass transition (Tg) to be observed in the
curves in the P-OCm (m ¼ 1 and 2). The second one, all the
samples exhibit a single transition peak in high temperature
region above Tg when m becomes 4, 6, or 8. These two types
are well agreement with the previous papers.60,62 However,
the last one, the samples with more longer length of alkoxy
tails (m ¼ 10, 12, 14, 16, and 18) exhibit more special tran-
sitional peaks, that is, besides the transitional peak in high
temperature, there are two transition peaks that appeared in
the lower temperature region and in slightly above the cor-
responding Tg. These two transition peaks correspond to the
crystallization of the long alkoxy tails and to the interconver-
sion between isotropic phase and new LC phase, respectively.
The corresponding transitions were observed during the sec-
ond heating process [see Fig. 2(b)], indicating the reversible
transitions for the polymers.
From the experiment results, we can observe that the alkoxy
tails of the P-OCm can be crystallized when the number of
carbon atoms in the side groups exceeds 10. Melting points
FIGURE 1 1H NMR spectrums of the monomer M-OC10 (a) and
TABLE 1 Molecular Characteristics of Polymers
the polymer P-OC10 (b) in CDCl3.
b
Sample Mn(ꢂ10ꢁ5 a
)
PDIa Tg (ꢀC) Td(N2)(ꢀC)c T1(ꢀC)d T2(ꢀC)e
example to elucidate the process. Figure 1(a,b) give the 1H
NMR spectra (CDClꢁ3 d) of the monomer M-OC10 and the
polymer P-OC10, respectively. The M-OC10 showed the char-
acteristic resonances of the vinyl group at 5.48–5.87 and
7.53–7.60 ppm. After polymerization, these signals disap-
peared completely. The chemical shifts of P-OC10 were quite
broad and consistent with the expected polymer structure.
To remove the influence of Mn on the LC behaviors, we syn-
thesized a series of polymers with the similar Mn. All the
polymers showed good thermal stability, that is, the temper-
atures at 5% weight loss of the samples under nitrogen
P-OC1 3.13
P-OC2 2.03
P-OC4 2.31
P-OC6 2.28
P-OC8 3.10
P-OC10 2.12
P-OC12 2.50
P-OC14 2.80
P-OC16 2.03
P-OC18 3.92
P-OC18f 2.22
a
2.21
1.78
1.98
2.05
2.31
2.22
1.96
2.12
2.21
1.88
2.21
120
112
102
100
105
109
108
104
101
92
396
398
406
391
418
416
418
424
409
408
409
–
–
–
–
–
169
203
200
252
253
247
258
218
242
–
–
130
130
131
123
113
118
ꢀ
ꢀ
were about 400 C measured by TGA at a rate of 10 C/min.
The molecular characterizations of the polymers are sum-
marized Table 1.
91
Relative Mn and PDI were measured by GPC using PS standards.
b
The glass transition temperatures were measured by DSC at a heating
rate of 10 ꢀC/min under nitrogen atmosphere during the second heating
process.
Phase Transitions and Phase Structures of the Polymers
The phase behaviors of the monomers M-OCm were exam-
ined by the conventional analyses including DSC and POM.
All the monomers can form LC phase during the heating and
cooling processes, but the monomers would suffer thermal
polymerization, so the mesophase was not stable. Therefore,
we mainly discuss the phase behaviors of polymers herein-
after. DSC experiments revealed that the melting point of
c
The temperatures at 5% weight loss of the samples under nitrogen
[Td(N2)] were measured by TGA heating experiments at a rate of 10 ꢀC/
min.
d
The transition temperature from first liquid crystalline phase to iso-
tropic phase measured by DSC at a heating rate of 10 ꢀC/min under
nitrogen atmosphere during the second heating process.
e
The transition temperature from isotropic phase to the second liquid
crystalline phase measured by DSC at a heating rate of 10 ꢀC/min under
nitrogen atmosphere during the second heating process.
