Hierarchical superstructures from a star-shaped molecule consisting of a cyclic oligosiloxane with cyanobiphenyl moieties

Article abstract of DOI:10.1039/c4sm02239d

Unconventional star-shaped liquid crystals (abbreviated as SiLCs) were successfully synthesized by chemically connecting four cyanobiphenyl anisotropic mesogens to the periphery of a super-hydrophobic and ultra-flexible cyclic tetramethyltetrasiloxane rin

Full text of DOI:10.1039/c4sm02239d

Volume 11 Number 1 7 January 2015 Pages 1–212
Soft Matter
www.softmatter.org
ISSN 1744-683X
PAPER
Kwang-Un Jeong et al.
Hierarchical superstructures from a star-shaped molecule consisting of a
cyclic oligosiloxane with cyanobiphenyl moieties

Soft Matter
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PAPER
Hierarchical superstructures from a star-shaped
molecule consisting of a cyclic oligosiloxane with
cyanobiphenyl moieties†
Cite this: Soft Matter, 2015, 11, 58
Dae-Yoon Kim,a Minwook Park,a Sang-A Lee,a Soeun Kim,a Chih-Hao Hsu,b
c
d
e
a
*
Namil Kim, Shiao-Wei Kuo, Tae-Ho Yoon and Kwang-Un Jeong
Unconventional star-shaped liquid crystals (abbreviated as SiLCs) were successfully synthesized by
chemically connecting four cyanobiphenyl anisotropic mesogens to the periphery of super-
a
hydrophobic and ultra-flexible cyclic tetramethyltetrasiloxane ring with flexible hexyl chains. Based on
the combined experimental techniques of differential scanning calorimetry (DSC), cross-polarized optical
microscopy (POM), solid-state carbon-13 (13C) nuclear magnetic resonance (NMR) spectroscopy and
one-dimensional (1D) wide-angle X-ray diffraction (WAXD), it was found that the SiLC molecule exhibited
the monotropic phase transition from a LC phase to a crystalline phase. The crystalline phase was only
detected during slow heating processes above its glass transition temperature, while a LC phase was
formed both during cooling and during heating processes. The hierarchical superstructures were
identified from the structure-sensitive 2D WAXD of the macroscopically oriented SiLC film and
confirmed by selected area electron diffraction (SAED) of the SiLC single crystals. The molecular packing
symmetry in the monoclinic unit cell was further investigated by computer simulations on the real and
reciprocal spaces. Macroscopically oriented SiLC hierarchical superstructures on the different length
scales may provide the targeted physical properties, which can allow us to apply SiLC molecules in the
fields of electro-optical devices and nonlinear optics.
Received 10th October 2014
Accepted 6th November 2014
DOI: 10.1039/c4sm02239d
www.rsc.org/softmatter
fundamental understanding and developments of partially
ordered so materials and their self-assembled superstruc-
tures.3 Due to the anisotropic physical properties combined
with the self-healing effects, LC materials play a key role in
determining the electrical and optical performances of LC
devices.4
Since the anisotropic physical properties of LC materials
intrinsically originate from the anisotropic molecular shapes,
the conventional LC compounds consist of a calamitic and/or
discotic rigid core and exible alkyl or alkoxy chains.5 The
1. Introduction
Molecular self-assembly is a fundamental bottom-up principle
to build up hierarchical systems along with the possible
involvement of growth kinetics.1 Formation of hierarchical
superstructures from noncovalent interactions of nucleic acids
on different length scales or host–guest interactions of cyclo-
amyloses with macromolecules can be representative examples
of the self-assembled bottom-up method found in nature.2
Since the rst discovery of liquid crystal (LC) materials in the
1880s, LCs have been considered as promising building block
molecules in the bottom-up method. The science and engi-
neering of LCs have signicantly contributed to the
property–structure
relationships
of
conventional
LC
compounds can be predicted by theoretical calculations based
on the experimental data.6 However, numerous unconventional
anisotropic LC materials including polycatenar-/metal-
lomesogen-/cyclic-compounds and swallow-/star-/banana-sha-
ped molecules have been newly synthesized.7
In this circumstance, the introduction of silicon connectors
into LC compounds is attractive in LC devices because they
improve the response time under external stimuli and the
mechanical exibility and durability.8 Anisotropic properties of
silicon-based LC materials are imparted by chemically attached
polymeric or low molar mass (LMM) materials. Since silicon-
based polymeric LC systems have some limitations such as a
complicated synthesis protocol and long response time to an
applied eld due to their high viscosity, a silicon-based
aDepartment of Polymer-Nano Science and Technology & Polymer Materials Fusion
Research Center, Chonbuk National University, Jeonju, Jeonbuk, 561-756, Korea.
E-mail: kujeong@jbnu.ac.kr
bDepartment of Polymer Science, The University of Akron, Akron, OH, 44325, USA
cSmart Materials R&D Center, Korea Automotive Technology Institute, Cheonan,
Chungnam 330-912, South Korea
dDepartment of Materials Science and Optoelectronic Engineering, National Sun Yat-
Sen University, Kaohsiung 804, Taiwan
eDepartment Materials Science and Technology, Gwangju Institute of Science and
Technology, Gwangju, Jeonnam 500-712, South Korea
† Electronic supplementary information (ESI) available: 1H NMR, 13C NMR, and
detailed experimental procedures. See DOI: 10.1039/c4sm02239d
58 | Soft Matter, 2015, 11, 58–68
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oligomeric LC has been proposed.9 The mesophase of LMM-LCs puried by column chromatography with silica gel using
is oen generated by the nanophase separation between the CH2Cl2 : n-hexanes ¼ 5 : 1. The resulting product was opaque
rigid aromatic mesogens and exible alkyl groups.10 Therefore, white oil (yield: 90%). 1H NMR (400 MHz, CDCl3, TMS): d ¼ 1.61
the formation of the mesophase can be enhanced by incorpo- (m, 2H), 1.84 (m, 2H), 2.14 (m, 2H), 4.02 (t, 2H), 5.03 (m, 2H), 5.85
rating short silicon segments such as linear oligocarbosilane, (m, 1H), 6.89 (d, 2H), 7.54 (d, 2H), 7.62 (d, 2H), 7.67 ppm (d, 2H).
