Stress-induced straightening of tie chains in oriented PE films as revealed by Raman (LAM) study

The Raman longitude acoustical mode (LAM) study of axially stressed polyethylene-made microfilms gave evidence of the formation of the straight tie molecules localized in two neighboring crystals and in an amorphous layer situated between them. The samples were bicomponent blends composed of polyethylenes with different MW (7 × 10⁴ and 2 × 10⁶). Each MW fraction exhibits an individual LAM band responding to crystals with the size specific to a given fraction. Under tensile stress, the bands shift insignificantly towards an exciting laser line. In addition, the LAM localized on all-trans sequencies with the length exceeding the mean crystallite size appear in the spectrum of a stressed sample. The reversibility of these spectral changes depends on the draw ratio of samples. We conclude the effect is due to stress-induced gauch-to-trans transformations taking place in the amorphous layers. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2829–2833, 1998     

Modelling the fractionation process in TREF systems: Thermodynamic simple approach

A simple thermodynamic model, based on an extension of Flory-Huggins theory, is applied to temperature rising elution fractionation (TREF). Dependence of the fractionation process on melting temperature, melting enthalpy, average crystallinity, average crystallizable sequence length, and polymer-solvent interaction parameter is predicted. Results from the model fit experimental TREF data, and correctly predict number-average branch points for TREF fractions. © 1995 John Wiley & Sons, Inc.     

Elution Temperatures of Polyethylene Copolymers in Temperature Rising Elution Fractionation (TREF): Comparison in Xylene and Trichlorobenzene Diluents

Temperature rising elution fractionation (TREF) became the preferred technique to characterize the short chain branching distribution of polyethylene copolymers. Due to technical limitations, preparative TREF (PTREF) is usually done in xylene, while trichlorobenzene is used in analytical TREF (ATREF). Attempts to correlate the TREF elution temperatures based on data published by different authors erroneously showed higher elution temperatures for xylene than for trichlorobenzene. Our study rectifies this error. The experiments were done in both solvents on the same analytical TREF instrument. For the analyzed polyethylene copolymers, we found that the average elution temperature in xylene is 3.7° ± 1°C lower than in trichlorobenzene.     

Crystallization of Silica Xerogels: A Study by Raman and Fluorescence Spectroscopy

Pure and europium doped silica xerogels were annealed at 1050°C to obtain full densification, and at 1300°C to induce crystallization. Raman spectroscopy, time resolved selective luminescence and lifetime measurements were performed on glassy and crystallized samples. We discuss the differences between the Raman spectra of the xerogel annealed at 1050°C and those of a commercial silica. The typical Raman structures of -crystobalite are evident for the 1300°C annealed samples, but a glassy phase coexists, indicating an incomplete crystallization. Fluorescence measurements give information on the environment of the Eu³⁺ ions in the glassy and crystallized sample.     

Phototransformation studies of halo-substituted cinnamates by Raman spectrometry and quantum-chemical computations

The comprehension of the cinnamic derivative phototransformation mechanisms is particularly important when these molecules are used as addressable photosensitive layers. In this work we show that the halo-substituted cinnamate sensitivity to the phototransformation is a function of the excitation wavelength and the substituent nature and position. With this intention, we underline the existence of various isomers and rotatomers by Raman spectroscopy and we assign the observed vibrational modes with the help of quantum-chemical calculations. These various aspects of our work clarify the relative roles of the steric, inductive and mesomeric effects according to the considered substitution.     

Correlation of the elution behavior in temperature rising elution fractionation and melting in the solid‐state and in the presence of a diluent of polyethylene copolymers

Compositionally homogeneous poly(ethylene‐α‐olefin) random copolymers with 1‐butene and 1‐hexene comonomers have been studied. The melting of solution‐crystallized specimens of these copolymers in the presence of trichlorobenzene as a diluent with differential scanning calorimetry (DSC) is well correlated with analytical temperature rising elution fractionation (A‐TREF) elution temperature profiles. This indicates that the A‐TREF experiment is essentially a diluent melting experiment. Furthermore, the correction of the corresponding solid‐state melting endotherms of these copolymers with Flory's diluent melting equation yields curves that also correlate very well with the DSC diluent melting curves and the A‐TREF elution temperature profiles. Values of χ, the Flory–Huggins interaction parameter, are determined for these copolymers in trichlorobenzene. χ decreases as short‐chain branching increases. The A‐TREF elution temperature profiles of one of these copolymers are the same, within experimental error, for dilute‐solution crystallizations of the copolymer performed over an extremely broad time schedule (10 s to 3 days). This indicates the profound effect of the branches, as limiting points of the ethylene sequences, in controlling the crystal thickness distribution, which in turn controls the melting point in the presence of the diluent, or the elution temperature from the A‐TREF. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2819–2832, 2001     