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FIGURE 2 DSC curves of P-OCm during the first cooling scan (a) and the second heating scan (b) at a rate of 10 ꢀC/min under
nitrogen atmosphere.
of the alkoxy tails of P-OCm increase with the increasing
length of alkyl, moreover, a comparison of the melting points
(Tm) of the alkoxy tails with those of n-alkanes [see Fig. 3(a)]
shows that the Tm of alkoxy tails are significantly higher than
the melting points of the same length n-alkanes. As well
known to all, the side chains of the comb-like polymers have
been found to be crystallized in hexagonal form, such as
poly(n-alkyl itaconate)s, N-alkylated poly(p-benzamide),64 pol-
y(octadecyl acrylate),65 and so on. So, we thought the long
alkoxy tails of the P-OCm can also be crystallized in hexagonal
form. After melting of the paraffin crystallites, the alkyl side
chains were from ordered packing to disordered packing.
FIGURE 3 (a) The influence of alkyl length on the melting points of the alkoxy tails of P-OCm and n-alkanes (CmH2mþ2) and (b) the
influence of alkyl length on the glass transition temperature (Tg) of P-OCm.
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FIGURE 4 Representative POM images of the texture of the P-OC1 maintained at 200 ꢀC (a), representative POM images of the tex-
ture of P-OC4 maintained at 130 ꢀC (b), 169 ꢀC (c) and representative POM images of the texture of the P-OC10 maintained at 127
ꢀC (d), 145 ꢀC (e), 260 ꢀC (f) (Magnification: ꢂ200).
However, the relationship between alkyl length and Tg of the
P-OCm is confusing. Usually, the Tg decreases gradually with
an increasing length of the alkyl tails as a result of decreased
packing density imposed by the enhanced internal plasticiza-
tion, however, it can be seen from Figure 3(b) that the
changes of Tg is not regular. First, the Tg of P-OCm (m ¼ 1, 2,
4, and 6) were decreased with the increase in the length of
alkyl; then were increased with the increase in m for P-OCm
(m ¼ 6, 8, and 10) and were decreased with further increase
in alkyl length for P-OCm (m ¼ 10, 12, 14, 16, and 18) in
the end. So, the changes of Tg is very interesting, we would
further prove the results through the solid 13C NMR.66
three types. First, for the samples P-OCm (m ¼ 1 and 2)
exhibited the stable texture. The liquid crystalline birefrin-
ꢀ
gence of P-OC1 unchanged even when heated to 300 C [Fig.
4(a)]. While cooled to room temperature from 300 ꢀC, the
birefringence of the sample remained unchanged. Second, for
the samples P-OCm (m¼4, 6, and 8) showed the isotropic
phase and LC phase. Figure 4(b,c) shows the observation for
sample P-OC4. On heating, the sample becomes soft above
the Tg but there is no birefringence [Fig. 4(b)]. The LC tex-
ture is formed only when the temperature reaches about
169 ꢀC [Fig. 4(c)] at which a corresponding endotherm
appears in DSC curve. On heating further, no visible change
of birefringence is ꢀobserved before decomposition (onset
temperature > 300 C). On the subsequent cooling, the origi-
nal scene is reestablished; that is, the birefringence
POM experiments further confirm the DSC results. From the
POM experiences, it also clears that they can be divided into
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FIGURE 5 One-dimensional WAXD patterns of the P-OC10 during the first heating process of the as-cast film [(a) in the low 2h
and (b) in the high 2h].
disappears. As for these, two kinds of the samples with short
alkoxy tails are well consistent with the previous works.60,62
Third, for the samples P-OCm (m ¼ 10, 12, 14, 16, and 18)
very diffuse and weak scattering halo, indicating a transition
from ordered state to isotropic state. However, with further
increase in the temperature, the diffraction peak gradually
developed again, indicating the formation of another ordered
structure. In Figure 5(b), the amorphous halo in the higher
2h range presented similar transitions as the diffraction at
low 2h range, that is, first, the halo gradually become
broader, then gradually become narrower and the center of
halo is found to shift to lower 2h range with the increasing
temperature. The similar features were observed during the
cooling process and the experiment results omitted here.
