Orientational Relaxation Research Articles

Behaviour of cis- and trans-N-methylformamide in liquid mixture: Dynamical properties at varying pressure and temperature, and ion solvation scenario

Classical molecular dynamics simulations have been carried out to investigate dynamical properties of cis-trans mixture of liquid N-methylformamide at eight different temperatures (278–423 K) and four pressures ranging from 0.1 MPa to 300 MPa. At 298 K and 0.1 MPa, it is observed that cis-NMF is impeded by approximately 4.12% in its diffusion rate than trans-NMF. Higher pressures evoke slower dynamics concomitantly with enhanced caging effect, which is appreciable in case of cis-NMF. Calculated activation energy values for diffusion (13.465 ± 0.21–15.641 ± 0.51 kJ/mol) are in agreement with experimental observations, particularly at 0.1 MPa. Unlike self-diffusion behaviour, the orientational relaxation time of NH bond vector follows Arrhenius behaviour even at lower temperature and higher pressure. Hydrogen bonding patterns are observed to be different for H-bonds involving amide-hydrogen of either trans-NMF or cis-NMF molecules and overall interaction probability is boosted by the participation of cis-NMF conformers. Solvation scenario of ions of different charge and size in cis/trans-NMF mixture has also been explored. It is found that cations favour oxygen of cis-NMF and anions are attracted towards hydrogen sites of trans-NMF while intermediately-sized potassium and bromide ions may deviate in their preferences from the usual trend.

(Keynote) Dimensional Change of Oriented Polymers upon Heating

Molecular orientation can be incorporated to the polymeric materials through either stretching in a solid state or elongation in a molten state. When a polymer has molecular orientation in a glassy state, shrinkage may proceed when temperature is increased above its glass transition temperature. Degree of shrinkage may vary depending on the temperature reached and the degree of molecular orientation. In the case of crystalline polymers, however, shrinkage may be suppressed because of the occurrence of crystallization prior to the shrinkage. It should be noted that the crystallization rate can be accelerated significantly with the increase of molecular orientation. Accordingly, there can be a critical degree of molecular orientation; below which orientation relaxation proceeds upon heating, whereas above which spontaneous orientation enhancement caused by crystallization proceeds upon heating. In some cases, even the spontaneous elongation of materials takes place along with the progress of crystallization. On the other hand, it is well known that the dynamics of polymeric materials is characterized by the wide range of relaxation time. This means that the state of molecular orientation can be manipulated through the combination of deformation/flow history and temperature history. Dimensional change of oriented polymeric materials upon heating is mainly governed by the oriented structure of relatively long relaxation time such as network structure of molecular entanglement. Based on this concept, we could prepare fibers which showed no birefringence but exhibited significant shrinkage with the increase of temperature. This is possible because birefringence merely reflects the degree of orientation of relatively short segments, i.e. short relaxation time, in the polymer chain. Concept of manipulating the oriented structure of wide range of relaxation period leads to the idea of controlling the state of molecular entanglement. Based on this concept, we speculated that the polymeric materials of high strength and high toughness can be prepared if the material has rather homogeneous state of molecular entanglement, i.e. narrow distribution of molecular weight between adjacent entanglement points. As-spun poly(ethylene terephthalate) (PET) fibers with rather homogeneous state of molecular entanglement was prepared through the control of melt spinning process. Applying the drawing and annealing processes to such as-spun fibers, high-strength PET fibers were prepared. To verify the applicability of the concept of homogeneous state of molecular entanglement, PET fibers prepared under various processing conditions were analyzed using the laser Raman spectroscopy. It was found that the narrow distribution of internal stress field and limited widening of such distribution upon applying the tensile stress to the fibers are the key factors for realizing the high-strength and high-toughness.

