Investigation of a polymer-dispersed liquid crystal system by NMR diffusometry and relaxometry

Polymer-eingebettete Flüssigkristalle (PDLCs) bestehen aus einer Dispersion flüssigkristalliner Tropfen in einer Polymermatrix. Sie sind wegen ihrer einzigartigen Eigenschaften und möglichen Anwendungen von Interesse. Die Eigenschaften von PDLCs hängen vom Grad der Phasenseparation und der Tropfengröße ab. Strukturuntersuchungen können zu einem besseren Verständnis und zur Optimierung von PDLCs beitragen.Hauptziel dieser Doktorarbeit war die Untersuchung von PDLCs durch NMR-Techniken. Diffusionskonstanten und Spin-Gitter-Relaxationszeiten im Labor- (T1) und im rotierenden Koordinatensystem (T1) wurden für PDLCs und Ausgangsmischungen, bestehend aus dem Monomer Trimethylolpropan-Triacrylat (TMPTA) und der kommerziellen nematischen Mischung E7, gemessen.Aus der temperaturabhängigen Variation der Dipolaufspaltung von 1H-Spektren wurden die nematisch-isotrope Umwandlungstemperatur und der nematische Ordnungsparameter von E7 und, zum Vergleich, dem Flüssigkristall 5CB bestimmt. Die Diffusionskonstanten in TMPTA/E7-Mischungen, bestimmt durch NMR mit gepulsten Feldgradienten, steigen, bedingt durch die geringere Viskosität von E7, für beide Komponenten mit dem Massenbruch an E7 an. Im PDLC diffundiert E7 wegen der Behinderung durch die Polymermatrix langsamer als im Bulk.T1 und T1 sind in der flüssigen bzw. flüssigkristallinen Phase von TMPTA und reinem E7 länger als im PDLC und im reinen Polymer. Grund ist die geringere Mobilität in den Polymerproben. Die im Vergleich zum reinen Polymer noch kürzere T1-Zeit im PDLC deutet auf eine Antiweichmacherwirkung der E7-Moleküle hin. In reinem E7 zeigen die gut geordneten stäbchenförmigen Moleküle eine starke C-H-Dipolkopplung, die zu Oszillationen in der Kreuzpolarisationskurve führt.Polymer-dispersed liquid crystals (PDLCs) are polymer composites containing a dispersion of liquid crystal droplets in polymer networks. PDLCs have attracted much attention due to their unique properties and potential usage. The properties of PDLCs depend on the degree of phase separation and the size of liquid crystal droplets. To investigate the structure will help us to better understand and optimize PDLCs.The main aim of this PhD thesis was to investigate PDLCs by NMR techniques. Diffusion constants and spin-lattice relaxation times in the laboratory (T1) and rotating frame (T1) were measured for PDLCs as well as precursor mixtuures based on the trifunctional monomer trimethylolpropane triacrylate (TMPTA) and the commercial nematic mixture E7.The variation of the main dipolar splitting of 1H spectra with increasing temperature was analyzed to obtain the nematic-to-isotropic phase transition temperature and the nematic order parameter of E7 and, for comparison, the nematic liquid crystal 5CB.Diffusion constants in TMPTA/E7 mixtures, measured by pulsed-field gradient NMR, increase for both E7 and TMPTA as the mass fraction of E7 increases, due to the lower viscosity of E7. E7 in the PDLC diffuses more slowly than in the bulk because of the hindrance by the polymer matrix.T1 and T1 relaxation times in the liquid or liquid-crystalline phases of TMPTA and bulk E7 are higher than in the PDLC and the pure polymer, due to the lower mobility in the polymer samples. T1 in the PDLC is even shorter than in the pure polymer, indicating an anti-softening effect caused by E7 molecules. In bulk E7, the well-ordered rod-like molecules exhibit a unique H-C dipolar coupling, which leads to oscillations in the cross-polarization curve. However, in the PDLC, the anchoring effect at the boundary between the polymer and LC droplets disturbs the molecular order resulting in a smooth cross polarization curve.Tag der Verteidigung: 26.02.2013Paderborn, Univ., Diss., 201

