Ordered nanostructures at two different length scales mediated by temperature: A triphenylene-containing mesogen-jacketed liquid crystalline polymer with a long spacer

Article abstract of DOI:10.1002/pola.27003

A mesogen-jacketed liquid crystalline polymer (MJLCP) containing triphenylene (Tp) moieties in the side chains with 12 methylene units as spacers (denoted as PP12V) was synthesized. Its liquid crystalline (LC) phase behavior was studied with a combination

Full text of DOI:10.1002/pola.27003

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Ordered Nanostructures at Two Different Length Scales Mediated by
Temperature: A Triphenylene-Containing Mesogen-Jacketed Liquid
Crystalline Polymer with a Long Spacer
1
1
1
1
1
1
Yu-Feng Zhu, Hai-Jian Tian, Hong-Wei Wu, De-Zhao Hao, Yu Zhou, Zhihao Shen,
1
2
1
1
De-Chun Zou, Ping-Chuan Sun, Xing-He Fan, Qi-Feng Zhou
1
Beijing National Laboratory for Molecular Sciences, Department of Polymer Science and Engineering, and Key Laboratory of
Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University,
Beijing 100871, China
2
Key Laboratory of Functional Polymer Materials for Adsorption and Separation, Polymer Chemistry Institute, Nankai University,
Tianjin 300071, China
Correspondence to: X.-H. Fan (E-mail: fanxh@pku.edu.cn) or Z. Shen (E-mail: zshen@pku.edu.cn)
Received 30 June 2013; accepted 25 October 2013; published online 21 November 2013
DOI: 10.1002/pola.27003
ABSTRACT: A mesogen-jacketed liquid crystalline polymer
MJLCP) containing triphenylene (Tp) moieties in the side
temperature range. Partially homeotropic alignment of the
polymer can be achieved when treated with an electric field,
(
chains with 12 methylene units as spacers (denoted as PP12V)
was synthesized. Its liquid crystalline (LC) phase behavior was
H
with the polymer in the U phase developed by the Tp moi-
eties. The incorporation of Tp moieties through relatively long
spacers (12 methylene units) disrupts the ordered packing of
the MJLCP at low temperatures, which is the first case for
main-chain/side-chain combined LC polymers with MJLCPs as
the main-chain LC building block to the best of our knowledge.
The relationship of the molecular structure and the novel
phase behavior of PP12V has implications in the design of LC
polymers containing nanobuilding blocks toward constructing
1
studied with a combination of solution H NMR, solid-state
NMR, gel permeation chromatography, thermogravimetric
analysis, polarized light microscopy, differential scanning calo-
rimetry, and one- and two-dimensional wide-angle X-ray dif-
fraction. By simply varying the temperature, two ordered
nanostructures at sub-10-nm length scales originating from
two LC building blocks were obtained in one polymer. The
low-temperature phase of the polymer is a hexagonal colum-
ordered nanostructures at different length scales. V
2013 Wiley
C
nar phase (U
H
, a 5 2.06 nm) self-organized by Tp discotic meso-
Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52,
gens. The high-temperature phase is
a
nematic columnar
295–304
0
phase with a larger dimension (a 5 4.07 nm) developed by the
rod-like supramolecular mesogen—the MJLCP chain as
a
KEYWORDS: liquid-crystalline polymer (LCP); nanocomposite;
whole. A re-entrant isotropic phase is found in the medium
structure–property relation; WAXD
INTRODUCTION Frustules with ordered structures across
length scales are of utmost importance in the life process of
diatoms for the mechanical protection, molecular and colloi-
approach is undoubtedly time-consuming, high capital and
operational cost, and not suitable for large-area patterning.
