Direct mapping of local director field of nematic Liquid crystals at the nanoscale

Article abstract of DOI:10.1073/pnas.1513348112

Liquid crystals (LCs), owing to their anisotropy in molecular ordering, are of wide interest in both the display industry and soft matter as a route to more sophisticated optical objects, to direct phase separation, and to facilitate colloidal assemblies.

Full text of DOI:10.1073/pnas.1513348112

Direct mapping of local director field of nematic liquid
crystals at the nanoscale
Yu Xiaa, Francesca Serraa,b,c, Randall D. Kamienb, Kathleen J. Stebec, and Shu Yanga,1
aDepartment of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104; bDepartment of Physics and Astronomy, University
of Pennsylvania, Philadelphia, PA 19104; and cDepartment of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104
Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved November 9, 2015 (received for review July 7, 2015)
Liquid crystals (LCs), owing to their anisotropy in molecular ordering,
are of wide interest in both the display industry and soft matter as a
route to more sophisticated optical objects, to direct phase separa-
tion, and to facilitate colloidal assemblies. However, it remains
challenging to directly probe the molecular-scale organization of
nonglassy nematic LC molecules without altering the LC directors.
We design and synthesize a new type of nematic liquid crystal mono-
mer (LCM) system with strong dipole–dipole interactions, resulting in
a stable nematic phase and strong homeotropic anchoring on silica
surfaces. Upon photopolymerization, the director field can be faith-
fully “locked,” allowing for direct visualization of the LC director field
and defect structures by scanning electron microscopy (SEM) in real
space with 100-nm resolution. Using this technique, we study the
nematic textures in more complex LC/colloidal systems and calculate
the extrapolation length of the LCM.
nematic textures in more complex LC/colloidal systems and esti-
mate the ratio between elastic and anchoring constants, i.e., the
extrapolation length, of LCMs in the nematic phase.
Results and Discussion
Visualization of topological defects has been achieved by dis-
persing polymer fibers in prealigned, low molecular weight NLC
(22, 23). However, the polymer fibers, fabricated in situ via po-
lymerization of LCMs, often phase separate from the nematic
host; thus maintaining the LC director field in a nonglassy ne-
matic host for direct visualization is thwarted. Moreover, the
fibers themselves can perturb the nematic phase, inducing arti-
facts and spurious defects. Of course, to visualize the native di-
rector field responding to geometric surface cues we must use
molecules that can be aligned reliably at interfaces of different
topology, topography, and surface chemistry. Typically, the
alignment of LCMs is controlled via mechanical rubbing of a
polymer layer, use of a photo-alignment layer (21), mechanical
stretching, or application of magnetic fields (24–28). However,
these techniques are limited in their spatial resolution, especially
in comparison with the highly refined ability to impart chemical
anchoring on boundaries with complex topography and topology
at the micro- and nanoscales, and also require high uniformity
across the large sample areas. Although functional LCMs have
been used for decades in conjunction with LCPs as actuators and
sensors (24–29), it remains challenging to achieve faithful anchor-
ing control at the level of the old stalwart, 4-cyano-4′-pentylbiphenyl
(5CB). Meanwhile, slow relaxation of the bulk director field will lead
to spurious defects in LCMs (or polydomain configurations) as
shown in SI Appendix, Fig. S1 from widely used LCMs in the
nematic liquid crystal polymers dipole–dipole interactions topological
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defects photo-cross-linking nanoscale imaging
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biquitous as they are, it sometimes escapes our attention
that liquid crystals (LCs) are the original nanomaterial. The
U
manipulation of these nanometer-size molecules into coherent,
centimeter-scale structures is now routine. Although we have
become adept at deducing textures indirectly through optical mi-
croscopies (1–5), probing the molecular-scale organization requires
either a glassy material or rapid cooling of samples, providing
metastable states that can be directly visualized through scanning
electron microscopy (SEM), transmission electron microscopy
(TEM), and atomic force microscopy (AFM) (6–8). Although
these techniques are effective to study a variety of more complex
LC phases, including smectic LCs (6, 7, 9), cholesteric and blue
phases (10), and biological LC polymers (11, 12), nonglassy, low
molecular weight nematic LCs (NLCs) reorient during fast freez-
ing. Polymer nematics can be quenched into metastable states but
organization of static configurations through surface alignment is
difficult. Flow alignment can be used but that often precludes
complex, molecular scale patterning of the boundary conditions,
essential for controlling and manipulating topological defects (13,
14). Although an undesired nuisance in NLC displays, topological
defects mediate many of the rich interactions between colloids in
NLCs (15). The defects, bearing quantized topological charge,
resemble the defects in superconductors (16), soft ferromagnets
(17), and even cosmic strings and monopoles (18). The fluidity of
the phase and the weakly first-order nematic–isotropic phase
transition allow for direct visualization of the creation/annihilation
of defects via the Kibble–Zurek mechanism (19). Further, defects
in a flat sheet of nematic gel or glass can be used to bend or twist
local directors, inducing 3D shapes (20, 21). It is therefore critical
to be able to characterize the defect structures at the nanoscale in a
variety of settings. Here, we report the design and synthesis of a
nematic liquid crystal monomer (LCM) system that has strong
homeotropic anchoring on silica surfaces and does not reorient its
director field during polymerization. Thus, the optical signatures
remain unchanged in the liquid crystal polymers (LCPs), allowing
for direct visualization of the defect structures by SEM in real
space with 100-nm resolution. Using this technique, we study the
Significance
It has been challenging to directly probe the molecular-scale
organization of nonglassy nematic liquid crystal (LC) molecules
without altering the LC directors. Here, we design and syn-
thesize a new type of stable nematic liquid crystal monomer
(LCM) system with strong dipole–dipole interactions. The new
LCMs can achieve faithful anchoring and alignment control at
various boundaries, analogous to that of small molecule LCs.
