Imaging of Two-Dimensional Distribution of Molecular Orientation in Poly(ethylene oxide) Spherulite Using IR Spectrum and Birefringence

Effects of Polarizer-Analyzer Configurations for FTIR Spectroscopy: Implications for Multiple Polarization Algorithms

Polarized Fourier transform infrared (p-FTIR) spectroscopy is a widely used technique for determining orientational information in thin organic materials. Conventionally, a single polarizer is placed in the path of the incident light (termed the polarizer). Occasionally, a second polarizer is also placed after the sample (referred to as the analyzer). However, this polarizer-analyzer configuration has the potential to induce polarization-dependent variances in the final spectra beyond those that are expected, i.e., the squared-cosine relationship of absorptance with respect to polarization angle is no longer accurate. These variances are due to changes in the polarization state of the transmitted light induced by the sample and have yet to be explored in the context of p-FTIR. Consequently, this study employs both theoretical and experimental approaches to identify the effects of including a second polarizer in p-FTIR analyses of anisotropic organic samples. For thin samples, the most significant spectral variance arising from only birefringence is observed on the shoulders of the dichroic peaks. By adopting a crossed polarizer configuration, it is shown that there is potential to identify anisotropy of samples that are generally considered too thick for p-FTIR analysis by exploiting this feature. Furthermore, the squared-cosine relationship of absorptance with respect to the polarization angle is also shown to be inapplicable when a second parallel-oriented polarizer is included. Accordingly, a function that accounts for the second polarizer is proposed for multiple polarization techniques.

Structural and Thermal Diffusivity Analysis of an Organoferroelastic Crystal Showing Scissor-Like Two-Directional Deformation Induced by Uniaxial Compression

A two-directional ferroelastic deformation in organic crystals is unprecedented owing to its anisotropic crystal packing, in contrast to isotropic symmetrical packing in inorganic compounds and polymers. Thereby, finding and constructing multidirectional ferroelastic deformations in organic compounds is undoubtedly complex and at once calls for deep comprehension. Herein, we demonstrate the first example of a two-directional ferroelastic deformation with a unique scissor-like movement in single crystals of trans-3-hexenedioic acid by the application of uniaxial compression stress. A detailed structural investigation of the mechanical deformation at the macroscopic and microscopic levels by three distinct force measurement techniques (including shear and three-point bending test), single crystal X-ray diffraction techniques, and polarized synchrotron-FTIR microspectroscopy highlighted that mechanical twinning promoted the deformation. The presence of two crystallographically equivalent faces and the herringbone arrangement promoted the two-directional ferroelastic deformation. In addition, anisotropic heat transfer properties in the parent and the deformed domains were investigated by thermal diffusivity measurement on all three axes using microscale temperature-wave analysis (μ-TWA). A correlation between the anisotropic structural arrangement and the difference in thermal diffusivity and mechanical behavior in the two-directional organoferroelastic deformation could be established. The structural and molecular level information from this two-directional ferroelastic deformation would lead to a more profound understanding of the structure-property relationship in multidirectional deformation in organic crystals.

Scattering correction for samples with cylindrical domains measured with polarized infrared spectroscopy

Scattering artifacts are one of the most common effects distorting transmission spectra in Fourier-Transform Infrared spectroscopy. Their increased impact, strongly diminishing the quantitative and qualitative power of IR spectroscopy, is especially observed for structures with a size comparable to the radiation wavelength. To tackle this problem, a wide range of preprocessing techniques based on the Extended Multiplicative Scattering Correction method was developed, using physical properties to remove scattering presence in the spectra. However, until recently those algorithms were mostly focused on spherically shaped samples, for example, cells. Here, an algorithm for samples with cylindrical domains is described, with additional implementation of a linearly polarized light case, which is crucial for the growing field of polarized IR imaging and spectroscopy. An open-source code with GPU based implementation is provided, with a calculation time of several seconds per spectrum. Optimizations done to improve the throughput of this algorithm allow the application of this method into the standard preprocessing pipeline of small datasets.

Super-Resolved 3D Mapping of Molecular Orientation Using Vibrational Techniques

When a sample has an anisotropic structure, it is possible to obtain additional information controlling the polarization of incident light. With their straightforward instrumentation approaches, infrared (IR) and Raman spectroscopies are widely popular in this area. Single-band-based determination of molecular in-plane orientation, typically used in materials science, is here extended by the concurrent use of two vibration bands, revealing the orientational ordering in three dimension. The concurrent analysis was applied to IR spectromicroscopic data to obtain orientation angles of a model polycaprolactone spherulite sample. The applicability of this method spans from high-resolution, diffraction-limited Fourier transform infrared (FT-IR) and Raman imaging to super-resolved optical photothermal infrared (O-PTIR) imaging. Due to the nontomographic experimental approach, no image distortion is visible and nanometer scale orientation domains can be observed. Three-dimensional (3D) bond orientation maps enable in-depth characterization and consequently precise control of the sample’s physicochemical properties and functions.

