Birefringence Relaxation Research Articles

Viscoelastic and Birefringence Relaxation of Individualized Cellulose Nanofibers in the Dilute and Semidilute Regions.

Viscoelastic relaxation mechanisms of individualized cellulose nanofibers (iCNFs) dispersed in glycerol in the dilute and semidilute regions were investigated by linear viscoelastic and dynamic birefringence measurements. The birefringence relaxation of the iCNFs was described by the orientational and curvature modes of an existing viscoelastic theory for ideal semiflexible polymers (Shankar-Pasquali-Morse theory). However, the Shankar-Pasquali-Morse theory could not fully describe the iCNF viscoelastic relaxation at high frequencies. Considering the results for birefringence relaxation, the experimental tension mode of the iCNFs was evaluated to be higher than the theoretical value. These results show that the viscoelastic relaxations of the iCNFs are different from those of ideal semiflexible polymers, in contrast to cellulose nanocrystals (CNCs). As the iCNF concentration increased, the orientational mode dramatically slowed, which was more drastic than other semiflexible polymers, including CNCs. This anomalous behavior is likely due to the nonideal nature of iCNFs.

Photoinduced birefringence in azopolymers measured at 1550 nm

Azobenzenes are a class of compounds presenting photoisomerization capabilities that allow the writing and erasure of birefringence along a desired direction. This feature enables applications requiring polarization control, which although have been extensively investigated in the visible light spectrum, poor emphasis has been paid to the infrared region. In this paper, a systematic characterization of induced birefringence creation and relaxation dynamics has been carried out in azopolymers thin films in the infrared telecommunications region of 1550 nm. This study covers both birefringence characterization in terms of wavelength and irradiance of birefringence writing beams. Preliminary results revealed remarkable maximum birefringence values as high as 0.0465 attained during the recording phase, that stabilized at 0.0424 during the relaxation phase, which is quite promising for many applications.

Combined Use of Ultrasonic and Electromagnetic Fields for the Study of Bonding Mechanisms between Dexamethasone Disodium Phosphate Molecules

We have investigated the ultrasonically induced birefringence traces of aqueous solutions of dexamethasone disodium phosphate, a derivative of hydrocortisone (cortisol). The stationary birefringence and the transient built-up and decay relaxation processes were studied as a function of solution concentration, ultrasound frequency and intensity, as well as a function of temperature. The results were analyzed in view of structural peculiarities of the system in an effort to gain further insights into the molecular relaxation dynamics and the proposed self-association process occurring in the system. The detected ultrasonically induced birefringence relaxation is motivated by the rotational diffusion of dexamethasone disodium phosphate aggregates due to self-association depending on the solution concentration. The observed relaxation mechanism is directly linked to the hydrodynamic size of the acoustic field-induced self-assembly. The systematic analysis of the transient birefringence signals caused by the applied ultrasonic field allowed us to evaluate the interplay between permanent and induced dipoles with changing concentration, temperature, and ultrasound properties. The birefringence traces are adequately fitted with a stretched exponential law indicating the polydispersive nature of the self-aggregated molecular structures. The obtained results are described in the light of recent studies performed on this system.

Enhancement of the photoinduced birefringence and inverse relaxation of a liquid crystal azopolymer by doping with carbon nanostructures

