Polymer Stabilize Liquid Crystals (PSLCs)

By: Ellen Kay R. Cacatian

POLYMER STABILIZED LIQUID CRYSTALS

Introduction Polymer Stabilized ( NEMATIC) Liquid

Crystals Polymer Stabilized (CHOLESTERIC)

Liquid Crystals Applications

INTRODUCTION

Displays & their associated technology havebecome an important part of everyday life, butdevelopment is not standing still. Lowerpower-consumption displays are increasingly indemand, as power demands from conventional display technologies still limit their usage.

October 2010 SHARP Microelectronics of the America

COMPARISON…

PLCs - Polymer Liquid Crystals

PDLCs- Polymer Dispersed Liquid Crystals

PSLCs –Polymer Stabilized Liquid Crystalline

WHAT is DIFFERENCE between the

three?

Reason Why PSLCs is produced..

PDLCs - deals with higher polymer concentration range above

20%wt.

- in display application presence of hazy images is a problem.

LOWERING the polymer concentration by 10%wt. can improve electro-optical performance.

WHY??? Because it has the ability to form POLYMER

NETWORKS that stabilize liquid crystals.

TWO kinds of (PSLCs) are:

1. Polymer Stabilized NEMATIC Liquid Crystals

2. Polymer Stabilized CHOLESTERIC Liquid Crystals

Polymer Stabilized NEMATIC Liquid Crystals…

TN- twisted Nematic

a significant reduction in operational voltage is

achievedin low polymer concentration.

STN- Super Twisted Nematic

undesirable stripping texture can

be eliminated & reduction indriving voltage is achieved.

TN- twisted Nematic

The most common LCD that is used for everyday items like watches & calculators.

consists of a nematic liquid crystal sandwiched between two plates of glass.

STN-Super Twisted Nematic

Require less power and are less expensive to manufacture 

However, STN displays typically suffer from lower image quality and slower response time 

 for mainstream laptops, some inexpensive mobile phones

 In early 90's they had been used in some portable computers such as Amstrad's PPC512 and PPC640

What are the things that need to understand?

Polymer Network Controlling Factors

1. morphology2. liquid crystal texture3. monomer concentration4. Photo polymerization temperature5. UV intensity 6. exposure time

Morphology of Polymer Network

How is PSLC is prepared?

1. Dissolve & photo polymerize the monomers ( < 5%wt.) in liquid crystal matrix. Formation of a polymer network occurs.

2. The homogenous alignment is achieved through:

a. Pretreatmentb. Coatingc. Rubbing

On the glass cell faces is provided.

Summary of preparation of PSLCs :

1

•Combining a small quantity of monomer & photo initiator to:

2

•Nematic or Cholesteric liquid crystal but in Cholesteric ,addition of Chiral Dopant is needed.

3

•After desired texture is achieved, field is applied

•U.V light is used to Photo polymerize the sample

•SEM for Morphology analysis

Morphology of Polymer Network

The solution contains: (monomer + photo initiator)

3% wt. of either (Diacrylate monomer) BAB or BAB6 with 0.3% wt. photo initiator BME dissolved in E48.

After being sandwiched between glass cell faces, the solution is photo polymerized under U.V light source.

Then polymer phase separation and network formation take place.

Finally the cell was carefully split open to permit study of polymer network using SEM.

FIGURE A

 SEM micrograph: Low network density areas appear near the bottom and again near the top of the image with much higher density in between. The molecular director of the nematic solvent during polymerization was in the

vertical direction.

FIGURE B

A high magnification SEM: image showing the fine scale structure. Note the nodular clusters that compose the aggregate network structure. This structure is unoriented but other polymerization conditions can yield oriented fibrils.

FIGURE C

 A high magnification SEM: image showing the fibrous network morphology

Effects of varying initiator concentration & UV photopolymerization (fixed monomer 2%wt. BAB)

FOUR PHASE-CONTRAST OPTICAL MICROGRAPH IMAGES OF THE SAME REGION AT VARIOUS STAGES OF POLYMERIZATION OF A 2 WT% BAB IN TOLUENE AFTER :

A) 8sB) 26sC) 28sD) 46s

Of UV irradiation, respectivelyfrom top to bottom.

ANIMATION …

The formation of the network from a uniform solution

(i.e. without an initiator concentration gradient) can be examined in situ as a function of UV photo polymerization time,

Control Size of large scale structure of polymer network from (3%WT. BAB in 5CB)

Optical micrographs below illustrate the results that the structure becomes finer as either the initiator concentration or the UV intensity increases.

Because the concentration of free radical species increases. This, in turn, increases the number of polymer aggregates (at fixed monomer concentration) and therefore reduces the size of the aggregate at the point of gelation.

Polymer Stabilized CHOLESTERIC Liquid Crystals…

Polymer Stabilized CHOLESTERIC Liquid Crystals…

Cholesteric Liquid Crystals- have many application as electro-optic materials in thin film devices in as much as nematic liquid crystals do.

The presence of polymer network formed at low concentrations provide similar advantages in enhancing the stability of structure & return of crystals director to desired stable configuration.

Reducing the switching time, and helping to determine and maintain the poly-domain size.

