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Facility
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Dr.
Sergij Shiyanovskii and Dr. Oleg Lavrentovich supervise
the Liquid Crystal Institute's facility for Materials and Surface
Characterization. This facility can determine material properties
of bulk liquid crystals, properties of the liquid crystal/substrate interface,
and properties of the substrate. Listed below are the measurements that can
be obtained by this facility. To learn more about a particular characterization
technique, click on the property link.
Substrate Properties If you are interested in contacting the Liquid Crystal Institute to have these measurements performed, contact the LCI Industrial Partner Liaison, Bentley Wall, tel: (330) 672-1555.
Dielectric
constants of the liquid crystal can be determined by the "one-cell method"*.
To measure the dielectric constants, a relatively thin (at least 3.5 mm)
empty cell with patterned electrodes must be used. The active area of the
cell should be smaller than 0.3 square cm; otherwise,
optically flat glass will be needed. If the liquid crystal has
a
positive dielectric anisotropy, the cell should have an alignment
layer that will induce planar alignment of the liquid crystal at
the surface. The thickness of the cell, d, will be measured using
an optical interference method, and the area, A, of the overlapping
patterned electrodes will be determined by measuring the capacitance
of the cell, where C=e0A/d.The
cell will then be filled with the provided liquid crystal. The
capacitance of the cell will be measured as a function of applied
voltage. Because of the energetic costs of supporting elastic deformations,
below a particular voltage, the liquid crystal will not deform.
The point where the cost of creating elastic distortion equals the energy
cost of the applied field is called the Frederiks transition (also
known as the Freedericksz transition), Vth.
At applied voltages lower than Vth, the capacitance measured is
C^ (since
the director is perpendicular to the electric field) and gives
e^ (C^ =e0e^A/d). At applied voltages much higher than the Frederiks transition
(V > 3Vth), the capacitance
can be plotted as a function of Vth/V. By
linearly fitting the data, the intercept (Vth/V=0)
gives the capacitance C|| (i.e., the director
is parallel to the electric field) which yields e||.The
tolerance on this technique should be quite good (± 2%);
however, aging and contamination can affect the dielectric constants
of the liquid crystal**. These measurements can be performed over
a temperature range of -20° C to 200° C.The client
need only provide the liquid crystal to have this experiment
performed. The
Frank elastic constants* are determined by applying an external
field to the liquid crystal cell in a direction perpendicular
to
the director orientation fixed by surface anchoring forces.
When the field is small, the liquid crystal will not deform because
the torque caused by the external field is not large enough to
overcome the energetic cost of the elastic distortion; however,
at some
point,
the field becomes large enough to overcome the elastic
energetic barrier, and any measured properties of the cell will
change
(i.e., optical retardation or capacitance). This point is called
the Frederiks transition, and is used to determine elastic constants.
If
the
preferred
direction is planar (perpendicular to the substrate normal)
and the external field is parallel to the substrate normal, then
the elastic
deformation will be a splay deformation, and the Frank
elastic constant
K11 can be determined. If the preferred direction
is planar and the external field is perpendicular to
both the substrate normal and the planar orientation, then the
deformation will
be a
twist deformation, and the Frank elastic constant K22
can be determined. If the preferred direction is homeotropic
(parallel
to the substrate normal) and the external field is parallel
to the substrate, then the deformation will be a bend deformation
and K33
can
be determined.The determination of the splay elastic
constant (K11) requires a liquid crystal cell with planar alignment.
K11 can
be determined by measuring the capacitance of the cell
as a function of voltage (which also can be used to determine
the dielectric
constants. With knowledge of the dielectric constants
of the liquid crystal and the Frederiks transition
voltage, K11
is
then determined. It should be noted that if the cell
is not planar (i.e., the pretilt angle is not 0°),
the change in any measured property of the liquid
crystal cell
will
be gradual, instead of
sudden, and will occur at any field strength smaller
than the true Frederiks
transition. However, with knowledge of the pretilt
angle, numerical analysis can be used to accurately
determine
the elastic constant.The
measurement of the twist elastic constant (K22)
requires a cell with planar alignment. K22 can
be measured by magnetic field or electric field techniques.
In the magnetic field technique, the critical magnetic
field Hth is
measured
by probing the liquid crystal cell for changing in
optical properties. This measurement can require
a thick cell since
Hth is
inversely proportional to the thickness. Typically,
a magnetic field of 10,000 Gauss is required for
a cell 10 mm
thick. The Liquid Crystal Institute Characterization
Laboratory is capable of creating magnetic fields
of 10,000 Gauss.
This technique requires knowledge of the diamagnetic
anisotropy. The electric field technique requires
that wires be placed in the planar cell perpendicular
to the rubbing
direction.
The threshold voltage which causes in-plane switching
is then determined, which allows for determination of
K22 with
knowledge of the dielectric properties of the liquid
crystal. The former method is easier to employ and more
accurate.The
bend elastic
constant (K33) can be determined
in two ways. It can be determined simultaneously with
K11 and the dielectric constants**, by examining the
slope of the line
when C
is plotted against V/Vth. It also can be determined
by using a homeotropic cell with an external electric
field parallel to the substrate to determine the Frederiks
transition.
