Development of Polymer Cholesteric Liquid Crystal Flake Technology for Electro-Optic

Devices and Particle Displays

Particles 2007Toronto

21 August 2007

T. Z. Kosc, K. L. Marshall, A. Trajkovska-Petkoska,C. Coon, K. Hasman, G. Babcock, R. Howe, and S. D. JacobsLaboratory for Laser Energetics, University of Rochester

• Environmentally and physically robust particles with unique wavelength-and polarization-specific optical properties

PCLC flake/fluid host suspensions are an exciting new medium for information display

Summary

• All materials are commercially available

• No polarizers or filters required

• Response times in 100’s of milliseconds and drive fields as low as millivolts/μm

• Shaped flakes modified through doping and/or with multimple layers display improved motion uniformity, enhance reflectivity >> 50% and altered dielectric properties

Microencapsulation could provide flexible reflective displays and conformal coatings with unique color and polarization properties

Presentation topics

• Cholesteric LC’s: structure, properties, and optical effects

• PCLC flake/fluid host suspensions:key properties and applications potential

• PCLC flake electro-optics: experiments and theory

• Engineering PCLC flakes: shaping and tweaking properties

• Microencapsulated suspensions: progress toward flexible, bistable displays

• Devices and future research directions

Cholesteric Liquid Crystals: Structure, Properties and Optical Effects

Nematic

Cholesteric

Pn

Brief liquid crystal overview

λ ϕ ϕo avg n i n sn pavg avg

= +− −[cos {sin ( sin ) sin ( sin )}]12

1 1 1 1

Selective reflection in cholesteric LC’s is a Bragg-like effect

0102030405060708090

100

300 400 500 600 700 800

Wavelength (nm)

Tran

smis

sion

(%)

ϕ i ϕ s

Δ Δλ = n p

, ,23

o ch e chavg

n nn

+=

• Reflected light is inherently circularly polarized

• Broad-band selective reflection is possible in systems with a pitch gradient

PCLC Flake/Fluid Host Suspensions:Key Properties and Applications Potential

Flakes are produced by fracturing a PCLC film and retain its unique physical and optical properties

1 cm

2.5 cm

*Polysiloxane materials provided by Dr. F. Kruezer, Wacker-Chemie, Consortium für Electrochemische Industrie GmbH.

1 cm

• Developed in early 1990’s. The initial form is a polycrystalline solid*.

• A solvent-free film is cast on a silicon substrate at an elevated temperature.

• Liquid nitrogen is poured over the substrate, and the film fractures, forming flakes.

Photos by E. Korenic, Ph.D. Thesis, University of Rochester, 1997.

Commercial applications for PCLC flakes in the 1990’s were mainly decorative

• Wacker polysiloxane PCLC flakes dispersed in a clear acrylic lacquer

The concept for an electro-optic device requires PCLC flakes to be suspended in a fluid host

K. L. Marshall, et al , “U. S. Patent No. 6,665,042 #B1 (16 December 2003).

Glass ITO Fluid host

FIELD OFF

Flake

v

+

-

v

• Bright selective reflection is shifted and diminished as flakes rotate.

• Ideally, flakes uniformly reorient 90º.

+

FIELD ON-

v

The unique properties of PCLC flakes open a host of possible device applications

• Information display– Reflective multi-color particle displays,

flexible displays, 3-D displays, “electronic paper”

• Electro-optics and Photonics– Switchable/ tunable color filters,

micropolarizers, modulators

• Coatings technology– Switchable “paints”, conformal coatings,

switchable “smart windows” for energy or privacy control

• Military/Security– Anti-counterfeiting, signature reduction,

camouflage, encoded and encrypted information storage

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Particle Display Technologies for Flexible Display Applications

A wide variety of display technologies are competing for dominance of the emerging flexible display market

• Particle displays

• Liquid crystal displays

• Organic light emitting diode (OLED)

• Polymer light emitting diode (PLED)

• MEM’s-based reflective displays

• Electro-chromic displays

• Electro-wetting technologies

Particle displays have a number of advantages over other competing technologies . . .

• Bistable switching- greatly decreases power consumption and reduces drive

electronics complexity

• Reflectivity - low power requirements as compared to emissive

displays (OLED’s, PLED’s, ) or backlighted LCD’s

• Environmental robustness- optical properties are relatively insensitive to temperature

and other environmental factors

• Flexibility - cost-effective fabrication of large area devices by liquid

coating techniques (web, slot, die) makes roll-to-roll manufacturing viable for large-volume applications

• Charged microparticles migrate towards the top or bottom of the microcapsule depending of the sign of the applied voltage.

• Microcapsule diameter ranges from 50 - 200 μm• Monochrome and two-color devices demonstrated

E-Ink technology is based on translational motion of microencapsulated charged particles

• Monochrome or two-color

• Multi-color only with color filters

SiPix Microcup devices use depressions stamped into a flexible substrate to confine an electrophoretic dispersion

• Microencapsulated bichromal plastic balls (< 100 μm) have a white and an oppositely charged black hemisphere.