This special phenomenon agrees well with the results of
POM and DSC. It is noted that the value of the diffraction
peaks at lower temperature and higher temperature was dif-
ferent, that is, the diffraction peaks remained about 2.81ꢀ (d
¼ 3.14 nm) with lightly thermal expansion at lower temper-
ature and the diffraction peaks appeared about 3.47ꢀ (d ¼
2.55 nm) at higher temperature, indicating two different LC
phases. At lower temperature, the ordered structure is mysti-
cal to us at this moment, however, at higher temperature,
the polymer form the typical column phase similar to the
other MJLCPs. Unfortunately, because of the temperature
control unit (Paar Physica TCU 100) in conjunction with the
diffractometer only cool to room temperature, herein, we
cannot observe the crystallization for the sample.
presented
a thermodynamically stable isotropic phase
between two LC phases at lower and higher temperature.
Herein, we use the P-OC10 as an example to describe the
textures of the third type of polymers observed by POM [see
Fig. 4(d–f)]. The phase behaviors and transition tempera-
tures from the POM results are perfectly consistent with the
DSC experiments. It is worth noticing that the two liquid
crystalline phases of P-OC10 that came up at lower and
higher temperature were different and the textures devel-
oped more perfectly at higher temperature than the lower
temperature. Moreover, as for the P-OC18, we can observe
the crystal of long alkoxy tail (see Supporting Information
Fig. S1).
To elucidate the phase structures and transitions clearly,
WAXD experiments at different temperatures were per-
formed to identify the corresponding structural evolutions.
In these experiments, about 30 mg of polymer were cast
from THF solution. The results of 1D WAXD data for P-OCm
(m ¼ 1, 2, 4, 6, and 8) are the same as the previous
papers,62 herein, we would not discuss the experiment
results again. Figure 5(a,b) illustrates the 1D WAXD patterns
of the as-cast sample (P-OC10) obtained during the first
heating process. As shown in Figure 5(a), below 135 ꢀC the
diffraction intensity at low 2h value of 2.81ꢀ (d ¼ 3.14 nm)
gradually increased with the increase in the temperature;
between 135 ꢀC and 190 ꢀC, the diffraction peak became a
With the increase in length of alkoxy flexible terminals, the
crystallization phenomenon of longer alkoxy tails could be
detected by the WAXD, so WAXD experiments for P-OCm
(m ¼ 16 and 18) provided the different information about
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FIGURE 6 One-dimensional WAXD patterns of the P-OC18 obtained during the first heating process of the as-cast film [(a) in the
low 2h and (b) in the high 2h].
the changes of structural relative to the P-OC10. Herein, we
use the P-OC18 as an example [see Fig. 6(a,b)] to elucidate
the structural evolution. At lower temperature range, the
WAXD patterns only rendered several peaks [as shown in
Fig. 6(b)] in the high 2h region of 10–35ꢀ, indicating the typ-
ical crystal formed by the long alkoxy tails.64,67,68 In the low
2h region of 3–6ꢀ [as shown in Fig. 6(a)], three obvious dif-
fraction peaks were observed. The ratio of scattering vectors
of the first diffraction peaks and the second was 1:2, indicat-
ing a lamellar structure which was similar to the structure
formed by hairy-rod.52,69 When the sample was heated to 70
ꢀC, several peaks in Figure 6(b) gradually disappeared and
the residual diffraction halo gradually shift to lower 2h
region, but the ratio of the scattering in Figure 6(a) was still
1:2, implying that the crystal between the lamellar structure
gradually melted, but the lamellar structure still remained
and the scale of d-spacing gradually increased. We can calcu-
late the value of (100) diffraction is 3.95 nm, which is
almost equal to the theoretic value 3.94 nm. As for the theo-
retic value, it is calculated by two points. One is that the
length of rigid core is about 1.92 nm. Conversely, with
increasing per two methylene, the length of side chain will
increase 0.25 nm when the alkane takes trans conformation.
