Développement des cristaux liquides stabilisé par des polymères pour les applications en photonique

LARISA ZOHRABYAN

DÉVELOPPEMENT DES CRISTAUX LIQUIDES

STABILISÉ PAR DES POLYMÈRES POUR LES

APPLICATIONS EN PHOTONIQUE

Mémoire présenté

à la Faculté des études supérieures de l'Université Laval dans le cadre du programme de maîtrise en Physique pour l'obtention du grade de Maître es Sciences (M.Se.)

DÉPARTEMENT DE PHYSIQUE UNIVERSITÉ LAVAL

QUÉBEC

2008

Résume

The aim of the présent project is to develop and investigate new types of polymer stabilized liquid crystals (PSLC), based on non-mesogenic monomers. The PSLC system is composed of few percent of monomer dissolved in the nematic liquid crystal (NLC) matrix. The photo-induced radical polymerization results in the formation of polymer network, the morphology of which dépends on the functionality of the monomer used as well as on the polymerization conditions. Thus, linear, 2D and 3D polymer networks are created, changing the elastic properties of the NLC, which can be used for the stabilization and control of liquid crystal (LC) cell's alignment and electro optical properties. Most importantly, the morphology of the polymer network near to the surface, in a sub-micrometric distances from the boundary of the cell and its interaction with the cell substrate plays crucial rôle in the electric field induced planar or 2D (in the plane of the electric field) and 3D molecular reorientations.

We will investigate a rich variety of PSLCs to find and demonstrate their unique electro-optic responses for différent photonic applications.

Résumé

De nouveaux types de cristaux liquides stabilisés par les réseaux de polymère (PSLC) sont étudiés, en utilisant des monomères non-mesogenic. Le système de PSLC est compose de quelque pourcent de monomère dissous dans la matrice à cristal liquide nematique (NLC). La polymérisation radicalaire photo-induite a comme conséquence la formation de réseau de polymère, la morphologie duquel dépend de la fonctionnalité du monomère utilisé. Ainsi, des réseaux du polymère linéaire, 2D et 3D sont créés en changeant les propriétés élastiques du NLC, qui peuvent être employées efficacement pour la stabilisation et le contrôle de l'alignement et des propriétés électro-optiques des cellules (LC) à cristal liquide.

D'une manière plus importante, la morphologie du réseau de polymère qui se forme près de la surface dans des distances sub-micrométriques de la frontière de la cellule et son interaction avec le substrat de la cellule joue un rôle crucial dans la réorientation moléculaire induite 2D (dans le plan du champ électrique) et 3D.

Une riche variété de PSLCs ont été étudiées. Des propriétés électro-optiques prometteuses de PSLCs ont été obtenues pour différentes applications photoniques.

Avant-propos

Ce travail a été réalisé dans le laboratoire de matériaux et composants optoélectroniques du centre d'optique, photonique et laser (COPL) du Département de Physique, de génie physique et d'optique de l'Université Laval. Il est le résultat de trois longues années de travail acharné d'exploration de nouveaux types de polymères cristaux liquides stabilisés (PSLC), basés sur les monomères non-mésognéique différents.

Ce travail est donc l'aboutissement de nombreuses collaborations enrichissantes les unes des autres dont je suis infiniment reconnaissante.

Je tiens tout d'abord de remercier mon directeur de recherche, Monsieur Tigran Galstian, pour sa disponibilité et son ouverture, dont les conseils et le support furent très fructueux pour la réalisation de l'ouvrage présent.

Ainsi, je voudrais remercier Vladimir Prisniakov, Karen Asatryan, Amir Tork pour leur aide et disponibilité tout au long de réalisation de ce travail de mémoire. Un merci tout spécial à Madame Ana-Maria Albu (Department of Polymer Science and Engineering, University of Bucharest, Romania), dont l'étroite collaboration et l'aide pour la partie des expériences chimiques fut très productive.

Enfin, je remercie tous les membres de ma famille et mes amis pour leur support et leur encouragement.

Table des matières

Résume 2 Avant-propos 4 Polymer stabilized liquid crystals for photonic applications

General scope 6

Introduction

1.1. LC based de vices. 8 1.2. Polymer dispersed and polymer stabilized liquid

crystal (PDLC and PSLC) based LCD's 19

1.3. Switchable Windows 22

Références 23

Chapter 1: Material System and cell

1.1. Introduction 27 1.2. Chemical composition of monomers used for PSLC préparation 29

1.3. PSLC cell fabrication procédure 33 1.4. Expérimental method for PSLC analyses: Optical polarizing microscopy 35

Conclusion 36 Références 37

Chapter 2: Morphologies of non-mesogenic polymer stabilized liquid crystals.

2.1. Introduction 39 2.2 Passive and active polarizing light microscopy observations:

expérimental conditions 40 2.3. Morphological observations 41

2.4. Discussions 49 Conclusion 52 Références 54

Chapter 3: Scattering of PSLC: expérimental results and comparative analyses.

3.1. Introduction 56 3.2. Light scattering theory 56

3.3. Expérimental conditions and setup for light scattering measurements 60

3.4. Electro-optic measurement results and discussions 62

Conclusion 68 Références 68

Chapter 4. Influence of polymer network to PSLC dynamic switching

4.1. Introduction 69 4.2. Nematic LC reorientation characteristic times: theory 70

4.3. Material system, expérimental conditions and setup used

for dynamic measurements 71 4.4. Electro-optic measurement results and discussions 72

Conclusion 76 Références 77 General conclusion 78

Polymer stabilized liquid crystals for photonic applications

General scope

Liquid crystals (LC) hâve been discovered more than one century. Due to the natural polarity of LC molécules, they exhibit high dielectric and optical anisotropy The value of optical biréfringence is more than one order of magnitude higher compared to the best solid state material's one and could reach the values of up to An=ne-no=0.4 for

the visible part of the spectrum, where the ne is extraordinary and the n0 is ordinary

refractive indexes of LC, respectively. In addition, most LC's hâve high optical quality. Major drawbacks of the LC based photonic devices are the slow response time and polarization dependence. Hence there is an urgent need to develop fast-response and polarization independent LC devices for the growing photonic applications.

Several approaches hâve been proposed to optimize the LC response time, using a high biréfringence (thus, thin cell gap) and low viscosity LC, elevating the operating température, or using a stressed (parallel sheared) LC cell. However, thèse methods either rely on the new LC materials development or complicate the fabrication process. There hâve been some reports that LC based device characteristics (like dynamic response, polarization sensitivity) can be significantly enhanced by using liquid crystal/polymer composites [1]. Liquid crystal/polymer composites are a relatively new class of materials. In the case of the polymer stabilized liquid crystals (PSLC) a small percentage (2-10%) of a reactive monomer is mixed with nematic liquid crystal (NLC). After mixing, the

mixture is injected into a LC cell with a proper surface treatment. The application of two types of PSLC gel Systems may be defined by the alignment used: homogenous [2-5] or homeotropic [6]. The homogenous aligned PSLC gel is used for modulating linearly polarized light and the homeotropic Systems is intended for unpolarized light. Each type has its own merits and drawbacks. In both cases the type and monomer concentration as well as cell gap hâve important effect on the operating voltage and contrast ratio of PSLC. Usually a higher monomer concentration or thickness of cell gap leads to a higher operating voltage.