Upon photo-cross-linking, the orientational order of mesogens
is effectively and faithfully locked, allowing for direct visuali-
zation of the LC director field and defect structures by scanning
electron microscopy (SEM) with 100-nm resolution. Further, we
use SEM imaging to calculate the extrapolation length of the
LCM for planar and homeotropic anchoring.
Author contributions: Y.X. and S.Y. designed research; F.S. helped with microscopy mea-
surements; Y.X. performed research; Y.X., F.S., R.D.K., K.J.S., and S.Y. analyzed data; and
Y.X., F.S., R.D.K., K.J.S., and S.Y. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
1To whom correspondence should be addressed. Email: shuyang@seas.upenn.edu.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1513348112/-/DCSupplemental.
www.pnas.org/cgi/doi/10.1073/pnas.1513348112
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literature; this inherent obstacle becomes increasingly obvious
under complex confinement.
SOCl2
ClOC
COCl
HOOC
COOH
1
DMF, 75o
C
To address this, we specially designed and synthesized a series of
LCMs (LCM_X1, LCM_X2, and LCM_X3) (Figs. 1 and 2) with
both strong surface anchoring and highly stable nematic phases.
These LCMs suppress spurious defects, leaving only those required
energetically and topologically. The monomers all have the same
aromatic ester-based mesogenic group, both common and inex-
pensive to synthesize, with an ortho-substituted nitro group but
different terminal groups, epoxy and alkene, both of which are
readily photo-cross-linkable at ambient conditions via pho-
toacids and “click” chemistry, respectively. We confirmed the
NO2
NO2
c
b
a
d
e
HO
COOH
HOOC
OOC
COO
COOH
2
3
KOH aq. solution, 0o
C
NO2
SOCl2
DMF, 75o
ClOC
OOC
COO
b
COCl
C
NO2
c
e
d
Glycidol, DMAP
1
O
OOC
OOC
COO
COO
3
3
chemical structures via H-NMR (SI Appendix, Fig. S2). Similar
DCM, 0o
C
O
a
LCMs terminated with epoxy or alkene groups have been
reported with nematic phases in the literature (30) although
they typically have very high phase transition temperatures,
making it difficult to maintain the director field during poly-
merization due to the relatively low viscosity of the LCMs in the
nematic phase. In addition, their anchoring properties have not
been well characterized.
NO2
4 (LCM_X1)
c'
b'
e'
d'
Allyl alcohol, DMAP
DCM, 0o
OOC
OOC
COO
COO
C
a'
5 (LCM_X2)
NO2
c''
b''
e''
d''
MCPBA
DCM, 48 h
O
OOC
OOC
COO
COO
5
We introduced a pendant nitro group on the aromatic ring because
it has been suggested that (i) LCs with a nitro group at the ortho
position to a linkage ester group typically form a nematic phase with a
relatively low phase transition temperature (31) and, more impor-
tantly, (ii) the introduction of a polar (e.g., nitro) or polarizable group
into the chemical structure often results in an increase of molecular
polarity, affecting molecular packing and, in turn, the phase stability
of LCs (31). In particular, aromatic ester-based mesogens could
achieve a stable nematic phase through strong intermolecular dipole–
dipole interactions between carbonyl groups of adjacent molecules,
an effect that increases with the strength of the molecular interaction
(32, 33). In our system, we expect strong dipole–dipole interactions
between carbonyl groups (dipole moment μ = 2.4D) (34) that are
evenly distributed along the molecule and a more polar nitro group
(μ = 4.01D) (34) in the middle of the molecule. Because each nitro
group bonds randomly to one of the four sites from the adjacent LC
molecule, crystallization is suppressed, as seen in analogous systems
(30, 33, 35), leading to a highly stable nematic phase (SI Appendix,
Fig. S3 and Movie S1) with strong surface anchoring properties for all
LCMs we synthesized (Fig. 3 A–C and SI Appendix, Fig. S4). Here,
we focused our study on LCM_X1 both because it has a large ne-
matic window (>100 K) with the nematic phase starting slightly above
its glassy transition temperature (∼10 °C) and because the terminal
epoxy groups could be rapidly and locally cross-linked by photoacids
without large volume change—effectively locking the orientational
order of mesogens for later SEM imaging.
a''
NO2
6 (LCM_X3)
Fig. 2. Schematic illustrations of the synthesis of the nematic liquid crystal
monomers. 1H-NMR spectra of protons labeled in each monomer can be
found in SI Appendix, Fig. S2.
