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Author: Dena Mae Agra-Kooijman Publisher: ISBN: Category : Liquid crystals Languages : en Pages : 126
Book Description
The physical and chemical nature of bare surfaces and of thin films of polymers deposited in solid substrates plays a key role in the alignment of liquid crystal (LC). The ability to define the polar and azimuthal direction of the LC director at substrate-LC interface is at the heart of electro-optical applications of liquid crystals. However, the precise mechanism(s) of LC alignment have remained shrouded in mystery. The research conducted under this project was aimed at gaining insight into the relationship between LC alignment and the surface structure/morphology and to understand the dynamics of polymer chains at the polymer-air interface and under confinement to small dimensions. The results presented here show that properties of thin polymer films, such as glass transition temperature, Tg, and chain mobility are significantly different from bulk polymers. However, the understanding of the underlying reasons for these effects is far from complete. The studies of various polymers (weight molecular weight, radius of gyration) in different film geometries (supported on substrate or free standing) using different techniques reveal either an increase or a decrease in Tg and/or chain mobility with decreasing film thickness. Although the shifts of the Tg are a clear indication of the differences between the polymer chain mobility in the bulk and thin films, the measurements of Tg's are only an indirect method to probe their relaxation dynamics. The measurement of optical retardation, a simple optical technique requiring no physical contact with the polymer film, has been employed to directly probe the relaxation dynamics of variously treated thin polymer films routinely used for liquid crystal alignment. Using the relaxation model based on Kohlrausch-Williams-Watts, the Tg's of polystyrene films were determined as a function of the thickness and rubbing strengths (depths). Tg's are significantly reduced by 15-20 K for films of thickness less than 250 ... This correlates very well with previous studies on similar systems but using different methods. In addition, Tg decreases with rubbing depth indicating that polymer molecules close to the film-air interface relax faster than the molecules farther from the interface. The dependence of Tg on the rubbing strength provides clear evidence that polymer chains at the surface relax more readily. The morphology of polymer films was determined using high-resolution x-ray reflectivity, which is a sensitive and direct probe of surface structure at submicron levels. Making use of the inherent anisotropy in the coherence area of the x-ray beam, the morphological anisotropy of the films was quantitatively determined. The films' surfaces were modified by a specific treatment (i.e., rubbing or exposing to linearly polarized UV). Knowledge of the vertical root mean square (rms) roughness parallel and perpendicular to the treatment direction is of great importance in understanding the exact role in liquid crystal alignment of the nature of the surface. Reflectivity scans on the treated surfaces reveal roughness anisotropy, i.e., roughness is significantly different in the two orthogonal directions on the substrate plane. This direction of a more roughness is perpendicular to the azimuthal direction of the liquid crystal alignment. Hence, liquid crystal molecules align in the smoother direction.
Author: Dena Mae Agra-Kooijman Publisher: ISBN: Category : Liquid crystals Languages : en Pages : 126
Book Description
The physical and chemical nature of bare surfaces and of thin films of polymers deposited in solid substrates plays a key role in the alignment of liquid crystal (LC). The ability to define the polar and azimuthal direction of the LC director at substrate-LC interface is at the heart of electro-optical applications of liquid crystals. However, the precise mechanism(s) of LC alignment have remained shrouded in mystery. The research conducted under this project was aimed at gaining insight into the relationship between LC alignment and the surface structure/morphology and to understand the dynamics of polymer chains at the polymer-air interface and under confinement to small dimensions. The results presented here show that properties of thin polymer films, such as glass transition temperature, Tg, and chain mobility are significantly different from bulk polymers. However, the understanding of the underlying reasons for these effects is far from complete. The studies of various polymers (weight molecular weight, radius of gyration) in different film geometries (supported on substrate or free standing) using different techniques reveal either an increase or a decrease in Tg and/or chain mobility with decreasing film thickness. Although the shifts of the Tg are a clear indication of the differences between the polymer chain mobility in the bulk and thin films, the measurements of Tg's are only an indirect method to probe their relaxation dynamics. The measurement of optical retardation, a simple optical technique requiring no physical contact with the polymer film, has been employed to directly probe the relaxation dynamics of variously treated thin polymer films routinely used for liquid crystal alignment. Using the relaxation model based on Kohlrausch-Williams-Watts, the Tg's of polystyrene films were determined as a function of the thickness and rubbing strengths (depths). Tg's are significantly reduced by 15-20 K for films of thickness less than 250 ... This correlates very well with previous studies on similar systems but using different methods. In addition, Tg decreases with rubbing depth indicating that polymer molecules close to the film-air interface relax faster than the molecules farther from the interface. The dependence of Tg on the rubbing strength provides clear evidence that polymer chains at the surface relax more readily. The morphology of polymer films was determined using high-resolution x-ray reflectivity, which is a sensitive and direct probe of surface structure at submicron levels. Making use of the inherent anisotropy in the coherence area of the x-ray beam, the morphological anisotropy of the films was quantitatively determined. The films' surfaces were modified by a specific treatment (i.e., rubbing or exposing to linearly polarized UV). Knowledge of the vertical root mean square (rms) roughness parallel and perpendicular to the treatment direction is of great importance in understanding the exact role in liquid crystal alignment of the nature of the surface. Reflectivity scans on the treated surfaces reveal roughness anisotropy, i.e., roughness is significantly different in the two orthogonal directions on the substrate plane. This direction of a more roughness is perpendicular to the azimuthal direction of the liquid crystal alignment. Hence, liquid crystal molecules align in the smoother direction.