Structure, polarity, dynamics, and vibrational spectral diffusion of liquid–vapour interface of a water–methanol mixture from first principles simulation using dispersion corrected density functional

The effects of dispersion interaction on the structural, dipolar and dynamical properties of liquid–vapour interface of an aqueous solution of methanol have been investigated through first principles simulations. A description of structural properties including the inhomogeneous density profiles of molecules, hydrogen bond distributions and orientational profiles are presented in detail. Among the dynamical properties, diffusion, orientational relaxation, hydrogen bond dynamics and vibrational spectral diffusion of molecules are determined. The variation of the polarity of the molecules from bulk to the interface are also calculated. The current first principles simulation results are compared with the results of dispersion uncorrected study. The dispersion corrected study predicts an increase in the number of molecules in their neighborhood, which are held together by non-hydrogen-bonded dispersion interactions. This results in the faster relaxation of hydrogen bonds, diffusion and rotational motion of molecules in comparison to the dispersion uncorrected study. Furthermore, the effects of dispersion correction on the dynamics of vibrational frequency fluctuations are emphasized, which clearly captures the dynamics of hydrogen bond fluctuations in the respective regions.

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.

Molecular polarizability anisotropy of liquid water revealed by terahertz-induced transient orientation

Reaction pathways of biochemical processes are influenced by the dissipative electrostatic interaction of the reagents with solvent water molecules. The simulation of these interactions requires a parametrization of the permanent and induced dipole moments. However, the underlying molecular polarizability of water and its dependence on ions are partially unknown. Here, we apply intense terahertz pulses to liquid water, whose oscillations match the timescale of orientational relaxation. Using a combination of terahertz pump / optical probe experiments, molecular dynamics simulations, and a Langevin dynamics model, we demonstrate a transient orientation of their dipole moments, not possible by optical excitation. The resulting birefringence reveals that the polarizability of water is lower along its dipole moment than the average value perpendicular to it. This anisotropy, also observed in heavy water and alcohols, increases with the concentration of sodium iodide dissolved in water. Our results enable a more accurate parametrization and a benchmarking of existing and future water models.

Dynamics of vibrational frequency fluctuations in deuterated liquid ammonia: roles of fluctuating hydrogen bonds and free ND modes

We present an ab initio molecular dynamics study of the roles of fluctuating hydrogen bonds and free ND modes in the dynamics of ND stretch frequency fluctuations in deuterated liquid ammonia. We have also looked at some of the other dynamical quantities such as diffusion and orientational relaxation and also structural quantities such as pair correlations and hydrogen bonding properties which are relevant in the current context. The time correlation function of ND stretch frequencies is found to decay with primarily two time scales: A short-time decay with a time scale of less than 100 fs arising from intermolecular motion of intact hydrogen bonds and also from fast hydrogen bond breaking and a longer time scale of about 500 fs which can be assigned to the lifetime of free ND modes. Unlike water, in liquid ammonia an ND mode is found to remain free for a longer period than it stays hydrogen bonded and this longer lifetime of free ND modes determines the long-time behaviour of frequency fluctuations. Our hole dynamics calculations produced results of vibrational spectral diffusion that are similar to the decay of frequency time correlation. Inclusion of dispersion corrections is found to make the dynamics slightly faster.

Parameters of LC molecules’ movement measured by dielectric spectroscopy in wide temperature range

Dielectric properties of a nematic liquid crystal (NLC) mixture ZhK-1282 were investigated in the frequency range of 102–106Hz and a temperature range of −20 to 80 °С. On the basis of the Debye’s relaxation polarization model dielectric spectra of temperature dependence of the orientational relaxation time τ and the dielectric strength δe were numerically approximated at the parallel orientation of a molecular director relative to alternating electric field. Influence of ester components in the mixture plays crucial role in relaxation processes at low temperature and external field frequency. The activation energy of the relaxation process of a rotation of molecules around their short axis was measured in a temperature interval of −20 to +15 °С in which the dispersion of a longitudinal component of the dielectric constant takes place. The energy of potential barrier for polar molecules rotation in the mesophase was calculated. The value of the transition entropy from the nematic to isotropic phase was obtained from this calculation. The values of the coefficient of molecular friction and rotational diffusion were obtained by different methods. The experimental data obtained are in a satisfactory agreement with the existing theoretical models.