cubic oligosilsesquioxane, and cyclic oligosiloxane.11
SiLC (2). 4-Cyano-40-hex-5-en-1-oxy-biphenyl (4.71 g, 17.0 mmol)
In this research, we newly designed and synthesized a star- was added to a solution of 2,4,6,8-tetramethyl-cyclotetrasiloxane (1
shaped cyanobiphenyl-substituted cyclic silicon LC molecule g, 4.17 mmol) in 40 ml of toluene. The solution was allowed to stir
(abbreviated as SiLC). A cyclic tetramethyltetrasiloxane ring for 30 min with nitrogen purge. Platinum(0)-1,3-divinyl-1,1,3,3-
was selectively chosen for the ultra-exible and super- tetramethyldisiloxane (0.3 g) was added as a solution in toluene
hydrophobic central core because the ordered structures with (3 ml). The reaction was heated to 100 ꢀC and stirred for 24 h. Aer
low viscosity are useful in LC displays, nonlinear optics, hydrosilation was complete, the reaction mixture was cooled to
information storage, and electro-optical devices.12 It was also room temperature. The dissolved solution was extracted with
possible to adjust the number of anisotropic cyanobiphenyl distilled water and dried over MgSO4. It was puried by column
mesogens chemically attached to the exible core for chromatography with silica (CH2Cl2 : n-hexanes ¼ 5 : 3). The
enhancing inter- and intra-molecular interactions. There- resulting product was white powder (yield: 76.4%). 1H NMR
fore, there were two main driving forces in the formation of (400 MHz, CDCl3, TMS): d ¼ 0.08 (s, 12H), 0.55 (t, 8H), 1.40
the ordered hierarchical superstructures in this system: the (m, 24H), 1.79 (m, 8H), 3.98 (t, 8H), 6.96 (m, 8H), 7.49 (m, 8H), 7.60
super-hydrophobic interaction among the cyclic siloxanes, (m, 8H), 7.67 ppm (m, 8H); 13C NMR (400 MHz, CDCl3, TMS): d ¼
and the nanophase separation between the rigid cyanobi- 0.57, 17.1, 22.9, 25.8, 29.1, 32.8, 68.1, 110.0, 115.0, 119.0, 126.9,
phenyl mesogens and the exible alkyl chains. The hierar- 128.2, 131.2, 132.5, 145.1, 159.7 ppm.
chical superstructures and phase behaviors of SiLCs were
investigated by the combination of differential scanning
calorimetry (DSC), one-dimensional (1D) wide-angle X-ray
Equipment and experiments
diffraction (WAXD), cross-polarized optical microscopy Phase transition behavior of SiLC was studied using a Perkin-
(POM), and solid-state carbon-13 (13C) nuclear magnetic Elmer PYRIS Diamond DSC with an Intracooler 2P apparatus.
resonance (NMR) techniques. Especially, the hierarchical The temperatures and heat ows were calibrated using indium
crystalline superstructure identied by 2D WAXD of the and zinc standards at cooling and heating rates ranging from
macroscopically oriented lm was conrmed by selected area 2.5 to 40 C minꢁ1. Heating experiments always preceded the
ꢀ
electron diffraction (SAED) of the SiLC single crystal and its cooling experiments at the same scanning rate in order to
molecular packing symmetry in the unit cell was further eliminate previous thermal histories. The transition tempera-
investigated by computer simulations on the real and recip- tures were determined by measuring the onset temperatures
rocal spaces.
from both the cooling and heating scans at different rates.
Optical textures of the ordered phases were observed with a
POM (Nikon ECLIPSE E600POL) coupled with a LINKAM LTS
350 heating stage in order to investigate morphology on the
micrometer scale.
2. Experimental
Materials
1D WAXD experiments were conducted in the reection
mode of a Rigaku 12 kW rotating-anode X-ray (Cu Ka radiation)
generator coupled with a diffractometer. The position and
width of diffraction peaks were calibrated with silicon crystals
in the high 2q-angle region (>15ꢀ) and silver behenate in the low
2q-angle region. To monitor the structural evolutions with
temperature changes, a hot stage calibrated to be within ꢂ1 ꢀC
error was coupled to the diffractometer. Samples were scanned
across a 2q-angle range of 1.5ꢀ to 35ꢀ at a scanning rate of
40-Hydroxy-4-biphenylcarbonitrile (CBP, 97%, Aldrich), 2,4,6,8-
tetramethylcyclotetrasiloxane (TMCTS, 95%, TCI), 6-bromo-1-
hexene (95%, TCI), platinum(0)-1,3-divinyl-1,1,3,3-tetramethyl-
disiloxane (Karstedt's catalyst, 2 wt% in xylene, Aldrich), and
potassium carbonate (K2CO3, 99.99%, Aldrich) were used as-
received unless otherwise noted. N,N-Dimethylformamide
(DMF, 99.8%, Sigma-Aldrich) and toluene (99.8%, Sigma-
Aldrich) were distilled by conventional methods prior to use.
Merck silica gel (9385 grade, 230–400 mesh) was used for all
column chromatography separations.
10ꢀ minꢁ1
.
