Low-frequency raman-active modes in methoxy-ended and hydroxy-ended oligo(oxyethylene)s

Low-frequency Raman spectra were recorded for α-methyl, ω-methoxy-oligo(oxyethylene)s, C₁E_mC₁ with m in the range 4–25 (i.e., 15–78 atoms). Longitudinal acoustical mode (LAM-1) frequencies were identified and compared with those determined previously for α-hydro, ω-hydroxy-oligo (oxyethylene)s. Nonlinear relationships between LAM-1 frequency and reciprocal chain length were explained as an effect of intermolecular end forces.     

Longitudinal acoustic mode in polymers. IV. LAM-3 of the general perturbed rod and applications to paraffins and polymers

The dependence of the LAM-3 frequency on the various parameters of a perturbed composite elastic rod has been calculated. Together with the previously calculated nonlinear variation of LAM-1 with these parameters, it can be expected that experimental measurements of both of these bands will permit a more detailed analysis of properties of long-chain molecules. The predictions of the theory are in complete agreement with experimental results on n-paraffins. For data on polyethylene and poly(ethylene oxide) reasonable solutions can be found, although the experimental information on polymers is not yet accurate enough or sufficiently devoid of assumptions to permit definitive conclusions to be reached.     

Hydrogen bonding effects on the skeletal optical and the longitudinal acoustical modes in long chain molecules and polymers

Raman spectra of stearyl alcohol (n-C₁₈H₃₇OH) indicate that vibrational bands in both the skeletal optical (1000-1200 cm^?1) and longitudinal acoustical (0–500 cm^?1) regions are considerably perturbed by intermolecular hydrogen bonding. The zone interior skeletal stretching mode reflecting phonon dispersion in the v₄ branch of the infinite polyethylene chain is found at 1105 cm^?1, approximately 20 cm^?1 higher than in the corresponding n-alkane. Similarly the single nodal longitudinal acoustical mode (LAM-1) is found shifted by 12 cm^?1 to higher frequency when the expected “mass effect” produced by the ? OH group is considered. This shift is further increased to 16 cm^?1 at ?100°C indicating a further perturbation on this accordion mode due to the increased strength of the hydrogen bond at the low temperatures. The positions of the higher multinodal vibrations, LAM-3 and LAM-5, are also perturbed by the hydrogen bond but by differing amounts. The observation of a low-frequency Raman-active LAM in polytetrahydrofuran [(? CH₂CH₂CH₂CH₂O? )_n] is discussed in conjunction with the expected effects of hydrogen bonding at the lamellar surface.     

Determination of the distribution of straight-chain segment lengths in crystalline polyethylene from the Raman LAM-1 band

It is shown how the shape of the longitudinal acoustic vibration observed in the low-frequency region of the Raman spectra of crystalline polymers can be used to obtain a quantitative distribution of lengths of straight-chain segments associated with polymer lamellas. The procedure is demonstrated for a “solid-state” extrudate of polyethylene and for a bulk-crystallized specimen of the same polymer. Equations relating the shape of the LAM-1 band to the shape of the distribution curve are given. The low intensity observed for the LAM-3 mode relative to LAM-1 is explained quantitatively without recourse to end effects. LAM-5 has been observed for bulk-crystallized polyethylene. For the extrudate we find the distribution of lengths of straight-chain segments to have a tail on the long-length side which is not present for the bulk-crystallized sample. The Raman technique is shown to provide new morphological data unattainable at present by other methods.     

Low-frequency Raman-active modes and temperature dependences in oligo-oxyethylene

Raman spectra have been recorded for monodisperse and polydisperse samples of oligooxyethylene at several temperatures in the range 310–77 K. Four scattering peaks are found with frequency dependent on temperature. One at 92–80 cm^?1 is unaffected by changes in chain length and is assigned to a lattice mode. The others are chain-length dependent and are assigned either to the longitudinal acoustical modes (LAM-1 and LAM-3)or, less certainly, to a transverse acoustical mode.     

Low-frequency Raman spectra of odd α,ω-disubstituted n-alkanes

Low-Frequency Raman spectra of odd α,ω-dibromo- and α,ω-dihydroxy-n-alkanes were recorded. The longitudinal acoustic mode (LAM-1) frequencies were assigned by references to the published results for n-alkanes and even α,ω-disubstituted n-alkanes and also by taking account of the effects of end intermolecular forces and end-group masses by use of the chain model of Minoni and Zerbi.     

