Birefringence Orientation Research Articles

Three-dimensional fiber orientation mapping of the human brain at micrometer resolution.

The accurate measurement of three-dimensional (3D) fiber orientation in the brain is crucial for reconstructing fiber pathways and studying their involvement in neurological diseases. Comprehensive reconstruction of axonal tracts and small fascicles requires high-resolution technology beyond the ability of current in vivo imaging (e.g. diffusion magnetic resonance imaging). Optical imaging methods such as polarization-sensitive optical coherence tomography (PS-OCT) and polarization microscopy can quantify fiber orientation at micrometer resolution but have been limited to two-dimensional in-plane orientation or thin slices, preventing the comprehensive study of connectivity in 3D. In this work we present a novel method to quantify volumetric 3D orientation in full angular space with PS-OCT. We measure the polarization contrasts of the brain sample from two illumination angles of 0 and 15 degrees and apply a computational method that yields the 3D optic axis orientation and true birefringence. We further present 3D fiber orientation maps of entire coronal cerebrum sections and brainstem with 10 μm in-plane resolution, revealing unprecedented details of fiber configurations. We envision that our method will open a promising avenue towards large-scale 3D fiber axis mapping in the human brain as well as other complex fibrous tissues at microscopic level.

Blending Bottlebrush Polymers for Compensation of Orientation Birefringence of Polystyrene for Use as an Optical Polymer

Blending Bottlebrush Polymers for Compensation of Orientation Birefringence of Polystyrene for Use as an Optical Polymer

Polarization phase unwrapping by a dual-wavelength Mueller matrix imaging system.

In this Letter, we report a dual-wavelength Mueller matrix imaging system for polarization phase unwrapping, allowing simultaneous acquisition of the polarization images at 633 nm and 870 nm. After phase unwrapping, the relative error of linear retardance is controlled to be 3% and the absolute error of birefringence orientation is about 6°. We first show that polarization phase wrapping occurs when the samples are thick or present obvious birefringence effects, and further analyze the effect of phase wrapping on anisotropy parameters via Monte Carlo simulations. Then, experiments on porous alumina with different thicknesses and multilayer tapes are performed to verify the feasibility of phase unwrapping by a dual-wavelength Mueller matrix system. Finally, by comparing the temporal characteristics of linear retardance during tissue dehydration before and after phase unwrapping, we emphasize the significance of the dual-wavelength Mueller matrix imaging system not only for anisotropy analysis in static samples, but also for determining the trend in polarization properties of dynamic samples.

Effect of propionyl group on birefringence and wavelength dispersion of propionylated cellulose acetate optical compensation films

Propionylated cellulose acetate (PCA) with different propionyl substitution degrees (DSPr) was synthesized, and the corresponding optical films were prepared by solution casting. The birefringence and its wavelength dispersion of PCA films stretched at 10 °C above and below the glass transition temperature (Tg) with different draw ratios (DR) were studied. The introduction of propionyl group at different substitution sites present different contribution to birefringence and its wavelength dispersion. The propionyl group at C-2 and C-3 sites have a larger positive orientation birefringence with stronger normal wavelength dispersion, while that at the C-6 site shows a smaller negative orientation birefringence with weaker normal wavelength dispersion compared with the acetyl group. Compared with CA film, the introduction of the propionyl group weakens the orientation birefringence of PCA film. With the increase of DSPr from 0.023 to 0.303, the occurrence of propionyl substitution only at the C-6 site turns to C-2, C-3 and C-6 sites and the wavelength dispersion of in-plane birefringence (Δnin) decreases. When DSPr is further increased to 0.537, the Δnin and out-of-plane birefringence (Δnth) of PCA films at different DR show a very weak wavelength dispersion with the relative horizontal curves. Current results indicate that when the substitution degree of the propionyl group and acetyl group is suitable, the wavelength dispersion of PCA film can be eliminated.

Regulation of orientation birefringence for cellulose acetate film: The role of crystallization and orientation

Cellulose acetate (CA) based films are widely used in liquid crystal displays due to their outstanding transparency, and a certain orientation birefringence of CA films is required when they are used as retardation films. The regulation of orientation birefringence is usually from the perspective of stretch-induced orientation, while the effects of crystallization behaviors of CA films remain obscure. In this study, the roles of crystallization and orientation on the orientation birefringence of CA films were elucidated. For cellulose diacetate films, the orientation birefringence is dominated by the orientation degree. In comparison, apart from the orientation degree, crystallinity is another key variable to regulate the orientation birefringence of cellulose triacetate and plasticized cellulose triacetate films, originating from the birefringence heterogeneity of the crystalline and amorphous phases. These results provide valuable guidelines for the production of CA-based optical films with excellent optical performance.