In this
case, knowledge of the diamagnetic anisotropy is
needed.The accuracy of each measurement is 5% and
they can be performed over a temperature range of
-20° C to 200° C.The
client need only provide the liquid crystal to have
this experiment performed. The indices of refraction of a liquid crystal are measured using a Kernco Model 60/HR Abbe refractometer. An Abbe refractometer measures the index of refraction by measuring the critical angle between a prism within the refractometer and the liquid crystal*. By employing monochromatic, polarized light, both the extraordinary (ne) and ordinary (n0) indices of refraction of the sample can be determined, respectively. The measurements can be done at a variety of discrete wavelengths, including l=633 nm (He-Ne laser light), l=589 nm (atomic spectra line of Sodium), l=546 nm (atomic spectra line of Mercury).The accuracy of this measurement is ± 0.001. This measurement can be performed over a temperature range of 5° C to 75° C. It is also possible to measure the indices of refraction over a wider temperature range (from -20° C to 200° C ) by examining the angular deviation of light through a wedge cell. This method is less accurate than the Abbe refractometer method.The client need only provide the liquid crystal to have this experiment performed.*R. E. Pepper and R. J. Samuels, Vol. 14 of Encyclopedia of Polymer Science and Engineering, 2nd edition, John Wiley and Sons, New York, 1988. The
diamagnetic anisotropy Dc of a liquid
crystal can be determined (the elastic constants
of the liquid crystal must be known) by
measuring the Frederiks
transition
using a magnetic
field*. A
properly prepared substrate will orient the
nematic liquid crystal in a preferred direction
called the
pretilt angle,
and we determine
it by the magnetic null method*.
A liquid crystal cell must be
prepared with antiparallel alignment layers
(i.e., the alignment layer of one substrate
is rubbed in the opposite
direction of the other
substrate).
A magnetic field is applied to
a cell to deform
the liquid crystal director within
the cell. The deformation
is
measured by looking
for changes in the optical properties
of the cell. If the magnetic field is parallel
to
the pretilt
angle at
the
surface, there
will be no change in the properties of the
liquid crystal in the cell, hence no change in the measured
retardation. To
ensure that
the two alignment layers are equivalent,
the optical response of the cell
is measured at different light
incidence angles, guaranteeing that the result
is not
a hybrid
of two
unequivalent alignment
layers**.The pretilt angle can
be measured to ± 0.1° over a temperature
range from -20° C to 200° C.The
client need only provide the liquid
crystal cell prepared
as
described above
to have this
experiment performed.* T. J. Scheffer
and J. Nehring, J. Appl. Phys., 48,
1783 (1977).
When the liquid
crystal is subjected
to an external torque,
the
orientation of
the molecular at
the substrate
surface will deviate
from its preferred position
at the pretilt angle
to partially
align
with the
field. For deviations
in the polar
plane,
the energetic
cost of this work is denoted
by the polar anchoring
coefficient W.
For cells
with planar orientation,
W can be measured
using the 'high-electric-field'
technique*. Recently, a protocol**
has been developed to
ensure that the
cell can
be used for the
measurement of W, and
the measurement
no longer requires the measurement
of capacitance***.This
measurement can
be performed over a
temperature range from
-20° C to 200° C.The
client must provide the liquid
crystal cell prepared
with the conducting
substrates having antiparallel
alignment
(as described
in the pretilt measurement)
to have this experiment performed.
Knowledge of the cell thickness
and
bulk physical properties (K11,
De, e||,
Dn)
are also necessary.*H. Yokoyama
and H. A. van Sprang, J. Appl. Phys. 57,
4520 (1985). The ability of the AFM to create topographical maps with resolution on a nanometer scale has made it an essential tool for the study of semiconductor, polymer and biological samples. Dr. Oleg Lavrentovich's group has a NanoScope III Atomic Force Microscope made by Digital Instruments. Measurements can be done in contact, tapping, and electrical or magnetic mode. The AFM has been used to study the topography of unrubbed and rubbed polyimide LC alignment layers, defects in polymer cholesteric films, and textures of lyotopic liquid crystal domains.The client must provide a sample of size not greater than 12mm x 12mm to have this experiment performed.  Click on image for larger view. Photo courtesy of Tod Schneider Back to top Another essential tool for the study of surfaces is the scanning electron microscope that reveals the topography of a surface on a nanometer scale. We have a JEOL 6300 SEM which can magnify a sample from 80 to 300,000 times (several hundred times greater than an optical microscope). The SEM has been used to study the morphology of polymer dispersed liquid crystals (PDLCs), polymer walls in cholesteric liquid crystal devises (see images below), pixel gaps in silicon substrates, and spacers studied for size distribution.The client must provide a sample of size not greater than 12mm x 12mm to have this experiment performed.
Click on image for larger view.
SEM
Images of polymer walls in cholesteric liquid crystal devises.
The walls were created by doping the liquid crystal with a monomer,
phase separating the monomer from
the liquid crystal by creating an electric field gradient and curing the
monomer.
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