Materials and Methods) to study the alignment of newly synthe-
sized LCMs in comparison with small molecule NLC, 5CB. As
seen from the polarized optical microscopy (POM) and bright-
field (BF) images shown in Fig. 3 A–C, both molecules are
readily aligned with nearly identical director configurations on
all patterned substrates despite the difference in surface treat-
ments to align LCM_X1 vs. 5CB. For LCM_X1, homeotropic
anchoring was achieved on silica-coated surfaces whereas planar
anchoring was achieved on polyimide-treated surfaces (SI Ap-
pendix, Fig. S6). This is in sharp contrast to the behavior of
commonly used LCMs, RM257 and LCM_AZO, which exhibit
polydomains on the same substrates regardless of surface
treatment (SI Appendix, Fig. S1). Note that the anchoring be-
havior of LCM_X1 shown in SI Appendix, Fig. S6 is visualized
from SEM images of its LCP. As we discuss later, the LC di-
rector is maintained from LCM to LCP.
In cylindrical pores with homeotropic anchoring on all bound-
aries, the nematic director either forms a bulk point defect with
integer topological charge ( 1) in the center or “escapes into the
third dimension” (36, 37) to separate two half-defects on the
boundaries. Likewise, in the pillar arrays, there could be a bulk
disclination ring surrounding the pillars (15) or an escape config-
uration where the defects are all concentrated next to the pillar
edges. However, neither POM nor BF microscopy can distinguish
these two modalities without further investigation of the director
field through the whole sample or through the use of a variety of
state-of-the-art tools such as confocal polarized microscopy.
To access the director information in these geometries,
we cross-linked the LCMs through cationic polymerization of ep-
oxy, a common reaction in negative-tone photoresists (38, 39).
Compared with acrylate and methacrylate groups that are often
used in radical polymerization of LCMs, epoxides have several
advantages that are key to our LCM design: (i) They are in-
sensitive to ambient oxygen. (ii) They can be cross-linked effi-
ciently through chemically amplified ring-opening reactions; each
photogenerated acid can initiate hundreds of reactions locally,
rendering fast polymerization without large volume shrinkage
seen in (meth)acrylate polymers. (iii) The bulky aromatic ester
mesogens limit acid diffusion at room temperature; therefore,
We used patterned substrates that were well characterized in
our laboratories, including membranes with cylindrical pores,
micropillar arrays, and square-shaped one-dimensional (1D)
microchannels with strong homeotropic anchoring (details in
A
B
Fig. 1. Schematic illustrations of (A) the chemical structures of various liquid
crystal monomers, LCM_X1, LCM_X2, and LCM_X3 and (B) random intermolecular
dipole–dipole interactions between nitro and carbonyl groups. Different colors
represent different functional groups: red for nitro and blue for carbonyl.
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Xia et al.

nanofiber-like fracture structures, the footprints of the director
field. We note that the polymer itself does not form fibers.
Rather, the oriented structures observed in SEM are a conse-
quence of the anisotropic mechanical properties of LCPs, in
which the bulk elastic modulus is usually much smaller per-
pendicular to the director (21, 42) and in which fractures occur
along surfaces parallel to the director field (43, 44). The same
type of fracture pattern can be observed in a nematic polymer
that has some similarity with our LCPs (42). Following the
fracture structures, we directly mapped the LC alignment with
100-nm resolution as illustrated by the red dotted line in Fig. 4
A–C. From Fig. 4A we determine the angle of molecular
alignment at the surface to be 90° inside a wide channel; likewise,
we can see that homeotropic anchoring breaks down when the
director field is tightly confined as shown in Fig. 4 B and C. We
can now directly visualize the defect structure in pores, pillar
arrays, and 1D channels and observe that the director most often
adopts an escaped configuration, avoiding defects in the bulk. By
contrast, in POM the observed point- and line-like structures
could just as well be the sign of a distorted director field or a
defect. Further, from SEM images we can estimate the extrap-
olation length of the LCM (45) for planar and homeotropic
anchoring (details in SI Appendix). We can estimate the elastic
constant, which is on the order of a few pico-Newtons, in line
with the elastic moduli of other LCs.
