Characterization Facility

 

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.

Bulk Material Properties

Liquid Crystal/Substrate Properties

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.


If you are interested in having Atomic Force Microscopy performed, contact Dr. Sergij Shiyanovskii, tel: (330) 672-1576.

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Dielectric Properties

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.
*S.-T. Wu, D. Coates and E. Bartmann, Liquid Crystals, 10, 635-646 (1991).
**S. Murakami and H. Naito, Jpn. J. Appl. Phys. 36, 2222 (1997).

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Frank Elastic Properties

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.
*W. H. DeJeu, Physical Properties of Liquid Crystals, Gordon and Breach, New York, 1980, Chapter 6.
**Y. Zhou Y. and S. Sato, Jpn. J. Appl. Phys., 36, 4397 (1997).

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Indices of Refraction

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.

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Diamagnetic Anisotropy

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*.

This measurement can be performed over a temperature range from -20° C to 200° C.The client need only provide the liquid crystal to have this experiment performed.

*W. H. DeJeu, Physical Properties of Liquid Crystals, Gordon and Breach, New York, 1980, Chapter 6.

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Pretilt Angle

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).
** D. Andrienko, Y. Kurioz, Y. Reznikov, C. Rosenblatt, R. Petschek, O. Lavrentovich, D. Subacius, J. Appl. Phys. 83, 50 (1998); P. Ziherl, D. Subacius, A. Strigazzi, V. M. Pergamenshchik, A. L. Alexe-Ionescu, O. D. Lavrentovich, S. Zumer, Liquid Crystals 24, 607 (1998).

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Polar Anchoring Coefficient, W

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).
**Yu. A. Nastishin, R. D. Polak, S. V. Shiyanovskii, V. H. Bodnar, and O. D. Lavrentovich, J. Appl. Phys., 86, 4199-4213 (1999).
***Yu. A. Nastishin, R. D. Polak, S. V. Shiyanovskii, and O. D. Lavrentovich, Appl. Phys. Lett. 75, 202-204 (1999).

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Atomic Force Microscopy (AFM)

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.
AFM image of a dried lyotropic liquid crystal texture
Photo courtesy of Tod Schneider

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Scanning Electron Microscopy

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.

Photos courtesy of
Dr. John West.

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