Time-dependent Birefringence Research Articles

Time‐Resolved Molecular Imaging: Optical Imaging of Coherent Molecular Rotors (Laser Photonics Rev. 14(5)/2020)

In article number 1900344, Jérémy Bert, Ilya Sh. Averbukh, Olivier Faucher, and co-workers demonstrate a novel optical method for visualizing rapidly spinning molecules in the gas phase. The method is based on detecting the time-dependent birefringence of the laser excited molecular gas, and it allows recording images similar to the one shown on the cover. A sequence of such images is combined into a high-resolution molecular movie providing information on molecular orientation with a resolution of few tens of femtoseconds.

Measurement of the internal stress and electric field in a resonating piezoelectric transformer for high-voltage applications using the electro-optic and photoelastic effects

The high output voltages from piezoelectric transformers are currently being used to accelerate charged particle beams for x-ray and neutron production. Traditional methods of characterizing piezoelectric transformers (PTs) using electrical probes can decrease the voltage transformation ratio of the device due to the introduction of load impedances on the order of hundreds of kiloohms to hundreds of megaohms. Consequently, an optical diagnostic was developed that used the photoelastic and electro-optic effects present in piezoelectric materials that are transparent to a given optical wavelength to determine the internal stress and electric field. The combined effects of the piezoelectric, photoelastic, and electro-optic effects result in a time-dependent change the refractive indices of the material and produce an artificially induced, time-dependent birefringence in the piezoelectric material. This induced time-dependent birefringence results in a change in the relative phase difference between the ordinary and extraordinary wave components of a helium-neon laser beam. The change in phase difference between the wave components was measured using a set of linear polarizers. The measured change in phase difference was used to calculate the stress and electric field based on the nonlinear optical properties, the piezoelectric constitutive equations, and the boundary conditions of the PT. Maximum stresses of approximately 10 MPa and electric fields of as high as 6 kV/cm were measured with the optical diagnostic. Measured results were compared to results from both a simple one-dimensional (1D) model of the piezoelectric transformer and a three-dimensional (3D) finite element model. Measured stresses and electric fields along the length of an operating length-extensional PT for two different electrical loads were within at least 50 % of 3D finite element simulated results. Additionally, the 3D finite element results were more accurate than the results from the 1D model for a wider range of electrical load impedances under test.

Investigation of the pH effect on the stability of biocompatible magnetic fluids using time-dependent birefringence measurements

In this work we used the static magnetic birefringence together with transmissivity measurements to investigate the effect of the pH on the stability of a water-based, citrate-coated, maghemite magnetic fluid as a function of the time. We observed a peak on the SMB measurement due to the high birefringence of the material. The peak position shifted to lower field values as a function of time and decreasing the pH of the sample. The phenomena is explained as a result of agglomerate formation. In addition, the conductivity constant decreases by 6% changing the pH value of the sample from pH = 10 to pH = 7 .

Orientational dynamics in dye-doped organic electro-optic materials

The time dependent birefringence of polymer-based electro-optic materials is investigated using ellipsometry. We show that the birefringence after switching off the poling field does not depend only on the induced refractive index, but also on how that level was reached. The role of the poling voltage and poling time is discussed in turn-on and turn-off experiments and an original curve-fit function is introduced. We also propose a schematic model of the polymer dynamics in the system, which is consistent with complementary dielectric measurements.

Extension of a model analysis of the degree of polarization of light

A theoretical approach is developed in which partially polarized radiation can be represented as a sum of coherent and incoherent components and also of the action of some imaginary device with time-dependent birefringence. This approach allows the Jones vector matrix method to be used for the calculations, which is important for theoretical calculations of the polarization state.

Apparatus for simultaneous observation of the electro-optic response and small angle x-ray scattering in liquid crystals

A novel apparatus allowing the simultaneous observation of small angle x-ray scattering and time dependent birefringence changes of a dynamic system is described. The apparatus incorporates a monochromatic light source, polarizers, a fast photodiode detector, and electric field generating equipment, such that electro-optic response times between 10 μs and 100 s may be measured. The electro-optic apparatus is incorporated in an x-ray camera sited at the Synchrotron Radiation Source at Daresbury Laboratory, UK. The x-ray apparatus allows the study of structures ∼30 Å in dimension and the detector is time resolved with a minimum response time of 10 μs. A hot stage and temperature controller are also incorporated into the apparatus, allowing the study of smectic liquid crystal devices. Results from the simultaneous acquisition of time resolved x-ray scattering data and electro-optic measurements from a thin film of smectic-A liquid crystal are presented.

