Measurement Of Molecular Orientation Research Articles

Hyperspectral Molecular Orientation Mapping in Metamaterials

The four polarisation method is adopted for measurement of molecular orientation in dielectric nanolayers of metal-insulator-metal (MIM) metamaterials composed of gold nanodisks on polyimide and gold films. Hyperspectral mapping at the chemical finger printing spectral range of 4–20 μμm was carried out for MIM patterns of 1–2.5 μμm period (sub-wavelength). Overlay images taken at 0,π4,π2,3π4 orientation angles and subsequent baseline compensation are shown to be critically important for the interpretation of chemical mapping results and reduction of spurious artefacts. Light field enhancement in the 60-nm-thick polyimide (I in MIM) was responsible for strong absorption at the characteristic polyimide bands. Strong absorbance A at narrow IR bands can be used as a thermal emitter (emittance E=1−R), where R is the reflectance and A=1−R−T, where for optically thick samples the transmittance is T=0.

Measurement of molecular orientation using longitudinal ultrasound and its first application in in-situ characterization

In-situ characterization of the molecular orientation during injection molding process remains a great challenge. Failing to penetrate the metallic mold, many conventional methods cannot realize the online measurements during the injection molding process. In comparison, longitudinal ultrasound provides a promising method due to its deep penetration depth and the ability to reveal the information of polymeric melts. In this study, a method for measuring the degree of molecular orientation with longitudinal ultrasonic waves is proposed. Based on the ultrasonic-wave equation and linear superposition principle, the relationship between the ultrasound velocity and degree of orientation was established. Online and offline experiments were performed to validate the proposed method by measuring the degrees of orientation with dichromic Fourier transform infrared spectroscopy and two-dimensional wide-angle X-ray diffraction. The experimental results show a negative correlation between the ultrasound velocity and degree of orientation, which agrees well with our mathematical model. Furthermore, the mechanism of the orientation measurement through the ultrasound velocity and the deviations of the proposed method were analyzed. The combined uncertainties of orientation in the offline measurement were found within the range of ±0.30–2.54%. In general, the proposed method is safe, non-destructive and valid for the in-situ characterization of molecular orientation.

Re-orientation of water molecules in response to surface charge at surfactant interfaces.

We present a measurement of molecular orientation of water at charged surfactant aqueous interfaces as a function of surface charge density. The polarization dependent spectral line shapes of the water bend mode were measured by vibrational sum-frequency generation at the positively charged surfactant cetyltrimethylammonium bromide (CTAB)/water interface and negatively charged surfactant sodium dodecyl sulfate/water interface. Orientational analysis using the water bend mode as a vibrational probe, within the electric dipole approximation, reveals structural differences between these surfaces and quantifies how different hydrogen bonded species re-orient around the surfactant head groups as the surface charge density changes. As the concentration of the positively charged surfactant (CTAB) increases, the surface water molecules with free-OH groups reorient their hydrogen away from the bulk water and the C2v axis closer to the surface normal. This suggests that these free-OH molecules are in general located above the positively charged head groups of CTAB, and thus, the charge-dipole interaction pulls their oxygen "down" and pushes hydrogen "up." On the contrary, water molecules with two donor hydrogen bonds re-orient their hydrogen toward the bulk water, likely because most of these molecules are below the CTAB surfactant head groups.

Precise Characterisation of Molecular Orientation in a Single Crystal Field-Effect Transistor Using Polarised Raman Spectroscopy.

Charge transport in organic semiconductors is strongly dependent on the molecular orientation and packing, such that manipulation of this molecular packing is a proven technique for enhancing the charge mobility in organic transistors. However, quantitative measurements of molecular orientation in micrometre-scale structures are experimentally challenging. Several research groups have suggested polarised Raman spectroscopy as a suitable technique for these measurements and have been able to partially characterise molecular orientations using one or two orientation parameters. Here we demonstrate a new approach that allows quantitative measurements of molecular orientations in terms of three parameters, offering the complete characterisation of a three-dimensional orientation. We apply this new method to organic semiconductor molecules in a single crystal field-effect transistor in order to correlate the measured orientation with charge carrier mobility measurements. This approach offers the opportunity for micrometre resolution (diffraction limited) spatial mapping of molecular orientation using bench-top apparatus, enabling a rational approach towards controlling this orientation to achieve optimum device performance.