Mueller Matrix Imaging Optimized by Uniform Illumination

In this study, we present a flat-field Mueller matrix imaging system to reduce the reconstruction error caused by critical illumination. This study demonstrates that the signal-to-noise ratio (SNR) of the reconstructed images is improved by about eight times by adding a beam shaping module made up of microlens arrays to a traditional Mueller system. The scalar diffraction theory and polarization numerical simulation show the ability of the new device in minimizing the adverse effects of light source noise on polarization reconstruction results. Finally, the experiment results on standard resolution board, porous anodic alumina, and real pathological slices further confirm the superiority of the flat-field Mueller system in precisely identifying sample structure and quantitative differences between various polarization parameters (depolarization ratio Δ, linear retardance δ, and birefringence orientation θ), demonstrating the potential of flat-field polarization imaging in pathological diagnosis and tissue characteristic extraction.

Compensation for Orientation Birefringence of PMMA by Blending Bottlebrush Polymers Composed of Well-Controlled Graft Chains.

The birefringence of optical polymers is a great issue in optical devices, inhibiting major applications of polymers to optical lenses and films. In this study, we have synthesized effective bottlebrush polymers with which we could attain almost zero birefringence when mixed with optical poly(methyl methacrylate) (PMMA). In detail, the PMMA bottlebrush polymers (PMMA-BBP) were synthesized by the ring-opening metathesis polymerization (ROMP) of norbornene-terminated PMMA macromonomers prepared via atom transfer radical polymerization (ATRP). Linear PMMA and PMMA-BBP were mixed to fabricate blend-film samples (PMMA/PMMA-BBP), which were uniaxially drawn to introduce molecular orientations. Linear PMMA possessed a negative value for its orientation birefringence, while the value of PMMA/PMMA-BBP increased as the PMMA-BBP content increased, whose orientation birefringence could reach almost zero when the ratio of the linear PMMA to PMMA-BBP became 73:27, regardless of the magnitude of the strain. The results reveal that the orientation birefringence of PMMA can be effectively controlled and removed by blending the appropriate content of PMMA-BBP.

Orientation Birefringence Regulation for the Binary Blend Film of Cellulose Triacetate and Rigid-Rod-Like 5CB Molecules

Orientation Birefringence Regulation for the Binary Blend Film of Cellulose Triacetate and Rigid-Rod-Like 5CB Molecules

Analysis and calibration of linear birefringence orientation parameters derived from Mueller matrix for multi-layered tissues

Mueller matrix polarimetry can provide abundant microstructural information of tissues. Mueller matrix polar decomposition (MMPD) is capable of extracting intrinsic polarimetric characteristics of tissue-like media, including the orientation of the linear birefringence axis. However, when measuring a multi-layered sample with linear birefringence of different orientation directions, there is a deviation between the measured and ideal values of the MMPD linear birefringence orientation parameter. Since the multi-layered anisotropic structures widely exist in biomedical samples, a modified MMPD parameter to calculate linear birefringence orientation more precisely for layered tissues is crucial. Here we propose a modified MMPD parameter to calculate linear birefringence orientation for multi-layered media. By measuring different customized phantoms and pathological tissue slices, we demonstrate that the modified parameter can show more accurately the linear birefringence orientation angle of multi-layered media. The proposed parameter can be helpful for the quantitative evaluation of not only the pathological tissue slices with fibers, but also many other material samples with layered birefringent structures.

Thermal, Mechanical, and Optical Properties of Maleimide Copolymers Containing Twisted N‐Phenyl Substituents in the Side Chain

AbstractUV absorption spectroscopic measurements and DFT calculations revealed that N‐(alkyl‐substituted phenyl)maleimides with substituents at the ortho position favored a twisted N‐phenyl conformation, due to the steric hindrance between the maleimide ring and the substituted N‐phenyl moiety. High‐molecular‐weight copolymers were synthesized by radical copolymerization of the N‐(alkyl‐substituted phenyl)maleimides with n‐butyl vinyl ether and n‐butyl acrylate. The thermal properties of the copolymers were determined by thermogravimetric analysis and differential scanning calorimetry. The mechanical tensile tests under temperature control revealed the mechanical features of the copolymers depending on their repeating structure and composition. The significant temperature dependence of the modulus and strain of the copolymers was also clarified. Then, the transparency, refractive indices, and birefringence properties of the copolymer films were evaluated. It was confirmed that the maleimide copolymers had negative orientational birefringence with small absolute values and positive photoelastic birefringence. The effect of the conformational structure of the N‐phenyl groups on the thermal, mechanical, and optical properties of the copolymers was discussed.

Realization of linear-mapping between polarization Poincaré sphere and orbital Poincaré sphere based on stress birefringence in the few-mode fiber.

