Brillouin scattering from polymers and gels

We report results from Brillouin scattering on highly swollen and crosslinked polymeric gels, i.e. methyl methacrylate (MMA) gels crosslinked with ethylene dimethacrylate (EDMA). The study is performed varying the crosslink amount of the gels from 0 to 6%. The Brillouin spectra are measured at different scattering wavevectors in the angular ranges 90°–150°. Ve also measured the spectra at constant wavevector (90°) changing the scattering volume, in particular we select the size and the positions of the sample scattering volume. The wavevector dependence of the measured quantities confirms that in our system a micro—phase separation phenomenon takes place. In particular, for samples with crosslink contents higher than 3%, Brillouin data (studied in terms of the group velocity) give information about the existence of well defined solid- and liquid-like islands (heterogeneities in the gel structure) with an extent of several hundred Angstroms (as confirmed by elastic scattering data). Furthermore, selecting different spatial positions of the scattering volume (of micrometric sizes) and the overall sizes of the same, and performing the measurements at different times, we are able to observe (for the first time with Brillouin scattering) the non–ergodic behavior typical of this system. We report here preliminary results of such a study.     

Brillouin scattering of oriented films of poly(vinylidene fluoride) and poly(vinylidene fluoride) -poly( methyl methacrylate) blends

Temperature dependent Brillouin scattering studies of PVF₂ films stretched to various stretch ratios have been carried out. Elastic constants for unstretched and stretched films have been obtained as functions of temperature. The elastic constant C₃₃ of the stretched films has a greater temperature dependence than that of unstretched films. To elucidate the effect of the surrounding amorphous matrix on the Brillouin spectrum of semicrystalline PVF₂ film, we carried out Brillouin scattering studies of films made from blends of PVF₂ and PMMA.     

Intensity of Brillouin scattering in molecular crystals

Expressions are derived for the intensity of Brillouin scattering from molecular crystals. A molecular theory of the electric susceptibility is used to obtain the elasto-optic coefficients, which govern the effect of acoustic phonons in modulating the polarization induced by the incoming light. The crystal optical properties are taken into account in treating both the scattering and transmission through crystal surfaces, in planes containing two principal dielectric axes. The optical properties fix the scattering mode for a given geometry, and together with the local electric field affect the intensity. The results complement those for Raman scattering.     

Rayleigh-brillouin spectroscopy of syndiotactic poly(n-butyl methacrylate) near the glass transition

We present Rayleigh-Brillouin light scattering data of highly syndiotactic poly(n-butyl methacrylate) [PBMA] whose glass transition temperature as measured by DSC is 55°C. The Brillouin peak shifts, Brillouin peak widths, and Landau-Placzek ratios from ?15 to 130°C are reported. The Brillouin peak widths decrease continuously through the glass transition region. This indicates a continual decrease in the strength of processes whose relaxation times are about 10^?10 s with decreasing temperature even as the system becomes glassy. The Landau-Placzek ratio above the glass transition is about 3, indicating the high optical purity of our sample. This low Landau-Placzek ratio arises from the sample's homogeneous stereochemistry. Some of the anomalous behavior observed around 40–50°C in previous PBMA studies is explained in terms of syndiotactic regions within a largely atactic sample.     

Hyperpolarizabilities for the one-dimensional infinite single-electron periodic systems. II. Dipole-dipole versus current-current correlations

Based on Takayama-Lin-Liu-Maki model, analytical expressions for the third-harmonic generation, dc Kerr effect, dc-induced second-harmonic optical Kerr effect, optical Kerr effect or intensity-dependent index of refraction, and dc-electric-field-induced optical rectification are derived under the static current-current (J0J0) correlation for one-dimensional infinite chains. The results of hyperpolarizabilities under J0J0 correlation are then compared with those obtained using the dipole-dipole correlation. The comparison shows that the conventional J0J0 correlation, albeit quite successful for the linear case, is incorrect for studying the nonlinear optical properties of periodic systems.     

Brillouin scattering study of gelatin gel

Brillouin spectra as functions of temperature for gelatin-water systems of various concentrations are reported. These new results are discussed in light of the model from Marqusee and Deutch. We found this model to be a reasonably good theory for Brillouin scattering from gels. The sound speed in a network and the frictional damping have been evaluated.     