According to the scale of d-spacing and the theoretic value,
the polymer basically formed one-layer structure. Further
increasing the temperature, only one diffraction halo left and
became a very diffuse and weak scattering halo in Figure
6(a,b), indicating a transition from LC state to isotropic state.
However, a new diffraction around 3.03ꢀ (d ¼ 2.92 nm) pre-
sented when the temperature was heated to 210 ꢀC, illumi-
nating a column liquid crystalline phase was formed. The
similar phenomenon was observed on the cooling process.
Based on the WAXD data, we can calculate the value of
(100) diffraction for samples P-OC10, P-OC12, P-OC14, and
P-OC16 are 3.14, 3.23, 3.43, and 3.67 nm, which are almost
equal to the theoretic value 2.94, 3.19, 3.44, and 3.69 nm.
Therefore, we may speculate the mystical phase should be
smectic A phase. So, four phases for the samples P-OCm (m
¼ 10, 12, 14, 16, and 18) subject to crystal, smectic A phase,
isotropic phase and column phase.
To further confirm the phase transition and information
about the nature of various phases, rheological measure-
ments were performed. Because the sample was not enough,
we renewably synthesized the sample P-OC18f (molecular
characteristics of P-OC18f can be seen in Table 1). The mo-
lecular weight of the polymer affected the transition temper-
atures which has been researched by our group, but did not
influence the change of the phase dynamics from the phase
to the other phase. Thereafter, the sample P-OC18f was
loaded into the instrument at 160 ꢀC and cooled to 90 ꢀC.
Temperature dependences of the shear storage modulus G0,
loss modulus G00 and complex viscosity Eta* were recorded
ꢀ
at a rate of 10 C/min in Figure 7(b). The transition temper-
atures evaluated by DSC at a rate of 10 ꢀC/min in Figure
7(a). It can be seen that two peaks shape of modulus and
complex viscosity are seen around T1 and T2, which agrees
well with the transition temperature of this polymer on DSC
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due to the low viscosity of isotropic phase, it also excludes
any known optically isotropic cubic phases which are highly
viscous. More ordered smectic phases are known to show
higher viscosity values than an isotropic phase.70,71 So, the
transition shape is a evidence of a smectic to isotropic phase
transition. Second, the modulus and viscosity slightly
increase between T1 and T2. Third, a dramatic increase in
the modulus and viscosity is observed around T2, indicating
isotropic phase to the other phase, followed by a slight
increase at higher temperature. In addition, the storage mod-
ulus is significantly greater than the loss modulus when the
temperature beyond T2.
The properties of the low temperature LC phase, isotropic
phase, and the high temperature LC phase are characterized
by the frequency dependence of G0 and G00 as well as the
Eta* which are shown in Figure 8. The frequency sweeps
(0.1–100 rad/s) for various states of the samples were per-
formed at 95, 105, 115, 170, and 260 ꢀC. It can be seen
from Figure 8(b–d), there are difference to G0, G00, and Eta*
versus frequency when the polymers enter the different
phase state. The data recorded at 105 ꢀC show the linear
viscoelastic response, highly shear thinning behavior for the
complex viscosity, with the storage modulus greater than the
loss modulus. The change of the viscoelastic properties of
the polymer is similar from 95 to 115 ꢀC, which are shown
FIGURE 7 Heat flow at a heating rate 10 ꢀC/min in DSC (a) and
temperature dependence of G0, G00, and Eta* for during heating
from 90 ꢀC to 290 ꢀC in rheology (b). Conditions: 1% strain, 10
rad/s frequency and 10 ꢀC/min heating rate.
scan. First, before the temperature T1, G0, G00, and Eta*
slightly decrease with the increase in temperature. Around
T1, a dramatic decrease in the modulus and viscosity is
observed. It clearly indicates a phase transition; moreover,
FIGURE 8 Dependence of the shear moduli on frequency; the temperature (ꢀC) is marked on each frequency sweep (a). Frequency
dependencies of the storage modulus, loss modulus, and complex viscosity at 105 (b), 170 (c), and 260 ꢀC (d).
