Birefringence Orientation Research Articles

A convenient way of interpreting steady shear rheo-optical data of semi-dilute polymer solutions

Rheo-mechanical and rheo-optical investigations were carried out with the aim of determining the influence of deformation and orientation or disentangling of polymer coils on the flow behavior in the non-Newtonian region of the flow curve, for a moderately concentrated network solution. To avoid the influence of polydispersity this was done on a series of narrowly distributed polystyrene standards (dissolved in toluene). By using steady state shear flow measurements it was possible to detect qualitatively a reduction in the entanglement density within the non-Newtonian flow region. Birefringence experiments were able to show that deformation of the polymer coils also occurs in the Newtonian flow region, which has no effect on the flow behavior in this range, whereas in the non-Newtonian flow region the increase in deformation is lower than in the Newtonian range. The flow birefringence and its orientation can be described over the whole range of the flow curve with a newly developed equation system (Eq. 8 and 14) derived from the stress states of a sheared solution using the stress-optical rule. Starting from these equations, it could be shown, that in the Newtonian flow region a mastercurve in form of a reduced birefringence Δn′/η0=f(γ˙) and a reduced orientation φ= f(γ˙/γ˙crit) can be plotted, independent from concentration and molar mass. A comparison of the experimentally determined orientation angle and birefringence curve form with theoretical deformations and orientations of polymer coils in a solution state, without intermolecular interactions, was able to demonstrate that the flow behavior of a moderately concentrated network solution is determined decisively (approximately to 85%) by the disentanglement.

Chain-length dependence of the optical properties of a series of push–pull polyenes: application to doped photorefractive polymers

Push–pull molecules are of considerable interest for their large quadratic hyperpolarizabilities. However, incorporated as doping chromophores in low glass-transition temperature photorefractive polymers, their Pockels contribution to the modulation of the refractive index is generally weak or even negligible. We present an experimental and analytical study of a series of push–pull polyenes with a number of double bonds in the chain increasing from 1 to 5. The molecules were dissolved in chloroform and the two contributions to the total birefringence, i.e. the orientational birefringence and the Pockels effect, were measured using a specific ellipsometric technique and analyzed in a two-form two-state formalism. For the longest polyene studied, a giant efficiency of the refractive index modulation, arising equally from the orientational contribution and from the Pockels effect, can be expected.

Direct observation of orientation limit in a fast photorefractive polymer composite

We report on a photorefractive polymer with a 4-ms-response time in transient four-wave mixing experiments at 0.5 W/cm2 writing irradiance, 95 V/μm applied electric field, and a grating period of 3.1 μm. Complementary transient ellipsometry, however, reveals orientational birefringence response which leads the four-wave mixing response all the way to its saturation, despite complex dynamics in these processes. Orientation does not limit the dynamic formation of photorefractive gratings in this polymer, which suggests that even faster photorefractive responses are possible for polymer composites with improved charge generation and transport properties.

Effect of Crystalline and Amorphous Structures on Biodegradability of Poly(Tetramethylene Succinate)

The effect of orientation in the amorphous and crystalline regions on the biodegradability of PTMS [poly(tetramethylene succinate)] was studied using the amorphous orientation function, birefringence, and crystallinity. The crystalline and amorphous intrinsic lateral sonic moduli, E t,c 0 and E t,am 0 , were 2.61 × 103 and 0.41 × 103 MPa, respectively. Using the data on birefringence, crystalline and amorphous orientation function (f ∈ and f am), crystallinity, and sonic modulus of the oriented PTMS fibers, the intrinsic birefringence of the crystalline (Δ c 0 ) and amorphous (Δ am 0 ) regions were evaluated to be 0.0561 and 0.0634, respectively. The biodegradabilities of oriented PTMS films were reduced as the elongation increased, i.e., the amorphous orientation increased. At low elongation (100 and 150%), however, biodegradabilities remained unchanged when the degradation test was performed in activated sludge, which was attributed to the amorphous orientation occurring even at 100% elongation, though the amorphous orientation direction was perpendicular to the fiber axis.

Ellipsometric measurements of the polarizability anisotropies of push–pull molecules in solution

We have developed an experimental setup for the measurement of the polarizability anisotropy of push–pull molecules in solution. We have used this technique to investigate prototype push–pull molecules that have been incorporated as doping chromophores in low glass transition temperature photorefractive polymers. Results obtained evidence for these compounds that the refractive index modulation is dominated by the orientational birefringence contribution while the electro-optic contribution remains small.

