Influences of hydrogen bonding and peripheral chain length on mesophase structures of mesogen-jacketed liquid crystalline polymers with amide side-chain linkages

Article abstract of DOI:10.1021/ma102444t

A series of mesogen-jacketed liquid crystalline polymers, poly{2,5-bis[(4-alkoxyphenyl)aminocarbonyl]styrene} (P-Cm, where m is the number of carbon atoms in the alkoxy groups, and m = 1, 4, 8, 12), with an amide core and flexible tails of varying lengths on the two ends in the side chain were designed and successfully synthesized via conventional radical polymerization. The mesophase structures of these polymers were dependent on the number of carbon atoms in the peripheral chains. Wide-angle X-ray diffraction and polarized light microscopy results revealed that the polymers with m ≥ 4 could form smectic A phases, while a columnar nematic phase could be formed for the polymer with methoxy end groups (P-C1). The hydrogen bonding among the side-chain amide groups might play an important role in forming and stabilizing these liquid crystalline phases, which was suggested by the results from variable-temperature FTIR and 2D IR analyses.

Full text of DOI:10.1021/ma102444t

ARTICLE
pubs.acs.org/Macromolecules
Influences of Hydrogen Bonding and Peripheral Chain Length
on Mesophase Structures of Mesogen-Jacketed Liquid Crystalline
Polymers with Amide Side-Chain Linkages
Yan-Hua Cheng,†,‡ Wen-Ping Chen,‡ Zhihao Shen,*,† Xing-He Fan,*,† Mei-Fang Zhu,*,‡ and Qi-Feng Zhou†
†Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
‡State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, China
ABSTRACT: A series of mesogen-jacketed liquid crystalline polymers, poly{2,5-bis-
[(4-alkoxyphenyl)aminocarbonyl]styrene} (P-Cm, where m is the number of carbon
atoms in the alkoxy groups, and m = 1, 4, 8, 12), with an amide core and flexible tails of
varying lengths on the two ends in the side chain were designed and successfully
synthesized via conventional radical polymerization. The mesophase structures of these
polymers were dependent on the number of carbon atoms in the peripheral chains.
Wide-angle X-ray diffraction and polarized light microscopy results revealed that the
polymers with m g 4 could form smectic A phases, while a columnar nematic phase
could be formed for the polymer with methoxy end groups (P-C1). The hydrogen
bonding among the side-chain amide groups might play an important role in forming
and stabilizing these liquid crystalline phases, which was suggested by the results from
variable-temperature FTIR and 2D IR analyses.
’ INTRODUCTION
To design LC materials with high complexity, versatility, and
functionality, supramolecular chemistry is a viable and facile
technique in building up complex systems based on noncovalent
bonds, such as hydrogen bonding, ionic interaction, π-π interac-
tion, coordination interaction, van der Waals force, and so on.12-14
The characteristics and advantages of such LC materials are that
they can change their structures by the dynamic and reversible
noncovalent interactions when responding to the different en-
vironment. Among these interactions, hydrogen bonding has been
extensively explored to tailor self-assembled nanostructures for
the applications in the fields of biology and materials science.15-18
Introducing amide groups, which can form hydrogen bonding,
into MJLCPs might bring about a new way to control phase
structures. Previously, we have synthesized 2-vinyl-1,4-phenyle-
nediamine-based poly-2,5-bis(4-alkoxybenzamido)styrene,
which exhibited a lyotropic nematic phase when methoxy groups
were appended to the two ends of the mesogens.19 However,
because of the time-consuming procedure in synthesizing the
polymers, their properties have not been fully investigated.
Herein, we report a novel series of MJLCPs, which could be
synthesized more conveniently, with amide linkages based on
2-vinylterephthalic acid.20 Hydrogen bonding could be formed
among the side-chain amide groups. The length of the peripheral
alkoxy tails was varied in these poly{2,5-bis[(4-alkoxyphenyl)-
aminocarbonyl]styrene} (P-Cm, where m is the number of carbon
Liquid crystalline polymers (LCPs) represent fascinating materials
of soft matter, combining anisotropic properties on molecular
and supramolecular levels and good mechanical properties on the
whole. Such liquid crystalline (LC) ordering depends on external
stimuli and is of great importance for numerous applications not
only in materials science but also in biosystems.1 Side-chain LC
polymers (SCLCPs), depending on the position of the mesogens
linking to the backbone, either longitudinally or laterally attached
into the polymeric system, are called end-on or side-on
SCLCPs.2 In order to achieve liquid crystallinity, flexible spacers
are usually introduced in SCLCPs to decouple the dynamics of
the main chain and the mesogenic side groups.3 In contrast to the
conventional route to design SCLCPs, Zhou et al.4 reported a
special type of side-on SCLCPs called mesogen-jacketed liquid
crystalline polymers (MJLCPs), which had a short spacer or with
a single covalent bond connecting the mesogen to the polymer
backbone. In our previous work, we have systematically investi-
gated the syntheses and LC properties of MJLCPs,5 revealing
that the MJLCP chain acts like a supramolecular cylinder because
of the so-called “jacketing” effect (the main chain takes an
extended chain conformation due to the steric effect of the
densely distributed bulky side groups), and many MJLCPs form
columnar phases.5-8 Recently, smectic phases have also been
reported when the length or the rigidity of the mesogen was
increased9,10 or when semifluorinated tails were introduced into
the mesogens.11 The previous researches on MJLCP systems
mostly focused on structures based on 2-vinylterephthalic acid
with ester linkages.