• Balls rotate 180o depending on the sign of the applied voltage, while translating toward the electrode.

Gyricon devices employ charged particles that both rotate and translate

Bridgestone’s Quick Response Liquid Powder Display is a unique electrophoretic technology

• Charged particles in air • Require large voltages• Chemical treatment allows dry

particles to flow like a liquid• Video frame rate capability• Color capability (???)

treated untreated

. . . . but the greatest single drawback of current particle displays is a lack of full-color capability

• Optical effect is a combination of absorption and scattering (white or colored particles in a transparent or dyed host fluid)

• Two-color systems are relatively easy to achieve, but obtaining multi-color devices has been much more difficult than anticipated.

• Multi-color particle displays have been demonstrated, but only by using color filters, which

- degrade display’s appearance - reduce reflectivity by as much as 60%- increase manufacturing cost

No commercial particle display device is currently capable of multi-color operation without employing color filters

PCLC flake technology offers advantages that are unmatched by existing particle display technology

• Selective reflection effect provides highly saturated colors without polarizers or filters

• Left- and right-handed PCLC materials along with other optical polymers form layered, composite flakes with a reflectivity exceeding 50%

• Broad-band and polarization-specific optical effects can be utilized for unique display properties and applications

• Drive fields are as low as millivolts/μm (applied voltage of only a few volts)

PCLC Flake Electro-Optics: Experiment and Theory

Motion was initially observed in a DC field

• Flake motion is random and sensitive to changes in field polarity

• Electric field requirements are modest (5 V/μm)• DC drive offers possibility for bistability

For εhost > εflakes(silicone oil host fluids)

• Cheap, commercial fluid

• Low conductivity• Commonly used for

microencapsulation

• Flake motion is coordinated and controlled

• Response is frequency dependent

• Electric field requirements are very small (< 10 mVrms/μm)• Flakes return to original orientation when the applied field is removed

• AC drive complicates the possibility for bistability

Uniform reorientation was seen using a conductive host fluid and an AC driving field

For εhost >> εflakes(propylene carbonate or polyethylene glycol)

• Commercial, but volatile fluid

• High conductivity• Not commonly used for

microencapsulation

Maxwell-Wagner polarization is the main mechanism for PCLC flake reorientation in an AC-field

+

_+ + + + +

- -- - -

+

_++ ++

-----

+

V = 0

T. Z. Kosc,”, PhD Thesis, University of Rochester, 2003

V=Vapp , increasing time

• Interfacial polarization induces a dope moment in the presence of an applied electric field.

• Frequency dependent behavior found over three decades.

• Response shows inverse quadratic dependence on electric field strength.

Dielectric anisotropy of the PCLC material plays no role in E-O reorientation.

0

20

40

60

80

100

0 0.5 1 1.5 2

Voltage (VRMS)

Res

ponc

e Ti

me

(s) small

The experimental data show an inverse quadratic dependence on the applied field

1

10

100

0.1 1 10

Slope of line = -2R2 = 0.78

4.4

44.6

3.1

1

10

100

1000

1 10 100 1000 10000Frequency (Hz)

Res

pons

e Ti

me

(s)

.

Real part modelImaginary part modelExperimental Data

4

5

10 100 1000Frequency (Hz)

Res

pons

e Ti

me

(s)

The experimentally observed frequency dependent behavior is not completely predicted by the model

{ }2 2

o 2 32 2* * 22 2 3 3 oh 2 3 3 2 o

4 η (a +a ) tan(φ)t = ln(a A +a A ) tan(φ )ε Re K K (A - A )E

⎛ ⎞⎜ ⎟⎝ ⎠

Real component:flake rotation much slower than E-field oscillation

Imaginary component:contains phase information on cross product of applied field and induced dipole moment

Flake shape and size affect reorientation time

0

10

20

30

40

50

60

1 1.5 2 2.5 3

Aspect Ratio (a.u.)

Reo

rient

atio

n Ti

me

(s

B

FC

A

GD

E

Reo

rient

atio

n Ti

me

(s) Flake surface shapes are drawn.

• Flakes with the largest aspect (length to width) ratio reorient the fastest.

*Optimum flake dimensions: 40 to 60 μm long, 3:1 aspect ratio, 3 to 5 μm thick

There are many aspects of the technology that can be developed and improved

Improve flake motion uniformity

Find a method or mechanism to drive PCLC flakes back to their original position

Find a method or mechanism for bistability

Create pixilated and multi-colored devices

Build flexible devices

Improve reflectivity and contrast of devices

There are several avenues being explored, and some address multiple problems

“Shape” PCLC flakes - improve flake motion uniformity- microencapsulation

Engineer materials - PCLC flake composites - bistability- reverse flake motion- improve reflectivity and contrast

Microencapsulation- bistability- reverse flake motion- pixilated and multi-colored devices- flexible devices

Specialized driving waveforms - bistability- reverse flake motion

Engineering PCLC Flakes: Shaping and Tweaking Properties

Reorientation times of commercial PCLC flakes vary due to differing size and shape