In addition, the construction of such nuanced materials at a
sub-10-nm length scale, which is vital for miniaturizing elec-
tronic and magnetic devices, is still challenging by the “top-
1
,2
dal sorting, and light harvesting. In addition to the signifi-
cance in nature, materials with ordered structures across
length scales play important roles in applications of catalysis,
waveguides, selective separations, sensor arrays, photonic
crystals, and nanotechnology. Several strategies have been
utilized to construct ordered structures across length scales
both in laboratory and for industrial applications. The
straightforward “top-down” approach is step-by-step photoli-
thography or scanning beam lithography, with which a layer-
4
down” approach. An alternative “bottom-up” approach has
emerged and offers opportunities to circumvent the above
5
problems. Self-assembly of small building blocks, such as
block copolymers, vesicles, micelles, and particles, allows for
6
fast prototyping of nanostructures. A delicate molecular
design on the basis of nanobuilding blocks can be expected
to construct ordered nanostructures at sub-10-nm length
scales.
3
by-layer coating and exposure process is required. This
Additional Supporting Information may be found in the online version of this article.
2013 Wiley Periodicals, Inc.
V
C
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Mesogen-jacketed liquid crystalline polymer (MJLCP)—a
supramolecular cylinder or tablet with excellent dimensional
tunability—is a superb nanobuilding block for self-assembly.
Taking a representative MJLCP with nonmesogenic side
chains as an example, poly[di(alkyl)vinylterephthalate]
(
PDAVT) with alkyl groups ranging from propyl/isopropyl to
hexyl forms highly ordered two-dimensional (2D) hexagonal
7
columnar (U ) phases at high temperatures. The diameter
H
of the overall molecular cylinder is 1–2 nm determined by
the length of alkyl tails. Symmetric incorporation of meso-
gens at the ends of the side chains of PDAVT does not influ-
ence the tendency of MJLCPs self-assembling into
supramolecular cylinders. For example, a combined main-
chain/side-chain liquid crystalline (LC) polymer with PDAVT
as the main chain and biphenyl meosgens in the side chains
exhibits hierarchically ordered structures at ambient temper-
ature. The biphenyl groups in the side chains pack into an
SmE phase with the ordering on subnanometer length scale,
and the MJLCP main-chain scaffold has a 2D rectangular
CHART 1 Chemical structure of PP12V.
target polymer displays ordered nanostructures with lengths
of 4.07 and 2.06 nm mediated by temperature. To the best
of our knowledge, this is the first LC polymer displaying two
LC phases with different ordered dimensions originating
from tailor-made LC building blocks.
8
positional order at a nanometer length scale. Similar hier-
archically ordered nanostructures are obtained by altering
9
the calamitic mesogen from biphenyl to azobenzene. A simi-
lar molecular design incorporates triphenylene (Tp), a model
discotic mesogen, into the side chains of PDAVT in our previ-
1
0
ous study. Tp moieties and the terephthalate core are
linked via 3 or 6 methylene units as flexible spacers (poly-
mers denoted as PPnV, n 5 3 or 6). At high temperatures
when Tp discotic liquid crystals (DLCs) are melting, the poly-
mers behave like typical MJLCPs, and 2D long-range-ordered
rectangular columnar (U ) or U phases are obtained. When
Materials and Measurements
Indium tin oxide (ITO) substrate was sequentially cleaned by
detergents, deionized water, acetone, and isopropanol, and it
was then dried in nitrogen at ambient temperature. All other
1
0
materials were pretreated as previously reported.
R
H
Tp DLCs enter the LC phase at low temperatures, a discotic
Solution NMR experiments, mass spectroscopy, elemental
analysis, gel permeation chromatographic (GPC) measure-
ments, thermogravimetric analysis (TGA), differential scan-
ning calorimetry (DSC), polarized light microscopy (PLM),
small-angle X-ray scattering (SAXS), and one-dimensional
nematic (N ) phase coexists in conjunction with the UR
D
phase, owing to the self-assembly of the Tp DLCs and the
rod-like MJLCP chains, respectively. Individual ordered struc-
tures developed by the two LC building blocks are not only
competitive but also promotive to each other in these two
PPnV samples. Flexible spacers play an important role in
tuning the phase behaviors of these polymers. With the flexi-
ble spacer length increasing, the two LC building blocks act
more independently. Therefore, the phase behavior of PPnV
may be more intriguing if the flexible spacers are sufficiently
long and the coupling between the two LC building blocks
diminishes.
