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Phase Separation of Liquid Crystal and Polymer Complex System and Its Applications
林怡欣 助理教授交通大學光電工程學系
website: http://www.cc.nctu.edu.tw/~yilin
04/28/2009
2
Honors and Awards
2008 Glenn H. Brown Prize (Top 4 in the world)2006 OSA New Focus/Bookham Award (Top 7 in the world)2005 Newport Research Excellence Award2005 SPIE Scholarship
3
Glenn H. Brown Prize
4
Outline
1. Introduction2. Phase separation of liquid crystal and polymer complex system:
a. Polymer dispersed liquid crystals: Surface pinning effect
b. Electric field induced phase separation
3. Other applicationsa. Tunable wettability of LC/polymer composite filmb. Polarizer-free display using dye-doped LC gels
4. Summary
5
Outline
1. Introduction2. Phase separation of liquid crystal and polymer complex system:
a. Polymer dispersed liquid crystals: Surface pinning effect
b. Electric field induced phase separation
3. Other applicationsa. Tunable wettability of LC/polymer composite filmb. Polarizer-free display using dye-doped LC gels
4. Summary
6
Introduction: Liquid Crystal Displays
100” LCD TV Philips
Kent Display
7
LC-based Phase Modulators
Airborne Laser Terminalhttp://www.seereal.com/
Tunable focus LC lenses 3D holographic displays
http://www.raytheon.com/http://alphamicron.com/
An Air Force helmet Directed Energy Weapons
8
Introduction to Liquid Crystals
Low temperature
High temperature
CNH2n+1Cn
20Å
5Å
n
Crystal Nematic LC IsotropicTemperature
9
Optical and Dielectric Anisotropy
ε∥
LC director n
ε⊥ ⊥ε−ε=εΔ ll
Dielectric anisotropy
E E
Δε >0 positive LC Δε
11
Deformation of Liquid Crystals
Twist (K22 )
Splay (K11)
Bend (K33)
( ) 0n ≠⋅∇ vv ( ) n//n vvv ×∇ ( ) nn vvv ⊥×∇Elastic constant: K33 > K11 > K22~S2The unit of Kii: pN=10-12 N
12
Reorientation of the Moleculesin Electric Fields
Electric field (E)+ ++
- --
Induced dipole (μ)
Torque E×= μτ
Δε >0 positive LC
•Liquid crystal directors reorient due to the torque.•Liquid crystal directors stop reorienting until they are parallel to the electric field.
13
Reorientation of the Molecules in Electric Fields
Electric field (E)+ ++
- --
Induced dipole (μ)
Δε >0
Electric field (E) + ++
- --
Induced dipole (μ)
Liquid crystals must be driven by AC fields; otherwise, ions accumulate on the alignment layer.
14
Induced Dipole
15
Polarizer
Liquid Crystal Cells
ITO
Alignment layer
LC
Glass
Glass
Polarizer
16
Example: Twisted Nematic LCD
17
Outline
1. Introduction2. Phase separation of liquid crystal and polymer complex system:
a. Polymer dispersed liquid crystals: Surface pinning effect
b. Electric field induced phase separation
3. Other applicationsa. Tunable wettability of LC/polymer composite filmb. Polarizer-free display using dye-doped LC gels
4. Summary
18
Polymer-dispersed Liquid Crystals
np
np~no=1.52
none
no
ne
ITO glass
LC droplet
Polymer
When voltage is off, the cell is translucent.When voltage is on, the cell is transparent.
Y. H. Lin et al., Appl. Phys. Lett. 84, 4083-5 (2004).Y.H. Lin et al., Optics Express,13, 468-474 (Jan. 24, 2005)
19
電控毛玻璃
20
Fabrication Process
LC /monomer = E48/NOA65
Cell filling at T= 60oC (isotropic state)
Thermal induced and Photo-inducedPhase separation
UV curing at T= 20oC
λ=365nmI = 60 mW/cm2
UV
Polymer-dispersed Liquid Crystal (PDLC)
Glass
ITO
Glass
ITO
21
TPDLC and PDLC Cells
PI with orthogonal rubbing cell(TN cell)
No PI cell
ITO Glass
Polyimide
PDLC cell TPDLC cell
22
Morphologies
100 μm 100 μm
PDLC in TN cell (TPDLC)PDLC in no PI cell (PDLC)
Microscope morphologies under crossed polarizersTPDLC has smaller droplet size and better uniformity.LC/monomer=70/30