Until recently, the studies of such materials offered information's mainly with orientation disturbances induced in liquid crystals by the présence of polymer networks. Usually, the polymers hâve been produced by photo-polymerization of diacrylate monomers, containing mesogenic séquences [7-15].

As it is known, the synthesis of mesogenic monomers is long, tiresome and costly; accordingly, new studies concerning the synthesis and characterization of PSLC Systems hâve been reported [16-18], using non-mesogenic monomers.

In the présent work we investigate optical properties of several non-mesogenic polymer stabilized liquid crystals, oriented to their use for différent photonic applications.

Introduction

1.1. LC based devices.

In introduction we will describe opération principles of différent types of LC based devices, particularly displays. A historical review of image displaying devices will be presented.

Différent type of LCs (smectic, nematic and cholesteric, including LCs consisting différent organic and inorganic materials, like polymers and inorganic nanoparticles) in combination with the ability of their initial orientation (planar, homeotropic, twisted, tilted, etc.) and electric control mechanism give a wide degree of freedom for LC based device design and further applications. LC composition, used cell with the surface treatment and électrode geometry détermine the electro-optic response of the LC system. There are many investigations showing, that LC mixtures with différent materials can significantly improve optical characteristics of LCs, such as response time, operating température range, induced phase delay magnitude,, etc. In the case of LC and polymer mixtures, thèse changes are related to the morphological change of LC composition, which détermines LC domain boundary interaction conditions with the mixture domains, hence final solution's electro-optical properties. As an example can be polymer dispersed liquid crystals (PDLC) and polymer stabilized liquid crystals (PSLC).

Thèse applications include displays, wavelength division multiplexing filters, light polarizer, tunable focal length lens, privacy Windows and finally, display applications.

Presently LCs are widely used in many scientific and industrial applications, particularly in electrically controlled tunable photonic devices. Excluding display applications, the short list of LC based devices consist of light modulators [19], wavelength filters [20-22], variable optical attenuators [23-24], optical switches [25], optical waveguides [26] etc.

As a promising application of LCs is the tunable focal length device. Authors of [27] hâve investigated the EO properties of polymer stabilized nematic LCs, in situ polymerized by Gaussian intensity distribution laser beam. The refractive index distribution in such structure under the action of homogeneous electric field reveals a non-homogeneous lens-like character. Obtained refractive index distribution is approximately reproducing the intensity transverse distribution of photopolymerazing Gaussian beam. Such a structure can be used for the création of electrically controllable focal length lenses. Another, close to this, approach has been proposed by authors of [28]. Instead of shaped polymerizing beam they use uniform polymerizing beam in combination with variable density filter photomask. As a resuit the gradient polymer network was obtained by photopolymerization through photomask. By changing the photomask pattern, both positive and négative lenses were fabricated. Another example of variable focal length lens hâve been reported by authors of [29], using Fresnel zone plate. They obtain Fresnel zone plate by photopolymerizing the polymer stabilizing LC cell through a patterned photomask, that was produced by etching a chromium oxide layer using électron beam lithography. Due to the polymer network effect, the created Fresnel lens has fast response time of less than 10ms. In ail described above devices a

drawback is that it is polarization sensitive. To overcome polarization dependence two orthogonal LC cells could be considered.

Another practical application of LCs is in light polarizing device. Hère both pure LC and LC based anisotropic gels were used. Pure LC was used by authors of [30] as an overlay material for side polished fiber blocks. When the surface of ordinary single-mode fiber is polished, the fiber is no longer cylindrically symmetrical. The state of linearly polarized light oriented parallel to a polished surface is the pseudo-TE mode and its orthogonal state is the pseudo-TM mode. So the propagating in the fiber light with différent states of polarization will see différent refractive indexes in overlaid LC. If the electrical induction's direction of linearly polarized light is aligned at an angle 9 with respect to LC director, it will see an index of refraction n(9)

1 _cos2(0) sin2(fl)

n]{0)~ n2u n[

where «j and n i are the extra-ordinary and ordinary refractive indexes of the LC, respectively. With such a LC, with its two principal indexes falling on both sides of a fiber index, a fiber polarizer of a low insertion loss can be fabricated.

An example of "non pure" LC application for light polarizer was presented by authors of [31]. They produce anisotropic nematic gel, which demonstrates electrically controlled anisotropy of scattering. The polarization ratio 500 and more was achieved, using developed anisotropic gel.

Authors of [32] hâve presented investigation results of chiral nematic liquid crystal compositions used for switchable mirrors application. Hère the présence of specially selected mono- and di-acrylates, in non reactive LC mixture, highly influences

Finally the most important application of LCs is in display technologies. Movies work because of persistence of vision, the fact that a human eye retains an image for about one-twentieth of a second after seeing it. The first movie was developed by William George Horner in 1834: Presently there are many complex devices for the présentation of still or moving information. Ail thèse devices require a tool or place to visualize information: displays. Generally there are two types of displays: direct view and projection displays. Cathode ray tube (CRT) based displays or plasma displays (PD) are typical examples of direct view technology. The most gênerai prototype of projection display is one used in early stages of cinematography. Hère, as a display, a simple white screen was used in combination with an optical projecting System. CRT based displays are limited in size. When screen size increases, the overall dimensions and weight increase accordingly. For display sizes greater than 40 inches, projection displays are more favorable than direct-view CRTs because projection displays offer much smaller size and weight. Plasma displays use as much power per square meter as a CRT télévision and has a limited lifetime. The lifetime of the latest génération of plasma displays is estimated at 60,000 hours.

After discovering liquid crystals (LC), they were intensively investigated as an attractive material for displaying applications. LCs promote their way towards the development of both above mentioned types of display applications, as well as switchable screen-like Windows (which will be discussed further). The brief list of LC based display applications include home electronics like TV, radio, watches, pocket calculators, flat screens for laptop computers, projectors as well as press, cultural announces, scientific and commercial présentations etc. In the very near future more and more space and

energy consuming CRT or other alternative displays like PD will be replaced by LC based flat screens with low energy consumption, and liquid crystal displays (LCDs) will take over more and more market shares. Another aspect of LCs is their potential application as switchable Windows, whose tint can be controlled through the use of electricity. Electronically switchable Windows are now finding their way into gênerai use, everything from privacy screens in office interiors to skylights that darken and lighten automatically.

LCDs are common because they offer some real advantages over other display technologies. They are thinner and lighter and draw much less power than CRTs, for example. Before to explain the main principles and opération conditions of modem LCDs, the brief description of liquid crystals will be presented below for better understanding.

Liquid crystals were first discovered in 1888, by Austrian botanist Friedrich Reinitzer. The molécules in liquid crystals tend to maintain their local orientation in the same direction, like the molécules in a solid, but also move around to différent positions, like the molécules in a liquid. This means that liquid crystals are anisotropy liquids. That's how they ended up with their seemingly contradictory name. Just as there are many varieties of solids and liquids, there is also a variety of liquid crystal substances.

