E. Mulazzi

Properties of stretched trans(CH)x systems: Analysis of polarized resonant Raman scattering

Abstract Polarized Resonant Raman Scattering (RRS) spectra of trans (CH) x taken at different excitation wavelengths ( λ L =647 nm and 458 mm) have been recorded for oriented samples obtained by using a new catalyst for the synthesis. The experimental results display a similar behaviour to that of RRS spectra recently analyzed. In particular, the ‖ ‖ spectra at λ L =458 nm show two bands with a double peaked sufficiently well resolved structure, due to the contribution of long and short conjugation length segments. On the contrary, the |; ‖, ‖ |; , |; |; polarized spectra only display two structureless bands peaked at higher frequencies with respect to the previous ones. These results confirm the theoretical predictions. Oriented trans (CH) x doped with iodine have also been studied in polarized RRS. A detailed analysis of the band shapes and of the depolarization ratios shows differences with respect to the undoped trans (CH) x case in agreement with a modification upon doping of the bimodal chain length distribution.

Steady state and transient photoluminescence in poly-p-phenylene vinylene films and nanofibers

We report in this paper experimental data on steady state and transient photoluminescence of poly-p-phenylene vinylene in the form of nanofibers prepared with a template method and converted at 110 degrees C. Results are compared to those obtained from films of different thicknesses converted at the same temperature. Data are analyzed by a model of bimodal distribution of conjugation lengths and the photoluminescence band shapes, evaluated in the framework of this model, are also presented.

Resonant Raman scattering of controlled molecular weight polyacetylene

Polyacetylene, (CH)x, films of 500, 5300, 10 500, and 100 000 Daltons number average molecular weights (Mn ) were synthesized using the titanium tetra‐n‐butoxide/triethyl aluminum‐catalyst/cocatalyst system and examined using resonant Raman scattering techniques. Before isomerization, trans segments are found to exist mainly as short, isolated sequences independent of Mn. After thermal isomerization, theoretical analysis of the RRS spectra using the Brivio, Mulazzi model indicate the ratio of long trans conjugated segments (N≥30) to short trans conjugated segments (N≤30) is significantly larger for 100 000 Dalton polymer in comparison to polymer of 10 500 Mn and below. For samples below 10 500 Daltons, no clear relationship between actual polymer molecular weight and G is observed. Optimization of the isomerization conditions for 100 000 Dalton polymer results in trans‐(CH)x with a G=0.80. These results suggest that not until very long molecular chains are obtained can samples composed principally of long...

Evidence of temperature dependent charge migration on conjugated segments in poly-p-phenylene vinylene and single-walled carbon nanotubes composite films.

We present new results of temperature dependence of photoluminescence spectra carried out on poly-p-phenylene vinylene (PPV) and on PPV composite films with single-walled carbon nanotubes. By performing studies at different temperatures (87 and 300 K), we show that a distribution of conjugated PPV segments is needed to interpret experimental data. At the microscopic scale, such a distribution corresponds to the morphological picture of poorly packed short chain segments and well-packed ordered long chain segments. Within this scheme, a new interpretation emerges for explaining the specific behavior of the photoluminescence bands. In particular, the two most intense components of the photoluminescence spectra of PPV thermally converted at 300 degrees C (2.23 and 2.43 eV at 300 K) change drastically their relative intensity when the observation temperature decreases. This effect is interpreted as due to the inhibition of charge migration to longer segments and to radiative recombination occurring mainly on n = 5 conjugated segments.

Spectroscopic properties of polyacetylenes synthesized via three modifications of Ziegler-Natta catalytic system

Abstract Results of spectroscopic measurements of three types of highly conducting polyacetylene are presented. A comparison is made of the traditional Shirakawa (or S-(CH) x ), Naarmann (or N-(CH) x ) and Tsukamoto (or v -(CH) x ) polyacetylenes. The non-conducting undoped materials have been characterized by 13 C CPMAS NMR. FT-IR and resonance Raman scattering. A higher conductivity for iodine-doped N- (CH) x and v -(CH) x ) in comparison with S-(CH) x is observed even though a greater sp 3 content is found in N-(CH) x and v -(CH) x films. Since the sp 3 hybridized carbons may belong to catalyst residues, the sp 3 content does not correlate with conductivity. On the other hand, it is shown from the resonance Raman scattering data that a longer average conjugation length in N-(CH) x and v -(CH) x may correlate with the higher conductivities of the iodine-doped films.