Armed with our careful study of the local director field of LCs
on patterned surfaces, we turned to more complex director field
structures: point defects and line defects created by colloids with
homeotropic anchoring suspended in NLC. POM images in Fig. 5
depict typical pictures of hedgehog and Saturn ring defects sur-
rounding silica colloids. Although numerous studies have focused
on these defects in LC systems with varying colloidal size, geom-
etry, and topology (46–49), direct visualization of the director
structure is difficult, as is measuring the precise position of the
induced topological defects. With fracture and SEM, here, we can
determine these features with 100-nm accuracy. Consider, for
example, the hedgehog in Fig. 5A; the defect can be seen as clearly
located 2 μm above the colloid. In the same sample, we also found
Saturn ring line defects around other silica colloids—Fig. 5B
shows a typical defect with a slightly tilted director field. This slight
tilting (tilting angle <15°) could not be detected by POM (Fig. 5B,
Inset); however, it can be easily read from SEM.
Fig. 3. POM and BF images of 5CB and LCM_X1. (A–C) POM and BF images of
5CB (Top panels) and LCM_X1 (Bottom panels) in (A) porous membranes (di-
ameter, 10 μm; pitch, 15 μm; depth, 20 μm), (B) pillar arrays (diameter,
10 μm; pitch, 20 μm; height, 19 μm), and (C) square channels (width, 10 μm; pitch,
40 μm; depth, 20 μm) with homeotropic anchoring imposed at all surfaces. (A,
Insets) BF image of LCs in pores where the dark dots at the center show
possible defects; (B, Insets) BF image with black circles showing possible bulk
line defects circumscribing the pillars; (C, Insets) BF image of a single ridge
where the gray color indicates a distorted director field but no defect; (D and
E) POM images of the schlieren texture of LCM_X1 before (D) and after (E) UV
curing. The nearly identical nematic schlieren textures indicate that the LC
director field is well maintained during polymerization. The slight change in
color can be attributed to the difference in UV-vis absorption of monomer vs.
polymer (SI Appendix, Fig. S5). After polymerization, the LCM-X1 appeared
somewhat yellowish. (Scale bars, 20 μm.)
We can use this technique to study the nematic textures in more
complex LC systems. For instance, in Fig. 6 we show a LC cell
treated for homeotropic alignment that we expect to image as
black in POM. Instead, we see the appearance of blue regions,
suggesting planar alignment, separated from the dark regions by
disclination lines. Although such disclination lines are metastable
in a neat NLC and relax very quickly, they can have a much longer
lifetime when they are stabilized by colloids trapped at the dis-
clinations. To observe the structures in Fig. 6 B–E with SEM, we
again cooled down the sample and then cross-linked the LCM_X1.
In Fig. 6B a disclination line generated at the boundary of the
homeotropic and planar regions is shown. The disclination line can
be identified by the fracture structure below and above the fracture
plane in SEM as shown in Fig. 6B because of the different me-
chanical properties of the defect. Similar line defects can also be
found in Fig. 6C and SI Appendix, Fig. S9, where silica colloids are
trapped at the boundary. From the SEM images, we can observe
the merging of the line defect in the bulk of LC and the Saturn ring
surrounding the colloid. Further, it should be noted that bulk
disclinations can also be stabilized by pinning to boundaries of the
LC cell, as illustrated in Fig. 6D. The direct, clear visualization of
the bending of the director field is useful to study the bending
energy of LCs in various settings. Fig. 6E shows the LC orientation
inside a planar-like region stabilized by silica colloids. The escaped
configuration is surprising because it requires a large bending of
the director field from the homeotropic boundaries to the center of
photopolymerization occurs locally. Together with the strong
dipole–dipole intermolecular interactions, our LC molecules can
lock in position effectively without altering the field direction
during polymerization. Closely related are earlier studies of
cationic photopolymerization of LC diepoxides (40, 41), which
show that the NLC phase is maintained in the densely cross-
linked network—in contrast to that from radical polymerization
of diacrylates. It was noted, however, that unintentional heating
by the high-intensity UV light could realign the mesogens, thus
increasing the order parameter after curing (41). To prepare
even cleaner data, we further suppressed reorientation during
cross-linking by performing the UV curing at room temperature
(∼25 °C) with a low UV light intensity (2 mW/cm2), keeping the
viscous timescales longer than the polymerization timescale
(details in Materials and Methods).
From the birefringence of the material (Materials and
Methods) and the POM images of samples before and after
polymerization (Fig. 3 D and E and SI Appendix, Fig. S7), we
confirmed that on the micrometer scale the director field was
preserved during polymerization. Once polymerized, we could
use SEM to directly visualize the nematic texture with nano-
scale resolution of the director configuration in various cross-
sections obtained by fracturing the sample at room tempera-
ture. As shown in Fig. 4 A–C and SI Appendix, Figs. S6 and S8,
the local director field is visible under SEM, represented by the
Xia et al.
