Liquid-Crystalline Polymers - American Chemical Society

Chapter 32

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: August 24, 1990 | doi: 10.1021/bk-1990-0435.ch032

Polymer-Dispersed Liquid Crystals John L. West Liquid Crystal Institute, Kent State University, Kent, OH 44242

Polymer-dispersed liquid crystals (PDLCs) are electrooptic materials that modulate light through electrical control of the refractive index similar to other liquid devices. Like dynamic scattering and smectic displays, PDLCs switch between scattering and clear states. PDLC -type devices consist of droplets of low-molecular weight liquid crystals dispersed in a solid polymer binder. They do not require polarizers and have a number of other unique advantages: ease of fabrication, suitability for large area devices, environmental stability and fast switching speeds. PDLCs may be tailored for a wide variety of applications, ranging from architectural glass to projection TV and shutters for infrared video cameras. PDLCs are light-scattering materials belonging to a class of liquid crystal devices that operate on the principle of electrically modulating the refractive index of a liquid crystal to match or mismatch the refractive index of an optically isotropic, transparent solid. The first demonstration of this type of device consisted of micron-sized glass particles dispersed in a liquid crystal film (Figure 1A) (1). Another approach imbibed liquid crystals in microporous polymer films (Figure IB) (2,3). Two recently developed approaches distribute the liquid aystalas droplets in a solid polymer binder. The liquid crystal may be encapsulated by standard microencapsulation or emulsification techniques which suspend it in a solid polymer film. These materials, termed "nematic curvilinear aligned phase" (NCAP), are shown schematically in Figure 1C (4.5). Tne newest technique involves phase separation of low-molecular weight liquid crystal from a prepolymer or polymer solution to form droplets of uniform size and controlled density. Materials formed by this final method are termed "polymer-dispersed liquid crystals" (PDLC) (Figure ID), the subject of this review (6.7). Only the NCAP films, being developed by the Taliq and RaycEem Corporations, and the PDLC films, being developed by Kent State 0097-6156/90/0435-00475$06.25/0 © 1990 American Chemical Society In Liquid-Crystalline Polymers; Weiss, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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LIQUID-CRYSTALLINE POLYMERS

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iU i Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: August 24, 1990 | doi: 10.1021/bk-1990-0435.ch032

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Figure 1. Schematic representation of operation of hetero geneous liquid crystal light shutters: A) glass beads in a liquid crystal film, B) liquid crystal imbibed in a microporous film, C) encapsulated liquid crystal, D) polymer dispersed liquid crystals.

In Liquid-Crystalline Polymers; Weiss, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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Polymer-Dispersed Liquid Crystals

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University in cooperation with industry, have shown promise for commercial application. These films can be sandwiched between conducting plastic films to form continuous sheets. They are highly scattering in the OFF state and window-glass clear in the O N state. They do not use surface alignment layers, polarizers or cell seals. They are durable and aesthetically pleasing. A new class of electro-optic materials, they are superior in many ways to conventional liquid crystal shutters and offer exciting new applications.


Principle of Operation PDLCs consist of micron-size droplets of a low-molecular weight nematic liquid crystal dispersed in a polymer binder. Figure 2 shows a scanning electron microphotograph of a cross section of a PDLC film. The size of the nematic droplets is on the order of the wavelength of light. Because of the droplet size and the refractive index mismatch between the liquid crystal in the droplet and the polymer binder, the films are highly scattering with a white, opaque appearance. A PDLC film sandwiched between substrates having a transparent conducting electrode, such as indium tin oxide, form a shutter. Upon application of a voltage across the electrodes of the shutter, it switches from an opaque, light scattering state to a clear, transparent state. The applied electric field aligns the droplets so that their refractive index matches that of the polymer, substantially reducing the light scattered by the droplets. The droplets return to their original alignment and the film returns to the scattering state upon removal of the field. The configuration of the liquid crystal in the droplets depends on the elastic constants of the liquid crystal, droplet size and shape, and surface anchoring of the liquid crystal at the droplet wall. The bipolar configuration is the most common; it occurs in droplets where the molecules are anchored tangentially to the droplet wall (Figure 3A). The director is assigned as the average orientation of the liquid crystal in the droplet. The bipolar droplet is birefringent with an extra ordinary refractive index, n , for light polarized parallel to the director and an ordinary refractive index, n , for light polarized perpendicular to the director. The liquid crystal is usually selected to have a positive dielectric anisotropy and therefore aligns parallel to an applied electric field. The polymer binder is selected to have a refractive index, n , essentially equal to n . In the absence of an applied field the directors are randomly oriented and because of the mismatch of the refractive index of the droplets and the polymer the films scatter light. Appli cation of an electric field aligns the directors normal to the film surmce and since n = n , the films are transparent for light incident normal to the film. The radial configuration occurs when the l i q u i d crystal molecules are anchored with their long axes perpendicular to the droplet wall (Figure 3B). The radial droplet is not birefringent. Application of an external field switches the radial droplet to an axial configuration. As with the bipolar case the films switch from scattering to transparent upon application of an electric field if n = n . ?

0

p

0

0

p

p

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Figure 3. Schematic of droplet configuration and PDLC device operation: A) bipolar configuration, B) radial configuration.