Visualization of Extra Melt Motion Induced in the Packing Stage and Its Effect on the Residual Stress Frozen in an Injection-Molded Polymer Product

This paper deals with the extra melt motion induced in the packing (or packing-holding) stage and its effect on the residual stress distribution frozen in an injection-molded polymer product. The packing-holding process is a technique commonly applied for avoiding shape deformation of the injection-molded products. Indeed this process is effective to obtain precise products, but, at the same time, it has been pointed out that the packing process increases the residual stress frozen in the final products. However, details of the phenomena occurring in the packing-holding process have not been made clear. In this paper, relations between the residual stress and the packing-holding process conditions were studied by visualizing the extra melt motion and the time-dependent birefringence distribution in the molded polymer during the packing-holding process.The visualized results clearly showed that the packing-holding process induces extra melt motion and that this melt motion results in the increase of residual stress (or birefringence) frozen near the gate of the final product. The extra melt motion observed in the packing process can be separated into two stages; “packing” flow during the packing-holding process and rapid inverse flow at the end of the holding process. The former melt motion is induced both by the compressibility of polymer melt and by the compensation of the polymer shrinkage due to solidification during the packing-holding stage. On the other hand, the later melt motion results from the compressed melt release from the mold gate. Therefore, the rapid inverse flow due to melt leakage should be avoided by a proper “gate-seal” technique, while the “packing” flow during the packing-holding process is essential for improving the quality of injection-molded products.

The time-dependent stress-optical behavior of polycarbonate in the glass transition region

The mechanical and stress-optical behavior of Bisphenol-A polycarbonate was investigated in the glass-transition region. For this purpose, optical creep experiments were carried out in shear and elongation on a tensile tester specially designed for use on a microscope state. A Kohlrausch Williams Watts equation (KWW) with a temperature-independent parameter β could successfully be applied to the curves describing the time-dependent values of the stress-optical coefficient for several temperatures. The temperature dependence of the corresponding retardation time could be established and described by the WLF equation. For variable stresses the time-dependent birefringence is obtained from a generalized linear stress-optical rule as modeled according to linear superposition. The time-temperature superposition principle was applied to all measurements. With the dynamic moduli some deviations were observed at the transition from the rubbery plateau to the α relaxation. The strain-optical coefficient was found to decrease with increasing time and strain. The strain dependence was found to be independent of temperature at constant stress.

A birefringence study of molecular and supermolecular structure in polyether urethane-ureas

Polyether urethane-ureas with alternating flexible and stiff-chain segments were studied by static and time-dependent birefringence. Four temperature regions characterized by different molecular mobility have been identified. The hard segments are assumed to be segregated to form domains embedded in a matrix of the soft, polyether segments. The stiff-chain segments inside the domains are oriented perpendicularly to the largest domain dimension. The length of the statistical segment in the stiff-chain blocks is A ⩾ 13 nm.

Time-dependent birefringence in glassy polymers during stress-relaxation and recovery

The birefringence of polystyrene and polycarbonate have been measured during uniaxial stress relaxation tests. The birefringence of polystyrene shows an instantaneous positive increment on applying a tensile deformation; during stress relaxation the value showed a time-dependent change in the negative direction and the total birefringence eventually became more negative than the value measured prior to the deformation. On unloading, the overall trend was for the birefringence to change towards the level measured in the as-prepared sample prior to deformation but an initial departure in the opposite direction was observed. With polycarbonate, the birefringence changed instaneously on applying the deformation and remained steady during stress relaxation. On unloading the birefringence fell instaneously by an amount less than the increment observed on applying the deformation. Thereafter the birefringence continued to fall, approaching the original value with a time constant similar to that governing stress relaxation. These results are interpreted with reference to the molecular structures, in particular the location of the highly polarizable aromatic rings.

Rheo-Optical Studies of Low-Density Polyethylene Films in the Nonlinear Viscoelastic Region

The time-dependent birefringence was measured simultaneously with stress relaxation for annealed low-density polyethylene films at various temperatures ranging from 15 to 80°C under relatively large strains of about 10%, at which the films showed nonlinear stress-strain and birefringence-strain curves during the tests with a constant rate of elongation.The master curves of the strain-optical coefficient at relatively large strains of about 10% were very similar to that obtained at a small strain of about 3%. The strain-optical coefficient increased with increasing time and reached an equilibrium at long time. However, the larger the strain was sustained the higher the equilibrium value became.

Rate dependence of the strain birefringence and ductility of polyethylene

A study of the ductile-brittle transition in the tensile behaviour of linear polyethylene is presented. Equipment for the simultaneous measurement of stress, strain and optical birefringence over a broad range of deformation rates and loading times is described. Low strain birefringence measurements showed that the total molecular orientation increased with decreasing test speed. High pressure polyethylene, which is not ‘brittle’ over the range of investigated test speeds, shows no time-dependent birefringence. Further, X-ray measurements show the same sequences of polymer chain orientation for both cold-drawn and brittle-fractured material. A detailed theory is advanced for explaining the low strain behaviour, the cold-drawing process and the mechanism of the ductile-brittle transition.