Optical Properties of PPV and PPP Polymers

We present experimental optical absorption and Raman data of PPV and PPP. Theoretical calculations of the band shapes of absorption and Raman spectra of both polymers are reported. In particular the dependence of the Raman band intensities as function of the excitation wavelengths is theoretically accounted for, in very good agreement with experimental data. These features together with the absorption band shapes can be simulated if a distribution of conjugated segments of different lengths is introduced in the calculation.

Interpretation of photoinduced infrared spectra and doping induced Raman scattering in trans-polyacetylene

Abstract We propose an interpretation of the photoinduced infrared spectra of the cis rich (CH) x and fully isomerized trans (CH) x in the framework of the perturbed Green function method in which the lattice dynamics of polyacetylene is considered. The perturbation induced by the photogenerated charge on the lattice is accounted for by a positive parameter Λ which simulates the changes of the force constants due to the modification of the electron-vibration and electron-electron interactions in the excited states with respect to the ground state. By using the same model and by considering a positive change Λ of the force constants, the doping induced Raman modes in n doped trans (CH) x are also well accounted for.

Polarized Raman spectra of oriented polymer systems: polyacetylene and copolymer polyacetylene-polynorbornene

Abstract In this paper, we present experimental and theoretical Resonance Raman studies carried out on oriented polymer systems. It is shown that different oriented trans polyacetylene samples exhibit Raman features which depend upon the stretching ratio. Also, the depolarization ratio of the Ra,an bands have been found to depend upon the isomerization temperature. Oriented copolymers polyacetylene/polynorbornene, (CH) x - (NBE) n - (CH) x , have also been studied. Raman spectra of the trans-(CH) x parts of these compounds have been analyzed in the frame of the bimodal distribution model. It is shown that whatever x and n values, results are rather similar in terms of conjugation length. In addition, from IR studies, one can deduce that in the stretched oriented copolymers, polynorbornene and polyacetylene segments have a different orientation degree.

Study of the properties of trans-polyacetylene through the Raman spectra analysis

Abstract We present a complete set of experimental results obtained by Resonant Raman Scattering from several samples of polyacetylene: different molecular weight and fully oriented trans- and cis-(CH) x films. The peak positions and the shapes of the Raman bands due to the vibrational modes of the trans conjugated segments either in a cis-rich sample or in a fully isomerized one, are interpreted by using a bimodal distribution of conjugated segments. On the other hand, the features of the Raman bands due to the vibrational modes of the cis chains are explained in terms of the properties of only long conjugated segments. Raman spectra of thermally isomerized trans-(CH) x obtained from a free standing oriented cis-(CH) x are also presented. The depolarization ratios of the Raman bands experimentally determined in the case of the oriented cis-rich (CH) x and trans-(CH) x and of the thermally isomerized trans-(CH) x are discussed. Finally, within the frame of the dynamical model used in this study, the new features observed in the Raman spectra of n-doped (CH) x are interpreted in terms of a perturbation due to the dopant on the polymeric chain.

A lattice dynamical calculation of photoinduced IR spectra of transpolyacetylene

Abstract Photoinduced IR spectra of trans (CH)x are interpreted in terms of a lattice dynamical calculation. We start from the lattice dynamical properties of trans (CH)x as an infinite dimerized chain and introduce an external perturbation to describe the new dynamics of the photoexcited system. By means of the Green function technique, we work out all the relevant dynamical properties of the perturbed system: the values of the IR photogenerated vibrational frequencies and their intensities. Effects of different conjugation length segments are also taken into account in such a model.