The rest of this thesis is organized as follows. The introduction and fabrication ways of polymer dispersed liquid crystal, and factors controlling alignment in LC droplets are presented in Chapter 2. In Chapter 3, the instruments used to measure the PDLC films will be described. The different fabrication ways of the PDLC films and the experiment results will be in Chapter 4 and the extended experiment, “biaxial extension” will be in Chapter 5. The conclusion and future work of this thesis will be given in Chapter 6.
Chapter 2
Overview of Polymer Dispersed Liquid Crystals
2.1 Components of Polymer Dispersed Liquid Crystals[12]
Polymer dispersed liquid crystals (PDLCs) are a relatively new class of materials which was discovered until the early 1980’s and hold promise for many application ranging from switchable windows to projection displays.
These materials, which are simply a combined application of polymers and liquid crystals, are the focus of extensive research in the display industry. Other different types of polymer-liquid crystal composites with low concentration polymer networks that stabilize the bulk liquid crystal domains have been developed, such as polymer stabilized liquid crystal (PSLC), polymer network liquid crystal (PNLC), polymer stabilized cholesteric liquid crystal (PSCLC).
PDLCs have solid polymer matrix with liquid crystal droplets embedded in it.
The LC droplets sizes (usually having bipolar configuration) range from hundred nano-meters to a few micro-meters. These tiny droplets are responsible for the unique behavior of the material. By changing the orientation of the liquid crystal molecules with an electric field[13], it is possible to vary the intensity of transmitted light.
PDLC windows are based on the ability of the nematic director of the piqued crystal droplets to align under an electric field. In a typical application, a thin
PDLC film (about 10 to 25 microns thick) is deposited between clear plastic covers. Transmission of light through a PDLC window depends primarily on scattering, which in turn depends on the difference in refractive index between droplets and their environment. In the case of high droplet density, the environment consists mainly of LC droplets, which makes the relative orientation of their directors an important factor. The droplets are anisotropic with the index of refraction parallel to the director different from that perpendicular to it. As sketched in figure 2.1, at voltage-off state, the random array of droplet orientation provides significant differences in indices and hence strong scattering. In this state, the cell appears opaque. On the contrary, light transparent the film at field-on state because the director of the individual droplets aligns with the field. There is now little difference in refrantive index for neighboring droplets, and the cell appears transparent.
Figure 2.1 Operation principle of a PDLC shutter; (a) V=0; scattering state, (b) V≠0; transparent state.
2.2 Fabrication Ways of PDLCs
Polymer dispersed liquid crystals are usually produced in two distinct ways:
encapsulation and phase separation[14,15]. Each method produces PDLCs with different properties and characteristics. Among the factors influencing the properties of the PDLC material are the size and morphology (shape) of the droplets, the types of polymer and liquid crystal used, and cooling and heating rates in production. The relationship between the method of production and these factors is explained below.
2.2.1 Encapsulation
Early attempts to produce PDLCs were made with a technique known as microencapsulation. In this method, a liquid crystal is mixed with a polymer dissolved in water. When the water is evaporated, the liquid crystal is surrounded by a layer of polymers. Thousands of these tiny “capsules” are produced and distributed through the bulk polymer. Droplets produced with this method tend to be non-uniform in size and can even be interconnected with each other. Materials fabricated by encapsulation are referred to as NCAP or nematic curvilinear aligned phase.
2.2.2 Phase Separation[16,17,18]
In order to obtain PDLCs by phase separation, a homogeneous mixture of polymer and liquid crystal is first produced. The liquid crystal droplets are then formed by the separation of the two phases. The separation can take place in one of the following three ways:
(1) Polymerization-Induced Phase Separation
Polymerization-induced phase separation (PIPS) occurs when a liquid crystal is mixed with a solution that has not yet undergone polymerization (a prepolymer).
Once a homogeneous solution is formed, the polymerization reaction is initiated either thermally (thermoset polymer) or optically (photocurable polymer).
Several PDLC sample have been obtained by an application of bias electric field intensity of 10-15 V/um and frequency of 150 Hz during curing the prepolymer.
Because all used LC mixtures have positive anisotropy of dielectric permittivity, the longer droplets’ axes have been aligned perpendicular to the glass plates. The principle of the method is shown in figure 2.2(a).
In addition, photopolymerization-induced phase separation (PPIPS) has been chosen as the most effective way to obtain ellipsoidal or flat LC droplets with extremely high aspect ration. The prepolymer can be cured by UV radiation. The curing rate, hence droplet size, has been adjusted by changing UV flux up to 20 W/cm2. A slow unidirectional shearing of the upper glass plate with the respect of bottom one has been applied when the system has been partly cured up to the moment of full curing and PDLC stabilization. The scheme of the respective setup is given in figure 2.2(b).