The orientation of the molécules in the nematic phase is described by the so called director. The initial orientation of LC molécules could be done for example by a surface that has microscopic grooves in it. Liquid crystals can be classified by the way how the molécules orient themselves with respect to each other. Smectic, the most common arrangement, créâtes layers of molécules. There are many variations of the smectic phase,

such as smectic C, in which the molécules in each layer tilt at an angle from the previous layer. Another well known phase is the cholesteric LC, also known as chiral nematic. In this phase, the molécules twist slightly from one layer to the next, resulting in a spiral formation [33].

Ferroelectric liquid crystals (FLCs) use liquid crystal substances that hâve chiral molécules in a smectic C type of arrangement. Because of the présence of spontaneous polarization thèse molécules allows the microsecond switching response time that makes FLCs particularly suited for advanced displays. Surface-stabilized ferroelectric liquid crystals (SSFLCs) apply controlled pressure through the use of a glass plate, suppressing the spiral of the molécules to make the switching even more rapid (see

http://electronics.howstuffworks.com/lcdl.htm).

Hère we will discuss liquid crystals in the nematic phase, the liquid crystals that made LCDs possible. One important feature of liquid crystals is that their alignments are affected by electric or magnetic fields. A particular sort of nematic liquid crystal display, so called twisted nematics (TN), is mechanically twisted. Applying an electric field to thèse liquid crystals will untwist them to varying degrees, depending on the applied to the spécial électrodes voltage. LCDs use thèse liquid crystals because they react predictably to electric field in such a way as to control light transmission.

LCD is a thin, lightweight device with no moving parts. First developed LCD was based on twisted nematic field effect in liquid crystals. Twisted (or supertwisted) nematics based technology is still the most prominent type of liquid crystal light modulators in modem displays. Fig. 1.1 shows the corresponding principle of opération. A nematic liquid crystal is filled between two glass plates, which are separated by thin

spacers, coated with transparent électrodes and orientation layers inside. The orientation layer usually consists of a polymer (e.g. polyimide) which has been unidirectionally rubbed e.g. with a soft tissue. As a resuit, the liquid crystal molécules are fixed with their alignment more or less parallel to the plates, pointing along the rubbing direction, which include an angle of 90 degree between the upper and the lower plate. Consequently, a homogeneous twist déformation in alignment is achieved. The polarization of a linearly polarized light wave is then guided by the resulting quarter of a biréfringent helix, if the orientation is not disturbed by an electrical field. The transmitted wave may pass therefore a crossed exit polarizer, and the modulator appears bright. If, however, an AC voltage of a few Volts is applied, the resulting electrical field forces the molécules to align themselves along the field direction and the twist déformation is unwound. Now, the polarization of a light wave is not affected and cannot pass the crossed exit polarizer. The modulator appears dark.

[moebius.physik.tu-Obviously, the inverse switching behavior can be obtained with parallel polarizer's. It must be noted further, that gray scale modulation is achieved easily by varying the voltage between the threshold for reorientation (which is a resuit of elastic properties of LCs) and the saturation field.

LCDs can be used in transmissive or reflective modes. A transmissive LCD is illuminated from one side and viewed from the opposite side. Most computer LCDs are equipped with built-in fluorescent tubes above, or illumination Systems behind the LCD. A white diffusion panel behind the LCD redirects and scatters the light evenly to ensure a uniform display. This is known as a backlight. Activated cells therefore appear dark while inactive cells appear bright. This type is used in high-brightness applications such as pocket télévision receivers. The lamp used to illuminate the LCD in such a product usually consumes more battery power than the LCD itself.

A reflective LCD (see Fig. 1.2), as used in pocket calculators and digital watches, is viewed by ambient light reflected in a mirror behind the display. This type has lower contrast than the transmissive type, because the ambient light passes twice through the display before reaching the viewer. The advantage of this type is that there is no lamp to consume power, so the battery life is long. A small LCD consumes so little power that it can run from a photovoltaic cell.

In the reflective mode of opération, a reflector is used, but not a mirror-like reflector, rather a scattering reflector (structured métal). Transflective LCDs use a combination of transmissive and reflective modes. Transflective mode of opération means reflective mode when ambient illuminance level is high and a low-power backlight provides transmissive illumination in dark and dim situations

Fig. 1.2. [chemistrydaily.com/chemistry/Liquid_crystaLdisplay]: a) Reflective twisted nematic liquid crystal display; b) A simple LCD display from a calculator. Hère:

1. Vertical filter film to polarize the light as it enters.

2. Glass substrate ITO électrodes. The shapes of thèse électrodes will détermine the dark shapes that will appear when the LCD is turned on. Vertical ridges are etched on the surface so the liquid crystals are in Une with the polarized light.

3. Twisted nematic liquid crystals.

4. Glass substrate with common ITO électrode film with horizontal ridges to line up with the horizontal filter.

5. Horizontal polarizer film to block/allow through light. 6. Reflective surface to send light back to viewer.

In gênerai LCDs can be divided into two groups: passive and active. So called passive-matrix LCDs use a simple grid to supply the charge to a particular pixel on the display. It consists of two glass layers called substrates. One substrate is given columns and the other is given rows made from a transparent conductive material. This is usually indium-tin oxide (ITO). The rows or columns are connected to integrated circuits that control when a charge is sent down a particular column or row. The liquid crystal material is sandwiched between the two glass substrates, and a polarizing film is added to the outer side of each substrate. To turn on a pixel, the integrated circuit sends a charge down the correct column of one substrate and a ground activated on the correct row of the other. The row and column intersect at the designated pixel, and that delivers the voltage to untwist the liquid crystals at that pixel.

LCDs with a small number of segments, such as those used in digital watches and pocket calculators use passive matrix with one electrical contact for each segment. The electrical signal to drive each segment is supplied from an external circuit. This passive

display structure becomes undesirable when the number of éléments increases. Medium-sized displays, such as those in monochrome personal organizers and pocket télévision sets, hâve a passive matrix structure. This type has one set of contacts for each row and column of the display, rather than one for each pixel. However, the disadvantage is that only one pixel can be addressed at any instant. The other pixels hâve to remember their last state until the control circuit has time to revisit them. This results in reduced contrast and a poor response to fast-moving images. As the number of pixels increases, this type of display becomes less and less attractive. The technology used in thèse displays is typically supertwist nematic (STN), or a double-layer version DSTN that corrects the color-shifting problem of STN. The simplicity of the passive-matrix System is beautiful, but it has significant drawbacks, notably slow response time and imprécise voltage control. Response time refers to the LCD's ability to refresh the image displayed. The easiest way to observe the slow response time in a passive-matrix LCD is to move the mouse pointer quickly from one side of the screen to the other. You will notice a séries of "ghosts" following the pointer. Imprécise voltage control prevents the passive matrix's ability to influence only one pixel at a time. When voltage is applied to untwist one pixel, the pixels around it also partially untwist, which makes images appear fuzzy and lacking in contrast.

For high-resolution color displays, such as large LCD monitors for computers, an active-matrix System is used. The LCD panel hère contains, besides the polarizing sheets and cells of liquid crystal, a matrix of thin-film transistors (TFT). They are arranged in a matrix on a glass substrate. Basically, TFTs are tiny switching transistors and capacitors. To address a particular pixel, the proper row is switched on, and then a charge is sent

down the correct column. Since ail of the other rows that the column intersects are turned off, only the capacitor at the designated pixel receives a charge. The capacitor is able to hold the charge until the next refresh cycle. And if we carefully control the amount of voltage supplied to a crystal, we can make it untwist only enough to allow some light through. This method provides a much brighter, sharper display than a passive matrix of the same size. Unavoidably, some LCD panels hâve defective transistors. This causes dark and bright pixels which are always off or always on. Unlike integrated circuits, LCD panels with a few defective pixels are usually still usable.