The response of entangled polymer solutions to step changes of shear rate: Signatures of segmental stretch?

Experiments measuring the orientation angle and birefringence in startup and double-step strain rate flows were conducted on a 3.0 wt % 8.42 × 106 molecular weight polystyrene solution in a Couette flow cell. A phase-modulated flow birefringence apparatus was used to noninvasively probe the sample. Upon startup from rest, the orientation angle undershoots its final steady-state value, as seen by earlier investigators. When the shear rate undergoes a step increase from one nonzero value to another, the amplitude of this undershoot is decreased. However, a more significant effect is a shorter time scale overshoot in the orientation angle that is highly counterintuitive in the sense that an increase of shear rate initially produces a rotation of chain segments away from the flow direction. Similarly, a step decrease in shear rate yields an initial transient rotation toward the flow direction. In both cases, the height of the peaks depends upon the magnitude of the shear rate jump, and the width of the peaks is a function of the final shear rate. The longer time transients in the startup and step increase experiments reflect an apparent change in the relaxation time for segment orientation, which we tentatively attribute to a combination of tube dilation and convective constraint release. The shorter time scale over- and undershoots in the orientation angle appear to be qualitatively explained by considering the differences in extension or contraction of segments along the polymer chain. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 265–280, 1998

Infrared photorefractive polymers and their applications for imaging.

Photorefractive polymers with high diffraction efficiency in the visible and near-infrared regions of the electromagnetic spectrum have been developed. These polymers, which have a large dynamic range because of their high orientational birefringence, incorporate a dye designed to have a large dipole moment and a high linear polarizability anisotropy. Such polymers have enabled demonstrations of imaging through scattering media, using a holographic time-gating technique at a wavelength that is compatible with the transparency of biological tissues and with the emission of low-cost semiconductor laser diodes.

Nanometer scale polarimetry studies using a near-field scanning optical microscope

We describe a new technique that incorporates polarization modulation into near-field scanning optical microscopy (NSOM) for nanometer scale polarimetry studies. By using this technique, we can quantitatively measure the optical anisotropy of materials with both the high sensitivity of dynamic polarimetry and the high spatial resolution of NSOM. The magnitude and relative orientation of linear birefringence or linear dichroism are obtained simultaneously. To demonstrate the sensitivity and resolution of the microscope, we map out stress-induced birefringence associated with submicrometer defects at the fusion boundaries of SrTiO3 bicrystals. Features as small as 150 nm were imaged with a retardance sensitivity of approximately 3 x 10(-3) rad.

A new technique for the determination of the polarizability anisotropy of intramolecular charge transfer molecules

We present an original set-up for the determination of the polarizability anisotropy Δα of polar molecules in solution. The technique is based on electric field-induced ellipsometry measurements and the analysis takes into account the contributions of the orientational birefringence and the electro-optic Pockels effect to the total refractive index change of the studied solutions.

Processing Characteristics of Recycled Poly(Ethylene Terephthalate) as Melt Spun Fibers

Recycled poly (ethylene terephthalate) (r-pet) used in blends with a fiber grade material (f-pet) is investigated. As-spun fibers of r-pet, f-pet, and r/f-pet blends are made at winding speeds ranging from 1000 to 4000 m/min (mpm), with subsequent drawing in the range of 1.6 to 4.0×. Spinning and drawing behaviors of the fibers are analyzed using orientation (birefringence), boil-off shrinkage, calorimetric (dsc), and mechanical measurements. The fully oriented yarns from the r-pet show a tensile strength of 70% of the fiber grade, with 3.2 g/d (r-pet) to 4.6 g/d (f-pet), which still meets and surpasses minimum industrial requirements. The dsc study of these materials indicates that the decreased crystallinity of the r-pet may be the main cause for the decreased tensile strength of the product.

Transparent zero-birefringence copolymer and its optical properties

Birefringence is caused by both orientation of polymer chains and photoelasticity. These birefringences were compensated by random copolymerization of negative birefringent methyl methacrylate and positive birefringent benzyl methacrylate, but orientational and photoelastic zero birefringences were achieved with quite different compositions of the copolymers. Note that the birefringence of the copolymer that occurred in the process of injection molding was almost completely eliminated with a composition for orientational zero birefringence. The orientational and photoelastic zero-birefringence copolymers possessed enough transparency as optical materials that the total scattering losses were 30.4 and 19.5 dB/km, respectively, competing with the transparency of homopolymers.