(a) (b)
Figure 2.2 Schematic presentation of the techniques used to elongate LC droplets in PDLC composite: (a) the deforming effect of electric field applied during PPIPS, (b) shearing of the system during PPIPS, an arrow represents the direction of the upper glass movement.
(2) Thermally-Induced Phase Separation
Thermally –induced phase separation (TIPS) can be used when the polymer binder has a melting temperature below its decomposition temperature. In this method, a homogeneous mixture of liquid crystal and a melted polymer is formed. The solution is cooled at a specific rate to induce phase separation.
Liquid crystal droplets begin to form as the polymer hardens. The droplets continue to grow until the glass transition temperature of the polymer is crossed.
Droplet size is affected the most by the cooling rate of the solution. Fast cooling rates tend to produce small droplets because there is not sufficient time for large particles to form. Therefore, droplet size and cooling rate are related inversely.
Then the PDLC foil is carefully separated from the substrate , placed on heating rod and caught by clips one of which is steady while the second one is mounted on the movable stage driven by micrometric screw. In the heated region the elongation reached about 150 percent. Then the LC droplets elongated and flattened are stabilized by fast foil cooling. The scheme of the respective setup is given in figure 2.3.
Figure 2.3 Schematic presentation of the techniques used to elongated LC droplets in PDLC composite: plastic stretching of the PDLC foil obtained by TIPS or SIPS.
(3) Solvent-Induced Phase Separation
The third common type of phase separation is called solvent-induced phased separation (SIPS). This process requires both the liquid crystal and polymer to be dissolved in a solvent. The solvent is then removed (typically by evaporation) at a controlled rate to begin the phase separation. Droplets start growing as the polymer and liquid crystal come out of solution and stop when all of the solvent has been removed. It still can be heated to elongate the LC droplets.
2.3 Factors Controlling Alignment in LC Droplets[19,20]
The configuration adopted by the (nematic) liquid crystal director field within a droplet reflects the subtle interplay of forces. The factors that determine the director configuration include the intrinsic, anchoring characteristics of the LCs, the presence of ecternal fields and the size (shape) of the droplets. The physical factors are:
Surface alignment
Surface alignment (between the liquid crystal and polymer binder) is the most important factor in determining the droplet configuration. Typically the anchoring energy which enforces this preferred alignment is quite strong compared to other elastic forces within the droplet. In the presence of strong surface anchoring force, the liquid crystals adopt a uniform tilt angle (either 0 or 90o) at all points on the droplet surface.
Elastic constants
The balance of elastic forces within the droplet is the second important factor.
To pack the liquid crystal into spherical shape, one or more defects were created.
The elastic forces determine the structure within the droplet and the number (type) of defects. The relative values of the elastic constants K11, K22 and K33 influence the preferred configuration of the director field within a cavity.
Droplet Size
The size of the cavity affects the elastic free energy density of the liquid crystals inside the droplets. In large droplets, the elastic forces (scale as curvature per unit length) are often too weak to force the LC directors. Many defects would exist to minimize the local free energy. The configurations in these droplets are complex.
Chapter 3
Measurement Systems
3.1 Overview of Measurement Systems
In this chapter, the measurement setups used in the experiments will be described in the following sections. The surface condition and the film thickness of PDLC films are observed by using α-step profilometer. Instrument such as polarizing optical microscope (POM), and laser optical system are utilized to characterize polarization and light selectivity. The size of LC droplets can be told by using the scanning electron microscope (SEM). The alignment of the LC droplets in polymer matrix can be observed by using Fourier transform infrared spectrometer (FT-IR). The major feature of the above mentioned instrument will be illustrated in this chapter.
3.2 α-Step Surface Profiler
α-Step surface profiler is a state-of –the –art, stylus-based surface profiler that combines high measurement precision with and economy. Ideal for applications such as semiconductor pilot lines and materials research, this advanced profiler enables faster process learning and higher yields. With guaranteed 8*10-10m (1 sigma) or 0.1% step height repeatability and sub-angstrom resolution, the α-Step provides excellent repeatability and performance to analyze and monitor processes. The diagram ofα-Step surface profiler is shown in figure 3.1.
Figure 3.1 The picture of α-Step surface profiler.
3.3 Polarizing Optical Microscope (POM)
The polarized optical microscope is designed to observe and photograph specimens that are visible primarily due to their optically anisotropic character.