The liquid crystal used in LCDs rotâtes ail visible wavelengths equally, but additional refinements hâve been added to the basic LCD to produce a color display. In a color LCD each pixel is divided into three sections as shown in Fig. 1.3, one with a red filter, one with a green filter and the other with a blue filter. The pixel can be made to appear an arbitrary color by varying the relative brightness's of its three colored sections. The color components are arranged in différent ways, forming a kind of pixel geometry depending on the monitor's usage.

Red Oreon Bluo Whitc Ycllow Imago

In spite of ail mentioned advantages of LC based devises one of major disadvantages still remains LC polarization dependence. To overcome thèse problems the materials called polymer dispersed and polymer stabilized liquid crystals are of great interest.

1.2. Polymer dispersed and polymer stabilized liquid crystal (PDLC and PSLC) based LCD's.

Polymer-dispersed liquid crystals (PDLCs) are intensively investigated for the use in many applications ranging from switchable Windows to internai partitions. Thèse materials, which are composed of polymers and liquid crystals, are still in the focus of extensive research in the display industry. PDLCs consist of liquid crystal droplets that are dispersed in a solid polymer matrix (Fig.1.4).

Fig.I. 4 Microphotography of typical PDLC cell trough crossed polarizaers.Bright sphères with crosses are LC droplets. [plc.cwru.edu]

The resulting material is a sort of "Swiss cheese" polymer with liquid crystal droplets filling the holes. Thèse tiny droplets are responsible for the unique behavior of the material controllable scattering of light. By changing the orientation of the liquid crystal molécules with an electric field, it is possible to vary the intensity of transmitted light. Below, their application as electro-optic light shutters in the construction of privacy Windows is presented. PDLC Windows are based on the ability of the nematic director of the liquid crystal droplets to align under an electric field. In a typical application, a thin PDLC film (about 25 microns thick) is deposited between clear plastic covers. The plastic substrates are coated with a very thin layer of a conducting material: ITO. Transmission of light through a PDLC window dépends primarily on scattering which in turn dépends on the différence in refractive index between droplets and their environment. In the case of high droplet density, the environment consists mainly of other droplets, which makes the relative orientation of their directors an important factor. The droplets are anisotropic due to ordered orientation of LC molécules inside of droplets. In the field "off ', the random array of droplet orientation provides significant différences in indices and hence strong scattering. In this state, the cell appears milky. When a voltage is applied, however, the directors of the individual droplets align with the field. There is now little différence in refractive index for neighboring droplets and the polymeric matrix, and the cell appears transparent. Usually, in PDLC, the polymer concentration range is above about 20-wt%. In display applications, thèse materials présent problems with hazy images for obliquely incident light.

Rapid development is occurring in the low polymer concentration range (10 weight % or less): so called polymer stabilized liquid crystalline (PSLC) materials. PSLC

cells are generally prepared by dissolving and photopolymerizing monomers (typically less than 5 wt%) in a liquid crystal matrix to form a polymer network. After being sandwiched between the alignment treated glass cell faces, the solution is photopolymerized using an UV light source. Hère, polymer phase séparation and network formation takes place (Fig.1.5). The formations of polymer network stabilize liquid crystal textures throughout the bulk of a device and improve its electro-optical performance, as will be discussed later. Particularly, a significant réduction in operational voltage and response time can be achieved at low polymer concentration. In ail possible applications of devices based on polymer stabilization, it is important to understand the rôle of the polymer network and its morphology as well as the factors controlling it, such as the used monomer structure and température of photopolymerization.

Fig.I.5. Magnified image of typical PSLC fibrous network morphology, after a removal of LC. [plc.cwru.edu]

1.3. Switchable Windows.

As mentioned above (1.2), scattering properties of PDLCs and PSLCS can be use for switchable Windows. Two différent switchable window technologies: electrochromic (EC) and suspended particle device (SPD) — can not only darken and lighten tints at the flick of a switch, they can also control and reduce the amount of solar energy transferred to the interior of a building. As a resuit, thèse Windows will hâve a major impact on energy conservation. Estimations shown that one can expect around 20 percent to as high as 30 percent réductions in total energy use in commercial buildings through the use of switchable Windows. Fine tuning the balance by automated control of variable tint Windows maximize energy savings and eliminate the need for some form of manual control of the Windows by the people who work in thèse office environments. The objective of the human factors testing is to détermine not only if the people are comfortable in the space with electrochromic Windows, but whether they are willing to accept the technology if it is fully automated.

Liquid crystal technology is a right one to create an electronic window shade. (LC Windows do not significantly reduce the amount of light transmission, but only the transparency of the Windows).

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1067-1080, 1996.

21. K.Hirabayashi, H.Tsuda, T.Kurokawa, "Tunable Liquid-Crystal Fabry-Perot Interferometer Filter for Wavelength-Division Multiplexing Communication Systems". Journal of Lightwave Technology, V. 11, pp. 2033-2043, 1993.

22. Sh.Matsumoto, K.Hirabayashi, S.Sakata, T.Hayashi, "Tunable wavelength filter using nano-sized droplets of liquid crystal", IEEE Photonic Technology Letters, V.l 1, No4, pp.442-444, 1999.

23. Y.-Q.Lu, F.Du, Y.-H.Lin, S.-T.Wu, "Variable optical attenuator based on polymer stabilized twisted nematic liquid crystal". K.Hirabayashi, M.Wada, Ch.Amano, "Liquid crystal variable optical attenuators integrated on planar lightwave circuits", IEEE Photonic Technology Letters, V.13, No6, pp.609-611, 2001. 24. A.Zohrabyan, D.Dumont, A.Tork, R.Birabassov, T.Galstian "In-fiber variable

optical attenuator with ultra-low electrical power consumption", Proc. SPIE Vol. 5724, p. 124-130, Organic Photonic Materials and Devices, 2005.

25. C.Mao, M.Xu.W.Feng, T.Haung, K.Wu, J.Wu, "Liquid-crystal applications in optical télécommunication", Liquid Crystal Materials, Devices and Applications IX, L.C.Chen, éd., Proc. SPIE 5003, pp.121-121-129, 2003.

26. M.Kawachi, N.Shibata, T.Edahiro, "Possibility of use of liquid crystals as optical waveguide material for l,3um and l,55|a,m bands", Japanese Journal of Applied Physics, V.21, No3, pp.L162-L164, 1982.

27. Vladimir V. Presnyakov, Karen E. Asatryan, Tigran V. Galstian, "Polymer-stabilized liquid crystal for tunable microlens applications", Optics Express, Vol. 10, No 17, pp.865-870, 2002.

28. Hongwen Ren, Shin-Tson Wu, "Tunable electronic lens using a gradient polymer network liquid crystal", Applied Physics Letters, Vol.82, No.l, pp.22-24, 2003. 29. Yun-Hsing Fan, Hongwen Ren, Shin-Tson Wu, "Switchable Fresnel lens using

polymer-stabilized liquid crystals", Optics Express, Vol.11, No23, pp.3080-3086, 2003.