The oriented 2D WAXD patterns were obtained using a
Rigaku X-ray imaging system with an 18 kW rotating anode X-
ray generator. The diffraction peak positions and widths were
Synthesis
4-Cyano-40-hex-5-en-1-oxy-biphenyl (1). A solution of 6-bromo- also calibrated with silicon crystals and silver behenate. A hot
1-hexene (5 g, 30.0 mmol), 40-hydroxy-4-biphenyl-carbonitrile stage was employed to obtain diffraction peaks at elevated
(6.32 g, 32.0 mmol), and K2CO3 (5.44 g, 32.8 mmol) in 150 ml of temperatures. At least 30 min exposure time was required for a
dried DMF was reuxed for 24 h in a N2 atmosphere. Aer high-quality pattern. In 2D WAXD experiments, background
reaction, the solvent was removed in a vacuum and the residue scattering was subtracted from the sample scans.
was dissolved in chloroform and washed with distilled water
Bright-eld TEM images (FEI Tecnai 12) were taken to
several times. The organic layer was dried over MgSO4. It was examine the lm morphology on the nanometer length scale at
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an accelerating voltage of 120 kV. The camera length for
selected area electron diffraction (SAED) analysis was set at 3.0
m, and the calibration of the SAED spacing smaller than 0.384
nm was carried out using evaporated thallous chloride, which
has a largest rst-order spacing diffraction of 0.384 nm. Spacing
values larger than 0.384 nm were calibrated by doubling the d-
spacing values of the rst order diffractions.
Conformational changes of the alkyl chain in SiLCs at
different temperatures were also studied using solid-state 13C
NMR spectroscopy (Chemagnetics CMX 200) operating at
201.13 and 50.78 MHz for 13C nuclei, respectively. The samples
were spun in nitrogen gas at 4.5 kHz at the magic angle. The
magic angle was optimized by the intensity calibration of the
aromatic carbon resonance of hexamethylbenzene. The 13C
NMR spectra with CP/MAS/DD were acquired to selectively
investigate the rigid components, and the Bloch decay spectra
with MAS/DD were used to particularly study the mobile
components. The CP contact time was 1 ms, while the recycle
time of the pulse was 5 s. Each spectrum consisted of an
accumulation of 500 scans. The temperature of the solid-state
13C NMR experiment was controlled using a REX-F900 VT unit.
Overlapped carbon peaks in solid-state 13C NMR were resolved
using the PeakFit peak separation program from Jandel Scien-
tic. Lorentzian functions were used to obtain the best
reasonable t.
Fig. 1 Chemical structure and calculated geometric dimensions of the
SiLC.
Crystallographic simulation was performed using Cerius2
(Version 4.6) simulation soware from Accelrys utilizing the
COMPASS force eld. The global equilibrium conformation of
the SiLC compound in the isolated gas phase was chosen as the
starting building block. Basic unit cell parameters determined
by crystallographic experimental data from 2D WAXD of the
macroscopically oriented SiLC lm and SAED of SiLC single
crystals were used to build the crystal unit cell. The molecular
packing symmetry in this unit cell was judged by comparing the
calculated diffraction patterns with those of experiments.
Scheme 1 Synthesis of SiLC. Reagents and conditions: (i) CBP, bro-
mohexane, K2CO3, DMF, 90 ꢀC, 24 h; (ii) Karstedt's catalyst, TMCTS,
toluene, 100 ꢀC, 24 h.
3. Results and discussion
Programmed star-shaped oligosiloxane LC molecule
A cyclic silicon-based LC molecule (abbreviated as SiLC) con-
taining four cyanobiphenyl moieties is newly designed and
synthesized, as illustrated in Fig. 1. By precisely locating the catalyst, divinyl(tetramethyldisiloxane) platinum.13 The detailed
exible alkyl connectors to the rigid cyanobiphenyl mesogens at synthetic procedures are described in the Experimental section
the periphery of the super-hydrophobic and ultra-exible cyclic of this paper. Chemical structures and purities of the SiLC and
siloxane ring, the nanophase separation between cyanobi- its intermediates are conrmed by 1H NMR (see Fig. S1 and S2
phenyl mesogens and cyclic siloxanes is maximized. From a in the ESI†), 13C NMR (Fig. S3†), and thin layer chromatography
synthetic point of view, the 2,4,6,8-tetramethylcyclotetrasilox- (TLC).
ane (TMCTS) cyclic ring containing four Si–H functions allows
us to synthesize a discrete and well-dened SiLC compound by a molecular dimensions and energy-minimized geometry in the
click chemistry. isolated gas phase are estimated by performing computer
The molecular weight of the SiLC is 1349.9 g molꢁ1. The
As shown in Scheme 1, the olen terminated alkyl ether of 40- simulation. As represented in Fig. 1, two cyanobiphenyl meso-
hydroxy-4-biphenyl-carbonitrile is rst prepared from the cor- gens connected at 1 and 3 positions of cyclic siloxane with hexyl
responding bromide compound with potassium carbonate and chains are located at one side of cyclic siloxane, while the other
then cyanobiphenyl substituted cyclic oligomeric siloxane was two cyanobiphenyl mesogens at 2 and 4 positions are located at
synthesized by the hydrosilation of olen terminated cyanobi- the other side. Therefore, the overall shape of the SiLC
phenyl mesogens with a multi-functional cyclic hydrosiloxane compound is ribbon-like. From the all-trans conformation in
TMCTS. The reaction is conducted in the presence of a Pt(0) the alkyl chains, the calculated length of the SiLC is 4.23 nm,
60 | Soft Matter, 2015, 11, 58–68
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while the width of the SiLC along the short axis is 1.25 nm, as information about structural changes.15 The 1D WAXD experi-
illustrated in Fig. 1.