Y. H. Lin et al, Appl. Phys. Lett. 84, 4083-5 (2004).
8 μm 6.5 μm
23
0
20
40
60
80
100
0 5 10 15 20 25Voltage, Vrms
Ref
lect
ance
, %0.00.40.81.2
0 2 4 6Voltage, Vrms
R,%
0
20
40
60
80
100
0 5 10 15 20 25Voltage, Vrms
Ref
lect
ance
, %0.00.40.81.2
0 2 4 6Voltage, Vrms
R,%
PDLC (8 μm)TPDLC (6.5 μm)
EO Properties: Reflective Mode
TPDLC has better dark state . CR~900:1 (TPDLC) and CR~250:1 (PDLC)Rise time~5ms and decay time~10 ms (TPDLC) Rise time~8ms and decay time~21 ms (PDLC)LC/Monomer=60/40
Y. H. Lin et al., Appl. Phys. Lett. 84, 4083-5 (2004).
λ=633nm
24
Application to Reflective Display
Dye-doped(2 wt%) TPDLCReflector: white paper; Ambient lightLC/monomer= 60/40; Contrast ratio~10:1
Voltage is on
Voltage is off
25
Summary for the Four Cells
Glass
ITO
Polyimide
Homogeneous cell
PI without rubbing cellNo PI cell
TN cellCell gap = 8 μm
26
38oC37oC36oC35oC34oC33oC32oC31oC30oC29oC28oC27oC26oC25oC24.5oC 39oC38oC36oC35oC34oC33oC32oC31oC30oC29oC28oC27oC26oC25oC24.5oC
Thermal-Induced Phase Separation (1)
No PI PI without rubbing
The LC droplets flow and coalesce.Cell gap= 8 μm.
Y.H. Lin et al, Optics Express,13, 468-474 (Jan. 24, 2005)
27
38oC37oC36oC35oC34oC33oC32oC31oC30oC29oC28oC27oC26oC25oC24oC 36oC34oC32oC31oC30oC29oC28oC27oC26
oC
Thermal-Induced Phase Separation (2)
Parallel rubbingOrthogonal rubbing
The LC droplets are fixed and nucleated.Cell gap= 8 μm.
28
Thermal-Induced Phase Separation in Thin Cells
Weak surface anchoring Strong surface anchoring
Nucleation and coalescence Nucleation
29
λ=365nmI = 60 mW/cm2
UV
The Dynamics of PDLC in Thin Cells
Thermal-induced phase separationPhoto-induced phase separation
PDLC
TPDLC
30
Surface Pinning Effect for PDLC
No PI PI without rubbing
Parallel rubbing Orthogonal rubbing
Cell gap= 8 μm
The polar anchoring energy must be larger than 2x10-4 J/m2 to have the surface pinning effects.
31
Cover Page on Optics Express
Y.H. Lin et al., Optics Express,13, 468-474 (Jan. 24, 2005)
32
Outline
1. Introduction2. Phase separation of liquid crystal and polymer complex system:
a. Polymer dispersed liquid crystals: Surface pinning effect
b. Electric field induced phase separation
3. Other applicationsa. Tunable wettability of LC/polymer composite filmb. Polarizer-free display using dye-doped LC gels
4. Summary
33
2008 Cover Pagein Physical Review Letters
•H. Ren, S. T. Wu, and Yi-Hsin Lin "In-situ observation of fringing field-induced phase separation in a liquid crystal and monomer mixture", Phys. Rev. Lett., 100, 117801 (2008).