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cules,73,74 oligomers,75 and poly(r-benzyl L-glutamate)76
which exhibits a re-entrant isotropic phase in certain mixed
solvents such as dichloroacetic acid þ dichloroethane, it is
difficult to find the re-entrant isotropic phase in the thermo-
tropic LC polymer system. Therefore, it may be useful to
understand the re-entrant phase in the thermotropic LC
polymer system through studying the phase behavior of the
MJLCPs.
CONCLUSIONS
We have successfully synthesized a series of vinyl monomers
M-OCm (m ¼ 1, 2, 4, 6, 8, 10, 12, 14, 16, and 18) containing
different length of alkoxy terminal chains. On using conven-
tional radical polymerization, we further obtained the corre-
sponding MJLCPs. The chemical structures of the monomers
and polymers were confirmed by various characterization
techniques. The phase structures and transitions of these
polymers were investigated by the combined technology of
DSC, POM, WAXD, and ARES rheometer. The experimental
results reveal that MJLCPs can form a re-entrant isotropic
phase which is separating smectic phase and columnar
phases in one polymer because of the microphase separation
and the driving force of the entropy. In low temperature,
with increasing length of alkoxy tails on the mesogen, the
flexible terminal substituents are increasingly immiscible
with the rigid core. Thus polymers form the smectic phase
based on self-assembly layers. In high temperature, due to
the driving force of the entropy, hairy-rod polymers form the
columnar phase in which alkoxy tails and rigid core become
a unit. Rheological measurement shows that the MJLCPs
have the lower dynamic moduli and complex viscosity in re-
entrant isotropic phase. As for the conformational transition
of the alkyl side chains, relationship between the alkyl side
chains and the main chain, and the phase behaviors and
structures of this type of MJLCPs with different density and
position of the long alkoxy tails attached to their side-chain
rigid-rods, detailed study will be done in our further
research.
FIGURE 9 Schematic drawing of the thermotropic phase
behavior of the polymers.
in Figure 8(a), which is very similar to those usually
observed for flexible homopolymers in smectic layer state.72
ꢀ
The data recorded at 170 C indicate the terminal regime at
low frequency which is observed in the isotropic state for
ꢀ
flexible polymers. The data recorded at 260 C show that the
storage modulus and the loss modulus are essentially inde-
pendent of frequency, with the storage modulus greater than
the loss modulus. As for this, it may be reasonable that the
single molecule acts as a rigid unit form columnar nematic
phase.60
Phase Structure Evolution: Influence of Difference of
Alkyl-Tail Lengths
Based on DSC, POM, XRD, and ARES rheometer experiments
results, P-OCm (m ¼ 1, 2, 4, 6, 8, 10, 12, 14, 16, and 18)
exhibits three types of different behaviors with the increas-
ing length of alkoxy flexible terminals, as shown in Figure 9.
The first type is that polymers show the transitional MJLCP
phase behavior when polymers have the short alkoxy termi-
nals containing one and two carbon atoms, which is attrib-
uted to a cooperative assembly of the backbone and its later-
ally attached mesogenic groups. The second type is that
polymers present a re-entrant isotropic phase and a LC
phase when the length of alkoxy tails become four to eight
carbon atoms. The LC phase of these polymers is formed
through a global change of the whole molecule from the
coiled to extended chain conformation accompanied by an
increase of the entropy which originates from the free mobi-
lities of bulky side chains in LC phase. The last type is that
the P-OCm (m ¼ 10, 12, 14, 16, and 18) exhibits a re-entrant
isotropic phase and two different kinds of LC. As for the
appearance and disappearance of this re-entrant isotropic
phase, it is considered to be the competing between the
microphase separation and the driving force of the entropy
over a specific temperature range. Although the reentrant
isotropic has been already reported in large disk-like mole-
ACKNOWLEDGMENT
This research was financially supported by the Innovation Plat-
form Open Foundation of University of Hunan Province
(10k067), the National Nature Science Foundation of China
(51073131, 21174117, 21104062). The authors thank Chen
Er-Qiang of Peking University for his help with WAXD experi-
ments measurements.
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