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PDLC Formation


PDLC materials are formed by phase separation of low-molecular weight liquid crystals from a homogeneous solution with a prepolymer or polymer. The liquid crystal forms droplets whose size, shape and density depend on the techniques used. The polymer binder gels around the droplets, locking in their morphology. It is possible to prepare PDLC materials with uniform droplet sizes ranging from 40 pm to less than 0.01 pm in diameter (8.9). For display applications droplet diameters in the range of 0.3 to 3 pm are usually desired, whereas for shuttering in the infrared droplet diameters up to 25 pm are required. Three general techniques have been developed for forming PDLCs. Polymerization-Induced Phase Separation (PIPS). Polymerizationinduced phase separation (PIPS) generally utilizes polymers formed by a step growth reaction. A low-molecular weight liquid crystal is dissolved in a prepolymer solution. Polymerization occurs either thermally or photochemically, changing the chemical potential of the solution and reducing the solubility of the liquid crystal (10.11). The PIPS process is illustrated in Figure 4. The solution passes through the miscibility gap and the liquid crystal phase separates into droplets. Phase separation occurs by either spinodal decomposition or droplet nucleation and growth (West, J . L.; Tamura-Lis, W. "Phase Separation of Low-Molecular Weight Liquid Crystals Dissolved i n a Polymer Melt," 12th International Liquid Crystal Conference, Frieburg, Germany, 1988). The droplets continue to grow until polymer gelation locks in the droplet morphology. Epoxies, polyurethanes and a variety of photopolymers have been used as binders in the PIPS process (12-14). The epoxies have been the most studied of the thermoset polymers because of the large number of commercially available epoxy resins and cure agents. The resins and cure agents can be blended to form copolymers with specified physical properties such as refractive index. Photopolymerization utilizes either free-radical chain reaction or a step-growth reaction. A free-radical chain reaction results in the high-molecular weight polymer phase separating from the low-molecular weight polymer precursor/liquid crystal solution. A polymer ball morphology results and the liquid crystal is the continuous phase. A step-growth reaction produces the desired morphology of liquid crystal droplets dispersed in a continuous polymer binder (15). Two majoFTactors affect droplet size and density in the PIPS process: types and relative concentration of materials used and cure temperature. The cure temperature influences the rate of polymeri zation, viscosity of the polymer, diffusion rate of the liquid crystal and solubility of the liquid crystal in the polymer. Each factor is affected differently by the cure temperature with the result that droplet size varies in a complex manner with cure temperature (Figure 5) and must therefore be empirically determined for each formulation. Thermally-Induced Phase Separation (TIPS). Thermally-induced phase separation (TIPS) results from cooling a l i q u i d crystal/ thermoplastic melt. The liquid crystal and thermoplastic are chosen to


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Phase Separation


Mix

Polymer Gelation

Final Cure

Time •

Figure 4. Schematic diagram of the polymerization induced phase separation process.

um

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50

75

100

Cure Temperature, °C

Figure 5. Graph of average diameter as a function of cure temperature for: A =1:1:1 mixture of epon 828, capcure 3800 and E7: X= 20% MK107,11% epon 828,28% capcure 3800 and 41% E7. (E7 is an cutectic mixture of cyanobiphenyls and cyanoterphenyls.)



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form a homogeneous solution above the melt temperature of the polymer. Figure 6 illustrates the phase diagram of a liquid crystal, thermoplastic mixture. At point A, the mixture forms a homogeneous solution of the liquid crystal dissolved in the thermoplastic melt. As the solution cools it passes through the miscibility gap at point B, resulting in liquid crystal droplet formation and growth. Gelation of the polymer binder locks in the droplet morphology. Droplet size and density are controlled by the types and relative concentrations of liquid crystal and thermoplastic and by the rate of cooling. Figure 7 is a plot of the average droplet diameter vs cooling rate for a system composed of E7 and a thermopolastic epoxy formed by curing Epon 828 with t-butylamine. In general rapid cooling results in smaller droplets and more liquid crystal remaining dissolved in the binder. Solvent-Induced Phase Separation (SIPS). Solvent-induced phase separation (SIPS) utilizes a liquid crystal and a thermoplastic dissolved in a common solvent. Evaporation of the solvent results in phase separation of the liquid crystal, droplet formation and growth, and polymer gelation. Figure 8 is a ternary phase diagram showing the SIPS process schematically. A system represented by point X consists of a polymer and a liquid crystal dissolved in a common solvent. Evaporation of the solvent moves the system alone line XA. As the system crosses the miscibility gap, droplets of liquid crystals form and grow until gelation of the polymer locks in the droplet morphology. Point A represents the final composition of the PDLC film. Droplet size and density depend on the types and relative concentration of liquid crystal ana thermoplastic, the type of solvent and the rate of solvent removal. Table I lists droplet formation time and the droplet size as a function of air flow rate over a thin film of a solution of E7 and polymethylmethacrylate dissolved in chloroform and coated on a glass substrate. The faster the rate of solvent removal, the smaller the droplets. Table I. Droplet Size and Formation Time

Air Flow Rate (ml/min)

Time to Droplet Formation (min)

Droplet Size (um)

100

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, =, or < n from the electro-optic response of the film. Figure 10 shows the transmission through a PDLC film in the ON state as a function of incident angle for n > n , n =n and np < n for vertical (V) and horizontal (H) polarized light. If n >n the film will not be of maximum clarity in the fully ON state for normally incident light. As the PDLC film rotates from the normal, the transmission for vertically polarized light increases because the effective refractive index of the droplet increases and more closely matches n . The transmission reaches a maximum at some angle and then decreases. For n =n the maximum clarity occurs for normally incident light. For n

Liquid-Crystalline Polymers - American Chemical Society

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