Magnetic field induced alignment of macroradical epoxy for enhanced electrical properties

Improving the electrical performance of macroradical epoxy thermosets to surpass the semiconductor threshold requires a comprehensive understanding of the electrical charge transport mechanisms and characteristics. In this study, we investigate the electrical properties of a non-conjugated radical thermoset in a rigid, three-dimensional (3D) motif cured under an external magnetic field. The outcomes of the four-angle analysis of the synchrotron IRM beamline provide for the first time quantitative insights into the molecular orientation at the atomic-scale level. These insights, in turn, were utilized to apply Quantum Computational modeling theories and Monte Carlo simulation to study the effect of the magnetic field-induced molecular alignment on tuning electrical charge transport characteristics. The results explored the impact of radical density on forming percolation networks, showing a robust protocol for designing polymers with high electrical/thermal conductivity.

Probe-based microscale measurement setup for the thermal diffusivity of soft materials

Based on the principle of the periodic heating method by using cantilever thermocouple nanoprobes, we developed a method and an apparatus to measure the thermal diffusivity of soft materials on a microscale. The contact position of the probe tip with the sample surface was defined by using the phenomenon that the DC component of the thermal electromotive force (EMF) of the probe changes significantly upon contact (i.e., the vertical temperature gradient near the sample surface changes significantly). This contact position was set as the surface reference position where the variation of the thermal contact conductance between the sample surface and the sensor probe is minimized. The phase shift from the micro-heater was measured by the AC component of the probe's thermal EMF and used to accurately determine the thermal diffusivity of micro-sized soft materials. The thermal diffusivity of the microstructured photoresist was determined with a deviation of ±3%.

Imaging 3D molecular orientation by orthogonal-pair polarization IR microscopy

Anisotropic molecular alignment occurs ubiquitously and often heterogeneously in three dimensions (3D). However, conventional imaging approaches based on polarization can map only molecular orientation projected onto the 2D polarization plane. Here, an algorithm converts conventional polarization-controlled infrared (IR) hyperspectral data into images of the 3D angles of molecular orientations. The polarization-analysis algorithm processes a pair of orthogonal IR transition-dipole modes concurrently; in contrast, conventional approaches consider individual IR modes separately. The orthogonal-pair polarization IR (OPPIR) method, introduced theoretically but never demonstrated experimentally, was used to map the 3D orientation angles and the order parameter of the local orientational distribution of polymer chains in a poly(ε-caprolactone) film. The OPPIR results show that polymer chains in the semicrystalline film are aligned azimuthally perpendicular to the radial direction of a spherulite and axially tilted from the film normal direction. This newly available information on the local alignments in continuously distributed molecules helps to understand the molecular-level structure of highly anisotropic and spatially heterogeneous materials.

Macromolecular Orientation in Biological Tissues Using a Four-Polarization Method in FT-IR Imaging

Fourier transform infrared spectroscopy has emerged as a powerful tool for tissue specimen investigation. Its nondestructive and label-free character enables direct determination of biochemical composition of samples. Furthermore, the introduction of polarization enriches this technique by the possibility of molecular orientation study apart from purely quantitative analysis. Most of the molecular orientation studies focused on polymer samples with a well-defined molecular axis. Here, a four-polarization approach for Herman’s in-plane orientation function and azimuthal angle determination was applied to a human tissue sample investigation for the first time. Attention was focused on fibrous tissues rich in collagen because of their cylindrical shape and established amide bond vibrations. Despite the fact that the tissue specimen contains a variety of molecules, the presented results of molecular ordering and orientation agree with the theoretical prediction based on sample composition and vibration directions.