Received: October 27, 2010
Revised:
January 20, 2011
Published: February 14, 2011
r
2011 American Chemical Society
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Scheme 1. Synthetic Route of Monomers and Polymers
atoms in the alkoxy groups, and m = 1, 4, 8, 12) polymers. We
examined the liquid crystallinity of these polymers using various
techniques, such as X-ray diffraction and optical microscopy, and
found that the LC phase structures were affected by both
hydrogen bonding and the peripheral chain length.
Synthesis of Monomers. The synthetic route of the monomers,
2,5-bis[(4-alkoxyphenyl)aminocarbonyl]styrene (M-Cm, where m is the
number of carbon atoms in the alkoxy groups, and m = 1, 4, 8, 12), is
shown in Scheme 1. The experimental details of monomer synthesis and
characterization are described below using M-C12 as an example.
Synthesis of N-(4-Dodecyloxyphenyl)acetoamide, 4d. A mixture of
4-acetoamidophenol (1.81 g, 12.0 mmol), 1-bromododecane (3.74 g,
15.0 mmol), and potassium carbonate (4.98 g, 36.0 mmol) in acetone
(50 mL) was heated under reflux for 24 h. The excessive potassium
carbonate was filtered off and washed with acetone for several times.
After evaporation of the solvent, the residue was recrystallized from
’ EXPERIMENTAL SECTION
Materials. The compound vinylterephthalic acid was synthesized
using the method previously reported.20 2,20-Azobis(isobutyronitrile)
(AIBN, Beijing Chemical Co., 95%) was purified by recrystallization from
ethanol before use. N,N0-Dimethylformamide (DMF, Beijing Chemical
Co., A.R.) used as the polymerization solvent was distilled over CaH2
prior to use. Tetrahydrofuran (THF) and triethylamine (Et3N) were
refluxed over CaH2 under argon and distilled out before use. Dichlor-
omethane (Beijing Chemical Co., A.R.) was dried over anhydrous
magnesium sulfate. All other reagents and solvents were used as received
from commercial sources.
1
ethanol to give 4d as a white crystal. H NMR (300 MHz, CDCl3, δ,
ppm): 0.88 (t, 3H, -CH3), 1.10-1.82 (m, 20H, -CH2-), 2.15 (s, 3H,
CO-CH3), 3.92 (t, 2H, -OCHH2-), 6,84 (d, 2H, Ar-H), 7.13 (br s,
NH), 7.36 (d, 2H, Ar-H). Anal. Calcd for C20H33NO2: C, 75.19; H,
10.41; N, 4.38. Found: C, 75.29; H, 10.32; N, 4.35.
Synthesis of 4-Dodecyloxyphenylamine, 5d. An ethanol (35 mL)
solution of 4d (2.59 g, 8.12 mmol) and a dilute HCl aqueous solution
were stirred at 75 °C. After 2 days, the solvent was evaporated in vacuo
to obtain a white crystal, 4-dodecyloxyphenylamide hydrochloride. 1H
NMR (300 MHz, CDCl3, δ, ppm,): 0.88 (t, 3H, -CH3), 1.17-1.85 (m,
20H, -CH2-), 3.93 (t, 2H, -OCHH2-), 6.90 (d, 2H, Ar-H), 7.44 (d,
Measurements. 1H NMR spectra, mass spectra (MS), gel permea-
tion chromatographic (GPC) measurements, thermogravimetric analyses
(TGA), differential scanning calorimetry (DSC), polarized light micro-
scopy (PLM), and one-dimensional(1D) andtwo-dimensional (2D) wide-
angle X-ray diffraction (WAXD) experiments were carried out according
to the procedures described previously.21
Hydrogen bonding was studied using variable-temperature FTIR.
The spectra were recorded on a Nicolet 8700 FTIR spectrometer with a
resolution of 4 cm-1 using a variable-temperature cell, and 32 scans were
2H, Ar-H), 10.38 (br s, NH2 HCl). Anal. Calcd for C18H32-
3
NOCl: C, 68.87; H, 10.28; N, 4.46. Found: C, 68.90; H, 10.16; N,
4.48. Then an aqueous solution of NaHCO3 (saturated, 50 mL) was
added to 4-dodecyloxyphenylamide hydrochloride until the pH g 7.
