Cholesteric Liquid Crystal Shear Response

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Joe Martel - Final Wiki Entry for APPHY 225, Fall 2008

Final Project: Cholesteric Liquid Crystal Shear Response

Image of liquid crystals in bulk.

Introduction

Goal

To understand a little bit more about the dynamics of liquid crystals and their response to shear stress in different known configurations: liquid crystal coating and partially exposed polymer dispersed liquid crystals (PEPDLCs).

Liquid Crystals (LCs)

Most people know the three main phases of materials: solid, liquid and gaseous. However, things are a little more complicated upon a more detailed inspection. For some materials as they melt (go from solid or crystalline phase to a liquid phase) they undergo more than a single transition in properties. These intermediate phases are known as liquid crystals where the material exhibits certain crystal-like properties and liquid like properties simultaneously. A more specific way to define liquid crystals is how the molecules order themselves.

Crystalline structures are characterized by a rigid three dimensional structure where the molecules or atoms are relatively fixed with respect to one another. In contrast, an isotropic liquid has no such structure. Molecules and atoms in this phase move freely past each other and bounce into one another. So in between these two phases one would expect some kind of order and some kind of disorder. DeGennes and Prost (1993, p.1-2) define the three known liquid crystal phases in a convenient manner:

Nematic liquid crystals are anisotropic liquids in that their density changes differently in perpendicular directions.

An important subset of this phase is the chiral nematic phase whose adjacent layers are on a slight angle with each other. (a.k.a. cholesteric LCs)

Smectic liquid crystals have one dimensional order in three dimensions.

Columnar liquid crystals have two dimensional order in three dimensions.

This is a most basic definition. A more common definition of liquid crystals is that they have long range orientational but not positional order. More information is available on this wiki as well.Phase_transitions_in_liquid_crystals

Liquid crystals are used in a variety of technologies from thermometers to liquid crystal displays (LCDs). Different chemical are know to respond to many stimuli including light, electrical, magnetic, thermal and most relevant to this report, shear stress.

Shear Response

Schematic showing how the orientation of the different layers of chiral nematic liquid crystals changes. [1]

In the case of chiral nematic liquid crystals over their different layers they form sort of helical shapes as in the image below. When a shear is applied to these helices they deform and untwist slightly. This untwisting changes the geometry or pitch of the helix. This coupled with a property called birefringence (having two distinct indicies of refraction) can alter the wavelength and/or polarization of incident light which can be measured as a color or intensity change.

Polymer Dispersed Liquid Crystals (PDLCs)

Polymer dispersed liquid crystals are liquid crystal droplets (of varying shapes and sizes) distributed in a solid polymer matrix. The goal of this material is to contain the liquid crystals (LCs) in discrete domains. Since LCs are liquid-like they can and do flow which is a potentially detrimental for certain technologies. For example, liquid crystal displays (computer monitors or televisions); If the LCs move within the display the picture would not be very consistent or if the LCs drained out of the display you would not see any images at all.

While introducing a polymer matrix solves this flowing problem it also creates many challenges in understanding the dynamics of the LCs and their response to different stimuli including:

  1. Creating uniform droplets throughout matrix
  2. Curved surfaces of polymer matrix interacting with LCs
  3. Flow of LCs through polymer matrix depending on local structure and proximity of droplets
  4. Getting all droplets to have a uniform alignment
  5. Effect of matrix material properties

Most PDLCs are made with two generalized methods

1. Emulsion methods: LCs dispersed as droplets into polymer solution then applied to a surface and the solvent evaporates.

Examples: Polyvinyl Alcohol systems, latex based systems (paint-like), pigskin gelatin and gum arabic and water?!?!?!

2. Phase separation methods: LCs mixed with organic solvent with polymers in it then induced to separate from solvent.

Three types:

  1. Polymerization-induced phase separation (PIPS) - Mixture of monomers and LCs is used and then polymerization is initiated by some energy source which then forms into pores and LC domains.
  2. Thermally-induced phase separation (TIPS) - LCs and thermoplastic polymers are heated to a high temperature and them cooled solidifying the polymer and creating LC domains.
  3. Solvent-induced phase separation (SIPS) - LCs are dissolved in an organic solvent and are then allowed to dry and this causes separation into polymer and LC domains.

The different methods of phase separation and the dynamics of this process are very important for the morphology of the final PDLC films.

In some cases, including for shear stress measurement it is desirable to have liquid crystals exposed at the surface of the PDLC membrane. These are called partially exposed polymer dispersed liquid crystals or PEPDLCs.

Partially Exposed Polymer Dispersed Liquid Crystals (PEPDLCs)

These materials are a subset of PDLCs where the pores or droplets of liquid crystals are found to be exposed at the surface of the membrane or film which leads to more complicated interactions.

Matrix formation and matrix properties are much more important than those of the liquid crystal for determining size, shape, and anchoring characteristics.

There are more complications for understanding the pores since they are open on one side. I have not found mention of anyone studying the dynamics of these type of pores. In fact due to the high difficulty most people have only looked at the response of spherical droplets of LCs which only occur in solution for emulsion based methods and early in the phase separation process for phase separation methods.

Droplet Size and Shape

In general, Droplet size ranges from 0.1 micron to 10 microns but the majority form in the size range 1-3 microns in diameter.

Emulsion Methods

When using emulsion methods drying causes the droplets to flatten into oblate spheroids. Also when using this technique the droplets seem to stay in a 2d distribution within the polymer.