34
Kelvin force:
])[(2
20 EF monomerLC
rεεε −∇=
P. Penfield and H. A. Haus, Electrodynamics of Moving Media ( MIT, Cambridge,1967)
E-field and UV-PIPS
LC Monomer
Monomer
MonomerLC
LC
E-field
EPFrrr
∇⋅= EPrr
)1(0 −= εε
)(21])[(
20
mm EEEEF εεεεε
−∇⋅−⋅−∇=
35
VV
J. West, APL 72, 2253 (1998)
Polymer Wall Formation
LC
1.Phase separation induced by vertical electric field2.LC molecules are in verical alignment during phase separation process
not easy trace the phase separation process
CNM/NOA65 ~90/10 with no field
(a) CNM/NOA65 ~90/10 with no field, (b) CNM/NOA65 ~90/10 with 17.8V/μm,(c) CNM/NOA65 ~80/20 with 17.8 V/μm, (d) (d) CNM/NOA65 ~70/30 with 17.8 V/μm
36
E-field Enforced Phase Separation
Vertical field (1) Little information is known during
phase separation (2) Forming polymer wall
Lateral field (Fringing field):(1) Phase separation is easily observed (2) Produce new LC devices
37
0V (b)
ITO LC Monomer
(a)
Phase Separation Process
H. Ren, S. T. Wu, and Y. H. Lin PRL 100, 117801(2008) (Cover page)
Cell Parameters:Width of ITO strip: 4 μmWidth of one period: 14 μmLC cell gap: ~7μmZig-zag=150o
ε(LC)> ε(monomer)1.LC moves toward the electrodes.2.Monomers are pushed away from electrodes.
38
Fringing E-field on Phase Separation
(c)
LC/Monomer =30/70
0V 3 V/μm 14 V/μm
3 V/μm 0V
39
Lateral Field induced Phase SeparationBL-003/NOA65=30/70
In situ Observation of Fringing-Field Phase Separation in LC/polymer Complex System
Movie
40
LC Concentration
(c)
(b)
LC Monomer PI ITO
W
(a)V 0
41
Phase Separation Before, Real-time, and After the Impact of Lateral E-field
LC/Monomer=75/25
0V 3 V/μm
12 V/μm 0V
42V. Vorflusev and S. Kumar, Science 283, 1903 (1999).
Phase-Separated Composite Films for Liquid Crystal Displays
43
Single Glass Substrate Liquid Crystal Device Using Electric Field-enforced Phase Separation and
Photoinduced Polymerization
CR=200:1
Mechanism: Electrodynamics of dielectric fluidsLC aggregates in strong EMonomer diffuses to weak EH. Ren, S. T. Wu, and Y. H. Lin,
Appl. Phys. Lett. 90, 191105 (May 7, 2007)
44
Outline
1. Introduction2. Phase separation of liquid crystal and polymer complex system:
a. Polymer dispersed liquid crystals: Surface pinning effect
b. Electric field induced phase separation
3. Other applicationsa. Tunable wettability of LC/polymer composite filmb. Polarizer-free display using dye-doped LC gels
4. Summary
4545
Introduction
1. K. Ichimura et. al., Science 299, 371 (2003).2. S. L. Gras et. al., Chemphyschem. 8, 2036 (2007).3. C. C. Cheng et. al., Opt. Express, 14, 4101 (2006).4.Y. H. Lin, et. al., Opt. Express, 16, 17591 (2008).
Electrically controlled surface tension
Photon-induced
Electrochemical
Thermal
PHSwitchable Surface
ElectrowettingEWOD
DielectrophoresisDEP
SAM
Phase separation(LC/polymer composite film)
Liquavista
ADT
46
Motivation
A switchable surface using the orientation of liquid crystal directors.