• Carbon nanotubes can enhance the photoinduced birefringence of azopolymers. • The in situ doping method provides a good polymer-nanotube interaction. • The occurrence of the inverse relaxation depends on the doping method. In this work, the effect of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) on the photoinduced behavior of one side-chain liquid crystal azopolymer is studied. Two distinct methods of doping the azopolymer with CNTs or CNFs are followed: during (in situ method) and after (mixing method) polymerization. Thermotropic studies show that dopants do not have a significant effect on the liquid-crystalline properties of the azopolymer, which develops a smectic A-type phase whose lamellar structure is preserved after cooling to room temperature. On the other hand, the light-induced birefringence of the azopolymer doped by the in situ method (∼ 0.095) is higher than that reached with the corresponding one doped by the mixing method (∼ 0.05). In switch-on/switch-off experiments, both the typical and the inverse relaxation processes of the photoinduced orientation are observed. The inverse relaxation occurs only in the azopolymer doped by the in situ method, which seems to provide a good interaction between the azobenzene units and the nanostructures. In addition, it was found that no minimal threshold value of irradiation dose is required to make the inverse relaxation happen. A model based on non-covalent interactions is proposed to explain the inverse relaxation promoted by nanostructures introduced by the in situ method.

Reduction of birefringence by dynamic asymmetry in miscible blends of dissimilar polycarbonates

We found that the birefringence of miscible blends of dissimilar polycarbonates was lower than those of the neat component polymers though the sign of the birefringence was same. The modulus and birefringence of the blends decreased more gently in the glass transition region but more steeply in the rubbery plateau region than those of the neat component polymers. The modulus and birefringence relaxation curve fitting with the KWW and modified Rouse equations revealed wider distortion relaxation and orientation distributions in the blends than in the neat component polymers. The wide distortion relaxation might be attributed to the dynamic asymmetry of the blends. The orientation relaxed faster in the blend than in the component polymers, indicating accelerated orientation relaxation, which could be explained by assuming cooperative relaxation of the component polymers strongly affected by fast relaxation component of the low-Tg one due to the dynamic asymmetry with a large glass transition temperature difference between the component polymers. Owing to the accelerated relaxation, the elongated blends exhibited lower birefringence than the component polymers.

Terahertz birefringence anisotropy and relaxation effects in polymer-dispersed liquid crystal doped with gold nanoparticles.

Terahertz (THz) birefringence anisotropy of the polymer-dispersed liquid crystal (PDLC) doped with gold nanoparticles (Au NPs) is investigated by using terahertz time domain polarization spectroscopy. Controlled by the electric field, the change rate of refractive index for PDLC doped with Au NPs is 0.91% V-1 as the voltage increases, smaller than the pure PDLC, which indicates that the response of the PDLC doped with Au NPs to electric field is more uniform than that of pure PDLC. Therefore, the PDLC doped with Au NPs is more suitable for tunable phase shifters. Furthermore, we found that under the high-frequency alternating electric field, the anisotropic polarization effect of PDLC will disappear to this electric field, namely polarization relaxation phenomenon. However, the results show that the PDLC doped with Au NPs can respond to an electric field with higher alternating frequencies, and the relaxation frequency of PDLC with an Au NPs concentration of 0.2 wt% was improved over two times compared with the pure PDLC and four times higher than that of the precursor mixture without ultraviolet radiation. This work has the significance for the potential applications of tunable THz liquid crystal phase and polarization devices, providing a more uniform and faster relaxation response to the operating electric field.

Viscoelastic Relaxation of Cellulose Nanocrystals in Fluids: Contributions of Microscopic Internal Motions to Flexibility.

We report the viscoelastic relaxation mechanisms of cellulose nanocrystals (CNCs) dispersed individually in fluids. Linear viscoelasticity and flow birefringence of softwood and tunicate CNC/glycerol dispersions in the dilute regime were measured. The obtained results were then compared with molecular theories for the linear viscoelasticity of rigid rods and semiflexible rods at infinite dilution by taking length distributions of CNCs into account. Although CNCs are traditionally regarded as rigid Brownian rods, the viscoelastic relaxation was not explained solely by the rotational motions of rods. Alternatively, molecular theories for semiflexible rods well-described the viscoelastic behavior; the CNCs showed additional relaxation modes derived from microscopic internal motions including "tension" and "curvature", which originated from the finite flexural rigidity. Birefringence relaxation of the CNCs was dominated by the rotational motions of rods. Length distribution functions of the CNCs were thus calculated from the birefringence relaxation and agreed well with their microscopy-determined histograms.