TEXTURES…

Refers to the orientation of liquid crystals molecules in the vicinity ofsurface

1.Planar texture - if messogens are confined between closely space plates oriented with rubbing directions

parallel to the liquid crystal sample

2. Homeotropic texture – if messogens are oriented normal to the surface with the presence of electric field

3. Focal conic Texture - If small electric field is apply normal to plates the messogens experience TORQUE

PLANAR cholesteric texture

no filed apply

Axis of helix formed by directors lies normal to the plates

Parallel to the rubbing direction

In the planar texture, they reflect circularly polarized light

HOMEOTROPIC texture

If electric field is increased above threshold value

Strong filed

Helical structure of focal conic structure is untwisted and it becomes perpendicular to the plates

FOCAL conic texture

If small electric field is apply normal to plates the molecules experience TORQUE

Due to interactions with adjacent molecules & with anchoring effects near the plates this texture formed

they scatter light in forward directions.

Selective Reflection

For planar texture

The Cholesteric Liquid crystals display selective reflection

The maximum reflection occurring at a wavelength equal to the pitch which may be pre-selected by choices of materials.

Focal conic texture - it scatters the light of all frequencies

Polymer Network

It is formed during the initial stage of film preparation

Processes is identical to Nematic but small amount of CHIRAL DOPANT is added

To produce the desired Cholesteric PITCH

It mimics the texture of ordinary Cholesteric mesophase

are sometimes referred to by the acronym PSCT (Polymer Stabilize Cholesteric texture)

Polydomains

A liquid crystal material that is broken up into small domains

Usually do not necessarily coincide with the boundaries between regions of common helicity in the focal conic texture

Even in the planar texture, polydomains have slightly different orientations relative to each other

important in many factor of Display purposes

Application

Reflective Cholesteric Liquid CrystalsLight Shutter

Bistable Reflective Cholesteric Liquid Crystal Display

HOW??

If pitch is at visible wavelength it obeys SELECTIVE REFLECTION At Planar state incident light = bright At Focal state incident light = true color

Planar and Focal state are @ E=O, means locked in and will remain intactuntil upon acted.

Switching from Planar state to focal state requires low voltage, BUT returnto focal to planar state requires HIGH voltage pulse.

HOW is Bistable Reflective Cholesteric cell prepared?

Same as with the formation of (PSLCs) to have short picth &photopolymerize in initial planar state.

.

Bistable Reflective Cholesteric Liquid Crystal Display:

ADVANTAGES:

1.Employing this would relatively lowers the power requirement

2.Does not require back lighting & polarizer like traditional LCD

3. Offers high multiplexed passive display and mechanical stability

Light Shutter

Need to lengthen the Cholesteric pitch

To the order of infrared wavelength and bistable effect at E=O is

avoided

Instead choice of either planar or focal conic stable is determined by

time of polymer network formation

The starting material is the same as with bistable but concentration

of chiral agent is need to, lengthen the PITCH.

WILL FOCUS ON:

1. Reverse mode Shutter2. Normal mode Shutter3. Illustration of characterization

Reverse mode light shutter: A sol’n of longer pitch w/ few percent of reactive monomer This is place in two glasses plate whose inner surface have

been coated w/ transparent conducting medium ex. ITO Formation of planar texture Photopolymerization is achieved w/o applying field yields

polymer network, w/ a stabilizing the planar between.

NOTE:

Applying very high electric field will switch to an untwisted texture. It

will also deform the polymer network and no longer return to its

original planar texture.

Application…

FIGURE (A)

A reverse mode cell with no

applied electric field (off state).

Application

FIGURE (B)

A reverse mode cell with an

applied electric field(on state).

Normal mode light shutter:• It is polymerized in homeotropic texture produce by strong

field• If Electric field is removed the cell settle into focal conic

structure• In the absence of an electric field the cell scatters light and

is opaque. With the application of an electric field the cell is once again transparent

With regard to electro-optic properties normal and reverse mode cells

depend on voltage applied to cell

NOTE: As the VOLTAGE across the NORMAL mode cell increases

the TRANSMITTANCE increases A lower voltage yield Low transmittance High voltage produces Homeotropic texture w/ high

transmittance

Application..

FIGURE (C)

A normal cell with anElectric field applied(on state)

Application…

FIGURE (D)

A normal cell without an

electric field (off-state)

Normal mode vs. Reverse mode

normal mode shutter which is opaque (light scattering) in the field‐OFF state, transparent in the field‐ON state.

reverse‐mode shutter with the opposite field conditions. The transparent state of both cells is haze‐free for all viewing angles making the material attractive for window applications and direct view displays.

Characterization of Light shutter:

Transmittance of reverse mode cell increases as the voltage

decreases

Maximum transmittance occurs at voltage = zero

In reverse mode cell’s minimum transmittance occurs at

high-voltage

LIGHT SHUTTER application:

The choice of mode depends upon on the desired operations

Polymer Walls in PSCTs

Polymer Stabilized Cholesteric Texture (PSCTs)

HOW IS IT FORMED??? Photo curable monomers dispersed in a cholesteric liquid crystal

mixture to form it Initially low concentration of monomer

WHY LOW concentration of Polymer?

1. If not it could yield dense polymer networks2. This would result to a significant light scattering in focal conic

state, but in planar state color and brightness is reduced by scattering of light.

3. However high concentration of interest for its structural benefit, it provides self-adhering & self-sustaining structure necessary for flexible devices on large area of polymer.

Two ways to produce polymer wall:

1st method: involves irradiating only selectives areas of cell containing ultra-violet (UV) curable monomers and light crystals with UV light through a photo mask.

Two ways to produce polymer wall:

2nd method: uses the same materials but the polymer is attracted to the interpixel region by a patterned electric field.

This is done by etching a cross pattern of indium tin oxide (ITO) on

The substrate. The temperature is then decreased to phase separate

the sample while an electric field is applied by means of the ITO.