V=0
ITO
LC/polymer composite film water
z
xy
θa
glass substrate
V>Vth
glass substrate
waterθb
Y. H. Lin, et. al., Opt. Express, 16, 17591 (2008).
4um14um
12um
10nm
國科會計畫:NSC 96-2112-M-009-019-MY2
474747
Mechanism
)cos()'cos( θθ ⋅= wR
Wenzel’s equation (1936)
)cos()cos()cos( 2211 θθθ ⋅+⋅= ff
Cassie’s equation (1944)
Young’s equation (1805)
LV
SLSV
γγγθ −=)cos(θ
γLV
γSV γSL
θ’
θ
David Quere, Rep. Prog. Phys. 68 , 2495 (2005)
4848
)cos())(cos()](cos[ PPrmsLCLCrms fVfV θθθ ⋅+⋅=LC/Polymer composite film
VaperLiquid
rmsLiquidLCrmsVaperLCrmsLC
VVV
−
−− −=γ
γγθ
)()())(cos(
Mechanism
Liquid crystal/polymer composite film
Liquid crystal/polymer composite film
Y. H. Lin, et. al., Opt. Express, 16, 17591 (2008).
49
AFM Images
10 wt% LC 20 wt% LC
30 wt% LC 40 wt% LC
50
Optical Microscopy
P
A
LC directors are reoriented by the electric field. Vth ~100 Vrms
R
70%LC
0V50V100V150V200V250V
51
Definition of Contact Angle
T. Young, Philos Trans. R. Soc. London, 95 , 65 (1805)
θ
LVγ
SLγSVγ
SLSVLV γγθγ −=cosYoung’s equation
γLV : the surface tension between liquid and air
γSV :the surface tension between surface and air
γSL :the surface tension between surface and liquid
Θ : contact angleContact angle: 代表著三相之間力平衡的狀態
52
0
30
60
90
0 1 2 3Time, second
CA
, deg
ree
-200
-100
0
100
200
300
400
App
lied
Volta
ge, V
0
30
60
90
0 1 2 3Time, second
CA
, deg
ree
-200
-100
0
100
200
300
400
App
lied
Volta
ge, V
0
30
60
90
0 1 2 3Time, second
CA
, deg
ree
-200
-100
0
100
200
300
400
App
lied
Volta
ge, V
0
30
60
90
0 1 2 3Time, second
CA
, deg
ree
-200
-100
0
100
200
300
400
App
lied
Volta
ge, V
1. The change of the contact angle increases with the applied electric field.
2. Max. Δ(CA): 80o- 65o=15o (0-200 Vrms)Surface tension: 13x10-3-30x10-3 [N/m]Response time~200 ms at 200 Vrms , f=1 kHz
Experimental Results
53
Tunable wettability
The change of contact angle is because of the orientation of liquid crystal directors.
0
40
80
120
0 1 2 3 4 5Time, sec
Con
tact
ang
le, d
egre
e
-300
0
300
600
900
1200
Volta
ge, V
rms
70%LC0%LCpure substrateapplied voltage
54
0
2
4
6
0 100 200 300Voltage, Vrms
Foca
l len
gth,
mm
TheoryExperiment
Other Applications: Liquid Lens
)n-)(ncos-cos-)(2cos-(13Vf
122
3
θθθπ=
Rθ
h
h=R-Rcosθ
r
R=r/sinθ
1. Electrically tunable focus liquid lens using LC/polymer composite film.f~ 4 mm-5.2 mm
2. Focal length is independent of droplet volume. Movie
55
Outline
1. Introduction2. Phase separation of liquid crystal and polymer complex system:
a. Polymer dispersed liquid crystals: Surface pinning effect
b. Electric field induced phase separation
3. Other applicationsa. Tunable wettability of LC/polymer composite filmb. Polarizer-free display using dye-doped LC gels
4. Summary
56
To mimic a white paper
Red ink in a white paper Dye-doped LC gels
Motivation
Goal: Polarizer-free, fast response and high CRflexible LCDs in reflective mode.
100μm
Y. H. Lin et al., J. Display Technology 1, 230 (2005).Y. H. Lin et al., Mol. Cryst. Liq. Cryst. 453, 371(2006).Y. H. Lin et al., Opt. Express 16, 1777 (2008)
57
Dye Absorption
When polarization // long axis of dye, light is strongly absorbed.When polarization // short axis of dye, light is weakly absorbed.
polarization
Dye
58
x x
z
Mechanism of Dye-doped LC Gels
The dye-doped LC gel is polarization-independent !!
