Depolarized Rayleigh Spectra Research Articles

Study of the volume phase transition of poly( N-isopropylacrylamide) gels in aqueous solutions of pyridine by depolarized Rayleigh and Raman scattering

The volume phase transition of neutral poly( N-isopropylacrylamide) gels in aqueous solutions of pyridine was studied by means of depolarized Rayleigh and Raman scattering. The Rayleigh reorientational relaxation time, τ Ray, of pyridine, calculated from the half-width of the depolarized Rayleigh spectrum, attains a maximum at approximately x=0.2 at which the gel undergoes the abrupt volume change. As the gel shrinks, the ratio, I G/ I S, of the intensity of the spectrum of the gel to that of the aqueous solution abruptly decreases, reflecting the decrease in the number of free pyridine in the gel network. A new Raman band, which shifts by 130 cm −1, appears for the shrunken gels. This new band is assigned to the C–H stretching vibrations of pyridine bound to the PNIPA chains by the hydrogen-bonding interaction. As the gel shrinks, the intensity, I B, of the new Raman band abruptly increases, on the contrary, the intensity, I F, of the free pyridine band decreases. The intensity ratio, I F/( I F+ I B), which represents the number fraction of free pyridine in the network, excellently agrees with I G/ I S. The results show that the hydrophilic solutes such as pyridine in PNIPA gels are bound by the hydrogen-bonding interaction between the solutes and the chains.

Orientational and induced contributions to the depolarized Rayleigh spectra of liquid and supercooled ortho-terphenyl

The depolarized light scattering spectra of the glassforming liquid ortho-terphenyl have been calculated in the low frequency region using molecular dynamics simulation. Realistic system configurations are produced by using a recent flexible molecular model and combined with two limiting polarizability schemes, both of them using the dipole-induced-dipole contributions at first and second order. The calculated Rayleigh spectral shape are in good agreement with the experimental results in a large temperature range. The analysis of the different contributions to the Rayleigh spectra emphasizes that the orientational and the collision-induced (translational) terms lie on the same time scale and are of comparable intensity. Moreover, the cross correlation terms are always found to be an important contribution to the scattering intensity.

Interaction-induced light scattering in xenon clusters: molecular dynamics study

Molecular dynamics (MD) simulations were used to calculate the time-correlation functions and spectra of the depolarized light scattering from the “magic” sizes xenon clusters Xe13 and Xe19. Three thermodynamic phases of these clusters were studied (solid, solid–liquid coexistence region, liquid) and significant differences between the depolarized Rayleigh spectra of these phases were found.

Interaction-induced light scattering in Lennard-Jones argon clusters: Computer simulations

Molecular-dynamics simulations have been used to calculate the time-correlation functions and spectra of the depolarized light scattering from a Lennard-Jones argon cluster Ar${}_{13}.$ Three thermodynamic phases of Ar${}_{13}$ have been studied (solid, solid-liquid coexistence region, liquid) and striking differences between the depolarized Rayleigh spectra of these phases were found.

Anomalous effects in the temperature dependence of depolarized Rayleigh spectra of benzene and quinoline.

Anomalous effects in the temperature dependence of depolarized Rayleigh spectra of benzene and quinoline.

Dynamics of water confined in non-ionic amphiphiles supramolecular structures

Light scattering studies of water properties in supramolecular structures of the aqueous solutions of polyoxyethylene non-ionic amphiphiles C 10E 5 are presented. Raman and Depolarized Rayleigh spectra are studied to obtain the OH stretching vibrational contribution and the rotational relaxational time of water along an isothermal path, crossing the isotropic one-phase region from 0 to 1 amphiphile volume fraction φ, and at the structural phase transitions at relatively large amphiphiles concentrations. Data analysis of the vibrational mode points out that the structure of bound water presents a local fourfold-coordinated environment, whereas the water rotational dynamics evidences a behavior that depends on the packing effects of the amphiphile aggregates.

Determination of the optical anisotropy of atactic polystyrene using a grating monochromator

AbstractTo extract the intrinsic molecular scattering from unwanted collision‐induced contributions as much as possible we propose a simple useful method recording the depolarized Rayleigh spectra using a double grating monochromator and correcting the spectra for the high‐frequency wings. For this purpose we tested the system polystyrene/CCl4 for which an asymptotic value of the optical anisotropy 〈γ2〉/x ≅ 30 Å6 with degree of polymerization x > 20 was found. The depolarized Rayleigh intensity of 2,4‐diphenylpentane and anionically polymerized atactic polystyrene standards was also measured in dilute solutions in the θ‐solvent bicyclohexyl (θ = 61°C). From the absolute depolarized Rayleigh ratio we obtained the configurational average optical anisotropy per number of monomer units, 〈γ2〉/x. We have chosen cumene as a model compound estimating that its optical anisotropy is similar to that of a quasi monomer unit. No significant temperature dependence of 〈γ2〉/x and of the static internal pair correlation in the temperature range from 25°C up to 80°C was found.