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Synthesis of Liquid Crystal Monomers (Chemical Structures in Fig. 2 and 1H-NMR
Spectra in SI Appendix, Fig. S2).
2-Nitro terephthaloyl dichloride(1). A total of 4.22 g 2-nitroterephthalic acid
(20 mmol) was mixed with 20 mL thionyl chloride under stirring. Two drops of
DMF were added as catalyst. The mixture was heated to 75 °C and refluxed until
no bubbles were generated from the solution. The reaction mixture was then
cooled down to room temperature, and excess thionyl chloride was removed
in vacuo. The crude 2-nitro terephthaloyl dichloride was obtained as an or-
ange oil and used in the next step without further purification.
2-Nitro, 1,4-benzenedicarboxylic acid, 1,4-bis(4-carboxyphenyl) ester(2). A total of
3.45 g 4-hydroxybenzoic acid (25 mmol) was gradually dissolved into 2.8 g KOH
(50 mmol) aqueous solution (100 mL), and the solution was cooled in an ice
water bath. A total of 2.48 g of (1) (10 mmol) was dissolved in 15 mL ethyl
acetate and added dropwise into the above prepared aqueous solution under
vigorous stirring. After 30 min, the reaction was stopped and the mixture was
neutralized by diluted HCl (5 vol%) aqueous solution to pH 7. The resulting
precipitate was then filtered off by vacuum filtration and washed with ethanol
to obtain 4.28 g of product (2) (9.5 mmol, 95% yield) as a white solid. 1H-NMR
(360 MHz, DMSO-d6): δ(ppm) = 7.41 (m, 4H, ArH of d), 8.02 (m, 4H, ArH of e),
8.36 (d, 1H, ArH of c), 8.65 (d, 1H, ArH of b), 8.79 (s, 1H, ArH of a).
2-Nitro, 1,4-benzenedicarboxylic acid, 1,4-bis[4-(chlorocarbonyl)phenyl] ester(3). A
total of 4.06 g of (2) (9 mmol) was mixed with 15 mL thionyl chloride under
stirring. Two drops of DMF were added as catalyst. The mixture was heated to
75 °C and refluxed until no bubbles were generated from the solution. The
reaction mixture was then cooled down to room temperature, and the excess
thionyl chloride was removed in vacuo. The crude product (3) was obtained as
a white solid and used in the next step without further purification.
2-Nitro, 1,4-benzenedicarboxylic acid, 1,4-bis[4-[(2-oxiranylmethoxy)carbonyl]phenyl]
ester(4, LCM_X1). A total of 1.3 g ( )-glycidol (17.6 mmol) and 3.90 g of (3)
(8 mmol) were dissolved in 30 mL DCM, and the solution was cooled in an ice
water bath at 0 °C. A total of 2.15 g DMAP (17.6 mmol) in 20 mL DCM was
added dropwise into the above prepared solution under stirring. The re-
action mixture was then gradually warmed up to room temperature and
kept for another 6 h. The completion of the reaction was monitored with
TLC until the reactant (3) was completely consumed. The mixture was then
filtered through a celite pad, and the solvent was removed in vacuo. The
resulting oil was purified by column chromatography (silica gel; eluent:
DCM, followed by ethyl acetate: DCM = 1:15 vol/vol; Rf ∼ 0.4) to obtain
3.06 g (4) (68% yield) as a white solid. 1H-NMR (360 MHz, CDCl3): δ(ppm) =
2.75 (dd, 2H, −(O)COCH2CHCH2O), 2.93 (dd, 2H, −(O)COCH2CHCH2O), 3.36
(m, 2H, −(O)COCH2CHCH2O), 4.20 (dd, 2H, −(O)COCH2CHCH2O), 4.71 (dd, 2H,
−(O)COCH2CHCH2O), 7.38 (m, 4H, ArH of d), 8.04 (d, 1H, ArH of c), 8.20
(m, 4H, ArH of e), 8.59 (d, 1H, ArH of b), 8.88 (s, 1H, ArH of a).
2-Nitro, 1,4-benzenedicarboxylic acid, 1,4-bis[4-[(2-propen-1-yloxy)carbonyl]phenyl]
ester(5, LCM_X2). A total of 1.02 g allyl alcohol (17.6 mmol) and 3.90 g of (3)
(8 mmol) were dissolved in 30 mL DCM, and the solution was cooled in an ice
water bath at 0 °C. A total of 2.15 g DMAP (17.6 mmol) in 20 mL DCM was
added dropwise into the above prepared solution under stirring. The reaction
mixture was then gradually warmed up to room temperature and kept for
another 6 h. The completion of the reaction was monitored with TLC until the
reactant (3) was completely consumed. The mixture was then filtered through a
celite pad, and the solvent was removed in vacuo. The resulting oil was purified
by column chromatography (silica gel; eluent: DCM, followed by ethyl acetate:
DCM = 1:30 vol/vol) to obtain 2.83 g (5) (66.5% yield) as a white solid. 1H-NMR
(360MHz, CDCl3): δ(ppm) = 4.87 (dd, 4H, −(O)COCH2CHCH2), 5.32 (d, 2H,
Fig. 4. (A–C) SEM images show the fracture structure of LCP after poly-
merization in a 1D microchannel (A), in a pore (B), and between two pillars
(C). Escaping director field of LC can be observed from the orientation of the
fracture structures in all three structures. The red dotted lines in A–C rep-
resent local director field of LCM mirrored in the other half of images. (Scale
bars: A, 5 μm; B and C, 2 μm.)