ments at different temperatures are combined with DSC results
to identify the corresponding structure evolutions. Fig. 3 shows
a set of 1D WAXD powder patterns obtained at cooling and
subsequent heating rates of 2.5 ꢀC minꢁ1. It is realized that the
SiLC undergoes the isotropization above Ti ¼ 119 ꢀC, exhibiting
three amorphous scatterings at 2q ¼ 2.91ꢀ (d ¼ 3.03 nm), 2q ¼
12.01ꢀ (d ¼ 0.74 nm) and 2q ¼ 19.59ꢀ (d ¼ 0.45 nm). The
amorphous halo at 2q ¼ 2.91ꢀ corresponds to the average
periodicity of electron density uctuations between the nano-
phase separated cyclic siloxane cores and cyanobiphenyl
mesogens while the amorphous halo at 2q ¼ 19.59ꢀ and at 2q ¼
12.01ꢀ comes from the average distances between the cyanobi-
phenyl mesogens and between the cyclic siloxane cores,
respectively. It is apparent that the nanophase separation
between cyanobiphenyl mesogens and cyclic siloxanes is
maintained even in the isotropic (I) phase by locating the ex-
ible alkyl connectors to the rigid cyanobiphenyl mesogens at the
periphery of super-hydrophobic and ultra-exible cyclic
siloxane. When the temperature is decreased below Ti ¼ 119 ꢀC,
the scattering halo at 2q ¼ 19.59ꢀ (d-spacing ¼ 0.45 nm)
suddenly shis to 2q ¼ 20.84ꢀ (d-spacing ¼ 0.42 nm) with a
smaller width at half height. In a low-angle region, a single
strong diffraction peak at 2q ¼ 2.91ꢀ (d ¼ 3.03 nm) is concur-
rently emerged but its high-order diffractions are very weak.
However, the amorphous halo at 2q ¼ 12.01ꢀ is not much
Thermodynamic phase transitions and their corresponding
structural evolutions
The DSC experiment was rst conducted at cooling and heating
rates ranging from 2.5 to 40 ꢀC minꢁ1 to determine the thermal
transition temperatures and quantitative thermodynamic
properties of the SiLC compound. As shown in Fig. 2, a single
ꢀ
exothermic rst-order thermal transition is observed at 119 C
with ꢁ7.34 J gꢁ1 (ꢁ9.91 kJ molꢁ1) during a rapid cooling at
40.0 C minꢁ1. Upon further cooling, a sudden step change of
ꢀ
heat capacity is detected at 22 ꢀC, which is attributed to the
glass transition temperature (Tg) of the SiLC. The DSC ther-
mogram obtained upon subsequent heating at 40 ꢀC minꢁ1
exhibits strong endothermic rst-order transition at 119 ꢀC with
7.35 J gꢁ1 (9.92 kJ molꢁ1). Similar thermal transition behaviors
ꢁ1
ꢀ
are observed at the scanning rates of 10 to 40 C min
.
When the scanning rate is reduced to 5.0 and 2.5 ꢀC minꢁ1
,
on the other hand, exothermic and endothermic thermal tran-
sitions are newly observed slightly above the Tg. Larger heat is
evolved at a slower heating rate: ꢁ10.85 J gꢁ1 (ꢁ14.64 kJ molꢁ1
)
ꢁ1
for 2.5 C min and ꢁ3.72 J gꢁ1 (ꢁ5.02 kJ molꢁ1) for 5.0 C
minꢁ1. Since the amount of heat released during the exothermic
transition is almost identical to that of heat absorbed during
the endothermic transition, the exothermic transition can be
due to the recrystallization of the SiLC. The onset temperature
ꢀ
ꢀ
ꢀ
changed. On further cooling below Tg ¼ 22 C, the 1D WAXD
patterns are not changed. Based on the 1D WAXD data
combined with DSC results, it is clear that a low-ordered
smectic LC phase is formed by the self-organization of rigid
cyanobiphenyl mesogens in the nanophase separated domains
below Ti ¼ 119 ꢀC and the molecular packing symmetry is
ꢀ
and the corresponding heat at 119 C show no change regard-
less of cooling and heating rates because the rst-order thermal
transition is close to the equilibrium state. From the POM
analysis, it is found that the transition at elevated temperature
is associated with liquid crystal (LC)-isotropic melt (I) transition
of the SiLC.14 The fast cooled SiLC reveals a glassy LC phase
ꢀ
still retained below Tg ¼ 22 C. Therefore, the structure below
Tg ¼ 22 ꢀC should be denoted as a metastable glassy smectic LC
ꢀ
below Tg ¼ 22 C.
phase.
Although DSC experiments are sensitive to heat absorption
and release events, this technique cannot provide direct
Fig. 2 Sets of DSC cooling and subsequent heating thermal diagrams Fig. 3 A set of 1D WAXD powder patterns of the SiLC molecule
of the SiꢁLC1 molecule at scanning rates ranging from 2.5 to obtained at different temperatures between ꢁ10 and 150 ꢀC during
40 ꢀC min
.
cooling and subsequent heating at a rate of 2.5 ꢀC minꢁ1
.