Relaxation of the Induced Orientational Order in the Isotropic Phase of a Nematic Polymer

The orientational dynamics in the isotropic phase of a comb-like nematic polymer with mesogenic and functional side groups is studied using the Kerr effect and dielectric spectroscopy methods. For the first time, it has been found that, in contrast to low-molecular-weight mesogens, the relaxation of the electric birefringence of a melt above the temperature of the nematic–isotropic phase transition in a mesogenic polymer can be represented by a sum of several exponential processes, two of which play a decisive role. These processes replace each other in the temperature range of about 50°C. Dielectric spectroscopy also makes it possible to distinguish two orientational relaxation processes, one of which is due to the rotation of the side mesogenic groups, and the second is associated with the motion of the main chain segments.

Structure-property relationship and photoinduced birefringence of the azo and azo-azomethine dyes thin films in PMMA matrix

Here, we present previously synthesized azo and azo-azomethine dyes as thin films in PMMA matrix in order to investigate their spectral and optical properties. The spectral properties of the vapour deposited 100 nm nanosized films of the dyes have been characterized by UV-VIS and Fluorescence spectroscopy to determine the excitation and emission energies of the electron transitions. Photoinduced birefringence was induced in the films by pump lasers with wavelengths 355 nm and 491 nm in the region of the absorbance band of the chromophores used. The kinetics of recording, relaxation and erasure of birefringence were presented. The photochromic behaviour of the chromophore groups in azo dyes (-N=N-) and azo-azomethine dyes (-N=N- and CH=N-) were evaluated as potential light driven reversible optical storage devices. The presence of two photoactive groups in azo-azomethines makes them appropriate candidates for optical photoswitches.

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.

Long-Time Relaxation of Stress-Induced Birefringence of Microcrystalline Alkali Halide Crystals

Alkali halide single crystals are most commonly used as the diluent matrix in the tablet method or disk technique for spectroscopic measurements. However, stress-induced birefringence (SIB) of alkali halides as well as intrinsic birefringence manifest during the disk formation process. Thus, the true chiroptical measurement is disturbed by optical anisotropies (OA) containing SIB and intrinsic birefringence, except in the case of optical homogeneity. SIB is generally larger than intrinsic birefringence and has a value of several thousand millidegrees in the ultraviolet-visible wavelength range, although this varies with disk type. Here, to investigate the SIB origin, alkali halide crystals were examined using polarized light, X-ray diffraction, Fourier-transform infrared, and electron backscattering diffraction spectroscopic measurements. It was found that, after stress release, the SIB exhibited nonlinear long-time relaxation, which roughly converged within several hours, with the only time-invariant intrinsic birefringence remaining being due to OA. This behavior was strongly related to an increase in the quasi-amorphous domain and the generation of an air gap between the crystallite boundaries and their pellets. Further, a straightforward correlation was found between amorphization and an increase in the disk water content caused by deliquescence. Thus, the OA of alkali halide single crystals was found to have two different origins yielding intrinsic birefringence and SIB.

Rapid structural changes that take place during constrained annealing of biaxially stretched pet films

The aim of this work is to study the structural changes that take place during the heat setting of simultaneously and sequentially biaxially stretched polyethylene terephthalate films. The rate of structural rearrangement, as detected by real-time birefringence and off-line the Raman spectroscopy, DSC thermal analysis and X-rays, was found to be dependent on the state of the orientation and crystallinity attained by the samples during the stretching step. This suggested that the annealing behavior can be categorized into three regimes according to the birefringence behavior of the samples. At low deformations, the films not crystallized by strain exhibit complete birefringence relaxation without showing any sign of crystallization. The intermediate ratio samples that were stretched to the onset of the strain-induced crystallization exhibited the highest levels of birefringence increase after a short relaxation. The samples stretched well above the onset of the strain-induced crystallization exhibited intermediate level of birefringence and crystallinity increase. Therefore, the classification of the three regimes was found to be related to the onset of the strain-induced crystallization during the stretching step. POLYM. ENG. SCI., 2016. © 2016 Society of Plastics Engineers

Kinetic Monte Carlo simulations for birefringence relaxation of photo-switchable molecules on a surface.