the LC cell, resulting in a high elastic energy in the bulk of this 2D
LC thin film (∼8 μm). A clarifying example of this behavior can be
found in SI Appendix, where we show a SEM image of a small
escape region whose boundaries could be both visualized in the
same picture (SI Appendix, Fig. S10). The SEM images provide us a
detailed director configuration in this small region, where the
planar region starts at the surface of a colloid with an associated
defect with charge −1, escapes in the direction determined by the
defect structure around the colloid, and finally ends at another
colloid surface. These SEM studies thus provide us the capability to
fully explore LC anchoring behaviors inside LC cells featuring
complex topology and also to “lock” and observe metastable states.
In conclusion, we designed and synthesized a new LCM system
with strong dipole–dipole interactions, resulting in a stable ne-
matic phase and precise control of molecular anchoring and
alignment on different boundary conditions. The director field
can be faithfully “locked” by photo-cross-linking, allowing for
direct mapping of the LC director field and defect structures by
SEM. In turn, we can calculate the extrapolation length and
estimate the elastic and anchoring constants of LCs (see SI
Appendix for detailed discussion and SI Appendix, Figs. S11 and
S12). This molecular design strategy can be extended to other
substituents (e.g., to induce hydrogen bonding) and bridging
groups that connect the mesogen and epoxy group to fine-tune
the mesomorphic properties. Moreover, our LC monomers are
low cost; e.g., LCM_X1 can be produced at a cost of ∼10% that
of 5CB, but with equal flexibility to be aligned on various
boundary conditions. This newfound ability provides us with
tools to enhance our understanding of anchoring, defects, and
elasticity of short-molecule nematics, which will enable the fur-
ther control of bulk structures via nanoscale patterning of the
substrate and surface chemistry. We anticipate that by directly
photopatterning a 2D sheet of NLC elastomer or glass with
embedded defects and elasticity, followed by actuation by heat or
light, we could induce folding into 3D.
Materials and Methods
Materials. All chemicals were used without further purification. Dime-
thylformamide (DMF), dichloromethane (DCM), potassium hydroxide (KOH),
sodium thiosulfate pentahydrate, and hydrochloric acid (HCl) were purchased
from Fisher Scientific. Thionyl chloride (SOCl2), 4-dimethylaminopyridine (DMAP),
2-nitroterephthalic acid, 4-hydroxybenzoic acid, metachloroperoxy-benzoic acid
(MCPBA), (3-Aminopropyl) triethoxysilane (APTES), allyl alcohol and ( )-glycidol
were purchased from Sigma Aldrich. 4-Cyano-4′-pentylbiphenyl (5CB) was
purchased from Kingston Chemicals Limited. Polyimide (Durimide 32A) was
purchased from Arch Chemicals. Liquid crystal monomers, 4-ethoxy-4′-(6-
acryloyloxyhexyloxy) azobenzene (LCM_AZO), were kindly provided by the US
Air Force Research Laboratory, and RM257 was obtained from Merck.
Fig. 5. (A and B) SEM images of silica colloids suspended in LCP where either a
point defect (A) or a line defect (B) was formed to screen the charge of the colloid.
(A and B, Insets) POM images of point (A) and line (B) defects circumscribing silica
colloids. The director field of the LC is represented by the red dotted line and the
red crosses show the positions of defects. (Scale bars: 2 μm.)
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(CVD) process according to the literature (52). In brief, patterned sub-
strates kept in a desiccator under vacuum were first exposed to chem-
ical vapors of silicon tetrachloride (SiCl4, 0.2 mL) for 10–15 min. Then
the samples were exposed to a humidity chamber (humidity ∼90%) for
10 min, followed by immersion in a pyridine aqueous solution (3 vol%)
for 5–10 min. The final substrates were obtained by washing with
ethanol and deionized (DI) water, respectively, three times, followed
by drying by an air gun and baking at 100 °C in a convection oven for
15 min to 1 h.