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On subsequent heating above the Tg of the SiLC molecule at recrystallization process. The fan shaped texture observed
a rate of 2.5 C minꢁ1, several sharp reection peaks suddenly during cooling appears again above 35 C due to the K / LC
ꢀ
ꢀ
appear in both low and wide-angle regions due to the develop- transition (Fig. 4c).
ment of highly ordered crystals. Phase transition at 30 ꢀC
From the combined experiments of DSC, 1D WAXD and
should be closely associated with the exothermic thermal POM, it is realized that there are two stable hierarchical
transition detected between Tg and 35 ꢀC in DSC results (Fig. 2). superstructures of the SiLC molecule depending on tempera-
The sharp diffraction peak at 2q ¼ 2.91ꢀ (d ¼ 3.03 nm) shis to ture: one is a low-ordered smectic LC phase at high temperature
2q ¼ 3.39ꢀ (d ¼ 2.60 nm) and its second-order diffraction peak is and the other is a crystalline structure formed by annealing a
also observed at 2q ¼ 6.78ꢀ (d ¼ 1.30 nm). In the wide-angle metastable glassy smectic LC phase above Tg. Note that the
region, the scattering halo at 2q ¼ 20.84ꢀ (d-spacing ¼ 0.42 nm) cyclic siloxane group of the SiLC molecule is involved in con-
at the glassy smectic LC phase is split into three distinct structing the hierarchical superstructures but the nanophase
diffraction peaks at 2q ¼ 19.59ꢀ, 22.74ꢀ and 27.24ꢀ, which separated cyclic siloxane groups are in a disordered state with a
correspond to the d-spacings of 0.45 nm, 0.39 nm and 0.32 nm, short-range positional order even in the crystalline phase. It
respectively. The diffraction peaks in wide-angle regions are means that the cyclic siloxane groups of the SiLC are in the
mainly related to the lateral molecular packing between cya- mobile and exible states in all hierarchical superstructures. To
nobiphenyl mesogens. However, the amorphous halo at 2q ¼ conrm this statement, the solid-state 13C NMR spectra with
12.01ꢀ detected in the smectic LC phase is not much changed Cross Polarization/Magic Angle Spinning/Dipolar Decoupling
even in the crystalline phase. This result refers to the fact that (CP/MAS/DD) (Fig. 5b) and Bloch decay (Fig. 5c) are acquired at
the nanophase separated cyclic siloxane groups are in a disor- different temperatures to selectively investigate the rigid and
dered state with a short-range positional order even in the mobile components of the SiLC at different phases,
crystalline phase. The 1D WAXD pattern at 30 C clearly indi- respectively.17
ꢀ
cates that the constructed hierarchical crystalline SiLC super-
At rst, chemical shi is identied by analyzing the solution
structure is a layered structure. However, its molecular packing 13C NMR. As indicated in Fig. 5a, the carbon atoms in cyano-
symmetry in the smectic layer should be identied by 2D WAXD biphenyl groups appear between 100 ppm and 170 ppm, while
of the oriented sample, which will be discussed in this paper. carbons in the alkyl chains are chemically shied between 100
When the temperature is increased above the endothermic ppm and 10 ppm. Specically, chemical shi of carbons in the
transition at 35 ꢀC, the SiLC crystal phase transforms to a cyclic siloxane ring is detected at 0 ppm. As we expected from
smectic LC phase. This explanation is justied by the fact that the DSC and 1D WAXD results, the mobilities of the cyclic
the 1D WAXD patterns at 50 ꢀC obtained during cooling and siloxane ring and alkyl chains as well as cyanobiphenyl groups
ꢀ
subsequent heating processes are almost identical. Above Ti ¼ in the low-ordered smectic LC phase (50 C) are much higher
ꢀ
119 C, a typical 1D WAXD pattern for the I phase is observed. than those of the glassy smectic LC phase (ꢁ10 ꢀC). Especially,
Phase transformations from the I phase to the smectic LC the cyclic siloxane rings in the crystalline phase are still mobile
phase and the crystalline K phase can be further supported by even though the rigidity is higher than that in the low-ordered
the morphological investigation on the micrometer length smectic LC phase. This result is matched well with those of 1D
scale. Optical textures oꢁf 1the SiLC upon cooling and subsequent WAXD. In Fig. 5, the 13C chemical shi at 63 ppm represents the
heating at 2.5 ꢀC min from ꢁ10 ꢀC to 130 ꢀC have been carbon atoms at the a position, and the b and g carbon atoms
recorded, and textures at ꢁ10 ꢀC, 30 ꢀC and 50 ꢀC are repre- appeared at 34 and 24 ppm, respectively. Especially, the b
sented in Fig. 4a–c, respectively. On cooling below Ti, the fan- carbon chemical shi in the Bloch decay method consists of two
shaped textures are grown against the dark state, suggesting the values. The chemical shi at 34–32 ppm in the Bloch decay 13
C
emergence of a low-ordered smectic LC phase.16 As shown in solid-state NMR spectra of the SiLC (Fig. 5c) represents the
Fig. 4a, the fully grown fan-shaped textures are still discernible methylene carbon atoms in the long trans zigzag segments of
even below Tg, which agrees well with DSC and 1D WAXD the alkyl chains, while the chemical shi at 32–30 ppm corre-
ꢁ1
ꢀ
results. Upon heating at 2.5 C min above Tg, the boundary sponds to the carbon atoms in the disordered, statistically
lines between the fan-shaped domains and the Maltese cross in distributed trans and gauche conformations. Upon increasing
each fan shaped domain get obvious, as shown in Fig. 4b, the the temperature from ꢁ10 ꢀC (a glassy smectic LC phase) to
phenomena of which are oen detected during
a
30 ꢀC (a crystalline phase), the area of the b peak at 33 ppm
corresponding to the carbon atoms in the ordered segments of
alkyl chains suddenly increases from 40% to 60%. When the
temperature is further increased up to 50 ꢀC (a low-ordered
smectic LC phase), the area of the b peak is abruptly decreased
down to 40%. Temperature-dependent solid-state 13C NMR
results agree well with those of DSC, 1D WAXD and POM.
To understand the thermodynamic phase transition behav-
iors of the SiLC and the formation of the stable K crystalline
phase, the free energy versus temperature at constant pressure is
schematically illustrated based on the combined experimental
data of DSC, 1D WAXD, POM and solid-state 13C NMR. As
Fig. 4 POM images of the SiLC molecule at (a) ꢁ10 ꢀC, (b) 30 ꢀC and
(c) 50 ꢀC during heating at 2.5 ꢀC minꢁ1
.