In PVA systems the polymer walls between LC droplets are usually very thin (0.1 - 0.3 microns) which have been known to break when put under stress causing flow of LCs between pores complicating the LC dynamics.

Phase Separation Methods

For some solvent-induced phase separation (SIPS) processes the film undergoes a large change in volume as the solvent evaporates leading to morphologies like those created by emulsion based methods. For both polymerization-induced phase separation (PIPS) and thermally-induced phase separation (TIPS) relatively small changes in volume occur and the changes are not usually directional. When using either PIPS or TIPS phase separation techniques there is also a 3D randomized distribution of droplets, however, some LC is absorbed into the polymer matrix swelling the polymer and altering its properties. Rate of cooling or rate of evaporation effects droplets size...In general the shorter the polymer cure time the smaller the droplets. This is due to a rapid increase in the viscosity of the polymer domains and less time for drops to coalesce.

PIPS

Depending on the mixture composition and the temperature of the film different morphologies are known to occur. Images of two different PDLC film morphologies are below...

Left: LC Droplet morphology, Right: Polymer Ball morphology

These morphologies have been well classified using the following phase diagram for a specific type of LC and polymer.

Phase diagram for LC polymer mixture.

TIPS

Droplet size can be controlled fairly well by setting the cooling rate. (Faster cooling rate...Smaller droplets)

Films are extremely sensitive to processing and therefore are difficult to reproduce.

Also these types of films are unstable at high temperatures where the polymer and LC are soluble in each other.

SIPS

Droplet size can be controlled fairly well by setting the evaporation rate. (Faster evaporation rate...Smaller droplets)

Polymers used in this process are usually thermoplastic and the final films can be reheated using a TIPS process to change the droplet sizes.

Phase Separation and Film Formation

The below information is specific to PIPS processes...

Generalized phase diagram for a PIPS system


Comparison of nucleation and growth versus spinodal decomposition.

Note that two different phase separation processes are possible; nucleation then growth or spinodal decomposition. The space between the spinodal and binodal curves, the activation threshold within the metastable region, speed of the polymerization and whether sites are available for nucleation are all determining factors for which phase separation process occurs. This is of great importance since the domain sizes and shapes as discussed earlier can effect the desired response of the film to different stimuli especially electromagnetic forces and shear forces.


















.

Within Droplet LC Configurations

Of course another important factor for the response of the LCs in PEPDLC films to shear is their orientation within the open pores. Since I have been unable to find a reference that specifically looks at PEPDLC films I will describe what researchers have found in spherical LC droplets and discuss them in relation to shear stress response.

In shear stress measurement with liquid crystals it is desirable to have the director of the LC phase to be pointed perpendicular to the shear force for the maximum color/intensity change to occur. This becomes complicated within droplets since each could have different director directions or even have patterns instead of uniform director direction. The different patterns come from different boundary conditions depending on the LC-polymer interactions and/or anchoring characteristics.

Overview of interesting occurrences in LC droplets that do not normally occur in bulk LCs on a flat surface...

  1. Curved director fields
  2. Director field dislocations and defects

Disclaimer from Paul Drziac in his text, "Liquid Crystal Dispersions"

"It is important to point out that the droplet configurations described in this chapter are most often studied in liquid solution. Suspended in a fluid, a nematic droplet can exist as a true sphere... These spherical configurations, however, are only approximations to the droplet configurations usually found in polymeric matrices."

Keeping this in mind and the fact that most LC droplets in PDLC and PEPDLC films are not exactly spherical look at these cool findings.

Director fields usually orient themselves with the major axis of the droplet, which is convenient for ellipsoidal droplets but for complicated droplets the director field is uncertain.
Director orientation following major axis

Now specifically for cholesteric (chiral nematic) LCs if their pitch is smaller than the droplet diameter...(This is just meant to give you a taste of the complexity. More detail is available in [2])

Parallel wall alignment:

Radial line defect structure:

Radial line defect

Diametrical defect structure:

Diametrical defect

Perpendicular wall alignment:

Finger print patterns when pitch << drop diameter

Radial pattern when pitch is infinite or greater than the diameter

Radial pattern with infinite pitch

No wall interaction:

Unidirectional director alignment but random between spherical droplets.

No wall interaction

Another reference goes into much more detail about LCs in curved domains [3].

Conclusion

The majority of work in this area has been completed with the focus on display technologies (LCD televisions and monitors). This is apparent in which areas of research have been looked at in the most detail. Most of the main reference for this report was related to the switching of the different LC domains due to electromagnetic fields. The geometries of PEPDLC films also make the morphologies less predictable and therefore it is more difficult to generalize about the films' shear response dynamics. More investigation is needed to get more detail into both shear response and electromagnetic response in these various domain shapes and their associated complexities... And while LCD TVs and monitors are still being used its a pretty safe assumption more research will be done in this area even if not directly for PEPDLC applications.

References

De Gennes, P.G. and Prost, J. "The Physics of Liquid Crystals - Second Edition." International Series of Monographs on Physics - 83. 1993.

Drziac, P. "Liquid Crystal Dispersions." World Scientific Series on Liquid Crystals Volume 1. January 1994.





Ideas:

Size and shape differences of droplets of polymer dispersed liquid crystals that are knife-bladed onto a surface versus sprayed.

Shear response dependence on pore size and shape - Some sort of scaling?

Pore dynamics versus flat surface under shear stress, stresses at boundaries of pores

How do cholesteric liquid crystals attach to a glass surface?

Why is polystyrene used in PEPDLC? -Material interactions -interfacial/adhesion forces

Keep track of forces and length scales -bulk vs. boundaries