The dynamics of succinonitrile in the plastic and liquid phases from the depolarized Rayleigh spectra

The paper reports the determination of the times of the orientational relaxation of a succinonitrile trans isomer in the plastic and liquid phase in the temperature range from 292.3 to 348.9 K, by the depolarized Rayleigh light scattering method. The activation energy of the process of the trans isomer reorientation in the plastic phase was found to be 11.6±4.2 kJ/mol. The decay constants of the interaction-induced processes in the liquid and plastic phase as well as the squares of the effective anisotropy of the optical polarizability of both trans and gauche isomers and the anisotropy due to the interaction-induced processes were determined.

Light-scattering study of phase transitions in aqueous solutions of nonionic amphiphiles.

The dynamical behavior of water in aqueous solutions of the nonionic polyoxyethylene surfactant ${\mathrm{C}}_{10}$${\mathrm{H}}_{21}$(${\mathrm{OCH}}_{2}$${\mathrm{CH}}_{2}$${)}_{5}$OH has been studied using Raman and depolarized Rayleigh-wing scattering, with particular attention to changes due to the structural phase transitions apparent at relatively large amphiphile concentrations. The frequencies and intensities of the two principal components evident in Raman spectra of the water O-H stretching band change at the hexagonal to isotropic phase transition. The width and intensity of the single resolvable line in the depolarized Rayleigh spectra similarly change at this transition: the relaxation time of the rotational water mode increases in the hexagonal phase. The changes are all consistent with an increase in the hexagonal phase of the proportion of water bound to the oxyethylene head groups: the environment of water in the system differs significantly from that in bulk water. At the lamellar to isotropic phase transition very much smaller changes were apparent, but only in the Raman scattering data. This lesser sensitivity suggests that in the lamellar phase almost all of the water is bound to the amphiphile head groups, as in the isotropic phase at such volume fractions. The changes in water dynamics reflect the structural changes in the solutions at the phase transitions, in particular the differing packing constraints in the various phases.

Depolarized Rayleigh scattering in water up to supercritical conditions

Depolarized Rayleigh spectra of water have been measured as a function of density along several isotherms in both liquid and supercritical phases. At all the thermodynamical states the spectral line shape can be fitted by a Lorentzian line superimposed to an exponential background. The behavior of the fitting parameters shows that at temperatures lower than ∼470 K the temperature is the most relevant parameter in determining the dynamics of the system, whereas above this temperature both temperature and density play a relevant role. Both the line shape and intensity of the spectra can be consistently interpreted in the framework of the first-order dipole induced dipole approximation to the collision-induced light scattering. The data show a clear evidence for cancellation effects.

Line shape analysis of the depolarized Rayleigh spectra of CS2/PS mixtures

Mixtures of carbon disulfide and polystyrene with the composition 0, 5%, 20%, 40%, 60%, 80%, 100% by weight of CS2 have been studied using depolarized Rayleigh scattering experiments within the temperature range of 263–313 K. The spectral ranges between 20 and 150 cm−1 were described in the reduced representation by the 3-parameter-Mori-function, indicating that most of the spectral features depend more on the CS2 compound than on the polymer matrix. The concept of analyzing the line shapes of liquid systems by separating the logarithmic spectrum into various distinct regions with characteristic slope parameters was successfully applied to the CS2/polystyrene mixtures and indicates the existence of liquid CS2 microstructures in the mixtures and the applicability of the dipole-induced dipole mechanism to the investigated system.

Effective optical anisotropies of simple molecular liquids from the depolarized Rayleigh spectra

Depolarized Rayleigh spectra (DRS) of the following liquids C 6H 6, C 6H 5CH 3, CS 2 CCl 4 and n-alkanes: C 5H 12, C 6H 14, C 7H 16, C 8H 18, have been studied. The applied apparatus permitted exact determination of the Lorentz type central line as well as its exponential wing. The percent share of the interaction induced contribution to DRSS as well as the square of the effective anisotropy of molecular polarizability due to interaction induced mechanisms have been determined.

A comparative study of the phenyl ring motion in styrene oligomers and polystyrene using FIR absorption and depolarized Rayleigh wing spectra

FIR absorption and depolarized Rayleigh wing spectra of styrene oligomer model compounds and polystyrene standards with a narrow molecular weight distribution were recorded at various temperatures below and above the glass transition point. For a comparative discussion of the spectral properties we have used the reduced depolarized Rayleigh spectrum R VH( ν ). In the case of cumene, 2,4-diphenylpentane and 2,4,6-triphenylheptane we were able to fit the well-known Mori function, which is often used to describe FIR absorption spectra of dipolar simple liquids, on the experimental data points. The analysis of the theoretical and band shape parameters obtained from the spectra measured as a function of chain length and temperature allows us to assume that the phenyl ring librational motion may be considered as the main dynamical process shaping the low-frequency part in the FIR absorption spectrum as well as in the depolarized Rayleigh wing of polystyrene.