ii) Strong homeotropic anchoring of LCM_X1 on micropillar arrays. To
achieve high homeotropic anchoring energy on micropillar arrays, we
first treated pillars based on step (i), followed by functionalization of
the SiO2 surface with APTES. Due to the Michael-addition reactions
between the amino groups in APTES and epoxy groups in LCM_X1,
the newly formed surface anchored LCM_X1, where the mesogenic
groups were orientated perpendicularly to the interface. As a result,
the interaction between the interface and LCM molecules was substan-
tially increased, showing stronger homeotropic anchoring than the
SiO2-coated surface. In detail, the sample consisting of the micropillar
array was immersed in a solution of 1 vol% APTES in an ethanol/water
mixture (90:10 vol/vol) for ∼10 min and then rinsed with ethanol and DI
water three times and dried by air gun. Finally, the sample was baked in
a convection oven at 100 °C for 15 min to 1 h.
iii) Planar anchoring. To create planar anchoring for LCMs, a glass substrate
was spin coated with a thin layer of polyimide from its xylene solution
at 5,000 rpm (on spin coater WS-650Hzb-23NPP-UD-3; Laurell) for 30 s,
followed by baking at 130 °C for 20 min. A uniform planar anchoring
sample was obtained by rubbing the polyimide-coated substrate with
a velvet cloth.
Fig. 6. Metastable configurations. (A) POM image of silica colloids suspended
in LCM resulting in coexistence of homeotropic and planar-like regions, where
the metastable planar regions (blue color) are stabilized by silica colloids. (B, C,
and E) SEM images characterizing the local director field at different positions
in A as indicated by the red dotted squares. (B) SEM image taken at the boundary
between homeotropic and planar regions of LC, where a bright line in the middle
of the image is shown, indicating the presence of a disclination line. (C) SEM
image of a silica colloid sitting at the boundary. The bright line in the image shows
a line defect in the bulk of LC that merged with the Saturn ring defect encircling
the colloid. The silica colloid was trapped in the middle of the LC cell, and sank
slightly downward. (D) SEM image of fiber-like structure that shows a bulk dis-
clination line pinned to the bottom surface, as indicated by the bending white
line. (E) SEM image of local LC director field inside an escaped region. Horizontally
aligned fiber-like fractures indicating planar alignment of LC director field were
found in the middle of the image, where the bending fiber-like fractures from top
and bottom boundaries merged. (Scale bars: A, 20 μm; B–E, 3 μm.)
Liquid crystal cell preparation.
i) On patterned substrates. Approximately 20–40 μL LCM_X1/DCM solution
was placed on the patterned substrate, and solvent DCM was evaporated
at 130 °C for at least 10 min. Then the LCM_X1 liquid was sandwiched
between the substrate and another cover glass (treat if needed) at 130 °C.
The sample was then cooled down to the desired temperature (cooling
rate not critical here) to align LC.
ii) With silica colloids. Five-micrometer diameter silica colloids (Sigma Aldrich)
were dispersed in a LCM_X1/DCM solution (note that a high concentration
of ∼20 wt% was preferred), and the mixture was sonicated for at least
15 min to obtain a homogeneous suspension. To prepare the LC cell with
silica colloids, several drops (∼20–40 μL) of the suspension were placed on a
clean glass slide, and the solvent was evaporated at 130 °C for at least 10 min.
Then another precleaned glass slide was placed on top to make a LC cell.
The thickness of the LC cell was roughly controlled as ∼10 μm. The sample
was then cooled down from 130 °C to the desired temperature (cooling
rate not critical here) to align the LC.
−(O)COCH2CHCH2), 5.44 (d, 2H, −(O)COCH2CHCH2), 6.06 (m, 2H,
−(O)COCH2CHCH2), 7.37 (m, 4H, ArH of d′), 8.04 (d, 1H, ArH of c′), 8.20 (m, 4H,
ArH of e′), 8.60 (d, 1H, ArH of b′), 8.88 (s, 1H, ArH of a′).
4-(4-(((Oxiran-2-yl)methoxy)carbonyl)phenyl) 1-(4-((allyloxy)carbonyl)phenyl) 2-nitro-
benzene -1,4-dioate (6, LCM_X3). A total of 6.8 g (12.8 mmol) of (5) was dissolved in
30 mL DCM at 0 °C. A total of 3.31 g MCPBA (20 mmol) was gradually added to the
solution. The reaction mixture was then warmed up to room temperature and
allowed to stir for additional 48 h. After reaction, the white precipitate was
filtered off and the resulting solution was washed twice with sodium thio-
sulfate pentahydrate aqueous solution and then washed twice with brine. The
solution was then dried with MgSO4 and the solvent was removed in vacuo.