The mixture was extracted with dichloromethane, and the extract was
dried with anhydrous Na2SO4 and concentrated to give 5d. After the
purity of 5d was tested by thin layer chromatography and 1H NMR, it
was directly used in the next step.
coadded for each spectrum in the wavenumber range 400-4000 cm-1
A sample of ∼1.5 mg was sandwiched between two KBr windows.
.
Furthermore, 2D IR correlation analysis, which is a spectral analysis
method originally proposed by Noda,22 was carried out to probe the
mechanism of the (LC) phase development. FTIR spectra collected in
the temperature range 80-280 °C with a 10 °C interval were utilized to
perform the 2D IR correlation analysis with the Omnic 7.2 SpectraCorr
software.
Synthesis of 2,5-Bis[(4-dodecyloxyphenyl)aminocarbonyl]styrene,
6d. Vinylterephthalic acid (0.75 g, 3.90 mmol), 10 mL of oxalyl
chloride, and a few drops of DMF were dissolved in dichloromethane
(25 mL) in a 50 mL round-bottom flask. The mixture was stirred for 5 h.
After evaporation of the solvent under reduced pressure, the residue was
dissolved in anhydrous THF. The resultant light yellow solution of
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Table 1. Molecular Weights, Polydispersity Indexes, and
Thermal Properties of P-Cm
sample
Mn (ꢀ104 g/mol)a
PDIa
Td (°C)b
Tg (°C)c
P-C1
P-C4
P-C8
P-C12
3.90
3.14
20.6
6.90
2.07
2.18
1.02
1.57
384
390
383
413
245
222
206
199
a The apparent number-average molecular weight (Mn) and polydisper-
sity index (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 °C/min. c The glass transition
temperatures were measured by DSC at a heating rate of 40 °C/min
under a nitrogen atmosphere during the second heating process.
Figure 1. 1H NMR spectrum of M-C12 in CDCl3.
vinylterephthal chloride was slowly dropped into an intensely stirred
solution of 50 mL of THF and 10 mL of triethylamine containing 5d for
1 h in an ice/water bath. The solution was further stirred for 24 h at ambient
temperature. After evaporation of the solvent, the residue was washed
with water and recrystallized from THF/petroleum ether (1/1, v/v) with
active carbon as the decolorant to obtain a white solid. Figure 1 shows
the 1HNMRof6d (M-C12) in deuterated chloroform. 1H NMR (400 MHz,
CDCl3, δ, ppm,): 0.88 (t, 6H, -CH3), 1.28-1.79 (m, 40H, -CH2-),
3.95 (t, 4H, -OCHH2-), 5.38 (d, 1H, =CH2), 5.75 (d, 1H, =CH2),
6.87 (d, 4H, Ar-H), 7.00 (q, 1H, -CH=), 7.45-7.60 (m, 5H, Ar-H),
7.61 (d, 1H, Ar-H), 7.79 (s, 1H, -NH-), 7.92 (s, 1H, -NH-), 8.13
(s, 1H, Ar-H). MS (EI): m/z 710. Anal. Calcd for C46H66N2O4: C,
77.70; H, 9.36; N, 3.94. Found: C, 76.96; H, 9.36; N, 3.86.
Figure 2. DSC thermograms of P-Cm’s at a heating rate of 40 °C/min
under a nitrogen atmosphere following cooling at 5 °C/min.
Compounds 6b and 6c were similarly prepared as mentioned for 6d,
while 6a was prepared from commercially available 5a.
2,5-Bis[(4-methyloxyphenyl)aminocarbonyl]styrene, 6a. 1H NMR
(400 MHz, DMSO-d6, δ, ppm,): 3.75 (d, 6H, -OCH3), 5.43 (d, 1H,
=CH2), 5.97 (d, 1H, =CH2), 6.92 (d, 4H, Ar-H), 7.01 (q, 1H, -CH=),
7.59-7.70 (m, 5H, Ar-H), 7.94 (d, 1H, Ar-H), 8.25 (s, 1H, Ar-H),
10.15 (s, 1H, -NH-), 10.21 (s, 1H, -NH-). MS (EI): m/z 402. Anal.
Calcd for C24H22N2O4: C, 71.63; H, 5.51; N, 6.96. Found: C, 71.08; H,
5.45; N, 6.75.
monomer, the precipitate was redissolved in THF and then reprecipi-
tated in methanol for three times. Finally, the polymer was dried to a
constant weight.
’ RESULTS AND DISCUSSION
1
2,5-Bis[(4-butoxyphenyl)aminocarbonyl]styrene, 6b. H NMR (400 MHz,
Synthesis and Characterization of Monomers and Poly-
mers. As shown in Scheme 1, the monomers were synthesized
in four steps. The structures of the monomers were confirmed by
DMSO-d6,δ, ppm,): 0.94 (t, 6H, -CH3), 1.41-1.47 (m, 4H, -CH2-),1.68-
1.72 (m, 4H, -CH2-), 3.95 (t, 4H, -OCHH2-), 5.44 (d, 1H, =CH2), 6.01
(d, 1H, =CH2), 6.91-6.96 (m, 5H, Ar-H, -CH=), 7.61-7.70 (m, 5H, Ar-
H), 7.94 (d, 1H, Ar-H), 8.28 (s, 1H, Ar-H), 10.27 (s, 1H, -NH-), 10.36(s,
1H, -NH-). MS (EI): m/z 486. Anal. Calcd for C30H34N2O4: C, 74.05; H,
7.04; N, 5.76. Found: C, 73.45; H, 6.97; N, 5.69.