Vibrational Raman optical activity of pi-conjugated helical systems: hexahelicene and heterohelicenes

Helicenes constitute a special class of molecules combining helical conformation with pi-electron delocalization. These confer to helicenes specific chirooptical properties. In this article, we investigate the vibrational signatures thanks to the simulation of vibrational Raman optical activity (VROA) spectra. For that, four representative helicenes: hexahelicene, tetrathia-[7]-helicene, and its pyrrole and furan analogs have been simulated and interpreted using a recently implemented analytical scheme. Helicenes show intense VROA peaks attributed to their pi-conjugated structure and associated with collective vibrational modes. In hexahelicene, the dominant VROA features are due to vibrational modes involving motions of the carbon skeleton and H-wagging, but the intensity finds its source almost exclusively in the former. In the case of the three heterohelicenes, the previous statement is also verified, and on changing the heteroatoms, similar modes presenting comparable atomic contribution patterns have been highlighted, though the vibrational and electronic properties are modified. Some fingerprints could therefore be associated with the helicity of the system. In particular, in forward spectra, most of the VROA bands are positive for left-handed helicenes. Nevertheless, the spectral patterns are quite complex, and no easy rule-of-thumb could distinguish between the different heterohelicenes. Then, considering the fact that most of the contributions originate from the C atoms (group coupling matrices decomposition), it can be concluded that the major role of the heteroatom is restricted to modifying the geometry and the normal modes. At last, the small impact of the gauge-origin on the calculated spectra using a relatively modest basis set (rDPS:3-21G) is demonstrated here in the case of the tetrathia-[7]-helicene molecule presenting a C(2) symmetry. This further demonstrates the adequacy of this basis set for VROA calculations.     

Direct calculation of solid-vapor coexistence points by thermodynamic integration: application to single component and binary systems

We present a new thermodynamic integration method that directly connects the vapor and solid phases by a reversible path. The thermodynamic integration in the isothermal-isobaric ensemble yields the Gibbs free energy difference between the two phases, from which the sublimation temperature can be easily calculated. The method extends to the binary mixture without any modification to the integration path simply by employing the isothermal-isobaric semigrand ensemble. The thermodynamic integration, in this case, yields the chemical potential difference between the solid and vapor phases for one of the components, from which the binary sublimation temperature can be calculated. The coexistence temperatures predicted by our method agree well with those in the literature for single component and binary Lennard-Jones systems.     

Diamond-like carbon films for polyethylene femoral parts: Raman and FT-IR spectroscopy before and after incubation in simulated body liquid

In artificial prosthetics for knee, hip, finger or shoulder joints, ultrahigh molecular weight polyethylene (UHMW-PE) is a significant material. Several attempts to reduce the wear rate of UHMW-PE, i.e. the application of suitable coatings, are in progress. A surface modification of polyethylene with wear-resistant hydrogenated diamond-like carbon is favourable, owing to the chemical similarity of polyethylene (–C–H₂–) _n and C:H or amorphous C:H (a–C:H) coatings with diamond-like properties. In the present study, the microstructure of a–C:H coatings on UHMW-PE substrates was investigated by Raman and Fourier transform infrared (FT-IR) spectroscopy. FT-IR spectroscopy shows very broad absorption lines, which point to the disorder and diversity of different symmetric, asymmetric aromatic, olefin sp ²-hybridized or sp ³-hybridized C–H groups in the amorphous diamond-like carbon coating. Following a long incubation of 12 months in a simulated body liquid, the structural investigations were repeated. Furthermore, fractured cross-sections and the wetting behaviour with polar liquids were examined. After incubation in simulated body liquid, Raman spectroscopy pointed to a reduction of the C–H bonds in the diamond-like carbon coatings. On the basis of these findings, one can conclude that hydrogenated diamond-like carbon is able to interact with salt solutions by substituting the hydrogen with appropriate ions.     