Dynamical Evidence of Aggregates in Isomeric Alcohols Mixtures by O—H Stretching Band Analysis

Abstract

Polarized and depolarized Raman data in mixed isomeric alcohols n-Pentanol and 2-Methyl-2-Butanol are presented. The spectra, obtained as a function of temperature and concentration, have been performed in the intramolecular O—H stretching region. The analysis of the spectral features of the deconvoluted bands, center frequency, percentage intensity and halfwidth, performed in the frame of a previous investigation on the pure components, shows that the aggregative processes, present in these H-bonded liquids, reflect themselves also on this spectral region furnishing evidence that, as evidenciated by other techniques, in the 2M-2BuOH rich region, the formation of hetero-structures, determining a strengthened respect to the more “fragile” pure tertiary alcohol, is present.     

Laser-Raman study of the longitudinal acoustic mode in polyethylene

A Raman band of low frequency, arising from an accordionlike vibration of all-trans \documentclass{article}\pagestyle{empty}\begin{document}$ \rlap{‐‐} ({\rm CH}_2 \rlap{‐‐})_n $\end{document} segments and previously observed in normal paraffins and in polyethylene single crystals, has now also been found in bulk and in cold-drawn polyethylene, both linear and branched. The accordionlike vibration, or longitudinal acoustic mode (LAM), in polyethylene is compared with the LAM in normal paraffins. Whereas the Raman bands corresponding to the third (LAM-3) and higher modes are quite intense in a long-chain paraffin such as n-C₉₄H₁₉₀, they are so weak in polyethylene as to be unobservable with the apparatus used. This is attributed to the presence of the chain fold in polyethylene. Of the two extreme structural models of the fold here discussed, namely the models of “tight folds” and of “loose loops,” only the latter seems capable of accounting for the weakness of LAM-3 and higher modes in polyethylene. A quantity called “nominal Raman length” is defined as the length of that all-trans n-paraffin that would have the same LAM-1 frequency as the polyethylene sample under consideration. The nominal Raman length is always greater than the average long spacing, deduced from discrete x-ray scattering at small angles after applying a Lorentz correction, and, after allowing for chain tilt, is found equal to the segment length between folds. This can be accounted for by both of the models mentioned. As a test of the theory of surface melting the frequency of the accordion vibration of annealed polyethylene single crystals was measured as a function of temperature up to the melting point; no frequency change with temperature was observable. On the basis of the naive idea that there is complete decoupling of the vibrations in the all-trans chain segment from the disordered (molten) surface layer, one would predict that upon surface melting and the concomitant shortening of the all-trans segment, the LAM-1 frequency should increase. A more careful analysis, taking into account the existence of coupling of the LAM to the surface layer, shows that the outcome of this experiment does not necessarily invalidate the idea of surface melting. Bulk polyethylene samples exposed to ⁶⁰Co γ-radiation for doses up to 100 Mrad show a slight shift of the Raman band to lower frequencies, whereas no such shift was observed upon absorption of a swelling agent. A search, without success, was made for a longitudinal acoustic mode in polypropylene, poly(vinylidene fluoride), nylon 66, and polyoxymethylene.     

Measurements of infrared frequency shifts in stressed polymers

A difference-spectrum method is used to measure vibrational frequency shifts in mechanically stressed polymers. It is shown that the peak-to-peak height of the difference-spectrum intensity profile is a linear function of small frequency shifts with a slope inversely proportional to the band halfwidth for Lorentzian or Gaussian bands. The method is applied to measure frequency shifts in uniaxially stressed ultraoriented isotactic polypropylene films, using a double beam infrared grating dispersion spectrometer with a nonstressed sample in the reference beam. It is shown that frequency shifts can be measured with an accuracy of better than 0.01 cm^?1 using the difference-spectrum method.     

Low-frequency raman scattering from methylene-oxyethylene-methylene triblock oligomers

Low-frequency Raman spectra are reported for monodisperse methylene-oxyethylene-methylene triblock oligomers. Assignments are made to the longitudinal acoustical modes (LAMs) of the oxyethylene block and the methylene block. The results for LAM-1 can be modeled by the vibrations of rods with perturbing forces and masses. The results for LAM-3 are not so readily modeled.     