1
conventional analyses, including H NMR, elemental analysis,
and mass spectrometry.
All the monomers were easily polymerized via the conven-
tional radical polymerization method. The molecular character-
istics of the polymers are summarized in Table 1. The apparent
number-average molecular weights (Mn’s) of the polymers
determined by GPC were higher than 3 ꢀ 104 g/mol, demon-
strating good polymerizability of the monomers. TGA measure-
ments showed that all polymers were quite stable under a
nitrogen atmosphere. The temperatures of 5% weight loss
(Td’s) were all higher than 380 °C as shown in Table 1. The
excellent thermal stability was probably due to the hydrogen
bonds among the amide moieties.
Phase Transitions and Phase Structures. All polymers were
amorphous when obtained from solution radical polymerization.
The DSC experiments of P-Cm’s were performed under a
nitrogen atmosphere at a heating rate of 40 °C/min following
a cooling process at a rate of 5 °C/min. Figure 2 shows DSC
thermograms of P-Cm’s. Only glass transitions could be ob-
served, and the glass transition temperature (Tg) was dependent
2,5-Bis[(4-octyloxyphenyl)aminocarbonyl]styrene, 6c. 1H NMR
(400 MHz, DMSO-d6, δ, ppm,): 0.87 (t, 6H, -CH3), 1.28-1.72 (m,
24H, -CH2-), 3.95 (t, 4H, -OCH2-), 5.43 (d, 1H, =CH2), 5.97 (d,
1H, =CH2), 6.89-6.93 (d, 4H, Ar-H), 7.01 (q, 1H, -CH=), 7.58-
7.68 (m, 5H, Ar-H), 7.94 (d, 1H, Ar-H), 8.25 (s, 1H, Ar-H). MS
(EI): m/z 598. Anal. Calcd for C38H50N2O4: C, 76.22; H, 8.42; N, 4.68.
Found: C, 76.29; H, 8.37; N, 4.62.
Polymerization. As shown in Scheme 1, all polymers were obtained
by conventional radical polymerization in solution. A typical polymer-
ization procedure was carried out as the following, again, using P-C12 as
an example. About 0.10 g (0.14 mmol) of M-C12, 10 μL of DMF solution
of 0.08 M AIBN, and 1.9 mL of DMF/THF (1/1, v/v) were transferred
into a polymerization tube. After three freeze-pump-thaw cycles, the
tube was sealed off under vacuum. Polymerization was carried out at
60 °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 completely eliminate the unreacted
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Figure 4. PLM micrograph of P-C1 at ambient temperature. The bulk
sample was cooled from 280 °C.
Therefore, we assigned the phase structure of P-C1 to columnar
nematic phase (ΦN), which was a formed by the supramolecular
rods of P-C1, although there might be other possibilities. The
d-spacing of the low-angle halo was 1.68 nm at 35 °C during
cooling. Thus, the columnar diameter was 1.94 nm, which was
close to the simulated length (2.00 nm) of the side group.
When the number of methylene groups in the end chains was
increased to 4, the segregation of the amide cores and the lipid
end chains took place. This was reflected in the WAXD patterns
of P-C4 as shown in Figure 5, which were different from those of
P-C1. During first cooling from the high temperature of 270 to
250 °C, a sharp peak appeared at 2θ = 3.24°, indicating the
existence of an ordered structure. However, the intensity of the
low-angle diffraction did not increase with the further decrease in
temperature. With annealing and heating back to 270 °C, the
intensity increased significantly, as shown in Figure 5a. When the
temperature was decreased to 250 °C again, two high-order
diffraction peaks were observed (Figure 5b). The three diffrac-
tion peaks had 2θ values of 3.24°, 6.50°, and 9.76°, and the ratio
of the scattering vectors of these three peaks was approximately
1:2:3, indicating a smectic structure of the sample P-C4, and the
d-spacing of the low-angle diffraction peak represented d100 of
the smectic phase. In addition, all these diffraction peaks almost
retained their intensity during the cooling process similar to the
cases of some other MJLCPs reported previously.9,10 No notice-
able change could be observed during the second heating and
cooling processes in 1D WAXD experiments, implying that there
was no isotropization for P-C4 in the temperature region we
studied.
