Relaxation Of Molecular Orientation Research Articles

Influence of Internal Bond Rotation on Ultrafast IR Anisotropy Measurements and the Internal Rotational Potential.

Measurement of molecular orientation relaxation using ultrafast infrared (IR) pump-probe experiments is widely used to understand the properties of liquids and other systems. In the simplest situation, the anisotropy decay is a single exponential reflecting diffusive orientational relaxation. However, the anisotropy decay is frequently biexponential. The faster component is caused by solvent caging restricting angular sampling until constraint release permits all angles to be sampled. Here, we describe another mechanism that limits the range of sampling, i.e., sampling of a restricted range of angles via internal bond reorientation on a rotational potential surface with barriers. If the internal angular sampling occurs faster than the entire molecule's diffusive orientational relaxation, it will produce a fast component of anisotropy decay with a cone angle determined by the shape of the internal rotation potential. We studied four molecules to illustrate the effects of internal bond rotations on anisotropy decay. The molecules are p-chlorobenzonitrile, phenylselenocyanate, phenylthiocyanate, and 2-nitrophenylselenocyante in the solvent N,N-dimethylformamide. The CN stretch is used as the IR chromophore. p-Chlorobenzonitrile does not have internal rotation; its anisotropy decays as a single exponential. The other three have bent geometries and internal rotation of the moieties containing the CN occurs; the anisotropies decay as biexponentials. The faster of the two decays can be understood in terms of motions on the rotational potential surface. A method is developed for extracting the intramolecular rotational potential surface by employing a modification of the harmonic cone model, and the results are compared to density functional theory calculations.

Effect of molding history on molecular orientation relaxation during physical aging of polystyrene injection moldings

Abstract This work examined the effect of changing molding conditions on the physical aging of polystyrene injection moldings. First, we investigated the relationship between the molecular orientation and the molding conditions. The molecular orientation near the surface changed with changing injection rate, so we hypothesized that this molecular orientation might form during the filling stage. Because this molecular orientation did not relax under heat treatment below the glass transition temperature (T g), the oriented molecules near the surface were thought to be elongated owing to the high strain rate during the filling stage. On the other hand, the molecular orientation in the core layer changed with changing holding pressure and relaxed under heat treatment below T g. Thus, the molecules in the core layer might become oriented during the holding stage and not be elongated owing to the slow strain rate. Furthermore, the molecular orientation in the core layer decreased with increasing mold temperature, and the physical heat resistance improved with increasing mold temperature. Meanwhile, the excess enthalpy did not change with changing molding conditions. Therefore, the improvement in physical heat resistance with increasing mold temperature was likely caused by the decrease in the molecular orientation in the core layer. Analyzing the relaxation behavior of the molecular orientation suggested that increasing mold temperature reduced the number of oriented molecules with large deformation in the core layer.

Relationship between Molecular Orientation Relaxation during Physical Aging and Heat Resistance of Polystyrene Injection Moldings

Abstract This work examined the relationship between changes in molecular orientation and the heat resistance (heat distortion temperature) of polystyrene injection moldings following heat treatment below the glass transition temperature. Molecular orientation around the core layer of the injection moldings was found to be relaxed by the heat treatment. Also, in the untreated specimen, the molecular orientation around the core layer was relaxed from 60°C during the heating process. Since loss tangent (tanδ) also increased from 60°C during the heating process in the untreated specimen, it was considered that the increase in tanδ occurred with the molecular motion due to the relaxation of molecular orientation from 60°C. After the heat treatment, because of the relaxation of molecular orientation around the core layer by the heat treatment, the relaxation of molecular orientation from 60 °C did not occur during the subsequent heating process, and the tanδ of the polymer between 60 and 90 °C was decreased. Because this decrease in the tanδ over this temperature range improved the heat resistance of the material, the enhanced heat resistance by the heat treatment was attributed to the suppression of the relaxation of molecular orientation from 60°C during the heating process. Furthermore, relaxation of molecular orientation and enthalpy relaxation were related to improvement in the heat resistance.