30. Ssu-Pin Ma, Shiao-Min Tseng, "High-Performance Side-Polished Fibers and Applications as Liquid crystal Clad Fiber Polarizers", Journal of Lightwave Technology, Vol. 15, No8, pp. 1554-1558, 1997.

31. V.V. Presnyakov, T.V. Galstian, "Light polarizer based on anisotropic nematic gel with electrically controlled anisotropy of scattering". Proceedings of 19th International Liquid Crystal Conférence, p.755,.Edinburgh UK, 30 June - 5 July 2002.

32. R.A.M. Hikmet, H. Kemperman, "Switchabkle mirrors of chiral liquid crystal gels", Journal of Liquid Crystals, Vol.26, Noll, pp.1645-1653, 1999.

Chapter1

Material System and cell

1.1. Introduction

Previous studies of PSLC devices, on the basis of mesogenic monomers, showed the importance of the morphology of the polymer domains involved in the control of electro optical properties of the material [1]. Reactive mesogens are difficult to synthesize and thus are relatively rare and costly. Accordingly, new studies concerning the synthesis and characterization of PSLC Systems hâve been reported [2-4] that use non-nematogenic monomers.

In PSLCs the concentration of the monomer is usually less than 10%. The monomer can be directly dissolved in the liquid crystal. Although any type of polymerization method can be used, photo-initiated polymerization is fast and is used most often. The monomer is usually an acrylate or a methyl-acrylate because of their fast reaction rate. In order to form stable polymer networks, a bi-functional monomer or a mixture of bi-functional and mono-functional monomers is used. A small amount of photoinitiating complex (dye, initiator and co-initiator) is added to the mixture. The concentration of the photo-initiator is typically 1-5% of the monomer (typically 10"3%

H-5xl0"3% w).

When the composite mixture of the liquid crystal (with the monomer and the photoinitiating complex) is irradiated by UV light, the monomer is polymerized to form a

polymer network. The photo-initiator produces free radicals which react with the 0=H double bonds of the monomer and start chain reaction of polymerization [5]. The UV intensity (300nirn450nm) is usually 1.2 mW/cm2 and the irradiation time is <30min (see

later).

SEM, neutron scattering, confocal microscopy, biréfringence study and the Freederickzs transition techniques hâve ail been used to study the morphology of polymer networks in PSCLs. The results suggest a bundle structure for polymer networks obtained. The bundle consists of polymer fibrils with a latéral size of a few nanometers and liquid crystals filling it. The morphology of the polymer network is affected by the following factors: type of monomer, the UV intensity, the photo-initiator type and concentration, and the température [6]. Polymer bundles with larger latéral sizes are formed at low UV intensity, low photo-initiator concentration and a higher température. The viscosity of the initial mixture is comparable to that of the nematic liquid crystal and that mixture can be easily put into cells in a vacuum chamber. During polymerization, external electric field can be applied to control the orientation of the mixture.

In this chapter material compositions of PSLC are presented, based on non-mesogenic monomers, as well as a motivation of chosen monomers. Sample préparation and further expérimental analyzes techniques as polarizing electronic microscopy, differential scanning calorimetric, etc. are also discussed briefly.

The possibility to obtain a rich variety of morphologies using non-mesogenic monomers is demonstrated, each showing a unique electro-optic response.

1.2. Chemical composition of monomers used for PSLC préparation.

Monomers used in PSLCs preferably hâve rigid cores and flexible spacers. They form anisotropic fibril-like networks [7]. If the monomer does not hâve flexible spacers, it forms bead-like structures which are not stable under perturbations such as externally applied field. If the monomer does not hâve a rigid core and is flexible, it can still form anisotropic networks.

In our study we used the monomers presented in Fig. 1.1; (from Sartomer and Merck), photoinitiation complex (dye: IR140, initiator: KBr4, and coinitiator: Aminé), described in [5, 8], in combination with commercially available NLC mixture E7 (Merck) used as a non-reactive liquid crystals matrix.

I

H3C— ÇJ^CH^- O-C— C=CH2 //

o

glycidile methacrylate (GMA): named as Ml: monofunctional.

ÇH^ J v C"3 / v ÇH3

Ô CH3

H

2

C - ^-°-<Qyi-^<Q}-

(

^

bc

=

Œ2

H2C = C H ^ ^ ( C H2C H20 )2^ Q y - Ç - Y Q ^ ( O C H2C H2)20 - € - H C =CH2

fH

ï"

3

/—\

(

(

H

H

C = e-(^ocH

cH

o—\0/—

—(O/—

2

20~c— c=

ô ^ / C H3 X / N0

Bisphenol A ethoxylate dimethacrylate (BFAEDMA): bifunctional.

CH3 CH3

I I

H2C = C ^ C - 0 - ( C H2)s- 0 - C - C = CH2

o o

1,6 hexamethylen dimethacrylate (HMDMA): bifunctional.

copolymer anhydride maleique- dicyclopentadiene: (AM-DCPD)

Fig. 1.1. Molecular structure of différent monomers used in the fabrication of PSLC.

In the table 1 the weight composition of the monomer-liquid crystal mixtures is summarized, which was used in our studies. The scattering observations was made on

freshly made samples without external excitation (V=0V). Ail the mixtures are based on glycidyle methacrylate (GMA); this choice is argumented by the previous studies done on this monomer [5]. The sélection of the "co-monomers" was relying on the similar reacting structures (the same family): the différence is the number of the reacting groups: one methacrilic for GMA, two methacrilic for BFADMA. Consequently, hâve been expected the différent network architectures: linear for GMA and crosslinking for BFADMA. The idea is to obtain an intermediate network structure generated by inclusion of linear structure of GMA and crosslinking structure for BFADMA. In this manner it's possible to generate more flexible and rigorous ordering network. The total concentration of the monomer's was 3-H7 % (wt) (see table 1). As shown in the table 1, the samples differ both in the weight fraction of liquid crystal and the ratios of the two types of monomers. For the solution Dfl, total monomer concentration was 7%, which brings to phase séparation. For other solutions the monomer total concentrations were close to 97%, according to reported in the other papers concentrations [5, 8].

code Monomer name % (w) monomer and LC Observations Dfl GMA (Ml) BFADMA (M2) 7: 3,5 Ml; 3,5 M2; LC: 93% Phase séparation, {ScatteringatV=OV} Bf3 GMA (Ml) BFADMA (M2) 3: 1,54 Ml; 1,46 M2 LC:97 Voltage induced scattering Ll GMA (Ml) 3,3: 3 Ml; 0,3 M2 Voltage induced

BFADMA (M2) LC: 96,7 scattering L2 GMA(Ml) BFADMA (M2) 2,8: 2,54 Ml; 0,26 M2 LC: 97,2 Voltage induced scattering

L3 AM-DCPD 3; LC : 97 Phase séparation,

{Scattering atV=0V} L4 GMA(Ml) BFAEDMA (M2) 2,51: 2,42 Ml; 0,09 M2; LC: 97,48 Voltage induced scattering L5 GMA(Ml) AM-DCPD (M2) 2,97: 1.13M1; 1,84 M2 LC: 97,03 Phase séparation, Scattering at V=0V

Table 1. Weight composition of the monomer/liquid crystal mixtures, used in our studies. Phase séparation

is appears in form of polymer or LC large domains (typical size is few tenths of micrometers and more, measured with microscope),

The application of two types of PSLC gel Systems dépend upon the alignment used: homogenous or homeotropic. The homogenous aligned PSLC gel is used for modulating linearly polarized light and the homeotropic Systems is intended for unpolarized light. After mixing, the solution is injected into a LC cell with rubbed PMMA (poly-methyl-methacrylate) surface, which provides strong aligning conditions. Each type has its own advantages and disadvantages. In the both cases the type and monomer concentration as well as cell gape hâve important effect on the operating voltage and contrast ratio of PSLC. Usually a higher monomer concentration or thickness of cell gap leads to a higher operating voltage.

