Skeletal cubic, lamellar, and ribbon phases of bundled thermotropic bolapolyphiles

Article abstract of DOI:10.1021/ja502410e

A series of T-shaped polyphilic molecules composed of a rigid linear biphenyl core with a polar glycerol group at each end and one swallow-tail semiperfluorinated lateral chain were synthesized and their thermotropic liquid crystalline (LC) phases were in

Full text of DOI:10.1021/ja502410e

Communication
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Skeletal Cubic, Lamellar, and Ribbon Phases of Bundled
Thermotropic Bolapolyphiles
Feng Liu,*,†,‡,§ Marko Prehm,||,§ Xiangbing Zeng,‡ Carsten Tschierske,*,|| and Goran Ungar*,‡,⊥
†State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, P. R. China
‡Department of Materials Science and Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, U.K.
||Institute of Chemistry, Organic Chemistry, Martin-Luther-University Halle-Wittenberg, D-06120 Halle, Germany
⊥Department of Physics, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou 310018, P. R. China
S
* Supporting Information
ABSTRACT: A series of T-shaped polyphilic molecules
composed of a rigid linear biphenyl core with a polar
glycerol group at each end and one swallow-tail semi-
perfluorinated lateral chain were synthesized and their
thermotropic liquid crystalline (LC) phases were inves-
tigated by X-ray diffraction, calorimetry, and microscopy.
The compounds have a long alkyl spacer between the
aromatic core and the fluorinated CnF2n+1 ends, where n =
4, 6, 8, and 10. Upon melting, all compounds start with
Figure 1. Sequence of LC phase types formed by rod-like
lamellar LC phases, followed on heating by a rectangular
columnar ribbon phase with c2mm symmetry. Unusually, a
ribbon is a flat bundle of molecular cores highly aligned
parallel to the ribbon axis. On further heating, for n = 8
and 10, this phase is succeeded by a bicontinuous cubic
bolapolyphiles with incompatible lateral and terminal groups, as seen
upon increasing the size of the lateral chain(s) relative to the rod
length: (a,b) polygonal cylinder phases (rods normal to column),1,3
(c) Lam phases (rods in plane of layer),6−8 and (d) axial-bundle
phases (bundles of rods parallel to column).9,10 Molecules displaying
these phases are sketched above: gray = aromatic, blue = glycerol,
black-green = side chain.
phase with Ia3d symmetry. This is a new variant of the
̅
“gyroid” phase, with axially oriented rod-like molecular
cores forming the skeleton of the two infinite networks
and junctions separated by exactly two molecular lengths.
In this tricontinuous core−shell structure (aromatic−
aliphatic−perfluoroalkyl), the polar glycerol domains of
appreciable size, contained within the skeleton, can be
considered as micellar.
they are incompatible; that can result in “multicolor”
honeycombs in which different cells contain different side
chains, or different mixtures thereof.5 However, it was found
that if the total side chain volume exceeds a certain threshold,
then the honeycomb arrangement becomes unsustainable, and
the discrete cylinders merge into a continuous flat layer (Figure
1c).3,6−8 In contrast to the usual layered smectic LCs, in these
lamellar (Lam) phases the rod-like mesogens lie parallel to the
plane of the layers.
If the side chain volume is increased still further, e.g., if the
two linear chains in Figure 1c are replaced by one or two
branched “swallow-tail” chains (Figure 1d), then a new class of
columnar LC phases is obtained that can be considered as
inverted honeycombs, with the rigid cores now in the center of
the cylinders and the side chains at the periphery.9,10 This type
of arrangement is in fact more akin to the classic columnar
phases of disk-shaped, slice-shaped, or polycatenar rod-like
mesogens,1,11−13 except that in the columnar bolapolyphiles the
long molecular axis is parallel rather than normal to the column
axis, and the rods are bunched together in bundles of about 10
(the “axial bundle” phases).9,10 In a simplistic representation,
the overall behavior described can be compared to that of
lyotropic LCs, whereby the aromatic moieties represent water
and the side chains represent the hydrophobic chains. The
od-like amphiphiles with laterally attached flexible chains
Rdisplay a large variety of thermotropic liquid crystalline
(LC) phases, foremost of which are honeycomb phases made