Abnormal brittle-ductile transition for glassy polymers after free and constrained melt stretching: The role of molecular alignment

By comparing the brittle-ductile transition (BDT) behavior between uniaxial width-Free Melt pre-Stretched (FMS) and width-Constrained Melt pre-Stretched (CMS) poly(methyl methacrylate) (PMMA), the effect of molecular alignment on the mechanical response of glassy polymers has been investigated. Unexpectedly, during further mechanical characterization, CMS samples exhibit higher ductility than FMS samples along both the machine (stretching) direction (MD) and the transverse direction (TD). Combining the results from molecular orientation and stress relaxation, it is proposed that chain stretching and aligned chain density is responsible for the ductile deformation along the MD and TD, respectively. Along the MD, CMS samples exhibit more significant chain stretching than FMS samples due to width constraint, leading to a lower chain activation energy barrier in CMS samples. Upon further deformation, chain activation is then accelerated and shear yielding can be achieved at lower temperature. Along the TD, the orientation function remains constant for CMS samples but negatively increases for FMS samples, and the lower aligned chain density in FMS samples increases the activation energy barrier for plastic deformation. As a result, crazing prevails during further deformation for FMS samples and causes weaker ductility. The mode of BDT in glassy polymers is considered from multiple scales.

Characteristic dynamic rheological responses of nematic poly(p-phenylene terephthalamide) and cholesteric hydroxypropyl cellulose phases

The liquid crystalline (LC) behavior of two lyotropic systems, poly(p-phenylene terephthalamide) solutions in sulfuric acid and hydroxypropyl cellulose (HPC) solutions in water, was compared on a dynamic rheological basis. Two systems showed similar behaviors in molecular stress relaxation but different orientation relaxation time behaviors. The helicoidal structure of the HPC systems is transformed to a nematic-like directional order via the damping process under very high shear.

Molecular Orientation Effect of Heat-Sealed PP Film on Peel Strength and Structure

Heat sealing properties are optimized by controlling temperature, pressure and dwell time, while film strength depends on the drawn ratio and the molecular orientation of the film. However, heat seal strength of polymer films with high drawn ratio shows lower peel strength, because the adhesion of films needs a higher heat sealing energy for molecular orientation relaxation at heat sealing. In the present study, polypropylene films with a drawn ratio of 1.0×, 1.5×, and 2.5× are heat sealed by using the heat sealing technique. The heat sealing condition is set to heat sealing time 1.0 s, sealing pressure 0.2 MPa, and heat sealing temperature 145°C. The effect of drawn ratio and stabilization temperature of PP films for peel strength are investigated using T-Peel test, DSC, FT-IR, and Raman spectroscopy. As a result, it is found that the peel strength is decreased with increasing the drawn ratio and stabilization temperature of PP films. The difference of ?H and melting point from the result of DSC measurement are exhibited for 1.5× drawn ratio film as compared with 2.5× one. In addition, FT-IR imaging and Raman line mapping reveal the influence and variation of high order structure for heat sealed parts of the drawn PP films.

Role of Molecular Orientation and Surface Relaxation on Vapor Growth Shape of Molecular Crystals

We report periodic ab initio total energy calculations to predict equilibrium morphology of molecular crystals. The approach is based on the adsorption of molecules upon the underlying crystal surfaces rather than the attachment of two-dimensional slices of thickness of interplanar distance. It takes into account the effect of molecular orientation and surface relaxation of the habit faces on the molecular attachment energy. The approach has been successfully applied to the case of urea and β-succinic acid crystals to calculate equilibrium shapes from the vapor phase. Hartree–Fock and density functional theory have been employed to obtain the atomic structure of different slices and molecular attachment energies. The relaxed morphologies obtained using the Hartree–Fock method yield good agreement with the experimental observations. Observed polar growth morphology of urea crystal has been discussed particularly in the context of different atomic environments of (111) and (111) faces.

Electrical Modulation of Coherent Scattering in Dual Frequency Liquid Crystal Tissue Models

Molecular orientation clusters are created in a cell of non aligned liquid crystal to study the light propagation in electrically controllable anisotropic non uniform media. Coherent scattering phenomenon and predominant role of large orientation clusters in such scattering are demonstrated. Fast relaxation of molecular orientation and modulation of light scattering in those clusters is obtained by using “dual frequency” liquid crystal mixtures and frequency modulation of the drive signal. The obtained results show the good potential for the creation of electrically tunable tissue models by using liquid crystalline materials.

Molecular orientation and stress relaxation during strain-induced crystallization of vulcanized natural rubber

Birefringence and tensile stress of vulcanized natural rubber samples were measured simultaneously. The effect of finite chain extensibility could be clearly detected. The increase in birefringence was related to the degree of orientation of amorphous chains to be incorporated into crystals. The normalized stress decreased in a manner that is almost independent of network-chain density. These experimental results could be successfully explained by assuming a certain fraction of network chains that behave in a manner similar to that of a fluid.