In order to accomplish this task, the microscope must be equipped with both a polarizer, positioned in the light path somewhere before the specimen, and an analyzer (a second polarizer), placed in the optical pathway between the objective rear aperture and the observation tubes or camera port. Image contrast arises from the interaction of plane- polarized light with a birefringent specimen to produce two individual wave components that are each polarized in mutually perpendicular planes. Liquid crystal microphotographs were observed under POM, Olympus BX51 as shown in figure 3.2, the magnifications of POM are 100X, 200X, 500X and 1000X with changeable object lens of 10X, 20X, 50X and 100X, respectively, and a 10X eyepiece. Two measurable modes depend on transparent and reflective substrates are utilized with bottom light source and top
light source, respectively. An adjustable and movable polarizer can be utilized in both modes. Images observed under POM can be captured under CCD, and the parameter such as distance, area and angle can be calculated with its software.
Figure 3.2 The concept and picture of POM Olympus BX51.
3.4 The Scanning Electron Microscope (SEM)
In light microscopy, a specimen is viewed through a series of lenses that magnify the visible-light image. However, the scanning electron microscope (SEM) does not actually view a true image of the specimen, but rather produces an electronic map of the specimen that is displayed on a cathode ray tube (CRT).
The diagram of SEM is shown in figure 3.3. electrons from a filament in an electron gun are beamed at the specimen in a vacuum chamber. The beam forms a line that continuously sweeps across the specimen at high speed. This beam
irradiates the specimen which in turn produces a signal in the form of either x-ray fluorescence, secondary or backscattered electrons.
The SEM at GMU has a secondary electron detector. The signal produced by the secondary electrons is detected and sent to a CRT image. The scan rate for
electron beam can be increased so that a virtual 3-D image of the specimen can be viewed. The image can also be captured by standard photography.
Figure 3.3 The diagram of the scanning electron microscope (SEM).
3.5 Fourier Transform Infrared Spectrometer (FT-IR)[21]
Fourier transform infrared (FT-IR) spectrometer has been extensively developed over the past decade and provides a number of advantages. Radiation containing all IR wavelengths (e.g., 4000-400cm-1) is split into two beams. One beam is of fixed length, the other of variable length (movable mirror). The diagram of FT-IR is shown in figure 3.4 and 3.5.
The varying distances between two path lengths result in a sequence of constructive and destructive interferences and hence variations in intensities: an interferogram. Fourier transformation converts this interferogram from the time domain into one spectral point on the more familiar form of the frequency domain. Smooth and continuous variation of the length of the piston adjust the position of sample mirror and varies the length of the beam; Fourier transformation at successive points throughout this variation gives rise to the complete IR spectrum. Passage of this radiation through a sample subjects the compound to a broad band of energies. In principle the analysis of one broadband pass of radiation through the sample will give rise to complete IR spectrum.
There are a number of advantages to FT-IR methods. Since a monochromator is not used, the entire radiation range is passed through the sample simultaneously and much time is saved; FT-IR instruments can have very high resultion (<0.001cm-1). Moreover since the data undergo analog-to-digital conversion, IR results are easily manipulated.
Figure 3.4 The scenograph of the FT-IR Spectrometer.
Light source
polarizer
sample Detector
Figure 3.5 The diagram of the FT-IR Spectrometer.
Chapter 4
Results & Discussion of Uniaxial Extension
4.1 Methods to Obtain Deformed LC Droplets
Each method of PDLC preparation could be used to obtain composites nonspherical LC droplets, however, only phase separation techniques secure the proper control of droplets’ size and shape. In this work we present four ways of preparation of elongated LC droplets in PDLC composites: R-to-R molding (by hand), spin coating, mold movement, and stretch and the results of each way will be described in this chapter..
4.2 Preparation of Elongated LC Droplets
General methods of PDLC preparation have been studied. In the case of encapsulation, the system is heterogeneous during the whole process. LC is dispersed in a polymer solution, the solvent of which does not dissolve LC. The solvent evaporation stabilizes the obtained composite structure due to polymer solidification.
4.2.1 Liquid crystal[22]
Nematic LC-E7 used as PDLC component has been prepared by Merck display technologies ltd.. It has been designed to meet standard PDLC requirements,
namely matching refractive indices of polymer and LC and fulfilling very low solubility of LC in cured polymer. The refraction indices of E7 are shown in table 4.1.
Table 4.1 Refraction indices of E7
ne n0 Δn
1.745 1.52 0.225
4.2.2 R-to-R molding (by hand)
The way R-to-R molding is used to get shearing force to elongated LC droplets in PDLC composites. NOA65 is used as the polymer here. The properties of NOA65 are shown in table 4.2 and table 4.3 shows the refraction indices of E7 and NOA65.