Minimization of Friction at Various Speeds Using Autonomous Viscosity Control of Nematic Liquid Crystal

To demonstrate the concept of the smart lubrication system using liquid crystal (LC) lubricant proposed by Nakano (Tribol Lett 14:17–24, 2003), the following three types of tests were performed: (1) film thickness measurements in pure rolling contacts, (2) friction coefficient measurements in rolling–sliding contacts, and (3) molecular orientation measurements in stationary cells. In all of these types of tests, a nematic LC [4-pentyl-4′-cyanobiphenyl (5CB)] and a carboxylic acid [hexadecanoic acid (HDA)] were used as the base fluid and the additive, respectively. The results of these tests confirm the following mechanism. First, surface films of the HDA additive that spontaneously adsorb onto contact surfaces induce surface anchoring, which has the effect of making 5CB molecules align themselves perpendicular to the surfaces, competing with the flow alignment of 5CB molecules and inducing an increase in the apparent viscosity of 5CB with decreasing entrainment speed. This increase in the apparent viscosity generates a constant friction coefficient region in the Stribeck curve on the left side of the minimum friction coefficient point, resulting in the minimization of friction at various speeds.

S051011 シースコア流れ内平面伸張流動場における粘弾性流体の流動挙動と分子配向([S051-01]複雑流体の流動現象(1))

It is proposed that the technique to generate planar elongational flow with high elongational rate by using core/sheath flow. In this study we carried out experiments to observe flow behavior for both viscous fluid and viscoelastic fluid in the flow. In case of viscous, we compared experimental results of velocity distribution along a centerline of the flow cell to analysis results under the two-dimensional Poiseuille flow and they show good agreement. On the other hand, it is observed for viscoelastic fluid that flow behavior of the core fluid shows four kind of flow patterns from result of flow visualization and velocity profile, that is, swelling, stretching, stretching before conjunction region and instable flow. Next, we evaluated molecular orientation profile of elongated fluid by using crossed nicols method. As the results, measure of molecular orientation doesn't depend on the distance to sidewalls of the flow cell in elongational flow region. It is shown that contribution elongational property to the flow can be discuss from results of distribution of molecular orientation along center line of the flow cell.

Mesomorphic−α-Monoclinic Phase Transition in Isotactic Polypropylene: A Study of Processing Effects on Structure and Mechanical Properties

We report the enthalpy for the mesomorphic to α-monoclinic phase transition in polypropylene under varying thermal treatments. The mesomorphic phase is created by fiber spinning and rapid quenching methods and identified using wide-angle X-ray diffraction and differential scanning calorimetry. Fiber mesomorphs are found to have a 3-fold increase in enthalpy of transition per gram of mesophase compared with our measurements of quenched polypropylene and previous reports of quenched polypropylene. In addition, systematic tensile testing over a range of spin speeds and polymer morphologies reveals that the presence of mesomorphic regions does not correlate with reduced fiber strength as has been previously suggested. Fiber true stress−true strain curves obtained at varying take-up velocities are compared to determine the “tensile strain shift”, which should theoretically provide a measure of molecular orientation. We find that the tensile strain shift correlates with birefringence, thereby providing an alternative method to assess molecular orientation in fibers, an important factor for fiber strength. This approach can prove useful for fibers in which measuring the molecular orientation via birefringence is not an option.

Quantitative polarized Raman spectroscopy in highly turbid bone tissue

Polarized Raman spectroscopy allows measurement of molecular orientation and composition and is widely used in the study of polymer systems. Here, we extend the technique to the extraction of quantitative orientation information from bone tissue, which is optically thick and highly turbid. We discuss multiple scattering effects in tissue and show that repeated measurements using a series of objectives of differing numerical apertures can be employed to assess the contributions of sample turbidity and depth of field on polarized Raman measurements. A high numerical aperture objective minimizes the systematic errors introduced by multiple scattering. We test and validate the use of polarized Raman spectroscopy using wild-type and genetically modified (oim/oim model of osteogenesis imperfecta) murine bones. Mineral orientation distribution functions show that mineral crystallites are not as well aligned (p<0.05) in oim/oim bones (28+/-3 deg) compared to wild-type bones (22+/-3 deg), in agreement with small-angle X-ray scattering results. In wild-type mice, backbone carbonyl orientation is 76+/-2 deg and in oim/oim mice, it is 72+/-4 deg (p>0.05). We provide evidence that simultaneous quantitative measurements of mineral and collagen orientations on intact bone specimens are possible using polarized Raman spectroscopy.