Effects of bonding type and interface geometry on coherent transport through the single-molecule magnetMn12

We examine theoretically coherent electron transport through the single-molecule magnet Mn$_{12}$, bridged between Au(111) electrodes, using the non-equilibrium Green's function method and the density-functional theory. We analyze the effects of bonding type, molecular orientation, and geometry relaxation on the electronic properties and charge and spin transport across the single-molecule junction. We consider nine interface geometries leading to five bonding mechanisms and two molecular orientations: (i) Au-C bonding, (ii) Au-Au bonding, (iii) Au-S bonding, (iv) Au-H bonding, and (v) physisorption via van der Waals forces. The two molecular orientations of Mn$_{12}$ correspond to the magnetic easy axis of the molecule aligned perpendicular [hereafter denoted as orientation (1)] or parallel [orientation (2)] to the direction of electron transport. We find that the electron transport is carried by the lowest unoccupied molecular orbital (LUMO) level in all the cases that we have simulated. Relaxation of the junction geometries mainly shifts the relevant occupied molecular levels toward the Fermi energy as well as slightly reduces the broadening of the LUMO level. As a result, the current slightly decreases at low bias voltage. Our calculations also show that placing the molecule in the orientation (1) broadens the LUMO level much more than in the orientation (2), due to the internal structure of the Mn$_{12}$. Consequently, junctions with the former orientation yield a higher current than those with the latter. Among all of the bonding types considered, the Au-C bonding gives rise to the highest current (about one order of magnitude higher than the Au-S bonding), for a given distance between the electrodes. The current through the junction with other bonding types decreases in the order of Au-Au, Au-S, and Au-H. Importantly, the spin-filtering effect in all the nine geometries stays robust and their ratios of the majority-spin to the minority-spin transmission coefficients are in the range of 10$^3$ to 10$^8$. The general trend in transport among the different bonding types and molecular orientations obtained from this study may be applied to other single-molecular magnets.

Crystallization behavior of melt-spun poly(vinyl alcohol) fibers during drawing process

The crystallization behavior of melt-spun poly(vinyl alcohol) (PVA) fibers during hot drawing process was studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and computer controlled electronic universal testing machine. The effects of drawing temperature and drawing speed on the crystallinity and the stress induced crystallization of PVA fibers were discussed. The results showed that the crystallization process of PVA fibers during hot drawing presented three stages: initial stage, stress induced crystallization stage and slowly increasing stage. And PVA fibers with high crystallinity can be obtained by properly increasing the drawing temperature and drawing speed, especially when the drawing temperature and speed were 453 K and 100 mm/min respectively. The stress induced crystallinity of PVA fibers during drawing process was the difference between the crystallinity of PVA fibers after drawing and after only heat treatment. At the low drawing speed, i.e. 50 mm/min, due to the strong molecular movement and orientation relaxation under high temperature, the effect of stress induced crystallization weakens with the increase of drawing temperature, the ratio between stress induced crystallinity and thermal induced crystallinity changed from 8.7%:0%(393 K) to 1.7%:5% (453 K). While at the high drawing speeds, i.e. 100 mm/min and 500 mm/min, with the decrease of available orientation relaxation time, the stress induced crystallization plays an important role during the drawing process, the ratio between stress induced crystallinity and thermal induced crystallinity were 8%:2.9% and 10.2%:0.5% at 453 K respectively.

Long-Term Relaxation of Molecular Orientation in Vacuum-Deposited ALQ3 Thin Films

Long-term measurement of noncentrosymmetric molecular orientation of vacuum-evaporated tris(8-hydroxyquinolinato) aluminum (III) (Alq3) thin film was performed by means of first-order electroabsorption (EA) spectroscopy. This molecular orientation is responsible for the giant surface potential of Alq3 film, and it might be utilized to nonlinear optical devices. Although the EA response presented reduction, the relaxation time constant was on the order of year. Possible mechanisms for the reduction are discussed.