Raman spectroscopy of tris-(hydroxymethyl)aminomethane as a model system for the studies of α-chymotrypsin activation by crown ether in organic solvents

Raman spectroscopy is employed to study tris-(hydroxymethyl)aminomethane and its complexes with 18-crown-6. The results obtained are used to interpret the known effect of α-chymotrypsin activation by crown ether in organic solvents. Raman spectra of the samples lyophilized from aqueous solutions at various pH values are measured in solid state, acetonitrile and cyclohexane.     

Doping of C60 fullerene peapods with lithium vapor: Raman spectroscopic and spectroelectrochemical studies

Raman spectroscopy and in situ Raman spectroelectrochemistry have been applied to the study of the lithium vapor doping of C60@SWCNTs (peapods; SWCNT=single-walled carbon nanotube). A strong degree of doping was proven by the disappearance of the radial breathing mode (RBM) of the SWCNTs and by the attenuation of the tangential (TG) band intensity by two orders of magnitude. The lithium doping causes a downshift of the Ag(2) mode of the intratubular C60 by 27 cm(-1) and changes the resonance condition of the encapsulated fullerene. In contrast to potassium vapor doping, the strong downshift of the TG band was not observed for lithium doping. The peapods treated with lithium vapor remained partially doped even when they were exposed to humid air. This was reflected by a reduction in the intensity of the nanotube and the fullerene modes and by the change in the shape of the RBM band compared with that of the undoped sample. The Ag(2) mode of the intratubular fullerene was not resolved after contact of the lithium-doped sample with water. Lithium insertion into the interior of a peapod and its strong interaction with the intratubular fullerene is suggested to be responsible for the air-insensitive residual doping. This residual doping was confirmed by in situ spectroelectrochemical measurements. The TG band of the lithium-doped peapods did not undergo an upshift during the anodic doping, which points to the formation of a stable exohedral metallofullerene peapod.     

Tuning of electronic properties in thienyl-phosphole pi-conjugated systems through P-functionalization monitored by Raman spectroscopy

Herein, a Raman spectroscopic study of a new family of 2,5-di(2-thienyl)phospholes and thienyl-capped 1,1'-diphospholes is presented. The Raman spectra have been carefully assigned with the help of density functional calculations. For di(2-thienyl)phospholes, two well-differentiated groups of Raman bands exist that arise either from the central phosphole ring or from the outer thiophene substituents. These data reveal a segmentation of the electronic structure. This paper reports interesting relationships between geometrical data such as the BLA (bond-length alternation) parameter and Raman band wavenumbers. These correlations are unprecedented in the chemistry of phospholes and have been used to interpret the evolution of the electronic structure (aromaticity=pi-conjugation) upon 1) substitution of the central sulfur atom of terthiophene by phosphorus and 2) P-functionalization. Increasing the coordination number of the phosphole ring results in intramolecular charge transfer. The best scenario for phosphole aromaticity is found for 1,1'-diphospholes.     

Analysis by Raman and infrared spectroscopy combined with theoretical studies on the identification of plasticizer in PVC films

Raman and Fourier Transform infrared spectroscopy were used to study the vibrational spectra in commercial polyvinyl chloride (PVC) flexible films. In order to have insights of the PVC and the plasticizer structures, density functional theory calculations were conducted via B3LYP hybrid functional. The infrared and Raman calculations took into account geometry optimization of the PVC film, the plasticizer DEHA and the convolution of both. The convoluted spectra were then compared with the experimental data. Finally, the thermal stability of the PVC films was checked through one-hour thermal treatment of the samples comprising temperatures ranges from 50 to 200 °C.     

Electrochemical formation of anodic oxide films on Nb surfaces: ellipsometric and Raman spectroscopical studies

Electrochemical formation of anodic oxide films on niobium (Nb) surfaces in 1 M H₂SO₄ solutions was studied using ellipsometry and Raman spectroscopy. By in situ ellipsometric measurements, the coefficient of film thickness growth and the complex index of refraction of anodic oxide films in the voltage range between 0 and 100 V were determined. The Raman spectra reveal that the thin passive films are amorphous. In the beginning of crystallization, the anodic oxide films consist of mixtures of NbO₂ and Nb₂O₅, while NbO₂ is completely transformed to Nb₂O₅ for thicker and well-crystallized films.     

