Phase Separation of Liquid Crystal and Polymer …2009/04/28 · Polymer dispersed liquid crystals:...

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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

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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





4. Summary

6

Introduction: Liquid Crystal Displays

100” LCD TV Philips

Kent Display

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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

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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

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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

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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.

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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.

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Induced Dipole

15

Polarizer

Liquid Crystal Cells

ITO

Alignment layer

LC

Glass

Glass

Polarizer

16

Example: Twisted Nematic LCD

17

Outline





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)

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電控毛玻璃

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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

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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

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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


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

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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

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Summary for the Four Cells

Glass

ITO

Polyimide

Homogeneous cell

PI without rubbing cellNo PI cell

TN cellCell gap = 8 μm

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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)

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38oC37oC36oC35oC34oC33oC32oC31oC30oC29oC28oC27oC26oC25oC24oC 36oC34oC32oC31oC30oC29oC28oC27oC26

oC

Thermal-Induced Phase Separation (2)

Parallel rubbingOrthogonal rubbing

The LC droplets are fixed and nucleated.Cell gap= 8 μm.

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Thermal-Induced Phase Separation in Thin Cells

Weak surface anchoring Strong surface anchoring

Nucleation and coalescence Nucleation

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λ=365nmI = 60 mW/cm2

UV

The Dynamics of PDLC in Thin Cells

Thermal-induced phase separationPhoto-induced phase separation

PDLC

TPDLC

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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.

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Cover Page on Optics Express

Y.H. Lin et al., Optics Express,13, 468-474 (Jan. 24, 2005)

32

Outline





4. Summary

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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).

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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

−∇⋅−⋅−∇=

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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

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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

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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.

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Fringing E-field on Phase Separation

(c)

LC/Monomer =30/70

0V 3 V/μm 14 V/μm

3 V/μm 0V

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Lateral Field induced Phase SeparationBL-003/NOA65=30/70

In situ Observation of Fringing-Field Phase Separation in LC/polymer Complex System

Movie

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LC Concentration

(c)

(b)

LC Monomer PI ITO

W

(a)V 0

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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

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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)

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Outline





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

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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).

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AFM Images

10 wt% LC 20 wt% LC

30 wt% LC 40 wt% LC

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Optical Microscopy

P

A

LC directors are reoriented by the electric field. Vth ~100 Vrms

R

70%LC

0V50V100V150V200V250V

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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

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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

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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


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

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Outline





4. Summary

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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)

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Dye Absorption

When polarization // long axis of dye, light is strongly absorbed.When polarization // short axis of dye, light is weakly absorbed.

polarization

Dye

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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

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EO Properties

0.0

0.2

0.4

0.6

0.8

1.0


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

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0

1

2

3


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

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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

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0.0

0.2

0.4

0.6

0.8

1.0


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)

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Flexible Displays

Kent Display

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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.

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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.

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Acknowledgment

Thank you for your kind attention !!

Phase Separation of Liquid Crystal and Polymer …2009/04/28 · Polymer dispersed liquid crystals:...

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