1.3 PSLC cell fabrication procédure.

In our investigations we used glass substrates FisherBrand, distributed from Fisher scientific. The dimensions of substrates are 75mmX25mmXlmm. Single substrates were eut in the middle by diamond peneil cutter (to hâve two substrates of 37.5x25x1 mm3) for one cell préparation. For further rubbing process eut substrates were

cleaned and mounted on a spécial holder. The holder with glass substrates was inserted into the solution of PMMA(10w%)/THF(90w%) (Poly-methyl-methacrilate: polymer and tetrahydrofurane as a solvent) to dip-coat an alignment layer. Then the holder was raised ' (with substrates fixed on it) from the PMMA/THF solution by a motorized mechanism. The speed of rising was 0.1 mm/s. The schematic of PMMA déposition equipment is presented on the figure. When substrates getting out from the solution, the THF is evaporated due to their high volatility and the layer of PMMA polymer remains on the surface of the substrates. The thickness of remaining PMMA polymer (typically ~lum) is a function of PMMA THF concentration in final solution, speed of raising solution température etc.

Motorized rising mechanism Holder Glass substrates PMMA/THF solution

The final LC (left side on Fig.1.3) cell and the schematic of the layers (right side) are presented on Fig. 1.3.

Fig. 1.3 : Optical cell photo

Where :

(a) Alignment layers (PMMA and/or fluorinated composite) (b) Glass substrates

(c) The LC E7 and monomer/photoinitiator solution, which is inserted by capillarity into the gap of the cell

(d) Spacers from 5um to lOum, cutted from polyethilen films of différent thickness, which controls the thickness of final cell. The maximum value of lOum was chosen, as alignment effect of PMMA is optimal between 5um and lOum values.

To avoid cell thickness errors, 4 screws are used to press with the same force and fix final cell both substrates parallel before LC/monomer/photoinitiator injection. After that injection cells stay for 2 hours for stabilization before any observation (in the same time we used cells, from Linkam, of thickness 5|im provided with transparent indium-tin-oxide électrodes which were also filled with the same mixtures).

The uniform planar orientation of NLC in the cell was obtained by rubbing poly-methyl-methacrylate (PMMA) layers. The photopolymerization of the monomer's mixtures was initiated by UV radiation 1.2 mW/cm2 emitted by high-pressure mercury

applied during irradiation. After polymerization, the sample was placed into thermo-stabilized oven for electro-optical measurements.

1.4. Expérimental method for PSLC analyses: Optical polarizing microscopy.

A uniformly aligned slab of nematic liquid crystal material can be considered to be a uniaxial slab, having its optical axis (c axis) directed along the long axis of the LC molécules. Due to this biréfringent nature of PSLCs, polarizing microscopy gives reach information about the local LC molécules alignment, surface effects as well as structural morphology of used composite materials. It's very useful to check alignment uniformity over the cell surface too. In our investigations we use Zeiss Axioskop 40pol' polarizing microscope. If the polarization of the incident probe beam is aligned at 45° to the rubbing direction of the cell, the phase différence of the zones is <b=2nAn/'k, where An is the différence of effective extraordinary and ordinary refractive indexes.

Fig. 1.4. Optical microphotography of PSLC cell through crossed polarizers: color change is related to phase change with applied voltage. Polymerized régions: top left, non-polymerized régions bottom right.

Under application of an electric field, the LC molécules reorient. So the induced phase shift is electrically tunable. This phase change may be seen in Fig.1.4 (as an example), as color change when observed the cell with a white lamp from an optical microscope through crossed polarizers. To enhance the effect of polymerization into the phase shift, cells were polymerized using a mask, which is non transparent for UV light, applied to the part of the cell. The diagonal différence on the figure shows the boundary between polymerized and non polymerized régions (bottom-right: non polymerized and top-left: polymerized). Under this visual inspection the rubbing direction of the LC cell was oriented at 45 degree with respect to the optical axis of the linear polarizer. The color change is due to the chromatic dispersion of phase retardation of the PSLC cell.

Conclusion

• Taking into account the fact, that reactive mesogens are difficult to synthesize and thus thèse are relatively rare and costly, in this work we investigate influence of non-mesogenic monomers on the morphology of the polymer domains involved in the control of electro optical properties of the PSLC material.

• Used in this work mixtures mainly based on non-mesogenic monomers glycidyle methacrylate (GMA) and BFADMA. The sélection of the "co-monomers" was relying on the similar reacting structures (the same family): the différence is the

number of the reacting groups: one methacrilic for GMA, two methacrilic for BFADMA.

• The différent network architectures hâve been expected: linear for GMA and crosslinking for BFADMA. The idea is to obtain an intermediate network structure generated by inclusion of linear structure of GMA and crosslinking structure for BFADMA.

Références

1. Hikmet, R.A. (1990). Electrically induced light scattering from anisotropic gels. J. Appl. Phys., 68(9), 4406-4412.

2. Kelly, S.M. (1998). Anisotropic Networks, Elastomers and gels. Liq. Cryst., 27(1), 71-82.

3. Hikmet, R.A.M. (1991). Anisotropic gels and plasticized networks formed by liquid crystal molécules. Liq. Cryst, 9(3), 403-416.

4. Hikmet, R.A.M. (1991). From liquid crystalline molécules to anisotropic gels. Mol. Crys. Liq. Cryst., 198, 357-370.

5. Presnyakov, V., Asatryan, K.A., Galstian, T.V. and Tork A. (2002). Polymerstabilized liquid crystal for tunable microlens applications. Optics Express, 10(11), 865-871.

6. Rajaram, C.V., Hudson, S.D. and Chien, L.C. (1996). Effect of polymerization température on the morphology and electrooptic properties of polymer stabilized liquid crystals. Chem.Mater., 8, 2451-2460.

7. Hikmet, R.A.M. and Lub, J. (1996). Anisotropic Networks and gels obtained by photo-polymerization in the liquid crystalline state: synthesis and applications. Prog. Polym. Sci., 21, 1165-1209.

8. Galstian, T.V. and Tork, A. (2002). Photopolimerizable composition sensitive to light in a green to infrared région of the optical spectrum. U. S. Patent, 6,398,981, June 4.

Chapter 2

Morphologies of non-mesogenic polymer stabilized liquid

crystals.