Table 4.2 The properties of NOA65
materiality materiality (25℃) np tensile ultimate
100% 1200cps 1.524 80%
Table 4.3 Refraction indices of E7 and NOA65
no ne Δn
E7 1.520 1.745 0.225 np
NOA65 1.524
To prepare a PDLC cell, we mixed nematic LC E7 with an UV-curable monomer NOA65 at 25:75 wt%. We used an agitator to emulsify the liquid crystal into the NOA65 solution and used an ultrasonic probe to drive bubbles out. After the emulsion was prepared, it was coated on a mode with grooving and covered with a polyethylene terephtalate (PET) substrate using a rod. The film was cured by the UV-irradiation machine and peeled from the substrate. We tried three different pitches of modes for getting different shearing force, such as W=50um, 10um, and 4um, and D= 25um, 5um, and 2um..
We used a scanning electron microscope (SEM) and a polarizing optical microscope (POM) to observe the shape of LC droplets and the orientation. The observation of SEM and POM are sown in figure 4.1. and 4.2. According to these figures, the amounts of LC droplets are only a few, the size is about 1um and the orientation of LC droplets is not uniform.
Figure 4.1 The SEM picture of E7/PVA PDLC film (magnification:
23000X)
(a) (b)
Figure 4.2 The pictures of PDLC films made by R-to-R method taken by POM: magnification in (a) is 200X, in (b) is 500X.
In order to know the polarizing property of PDLC films, we used UV/Vis spectrophotometer- LAMBDA 950 and Fourier Transform Infrared Spectrometer (FT-IR) to observe. ‘-CN’ is a representative functional-group of E7 and it absorbs a specific wave band at 2230cm-1. We divided PDLC film into five parts shown in figure 4.3 and used FTIR to observe their absorption spectrums shown in figure 4.4.
Figure 4.3 PDLC film is divided into five parts.
Figure 4.4 An absorption spectrum with light parallel (perpendicular) to the grooving of the film taken by UV/Vis spectrophotometer.
Use a formula as follow. We can get order parameters of each part.
//
//
order parameter
2 A A
A A
⊥
⊥
= − +
A∥ (⊥): absorption with polarization of light parallel (perpendicular) to the grooving of the film.
We found that each part of this film approximated to zero. It meant that LC droplets oriented almost randomly.
Figure 4.5(a) was taken by FT-IR with a tunable polarizer. The definitions of A∥ and A⊥ are the same as that used in UV/Vis spectrophotometer. From figure 4.5(b) which is given by stacking two absorption spectrums in figure 4.5(a) at 2230cm-1, polarization selectivity can be seen but not obviously.
(a) (b)
Figure 4.5 (a): An absorption spectrum with light parallel (perpendicular) to the grooving of the film taken by FT-IR. (b): Stack up (a) at 2230cm-1
According to the pictures taken by SEM and POM, and absorption spectrums taken by UV/Vis spectrophotometer and FT-IR, we can presume that the orientation of LC droplets in PDLC films made by R-to-R method is not uniform or LC droplets are not stretched during the process.
4.2.3 Spin Coating
The way-spin coating is used to get shearing force to elongated LC droplets in PDLC composites. NOA65 and E7 are the components of the PDLC films.
The preparation of LC/polymer solution is the same as which in R-to-R method.
After the emulsion was prepared, it was coated on a PET substrate using a spin coater. During the process, we changed rotation rates to get different shearing force. The film was cured by the UV-irradiation machine.
In order to know the polarizing property of the PDLC film, we used spectrophotometer-LAMBDA 950 and FT-IR to check each part of films. The result is that we could not get effective polarizing property by spin coating method. The shearing force made by spin coating might not be influential enough to elongate and orient LC droplets.
4.2.4 Mold movement
The way-mold movement is used to get shearing force to elongated LC droplets in PDLC composites. NOA65 and E7 are the components of the PDLC films.
The preparation of LC/polymer solution is the same as which in R-to-R method.
After the emulsion was prepared, put some emulsion on the substrate (PET) covered by the mold. A slow unidirectional shearing of the mold with respect to the substrate was applied. The film was cured by the UV-irradiation machine.
We used spectrophotometer-LAMBDA 950 and FT-IR to check each part of films. We still could not get effective polarizing property by way of mold movement. The reason we presume is that the elongated LC droplets recovered before cured and it’s easy to get a lot of bubbles during mold movement.
4.2.5 Stretch
4.2.5 Stretch