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Fig. 6 Illustration of the free energy versus temperature at constant
pressure for the SiLC. Here, blue and red arrows represent the cooling
and subsequent heating tracks, respectively.
ordered crystalline K phase transform to the smectic LC phase
at TK-LC and to the I phase at TLC-I. This monotropic phase
transition behavior and the development of the metastable
smectic LC phase below TK should originate from the kinetic
competition between the smectic LC phase and the crystalline K
phase.
Identication of hierarchical superstructures at different
temperatures
The molecular packing structures and symmetries in the hier-
archical superstructures of the SiLC are identied by conduct-
ing the 2D WAXD experiments. Macroscopically oriented SiLC
samples for 2D WAXD experiments are rst prepared by
mechanical shearing at the temperature between TK-LC and TLC-I
and then quenched below the Tg of the SiLC. The oriented and
quenched sample is further annealed at the temperature
between Tg and TK-LC to prepare the macroscopically oriented K
crystalline lm for 2D WAXD.
A series of 2D WAXD patterns for the oriented SiLC crystal
(Fig. 7d) were obtained with the incident X-ray beam directions
parallel to the x (Fig. 7a), y (Fig. 7b) and z (Fig. 7c) axes,
respectively. Diffraction arc positions and widths in Fig. 7 are
calibrated based on the diffraction ring of the silicon power
crystal at 2q ¼ 28.466ꢀ (d ¼ 0.314 nm). As discussed in 1D WAXD
(Fig. 3), diffraction information about SiLC molecular arrange-
Fig. 5 Solution 13C NMR spectrum of the SiLC molecule (a) and sets of
solid-state 13C NMR spectra of the SiLC molecule during heating
between ꢁ10 ꢀC and 50 ꢀC: CP/MAS/DD method (b) and Bloch decay
method (c).
represented by blue arrows in Fig. 6, the I phase is transferred to ments appears on two different length scales. One is on the
the smectic LC phase by cooling the SiLC. Since the I 4 LC nanometer length scale in the low 2q-angle region between 1.5ꢀ
transition is close to the equilibrium state, the transition and 11ꢀ, which is related to hierarchical superstructures of the
temperature and its heat change are independent of cooling SiLC. Diffractions on the sub-nanometer length scale between
and heating rates. Upon lowering the temperature below the 11ꢀ and 30ꢀ can provide information about the lateral molecular
Tg of the SiLC, the symmetry and order of the smectic LC phase packing structures between the nanophase separated cyclic
are retained to be a glassy smectic LC phase. As illustrated in siloxane cores and between the cyanobiphenyl mesogens. As
Fig. 6, on subsequent heating above the Tg of the SiLC, the represented in Fig. 7, the X-ray patterns along the x and y axes
metastable glassy smectic LC phase transforms to the stable K are identical while the 2D WAXD along the z axis (Fig. 7c) shows
crystalline phase. To conrm this monotropic phase transition, disoriented ring patterns in wide-angle regions without any
the SiLC compound is isothermally annealed at 30 ꢀC right aer detectable diffractions in low-angle regions. Therefore, it is safe
ꢁ1
ꢀ
cooling at 10 C min using DSC. The subsequent heating at to conclude that 2D WAXD along the x and y axes is a uniaxially
10 ꢀC minꢁ1 indicates that the crystallization starts aer 10 min oriented ber pattern, in which the molecular long axis is
and nishes in 2 h. The continuous heating makes the 3D parallel to the z axis (perpendicular to the shear direction, SD).
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thin SiLC lm prepared by casting the 0.05 wt% chloroform
solution is cooled down to ꢁ10 ꢀC from Ti ¼ 119 ꢀC and
subsequently heated up to 30 C at a rate of 2.5 C minꢁ1. The
SAED pattern of the single crystal obtained by a low dose TEM
technique is shown in Fig. 8. Two pairs of electron diffractions
are detected at d ¼ 0.45 nm on the quadrants while a pair of
electron diffractions at d ¼ 0.39 nm is observed on the a0* axis.
The (020) diffraction is also detected at d ¼ 0.275 nm, as shown
in Fig. 8. The calculated g0-angle (g0 ¼ 90ꢀ) based on 2D WAXD
is matched well with the measured value in SAED.
ꢀ
ꢀ
The molecular packing structure of cyanobiphenyl mesogens
in the hierarchical crystalline superstructure of the SiLC deter-
mined by 2D WAXD and SAED is further veried by a computer
simulation. At rst, we only take care of the molecular packing of
cyanobiphenyl mesogenic parts in SiLC molecules. As shown in
Fig. 9, four cyanobiphenyl mesogens located at the corners are
arranged along the c0 axis (long axis of the molecule, z axis) while
the cyanobiphenyl mesogen at the center points the opposite
direction. This molecular arrangement exactly satises the
diffraction patterns of the experimentally observed 2D WAXD and
SAED. Furthermore, the dipole moment induced by the indi-
vidual cyanobiphenyl mesogen can be canceled out to make the
system stable. Note that the cyanobiphenyl mesogen at the center
and at the corners should come from the different SiLC mole-
cules. The unit cell created only from cyanobiphenyl mesogens
cannot represent the K phase, but it is considered as a sub-unit
cell of the hierarchical crystalline superstructure of the SiLC.
Fig. 7 2D WAXD patterns of the crystalline phase (K) of the SiLC at
25 ꢀC: the incident X-ray beam directions are parallel to the x (a), y
(b), and z axes (c), respectively. Schematic illustration of the SiLC
specimen (d).