2.1. Introduction.

Previous studies of PSLC polymerization process show that the final morphology of PSLC mixture strongly dépends on following factors: solubility of used monomer compositions with host LC mixture, functionality and concentration of monomers, UV curing radiation intensity, polymerization température, alignment boundary conditions, applied electric field, photoinitiator's concentration, etc. [1-4]. In this chapter we are investigating non-mesogenic monomer's final morphology and phase transition level dependence upon concentration of each monomer. Used co-monomers are characterized with différent functionality and spacer lengths, which suppose différent final polymeric structure and flexibility. Polymerizing light intensity and température are fixed and described in détail in Chapter 1. We are using the same LC E7 as a host for différent polymer compositions inside of planar aligned cells. Results of polarization light microscopy and DSC are presented to examine polymer morphology. We demonstrate the possibility to obtain a rich variety of morphologies using non-mesogenic monomers.

2.2 Passive and active polarizing light microscopy observations: expérimental conditions.

Microscope observations were done for each monomer composition in 3 différent configurations, as presented on the Fig. 2.1: LC/monomer composition director is aligned at 45° between crossed polarizer and analyzer (a), LC director is parallel or perpendicular (dashed arrow) to the incident light polarization direction and without analyzer (b). For ail samples, observations done with (active) and without (passive) voltage. More detailed influence of observed morphologies to the light scattering properties of cells will be discussed in détail in the next chapter.

Analyzer 45° placed PSLC cell Polarizer No analyzer Parallel or perpendicular Polarizer a) b)

Fig. 2.1: Scheme used for polarimetric microscopy: relative orientation of analyzer, polarizer and PSLC cell.

a) LC/monomer composition director is in 45° is between crossed polarizer and analyzer to examine the System biréfringence;

b) LC/monomer composition director is parallel or perpendicular (dashed arrow) to the incident light polarization direction and without analyzer to examine scattering.

Samples initially made were checked for their alignment by placing them between crossed polarizers. The homeotropic alignment (obtained with relatively high voltage

applied to the cell voltages), is assured when the cell appears dark under crossed polarizers, while the homogeneous alignment is ensured when the cell appears alternatively dark and bright under crossed polarizers as the angle between the liquid crystal alignment director and incident light polarization vector was changed.

2.3. Morphological observations.

Before polymerization, the samples hâve a homogeneous texture corresponding to the planar orientation of the nematic. To avoid ambiguities related to LC alignment defects, tested samples were verified for alignment quality and ail measurements were done with cells without évident alignment defects. Application of electric field results in the réversible planar- homeotropic reorientation of the samples before polymerization.

After polymerization, the initial texture is conserved and the cells remain homogenous in gênerai. The samples DF1 and L5 make exception. In thèse cases a phase séparations appear, similarly to the PDLC Systems. In Fig. 2.2 are presented microphotographs of the sample with DF1 and L5 solutions after polymerization and without applied voltage. Hère LC aggregation zones in the photoexposed région were présent (phase séparation). Nevertheless, phase séparation reasons for the case of DF1 (a) and in the case of L5 (b) are différent. For DF1 thèse évolutions seem to be related to the high global concentration of monomers (7%). The monomer concentration is 2.97 %w only for L5 composition. In this case, liquid crystal phase séparation (blue séparation Une between LC and polymer in Fig. 2.2 b) is related rather to the used AM-DCPD polymer solubility with LC E7.

a) b)

Fig. 2.2. Optical micrographs of the sample DFl (a) and L5 (b): LC phase séparation is présent due to the high concentration of monomer: 7% (a) and low solubility of polymer (b).

After observing those phase séparations thèse solutions were excluded from further analyzes. Nevertheless the importance of the solubility and polymer concentration in final solution are well expressed in thèse 3 solutions.

The aggregation phenomena are reduced (Fig. 2.3) by decreasing monomers' relative concentration (samples Ll, L2) and increasing the weight ratio (Mi/M2=10 see table 1), Moreover, the sample L2 préserves the morphological aspect, us the electro-optics characteristic remains the same after 2.5 months.

c) Cell L2; U=OV; d) Cell L2; U=7V;

e) Cell BF3; U=OV; f) Cell BF3; U=6.8V;

h) Cell L4; U=OV; i) Cell L4; U=7V.

Fig. 2.3: Microphotographs of compositions Ll, L2, BF3, L4 between crossed polarizers. Director of LC

is placed in 45° polarizer. Microphotographs done with and without voltages.

As mentioned in Chapter 1 (§ 1.4), to enhance the effect of polymerization into a phase shift, often cells were polymerized using a non transparent for UV light mask, applied to the part of the cell, where monomer solution remains non polymerized. The separated areas on the Fig. 2.3 (a, b, e, f, g) are related to this fact (red contours on the Fig. 2.3 (c, d, h, i) are related to spacer of mechanical holder of PSLC cells, and don't relate to polymerization). In thèse photographs polymeric micro-domains are observed for ail compositions. With the application of the electric field the différent change of color for polymerized and non polymerized (just after UV curing, excluding "dark" polymerization) areas is due to the différent phase retardation between both areas. Electro-optical measurements for the mixtures show strong phenomena of diffusion of the light.

For ail samples the electric field induced texture doesn't relax into the initial state after the field removed for polymerized area. In contrast, in non polymerized part, full relaxation takes place, which does not dépend on applied voltage value. For example, in

Fig. 2.4 are presented electro-optic measurement results for the cell with polymerized solution of BF3. Incident light polarization is parallel to the rubbing direction. Expérimental setup (for full description see §3.3) based on He-Ne laser (k=543 nm) of diameter 0.7 mm (used as a probe at normal incidence on the sample). The polarization state and intensity of probe beam were varied with À/2 plate and Glan prism. Light transmission through the sample was measured using photodiode with a total collection angle of 0.5° and computer controlled LabView data acquisition system. The frequency of driving electric field was AC, 1 kHz with sinusoidal form. Far field power détection was made without analyzer. To find a voltage value starting from which PSLC don't relax into the initial state the voltages are increased from 0V to certain voltage values (5V, 14V, 52V and 100V), as shown in figures a), b), c) and d) respectively. For voltages less than 5V there is no essential change of transmitted radiation (a). Starting from approximately 7V, strong scattering and corresponding signal réduction was observed for parallel (to the ribbing direction) polarization component of light (b). Effect of non réversible changes inside of PSLC cell is more obvious for voltages >52V, where the light transmission is changed significantly after electric field removal. This non réversible change of PSLC structure is probably related to the polymer chain non réversible déformation or dégradation (particularly its interaction with the alignment layer) due to the applied strong electric field. The effect could be related to the rather rigid bead like structure of PSLC based on non-mesogenic monomers.

S 3 0,4 Temps [s] a) Temps[s] b) 1,0-——s -

0,8-1

0,6-1

0,6-1

/ " \ 1

Fig. 2.4: electro-optic measurement results for the cell with polymerized solution of BF3. Initial light

polarization is parallel to the rubbing direction, without analyzer. The maximum applied voltages (1kHz ,AC) are 5V (a); 14V (b); 52V (c) and lOOv (d).

As one can estimate from figures for BF3 composition the voltage value, starting from which non réversible changes occur (initial and final (corresponding to 0V) signal values (black curves) are not equal, except in case of 5V, a), is around 10V, as for 14V still the change of the signal (the reversibility) is not so critical and less than 3% (below that value the phenomenon is réversible). This phenomenon is related probably to the breaking of polymer chains due to the applied high voltage, which permanently créâtes local defects.