A New Method for the Time-Resolved Analysis of Structure and Orientation: Polarization Modulation Infrared Structural Absorbance Spectroscopy

Polarization modulation infrared linear dichroism (PM-IRLD) is often used for measurements of molecular orientation with high sensitivity and good time resolution. However, PM-IRLD is unable to provide the structural absorbance spectrum because it does not measure separately the parallel and perpendicular spectra. Here we propose a new method, named polarization modulation infrared structural absorbance spectroscopy (PM-IRSAS), to overcome this limitation of PM-IRLD. PM-IRSAS measures the dichroic difference and structural absorbance spectra simultaneously and, therefore, allows quantitative analysis of molecular orientation and conformation with 200 ms time resolution. The PM-IRSAS method was first validated through comparison with conventional polarized FT-IR spectroscopy using drawn polymer films. Second, it was demonstrated that the PM-IRSAS method can provide a quantitative analysis of dynamic orientation and conformation changes in PET films during deformation and crystallization processes.

Measurement of Molecular Orientation in a Subcellular Compartment by Synchrotron Infrared Spectromicroscopy

Subcellular dichroism at the diffraction limit: Synchrotron infrared spectromicroscopy provides a snapshot of the axial molecular architecture in the outer segment of an intact retinal rod cell (see picture), which reinforces the role of this technique as a tool for in vivo studies of biomolecular structure–function relationships.

A Relationship between Molecular Orientation and Current–Voltage Characteristics in Poly(3-hexylthiophene) Thin Film

Abstract Simultaneous measurement of molecular orientation and current as a function of voltage has been conducted. Molecular orientation was measured using the polarization dependence of intramolecular electronic transition. In ITO/poly(3-hexylthiophene)/Al structure, it was found that the molecular orientation change assists the hysteretic current–voltage (J–V) characteristics. The result indicates that this measuring method can be useful in investigating the effects of molecular orientation change on unusual J–V characteristics.

Molecular orientation in individual electrospun nanofibers measured via polarized Raman spectroscopy

Using polarized Raman spectroscopy, we have recorded Raman spectra from individual electrospun Nylon-6 nanofibers. Analysis of these single-fiber spectra, compared to those of unoriented and oriented Nylon-6 films, indicates significant molecular orientation. Because electrospinning produces fibers in a jet with a large strain rate, this molecular orientation is expected. We present quantitative measurements of molecular orientation in a single nanofiber and compare these to those of film samples. Such measurements could yield information about the uniformity of the electrospinning process and resulting fibers, and may also allow comparison between spectrally measured orientation functions and single-fiber mechanical properties.

Orientation dynamics in commercial thermotropic liquid crystalline polymers in transient shear flows

In situ X-ray scattering measurements of molecular orientation under shear are reported for two commercial thermotropic liquid crystalline polymers (TLCPs), Vectra A950® and Vectra B950®. Transient shear flow protocols (reversals, step changes, and flow cessation) are used to investigate the underlying director dynamics. Synchrotron X-ray scattering in conjunction with a high-speed area detector provides sufficient time resolution to limit the total time spent in the melt during testing, whereas a redesigned X-ray capable shear cell provides a more robust platform for working with TLCP melts at high temperatures. The transient orientation response upon flow inception or flow reversal does not provide definitive signatures of either tumbling or shear alignment. However, the observation of clear transient responses to step increases or step decreases in shear rate contrasts with expectations and experience with shear-aligning nematics and suggests that these polymers are of the tumbling class. Finally, these two polymers show opposite trends in orientation following flow cessation, which appears to correlate with the evolution of dynamic modulus during relaxation. Specifically, Vectra B shows an increase in orientation upon flow cessation, an observation that can only be rationalized by the assumption of tumbling dynamics in shear. Together with prior observations of commercial LCP melts in channel flows, these results suggest that this class of materials, as a rule, exhibits director tumbling.