(
(
1D) and two-dimensional wide-angle X-ray diffraction
WAXD) experiments were performed according to the pro-
10,11
cedures previously described.
Solid-state NMR (ssNMR) experiments were performed on a
Varian Infinityplus-400 wide-bore (89-mm) NMR spectrome-
ter operating at a proton frequency of 399.7 MHz. A conven-
tional 4 mm double-resonance HX CP/MAS probe was used,
and the sample was placed in a 4 mm zirconia PENCIL rotor
with a 52-lL sample volume. The magic angle spinning (MAS)
was automatically controlled at 5 kHz within 6 2 Hz with a
In this work, we investigated the phase behavior of an
MJLCP containing Tp moieties with a spacer of 12 methylene
units, denoted as PP12V (Chart 1). Interestingly, PP12V has
a completely different phase behavior compared with PP3V
and PP6V. With the variation of temperature, a nematic
13
MAS speed controller. C cross polarization MAS (CP/MAS)
NMR experiments with total suppression of sidebands were
ꢀ
carried out at a temperature of 26 C. Variable-temperature
columnar (U ) phase and a UH phase with different length
13
N
C direct polarization MAS (DP/MAS) NMR experiments
were carried out at a temperature of 140 C within 6 0.1 C
with a Varian Model-L950 temperature controller. The
scales were observed. On the basis of the relationship
between the molecular structure and the novel phase behav-
ior of PP12V, a molecular design rule can be derived so that
ordered nanostructures across length scales can be obtained
by incorporating nanoscaled LC building blocks into the side
chains of MJLCPs with a sufficiently long flexible spacer.
Depending on the temperature range of the LC phase of the
individual LC building block, namely MJLCP and DLC, the
ꢀ
ꢀ
13
C
chemical shifts were referenced to external HMB (hexamethyl-
benzene) and the recycle delay was set to 3 s.
PLM experiments on electrically aligned samples were per-
formed between ITO-coated glass slides spaced by ꢁ30 lm.
The cell was equipped with electrode leads and connected to
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SCHEME 1 Synthetic route of PP12V.
ꢀ
ꢀ
a direct current source. A typical heating or cooling rate of
from ambient temperature to 250 C at a rate of 20 C/min
to erase the thermal history. The DSC thermogram for the
ꢀ
1
C/min was applied.
ꢀ
second heating process with a heating rate of 40 C/min
was recorded after a slow cooling process. As shown in
1
0
RESULTS AND DISCUSSION
Figure 2, the glass transition temperature (T ) of PP12V is 6
C, which is lower than those of PP3V and PP6V owing to
g
Synthesis and Thermal Property of PP12V
ꢀ
The synthetic route for PP12V is analogous to that previ-
the weaker steric hindrance and the stronger internal plasti-
cization effect of the longer alkyl spacers, as expected. Upon
1
0
ously reported for PPnV (n 5 3 or 6) and is depicted in
Scheme 1. Details of the experimental procedures can be
found in the Supporting Information. The chemical structure
of the monomer P12V was confirmed by elemental analysis,
ꢀ
heating, an endothermic process appears at 133 C with an
enthalpic change of 2.3 J/g. According to our previous
report, this process may be related to the isotropization of
Tp DLCs. The temperature of the isotropization of Tp DLCs
1
13
high-resolution mass spectrometry, and H/ C NMR. The
molecular characterizations of PP12V are summarized in
ꢀ
10
for PP6V is 114 C,
2
which is higher than that of
1
Table 1. H NMR spectrum of PP12V (Fig. 1) indicates that
ꢀ
,3,6,7,10,11-hexahexyloxytriphenylene (ca. 100 C) and Tp-
containing poly(meth)acrylates (ca. 40280 C). The isotrop-
ization enthalpy of Tp DLCs for PP6V is 0.3 J/g. For PP12V,
the residue monomer has been separated completely from
the polymer. GPC result shows that PP12V has a number-
ꢀ
10
5
average molecular weight (M ) of 1.3 3 10 g/mol, with a
n
the higher temperature and larger enthalpic change of transi-
tion than those of PP6V indicate that more ordered packing
of Tp moieties may have formed in PP12V.
polydispersity index of 1.81.