When m was further increased to 8 and 12, the polymers also
formed smectic phases upon cooling from high temperatures,
similar to P-C4. As mentioned before, the longer alkyl chains
decreased the glass transition of the polymer due to the enhanced
internal plasticization. The softening of the backbone could also
be confirmed by the reduction of temperature in isotropization
from the 1D WAXD experiments (Figure 6). One difference
between P-C12 and P-C4 was that P-C12 went into the isotropic
liquid at high temperatures, as reflected by the second heating
and cooling processes in the 1D WAXD study in Figure 6a,
although the transition could not be detected in DSC. After the
glass transition, the low-angle scattering halo appeared, accom-
panied by the decrease in intensity of the sharp first-order diffrac-
tion peak, implying that the smectic phase started to disorder. At
280 °C, the sharp first-order diffraction peak completely dis-
appeared, with only a scattering halo remaining, indicating that
Figure 3. Sets of 1D WAXD patterns of P-C1 obtained during the first
heating (a) and the first cooling (b) processes.
on the number of methylene groups, m, in the flexible alkyl tails
for the P-Cm series. Tg decreased with increasing alkyl chain
length due to the increased plasticization by the alkyl end chains,
as expected.
To study the phase transitions and the phase structures of
P-Cm’s at different temperatures, 1D WAXD experiments were
performed. About 30 mg of the polymer was dissolved in acetone
(for P-C1) or THF (for P-C4, P-C8, and P-C12) and cast onto a
copper substrate, and then the solvent was allowed to evaporate
at ambient temperature. The temperature was not higher than
280 °C during the experiments for all samples to avoid thermal
degradation. Parts a and b of Figure 3 show two sets of 1D
WAXD patterns of P-C1 during the first heating and the
subsequent cooling processes, respectively. In Figure 3a, there
were only two scattering halos in the low-angle and high-angle
regions at 35 °C, indicating that P-C1 was in the amorphous
state. The intensity of the low-angle halo increased upon the first
heating to 280 °C and decreased upon the subsequent cooling
(Figure 3b). It was difficult to judge whether there were phase
transitions during heating and cooling from 1D WAXD experi-
ments. On the other hand, schlieren texture started to develop at
278 °C (Figure 4a) when P-C1 was heated, and the texture
remained during further cooling, indicating possible existence of
an LC phase. Schlieren texture is frequently found in nematic and
smectic C (SC) phases, but not in smectic A (SA) phases. In our
pervious study, MJLCPs having smectic phases exhibit SC phases
only when relatively long alkyl chains are appended to the ends of
the mesogens, while those with short alkyl ends exhibit SA phases.23
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Figure 5. 1D WAXD patterns of P-C4 during the formation of the smectic
phase (a), with the arrow pointing out the direction of temperature change,
and the patterns recorded during the last cooling process (b).
Figure 6. 1D WAXD patterns of P-C12 obtained during the second
heating and subsequently cooling processes (a), with the arrow pointing
out the direction of the temperature change, and the patterns for P-C4,
PC-8, and P-C12 at 180 °C during cooling (b), with the inset showing
the (200) and (300) diffractions.
the polymer entered into the isotropic state. The smectic LC
phase formed again upon cooling. The phase behavior of P-C8
was similar to that of P-C12, although only partial disordering of
the smectic phase of P-C8 could be observed at 280 °C, which
was the highest temperature in our experiments in order to avoid
thermal decomposition. We reasoned that the isotropization
temperatures were higher than 280 °C for P-C8 and P-C4. The
diffraction patterns of the smectic phases at 180 °C during the
cooling process for all the samples are shown in Figure 6b. The
layer spacing values of the polymers are listed in Table 2, which
also increased with increasing length of the side chain, similar to
the cases of other MJLCPs forming smectic phases.9,10
To identify the smectic structures of the polymers, 2D WAXD
experiments were carried out. Taking P-C12 as an example, the
oriented sample for the experiments was prepared by mechani-
cally shearing the solution-cast film at 250 °C. The sample was
put on the sample stage, and the point-focused X-ray beam was
aligned perpendicular to the shear direction. Figure 7 shows the
pattern of the sheared P-C12. A pair of strong low-angle
diffraction arcs appeared on the equator, which was perpendi-
cular to the shear direction, indicating that the normal of the
smectic layers was perpendicular to the shear direction. Mean-
while, the high-angle scattering halo was more or less concen-
trated on the meridian with rather broad azimuthal distributions.
The pattern in Figure 7 proved that the smectic phase of P-C12
was SA instead of SC. Similar patterns were obtained from
samples of P-C4 and P-C8, suggesting that these two polymers
also formed SA phases.