Author: Igor Muševič Publisher: Springer Science & Business Media ISBN: 9783540207894 Category : Mathematics Languages : en Pages : 320
Book Description
This book describes the state of the art of our understanding of liquid-crystal interfaces on a molecular level. The interactions of liquid crystal molecules with a surface play an essential role in the operation of liquid crystal displays (LCD's) and other LC devices that are based on the controllable anchoring of LC molecules on polymer coated surfaces. This book addresses the microscopic interaction between a macromolecule (liquid crystal, polymer) and a wall, using state of the art surface and interface-sensitive experimental techniques, such as Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM), Linear and Nonlinear Optical Microscopy and (Dynamic) Light Scattering (DLS). These experimental techniques were complemented with computer simulations and supra molecular chemistry methods to develop controllable polymeric surfaces.
Author: Toshiji Kanaya Publisher: Springer ISBN: 3642343392 Category : Technology & Engineering Languages : en Pages : 150
Book Description
Mobility Gradient of Polystyrene in Films Supported on Solid Substrates, by Yoshihisa Fujii, Hiroshi Morita, Atsushi Takahara and Keiji Tanaka Probing Properties of Polymers in Thin Films Via Dewetting, by Günter Reiter Heterogeneous and Aging Dynamics in Single and Stacked Thin Polymer Films, by Koji Fukao, Takehide Terasawa, Kenji Nakamura, Daisuke Tahara Heterogeneous Dynamics of Polymer Thin Films as Studied by Neutron Scattering, by Rintaro Inoue and Toshiji Kanaya
Author: Dongping Qi Publisher: ISBN: Category : Languages : en Pages : 230
Book Description
Studies show that dynamical properties of ultra-thin polymer films deviate from those of bulk materials. Despite some controversial issues, there is growing evidence indicating that the interfacial properties play a key role for observed dynamical anomalies. However, how and how much the interfacial properties affect the average dynamics of the nanometer scale systems are still elusive. In this work, we developed several novel techniques to investigate near-free-surface dynamics of thin polymer films. We studied surface dynamics of glassy i-PMMA films using a nano surface hole relaxation technique: a strong substrate property dependence and an unexpected molecular weight dependence were observed; we found that a local Tg of ~40K below bulk Tg could be assigned to the surface region. We used nano gold particle embedding to study PS surface dynamics: enhanced surface dynamics and weak temperature dependence were observed for the surface region; a depth profile with the nm resolution was observed; viscous liquid-like and soft solid-like properties were observed in the first 5.5nm and next 3.3 nm regions in PS films; no molecualr weight dependence was found in glassy PS films. We built a low level noise measurement system to study the thermal polarization noise in PVAc films: cooperative rearranging dynamics were evidenced; the noise power spectral density (PSD) is found to fluctuate around a certain average level without discernable peak shift; we observed some relatively big jumps or fluctuations in successive integrated PSD's, which indicate some energy exchange between different microscopic domains in glassy polymer systems. We developed a novel nano rheology AFM technique to study the near-free-surface dynamics of thin polymer films: enhanced near-free-surface dynamics with weak temperature dependence are observed for PVAc films, which is similar with the PS case.
Author: Wolfgang Haase Publisher: Springer Science & Business Media ISBN: 3662097478 Category : Science Languages : en Pages : 732
Book Description
The authors describe the electric, magnetic and other relaxational processes in a wide spectrum of materials: liquid crystals, molecular magnets, polymers, high-Tc superconductors and glasses. The book summarizes the phenomenological fundamentals and the experimental methods used. A detailed description of molecular and collective dynamics in the broad range of liquid crystals is presented. Magnetic systems, high-Tc superconductors, polymers and glasses are an important subject of matter. It is shown that the researchers working on relaxation processes in different fields of materials sciences are dealing with the same physical fundamentals, but are sometimes using slightly different terms. The book is addressed to scientists, engineers, graduate and undergraduate students, experimentalists and theorists in physics, chemistry, materials sciences and electronic engineering. Many internationally well known experts contribute to it.
Author: Ingo Dierking Publisher: Royal Society of Chemistry ISBN: 1782629823 Category : Science Languages : en Pages : 386
Book Description
Describing all aspects of polymer-dispersed and polymer-stabilized liquid crystals, this book is a must-have resource for practitioners in the area.