Molecular structure and dynamics of liquids: aqueous urea solutions

In this review a multi-technical approach to the analysis of the structure and dynamics of the urea/water system is described. The reorientational movement of the solute molecule is investigated by the analysis of spectral band-shapes, as well as with the use of the optical Kerr effect (OKE) and molecular dynamics simulation (MDS). The effect of solute concentration on the structure and dynamics of the aqueous solutions (aggregation, orientational distribution, solvation...) is studied by molecular dynamics simulation and neutron scattering. The results obtained by other techniques are included to provide a critical analysis. Finally, the low-frequency Raman spectra of the system are interpreted on the basis of the semi-quantitative information obtained by molecular dynamics simulation.     

Studies of molecular relaxation of poly(propylene oxide) solutions by dielectric relaxation and brillouin scattering

Dielectric relaxation and Brillouin scattering are jointly used in studying molecular relaxation in poly(propylene oxide) (PPO) and its solutions in methylcyclohexane. The dielectric method was applied to the more concentrated (100%, 80%, 60%, by volume) solutions over a wide temperature and frequency range (30 Hz to 8 GHz) in order that the variation in activation energy characteristic of a glass-forming substance could be delineated. The present work extends previous work on the undiluted polymer to higher frequencies so that range of 12 decades in the dielectric loss maximum f_max as a function of temperature is now available. The “Antoine” equation is found to represent the behavior of log f_max, of the bulk concentrated solutions very well. The more dilute (40%, 20%) solutions were studied only in the high-frequency (GHz) region since phase separation occurred at low temperatures. Both the temperature and dilution effects were interpreted in terms of free-volume theory. Brillouin scattering spectra were obtained at several scattering angles and a wide range of temperatures. A maximum in the curve of hypersonic attenuation versus temperature was observed in each polymer solution. The attenuation maximum shifts toward lower temperature upon dilution, in agreement with the dielectric relaxation result. The Brillouin scattering follows different activation parameters and evidences a more rapid process than does the dielectric relaxation. It is speculated that it monitors a secondary or subglass relaxation, due perhaps, to damped torsional oscillations.     

Correlation between quasielastic Raman scattering and configurational entropy in an ionic liquid

Low-frequency (5-200 cm(-1)) Raman spectra are reported for the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim]PF(6), in glassy, supercooled liquid, and normal liquid phases (77-330 K). Raman spectra of [bmim]PF(6) agree with previous results obtained by optical Kerr effect spectroscopy and molecular dynamics simulation. Both the superposition model and the coupling model give reasonable fit to low-frequency Raman spectra of [bmim]PF(6). The configurational entropy of [bmim]PF(6) has been evaluated as a function of temperature using recently reported data of heat capacity. The calculated configurational entropy is inserted in the Adam-Gibbs theory for supercooled liquids, giving a good fit to non-Arrhenius behavior of viscosity and diffusive process, with the latter revealed by a recent neutron scattering investigation of [bmim]PF(6). There is a remarkable linear dependence between intensity of quasielastic Raman scattering and configurational entropy from 77 K up to the melting point of [bmim]PF(6). This correlation offers insight into the nature of dynamical processes probed by low-frequency Raman spectra of ionic liquids.     

Brillouin scattering from amorphous bisphenol-A polycarbonate

Brillouin scattering has been studied from amorphous bisphenol-A polycarbonate in the temperature interval 60–240°C. Both longitudinal and transverse Brillouin peaks are observed over the entire range. The behavior of both types of Brillouin splittings, Δω_l and Δω_t, in the region of the glass–rubber relaxation is typical of an amorphous polymer. Equilibrium values of Δω_l and Δω_t were obtained 20°C below the glass-transition temperature T_g determined at cooling rates of 20°C/hr. Comparison of the present results with previous ultrasonic data reveals a considerable dispersion in the longitudinal phonon velocity below T_g. The origin of the large transverse Brillouin intensities is related to the structure of polycarbonate.     