The crude solid was purified by column chromatography (silica gel; eluent,
DCM, followed by ethyl acetate: DCM = 1:30 vol/vol) to obtain 2.1g (6) (30%
yield). 1H-NMR (360MHz, CDCl3): δ(ppm) = 2.75 (dd, 1H, −(O)COCH2CHCH2O),
2.93 (dd, 1H, −(O)COCH2CHCH2O), 3.36 (m, 1H, −(O)COCH2CHCH2O), 4.20 (dd,
1H, −(O)COCH2CHCH2O), 4.71 (dd, 1H, −(O)COCH2CHCH2O), 4.87 (dd, 2H,
−(O)COCH2CHCH2), 5.32 (d, 1H, −(O)COCH2CHCH2), 5.44 (d, 1H, −(O)COCH2CHCH2),
6.06 (m, 1H, −(O)COCH2CHCH2),7.38 (m, 4H, ArH of d′′), 8.04 (d, 1H, ArH of c′′),
8.20 (m, 4H, ArH of e′′), 8.59 (d, 1H, ArH of b′′), 8.88 (s, 1H, ArH of a′′).
UV Cross-Linking. To prevent the reorientation of the directors of LCM_X1
during photopolymerization, UV curing was carefully performed in three
steps, including (i) samples were slowly cooled down at 1 °C/min to room tem-
perature (∼25 °C); (ii) samples were exposed to a low UV power (∼2 mW/cm2 at
365 nm, Hg lamp), overnight; and (iii) after UV exposure, samples were
slowly heated up to 100 °C at a ramping rate of 1 °C/min on a Mettler FP82
and FP90 thermo-system hot stage in ambient air, followed by baking at
120 °C for another 1–2 h to completely cure the epoxy groups.
Preparation of Liquid Crystal Monomer Solutions. A total of 200 mg liquid crystal
monomer (4, LCM_X1) and 4 mg Iragcure 261 (2 wt%; Ciba Specialty Chemicals) as
photoacid generator (PAG) were dissolved in 10 g DCM and kept in a cool and dark
place before use.
Characterization. Chemical structures of the synthesized chemicals were
confirmed with 1H-NMR performed on a Bruker Advance DMX 360 (360
MHz) spectrometer at 25 °C and analyzed with TOPSPIN software. Thermo
analysis of the synthesized LCMs was performed on a differential scanning
calorimetry (DSC) Q2000 (TA Instruments). Samples were heated and cooled
under nitrogen with a ramping rate of 10 °C/min for three cycles. Data
from the second cycle were reported. Liquid crystal phases and alignments
were observed under an Olympus BX61 motorized optical microscope
with crossed polarizers, using CellSens software. Liquid crystal polymer
samples were manually broken into pieces or cut with a razor blade. The
cross-section was coated with a 4-nm iridium layer for SEM. Imaging was
performed on a dual-beam FEI Strata DB 235 Focused Ion Beam (FIB)/SEM
instrument with a 5-kV electron beam.
Fabrication of Patterned Substrates. All of the patterned substrates were
fabricated by replica molding from commercially available epoxy (D.E.R. 354;
Dow Chemical) on glass slides, using poly(dimethylsiloxane) (PDMS) molds,
following the procedure reported in the literature (50).
Preparation of substrates with desired LC anchoring.
i) Homeotropic anchoring of LCs on flat substrates, porous membranes,
and square channels. The anchoring type of the LC (homeotropic or
planar) largely depends on the surface energy of the interface (51).
For many types of LCMs, hydrophilic surfaces with high surface energy
usually give homeotropic anchoring. In our system, all liquid crystal
monomers, LCM_X1, LCM_X2, and LCM_X3, were found to have
homeotropic anchoring on a SiO2 surface. Here, we used precleaned
glass slides as the flat substrates or coated patterned polymer sub-
strates with a thin layer of SiO2 through a chemical vapor deposition
Measurement of Refractive Index and Birefringence. We used Snell’s law to
estimate the refractive indexes of LCM_X1 by comparing the apparent thick-
ness of the LC cell, DLC, with the actual cell thickness without LC, D0; that is,
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ne,o = D0/DLC. We prepared a uniform planar cell in the way same as described
earlier. D0 and DLC were measured before and after LC was infiltrated, re-
spectively. Accordingly, we obtained the extraordinary refractive index, ne
∼1.67 0.02 and the ordinary refractive index, no ∼ 1.5 0.02 of LC when the
polarized light was parallel and perpendicular to the LC director, respectively.
The birefringence of LCM_X1 was estimated as ∼0.17. We also estimated bi-
refringence of LCM_X1 from a wedge cell, using the Michael–Levy chart be-
measured from optical microscopy. Again, the consistent birefringence before
and after polymerization as shown in Fig. 3 D and E clearly indicates that
polymerization did not affect the nematic order parameter.
ACKNOWLEDGMENTS. We acknowledge support by the National Science
Foundation Materials Science and Engineering Center grants to the Univer-
sity of Pennsylvania, DMR-1120901, DMR-1410253 (to S.Y.), and DMR12-
62047 (to R.D.K.). This work is also partially supported by
Investigator grant from the Simons Foundation (to R.D.K.).
a Simons
fore and after UV curing, as 0.16
0.02, in good agreement with that
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