Water-separated ion pairs cause the slow dielectric mode of magnesium sulfate solutions.

We compare the dielectric spectra of aqueous MgSO4 and Na2SO4 solutions calculated from classical molecular dynamics simulations with experimental data, using an optimized thermodynamically consistent sulfate force field. Both the concentration-dependent shift of the static dielectric constant and the spectral shape match the experimental results very well for Na2SO4 solutions. For MgSO4 solutions, the simulations qualitatively reproduce the experimental observation of a slow mode, the origin of which we trace back to the ion-pair relaxation contribution via spectral decomposition. The radial distribution functions show that Mg2+ and SO42- ions form extensive water-separated-and thus strongly dipolar-ion pairs, the orientational relaxation of which provides a simple physical explanation for the prominent slow dielectric mode in MgSO4 solutions. Remarkably, the Mg2+-SO42- ion-pair relaxation extends all the way into the THz range, which we rationalize by the vibrational relaxation of tightly bound water-separated ion pairs. Thus, the relaxation of divalent ion pairs can give rise to widely separated orientational and vibrational spectroscopic features.

Picosecond Proton Transfer Kinetics in Water Revealed with Ultrafast IR Spectroscopy.

Aqueous proton transport involves the ultrafast interconversion of hydrated proton species that are closely linked to the hydrogen bond dynamics of water, which has been a long-standing challenge to experiments. In this study, we use ultrafast IR spectroscopy to investigate the distinct vibrational transition centered at 1750 cm-1 in strong acid solutions, which arises from bending vibrations of the hydrated proton complex. Broadband ultrafast two-dimensional IR spectroscopy and transient absorption are used to measure vibrational relaxation, spectral diffusion, and orientational relaxation dynamics. The hydrated proton bend displays fast vibrational relaxation and spectral diffusion timescales of 200-300 fs; however, the transient absorption anisotropy decays on a remarkably long 2.5 ps timescale, which matches the timescale for hydrogen bond reorganization in liquid water. These observations are indications that the bending vibration of the aqueous proton complex is relatively localized, with an orientation that is insensitive to fast hydrogen bonding fluctuations and dependent on collective structural relaxation of the liquid to reorient. We conclude that the orientational relaxation is a result of proton transfer between configurations that are well described by a Zundel-like proton shared between two flanking water molecules.

Raman non-coincidence effect of boroxol ring: The interplay between repulsion and attraction forces in the glassy, supercooled and liquid state

Temperature dependent Raman spectra of boric oxide have been measured in a temperature range covering the glassy, supercooled and liquid state. The shift of the isotropic band assigned to boroxol rings relative to the anisotropic component upon heating the glass is measured and attributed to the Raman non-coincidence effect. The measured shift is associated with the competition between attraction and repulsion forces with increasing temperature. The relation of dephasing and orientational relaxation times to the non-coincidence effect of the condensed phases has been examined. We discuss our results in the framework of the current phenomenological status of the field in an attempt to separate the attraction and repulsion contributions corresponding to the observed non-coincidence effect.

Born-Oppenheimer Molecular Dynamics Simulations of a Bromate Ion in Water Reveal Its Dual Kosmotropic and Chaotropic Behavior.