Recent experiments have demonstrated that in a dense monolayer of photo-switchable dye methyl-red molecules the relaxation of an initial birefringence follows a power-law decay, typical for glass-like dynamics. The slow relaxation can efficiently be controlled and accelerated by illuminating the monolayer with circularly polarized light, which induces trans-cis isomerization cycles. To elucidate the microscopic mechanism, we develop a two-dimensional molecular model in which the trans and cis isomers are represented by straight and bent needles, respectively. As in the experimental system, the needles are allowed to rotate and to form overlaps but they cannot translate. The out-of-equilibrium rotational dynamics of the needles is generated using kinetic Monte Carlo simulations. We demonstrate that, in a regime of high density and low temperature, the power-law relaxation can be traced to the formation of spatio-temporal correlations in the rotational dynamics, i.e., dynamic heterogeneity. We also show that the nearly isotropic cis isomers can prevent dynamic heterogeneity from forming in the monolayer and that the relaxation then becomes exponential.

Influence of the strength of polarizing electric field on free relaxation of electric birefringence in poly(butyl-isocyanate) solutions

Free relaxation of electric birefringence in tetrachloromethane solution of high molecular weight poly(butyl-isocyanate) was studied. The effect of electric field strength on the average relaxation time was observed. The relaxation spectrum was analyzed using the Rouse and Zimm theories. With increase in the electric field strength, the contribution of fast (deformation) relaxation modes also increased significantly. It is assumed that certain changes in intramolecular mobility occur under the influence of electric field.

Contribution of counterions and degree of ionization for birefringence creation and relaxation kinetics parameters of PAH/PAZO films

Photo induced birefringent materials can be used to develop optical and conversion energy devices, and consequently, the study of the variables that influences the creation and relaxation of birefringence should be carefully analyzed. In this work, the parameters of birefringence creation and relaxation kinetics curves obtained on layer-by-layer (LBL) films, prepared from azo-polyectrolyte poly[1-[4-(3-carboxy-4 hydroxyphenylazo) benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) and poly(allylamine hydrochloride)(PAH), are related with the presence of counterions and the degree of ionization of the polyelectrolytes. Those kinetics curves obtained on PAH/PAZO LBL films, prepared from PAH solutions with different pHs and maintaining the pH of PAZO solution constant at pH = 9, were analyzed taking into account the films composition which was characterized by X-ray photoelectron spectroscopy. The creation and relaxation birefringence curves are justified by two processes: one associated to local mobility of the azobenzene with a characteristic time 30 s and intensity constant and other associated with polymeric chains mobility with the characteristic time and intensity decreasing with pH. These results allow us to conclude that the birefringence creation process, associated to local mobility of azobenzenes is independent of the degree of ionization and of number of counterions or co-ions present while the birefringence creation process associated to mobility of chains have its characteristic time and intensity dependent of both degree of ionization and number of counterions. The birefringence relaxation processes are dependent of the degree of ionization. The analysis of the films composition revealed, in addition, the presence of a protonated secondary or tertiary amine revealing that PAZO may have positive charges and consequently a zwitterionic behavior.