Analysis of molecular orientation in polymeric spherulite using polarized micro attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopic imaging

Micro ATR-FTIR spectroscopic imaging enables the visualization of two-dimensional chemical distribution at a higher spatial resolution than macro-transmission FTIR imaging approach. In this study, micro ATR-FTIR imaging was applied for analysis of a specific morphology in a spherulite of poly(3-hydroxybutyrate) (PHB). The PHB spherulites crystallized at an isothermal condition, showed the fine band structure due to the twisting lamellar crystals during the spherulite growth under the polarized optical microscope (POM). In addition, the band structure observed in the PHB spherulite was the double band pattern in which the higher and lower birefringence banded areas alternatively appear due to the three-dimensional orientation of crystallographic axes and the biaxial refractive index ellipsoid of PHB crystalline structure. Micro ATR-FTIR spectroscopic imaging was employed for detecting the double band structure in the PHB spherulite. However, the obtained spectral images did not indicate any band structures. To detect the difference of molecular orientation among the double band structures, the micro ATR-FTIR imaging was performed with a linear polarizer at four different angles. The mean values of absorbance in each measured area changed depending on the polarizer angle. The in-plane molecular orientation to the tangential direction of spherulite, caused by the dependence of the average absorbance on the polarizer angles, was determined by the position of measured area in the spherulite and the linear dicroism of each of the spectral band used. To visualize the small difference of molecular orientation in the double band structure, micro ATR-FTIR images of the dichroic differences at three spectral bands were calculated from two different sets of polarizer angles. The micro ATR-FTIR images representing the dichroic differences displayed their corresponding distributions among three spectral bands. The complementary distributions of the dichroic difference were caused by the crystallographic orientation of b- and c-axes and were successfully visualized to reveal the pattern with the features less than 10 μm in size. The results achieved in this study were due to two advantages of the polarized micro ATR-FTIR imaging: the high spatial resolution of micro ATR-FTIR imaging technique, and the high sensitivity of polarization measurements. Thus, this work demonstrates the power of this spectroscopic approach for such analytical investigation.

Analysis of interference fringes based on three circularly polarized beams targeted for birefringence distribution measurements

As a potential means of measuring birefringence distribution, we analyzed the interference fringes based on three circularly polarized beams: a right-handed signal beam, a left-handed reference beam, and a right-handed reference beam. All beams were crossed at the same angle on the interfering plane, creating a two-dimensional interference fringe with three grating vectors. We proposed that by analyzing the interference fringes, we can measure the anisotropic phase shift in the signal beam. The obtained features of the anisotropic phase shift can be extended to the measurement of two-dimensional birefringence distributions without rotational manipulations of the objectives or polarizers. The fringes were generated by monolithic gratings, which can generate three-beam interfering fields precisely and easily. Finally, we confirmed the feasibility of a birefringence measurement system without any rotational manipulations of optics.

Rapid visualization of macromolecular orientation by discrete frequency mid-infrared spectroscopic imaging

Infrared (IR) spectroscopic imaging has been used to measure the composition and orientation of polymeric systems for decades. IR microscopy can provide detailed views of microscopic regions, allowing the observation of both morphology and molecular properties of a sample, but involves a trade-off between the spatial extent and details of molecular content. Here we describe an approximately two orders of magnitude faster approach to measure the spherulitic structure and molecular orientation in large semi-crystalline polymer samples compared to extant Fourier transform infrared (FT-IR) spectroscopic imaging. This discrete frequency approach utilizes individual narrowband emission lines of a quantum cascade laser (QCL) source to spectrally image large areas rapidly. The inherent polarization of the laser beam is employed to measure orientation, enabling calculation of Hermans in-plane orientation function along with molecular chain angles distribution.

Orientational Mapping Augmented Sub-Wavelength Hyper-Spectral Imaging of Silk

Molecular alignment underpins optical, mechanical, and thermal properties of materials, however, its direct measurement from volumes with micrometer dimensions is not accessible, especially, for structurally complex bio-materials. How the molecular alignment is linked to extraordinary properties of silk and its amorphous-crystalline composition has to be accessed by a direct measurement from a single silk fiber. Here, we show orientation mapping of the internal silk fiber structure via polarisation-dependent IR absorbance at high spatial resolution of 4.2 μm and 1.9 μm in a hyper-spectral IR imaging by attenuated total reflection using synchrotron radiation in the spectral fingerprint region around 6 μm wavelength. Free-standing longitudinal micro-slices of silk fibers, thinner than the fiber cross section, were prepared by microtome for the four polarization method to directly measure the orientational sensitivity of absorbance in the molecular fingerprint spectral window of the amide bands of β-sheet polypeptides of silk. Microtomed lateral slices of silk fibers, which may avoid possible artefacts that affect spectroscopic measurements with fibers of an elliptical cross sections were used in the study. Amorphisation of silk by ultra-short laser single-pulse exposure is demonstrated.