Spectrum of light scattered from polydisperse polymer solutions

We examine several methods for analyzing the spectrum of light scattered from polydisperse polymer solutions. General expressions are reviewed for the inelastic scattering spectra and integrated intensities due to the pure translational normal mode of motion, using both heterodyne and homodyne detection, in terms of the molecular weights, concentrations, scattering form factors, and diffusion coefficients of the individual polymeric species, These results are used to obtain general expressions for the limiting slopes and intercepts of various rearranged versions of the equation I(v) = (A/π)²/(v² + γ²) that permit linear plotting: I(v) is the intensity of light scattered at frequency v, A is the integrated intensity, and γ is the spectral halfwidth, K²D/2π, where K is the scattering vector and D the diffusion coefficient. These results are applied to the special case of a Schulz-Zimm distribution, neglecting form factors, to obtain explicit expressions relating the average diffusion coefficients determined by these procedures to other measurable quantities: the mean polymer radius; the diffusion coefficient of the weight-average species; and, together with the weight-average sedimentation coefficient, the weight-average molecular weight. Numerical calculations for two particular cases indicate the relative merits of the various data analysis procedures. Homodyne detection gives average values that are closer to weight averages than does heterodyne detection.     

Thermo-stimulated luminescence analysis of powder of the gamma-irradiated tetrafluoroethylene/ethylene copolymer

The thermo-stimulated luminescence of Γ-irradiated commercial copolymers of tetrafluoroethylene with ethylene were explored. There are five intense maxima in the luminescence curves. Position, halfwidth and number of maxima remain constant at doses of irradiation from 10 kGy up to 30 kGy. The first peak at 112 K to 119 K has the greatest intensity and is found on the luminescence curves of all of the investigated copolymers except for Tefzel®. For Tefzel®, the maximum occurs at 141 K. The differences of the glow curves in the different copolymers may be the result of impurities capable of reacting with ions or radicals formed under irradiation and by differences in the copolymer’s topological structures.     

Thermophysical properties of macromolecules in the block state

The macromolecules of linear polymers usually consist of regular helices. The paper presents the results of investigation of the concentration, thermal expansion coefficient and vibration amplitude of such helices at different temperatures. These parameters were determined from temperature dependences of the intensity, frequency and halfwidth of regular bands in IR and Raman spectra of polymers.

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Zusammenfassung Die Makromoleküle linearer Polymere zeigen gewöhnlich Helixstruktur. Es wurden Konzentration, Wärmeausdehnungskoeffizient und Schwingungsamplitude derartiger Helizes bei verschiedenen Temperaturen untersucht. Diese Parameter wurden anhand von Intensität, Frequenz und Halbwertbreite der Normalbanden in IR- und Raman-Spektren von Polymeren bestimmt.

     

Optical properties of silver microcrystals prepared by means of the gas aggregation technique

Spherical silver microcrystallites with mean diameters between 10 and 200 Å have been prepared by gas aggregation techniques using argon gas in a dynamic flow system. The crystallites were isolated in a growing argon matrix at 4 K. Typically, a metal volume fraction below 0.1% was used, which was low enough to prevent coagulation into large aggregates and to supress substantially the role of dipole—dipole coupling. In each experiment a small quantity of these crystallites was allowed to condense onto an umorphous carbon foil, which allowed a direct characterisation of the size distribution by evaluating the corresponding electron micrographs. The optical absorption spectra exhibit mainly the commonly observed band due to surface plasma resonance oscillations. The peak position of this band shifts from 3800–3600 Å as the mean microcrystallite diameter decreases from 100 Å to 10 Å. The frequently reported increase of the halfwidth from 0.2 eV to 1 eV or more with decreasing crystallite size could not be confirmed. Rather, a much smaller increase of the halfwidth from 0.1 eV to 0.16 eV was measured.     