Author: Kohki Takatoh Publisher: CRC Press ISBN: 1420023012 Category : Science Languages : en Pages : 280
Book Description
Alignment phenomena are characteristic of liquid crystalline materials, and understanding them is critically important in understanding the essential features and behavior of liquid crystals and the performance of Liquid Crystal Devices (LCDs). Furthermore, in LCD production lines, the alignment process is of practical importance. Alignment Technologies and Applications of Liquid Crystal Devices demonstrates both the fundamental and practical aspects of alignment phenomena in liquid crystals. The physical basis of alignment phenomena is first introduced in order to aid the understanding of the various physical phenomena observed in the interface between liquid crystalline materials and alignment layer surfaces. Methods for the characterization of surfaces, which induce the alignment phenomena, and of the alignment layer itself are introduced. These methods are useful for the research of liquid crystalline materials and devices in academic research as well as in industry. In the practical sections, the alignment methods used in the LCD production lines are introduced with various other trials for the alignment technologies. LCD performances are also discussed in relation to alignment phenomena. The authors have a wide range of experience in both academic research and in industry. This book will be of interest to researchers and engineers working in the LCD industry, and for physics and chemistry researchers studying liquid crystalline materials.
Author: Morteza Sasani Ghamsari Publisher: BoD – Books on Demand ISBN: 1789853672 Category : Science Languages : en Pages : 200
Book Description
Liquid crystals have attracted scientific attention for potential applications in advanced devices. Display technology is continuously growing and expanding and, as such, this book provides an overview of the most recent advances in liquid crystals and displays. Chapters cover such topics as nematic liquid crystals, active matrix organic light-emitting diodes, and tetradentate platinum(II) emitters, among others.
Author: Jin Kuk Lee Publisher: ISBN: Category : Glass transition temperature Languages : en Pages : 315
Book Description
The surface dynamics of thin polymer films were tailored via tethering a fraction of the chains to the supporting substrate. This fractionally tethered state was called a "partially tethered film". A partially tethered film was prepared by coating an untethered chain layer on top of a tethered chain layer, followed by annealing. The tethered chain layers for this study had values of grafting density smaller than 0.15 chains/nm2. This low grafting density provided better miscibility between untethered and tethered chains, as compared to previous work with densely grafted brushes, therefore allowing the untethered chains to penetrate to the substrate region. Partially tethered films had slower surface fluctuation dynamics than did corresponding untethered reference films. These tailored surface dynamics after partial tethering were indicated by variations in surface relaxation time measured with X-ray photon correlation spectroscopy. Using the hydrodynamic continuum theory, the surface relaxation behavior of each sample was rationalized. However, since the hydrodynamic continuum theory hypothesizes a homogeneous viscous layer, it is not perfectly applicable to partially tethered films. To address this problem, the surface dynamics of the partially tethered film were described with models designed to account for various effects. A confinement effect due to the covalent bonds between the polymer layer and the substrate was described by modeling the hydrodynamics of a layer adjacent to the substrate and containing a substantial fraction of tethered chains as being extremely slow. The surface dynamics of a partially tethered film for which surface relaxation could still be observed was explained using a two-layer model composed of a top layer of untethered chains moving essentially like bulk untethered chains on top of the bottom layer with very slow dynamics designed to capture the effect of tethering. For sufficiently thick films this model looks very much like that already described by others for the flow of solvent past a solvent swollen layer of adsorbed polymer chains. However, as the partially tethered film becomes thinner, a more complicated model is required. At the least, the thickness of the layer with very slow hydrodynamics has to be varied as a fitting parameter and can become as large as the entire film thickness in some cases. We attribute this behavior to a "neighboring layer" effect. That is, the dynamics of the nominally untethered top layer are slowed by the influence of the practically immobile lower layer. The width of the interface between the top region of nearly pure untethered chains and the bottom region rich in tethered chains apparently plays an important role. When the degree of interpenetration between the two regions is altered by varying miscibility between the tethered and untethered chains, through the control of molecular weights or the segment-segment exchange interaction parameter, [chi], the surface dynamics can be strongly impacted. The relationship of the glass transition temperatures (T [subscript g]) of the tethered and untethered chains to one another was yet another parameter found to influence the tethering effect. When an oligiomeric untethered chain was used, the plasticization of the overall film by the low T[subscript g] untethered material dominated in determining the surface dynamics in the range of temperatures studied. More realistically accounting for all these effects requires a gradient model envisioning a gradual change in dynamical properties with depth into the film.
Author: Dena Mae Agra-Kooijman
Author: Dena Mae Agra-Kooijman
Author: Igor Muševič
Author: Toshiji Kanaya
Author: Dongping Qi
Author: Wolfgang Haase
Author: Ingo Dierking
Author: Kohki Takatoh
Author: Morteza Sasani Ghamsari
Author: Kathryn Ellen Foster
Author: Jin Kuk Lee