The thermal properties of PP12V were investigated by TGA
and DSC. As shown in Table 1, the thermal stability of PP12V
is excellent with the temperature at 5% weight loss of 387
Liquid Crystalline Phase Behavior of PP12V
Textures of PP12V were investigated by PLM with a powder
sample. Typical textures at different temperatures were
ꢀ
C in nitrogen. DSC experiments were carried out to study
the phase transitions of PP12V. The sample was first heated
TABLE 1 Molecular Characterizations and Thermal Properties of PP12V
ꢀ
transition ( C)
T
5
a
a
ꢀ
b
ꢀ
c
c
Yield (%)
M
n
(3 10 g/mol)
1.30
Determined by GPC in tetrahydrofuran using polystyrene standards.
M
w
/M
n
T
d
( C)
T
g
( C)
(Corresponding Enthalpy Change, J/g)
66
1.81
387
6
133 (2.3)
a
c
ꢀ
Evaluated by DSC during the second heating cycle at a rate of 40 C/
b
5%
weight loss temperature evaluated by TGA under
atmosphere at a heating rate of 10 C/min.
a
nitrogen
min.
ꢀ
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1
3
FIGURE 1 H NMR spectra of P12V (a) and PP12V (b) in CDCl .
recorded during cooling, as shown in Figure 3. At high tem-
peratures before degradation, PP12V shows birefringence
within the whole sample area. This birefringence becomes
ꢀ
weaker when the sample is cooled down to below 200 C. At
ꢀ
1
50 C, the sample goes into the isotropic state. When the
temperature decreases further, strong birefringence emerges
again [Fig. 3(c)]. However, no characteristic texture is
observed. Upon heating, the sample shows similar textures
as those at different temperatures in the cooling process.
Notably, an isotropic state appears between two LC phases
from the PLM observation. This unusual LC-to-isotropic tran-
sition during cooling, which is well known as re-entrant iso-
tropic, was also observed for other polymers, such as
1
2,13
poly{2,5-bis[(4-butoxyphenyl)oxycarbonyl]styrene}s
poly[n-decyl-(RS)-2-methylbutylsilane]s.
and
1
4
Phase Structure Identification
Because the phase structures of PP12V cannot be directly
identified from PLM textures, variable-temperature 1D and
FIGURE 2 DSC thermogram of PP12V at a heating rate of
ꢀ
40 C/min under a nitrogen atmosphere following a cooling
ꢀ
process at 2 C/min.
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ꢀ
FIGURE 3 PLM micrographs of PP12V at (a) 250, (b) 150, and (c) 80 C during cooling.
ꢀ
2
D WAXD experiments were performed. Solution-cast sam-
further to 300 C, a small diffraction peak at a 2h value of
ꢀ
ples from tetrahydrofuran were prepared for 1D WAXD
2.17 (d 5 4.07 nm) develops in addition to the amorphous
halo at a 2h value of ꢁ5 , indicating the formation of an
ꢀ
experiments. A typical sample contains ꢁ40 mg of PP12V.
ꢀ
The samples were dried overnight at 35 C in a vacuum
ordered structure with a dimension of 4.07 nm at elevated
temperatures. As the diffraction peak in the low-angle region
is quite close to the detection limit of the 1D WAXD equip-
ment, SAXS experiments were conducted to ensure that no
other diffraction peaks exist in even lower angles. As
expected, SAXS results (Supporting Information Fig. S1) are
consistent with the 1D WAXD ones. Owing to the weak inten-
sity and the absence of higher-order diffractions, this
ordered structure can be identified as a UN phase developed
by the rod-like supramolecular mesogen—the MJLCP chain
as a whole. Herein, the molten Tp moieties are regarded as
parts of the MJLCP side chains.
oven after most of the solvent evaporated at ambient tem-
perature. To avoid thermal decomposition, the sample cham-
ber was charged with nitrogen during characterization.