As in other MJLCPs, the microphase separation between the
flexible alkyl ends and the rigid core in the side chains might play
an important role in the formation of LC phases. On the other hand,
P-C4 formed an SA phase, different from the hexatic columnar
nematic phase of poly{2,5-bis[(4-butoxyphenyl)oxycarbonyl]-
styrene}24 which had the similar chemical structure to that of
P-C4, but with ester linkages instead of amide linkages in the side
groups. Therefore, the amide linkage could have an important
effect in forming the smectic phase. Hydrogen bonding might
exist among the amide linkages, resulting in more rigid side
chains and more sheetlike conformation for the polymers, both
of which favored the formation of smectic phases. It should be
noted that all three samples (P-C4, P-C8, and P-C12) formed
their SA phases at an almost identical temperature, irrelevant to
their alkyl end chain lengths. This phenomenon could be related
to the effect of hydrogen bonding, which will be discussed in the
following section. Contrary to increasing the rigidity of the side
chain by increasing the length of the rigid core in the mesogen to
form smectic phases, in the P-Cm polymers, the side-chain
hydrogen bonding was an important factor. However, for P-C1
with methoxy groups on two ends, a columnar nematic phase was
formed. Therefore, both the hydrogen bonding and the terminal
alkyl chains with enough lengths might be responsible for the
formation of the observed SA phases.
Molecular Origin of the Phase Transitions. To understand
the role of the side-chain hydrogen bonding in phase transitions,
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Table 2. Summary of X-ray Diffraction Results of P-Cm’s
sample
molecular length of side chain (nm)a
layer spacing (nm)b
phasec
TLC (°C)
P-C1
P-C4
P-C8
P-C12
2.00
2.65
3.51
4.37
ΦN
SA
278d
250e
250e
265-250e
2.72
3.23
3.65
SA
SA
a Molecular length calculated with the assumption of all-trans chain conformation. b Values of d100. c ΦN: columnar nematic phase; SA: smectic A phase.
d The temperature at which schlieren texture appeared under PLM during the heating process. e The temperature at which the sharp diffraction peak
appeared in 1D WAXD experiments during the cooling process.
Figure 7. 2D WAXD pattern of P-C12 with the X-ray beam perpendi-
Figure 8. FTIR spectra of P-C12: N-H stretching region 3500-3090
cm-1 in the temperature range 35-280 °C during heating (a) and
cooling (b).
cular to the shear direction (a) and the shearing geometry (b).
we carried out variable-temperature FTIR experiments. The
information about the hydrogen-bonding state was obtained
from the N-H stretching (νN-H) vibration and amide I bands.
Taking P-C12 as an example, it exhibited two infrared bands at
3426 cm-1 (weak shoulder) and 3275 cm-1 (very broad) of νN-H
representative FTIR spectra (“free” and hydrogen-bonded N-H
stretching vibrations) for P-C12 at selected temperatures. The
increased frequency of the N-H stretching band could be
attributed to the decrease in the average strength of the hydro-
gen-bonded N-H groups. The intensity and frequency of the
N-H stretching (νN-H) vibration absorption were recovered
when the temperature was decreased (Figure 8b).
A similar phenomenon was also observed for other P-Cm
polymers. This temperature-dependent behavior could be ratio-
nalized by the following. Before the polymers were heated, there
was relatively strong hydrogen bonding in these samples, which
stabilized the nascent amorphous structures from the preparation
process. However, the side-chain hydrogen bonding became
weaker upon heating, which was necessary for the mesogenic
molecules to obtain mobility in the amorphous state. Upon
cooling, the hydrogen bonding was recovered, which enhanced
(Figure 8a), which were assigned to “free” (higher than 3400 cm-1
)
and hydrogen-bonded N-H stretching modes, respectively. The
peak positions of the absorption bands of νN-H and amide I
(1652 cm-1) clearly indicated that at ambient temperature most
N-H groups were associated with the CdO groups, reflecting
the presence of relatively strong hydrogen bonding.25,26 The N-
H stretching vibration was dependent on the temperature. The
νN-H band became weaker and shifted toward higher frequen-
cies upon heating to 280 °C, showing a very broad absorption
band with a peak at 3292 cm-1, which indicated that the
hydrogen bonding became very weak in the isotropic state at
280 °C.27 This is demonstrated in Figure 8a, which show
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Figure 9. Schematic drawing of the LC phase structures of the poly-
mers. P-C1 formed a columnar nematic phase, while P-C4, P-C8, and
P-C12 formed SA phases.
the attractive interaction between the neighboring mesogens to
form an ordered phase. This dynamic and reversible nature of the
hydrogen bonding was essential for the formation and stabiliza-
tion of the LC phases.28
We also examined the band centered around 1600 cm-1
representing the C-C stretching mode of the benzene ring in
P-C12, which slightly shifted toward lower frequencies from
1603 to 1596 cm-1 upon heating. The shifts might be related to
the strengthening of the intermolecular π-π interaction be-
tween the amide cores.28,29 On the contrary, the band had a
reversible change and recovered the absorption upon cooling.
Figure 9 depicts a schematic drawing of the molecular packing
of P-C1 in the columnar nematic phase and P-Cm (m = 4, 8, 12)
in the smectic A phases at ambient temperature. For P-C1 with
the columnar nematic LC order, a relatively strong hydrogen
bonding existed among the rigid amide cores within the rodlike
polymer chain. As described by the mesogen-jacketed model, the
whole polymer chain acted as a supramolecular rod to form
the nematic phase. Meanwhile, for P-C4, P-C8, and P-C12
with the smectic LC ordering, hydrogen bonding also formed
among the amide cores, and yet the polymer chain took a more
sheetlike conformation to pack into the smectic structures.