In Fig. 7a and b, a pair of strong diffraction at 2q ¼ 3.39ꢀ (d ¼
2.60 nm) is detected on the meridian with its high order
diffraction at 2q ¼ 6.78ꢀ (d ¼ 1.30 nm). This result indicates that
the hierarchical superstructure of the SiLC in the K phase is a
basically layer structure with the layer d-spacing ¼ 2.60 nm.
Therefore, Miller indices of the diffractions at 2q ¼ 3.39ꢀ and
6.78ꢀ are identied as (001) and (002), respectively. Note that the
layer normal is parallel to the z axis which is perpendicular to
the SD. Since the calculated length of the SiLC is 4.23 nm (as
shown in Fig. 1), SiLC molecules may overlap each other along
the layer normal. On the equator of 2D WAXD along the x and y
axes, three diffraction arcs are detected at 2q ¼ 19.59ꢀ (d ¼ 0.45
nm), 22.74ꢀ (d ¼ 0.39 nm) and 27.24ꢀ (d ¼ 0.32 nm) with a pair
of scattering halo at 2q ¼ 12.01ꢀ (d ¼ 0.74 nm). Note that all
diffractions and scatterings on the equator of 2D WAXD along
the x and y axes appear as ring patterns in Fig. 7c (along the z
axis). The scattering halo detected at 2q ¼ 12.01ꢀ originates
from the average periodicity of electron density uctuations
between the nanophase separated cyclic siloxane cores with a
short-range positional order. Since the scattering halo is
detected on the equator, the layer normal of siloxane core
aggregates is parallel to the z axis (meridian direction). When
the molecular packing among cyanobiphenyl mesogens in the
hierarchical crystalline superstructure of the SiLC is only
considered, three diffractions on the equator at 2q ¼ 19.59ꢀ,
22.74ꢀ and 27.24ꢀ can be assigned to the (200), (110) and (120)
planes, respectively, based on the triangulation method of
building a 2D a0*b0* lattice of the unit cell. Through the
renement of the reciprocal 2D a0*b0* lattice using the diffrac-
tions on the equator, a real space 2D unit cell is evaluated with
dimensions of a0 ¼ 0.78 nm, b0 ¼ 0.55 nm, and g0 ¼ 90ꢀ.
To conrm the 2D molecular packing of cyanobiphenyl
mesogens in the hierarchical crystalline superstructure of the
SiLC, SAED experiments from single crystals are conducted. The
Fig. 8 SAED pattern of the SiLC single crystal in the K phase.
Fig. 9 Simulated molecular packing in the real space by considering
only cyanobiphenyl mesogens in the SiLC.
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Soft Matter
Next, to identify the hierarchical crystalline superstructure of opposite to the neighboring ones, as shown in Fig. 9. Finally,
SiLC diffractions generated from all parts of the SiLC molecule the cyclic siloxane groups of the SiLC molecule are self-assem-
should be considered together. In Fig. 7a and b, the a* and b* bled to form the 2D platform with the average distance among
axes are assigned parallel to the equator while the c*-axis is cyclic siloxane cores ¼ 0.74 nm. Since the scattering halo from
along the meridian direction, which is parallel to the layer cyclic siloxane groups is detected on the equator of 2D WAXD
normal. Based on the triangulation method of building a 2D (Fig. 7a and b), the 2D platform normal is parallel to the z-axis
a*b* lattice of the unit cell, a scattering halo at 2q ¼ 12.01ꢀ and (meridian direction). Without disturbing the molecular packing
three diffraction arcs at 2q ¼ 19.59ꢀ, 22.74ꢀ and 27.24ꢀ on the symmetry of cyanobiphenyl mesogens in the 2D orthogonal unit
ꢀ
equator of 2D WAXD (Fig. 7a and b) can be indexed to (110), cell, the locations of cyclic siloxane groups are rst estimated by
ꢀ
ꢀ
(200), (220) and (310), respectively. Through the renement of overlapping two sub unit cells, as illustrated in Fig. 10a. By
the reciprocal 2D a*b* lattice using the diffractions on the connecting four cyanobiphenyl mesogens to the periphery of a
equator, a real space 2D unit cell is evaluated with dimensions cyclic siloxane ring with hexyl chains, the 3D molecular
of a ¼ 0.95 nm, b ¼ 0.95 nm and g ¼ 109.62ꢀ.18
arrangements of the SiLC molecule in the K phase are recog-
Since the c*-axis along the meridian is perpendicular to the nized in the real space. This simulated molecular packing
a* and c* axes along the equator, both a and b angles are 90ꢀ. model satises all the experimental data observed in 2D WAXD
From the strong diffraction at 2q ¼ 3.39ꢀ with its high order and SAED.
diffraction at 2q ¼ 6.78ꢀ, the dimension of c-axis is determined
In order to examine whether the proposed molecular
to be 2.60 nm. Table 1 lists the experimentally observed and the packing scheme ts to the real structure, the calculated 2D
calculated d-spacings based on this monoclinic unit cell lattice. WAXD and SAED diffraction patterns are generated and
With one SiLC molecule in one unit cell, its calculated crystal- compared with the experimentally obtained diffraction
lographic density is 1.00 g cmꢁ3. The experimentally observed patterns.19 On comparison, the precise atom positions in the
density is 0.98 g cmꢁ3, which ts well with the calculated one. monoclinic unit cell are determined. The calculated 2D WAXD
Therefore, this hierarchical crystalline superstructure (K) of the ber pattern (Fig. 11a) and the SAED pattern with the [001] zone
SiLC is identied as a monoclinic crystalline phase.