Heavy-atom tunneling in cyclobutadiene:Ab initio calculation of the intensities ofa g Raman lines

Summary GVB/[5s3p1d/3s1p] calculations were performed on the polarizability surface (₁, ₂, ₃) of cyclobutadiene. The threea _g coordinates refer to automerization (₁), symmetric CC stretch (₂), and symmetric CCH bend (₃). This surface was used together with the previously obtained variational vibrational wave functions for the calculation of Raman intensities. The calculation predicts comparable intensities for the two split components of the automerization Raman line in an isolated molecule and disagrees with observations on matrix-isolated cyclobutadiene. The disagreement is attributed to the asymmetry of the double-well potential imposed by the effect of the Ar matrix.Dedicated to Professor Ruedenberg     

Solute-solvent interactions in formamide and zinc chloride solutions: An investigation by Raman spectroscopy

Raman experiments of formamide and zinc chloride solutions in a wide concentration range (0.1-5.0 mol kg⁻¹) have been carried out. The spectral changes were interpreted on three different ways: (i) the rupture of the H-bonds of FA was evidenced by the trend observed in the ν_CO, δ_HNH and restricted (translation or libration) modes; (ii) the appearance of a new band in the ν_CN region (∼1338 cm⁻¹) was assigned to FA coordinated to Zn (II) through nitrogen atom; (iii) the electronic delocalization in the FA structure upon complexation provided the appearance of new features in the δ_CH and ν_CH regions. The quantitative treatment performed at the ν_CN region of FA allowed the determination of an average number of 3 FA molecules per Zn (II) in the first solvation shell. This value is supported by the appearance of features assigned to ZnCl⁺ and ZnCl₂ entities that also occupy vertices of the tetrahedron at higher salt concentrations. The present study may be useful for a better understanding on electrochemical processes employed in the production of dendritic zinc films as well as FA hydrolysis catalyzed by this metal.     

Phase behavior, conformations, thermodynamic properties, and molecular motion of multicomponent paraffin waxes: A Raman spectroscopy study

Variable-temperature (72–20 °C) studies of Raman spectra (3100–800 cm⁻¹) and thermal analysis of multicomponent paraffin wax have been carried out. The disorder–order transition under liquid–solid transition was observed and their temperature ranges were obtained through the S_lateral order parameter as a function of temperature. From 56 to 43 °C, the paraffin undergoes a conformational state transition of non-extended chain state (NECS) to extended chain state (ECS). The enthalpy and entropy change for the transition obtained by van’t Hoff analysis were 214.286 ± 21 kJ/mol and 0.661 ± 0.066 kJ/mol/K, respectively. The enthalpy determined by differential scanning calorimetry (DSC) was 52.165 ± 5.2 kJ/mol, which is smaller than the van’t Hoff enthalpy due to larger effective non-extended chain state. The variation of Raman spectra with decreasing temperature presents the structure evolution and molecular motion during the crystallization of paraffin wax.     

Raman and differential thermal analysis studies of aqueous CsF solution in the glassy state

The glass transition temperature T _g of an aqueous CsF solution was measured as a function of CsF concentration, and Raman spectra of the solution were obtained for both liquid and glassy states. The results were discussed in comparison with those for aqueous LiX (X=Cl, Br and I) and HF solutions.     

Order-disorder phase transition in p-terphenyl and p-terphenyl: tetracene doped crystals as studied by Raman spectroscopy

Phase transition in crystalline pure p-terphenyl and p-terphenyl: tetracene doped crystals was studied with Raman spectroscopy, for temperatures from 295 to 10 K. In particular, the torsional Raman vibrational mode with a “hard-core frequency” of 230.8 cm⁻¹ was investigated in its frequency and bandwidth dependence upon temperature. The results were analyzed based on an order-disorder model allowing the determination of the activation energies and orientational correlation times of the molecular diffusive process in the monoclinic (above 193 K) and triclinic (below 193 K) phases of the crystals. The activation energy is observed to decrease from the monoclinic to the triclinic phase, whereas the orientational correlation times increase, both in the undoped and the doped crystals. The doping of p-terphenyl with tetracene appears to affect the activation energy and the orientational correlation times in a different way in each phase.