Brillouin study of photopolymerization process of two-monomer systems

The results of Brillouin scattering investigations of two-component system: 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane/polyethylene glycol methacrylate (bis-GMA/PEGMM) containing 0, 30, 50, 70, 85 and 100 mol% of PEGMM are presented. For the first time the Brillouin spectroscopy was used to monitor the progress of the polymerization process. The polymerization was initiated by ultraviolet radiation (λ=365 nm), at temperature 20°C and 40°C. Some of the physical parameters characteristic for this system such as velocity V, adiabatic compressibility β(ad) and attenuation coefficient α of the acoustic waves have been estimated from Brillouin spectra as a functions of the polymerization time. The obtained results have been discussed in terms of changes of the elastic properties of the two-component system occurring during polymerization process and their dependence on bis-GMA/PEGMM system composition.     

Femtosecond molecular motions and implications for electron solvation dynamics in liquids

Molecular motions at femtosecond times are observed experimentally, via nonlinear laser spectroscopy, as responses to the imposition of a sudden, intense laser field in the optical Kerr effect. The electronic, librational, collision-induced translational and reorientational diffusional motions identified through the nonlinear polarizability responses are linked to the probable responses of the disordered medium to an excess electron undergoing multiple scattering at times <10^-12s.     

Conformational and optical properties of macromolecules of some aliphatic-substituted cellulose esters

Synthesized cellulose pelargonates, tridecanoates, valerates, and acetovalerates of various molecular weights are studied in chloroform, dioxane, and tetrachloroethane solutions by the methods of isothermal translational diffusion, sedimentation velocity method, flow birefringence (dynamooptical Maxwell effect), viscometry, and equilibrium electric birefringence (Kerr effect). The equilibrium polymer rigidities are determined and the role of the solvent and temperature in the formation of the conformational characteristics of the macromolecules under study is analyzed. The values of the intrinsic optical anisotropy of the monomeric units of the studied cellulose esters are experimentally determined. The contribution of the side chains to the optical anisotropy of the macromolecules of cellulose esters with aliphatic substituents is analyzed. The results obtained in this study are compared with the data on the cellulose esters with the aliphatic side substituents studied earlier. For the studied samples, the values of the longitudinal components of the monomeric unit dipoles in a nonpolar solvent are estimated.     

Time-resolved optical Kerr-effect investigation on CS2/polystyrene mixtures

The relaxation dynamics of carbon disulfide are investigated in mixtures with polystyrene (PS) using the time-resolved optical heterodyne-detected optical Kerr effect (OHD-OKE). The data are analyzed using both the model-dependent approach, which assumes four distinct temporal responses, and the model-independent Fourier transform approach, which generates a spectral response that can be compared with results obtained by depolarized Rayleigh scattering. A slow dynamics is observed for the OHD-OKE transient decaying exponentially with a time constant that varies from 1.68 ps for neat CS2 to 3.76 ps for the most concentrated CS2PS mixture. The increase of this time constant accompanies an increase in the viscosity of the mixture, so we can associate this component with the diffusive reorientation process of the induced polarizability anisotropy of the carbon disulfide in the mixture. The short-time nuclear response is characterized in the frequency domain by a broad band that peaks around 30 cm(-1) for neat carbon disulfide, and is associated with a complex relaxation pattern. The vibrational distribution shifts to higher frequencies when the PS concentration is increased in the mixture. This result is discussed in terms of an increase in the interaction strength between the PS phenyl rings and the carbon disulfide molecules.     

Brillouin spectra and intermolecular potential of crystalline benzene

Benzene single crystals suitable for Brillouin scattering experiments have been prepared. The Brillouin spectra have been obtained at 0°C for all the crystal orientations needed for the determination of the nine elastic constants of the orthorhombic crystal. These elastic constants are compared to values determined from ultrasonic experiments, the Brillouin results are in general 10 to 15% higher than ultrasonic results. The dynamic properties of the crystal lattice have been evaluated, for small nonzero wave vector, using a Buckingham (exp—6) potential and a coulombic (exp—6—1) potential with parameters from different authors. The agreement is not satisfactory with the (exp—6) potential while the (exp—6—1) model is appropriate for calculation of the long-wave dynamics of the crystal.     