To investigate the mechanism of the development of the
liquid crystalline phases at molecular levels, 2D IR was per-
formed to probe the sequential order of changes of chemical
groups in the backbone and side chain with respect to tem-
perature change. We chose P-C1 as an example because the
vibrational bands of the alkyl end chains for other P-Cm
polymers were overlapped with those of the backbone, while
the -OCHH3 stretching modes of P-C1 were independent
from the alkyl C-H stretching modes of the backbone. On the
other hand, although the liquid crystalline phase of P-C1 was
different from those of the others, the interactions between the
chemical groups were similar. Figure 10 presents two sets of
FTIR spectra of P-C1 at different temperatures between 80 and
280 °C for heating and cooling processes. For the sake of clarity,
spectra with a 20 °C interval are shown. According to Noda's
rule,22,30,31 the 2D IR correlation spectroscopy provides 2D
synchronous and 2D asynchronous spectra. Ψ(ν1/ν2) is de-
fined as the cross-peak in an asynchronous spectrum, and
Φ(ν1/ν2) is the cross-peak in a synchronous spectrum. When
Φ(ν1/ν2) > 0, if Ψ(ν1/ν2) is positive, ν1 varies prior to ν2; if
Ψ(ν1/ν2) is negative, ν1 varies after ν2. And the sequence is
reversed when Φ(ν1/ν2) < 0. We concentrated on the region
3450-2811 cm-1 which presented the C-H vibration of P-C1.
It should be noted that all the absorbance bands decreased
Figure 10. FTIR spectra of P-C1 in the 3500-2700 cm-1 region
between 80 and 280 °C during heating (a) and cooling (b).
Table 3. Assignments of Infrared Frequencies for P-C1 at
Ambient Temperature25,26,28,32
peak position (cm-1
)
assignment
3281
ν(N-H) in amide bonds
νar(CH) in the aromatic ring
νas(CH3) of -OCHH3
3126, 3043
2999
2933
νas(CH2) of -CH2- in backbone
νs(CH3) of -OCHH3
2835
gradually with increasing temperature, and a reverse change was
observed during the subsequent cooling process. According to
the 1D WAXD results and the analysis mentioned before, the
spectral data sets during heating and cooling were used sepa-
rately for the 2D IR correlation analysis in the temperature
range 80-280 °C.
The bands located at 3281, 3126, 3043, 2933, and 2835 cm-1
represented the characteristic vibrations of chemical groups in
P-C1. The band assignments are listed in Table 3. The cross-
peaks that appeared in synchronous and asynchronous maps
were calculated by 2D IR, and the results are summarized in
Tables 4 and 5. Signs of each cross-peak in the synchronous and
asynchronous maps are shown in the top left corner of the tables,
with the signs in the asynchronous map in parentheses. The
reversed signs of their products are shown in the lower right
corner.
According to the calculated 2D IR results in Table 4, during
the heating process, the sequential order of spectral intensity
changes was ν(CH3) of -OCHH3 f ν(CH) in the aromatic
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Table 4. Change Sequence of Chemical Groups in ν(C-H) Region (3500-2700 cm-1) during Heatinga
a Calculated from 2D maps within the temperature range 80-280 °C during heating.
Table 5. Change Sequence of Chemical Groups in ν(C-H) Region (3500-2700 cm-1) during Coolinga
a Calculated from 2D maps within the temperature range 80-280 °C during cooling.
ring f ν(N-H) in amide bonds f ν(C-H) of polymer back-
bone. In other words, in this temperature range 80-280 °C, the
response of the chemical groups began from the end groups in
the mesogenic units, followed by the movement of the benzene
rings of the mesogenic cores. Because the end groups were
individually connected to the mesogenic units at the peripheral
position, they had larger freedom than the rigid amide cores and
the backbones, and the side-chain hydrogen bonding gradually
decreased in strength or even dissociated upon heating. The
backbone had a delayed response compared with the side chains
attaching to it because of the restriction by side-chain hydrogen
bonding among the rigid amide cores in the side chains.
On the basis of the results listed in Table 5, during the cooling
process, the motional order of the chemical groups was discerned
as ν(N-H) in amide bonds f ν(CH) in the aromatic ring f
ν(CH3) of -OCHH3 f ν(C-H) of polymer backbone.
The ν(N-H) in amide bonds responded first, which drove
the neighboring mesogens to interact with one another. The
strengthening of the hydrogen bonding further stabilized the LC
phase. The motions of the connected benzene rings and end
groups followed the movement of the amide bonds. The back-
bone was still the last to respond to the temperature change.