The solvation structure and dynamics of a bromate (BrO3-) ion in water are studied by means of Born-Oppenheimer molecular dynamics simulations at two different temperatures using the Becke-Lee-Yang-Parr functional with Grimme D3 dispersion corrections. The bromate ion possesses a pyramidal structure, and it has two types of solvation sites, namely, the bromine and oxygen atoms. We have looked at different radial and orientational distributions of water molecules around the bromate ion and also investigated their hydrogen bonding properties. The solvation structure of the bromate ion is also compared with that of the iodate (IO3-) ion, which is structurally rather similar to the bromate ion and was found to have some unusual solvation properties in water. It is found that the bromate ion follows a similar trend as that followed by the iodate ion as far as the solvation structure is concerned. However, the effect of the former on surrounding water is found to be much weaker than that of the latter. On the dynamical side, we have looked at diffusion, residence dynamics, and also the orientational and hydrogen bond relaxation of water molecules around the BrO3- ion and compared them with those of the bulk. Dynamical results are presented for both H2O and D2O around the BrO3- ion. Interpretation of the dynamical results in terms of structure-making (kosmotropic)/-breaking (chaotropic) properties of the BrO3- ion reveals that the bromine atom of this ion acts as a water structure breaker, whereas the three oxygens act as water structure makers. Thus, in spite of being a single ion, the bromate ion has dual characteristics and the experimentally observed kosmotropic ability of this ion is actually a trade-off between a chaotropic site (the bromine atom) and three kosmotropic sites (three oxygen atoms) that are present in the ion.

Effects of dispersion interactions on the structure, polarity, and dynamics of liquid-vapor interface of an aqueous NaCl solution: Results of first principles simulations at room temperature.

The effects of dispersion interaction on the structure, polarity, and dynamics of liquid-vapor interface of a concentrated (5.3M) aqueous NaCl solution have been investigated through first-principles simulations. Among the structural properties, we have investigated the inhomogeneous density profiles of molecules, hydrogen bond distributions, and orientational profiles. On the dynamical side, we have calculated diffusion, orientational relaxation, hydrogen bond dynamics, and vibrational spectral diffusion of molecules. The polarity of water molecules across the interface is also calculated. Our simulation results are compared with those when no dispersion corrections are included. It is found that the inclusion of dispersion correction predicts an overall improvement of the structural properties of liquid water. The current study reveals a faster relaxation of hydrogen bonds, diffusion, and rotational motion for both interfacial and bulk molecules compared to the results when no such dispersion corrections are included. The dynamics of vibrational frequency fluctuations are also calculated which capture the relaxation of hydrogen bond fluctuations in the bulk and interfacial regions. Generally, the hydrogen bonds at the interfaces are found to have longer lifetimes due to reduced cooperative effects.

Magnetic field effects in nematic and smectic liquid crystals probed by time resolved observation of orientation relaxation of the spin probe

The kinetics of reorientation of the liquid crystal HOPOOB in nematic and smectic phases in the magnetic field was studied by the spin probe technique. The time evolution of the EPR spectrum shape in the course of reorientation in the nematic phase indicates that the mechanism of alignment comprises redistribution between domains of different orientations rather than the turn of the orientation distribution function. A microscopic model of domain structure rearrangement is provided to qualitatively describe the experimental data. In the smectic C phase, partial reorientation is observed upon the action of the magnetic field. A possible mechanism of this process is the reorientation of tilt direction within the smectic layers. It is shown that this process is constrained, and the value of the corresponding elastic constant is estimated.

A constitutive model for entangled polydisperse linear flexible polymers with entanglement dynamics and a configuration dependent friction coefficient. Part II. Modeling “shear modification” following cessation of fast shear flows