WAXD profiles of PP12V during the first heating and subse-
quent cooling processes are shown in Figure 4.
As shown in Figure 4(a), a sharp diffraction peak is observed
at 2h 5 4.96 (d 5 1.78 nm) in the low-angle region of the
powder pattern from the as-cast film at ambient tempera-
ture. In the high-angle region, two amorphous halos are
ꢀ
ꢀ
ꢀ
found at 2h values of ꢁ20 (d ꢂ 0.44 nm) and ꢁ25 (d ꢂ
0
.36 nm). This diffraction pattern is highly reminiscent of
those from Tp derivatives forming a columnar phase. Gener-
ally, Tp DLCs tend to stack on one another to self-organize
into a well-ordered supramolecular column because of the
p–p stacking of the planar aromatic cores and the van der
Waals interactions of the peripheral alkyl chains. Many Tp
As shown in Figure 4(b), PP12V follows the opposite transi-
tion pathway upon cooling compared with that in the heat-
ing process, indicating the enantiotropic phase behavior of
PP12V. The only difference between the powder patterns of
PP12V in the first heating process and those in the subse-
1
5,16
ꢀ
derivatives including Tp-based molecules,
side-chain LC
quent cooling process is that the peak at a 2h value of ꢁ5
1
7–26
27–32
polymers,
and main-chain LC polymers
have been
is more intense and the amorphous halo at a 2h value of
ꢀ
reported to form 2D UH phases at ambient temperature. The
d-spacing of ꢁ0.44 and ꢁ0.36 nm for the two halos in the
high-angle region represent the characteristic dimensions of
the amorphous packing of the alkyl chains and the p–p
stacking in a discotic LC column. The d-spacing of 1.78 nm
in the low-angle region is the d100 of the hexagonal structure
formed by the discotic LC columns. Therefore, the diameter
of the column is 2.06 nm, which agrees well with the simu-
lated dimension of Tp containing six hexyl arms (ꢁ2.3 nm)
with the interdigitation of the alkyl chains. However, no
higher-order diffractions were observed even with a syn-
chrotron radiation X-ray source. Hence, we tentatively
ꢁ25 becomes narrower after thermal annealing. This result
indicates that thermal annealing can increase the degree of
order of the discotic columns, which is beneficial for elec-
3
3–36
tronic applications.
H
hypothesize that a U phase with 2D ordering is formed at
ambient temperature. The building block of the UH phase is
the supramolecular discotic column formed by the stacking
Tp moieties. The polystyrene main chains along with the
alkyl parts of the polymer fill the spaces among the columns
and stabilize the columnar phase at ambient temperature.
ꢀ
As the temperature increases to 150 C, the sharp diffraction
FIGURE 4 One-dimensional WAXD patterns of PP12V during
the first heating (a) and subsequent cooling (b). [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
ꢀ
at a 2h value of ꢁ5 evolves into an amorphous halo, indi-
cating that the aforementioned UH structure and the discotic
columns self-organized by Tp moieties melt. Upon heating
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ꢀ
FIGURE 5 Two-dimensional WAXD pattern of PP12V at 30
C
with the X-ray beam perpendicular to the fiber direction.
Macroscopically oriented samples were used for 2D WAXD
experiments. Fiber samples of PP12V were drawn with a
ꢀ
pair of tweezers at ꢁ190 C. Figure 5 shows the 2D WAXD
ꢀ
fiber pattern of PP12V at 30 C, recorded with the X-ray inci-
dent beam perpendicular to the fiber direction. 2D WAXD
experiments at higher temperatures were not conducted
because the fibrous sample began to flow at elevated tem-
peratures. As shown in Figure 5, two pairs of diffraction arcs
with d-spacing values of 1.78 and 0.36 nm are located on
the equator and the meridian, respectively. This result fur-
ther confirms the conjecture on the basis of 1D WAXD
experiments that the diffraction with the d-spacing of 1.78
nm in the low-angle region is the d100 of the UH phase and
the d-spacing of 0.36 nm represents the average distance of
FIGURE 6 Schematic drawing of the molecular packing of
PP12V at high temperatures (a) and at ambient temperature
(
b). [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
1
3
FIGURE 7 C DP/MAS (up) and CP/MAS (middle) ssNMR spectra and liquid-state NMR spectrum in CDCl
Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
3
(bottom) of PP12V.