The sequences obtained from the 2D IR analysis suggested the
molecular mechanism in forming the LC phases. During the
heating and cooling processes, the polymer backbone of P-C1
responded later than the chemical groups in the side chains
because the backbone could not change its conformation easily
owing to the attractive interactions among the side chains. This
was different from the case of poly{2,5-bis[(4-methoxyphenyl)-
oxycarbonyl]styrene} (PMPCS) which had the similar chemical
structure but with the ester linkages in the side chains. For
PMPCS, the backbone changed its conformation more easily to
adopt the helical conformation after the phase transition to form
the columnar phase because of the independent side chains.32
To confirm the role of hydrogen bonding in the formation
and stabilization of the mesophases of P-Cm’s (m g 4), we also
analyzed the spectra data set of P-C12 in the region 3450-3000
cm-1 during the cooling process. The sequential order was also
discerned as ν(N-H) in amide bonds f ν(CH) in the
aromatic ring, which was the same as that in P-C1 without
considering the vibration of CH2 because of the overlapped
absorptions of the alkyl tails and backbone. The earlier response
of the amide bonds enabled the recovery of hydrogen bonding
among the side chains, resulting in the enhanced rigidity of the
side chains. On the other hand, the increased content of alkyl
tails for these P-Cm (m g 4) polymers promoted the micro-
phase separation between the alkyl tails and the rigid cores of
the side chains, consequently stabilizing the smectic phases. In
addition, the role of hydrogen bonding in the formation and
stabilization of LC phases was also the reason for the similar
transition temperatures of P-C4, P-C8, and P-C12 from vari-
able-temperature WAXD experiments.
’ CONCLUSIONS
In summary, we found that hydrogen bonding could be
applied to construct new MJLCPs and control their phase
structures. MJLCPs with amide linkages in the side groups were
synthesized via conventional radical polymerization. The poly-
mer with methoxy end groups, P-C1, formed a columnar nematic
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phase, while P-C4, P-C8, and P-C12 with longer alkyl tails
formed smectic A phases. In addition to being affected by the
length of the alkoxy peripheral chains, the phase structures of the
polymers were also influenced by the side-chain hydrogen
bonding. The new MJLCPs with side-chain hydrogen bonding
may become potential functional materials as liquid crystalline
elastomers and engineering plastics owing to their stable meso-
phase structures.
(21) Xu, Y.; Yang, Q.; Shen, Z.; Chen, X.; Fan, X.; Zhou, Q.
Macromolecules 2009, 42, 2542–2550.
(22) Noda, I. J. Am. Chem. Soc. 1989, 111, 8116–8118.
(23) Chen, S.; Zhang, L.-Y.; Gao, L.-C.; Chen, X.-F.; Fan, X.-H.;
Shen, Z.; Zhou, Q.-F. J. Polym. Sci., Part A: Polym. Chem. 2009, 47,
505–514.
(24) Zhao, Y.-F.; Fan, X.-H.; Wan, X.-H.; Chen, X.-F.; Yi; Wang,
L.-S.; Dong, X.; Zhou, Q.-F. Macromolecules 2006, 39, 948–956.
(25) Skrovanek, D. J.; Howe, S. E.; Painter, P. C.; Coleman, M. M.
Macromolecules 1985, 18, 1676–1683.
’ AUTHOR INFORMATION
(26) Xue, C.; Jin, S.; Weng, X.; Ge, J. J.; Shen, Z.; Shen, H.; Graham,
M. J.; Jeong, K.-U.; Huang, H.; Zhang, D.; Guo, M.; Harris, F. W.;
Cheng, S. Z. D.; Li, C. Y.; Zhu, L. Chem. Mater. 2004, 16, 1014–1025.
(27) Pang, D.; Wang, H.; Li, M. Tetrahedron 2005, 61, 6108–6114.
(28) Kutsumizu, S.; Mori, H.; Fukatami, M.; Naito, S.; Sakajiri, K.;
Saito, K. Chem. Mater. 2008, 20, 3675–3687.
Corresponding Author
*E-mail: fanxh@pku.edu.cn (X.-H.F.); zshen@pku.edu.cn (Z.S.);
zhumf@dhu.edu.cn (M.-F.Z.).
(29) Shi, J.; Wu, P.; Yan, F. Langmuir 2010, 26, 11427–11434.
(30) Noda, I. Appl. Spectrosc. 1993, 47, 1329–1336.
(31) Noda, I.; Dowrey, A. E.; Marcoli, C.; Story, G. M.; Ozaki, Y.
Appl. Spectrosc. 2000, 54, 236–248.
(32) Shen, Y.; Chen, E.; Ye, C.; Zhang, H.; Wu, P.; Noda, I.; Zhou, Q.
J. Phys. Chem. B 2005, 109, 6089–6095.
’ ACKNOWLEDGMENT
Financial support from the National Natural Science Founda-
tion of China (Grants 50925312, 50973016, and 20990232), the
Programme of Introducing Talents of Discipline to University
(No. 111-2-04), and National “973” Project (2011CB606004) is
gratefully acknowledged.
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