Orientational relaxation of three different dendrimers in polycarbonate matrix investigated by optical poling

We report on all-optical poling relaxation of E-O active dendrimers photo-patterned in the polycarbonate matrix. Three kinds of polyester type dendrimers G0, G1 and G2 containing azobenzene in the core were used as second-order nonlinear optical active chromophores to create noncentrosymmetry inside the material. We used a guest–host type system of these structures spin-coated on a glass substrate. These samples were poled by using an all-optical poling method, where the polar electric field of electromagnetic waves enables us to orient dendrimer molecules creating a quasi-phase-matched grating of nonlinear second-order susceptibility χ(2). Molecular orientation and subsequent dark relaxation kinetics in the samples with different poling beam applied energies were measured. Theoretical modeling of the poling relaxation kinetic revealed a set of characteristic time constants, which were interpreted by different relaxation mechanisms.

Optical nanotomography of anisotropic fluids

A technique that combines ideas taken from conventional scanning near-field optical microscopy and medical tomography enables structures within an anisotropic fluid to be imaged in 3D with sub-wavelength resolution. The physical properties of anisotropic fluids can be manipulated on very short length scales of 100 nm or less by appropriate treatment of the confining substrate(s)1,2. This facilitates the use of ordered fluids in a variety of scientific endeavours and applications. Although future advances will require a complete understanding of their structure at the nanoscale level, high-resolution three-dimensional optical imaging of the fluid’s molecular orientation profile is beyond the reach of extant techniques3. Here, we report a powerful imaging approach based on the collection of transmitted light in the far field that is emitted from a light source with a subwavelength aperture. We acquire high-resolution images by raster-scanning, at multiple heights, an optical fibre immersed inside a thin layer of anisotropic fluid, thereby facilitating the visualization of the fluid’s structure with a resolvable volume ∼1/500 of that attainable by current methods. This novel technique offers the intriguing possibility of three-dimensional nanoscale reconstruction of a variety of soft materials, here the first direct visualization and measurement of the liquid-crystal molecular orientation relaxation length.

Influence of Molecular Orientation and Melt Relaxation Processes on Glassy Stress−Strain Behavior in Polystyrene

This paper explores the relationship between the level of preferred molecular orientation at different length scales and the subsequent glassy stress/strain behavior of polystyrene. To produce samples with a range of orientation states, samples of a commercial polydisperse polystyrene (M w = 274 000 g/mol, polydispersity = 2.74) were first drawn to a set draw ratio and then annealed for a range of times: subsequent to this the glassy stress-strain behavior of the various annealed samples was then measured. Appropriate annealing times, and temperatures, were informed by the molecular based theories for the melt state. In particular, annealing times were chosen to span the range between the entanglement time, τ e , and the typical reptation time, τ d . Differences in the deformation behavior were linked to measurements of the birefringence and thermal shrinkage, reflecting short-range orientation and orientation of the order of the entanglement length, respectively. Results showed that for short annealing times (∼τ e ) the birefringence fell almost immediately and that the true stress-strain curves measured for these samples could be superposed by shifting along the true strain axis by an amount which correlated with the sample birefringence. For longer annealing times, of the order of the Rouse time τ R , the stress/strain curves were found to no longer superpose in the higher true strain region. The shape of the strain hardening curve was found to correlate very well with the measurement of thermal shrinkage of the samples obtained after annealing. Samples with different initial draw ratios (and hence different initial birefringence and shrinkage), but then annealed to have the same shrinkage, were found to show the same shaped strain hardening curve: that is, the higher strain region of the curves superposed.

Classifying the Combined Influence of Shear Rate, Temperature, and Pressure on Crystalline Morphology and Specific Volume of Isotactic (Poly)propylene

The use of the Deborah number in classifying the effect of shear flow on the evolution of the specific volume and resulting crystalline morphology is investigated. We distinguish between flows applied at large undercoolings and flows applied at low undercoolings. In the first case, the Deborah number provides a good classification tool to determine the influence of flow on the evolution of orientation and structural properties of the crystalline morphology and specific volume. Both the Deborah number related to the process of chain retraction (Des) or the Deborah number related to reptation of chains (Derep) can equally well be used to classify the influence of flow on the evolution of specific volume, as characterized by the dimensionless transition temperature and dimensionless rate of transition. If, however, flow is applied at relatively low undercoolings, remelting of flow-induced crystalline structures and relaxation of molecular orientation play a significant role after cessation of flow. Therefore...