An Improved Optical Method for Determining the Order Parameter in Thin Oriented Molecular Films and Demonstration of a Highly Axial Dipole Moment for the Lowest Energy π–π* Optical Transition in Poly(9,9‐ dioctylfluorene‐co‐bithiophene)

AbstractUnderstanding the complex interplay between the 3D structural hierarchy within thin films of conjugated polymers and the properties of devices based thereon is starting to be recognized as an important challenge in the continued development of these materials for a range of applications. As a result, for example, accurate measurements of molecular orientation and elucidation of its influence on optical characteristics are of significant interest. Here we report an improved optical method to determine both the order parameter and the angle between the polymer backbone director and the optical transition dipole moment for the lowest energy π–π* absorption peak in uniaxially aligned thin films of conjugated polymers. The method uses a combination of polarized Raman spectroscopy and UV‐vis spectroscopy and is based on a general theoretical treatment to describe the expected Raman and optical absorption anisotropies of such films. It is applied to study the orientation within thermotropically aligned films of the electroluminescent fluorene‐based copolymer poly(9,9‐dioctylfluorene‐co‐bithiophene) (F8T2). A more highly axial transition dipole moment is found for the dominant long wavelength absorption peak of F8T2 compared to that of other fluorene‐based (co)polymers. The angle between the polymer backbone director and the transition dipole is estimated to be β ≤ 3°, a deduction that helps to explain the relatively large optical dichroism for aligned films of F8T2 and that offers the prospect of highly polarized electroluminescence from F8T2‐based light‐emitting diodes.

Tearing energy study of “oriented and relaxed” polystyrene in the glassy state

AbstractTo study the effect of processing history, molecular weight/molecular weight distribution, and thermal history on solid state properties (in particular fracture properties and orientation), carefully characterized polydisperse and monodisperse polystyrene samples were drawn above Tg and the orientation frozen in. The objective was to simulate the incidental orientation of polymer chains after processing, molding, and so forth (e.g., injection or compression, blow molding) as a result of melt flow. A series of polystyrene samples was produced by hot drawing at temperatures of 113 and 148 °C, followed by a relaxation period, and then a quench to below Tg. The level of segmental orientation imposed in the samples was determined by birefringence measurements. The tear energy of the sheets was measured at 20 °C by tearing along the draw direction, ultimately giving a value for the fracture energy, G3C. Samples of high draw ratio and low segmental orientation were unexpectedly found to have highly anisotropic fracture properties despite the low level of optical anisotropy. The fracture properties also depended significantly on whether the samples were drawn with or without lateral constraint. The results are compared with measurements of isotropic samples and the findings of a previous investigation utilizing SANS and birefringence. Modeling the drawing conditions at the chain level using a recent nonlinear tube theory explains how birefringence alone is an inadequate measure of molecular orientation. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 377–394, 2007

In-situ measurement of molecular orientation of the pentacene ultrathin films grown on SiO 2 substrates

Molecular orientations of pentacene ultrathin films grown on SiO 2 substrates were studied without the influence of the atmosphere by vacuum atomic force microscopy (V-AFM) and near edge X-ray absorption fine structure (NEXAFS). The experimental processes from deposition of pentacene to characterization of films were performed under vacuum condition without exposure to the atmosphere. V-AFM and NEXAFS measurements showed that pentacene molecules tend to grow on SiO 2 surface with their molecular long axes perpendicular to the substrate surfaces (standing-mode) irrespective of preparation procedure of SiO 2 substrate.

Molecular structure and orientation of gel‐spun polyethylene fibers

AbstractCharacterization of molecular structure and orientation of six commercially available gel‐spun polyethylene fibers have been carried out using infra‐red and Raman spectroscopy, thermal and X‐ray diffraction analysis, together with optical microscopy techniques. Thermal and X‐ray diffraction analysis revealed the existence of highly oriented orthorhombic and monoclinic crystallites together with a highly oriented intermediate phase known as pseudohexagonal mesophase structure. The results suggest the existence of a three‐phase structure consisting, at room temperature, of orthorhombic and monoclinic polymorphic crystallites, oriented noncrystalline and un‐oriented noncrystalline (amorphous) phases, respectively. The crystallinity measurements have been carried out using density, thermal and X‐ray diffraction analysis together with infra‐red and Raman spectroscopy techniques whereas the molecular orientation measurements have been carried out using birefringence and polarized IR spectroscopy, respectively. The results obtained from density, thermal analysis, and Raman spectroscopy based on a simple two‐phase modeling approach lead to the overestimated amorphous fractions and appears to ignore the presence of an intermediate phase known as oriented noncrystalline structure. X‐ray analysis has also been used for the measurement of the apparent crystallite sizes. The birefringence values have been used to determine the overall orientation parameters whereas the dichroic measurements of IR bands have been used to determine the crystalline and oriented noncrystalline orientation parameters. The results show that the orthorhombic and monoclinic phases are more highly oriented than the oriented pseudohexagonally packed noncrystalline chains. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1317–1333, 2006