up of cylinders with different polygonal cross sections.1,2 The
most representative group of compounds forming these phases
are bolapolyphiles having a rod-like core, with a glycerol group
at each end. The side chains, usually n-alkyl, semiperfluoroalkyl,
or carbosilane, fill the interior of the polygonal cylinders. The
walls are assembled from the aromatic rods lying normal to the
cylinder axis and linked by the H-bonding glycerols. For small
side chains, polygons could be triangles, rhombi, or squares,
and pentagons or hexagons may be found for larger chains
(Figure 1a). In all the above cases, the side of the polygon is
one molecule long, but there are instances where the side
contains two rods linked end-to-end, as in the example of
stretched or “giant” hexagons (Figure 1b),3 or in the recently
reported complex honeycomb consisting of giant squares and
octagons.4 The complexity increases when two chains are
attached to the opposite sides of the rod and, in particular, if
Received: March 10, 2014
Published: April 21, 2014
© 2014 American Chemical Society
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a
Table 1. Transition Temperatures and Lattice Parameters
compd
n
T/°C
lattice parameter/nm
1/4
4
Cr 60 LamSm 129 Iso
d = 3.68
1/6
1/8
6
8
Cr 57 LamSm 119 LamN 125
d = 4.04
Colrec/c2mm 131 Iso
a = 7.64, b = 4.91
Cr 75 LamSm 102 LamN 106
d = 4.48
Colrec/c2mm 132
a = 8.28, b = 5.75
Cub/Ia3d 154 Iso
acub= 10.08
̅
1/10
10
Cr 108 LamSm 132 LamN 133
d = 5.15
Colrec/c2mm 145
a = 9.02, b = 6.14
Cub/Ia3d 178 Iso
acub= 10.80
̅
a
From first DSC heating (10 K min−1) and POM; Cr, crystalline;
LamSm, lamellar smectic; LamN, lamellar nematic; Colrec/c2mm,
rectangular c2mm; Cub/Ia3d, gyroid cubic; Iso, isotropic liquid. For
transition enthalpies see Table S1.
̅
Figure 2. Compound 1/8. (a−c) Texture as seen between crossed
polarizers in (a) the LamN phase at T = 105 °C; (b) the Colrec/c2mm
phase at T = 131 °C; and (c) the Cub/ Ia3d phase at T = 140 °C. (d−
̅
honeycomb−Lam−axial bundle phase sequence can then be
compared to the lyotropic sequence, direct hexagonal−
lamellar−inverse hexagonal, as the hydrophobic fraction
increases (water fraction decreases). As is well known, in the
lyotropic range between lamellar and hexagonal phases, several
“intermediate” phases may appear, of which the bicontinuous
cubic (Cubbi) phases are best known.14 It is tempting to ask
whether an equivalent intermediate phase can be found also in
the thermotropic bolapolyphiles. Several examples of the
double gyroid Cubbi phase have been found in thermotropic
LCs, mostly in the “polycatenar” rod-like compounds bearing
between one and three terminally attached chains.15−18 In this
Communication we indeed report, for the first time, two
thermotropic intermediate phases in laterally substituted
bolapolyphiles. While they share a number of geometrical
features with the intermediate phases in lyotropic and
polycatenar systems, as will be shown, the molecular
organization is quite different.
Compounds 1/n investigated in this work contain a
symmetric “swallow-tail” lateral group, with a −(CH2)11−
spacer and a perfluorinated end-segment, −CnF2n+1, in each
branch (see Table 1). Details of the synthesis are given in the
Supporting Information (SI). The compounds were inves-
tigated by polarized optical microscopy (POM), differential
scanning calorimetry (DSC), powder X-ray diffraction (XRD),
and grazing-incidence small-angle X-ray scattering (GISAXS)
on thin surface-aligned films on glass or silicon. For selected
compounds synchrotron radiation was used. Electron density
(ED) maps were reconstructed using powder XRD intensities.
For details of experiments and analysis, see SI.2
f) GISAXS patterns of (d) LamSm at T = 100 °C, (e) Colrec/c2mm at T
= 110 °C; and (f) Cub/ Ia3d at T = 140 °C. For indexing see Figure
̅
S13. (g−i) Histogrammatic electron density maps with added
schematic molecules for (g) Lam, (h) Colrec/c2mm, and (i) Cub/
Ia3d. For simplicity, discrete solid colors are allocated to three
̅
different ED ranges, purple for the highest, green for the middle, and
red for the lowest; they correspond roughly to chemical group colors
in Table 1. The continuous color scale is shown underneath panel (i);
intermediate density ranges (yellow, blue) were cut out in order to
visualize the cell interior.