Molecular Orientation of Poly(Ethylene Terephthalate) and Poly(Butylene Terephthalate) Probed by Polarized Raman Spectra: A Parallel Study

Poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) samples uniaxially drawn above Tg and beyond the yield point exhibit significant differences in their molecular orientation behavior as probed by polarized Raman spectra. The quasi-amorphous PET samples, drawn close to the Tg, manifest considerable molecular orientation development; however, when drawn above Tg + 30 degrees C, they exhibit significant molecular orientation relaxation. The semi-crystalline PBT samples maintain prominent molecular orientation even when drawn 110 degrees C above Tg. The drawing process, in PET samples, when resulting in molecular orientation, is accompanied by a gauche-trans transformation of the glycol linkage and a concurrent initiation of crystallinity development. In PBT specimens, it gives rise to a coexistence of alpha- and beta-type crystalline phases. Phase alpha is predominant at high draw temperatures, i.e., Tg + 110 degrees C, while phase beta dominates at low draw temperatures, i.e., Tg + 10 degrees C. PBT samples, with beta-phase predominance, left at relevant draw temperatures without stress, exhibit a beta-alpha phase change though no molecular orientation relaxation occurs. A note is made of the fact that complete molecular orientation analysis of PBT segments utilizing the depol method gives more reliable results than the simplified analysis assuming a cylindrical tensor for the 1614 cm(-1) symmetric stretch of the para-disubstituted benzene ring of PBT. In this context, segments of PBT specimens rich in alpha-phase exhibit higher molecular orientation than those with beta-phase predominance.

Zone-reacted poly( p-phenylene vinylene) films. Changes in mechanical properties and microstructure by acid doping

The doping of highly oriented poly( p-phenylene vinylene) films prepared by the zone-reaction method was performed with various acids, HCl, H 2SO 4, and H 3PO 4. The changes in the microstructure, electrical, tensile, and viscoelastic properties of the resulting films were studied. It was found that the doping of HCl and H 2SO 4 increased the electrical conductivity to 186 and 61 S cm −1 at the doping time of 6 and 1.5 h, respectively. A further doping, however, lowered the conductivity, which was associated with the relaxation of molecular orientation. The zone-reacted film doped with HCl preserved Young's modulus and tensile strength as high as 44.7 and 0.73 GPa, where dynamic storage modulus was 49 GPa at 25 °C and held 21 GPa at 400 °C.

The effect of the molecular orientation on the release of antimicrobial substances from uniaxially drawn polymer matrixes

A new method of controlled release of low molecular weight biocides incorporated in polymer matrixes is described. The molecular orientation of uniaxially drawn biocide doped polymer films is suggested as a significant parameter for controlled release monitoring. Triclosan, a well-established widespread antibacterial agent, has been incorporated into high density polyethylene (HDPE) films that have been subsequently uniaxially drawn at different draw ratios. The molecular orientation developed was estimated utilizing polarized μ-Raman spectra. Biocide incorporated polymer films, drawn at different draw ratios, have been immersed in ethanol–water solutions (EtOH) and in physiological saline. The release of Triclosan out of the polymer matrix was probed with UV-Vis absorption spectroscopy for a period of time up to 15 months. In all cases, although the film surface of the drawn samples exposed to the liquid solution was higher than the undrawn one, the relevant release rate from the drawn specimens was lower than the non-stretched samples depending on the molecular orientation developed during the drawing process. A note is made of the fact that no significant molecular orientation relaxation of the polyethylene films has been observed even after such a long time of immersion of the drawn films in the liquid solutions.

A system of Kerr effect spectroscopy for light absorbing liquids

We developed experimental system of the optical Kerr effect spectroscopy for the light absorbing liquids. To avoid the problems due to the light absorption, such as the thermal lens effect, our new system used a gated rectangular light pulse with the alternative polarization modulation as the pump light. The experimental results of the transient behavior of the signal showed that our system could clearly distinguish the optical Kerr effect from the thermal lens effect. Careful monitoring of the signal enables us to suppress the temperature increase within the experimental latitude. We examined the previous results that the optical anisotropy of the isotropic liquid crystals induced by the external field is enhanced dramatically compared with the pure liquid crystals by doping a small amount of the dye from the viewpoint of relaxation dynamics. The information on the dynamic molecular orientation relaxation obtained by our spectroscopy would help us understand a microscopic interaction among liquid crystals and dye molecules, especially in the vicinity of the transition temperature. The system would be useful widely for the measurement of the nonlinear optical properties in the materials, which show strong light absorption or scattering.