Nonlinear Susceptibility of a Cyanobiphenyl Derivative at the Air/Water Interface: Improved Measurement of Molecular Orientation by Optical Second Harmonic Generation

The nonlinear susceptibility tensor, chi(2), for second harmonic generation (SHG) by a monolayer of the cyanobiphenyl derivative 4-n-octyl-4'-cyanobiphenyl (8CB) at the air/water interface has been measured with the fundamental frequency variant Planck's over 2piomega=1.55 eV (lambda=800 nm). The contribution of the water subphase was estimated by measuring the magnitudes and relative phases of the nonlinear susceptibilities of the bare and monolayer-covered water surfaces. All nonzero elements of chi(2) are placed on an absolute scale by comparison with SHG in reflection from a z-cut quartz crystal. The experimental measurements for the 8CB monolayer are compared with computed susceptibilities derived by using standard time-dependent perturbation theory in conjunction with a semiempirical electronic structure model. Good agreement has been found between experimental and computational results when the average tilt angle of the cyanobiphenyl chromophore of 8CB is in the range 60-70 degrees relative to the surface normal. A critique is given of an alternative, simplified measurement procedure of the tilt angle in which only the dominant element of the molecular hyperpolarizability tensor is considered. It is shown that the simplified procedure is invalid for 8CB monolayers when the tilt angle is greater than approximately 70 degrees.

Linear Dichroism in the X-ray Absorption Spectra of Linearn-Alkanes

The nature of the linear dichroism in the near-edge X-ray absorption fine structure (NEXAFS) spectra of linear n-alkanes is a matter of long-standing controversy. Linear dichroism in the carbon 1s --> sigma*(C-C) transition has been interpreted within a building block model and a molecular orbital model, leading to two different descriptions for the angular dependence of this feature. When used for measurement of molecular orientation, the application of these two different models will lead to different results. We have explored the linear dichroism in the carbon 1s NEXAFS spectra of single crystals of the linear n-alkane hexacontane (n-C60H122). An analysis of the angular dependence in this spectrum shows that the transition dipole moment associated with the carbon 1s --> sigma*(C-C) transition is oriented along the macromolecular chain axis, contradicting the predictions of the building block model. However, other transitions are observed in the sigma*(C-H) and the sigma*(C-C) bands that are orthogonal to the dominant transitions for each band. We also observe that radiation damage can be manifest in the form of molecular reorientation in highly ordered organic thin films.

Mechanical and morphological anisotropy in injection molding of thermotropic liquid crystalline copolyesters

Relationships among mechanical properties, degree of molecular orientation and molding conditions are investigated in injection molded plaques fabricated from a 4,4′-dihydroxy-α-methylstilbene (DHαMS)-based thermotropic liquid crystalline copolyester. Wide-angle X-ray scattering (WAXS) patterns reveal bimodal orientation states at most locations in the plaques. One population aligns roughly along the anticipated flow direction while a separate population is generated as a result of transverse stretching associated with diverging streamlines during mold filling. Micro-tensile bars are cut from the plaques both parallel and perpendicular to the filling direction to assess anisotropy in properties. Enhanced molecular orientation and properties ‘in-shear’ are observed for thinner plaques fabricated at relatively low mold temperatures and melt temperatures slightly above the nominal melting point of the polymer. Injection fill speed is not found to have a significant effect on anisotropy in tensile strength/stiffness. Mechanical properties such as tensile modulus and fracture stress are found to obey a ‘universal’ correlation with X-ray measurements of molecular orientation projected onto the axis of the testing specimens. These results suggest that even in the presence of complex, spatially heterogeneous orientation states, simple average measures of orientation can provide a robust means of anticipating macroscopic properties.