Orientation determination and morphological study of high density polyethylene (HDPE) extruded tubular films: effect of processing variables and molecular weight distribution

The structure—property behaviour of high density polyethylene (HDPE) extruded tubular films having a blow up ratio of unity was investigated. Two different HDPE resins with identical M n (14 600 g/mol) values but different distributions ( M w M n = 10.3, 15.1 ) were utilized in this extrusion process. Systematic changes were also made in the process variables—these being the melt temperature at the die exit, the quench height, which is the distance from the exit of the die to the cooling ring, the flow rate of the air through the cooling ring, and the film line speed. The morphological features and state of orientation of the extruded tubular films were examined by transmission electron microscopy (TEM), high resolution scanning electron microscopy (HSEM), birefringence, wide and small angle X-ray scattering studies (WAXS, SAXS), and linear infra-red (i.r.) dichroism. A stacked lamellar or, in some cases, a fibril nucleated morphology was observed in the melt-extruded films by TEM. This technique clearly showed that the broader molecular weight samples possessed fibril nuclei that were not evident in the narrower molecular weight distribution films. The existence of the fibril nuclei was further confirmed from WAXS by the intense sharp spots superimposed on the equatorial (110), (200), and (020) reflections. The origin of this pronounced fibril nucleation was clearly believed to be due to the longer relaxation time behaviour of the broader distribution resin. The amorphous phase orientation and form birefringence for all films were found to be very small, and the observed birefringence arose almost exclusively from the crystalline phase. It was also found that the morphology and the orientation of the chain axis in the crystalline phase were particularly sensitive to the quench height and melt temperature for the narrower distribution HDPE films, but much less so for the broader distribution HDPE samples. Little dependence of structure/orientation was noted for either resin for the variables of the flow rate of the air through the cooling ring and the film line speed. These findings show the sensitivity of the molecular weight distribution to be a very important parameter, as are the specific processing variables of quench height and melt temperature, in determining the final solid state structure of the HDPE extruded tubular films.

Polarization mode dispersion, decorrelation, and diffusion in optical fibers with randomly varying birefringence

Polarization mode dispersion and the polarization decorrelation and diffusion lengths are calculated in fibers with randomly varying birefringence. Two different physical models in which the birefringence orientation varies arbitrarily are studied and are shown to yield nearly identical results. These models are appropriate for communication fibers. We show that both the length scales for polarization mode dispersion and polarization decorrelation measured with respect to the local axes of birefringence are equal to the fiber autocorrelation length. We also show that the coupled nonlinear Schrodinger equation which describes wave evolution over long length along a communication fiber can be reduced to the Manakov equation. The appropriate averaging length for the linear polarization mode dispersion is just the fiber autocorrelation length but the appropriate averaging length for the nonlinear terms is the diffusion length in the azimuthal direction along the Poincare sphere which can be different, The implications for the nonlinear evolution are discussed.

Differential Conoscopy: Local Birefringence and Molecular Orientation in the Liquid Crystals

Abstract A new experimental technique of observation and quantitative characterization of interference states in polarizing light is proposed. It permits the determination of the main optical features of the heterogeneous media, namely birefringence, rotatory power, biaxiality…etc, even of the microdomains. The method consists in superposition of unknown interference state and a standard interference state which presents axial symmetry, and permits observation of extinction zones.

Macroscopic Properties of Ferronematics Caused by Orientational Interactions on the Particle Surfaces. II. Behavior of Real Ferronematics in External Fields

Abstract On the basis of the worked out modification of the continuum theory of ferronematics the Fredericksz transitions and orientational birefringence induced by magnetic and/or electric field are studied. A comparison with the available experimental data is made.

Stability and degradation of poly(naphthoyleneimidobenzimidazole)

It has been shown that the changes with time of the mechanical characteristics of films and fibres of poly(naphthoyleneimidobenzimidazole) (PNIB) under atmospheric conditions at room temperature are accompanied by slow degradation of the macromolecules. This is indicated by a drop in intrinsic viscosity, an increase in translational diffusion of the macromolecules and a decrease in characteristic orientation of flow birefringence (FB) in PNIB solution in concentrated H 2SO 4. The decrease in molecular ( M) is considerably accelerated for PMIB in the powdered state. Resynthesis (heating under vacuum at 250°C) partially restores M of the sample. However, both resynthesized and precipitated (into water) PNIB samples retain a tendency to self-degradation.