Elastic moduli of ReO3 by Brillouin scattering

Brillouin light scattering has been used to measure elastic stiffness constants in the metal ReO₃: c₁₁ = (71.7 ± 3.5) × 10¹¹dynes/cm², c₄₄ = (7.11 ± 0.25) × 10¹¹dynes/cm², and c₁₂ = (10.3 ± 3.4) × 10¹¹dynes/cm². These values are significantly higher than those obtained ultrasonically at room temperature by Pearsall and Coldren. Since the Brillouin study employs light which heats the scattering volume, it probes lattice forces at an effectively higher temperature. Additional data taken at lower light intensity confirms the positive acoustic velocity temperature gradient.     

Effects of counterion valency on the damping of phonons propagating along the axial direction of liquid-crystalline DNA

The phonon propagation and damping along the axial direction of films of aligned 40 wt % calf-thymus DNA rods are studied by inelastic x-ray scattering (IXS). The IXS spectra are analyzed with the generalized three effective eigenmode theory, from which we extract the dynamic structure factor S(Q,E) as a function of transferred energy E=variant Planck's over 2piomega, and the magnitude of the transferred wave vector Q. S(Q,E) of a DNA sample typically consists of three peaks, one central Rayleigh scattering peak, and two symmetric Stokes and anti-Stokes Brillouin side peaks. By analyzing the Brillouin peaks, the phonon excitation energy and damping can be extracted at different Q values from about 4 to 30 nm(-1). A high-frequency sound speed is obtained from the initial slope of the linear portion of the dispersion relation below Q=4 nm(-1). The high-frequency sound speed obtained in this Q range is 3100 ms, which is about twice faster than the ultrasound speed of 1800 ms, measured by Brillouin light scattering at Q approximately 0.01 nm(-1) at the similar hydration level. Our observations provide further evidence of the strong coupling between the internal dynamics of a DNA molecule and the dynamics of the solvent. The effect on damping and propagation of phonons along the axial direction of DNA rods due to divalent and trivalent counterions has been studied. It is found that the added multivalent counterions introduce stronger phonon damping. The phonons at the range between approximately 12.5 and approximately 22.5 nm(-1) are overdamped by the added counterions according to our model analyses. The intermediate scattering function is extracted and it shows a clear two-step relaxation with the fast relaxation time ranging from 0.1 to 4 ps.     

temperature-dependent orientational dynamics of 1,n-dicyano n-alkanes

Ultrafast optical Kerr effect spectroscopy has been used to study the temperature-dependent orientational dynamics of 1,n-dicyano n-alkane liquids ranging from dicyanomethane to 1,8-dicyanooctane. The dependence of the reorientational times on temperature and viscosity is consistent with the molecules adopting a largely extended structure in the liquid state, with a preference for gauche conformations at the methylenes bonded to the cyanide groups. The data are also suggestive of temperature-dependent, collective structural rearrangements in these liquids.     

Morphological patterns of poly(N-isopropylacrylamide) derivatives synthesized with EGDMA, DEGDMA, and TEGDMA crosslinkers for application as thermosensitive drug carriers

A number of poly(N-isopropylacrylamide) (polyNIPAM) microgels were prepared with dimethacrylate cross-linking agents of various lengths, ether and ester groups in the backbone, and pendant vinylidine functionality. These materials were characterized by examining their morphological patterns using optical and scanning electron microscopy. When ethylene glycol dimethacrylate (EGDMA) was used as a cross-linking agent, microspheres of approximately 1 μm in diameter were obtained. Diethylene glycol dimethacrylate (DEGDMA) cross-linking resulted in relatively large spherical structures (1–5 μm) as well as spherical nanostructures (200 nm). Using triethylene glycol dimethacrylate (TEGDMA) resulted in spheres with diameters between 1 μm and 3 μm. The hydrodynamic particle diameter decreased with the increasing chain length of the dimethacrylate cross-linking agents. The turbidity increased with the temperature of transition points occurring at approximately 31–32°C confirming the thermosensitivity of the obtained polymeric structures.     

Hypersonic relaxation in polybutadiene and polyisoprene

Brillouin scattering is used to study hypersonic relaxation in 1,2-polybutadiene and cis-polyisoprene. The temperature of maximum loss was determined to be 125°C for a frequency of 5.0 GHz for polybutadiene and 100°C at a frequency of 5.9 GHz for polyisoprene. Hypersonic loss maxima are examined for many polymers and general conclusions are reached concerning the factors that can be used to predict the temperature of maximum loss.