[
300
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1
3
TABLE 2 Observed C Chemical Shifts of the Aromatic Car-
chains are in an anisotropic motion or with orientation ani-
sotropy when the polymer is in the LC state. The ring cur-
rent effect also plays an important role in line broadening of
aromatic systems because the radial component of the field
bons in Tp Moieties
Chemical Shift (ppm)
Solution (in CDCl , at
C2
C4
C5
3
8
produced by the ring current is not averaged by MAS. Sev-
3
148.5
123.2
106.6
ꢀ
eral peaks are found from the spectrum at 26 C. Because no
ambient temperature)
crystallographic splittings are observed, the lines may be
easily assigned as shown in Figure 7. When the temperature
ꢀ
Solid-state (140 C)
149.6
147.8
1.8
124.3
123.1
1.2
108.7
103.8
4.9
ꢀ
Solid-state (26 C)
ꢀ
is increased to 140 C, some peaks related to Tp moieties
a
D
ꢀ
are remaining, while Peaks 1 and 3 at 26 C are too weak to
ꢀ
a
ꢀ
be observed. The spectrum at 140 C consists of nine well-
Difference in chemical shifts between 140 and 26 C.
resolved, narrower lines, presumably because of the higher
mobility of the side chains. The chemical shifts are similar
when the polymer is in the isotropic state or in a homogene-
the p–p stacking in the discotic LC columns. In addition, the
relative positions of these two pairs of diffraction arcs prove
that the plane of Tp molecules is perpendicular to the fiber
direction. Considering that the polymer main chain is
inclined to be parallel to the fiber direction, the discotic
supramolecular columns self-organized from Tp moieties are
oriented parallel to the main chains. Namely, the Tp columns
can be oriented by orienting the polymer main chains. Thus,
orienting the polymer main chains via mechanical force
offers a facile technique among others to orient DLCs, which
3
ous solution (CDCl ), while the chemical shifts of the
resonances corresponding to the aromatic carbons of Tp
moieties in the range of 100–160 ppm (C2, C4, C5) move to
ꢀ
high field at 26 C (Table 2). Notably, this systematic change
of carbon resonances is similar to that of octa-n-undecoxyph-
3
9
thalocyanine that forms a columnar mesophase. The chemi-
cal shift differences stem from the anisotropic shielding by
intermolecular ring current effects between neighboring Tp
moieties in the columnar LC phase. The ssNMR results fur-
ther confirm the formation of the discotic supramolecular
columns self-organized from Tp moieties at ambient temper-
ature, in accordance with WAXD results.
3
7
has not been reported before to the best of our knowledge.
Combining the WAXD and PLM results, we propose that
PP12V shows a UN phase at high temperatures, and it
behaves like a regular LC polymer with Tp-pendant groups
self-organized into a 2D UH phase at low temperatures. Fig-
ure 6 depicts the molecular packing of PP12V at high tem-
peratures and at ambient temperature.
Influence of Electric Field on Alignment of PP12V
Although DLCs can be regarded as quasi-1D conductors
because of the large p-orbital overlap in the supramolecular
columns, the control of the structural order and alignment of
3
7,40
Variable-Temperature Solid-State NMR Study
columns is critical to attain high charge-carrier mobility.
To understand the phase behavior of PP12V at the molecular
level, variable-temperature ssNMR experiments were carried
out. Owing to the hardware limit of the instrument, the high-
The alignment and LC ordering can be induced by applying
an electric field owing to the permanent dipole moment of
4
1,42
the LC building blocks.