As typically observed for T-shaped polyphiphiles with large
lateral substituents,1,6−8 all four compounds display lamellar
phases at lower temperatures. This is illustrated for compound
1/8 by the GISAXS pattern in Figure 2d. A series of strong
Bragg reflections with associated d-spacings of 4.48, 2.25, and
1.50 nm, ratio 1:1/2:1/3, are seen along the meridian. This
implies layers parallel to the (horizontal) substrate (see also
Figures S8a and S9a). At the same time, the sample shows
strong birefringence (Figure 2a; see also Figures S5 and S6),
consistent with the aromatic rods lying in the layer plane, as is
typical for the Lam phases in T-shaped bolapolyphiles and
unlike in conventional smectics. The observed lattice
parameters increase with increasing length of the lateral chains
(see Table 1).
From powder XRD intensities we obtained a 1D electron
density profile along the layer normal, shown as a 3D color map
in Figure 2g. As shown by the superimposed schematic
molecules, the aromatic double-layer (green) is surrounded on
both sides by a layer of aliphatic spacers (lowest density, red)
followed by a fluorocarbon double-layer (highest density,
purple; see the predicted electron densities for the aromatic,
aliphatic, and perfluoroalkyl regions in Table S13). There is
positional order of the aromatic mesogens within the aromatic
layer plane in the LamSm phase (T < 102 °C for 1/8, Table 1),
whereas in the LamN phase (T > 102 °C) the mesogens are
positionally disordered, giving a 2D nematic.
Phase transition temperatures and associated enthalpy values
are collated in Tables 1 and S1, respectively. All compounds
display broad regions of enantiontropic (reversible) LC phases.
As expected, in the LC temperature range the XRD patterns
show only diffuse wide-angle scattering (e.g., maximum at d =
0.56 nm for 1/10, see Figure S9f). Indexing of the small-angle
Bragg reflections (see Tables S2−S10 and Figures S10−S12)
led to the phase assignments, plane and space groups shown in
Table 1.
At higher temperatures all compounds except 1/4 transform
to a centered rectangular columnar phase, plane group c2mm, as
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branched networks contain the aromatic cores and glycerol
groups, while the lowest density red regions interposed
between the above two contain mainly the aliphatic spacers.
EDs and volume fractions calculated from molecular models are
listed in Table S13. The electron density histogram of the Cub/
Ia3d phase, supporting the chosen phase combination, is shown
̅
in Figure S15. We note that, while in most reported cases
maximum and minimum ED regions are the continuous
networks and the continuous G-surface, respectively,15,20 in the
current “tricontinuous” structure the minimum density
continuum is intercalated between the two.
Based on the lattice parameter, the distance between two
junctions of the network is 3.56 nm. This is twice the molecular
length, L = 1.80 nm, measured between the diol units.
Accordingly, we suggest that each network segment contains
two successive coaxial bundles of molecules (Figure 2i). Based
on the volumes of a molecule and the unit cell, there are ∼566
molecules per cell. As there are 24 network segments or 48
bundles per cell, each bundle contains 12 molecules on average,
half the number in a Colrec ribbon (compare Tables S11 and
S12).
Furthermore, the closest distance between the two infinite
networks is a×31/2/4 = 4.37 nm, half that value (2.2 nm) being
the distance from the network to the minimal surface. This is
slightly shorter than an extended lateral chain (∼2.50 nm), so
there is no problem for the chains to reach the minimal surface
and fill the space of the cubic cell.
As can be seen in Figure 3a, all cubic domains lie with a
{211} plane facing the substrate; no other orientation is
observed. This is explained in Figure 3b: more than half of the
network segments can be made to lie in a {211} plane with
little distortion. Thus, cropping the network along a {211}
plane requires the least distortion and the lowest number of
“broken bonds”, while allowing most of the polar glycerol-based
hydrogen-bonding clusters to stay in contact with the oxidized
Si surface. Incidentally, it is no coincidence that the contact
plane, a densely packed plane with high-amplitude ED
modulation along its normal, is also the strongest diffracting
plane (see Figure 3a), as often found in such systems.21,22
Referring to the scheme in Figure 1, we can now insert two
new phases between the Lam (c) and axial bundle Colh phases
(d): the Colrec/c2mm ribbon phase (Figure 2h) and what we
Figure 3. (a) Experimental and simulated GISAXS patterns of the Ia3d
̅
phase of 1/8. Spots of different colors belong to different reflection
groups (see legend). Simulated spots within a group are generated by
permutation of {hkl} indices while keeping one of the {211} planes
horizontal. Solid circles, observed; open circles, hidden below horizon.