Depolarized Rayleigh scattering from short-chain polystyrene in a .THETA.-solvent

Depolarized Rayleigh spectra of low molecular weight polystyrene samples in dilute solution in bicyclohexyl, a θ-solvent at 61°C, were obtained. The spectra fit well to single Lorentzian line shapes which were assigned to the overall reorientation motion of the highly anisotropic polymer. The relaxation times at different molecular weights scale with the Stokes-Einstein-Debye equation. The obtained hydrodynamic volumes reduced to a simple monomer unit, show a maximum at w ≃ 2000, apparently reflecting increasing coiling with increasing chain length

Depolarized dynamic light scattering studies of ortho-terphenyl dynamics above T g

Depolarized Rayleigh spectra of ortho-terphenyl (OTP) were measured in the temperature range from Tg to Tg+190 K. Two samples prepared with and without ‘‘clusters’’ were used for the measurement. Four different Fabry–Perot interferometers covering the time range from about 0.5 ps to 100 ns were employed. Two relaxation modes were observed: a slow mode with all its characteristics of the α process, and a fast mode with a constant, temperature independent relaxation time of about 3 ps. The fast mode has not been reported heretofore. The intensity of the fast mode vanishes at about the Vogel–Fulcher–Tamman temperature T0. The relaxation times of these two processes are found to be identical for both OTP with and without clusters. The temperature dependence of the relaxation time of the α process is Arrhenius at high temperatures, but shows a Williams–Landel–Ferry (WLF) behavior in the range from Tg to Tg+80 K for both samples with and without the long-range density fluctuations (i.e., cluster).

Optical anisotropies of methyl benzoate and diesters of benzenedicarboxylic acids as determined from the analysis of Kerr constants and depolarized light scattering measurements

Mean-square optical anisotropies of methyl esters of benzoic, terephthalic, isophthalic and phthalic acids have been measured at T= 25°C by analysis of the depolarized Rayleigh spectra of their dilute solutions in CCl4 and CHCl3. A planar Fabry–Perot interferometer was used to record the spectra and the VH spectrum of benzene was used as standard. The experimental values thus obtained were 〈γ2〉= 46, 136, 97 and 58 A6 respectively, for methyl benzoate, dimethyl terephthalate, dimethyl isophthalate and dimethyl phthalate. The analysis of these experimental results, together with values of Kerr constants of the same molecules taken from the literature, allows the formulation of the anisotropic part of the optical polarizability tensors of phenyl and ester groups, including the inductive effects produced when these groups are linked together. The results seem to indicate that the inductive effects do not depend on the number of ester groups or the positions by which they are attached to the benzene ring; however they are very sensitive to whether both groups are coplanar.

Evidence of modification of solvent reorientation dynamics from depolarized Rayleigh spectra of polymer in Aroclor solutions

Depolarized Rayleigh spectra of neat Aroclor and Aroclor solutions of polystyrene and 1,4-polybutadiene obtained experimentally provide clear evidence of modification of the dynamics of orientational motion of the solvent by the dissolved polymer. The modification of the dynamics was monitored in a time window of nine decades. The experimental data can be explained by a modified Adam-Gibbs theory in which coupling between the cooperative rearranging regions has been incorporated.

Concentration effects on collision-induced depolarized Rayleigh lineshapes in CS2/CCl4 mixtures

We report measurements of depolarized Rayleigh spectra of liquid CS2/CCl4 mixtures following the procedure proposed by Cox and Madden and recently used by Hegeman and Jonas. We fit a linear function on two distinct regions of the logarithmic spectra. The slopes Δ depend nonlinearly on the mixture composition. A comparison of our results with those of Hegeman and Jones where we follow the number-density dependence seems to be straightforward for further exploration of our experimental data. Our results indicate that, while in the high-frequency range (region III) of the spectrum changes of pressure and number density have very similar effects, these two parameters affect the lower-frequency range (region II) in a quite different way.

Solvent reorientation in polystyrene/A1248 and polystyrene/DOP systems

The depolarized Rayleigh spectra IVH( omega ) of the solvents di-2-ethylhexylphthalate (DOP) and polychlorinated biphenyl (A1248) in undiluted state and of the solutions A1248/polystyrene (PS) (cPS up to 15%) and DOP/PS (cPS up to 75%) were measured over the temperature range 30-140 degrees C using Fabry-Perot interferometry (FPI). The experimental IVH( omega ) of the undiluted solvents were well represented by a single Lorentzian whereas two Lorentzians were found necessary to fit the IVH( omega ) of the PS solutions. The variation of the intensity ratio of the two Lorentzians with temperature and the increase of the orientation time tau 0 of the solvents, upon addition of PS, show evidence of polymer-induced modification of solvent mobility. The rate of change of tau (c,T)/ tau 0 with cPS, which is a measure of the solvent local friction assumes similar values for both DOP/PS and A1248/PS solutions at high temperatures.

Depolarized Rayleigh scattering study of pyrimidine

The depolarized Rayleigh spectrum from pyrimidine has been obtained from 296K to 395K. The spectra do not display a shear wave dip at zero frequency due to translation-rotation coupling. The Stokes - Einstein - Debye plot of the reorientation time versus viscosity/temperature displays a slightly non-linear relation. The light scattering orientation pair correlation factor obtained by comparing the depolarized Rayleigh reorientation to the single particle time from NMR for pyrimidine is 2.3. This value is compared to other azobenzenes.