Based on the spatial profiles and polarization states evolution process of the first-order modes resulted from stress-induced birefringence in the few-mode fiber (FMF), we analyze the mapping relationship between the input polarization states represented on polarization PS and the output spatial profiles represented on the orbital PS of the FMF with respect to the magnitude and orientation of birefringence. When the input mode lobe orientation and the phase differences between the four eigenmodes of FMF induced by the stress birefringence satisfy a given condition, the mapping relationship between the input polarization PS and the output orbital PS is linear. Thus, the arbitrary points on the orbit PS can be generated at the output of stressed FMF by controlling the polarization state of the input modes. Then we experimentally verify that, an electrical single-mode polarization controller, a mode converter for converting fundamental mode to higher-order mode, a polarization controller mounting a coil of two-mode fiber and a polarizer can be employed to generate arbitrary first-order spatial modes on the orbital PS by controlling the input single-mode polarization states. The positions on the orbital PS of the generated first-order modes, which are obtained by calculating the three normalized Stokes parameters of output modes, agree well with the simulation ones. The correlation coefficients between the theoretical mode profiles and the experimental ones are higher than 80%. Since the spatial profile evolutions depend on the variations of the input polarization states, a potential advantage of this method is high-speed switching among desired first-order modes by using the commercial devices switching the state of polarization.

High refractive and low birefringent materials based on Poly(arylene ether phosphine oxide)s and Poly(arylene ether phosphine sulfide)s

Poly(arylene ether phosphine oxide)s and poly(arylene ether phosphine sulfide)s with high refractive indices and low birefringences have been developed. The poly(arylene ether phosphine oxide)s were synthesized by the polycondensation of bis(4-fluorophenyl)phenylphosphine oxide with various bisphenols. Subsequently, the poly(arylene ether phosphine sulfide)s were synthesized by the sulfurization reaction of the poly(arylene ether phosphine oxide)s with Lawesson's reagent. These polymers have excellent thermal stabilities (Td5 > 490 °C), high glass transition temperatures (Tg = 198–281 °C), and no absorption in the visible light region. In addition, they achieve high refractive indices of 1.649–1.675 for the poly(arylene ether phosphine oxide)s and 1.671–1.691 for the poly(arylene ether phosphine sulfide)s owing to the highly polarizable P X (X = O, S) groups in their backbones. Their orientational birefringences (CR values) are in the range from +0.7 × 10−9 to +3.0 × 10−9 Pa−1, which are lower than those of the conventional poly(arylene ether ketone) and poly(arylene ether sulfone) (+7.8 × 10−9 and +4.0 × 10−9 Pa−1, respectively).

The impact of processing conditions on the structural and optical properties of the as-spun polyamides fibers.

The development of crystallinity and orientation during the melt spinning of polyamides (nylon 66) was investigated. This study reports the question of the orientation and crystallinity determination of nylon 66 by means of differential scanning calorimetry, optical polarized microscope, and X-ray diffraction techniques during the cold drawing process. The different structural properties such as crystallinity, crystal size, birefringence, and different orientation functions were measured as a function of the draw ratio. From the obtained thermogram, it is clear that the melting temperature of the drawn nylon 66 fibers did not show a significant change due to the cold drawing process. As the draw ratio increase, the crystallinity and crystal size increase. The drawing process improves the chain orientations along the fiber axis of nylon 66 fibers. The orientations of the chain segments enhanced due to the cold drawing process of nylon 66 fibers.

Birefringent properties of poly(phosphonate)s and poly(thiophosphonate)s derived from various bisphenols

The birefringent properties of poly(phosphonate)s and poly(thiophosphonate)s possessing various bisphenol structures were investigated. The CR values (orientational birefringence) of the poly(phosphonate)s and the poly(thiophosphonate)s range from −0.3 × 10−9 to + 1.3 × 10−9 Pa−1 and from −0.6 × 10−9 to + 0.8 × 10−9 Pa−1, respectively, which are lower than those of the corresponding bisphenol A-based polymers. The CD values (photoelastic birefringence), which range from + 4.0 × 10−11 to + 4.9 × 10−11 Pa−1, are also lower than those of the corresponding bisphenol A-based polymers. The birefringent properties of poly(phosphonate)s and poly(thiophosphonate)s possessing various bisphenol structures were investigated. The CR values (orientational birefringence) of the poly(phosphonate)s and the poly(thiophosphonate)s range from −0.3 × 10−9 to + 1.3 × 10−9 Pa−1 and from −0.6 × 10−9 to + 0.8 × 10−9 Pa−1, respectively, which are lower than those of the corresponding bisphenol A-based polymers. The CD values (photoelastic birefringence), which range from + 4.0 × 10−11 to + 4.9 × 10−11 Pa−1, are also lower than those of the corresponding bisphenol A-based polymers.