V1
V2
ITO glass substrate
Diffusive reflectorLCDye
Alignment layerPolymer network
V
Y. H. Lin et. al., IEEE/OSA J. Display Technology ,1 230 (2005).Y. H. Lin et al., Mol. Cryst. Liq. Cryst. 453, 371(2006).Y. H. Lin et. al.,Opt. Express 16, 1777-1785 (2008)
dd aveave eeR 2)(2)()( ⋅−⋅− ⋅≈ θβθαθ
1. V=0 No scattering, little absorption (np~no)2. V>Vth : Multi-domain mode3. V>>Vth: Strong scattering + strong absorption
59
EO Properties
0.0
0.2
0.4
0.6
0.8
1.0
0 10 20 30Voltage, Vrms
Ref
lect
ance
10℃20℃30℃40℃
10 oC
20 oC
30 oC
40 oC
Unpolarized light λ=543.5 nm
EO properties depend on the curing temperature.
CR: 450:1; R(0V)~55%, Vth~7 VrmsResponse time: 6.5 ms @10oC
20oC curing
30 Vrms 100 μm
30oC curing
30 Vrms 100 μm
40oC curing
30 Vrms100 μm
30 Vrms 100 μm
10oC curing 20oC curing
30 Vrms 100 μm
20oC curing
30 Vrms 100 μm
30oC curing
30 Vrms 100 μm
30oC curing
30 Vrms 100 μm
40oC curing
30 Vrms100 μm
40oC curing
30 Vrms100 μm
30 Vrms 100 μm
10oC curing
30 Vrms 100 μm
10oC curing
30 Vrms 100 μm
10oC curing
Typical GH LCD: CR~5:1; reflectance
61
0
1
2
3
0 10 20 30Voltage, Vrms
Tran
smitt
ance
, a.u
.
beforeafter
A Polarizer-free Flexible Electro-Optical Switch
T mode CR :20:1; response time :12ms;Min. Radius~21mm
It is polarizaer-free, bendable and trim-able.
30 Vrms30 Vrms 0 Vrms0 Vrms 30 Vrms30 Vrms
Y. H. Lin et. al.,Opt. Express 16, 1777-1785 (2008)
Movie
62
r = 0.5 μmr = 1.0 μmr = 2.0 μmr = 3.0 μm
r = 0.5 μmr = 1.0 μmr = 2.0 μmr = 3.0 μm
r = 0.5 μmr = 1.0 μmr = 2.0 μmr = 3.0 μm
r = 0.5 μmr = 1.0 μmr = 2.0 μmr = 3.0 μm
Simulation Results
1. The dye-doped LC gel is polarization independent.2. The reflectance decreases with tilt angle.
dd aveave eeR 2)(2)()( ⋅−⋅− ⋅≈ θβθαθ
θ= 0o
θ= 30oθ= 45o
θ= 90oθ= 0o
θ= 30oθ= 45o
θ= 90oθ= 0o
θ= 30oθ= 45o
θ= 90oθ= 0o
θ= 30oθ= 45o
θ= 90o
63
0.0
0.2
0.4
0.6
0.8
1.0
0 10 20 30Voltage, Vrms
Ref
lect
ance
High density
Low density0 V
9 V
30 V
3-Step Switch Using Distinct Dye-doped LC Gels
10.68.6
Response, ms
220:1170:1
CR
4.60.50Low density5.40.49High density
Vth, VrmsRmax
Movie
Y. H. Lin, and C. M. Yang,Appl. Phys. Lett. 94, 143504 (2009)
64
Flexible Displays
Kent Display
65
Decorative Displays
0 V 9 V 30 V
0 V 5 V
9 V 30 V
1. The potential application is monochromatic decorative displays, and e-label.
2. Increase the value of fashion products in a simple way.
66
Summary 1. We have introduce phase separation in LC/polymer complex
system including surface pinning effect, electric field effect.
2. Surface pinning effect results in smaller and uniform droplets and better scattering in PDLC, even though the cell gap is thin.
3. The phase separation induced by fringing electric field depends on LC concentration. Such phase separation can in situ observe the process.
4. Our LC/polymer composite film is electrically tunable due to theorientation of LC directors. The applications are polarizer-free displays and microfluidic channel.
5. By combining scattering and absorption, dye-doped LC gels is polarizer-free and is useful in flexible displays.
67
Acknowledgment
Thank you for your kind attention !!