The response of PP12V to an
ꢀ
temperature DP/MAS experiment was conducted at 140 C,
applied longitudinal electric field was investigated. In a typi-
cal sample treatment, PP12V was sandwiched between two
ITO-coated glass plates separated by ꢁ30 lm. To obtain a
better effect of alignment or annealing, a typical processing
procedure was applied as follows. The cell was cooled down
1
3
with PP12V in the isotropic state. For comparison, C CP/
ꢀ
MAS spectra were also recorded at 26 C before heating. Fig-
1
3
ure 7 shows the C ssNMR spectra of PP12V at both tem-
1
3
peratures and
C solution NMR spectrum of PP12V in
ꢀ
CDCl . As is clearly shown, the lines are broadened in ssNMR
from 150 C, with the sample remaining in the isotropic
3
ꢀ
spectra due to the anisotropic bulk magnetic susceptibility
state, to ambient temperature at a cooling rate of 1 C/min
3
8
13
6
effects compared with the C solution spectrum. In addi-
tion, the other reason for the line broadening is that all the
with (or without) a longitudinal electric field of about 10
V/m. PLM was utilized to monitor the aligning process.
ꢀ
FIGURE 8 PLM micrographs of PP12V at 100 C before (a) and after (b) electric field treatment and during annealing after the elec-
tric field was removed (c).
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6
does not change when an electric field of about 10 V/m is
applied at ambient temperature for hours. This suggests that
the alignment only occurs when the temperature is much
higher than the glass transition temperature of the polymer.
Effective alignment takes place when the polymer is mobile
enough at relatively high temperatures. The remaining orien-
tation of the polymer main chains may render the partially
homeotropic alignment of the discotic columns.
Correlation Length of the Discotic Columns in PP12V
One-dimensional WAXD experiment on the electrically
aligned sample was conducted to investigate the influence of
electric field on the phase structure of PP12V. Similar 1D
WAXD profile with that from the as-cast sample was
obtained. To study the ordering of the discotic columns in
PP12V, the correlation lengths of the diffraction peak at the
d-spacing of 1.78 nm and the amorphous halo at the d-spac-
ing of ꢁ0.36 nm, which represent the apparent diameter of
the columns and the average distance between neighboring
Tp discs along the long-axis direction of the columns, respec-
FIGURE 9 One-dimensional WAXD patterns of the as-cast,
thermally annealed, and electrically aligned PP12V at ambient
temperature (a) and the patterns in the low-angle region (b).
[
Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
ꢀ
44
Figure 8 shows the PLM micrographs of PP12V at 100
C
tively, were estimated with the Scherrer equation. Assum-
before and after electric field treatment. A relatively uniform
texture [Fig. 8(a)] always remains the same after annealing
for several times before the electric field treatment. However,
ing that the diffraction peak shape obeys a Gaussian
function, we used the Warren’s correction to account for
4
5
instrument broadening. Figure 9 shows the 1D WAXD pat-
terns of the as-cast, thermally annealed, and electrically
aligned PP12V at ambient temperature. To precisely identify
the center d-spacings and the values of the full width at half-
maximum (FWHM) of the amorphous halo at the d-spacing
of ꢁ0.36 nm, the two scattering halos in the high-angle
region were deconvoluted into two components using Gaus-
sian functions (Supporting Information Fig. S2). The correla-
tion lengths and correlation numbers (apparent numbers of
the building blocks, that is, the discotic columns for the dif-
fraction peak at the d-spacing of 1.78 nm and Tp discs for
the amorphous halo at the d-spacing of ꢁ0.36 nm, along the
normal of the diffraction planes) are listed in Table 3.
6
when an electric field of about 10 V/m is applied to the
sample during the cooling process, the bright areas partially
turn dark as shown in Figure 8(b). This is likely due to a
partially homeotropic alignment of discotic columns. Notably,
a completely homeotropic alignment cannot be achieved
even when the sample was carefully annealed for several
times under the aforementioned condition. In addition, this
alignment is reversible. When the voltage across the cell is
removed, the sample slowly relaxes back to the homogene-
ous alignment during annealing, and the similar uniform tex-
ture as that observed before applying the electric field
emerges again as shown in Figure 8(c).