(b) Model of the infinite networks in an Ia3d unit cell intersected by
̅
horizontal (211) planes.
shown by the additional hk diffraction spots on “half” layer lines
(Figure 2e; also Figures S8c and S9c). The indices,
experimental and calculated d-spacings, diffraction intensities,
and phase angles of the Colrec/c2mm phase of compound 1/8
are listed in Table S6. In the ED map in Figure 2h,
reconstructed using these intensities and phases, it is evident
that the continuous aromatic layers of the Lam phase break up
into ribbons, while the fluoroalkyl layers become slightly wavy
(see also maps in Figure S14). The Lam−Colrec transition is
believed to be the result of the thermal expansion of the side
chains requiring a positive mean curvature of the aromatic−
aliphatic interface. The use of POM with λ-plate confirmed that
the rod-like cores are parallel to the ribbon axis (see Figure
S5c). According to volume calculation, a flat ribbon in 1/8
contains a bundle of 24 molecules in cross section (Table S11).
The orientation of the director remains mainly unchanged
upon the transition, as seen from optical micrographs in Figure
2a,b; only the smoothly curving director field of the LamN
phase gives way to sharp breaks at grain boundaries of the
Colrec phase, indicating a greatly increased bend modulus.
Unusually for a transition on heating, the orientational order
parameter increases in the Colrec phase, as judged by the change
in color from yellow-green to orange (Figures 2a,b and
explanation in SI). This can be understood as the highly
oriented Colrec ribbons acting as nanochannels, confining the
mesogenic cores within to lie along the channel axis.
may call the “skeletal” bicontinuous gyroid Cub/Ia3d phase
̅
At still higher temperatures, compounds 1/8 and 1/10
transform to an optically isotropic LC phase (Figure 2c). The
small-angle powder diffraction patterns show a number of sharp
reflections (Figures S11b and S12c), their reciprocal spacings
(Figure 2i). Ribbon23 and Cubbi phases14 are two alternative
solutions when the required interface curvature is too high for
flat layers and too low for circular columns (Colh).19 However,
instances of the two phases occurring successively in the same
system are quite rare. As shown above, the number of
molecules side-by-side in a Colrec bundle (24) is twice that in
a Cubbi bundle (12). One can ask why a larger Cubbi structure,
with thicker bundles containing 24 molecules, did not form
instead of the Colrec phase. The likely answer is that all
molecules must reside at the surface of the bundle to allow their
side chain access to the non-aromatic continuum. For circular
bundles with 24 molecules this is impossible, hence the flat
bundles of the Colrec phase. At higher temperatures the
necessary mean curvature increases, stabilizing thinner bundles
and allowing them to be closer to circular, as required by the
Cubbi geometry.
being in the ratio 61/2:81/2:141/2:161/2:201/2:221/2:241/2:261/2
,
conforming to a cubic phase with Ia3d symmetry. The GISAXS
̅
pattern of a highly aligned film of 1/8 is shown in Figures 2f
and 3a. In Figure 3a it is juxtaposed against a simulated pattern
based on fiber-type orientation, with [211], as the unique axis,
perpendicular to the substrate. Spots with different colors
represent different {hkl} reflection groups. Thirteen layer lines
of fiber diffraction are visible. The simulated and experimental
patterns are in excellent agreement.
The reconstructed ED map of the Cub/Ia3d phase of
̅
compound 1/8, which was obtained using the corrected
diffraction intensities and structure factor phase angles (0 or π)
listed in Table S7, is shown in Figure 2i. The high-density
purple regions contain the fluorinated segments of the
semiperfluorinated lateral chains and are centered around the
infinite periodic minimal gyroid (G) surface, characteristic of
this “double gyroid” structure.19 The green medium-density
Finally, the current Cub/Ia3d phase is the first example
̅
where the mesogen long axis lies along the axis of the network
segments, creating a relatively rigid skeleton with flexible H-
bonded hinges. Such an arrangement imposes the extra
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (No. 21374086), the NSF-EPSRC PIRE
program RENEW (EP/K034308/1), DFG (FG 1145, TS 39/
21-2), and the Leverhulme Foundation (RPG-2012-804). For
help with synchrotron experiments we thank Drs. Nick Terrill
and Jen Hiller (I22) at Diamond, Drs. Xiuhong Li and Feng
Tian (BL16B1) at SSRF, and Drs. Oier Bikondoa, Simon
Brown, and Paul Thompson (BM28-XMaS) at ESRF.
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedures with complete spectral and structural
analysis. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
feng.liu@mail.xjtu.edu.cn
(24) Ungar, G.; Percec, V. Presented at the 14th International Liquid
Crystal Conference, Pisa, Italy, 1992.
(25) Tschierske, C. Angew. Chem., Int. Ed. 2013, 52, 8828.
■
carsten.tschierske@chemie.uni-halle.de
g.ungar@sheffield.ac.uk
Author Contributions
§F.L. and M.P. contributed equally.
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