(Fig. 11b) based on the molecular packing in the real space
The 3D molecular arrangement and packing symmetry in the (Fig. 10) are qualitatively matched well with the experimental
monoclinic unit cell of the SiLC hierarchical crystalline super- observations (Fig. 7a for 2D WAXD and Fig. 8 for SAED).
structure are further investigated by computer simulation. The
The molecular packing structure of the low-ordered LC
simulated molecular packing should satisfy the experimentally phase between TK-LC ¼ 35 ꢀC and TLC-I ¼ 119 ꢀC is also studied
observed 2D WAXD and SAED results. First of all, there is a by the 2D WAXD patterns obtained by irradiating X-ray along
strong nanophase separation between cyanobiphenyl mesogens the x (Fig. 12a), y (Fig. 12b) and z (Fig. 12c) axes, respectively.
and cyclic siloxane groups of the SiLC molecule, and four cya- The 2D WAXD along the x and y axes are identical and the 2D
nobiphenyl mesogens are chemically connected to the WAXD along the z-axis is the amorphous ring pattern without
periphery of a cyclic siloxane ring. Second, the nanophase exhibiting low-angle diffractions. Similar to the 2D WAXD of
separated cyanobiphenyl mesogens are laterally aggregated to the K phase, the 2D WAXD of the low-ordered LC phase is a
form the 2D orthogonal unit cell with dimensions of a0 ¼ 0.78
nm, b0 ¼ 0.55 nm, and g0 ¼ 90ꢀ. Here, note that the cyanobi-
phenyl mesogens on the (11) 2D plane in the sub unit cell are all
arranged parallel to the layer normal but their polar axes are
Table 1 Experimental and calculated crystallographic parameters of
the monoclinic crystalline (K) phase of the SiLC
2q (degree)
d-spacing (nm)
(hkl)
Exptla
Calc.b
Exptla
Calc.b
ꢀ
110
12.01
19.59
19.59
22.74
27.24
27.24
3.39
11.82
19.65
19.65
22.85
27.51
27.51
3.40
0.74
0.45
0.45
0.39
0.32
0.32
2.60
1.30
0.75
0.45
0.45
0.39
0.32
0.32
2.60
1.30
200
020
ꢀ
220
ꢀ
310
ꢀ
130
001
002
6.78
6.80
a
The accuracy of the experimental data is ꢂ0.005 nm. b The calculated
data listed are based on the crystalline (K) monoclinic unit cell with a ¼
0.95 nm, b ¼ 0.95 nm, c ¼ 2.60 nm, a ¼ b ¼ 90.0ꢀ and g ¼ 109.62ꢀ.
Fig. 10 Schematic illustration of the overlapping the 2D orthogonal
sub unit cell of cyanobiphenyl mesogens on the 2D self-assembled
SiLC platform (a). Computer simulated molecular packing in the SiLC
crystal phase: [001] zone (b) and [010] zone (c).
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maintains a layer structure with d-spacing ¼ 3.03 nm. Even
though the exible alkyl chains are assumed to be all trans
conformation, the calculated length (4.23 nm) of the SiLC is
signicantly greater than the detected layer spacing (d ¼ 3.03
nm). The smaller layer thickness compared with the length of
the SiLC molecule indicates the fact that the SiLC molecule is
tilted with respect to layer normal or that the SiLC molecules
are interdigitated with the molecules in the neighboring
layers. Since the scattering halos at 2q ¼ 20.84ꢀ (d ¼ 0.42 nm)
and 2q ¼ 12.01ꢀ (0.74 nm) that originated from the average
distances between the cyanobiphenyl mesogens and between
cyclic siloxane cores, respectively, are on the equator in
Fig. 12a and b, the smectic layer should be interdigitated and
the long axis of the SiLC molecule is parallel to the layer
normal. Therefore, this low-ordered LC phase can be
identied as a SmA phase, as schematically illustrated in
Fig. 12e.
4. Conclusions
A star-shaped liquid crystalline (LC) molecule (abbreviated as
SiLC) has been newly designed and successfully synthesized
by chemically connecting four cyanobiphenyl mesogens to
the periphery of a cyclic tetramethyltetrasiloxane ring with
exible hexyl chains. Utilizing the combined techniques of
thermal, microscopic, spectroscopic, and scattering experi-
ments, it was found that the SiLC molecule exhibited the
peculiar monotropic phase transition behavior. The hierar-
chical crystalline (K) superstructure was only formed during
heating processes above its glass transition temperature (Tg
¼ 22 ꢀC), while the low-ordered LC phase is formed both
during cooling and during heating processes. Based on the
structure-sensitive 2D WAXD and SAED, the 3D hierarchical
superstructures of the SiLC molecule at different tempera-
tures were identied: one is the crystalline phase with a
monoclinic unit cell at low temperatures and the other is the
SmA LC phase at high temperatures. The molecular packing
symmetry in the hierarchical crystalline superstructure was
further investigated by performing computer simulations on
the real and reciprocal spaces. The hierarchical superstruc-
tures controlled on the different length scales can allow us to
apply SiLC molecules in the elds of electro-optical devices
and nonlinear optics.
Fig. 11 Computer simulated 2D WAXD fiber pattern (a) and SAED on
the [001] zone (b) of the SiLC in the K phase. The circled diffractions are
experimentally observed.
Acknowledgements
This work was mainly supported by Basic Science Research
(2013R1A1A2007238) and BK21 Plus program, Korea. D.-Y. Kim
appreciates the support from Global Ph.D. Fellowship Program.
Fig. 12 2D WAXD patterns of the liquid crystalline phase (LC) of the
SiLC at 70 ꢀC: the incident X-ray beam directions are parallel to the x
(a), y (b), and z axes (c), respectively. Schematic illustrations of the SiLC
specimen (d) and molecular packing models in the SmA LC phase (e).
Notes and references
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66 | Soft Matter, 2015, 11, 58–68
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