A methodology for the systematic and quantitative study of cell contact guidance in oriented collagen gels. Correlation of fibroblast orientation and gel birefringence.

Despite the likely role of contact guidance in every physiological process involving cell migration, its study in a three-dimensional tissue-equivalent environment has been precluded, heretofore, by inherent difficulties in systematically preparing well-defined contact guidance fields and quantifying the resultant contact guidance. Here, we describe a novel use of a magnetic field to orient collagen fibrils during fibrillogenesis, entrapping cells dispersed in the collagen solution. Using computer-controlled staging and image analysis, we show from automated birefringence measurements of the resultant slab of cell-populated gel contained in a specially designed observation chamber that the fibril orientation is biased along the long axis of the chamber uniformly throughout the chamber. Further, we show that the degree of fibril orientation, and consequently the elicited contact guidance, can be controlled by independently varying the magnetic field strength or temperature during fibrillogenesis. We characterize the contact guidance response to the imposed contact guidance field by measuring cell orientation relative to the axis of fibril orientation from still images obtained in time-lapse via automated image analysis. We present the first quantitative correlation of contact guidance (based on cell orientation) with collagen fibril orientation (based on birefringence) for human foreskin fibroblasts cultured in a collagen gel, by using gels of varying orientation resulting from different magnetic field strengths and temperatures during fibrillogenesis, and by using sufficiently low cell concentrations and early observation times.

Orientational anisotropy in the optical Stark effect: An FMD computer simulation

Abstract Field applied molecular dynamics (FMD) computer simulation is used to investigate the optical Stark effect in a liquid ensemble of chiral molecules, using left and right circularly polarised laser radiation, interacting with each molecule through the appropriate torque. A novel orientational anisotropy develops under the influence of this torque, an anisotropy which indicates bi-axial birefringence. The latter is the same for right and left circularly polarised laser components, even when a contribution is included from the imaginary part of molecular polarisability. The torque used in the FMD simulation is independent of the phase of the laser implying that the orientational birefringence persists at all frequencies.

X-ray and birefringence orientation measurements on uniaxially deformed polyethylene film

Characterization of the crystalline and amorphous orientation of a uniaxially deformed low-density polyethylene (LDPE) film has been accomplished by combining pole figure X-ray diffraction and birefringence measurements. Characterization of the crystalline regions of the extruded film reveals the existence of a row-nucleated morphology with the a axis preferentially oriented in the plane containing the machine (MD) and transverse (TD) directions and the b axis strongly oriented along the film normal. This morphology is embedded in an amorphous matrix, biaxially oriented in nature. Low uniaxial extension along the MD produces two major orientational states in the crystalline regions which seem independent of the corresponding amorphous orientation. The pole figure analysis clearly shows that the c axis weakly orients towards the MD. Upon further uniaxial extension, it begins to tilt progressively away from the stretching direction inducing a complementary orientational behaviour between the c and b crystallographic axes on account of the stationary a-axis orientation perpendicular to the stretching direction. At higher draw ratios, the c axis gradually realigns itself with the stretching direction and uniaxial orientational states in all regions of the microstructure are obtained, indicating the existence of a fibrillar-type morphology. Quantification of the induced orientation was carried out and compared in terms of Hermans, Nomura/Kawai and White/Spruiell orientation functions.

An analysis of the orientational birefringence of poly(methyl methacrylate) in terms of the Mooney-Rivlin equation

The orientational birefringence of poly(methyl methacrylate) (PMMA) was analysed using the Mooney-Rivlin equation. It is shown that its dependence on strain can be well described by this equation over more than three decades of time (in seconds). More interestingly, the B 1 term (usually found and ascribed to some temporary affine deformation upon stretching in uncrosslinked polymers) is absent for PMMA within the error of experimental measurements. The B 2 term is shown to be governed by the chain relaxation that takes place during stretching, emphasizing its molecular origin associated with the entanglement network. Also, a comparison of B 1 and B 2 for PMMA and polystyrene (PS) indicates clearly a significant difference between the deformation behaviour of the entanglement network for the two polymers. The B 1 term of PS gives an important contribution to the orientational birefringence, and relaxes with time in a similar way to B 2.