The polydisperse Mead–Park (MP) “toy” molecular constitutive model developed in Paper I [Mead et al., J. Rheol. 62, 121–134 (2017)] as well as our previously published work [e.g., J. Rheol. 59, 335–363 (2015)] is used in the “forward” direction to study model polydisperse melts of entangled linear flexible polymers in severe, fast shear flows. The properties of our new model are elucidated by way of numerical simulation of a representative model polydisperse polymer melt in step shear rate and interrupted shear flow. In particular, we demonstrate how the MP model simulates the individual molecular weight distribution (MWD) component dynamics as well as the bulk material properties. Additionally, we demonstrate that the polydisperse MP model predicts the phenomenon of “shear modification” for model MWD's with a long, high molecular weight tail. Specifically, the terminal dynamic moduli following cessation of severe, disentangling deformation, are shown to slowly heal/recover on the orientational relaxation time scale of the longest chains in the MWD. This is the first molecular constitutive equation to predict the phenomenon of shear modification. We provide detailed insight into the molecular mechanism responsible for this previously enigmatic and important phenomenon. Additionally, the presence of shear modification is not necessarily associated with the presence of shear stress peak overshoot transients in interrupted shear flow. Specifically, we examine and analyze the interrupted shear experiments reported by Tsang and Dealy [J. Non-Newtonian Fluid Mech. 9, 203–222 (1981)] and demonstrate quantitatively their lack of a relationship to shear modification. We also demonstrate that the new MP model accurately predicts the Cox-Merz rule, Laun's rule and Gleissele's mirror relations in steady shear.

Preferential solvation, ion pairing, and dynamics of concentrated aqueous solutions of divalent metal nitrate salts.

We have investigated the characteristics of preferential solvation of ions, structure of solvation shells, ion pairing, and dynamics of aqueous solutions of divalent alkaline-earth metal nitrate salts at varying concentration by means of molecular dynamics simulations. Hydration shell structures and the extent of preferential solvation of the metal and nitrate ions in the solutions are investigated through calculations of radial distribution functions, tetrahedral ordering, and also spatial distribution functions. The Mg2+ ions are found to form solvent separated ion-pairs while the Ca2+ and Sr2+ ions form contact ion pairs with the nitrate ions. These findings are further corroborated by excess coordination numbers calculated through Kirkwood-Buff G factors for different ion-ion and ion-water pairs. The ion-pairing propensity is found to be in the order of Mg(NO3)2 < Ca(NO3)2 < Sr(NO3)2, and it follows the trend given by experimental activity coefficients. It is found that proper modeling of these solutions requires the inclusion of electronic polarization of the ions which is achieved in the current study through electronic continuum correction force fields. A detailed analysis of the effects of ion-pairs on the structure and dynamics of water around the hydrated ions is done through classification of water into different subspecies based on their locations around the cations or anions only or bridged between them. We have looked at the diffusion coefficients, relaxation of orientational correlation functions, and also the residence times of different subspecies of water to explore the dynamics of water in different structural environments in the solutions. The current results show that the water molecules are incorporated into fairly well-structured hydration shells of the ions, thus decreasing the single-particle diffusivities and increasing the orientational relaxation times of water with an increase in salt concentration. The different structural motifs also lead to the presence of substantial dynamical heterogeneity in these solutions of strongly interacting ions. The current study helps us to understand the molecular details of hydration structure, ion pairing, and dynamics of water in the solvation shells and also of ion diffusion in aqueous solutions of divalent metal nitrate salts.

Order and gelation of cellulose nanocrystal suspensions: an overview of some issues.

Cellulose nanocrystals (CNCs) are polydisperse rod-shaped particles of crystalline cellulose I, typically prepared by sulfuric acid hydrolysis of natural cellulose fibres to give aqueous colloidal suspensions stabilized by sulfate half-ester groups. Sufficiently dilute suspensions are isotropic fluids, but as the concentration of CNC in water is increased, a critical concentration is reached where a spontaneously ordered phase is observed. The (equilibrium) phase separation of the ordered chiral nematic phase is in competition with a tendency of the CNC suspension to form a gel. Qualitatively, factors that reduce the stability of the CNC suspension favour the onset of gelation. The chiral nematic structure is preserved, at least partially, when the suspension dries. Solid chiral nematic films of cellulose are of interest for their optical and templating properties, but the preparation of the films requires improvement. The processes that govern the formation of solid chiral nematic films from CNC suspensions include phase separation, gelation and also the effects of shear on CNC orientation during evaporation. Some insight into these processes is provided by polarized light microscopy, which indicates that the relaxation of shear-induced orientation to give a chiral nematic structure may occur via an intermediate twist-bend state.This article is part of a discussion meeting issue 'New horizons for cellulose nanotechnology'.

