Development of Polymer Cholesteric Liquid Crystal Flake

Development of Polymer Cholesteric Liquid
Crystal Flake Technology for Electro-Optic
Devices and Particle Displays
T. Z. Kosc, K. L. Marshall, A. Trajkovska-Petkoska,
C. Coon, K. Hasman, G. Babcock, R. Howe, and S. D. Jacobs
Laboratory for Laser Energetics, University of Rochester
Particles 2007
Toronto
21 August 2007
Summary
PCLC flake/fluid host suspensions are an exciting new
medium for information display
• Environmentally and physically robust particles with unique wavelengthand polarization-specific optical properties
• 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
ƒ
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
Brief liquid crystal overview
Cholesteric
Nematic
n
P
Selective reflection in cholesteric LC’s is
a Bragg-like effect
λ o = navg p[cos 12 {sin −1( n1 sin ϕ i ) + sin −1( n1 sin ϕ s )}]
avg
2 n o , ch + n e , ch
3
ϕi ϕs
100
90
Transm ission (%)
n avg =
avg
80
70
60
50
40
Δλ = Δn p
30
20
10
0
300
400
500
600
700
800
Wavelength (nm)
• 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
•
Developed in early 1990’s.
The initial form is a polycrystalline solid*.
1 cm
•
A solvent-free film is cast on a silicon
substrate at an elevated temperature.
2.5 cm
•
Liquid nitrogen is poured over the substrate,
and the film fractures, forming flakes.
1 cm
*Polysiloxane materials provided by Dr. F. Kruezer, WackerChemie, Consortium für Electrochemische Industrie GmbH.
Commercial applications for PCLC flakes in
the 1990’s were mainly decorative
• Wacker polysiloxane PCLC flakes dispersed in a clear acrylic lacquer
Photos by E. Korenic, Ph.D. Thesis, University of Rochester, 1997.
The concept for an electro-optic device requires PCLC
flakes to be suspended in a fluid host
FIELD OFF
v
Glass
ITO
Fluid host
Flake
• Bright selective reflection is shifted and diminished as flakes rotate.
• Ideally, flakes uniformly reorient 90º.
+
+
v
v
FIELD ON
-
K. L. Marshall, et al , “U. S. Patent No. 6,665,042 #B1 (16 December 2003).
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 a
TIFF (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
E-Ink technology is based on translational motion
of microencapsulated charged particles
• 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
SiPix Microcup devices use depressions stamped into a
flexible substrate to confine an electrophoretic dispersion
• Monochrome or two-color
• Multi-color only with color filters
Gyricon devices employ charged particles
that both rotate and translate
• 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.
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
For εhost > εflakes
(silicone oil host fluids)
•
Cheap, commercial fluid
•
Low conductivity
•
Commonly used for
microencapsulation
•
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
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
•
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
Maxwell-Wagner polarization is the main mechanism
for PCLC flake reorientation in an AC-field
• Interfacial polarization induces a dope moment in the presence
of an applied electric field.
+
+
-- - -
- - - - ++ + + +
_
V=0
_
+ ++
++
V=Vapp , increasing time
• 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.
T. Z. Kosc,”, PhD Thesis, University of Rochester, 2003
The experimental data show an inverse quadratic
dependence on the applied field
Responce Time (s)
100
Slope of line = -2
R2 = 0.78 small
100
80
10
60
40
1
0.1
1
10
1.5
2
20
0
0
0.5
1
Voltage (VRMS)
The experimentally observed frequency dependent
behavior is not completely predicted by the model
⎛ tan(φ) ⎞
4 ηo
(a +a )
ln
⎜
⎟
2
ε h Re {K*2 K*3 } (A3 - A 2 )E o2 (a 2 A 2 +a A3 ) ⎝ tan(φo ) ⎠
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
Real part model
Imaginary part model
Experimental Data
1000
Response Time (s) .
2
3
2
3
100
Response Time (s)
t=
2
2
44.6
10
4.4
3.1
1
1
10
100
Frequency (Hz)
1000
10000
5
4
10
100
Frequency (Hz)
1000
Flake shape and size affect reorientation time
Reorientation Time (s
(s)
60
E
50
40
Flake surface shapes are drawn.
G
D
30
B
A
20
C
10
F
0
1
1.5
2
2.5
3
Aspect Ratio (a.u.)
•
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
POM
X polars
SEM
10 um
50 um
Small, elongated flakes reorient faster than larger, symmetrical flakes
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
Squares
(2:1)
(1:1)
The physical, optical, and electrical properties of
PCLC flakes are modified with particle dopants
•
•
•
•
Increases difference between εflake and εhost
Adjust flake density
Provide color enhancement
Produce a homogeneous or non-uniform charge distribution
• Small amounts of
carbon black or carbon
nanotubes dramatically
increase flake
conductivity
Layered composite PCLC flakes could produce
reflective displays with both highly saturated
colors and reflectivity greatly exceeding 50%
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
•
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
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
Microencapsulation of the two-component
(flake and fluid) host suspensions represents a
significant challenge
PCLC flakes and silicone oil are strongly hydrophobic,
so water-based polymer binders are ideal
Direct emulsification (PVA)
Complex coacervation (gelatin)
100 μm
•
•
•
Disperse at low-shear in PVA
Knife-coat onto substrate
Dry in air or nitrogen stream
•
•
•
Form gelatin microcapsules (pH or
concentration change)
Chemically “harden” and isolate
Re-disperse in a compatible binder
Microencapsulation using silicone oils with εh > 3 requires surfactants
PCLC flakes reorientation is seen in both gelatin and
PVA microencapsulation matrices
Flake/fluid “gelcaps” in a PVA binder
Film thickness (total): 300 um
Drive voltage: 50-125 V
•
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?
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
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
• 3-Vpp drive voltage
• 0.4-V holding voltage
• 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
• 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*
*80 Hz sine wave
PCLC flake technology offers unique features for
particle displays and other applications
• PCLC selective reflection effect provides highly saturated
colors at low flake concentrations (3-5%) without polarizers or
filters
• Response times are on the order
of 100’s of ms
• Drive fields (mV/μm) are
remarkably low
• Microencapsulation will enable
flexible devices
• Solutions for motion reversal and
bistability are being actively
pursued
• Possibilities are limitless . . .
Acknowledgements
Laboratory for Laser Energetics
University of Rochester
U.S. Department of Energy
Office of Inertial Confinement Fusion
(Cooperative Agreement DE-FC52-92SF19460)