Effects of a flow field on amyloid fibrillogenesis in a β-lactoglobulin solution

The effects of a flow field on the amyloid fibrillogenesis of β-lactoglobulin (βLG) were investigated using a flow birefringence method and AFM imaging experiments. A 4wt% βLG aqueous solution was incubated at pH 2 and 80°C. A flow field was then applied by stirring at 250 and 474rpm. An incubation without stirring was used as a control sample. Flow birefringence measurements were taken at room temperature from the incubated sample solutions in which an elongational flow field was used. The birefringence pattern obtained indicated that the fibrils formed by the incubation were rigid rod-like molecules. Birefringence relaxation experiments revealed at least two relaxation processes, suggesting a double peaked distribution function for fibrils length. The length distribution of fibrils expected from the birefringence experiments was confirmed by the AFM images of amyloid fibrils. The order of the expected length of the resultant fibrils in both longer and shorter length distributions was those stirred at 250rpm≅474rpm>0rpm. The effects of the flow field applied during the incubation on amyloid fibrillogenesis was discussed on the basis of the rate process consideration. The present results demonstrated that the flow field should be considered as an important factor that regulates the fibrillogenesis of globular proteins.

Analysis of optical properties in injection-molded and compression-molded optical lenses.

Numerical mold-flow simulations and experimental measurements for injection-molded lenses have been investigated in form accuracy on a two-cavity mold with various process conditions. First, form profiles of the molded lenses have been measured together with the corresponding simulated mold-temperature distribution and displacement distribution of the lens in the z direction. A flow-through type layout of cooling channels has been devised for balance of mold-temperature distribution in mold cavities with various parametric distances for assessments in uniformity of temperature distribution. Finally, a compression-molding process is proposed for the post-process of birefringence relaxation as well as adequate form accuracy of lenses. In conclusion, optimization of process parameters to achieve good form accuracy in a multicavity mold with symmetric geometry but nonuniform cooling conditions is difficult. A good design of cooling channels plus optimized process conditions could provide uniform mold-temperature distribution so that molded lenses of good quality would be possible. Then, the profile deviation of lenses could be further compensated by profile geometry corrections. In conclusion, the post-compression-molding process could make birefringence-free plastic lenses with good form accuracy.

Temperature effects in optical fiber dispersion compensation modules

Differential group delay measurements of a fiber-based dispersion compensation module under different controlled temperature variations experience long birefringence relaxation times (>10 h) in response to temperature changes. These are interpreted here qualitatively with a stress strain behavioral model based on silica's viscoelastic property.

A simple strategy to generate light-responsive azobenzene-containing epoxy networks

Azobenzene containing epoxy networks are a class of photosensitive materials characterized by high thermal, optical and mechanical stability, promising for reversible optical storage applications. Here, we propose an encouraging two-step method to fabricate crosslinked coatings by simply reacting an amine-functionalized azobenzene and an epoxy resin in bulk for specified times to get soluble products (network precursors). Thin films based on these precursors were prepared, and thermally crosslinked in order to obtain high-Tg materials. The optical response of the materials was determined, both before and after crosslinking. In the case of the samples as prepared, the dynamic time response of the system is fast, as well as the relaxation of the photoinduced birefringence, as expected due to the high mobility of the chromophore. On the other hand, crosslinked systems have a slightly slower response, but higher values of remnant birefringence, providing stability of the photoinduced orientation, what makes them promising materials to use in optical storage applications. Besides, further analysis on the effect of temperature on the induced birefringence of the polymeric networks was also conducted to help optimization of material design. Finally, we had presented some preliminary investigations of surface relief grating recording in the obtained new materials.

Plasticization of Fibrous Cellulose Acetate: Part II – Physical Property Measurement

Birefringence, broadband dielectric and dynamic mechanical relaxation measurements are reported on fibrous cellulose acetate (CTA) plasticized with dibutylphthalate (DBP). Marked differences in physical properties are attributed to retention of nano-structure. Dielectric and mechanical relaxation data show two relaxation processes. The lowest temperature process is attributed to segmental type motion of the polymer chain and the higher temperature process to side-chain/plasticizer relaxation. The plasticization behavior is very different from that observed with other materials. The high-temperature relaxations are consistent with relative sliding of the polymer chain or nano crystallites past one another as in a liquid crystalline material.