An unusual promotion of γ-crystals in metallocene-made isotactic polypropylene from orientational relaxation and favorable temperature window induced by shear

Combined effects of shear flow and non-isothermal processing, which were akin to the practical industrial processing, on the crystallization of metallocene-made isotactic polypropylene (m-iPP) were investigated by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. The simultaneous appearance of α- and γ-crystals was observed in quiescent samples, while shear flow led to the occurrence of α-crystals prior to γ-crystals at the initial stage of crystallization. Although the local alignment of chain segments promoted by shear flow facilitated the α-crystals, the relaxation of oriented chains was experimentally confirmed to boost the formation of γ-crystals. Interestingly, the resultant γ-crystal content was higher in the sheared samples than the quiescent counterparts at a given cooling rate and further increased with the decrease of cooling rates. Such a discrepancy was not only related to shear-induced orientation and relaxation, but also was accredited to the advanced crystallization temperature window, which was favorable for the γ-crystal growth. It was quite different from the isothermal crystallization situation, where the formation of γ-crystals was depressed in presence of shear flow. The dynamic competition between α-crystal and γ-crystal formation under the effects of shear during the non-isothermal crystallization was discussed. Our work is of valuable reference to tailor the crystalline structure of m-iPP during practical processing and hence to acquire the desired performance of products.

Raman Spectra and Dynamics of Thiocyanate Ion in Poly(Vinyl Alcohol)–KSCN Films

Raman spectra of poly(vinyl alcohol)–potassium-thiocyanate films are studied. Parameters of vibrational and orientational relaxation of thiocyanate ion in the polymer matrix are determined. The character and rate of vibrational dephasing become identical to SCN– vibrations in aqueous solution at salt concentrations ≥0.3 M.

Role of Dielectric Drag in Polaron Mobility in Lead Halide Perovskites

Hybrid organic–inorganic lead halide perovskites (HOIPs) have attracted much attention because of their remarkable carrier lifetimes and diffusion lengths. These properties have been attributed to the efficient screening of charge carriers via polaron formation in the highly polar and dynamic environment. Polaron formation explains, at least in part, the moderate charge carrier mobility, but the calculated mobilities are somewhat higher than experimental values. Here we discuss a factor that has been previously overlooked and can potentially account for the discrapency: the effect of dielectric drag. While optical phonon modes of the lead halide sublattice are mainly responsible for polaron formation, slower orientational relaxation of surrounding dipoles adds a dielectric drag to the moving charge. We discuss the role of this dielectric drag based on the measured dielectric function in the gigahertz to terahertz frequency range and how we can understand the unique carrier physics in HOIPs in view of its ...

Molecular dynamics simulation study of hydration of uranyl nitrate in supercritical water: Dissecting the effect of uranyl ion concentration from solvent density

All atom molecular dynamics simulations of uranyl ions in supercritical water are used to dissect the effects of concentration of uranyl ions and density of water on various structural and dynamic properties of the solutions. The analyses of radial distribution functions as a function of concentration of the uranyl ion and water density reveal that the effect of the former on the local structure is negligible as compared to the same of the later. The number of hydration water of the uranyl ion has been observed to increase with increasing density of the water, but it decreases with the increasing concentration of the uranyl ions. The orientational distributions are observed to be independent of variation in concentration of the uranyl ion, same as the case was with water density. The translational and rotational dynamics of the water molecules have been investigated from the respective mean squared displacements and time correlation functions. Although increase of both the concentration of the uranyl ions and the density of water reduces translational diffusivity of water as well as uranyl ions, the effect of changing water density is more than that of uranyl concentrations. However, orientational relaxation of various molecular vectors of the water molecule is practically unchanged with any variation in concentration of the uranyl ions and it changes only slightly with the change in water density. Unlike at ambient condition, orientational dynamics at supercritical conditions remains virtually unchanged with the change in uranyl ion concentration.