Fatty acids impact sarcomere integrity through myristoylation and ER homeostasis

Decreased ability to maintain tissue integrity is critically involved in aging and degenerative diseases. Fatty acid (FA) metabolism has a profound impact on animal development and tissue maintenance, but our understanding of the underlying mechanisms is limited. We investigated whether and how FA abundance affects muscle integrity using Caenorhabditis elegans. We show that reducing the overall FA level by blocking FA biosynthesis or inhibiting protein myristoylation leads to disorganization of sarcomere structure and adult-onset paralysis. Further analysis indicates that myristoylation of two ARF guanosine triphosphatases (GTPases) critically mediates the effect of FA deficiency on sarcomere integrity through inducing endoplasmic reticulum (ER) stress and ER unfolded protein response (UPRER), which in turn leads to reduction of the level of sarcomere component PINCH and myosin disorganization. We thus present a mechanism that links FA signal, protein myristoylation, and ER homeostasis with muscle integrity, which provides valuable insights into the regulatory role of nutrients and ER homeostasis in muscle maintenance. &nbsp;</p

Measuring Nano- to Microstructures from Relayed Dynamic Nuclear Polarization NMR

We show how dynamic nuclear polarization (DNP) NMR can be used in combination with models for polarization dynamics to determine the domain sizes in complex materials. By selectively doping a source component with radicals and leaving the target undoped, we Can measure experimental polarization buildup curves which can be compared with simulations based on heterogeneous distributions of polarization-within the sample. The variation of the integrated DNP enhancement as a function of the polarization time is found to be characteristic of the geometry. We demonstrate the method experimentally on four different systems where we successfully determine domain sizes between 200 and 20 000 nm, specifically in powdered histidine hydrochloride monohydrate) pore lengths of mesoporous silica materials, and two domain sizes in two component polymer film coatings. Additionally, we find that even in the apparently homogeneous frozen solutions used as polarization sources in most DNP experiments, polarization is relayed from protons near the radicals to the bulk of the solution by spin diffusion, which explains the experimentally observed buildup times in these samples

Descope of the ALIA mission

The present work reports on a feasibility study commissioned by the Chinese Academy of Sciences of China to explore various possible mission options to detect gravitational waves in space alternative to that of the eLISA/LISA mission concept. Based on the relative merits assigned to science and technological viability, a few representative mission options descoped from the ALIA mission are considered. A semi-analytic Monte Carlo simulation is carried out to understand the cosmic black hole merger histories starting from intermediate mass black holes at high redshift as well as the possible scientific merits of the mission options considered in probing the light seed black holes and their coevolution with galaxies in early Universe. The study indicates that, by choosing the armlength of the interferometer to be three million kilometers and shifting the sensitivity floor to around one-hundredth Hz, together with a very moderate improvement on the position noise budget, there are certain mission options capable of exploring light seed, intermediate mass black hole binaries at high redshift that are not readily accessible to eLISA/LISA, and yet the technological requirements seem to within reach in the next few decades for China

Analysis of Weak Zones in Friction Stir Welded Magnesium Alloys from the Viewpoint of Local Texture: A Short Review

Friction stir welding (FSW) is a promising approach for the joining of magnesium alloys. Although many Mg alloys have been successfully joined by FSW, it is far from industrial applications due to the texture variation and low mechanical properties. This short review deals with the fundamental understanding of weak zones from the viewpoint of texture analysis in FSW Mg alloys, especially for butt welding. Firstly, a brief review of the microstructure and mechanical properties of FSW Mg alloys is presented. Secondly, microstructure and texture evolutions in weak zones are analyzed and discussed based on electron backscatter diffraction data and Schmid factors. Then, how to change the texture and strengthen the weak zones is also presented. Finally, the review concludes with some future challenges and research directions related to the texture in FSW Mg alloys. The purpose of the paper is to provide a basic understanding on the location of weak zones as well as the weak factors related to texture to improve the mechanical properties and promote the industrial applications of FSW Mg alloys

Kinetics of the grating formation in holographic polymer-dispersed liquid crystals: NMR measurement of diffusion coefficients

Polymer films with embedded liquid crystal inclusions (polymer-dispersed liquid crystals) are superb composites for addressable windows, flexible displays and optical storage. Their scattering behavior and electro-optic properties depend essentially on the shape and size of the liquid crystal inclusions, which are typically formed by phase separation from a multicomponent homogeneous mixture. Here, pulsed field gradient NMR is used to measure the self-diffusion coefficients of the liquid crystal and a photo-reactive monomer, which compose such a precursor mixture. The kinetics of holographic grating formation in this mixture can be predicted by inserting the NMR diffusion coefficient of the monomer and the polymerization rate in a reaction diffusion model. The ratio of diffusion rate over reaction rate is found to be in the limiting regime in which the kinetics of the grating formation is not sensitive to this parameter