Structure and optical properties of transparent polyamide 6 containing lithium bromide

ABSTRACTWe investigated the effects of lithium bromide (LiBr) on the structure and properties of polyamide 6 (PA6). The strong ion–dipole interactions between lithium cations and the amide groups in PA6 greatly increased the glass transition temperature (Tg) and retarded the crystallization rate of PA6. As a result, compression‐molded PA6 blends were highly transparent and had high Tg values. The rheological terminal region was obvious in the blends because the ion–dipole interactions weakened at high temperatures. This indicates that the melt processability was barely affected by LiBr. We also evaluated the optical anisotropy of the polymer to determine its suitability as a functional optical film. We found that hot‐stretched blend films had large positive orientation birefringence with significantly weak wavelength dispersion, which can be attributed to the enhanced anisotropic polarizability of PA6. We also found that the stress‐optical coefficient in the glassy region decreased with increasing LiBr content. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1513–1520

Robust Anisotropic Cellulose Hydrogels Fabricated via Strong Self-aggregation Forces for Cardiomyocytes Unidirectional Growth

The development of a facile and fast method to construct anisotropic hydrogels with the ability to induce unidirectional growth of cells remains challenging. In this work, we demonstrated anisotropic cellulose hydrogels (ACHs) that are composed of nanoscale aligned nanofibers by dissolving cotton liner pulp in alkali/urea aqueous solution. On the basis of directionally controlling the architecture of cellulose chains with a facial prestretching strategy in chemical gel state and locking the highly ordered nanostructure through the formation of close physical networks via strong self-aggregation forces among neighboring cellulose nanofibers, ACHs, combing with a long-range aligned structure, entirely differential mechanical performances along the parallel and perpendicular directions of the hydrogel orientation and optical birefringence, were constructed. The aggregation of hydrogen bonds in anisotropic and isotropic hydrogels are of significant difference, confirmed by nuclear magnetic resonance technolog...

Low Birefringent Properties of Poly(phosphonate) Derivatives

The birefringent properties of bisphenol A-based poly(phosphonate) and poly(thiophosphonate) were investigated. The orientational birefringences (CR values) of the poly(phosphonate) derivatives are in the range from +1.2 × 10–9 to +1.5 × 10–9 Pa–1, which are less than half of those of the conventional bisphenol A-based polymers. In addition, their photoelastic birefringences (CD values) are in the range from +5.7 × 10–11 to +6.1 × 10–11 Pa–1, which are also lower than those of the conventional bisphenol A-based polymers. From the DFT calculations of the polarizability anisotropies of the model compounds, the low birefringent properties of the poly(phosphonate) derivatives are most probably due to the presence of the P═O and P═S groups and the phenyl substituents perpendicularly oriented to the main chain direction.

Effect of acetyl substitution on the optical anisotropy of cellulose acetate films

The effect of acetyl substitution on the optical properties of cellulose acetate (CA) was investigated in the present study, because a strong demand for advanced retardation films increases greatly these days. The hot-stretched films with high acetyl substitution had negative orientation birefringence, whereas those with low acetyl substitution had positive orientation birefringence with extraordinary wavelength dispersion. It should be noted that orientation birefringence hardly relaxed even after cessation of hot-stretching. The slow relaxation of crystal orientation was responsible for the anomalous optical anisotropy, as confirmed by two-dimensional X-ray diffraction. Moreover, the slow relaxation of orientation birefringence would greatly benefit the preparation of CA optical retardation films by hot-stretching, because it would simplify the precise control of retardation. The stress-optical coefficient in the glassy state was also evaluated, and was found to decrease with the degree of acetyl substitution. This is an attractive property for optical film applications.

Wavelength dispersion of birefringence of oriented polyethylene films.

Oriented polyethylene films, in which stacked lamellae grow perpendicular to the flow direction, can show extraordinary wavelength dispersion of birefringence; i.e., birefringence increases with wavelength. Owing to the periodical change of the refractive indices between the crystalline and amorphous layers along the flow direction, with oriented long lamellae having a high refractive index, the form birefringence has negative values. Furthermore, the refractive indices of the crystalline region show a stronger wavelength dependence than those of the amorphous region, which is responsible for the marked wavelength dispersion in the form birefringence. The combination of negative form birefringence with strong wavelength dispersion and positive orientation birefringence can give extraordinary wavelength dispersion.

Reliability of intrinsic birefringence estimated via the modified stress-optical rule

Intrinsic birefringence, Δn 0, is a material constant for the orientational birefringence of polymers, and has been measured by many researchers with various methods. However, the reported values of the same polymer sometimes disagree among the researchers because determination of the orientation degree is somehow complicated. Alternatively, a new method using the modified stress-optical rule (MSOR) was proposed for Δn 0 measurement. In this study, literature data of the Δn 0 for various polymers are assessed and the reliability of Δn 0 estimated via the MSOR is discussed.