In non-polymerized régions the NLC director keeps its initial state. The samples look dark when the direction of molecular orientation is set parallel to one of the crossed

(Fig. 2.3). The microscopic observations show thèse formations as optical non-homogeneous, as shown in the Fig. 2.5.

Fig. 2.5: Microphotographs of cell BF3:

a) cell's director is set to 45° relative to crossed polarized and analyzer; b) cell's director is parallel to polarizer axes, without analyzer;

c) cell's director is perpendicular to polarizer axes, without analyzer.

Farther examination of PSLC was continued with BF3 solution, which shows promising electrically controlled scattering properties. Tests hâve been done with reduced voltage, where the electro-optic properties are réversible. To analyze polymer chain influence to the visco-elastic properties of LC, the comparative electro-optic response of PSLC and pure LC is presented in Fig. 2.6. In both cases we are using 5|im thickness planar aligned cells. Maximum 6.2V 1kHz sin AC voltage was applied to both cells.

0.55 0,60 0.45 0.40' 0.36 cô 0.30 l e . S Time [s] a) b)

Fig. 2.6: Comparative electro-optic response of (a) PSLC BF3 and (b) pure LC E7. In both cases polarizer and analizer are crossed and the LC cell director is oriented at 45°.

In this case transmitted power is given by [5]:

T=Sin

[*], where <I>=y ^ [ n . f r ) - n

]dx; (1)

Hère the phase change of n corresponds to 2 adjacent extremumes (max & min) in transmission curve (points A and B on Fig.2.6). As indicated on the Fig. 2.6, voltage change, which corresponds to the phase delay of % for pure LC and PSLC are quite différent. In the case of PSLC this value is higher (-1.6V, instead of -0.4 V for pure LC), which relates to the decrease of efective thickness of LC cavities Leff and decrease of

effective anisotropy of dielectric constant £a (sa=£||-e±) of PSLC due to polymer chain/LC

molécules boundary interaction and réduction of order parameter. The change in the case of PSLC of modulation depth of transmission is related to higher (compared to the pure LC) scattering of PSLC. Another observation is that the maximum transmission of PSLC

is smaller compared to pure LC one, which is related to the more scattering property of PSLC.

In the table 1 we présent observations concerning to DSC measurements, without curves.

code Monomer T(°C) Type of transition Observations E7 - 61,9 Endothermal Nematic-Isotrope L4 GMA BFAEDMA 51,6 Endothermal Nematic-Isotrope L4 GMA

BFAEDMA _{102,5 Exothermal Polymerization} L4 GMA BFAEDMA 112 Exothermal Crosslinking L4 GMA BFAEDMA 117 Endothermic Maybe Tg L3 AM-DCPD 41,2 Endothermal Nematic-Isotrope L3 AM-DCPD 54,7 Exothermal Polymerization L2 GMA BFADMA 52,3 Endothermal Nematic-Isotrope L2 GMA

BFADMA 88,3 Exothermal Polymerization L2 GMA

BFADMA

118 Endothermal Physical transition L2 GMA

BFADMA

135 Endothermal Maybe Tg

- GMA 113,8 Exothermal Polymerization

- BFAEDMA 85,1 Exothermal Polymerization

- BFAEDMA

114 Exothermal Crosslinking

Table 1 : Transition températures of few PSLC analyzed compositions

According to results of DSC analyses, adding the small amounts of the monomer to the NLC, is not changing dramatically the thermal évolution of the sample. In ail the cases the nematic-isotropic (N-I) transition appears at the small température range. This phenomenon is related rather to the decrease of orientational order of LC due to polymer chains.

2.4. Discussions.

According to polarimetric microscopy and DSC observations, during the polymerization process, in ail the cases, the polymer network doesn't break down the

initial planar structure of NLC. This happens thus even for non-mesogenic monomers we used. The electric field application leads to the reorientation of nematic director to the homeotropic state since the LC E7 has positive anisotropy of dielectric constant. Fig.2.6 présents comparison of accumulated phase retardation with voltage for pure LC and PSLC. Obtained not complète reorientation in case of PSLC points out the strong bounding (corrélation) between LC molécules and polymer network. This aspect is most évident for samples with BFDMA. This monomer is characterized by a great order of phenyl core caused by the absence of the spacer between phenyl rings and reacting groups. Often (except solution BF3), after switching, the présence of polymer network prohibits the complète nematic relaxation. Thèse observations show that the polymer domains create discontinuities in the orientation process of NLC.

Similarly to the studies, based on the nematogene monomers, we imagine the morphological évolution showed in the Figure 2.7 [6-10].

monomère NIX! ^ (monomcrc's unit)

c)

Fig. 2.7. Morphological évolution of polymer network in PSLC Systems: a) microscopic phase séparation; b) macroscopic phase séparation (aggregation); c) final polymer network.

In the initial stage (starting from the homogeneous monomers-NLC mixture) a microscopic polymer area, still compatible with NLC is formed. At this moment, the anisotropy of the System is not very important, but exists, as we hâve used from the beginning a "rigid monomer System", which is not easy to be orientated. The subséquent development of the process (b) involves an increase of the network size. At the same time, "gel" is formed, likely due to "aggregation" of polymer particles. It is conceivable that during the last stages of networking, characterized by an increase of the crosslinking degree, NLC is consequently "expelled" from the network.

For the Systems in which the flexible monomer has a high concentration, in intermediate évolution of polymerization process there are two spécifie morphologies: rod-like (for the rigid monomer) and fibril type (for the flexible monomer) [see réf. N7 in Chapter 1], That could be explained by the réduction of a translational and rotational mobility of the monomer units in the polymer chain. That is a resuit of a high density of network structure. When the phenyls core is very near one to other, we must consider the steric hindrance.

By decreasing the global concentration of monomers (see chaper 1), the dimension of the polymer micro- domain decreases and so the intensity of scattering phenomenon.

Like conséquence of this différent orientation, the System is characterized by a biréfringence that détermines a local anisotropy. This aspect can be interpreted like conséquence has some particular interactions of interface tension type, between the very polar molécules of liquid crystal and the polymer network structure. While the

electo-optic measurements, as well as the polarizing microscopy demonstrate that, this phenomenon is decreased for a long distance of the polymerized région.

The apparition of polymers domains involves a discontinuity to the orientation of liquid crystal (continuous phase) because apparition of two nematic liquid crystalline populations:

• A LC population, included into polymer domain, characterized by high "anchoring" énergies on the polymer surface; this population does not undergo the N-I transition, the effect being the lowering of the distinctive transition température.

• A second population, external to the polymer network, practically "un-bonded", that préserves the alignment properties of pure NLC.

Ail thèse observations represent the first data in a study that is still in progress in order to elucidate on one hand the structure of doping copolymer, and on the other the network morphologies (for both solutions analyzed). SEM assessments and X-ray measurements, still in progress, would surely contribute to elucidation of the hypothèses presented, and partially demonstrated in this work.

Conclusion

• After polymerization of LC/monomer composition, the initial texture is conserved and the cells remain homogenous in gênerai. The samples DF1 and L5 make exception. In thèse cases the phase séparations appears. For DF1 solution phase séparation seem to be related to the high global concentration of monomers (7%).