A solid-state NMR method to determine domain sizes in multi-component polymer formulations

Polymer domain sizes are related to many of the physical properties of polymers. Here we present a solid-state NMR experiment that is capable of measuring domain sizes in multi-component mixtures. The method combines selective excitation of carbon magnetization to isolate a specific component with proton spin diffusion to report on domain size. We demonstrate the method in the context of controlled release formulations, which represents one of today's challenges in pharmaceutical science. We show that we can measure domain sizes of interest in the different components of industrial pharmaceutical formulations at natural isotopic abundance containing various (modified) cellulose derivatives, such as microcrystalline cellulose matrixes that are film-coated with a mixture of ethyl cellulose (EC) and hydroxypropyl cellulose (HPC). (C) 2015 Published by Elsevier Inc

Nanostructure of Materials Determined by Relayed Paramagnetic Relaxation Enhancement

Particle and domain sizes strongly influence the properties of materials. Here we present an NMR approach based on paramagnetic relaxation enhancement (PRE) relayed by spin diffusion (SD), which allows us to determine lengths in the nm pm range. We demonstrate the method on multicomponent organic polymer mixtures by selectively doping one component with a paramagnetic center in order to measure the domain size in a second component. Using this approach we determine domain sizes in ethyl cellulose/hydroxypropyl cellulose film coatings in pharmaceutical controlled release formulations. Here we measure particle sizes ranging from around 50 to 200 nm

Tuning Low Cycle Fatigue Properties of Cu-Be-Co-Ni Alloy by Precipitation Design

As material for key parts applied in the aerospace field, the Cu-Be-Co-Ni alloy sustains cyclic plastic deformation in service, resulting in the low cycle fatigue (LCF) failure. The LCF behaviors are closely related to the precipitation states of the alloy, but the specific relevance is still unknown. To provide reasonable regulation of the LCF properties for various service conditions, the effect of precipitation states on the LCF behaviors of the alloy was investigated. It is found that the alloy composed fully of non-shearable &gamma;&prime; precipitates has higher fatigue crack initiation resistance, resulting in a longer fatigue life under LCF process with low total strain amplitude. The alloy with fine shearable &gamma;&prime;I precipitates presents higher fatigue crack propagation resistance, leading to a longer fatigue life under LCF process with high total strain amplitude. The cyclic stress response behavior of the alloy depends on the competition between the kinematic hardening and isotropic softening. The fine shearable &gamma;&prime;I precipitates retard the decrease of effective stress during cyclic loading, causing cyclic hardening of the alloy. The present work would help to design reasonable precipitation states of the alloy for various cyclic loading conditions to guarantee its safety in service

In Situ Electron Paramagnetic Resonance Correlated Spectroscopy and Imaging: A Tool for Lithium-Ion Batteries to Investigate Metallic Lithium Sub-Micrometric Structures Created by Plating and Stripping

Monitoring the formation of dendrites or filaments of lithium is of paramount importancefor Li-based battery technologies, hence the intense activities in designing in situ techniquesto visualize their growth. Herein we report the benefit of correlating in situ electron para4 magnetic resonance (EPR) spectroscopy and EPR imaging to analyze the morphology andlocation of metallic lithium in a symmetric Li/LiPF6/Li electrochemical cell during polariza6 tion. We exploit the variations in shape, resonance field and amplitude of the EPR spectrato follow, operando, the nucleation of sub-micrometric Li particles (narrow and symmetricalsignal) that conjointly occurs with the fragmentation of bulk Li on the opposite electrode(asymmetrical signal). Moreover, in situ EPR correlated spectroscopy and imaging (spectral10 spatial EPR imaging) allows the identification (spectral) and localization (spatial) of the sub11 micrometric Li particles created by plating (deposition) or stripping (altered bulk Li surface).We finally demonstrate the possibility to visualize, via in situ EPR imaging, dendrites formedthrough the separator in the whole cell. Such a technique could be of great help in masteringthe Li-electrolyte interface issues that plague the development of solid-state batteries.</div