LIQUID-CRYSTALLINE PHASE TRANSITION OF MONOMOLECULAR LAYERS OF CHLOROPHYLL a*

Monomolecular layers of chlorophyll a at the air-water interface were investigated by absorption spectroscopy and simultaneous thermodynamic measurements. Whereas at temperatures near 20°C at all pressures, only a liquid phase exists, at a temperature of 4°C, a liquid-crystalline phase transition is observed at a surface pressure of 5 dynes/cm. Pressure-induced changes in the chlorophyll arrangement become evident from a drastic change in the absorption spectra, accompanying the phase transition. The crystalline phase exhibits an extremely narrow absorption band (halfwidth below 9 nm) centered at 698 nm, indicative of a coplanar chlorophyll arrangement in a well-ordered environment. It is highly probable that in these model membranes a chlorophyll arrrangement could be established that is equivalent to the one proposed for the reaction center P700.     

Quasielastic scattering of neutrons from freely jointed polymer chains in dilute solutions

Starting with Kirkwood's Fokker–Planck equation for the polymer configuration-space distribution function and using the Zwanzig–Mori projection operator technique we have calculated the scattering law S_(q,w) for a freely jointed model polymer chain in a dilute solution. When memory effects are neglected, the theory predicts a Lorentzian for S_(q,w) with a halfwidth Ω_(q), which we have determined as a function of the momentum transfer q for all values of q. The results are compared with recent neutron scattering experiments on deuterated polytetrahydrofuran and polystyrene in dilute solution in CS₂. It is found that the observed q dependence of Ω(q) is represented satisfactorily by the present theory with a bond length b of about 6.3 Å for polystyrene and 3.8 Å for polytetrahydrofuran, and a friction coefficient ζ = 4πη0b where η₀ is the viscosity of the solvent.     

Electron hopping and magnetic field dependent spin dynamics of radical ions in solution

The role of hopping on the geminate recombination of radical ions (N,N-dimethylaniline cation and anthracene anion) in acetonitrile is studied via the nanosecond time-resolved magnetic field effect on the triplet yield and the influence of donor concentration thereon. Increasing donor concentration leads to lifetime broadening of the magnetic field dependence of the triplet yield. Responsible for this effect is the perturbation of the coherent spin motion caused by hopping of the electron spin between donor sites of different nuclear spin configuration. Comparison of experimental results with calculations based on the semiclassical theory of spin motion yields an estimate of the hopping rates. Deuteration of both radicals influences the halfwidth of the magnetic field effect: at long probing times and low donor concentrations the halfwidth measured for protonated radical ions exceeds the one for the deuterated species: at short delay times and large donor concentrations, i.e. high hopping rates, this isotopic effect is reversed.     

Intensity and polarization of light emitted in slow ion-atom collisions

We study the intensity and polarization of light emitted during slow ion-atom collisions. We describe the nuclei as moving along classical trajectories while the electronic rearrangement is treated using time-dependent molecular orbitals. The intensity of emitted light is calculated from the diatomic time-dependent dipole. We evaluate the diatomic dipole matrix elements involving 1s, 2s, and 2p traveling atomic orbitals suitable for time-dependent collision studies. We calculate the intensity and the polarization of light emitted in p + H(1s) collisions at kinetic energies from 10 to 1000 eV, for several impact parameters, changing over time. The emitted intensity goes through a maximum as the collision energy increases and lasts between 10 and 1 fs; the polarized light components parallel and perpendicular to the incoming beam direction show pronounced dependences on impact parameters and time. © 1996 John Wiley & Sons, Inc.     

Spectral properties of intramolecular charge transfer fluorescence probe 1-keto-2-(p-dimethylaminobenzal)-tetrahydronaphthalene

The photophysical properties of a new compound 1-keto-2-(p-dimethylaminobenzal)-tetrahydronaphthalene in various solvents at room temperature were characterized by the absorption and steady-state fluorescence technique. The bathochromic shift on the emission spectra, the broad halfwidth of the fluorescence band and the increase in the excited state dipole moment occurred. These results gave the evidence about the intramolecular charge transfer (ICT) character in the emitting singlet state of the compound.     

Theoretical fundamentals of high resolution Auger electron spectroscopy

A method is discussed for obtaining narrow Auger electron lines of atoms, molecules and solids with a halfwidth much smaller than the natural width Γ_n of a respective intermediate X-ray excited state |n>. The method provides a possibility of resolving diverse fine structures in Auger spectra.