The disconnected arrangements of the microstructures have
been found in the electrical alignment of 2,3,6,7,10,11-hexa-
pentaloxy Tp on ITO glasses. Thus, this partial alignment
observed from PLM should be mainly due to the alignment
of the Tp moieties in side chains. Furthermore, the texture
As shown in Table 3, the comparison of the 1D WAXD results
of the three samples demonstrates that PP12V has ordered
domains of different sizes, although they all form the same
UH phase. Correlation length derived from the diffraction
with the d-spacing of 1.78 nm represents the apparent
4
3
TABLE 3 One-Dimensional WAXD Results of PP12V Obtained from Different Conditions
a
ꢀ
a
b
c
Sample
As-cast
d-Spacing (nm)
FWHM ( )
Correlation Length (nm)
Correlation Number
1.78
0.250
38.1
–
18
–
d
d
ꢁ
0.36
–
Annealed
1.78
0.210
1.74
48.8
4.89
78.3
13.4
24
14
38
38
0.354
Electrically aligned
1.78
0.143
1.58
0.357
a
c
Deconvolution with two Gaussian functions was utilized to precisely
identify the center d-spacings and FWHMs of the two scattering halos
in the high-angle region.
Determined by the ratio of the correlation length to the intercolumn
distance or the disc-to-disc distance.
d
2h value of ꢁ25ꢀ is too broad to be
The scattering halo at
deconvoluted.
a
b
Estimated from the FWHM of the X-ray diffraction using Scherrer
equation.
302
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ordered dimension along the (100) direction of the UH
phase, namely the direction perpendicular to the discotic col-
umns. The correlation length gradually increases in the order
of the as-cast, thermally annealed, and electrically aligned
samples, corresponding to 18, 24, and 38 columns in the
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(
100) direction, respectively. The correlation length obtained
5
B. D. Gates, Q. Xu, M. Stewart, D. Ryan, C. G. Willson, G. M.
from the scattering halo with the d-spacing of ꢁ0.36 nm,
which represents the apparent length of the discotic column,
shows that 14 and 38 Tp moieties stack together in the col-
umns of the thermally annealed and electrically aligned sam-
ples, respectively. The correlation length estimation of the
three samples shows that thermal annealing increases the
ordered domain size by a third, while electrical alignment
doubles the ordered domain size. Electrical alignment also
increases the column length to a large extent by inducing
more Tp moieties stacking together, which is much more
effective than thermal annealing. The above results clearly
show that electrical alignment offers a better way for align-
ing discotic LCs, which is crucial for electronic applications.
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1
CONCLUSIONS
1
In summary, we have synthesized an MJLCP, PP12V, which
contains Tp moieties with 12 methylene units as spacers, and
investigated its novel LC phase behavior. With the incorpora-
tion of Tp moieties in the side chains of the MJLCP, PP12V
forms two ordered nanostructures at sub-10-nm length
scales at different temperatures. The low-temperature phase
1
1
2
4 K. Okoshi, T. Hagihara, M. Fujiki, J. Watanabe, Liq. Cryst.
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1
6 I. Paraschiv, K. de Lange, M. Giesbers, B. van Lagen, F. C.
0
a U phase with a larger dimension (a 5 4.07 nm) developed
N
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by the rod-like supramolecular mesogen—the MJLCP chain as
a whole. Partially homeotropic alignment of the polymer in
the UH phase formed by the Tp moieties is achieved when
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,9
(
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The delicate molecular design based on MJLCPs containing
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modular strategy to construct ordered nanostructures with
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The authors acknowledge Yao Liu at ICCAS and Prof. Xiao-Wei
Zhan at the College of Engineering at Peking University for
helpful discussions. This work was supported by the National
Natural Science Foundation of China (Grants 21134001 and
2
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