Small, elongated flakes reorient faster than larger, symmetrical flakes

50 um

10 um

POMX polars

SEM

Specialized flakes have been manufactured to investigate theoretical predictions and improve device characteristics

Soft lithography using polydimethilsiloxane molds is employed to produces flakes of various (uniform) sizes and shapes

PCLC materials are doped with conductive or highly dielectric dopants

Two or more PCLC layers are fused to greatly enhance flake reflectivity

Experimental data verifies theoretical prediction that shaped flakes with greater aspect ratios reorient faster

• Shaped PCLC flakes prepared from Wacker Helicone® PCLC

• Host fluid: γ-butyrolactone

• Cell thickness: 80 μm

• Applied voltage: 3.2 Vrms (50 Hz)

• Resonse time: 280 ms

• Aspect ratios: Rectangles (3:1)Ellipses (2:1)Squares (1:1)

The physical, optical, and electrical properties of PCLC flakes are modified with particle dopants

• Small amounts of carbon black or carbon nanotubes dramatically increase flake conductivity

• Adjust flake density• Provide color enhancement• Produce a homogeneous or non-uniform charge distribution

• Increases difference between εflake and εhost

Layered composite PCLC flakes could produce reflective displays with both highly saturated colors and reflectivity greatly exceeding 50%

• Response is frequency dependent

• Flake motion is coordinated and controlled

• Drive voltage requirements are very small (<5 mV/μm)• DC drive offers possibility for bistability

• Motion reversal has been observed for opposite polarity

A silicone oil with a high dielectric permittivity allowed uniform reorientation using a DC field

For εhost > εflakes(silicone oil)

• Commercial fluid

• Low conductivity• Commonly used for

microencapsulation

Microencapsulation of Suspensions: Toward Flexible, Bistable Displays

Segregation of electro-active particles into “microcompartments” has been crucial in achieving commercial viability for particle displays

• Prevent particle agglomeration

• Effect bistable operation

• Cost-effective fabrication of large area devices (roll-to-roll )

• Enables applications in flexible displays (roll-up displays, e-paper)

• Electrically addressable conformal coatings

PCLC flakes and silicone oil are strongly hydrophobic, so water-based polymer binders are ideal

100 μm100 μm

Direct emulsification (PVA) Complex coacervation (gelatin)

• Form gelatin microcapsules (pH or concentration change)

• Chemically “harden” and isolate• Re-disperse in a compatible binder

• Disperse at low-shear in PVA • Knife-coat onto substrate• Dry in air or nitrogen stream

Microencapsulation using silicone oils with εh > 3 requires surfactants

Microencapsulation of the two-component (flake and fluid) host suspensions represents a significant challenge

PCLC flakes reorientation is seen in both gelatin and PVA microencapsulation matrices

• Greatest motion in large capsules and when field polarity changes

• Flakes become “trapped” in capsules of comparable size

• Flakes display “sticking” effect• Become attached to capsule

wall• Released with passing time,

higher voltage, opposite polarity• Possible latching mechanism?Flake/fluid “gelcaps” in a PVA binder

Film thickness (total): 300 umDrive voltage: 50-125 V

Building PCLC Flake Devices: Waveforms, Pixels, and Curves

An optimized 3-V, 1.5-s saw-tooth pulse shows acceleration compared with natural relaxation of flakes

• A pulse with a sharp leading edge and a gradual trailing edge produced optimal results• Accelerated relaxation occurs within 4 seconds• Natural relaxation requires ~70 seconds to reach the same brightness level• Waiting several minutes to gain the additional brightness would not be useful for most

applications

A holding voltage lower than the drive voltage prevents flake relaxation while consuming less power

• 3-Vpp drive voltage• 0.4-V holding voltage

PCLC flakes reorient most quickly at 80 Hz in this PC test system.< 80 Hz : many flakes relax and significant amount of reflectivity regained> 80 Hz : magnitude of the brightness barely changes from its minimum level

• Free ions cannot move far at high frequencies, so the induced dipole remains.• Holding voltages do not diminish for frequencies above 80 Hz.

ITO-coated Mylar substrates were used to fabricate flexible devices

*80 Hz sine wave

• Commercial PCLC flakes (40 to 60 μm long, 3:1 aspect ratio, 3 to 5 μm thick)• Flake concentration: 4% to 5%• Path length = 120 μm V = 3.0 Vrms*

PCLC flake technology offers unique features for particle displays and other applications

• Response times are on the order of 100’s of ms

• Drive fields (mV/μm) are remarkably low

• Microencapsulation will enableflexible devices

• Solutions for motion reversal and bistability are being actively pursued

• Possibilities are limitless . . .

• PCLC selective reflection effect provides highly saturated colors at low flake concentrations (3-5%) without polarizers or filters

Acknowledgements

Laboratory for Laser EnergeticsUniversity of Rochester

U.S. Department of Energy Office of Inertial Confinement Fusion

(Cooperative Agreement DE-FC52-92SF19460)