Conjugated Polymers and Aromaticity

Interfacial studies on model composites by laser Raman spectroscopy

The visible optical spectrum of two typical polydiacetylene (PDA) crystals, $\mathrm{poly}[2,4\ensuremath{-}\mathrm{h}\mathrm{e}\mathrm{x}\mathrm{a}\mathrm{d}\mathrm{i}\mathrm{y}\mathrm{n}\mathrm{e}\ensuremath{-}1,6\ensuremath{-}\mathrm{d}\mathrm{i}\mathrm{o}\mathrm{l}\ensuremath{-}\mathrm{b}\mathrm{i}\mathrm{s}\ensuremath{-}(p\ensuremath{-}\mathrm{t}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{e}\mathrm{n}\mathrm{e}\phantom{\rule{0ex}{0ex}}\mathrm{s}\mathrm{u}\mathrm{l}\mathrm{f}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{e})]$ (PTS) with an acetylene-like structure and poly[5,7-dodecadiyne-1,2-diol-bis-(phenylurethane)] (TCDU) with a butatriene-like structure, is investigated with the use of a first-principles Green's-function formalism of charge-transfer exciton theory The energy-band structures of the ground state are calculated first at the Hartree-Fock level with nonlocal exchange and they are corrected afterward for electron correlation effects using the electron-polaron model of Toyozawa The electron-hole interaction ${\stackrel{^}{H}}_{e,h}$ is treated by first-order perturbation theory in the framework of the Lax-Koster-Slater resolvent method as proposed by Takeuti The influence of screening on ${\stackrel{^}{H}}_{e,h}$ is also investigated with the help of an $\stackrel{\ensuremath{\rightarrow}}{\mathrm{r}}$-dependent static dielectric function The results show that correlation corrections play an important role in the calculation of the optical spectrum since they substantially modify the Green's function of the host crystal as well as ${\stackrel{^}{H}}_{e,h}$ Both the singlet and triplet excitons are found to be delocalized with a radius of 25-30 \AA{} in PDA's: the Frenkel exciton contributes only \ensuremath{\sim}30% of the binding energy The calculations predict the first-singlet absorption maximum for PTS at 21-22 eV and for TCDU at 17-18 eV, with an exciton bandwidth of \ensuremath{\sim}29 and \ensuremath{\sim}35 eV, respectively The exciton binding energy is found to be \ensuremath{\sim}04 eV for both systems Triplet levels are located at 08-09 eV below the singlets at $K=0$ and they form relatively flat bands The theoretical ionization potentials are 57 and 50 eV for PTS and TCDU crystals, respectively

Green's-function study of optical properties of polymers: Charge-transfer exciton spectra of polydiacetylenes

Resonance Raman excitation profiles have been measured between 1.8 and 2.8 eV for four zero-wavevector phonons of a single crystal of toluene sulphonate diacetylene polymer. The strength of the coupling between the electronic excited state at 2 eV and these phonons has been estimated with the aid of a simple theoretical model. The shifts in the phonon frequencies between the ground and excited states have been determined and a lattice dynamical model has also been used to estimate the interatomic separations and force constants along the polymer backbone in the excited state.

https://iopscience.iop.org/article/10.1088/0022-3719/15/13/025/pdf

An investigation of the electronic excited state of a polydiacetylene by resonance Raman spectroscopy

The technique of Laser Raman Spectroscopy has been applied in the study of aramid fibers, such as Kevlar 49, and aramid/epoxy interfaces. A linear relationship has been found between Raman frequencies and strain upon loading a single Kevlar 49 filament in air. Model composites of single Kevlar 49 fibers embedded in epoxy resins have been fabricated and subjected to various degrees of mechanical deformation. The transfer lengths for reinforcement have been measured at various levels of applied tensile load and the dependence of transfer length upon applied matrix strain has been established. Finally, by balancing the tensile and the shear forces acting along the interface, the interfacial shear stress (ISS) distribution along the embedded fiber was obtained. 52 refs.

Interfacial Shear Stress Distribution in Model Composites, Part 1: A Kevlar 49® Fibre in an Epoxy Matrix

The second order phase transition at 195 K which occurs in bis (p-toluene sulfonate) diacetylene polymer crystals has been studied by optical reflection and transmission, Raman and far-infrared spectroscopy. Order parameter fitting to the data suggests that at low temperatures the transition has a two dimensional character while closer to the transition it appears to be three dimensional. There are indications of a broad transition region which might be expected for a predominately two dimensional phase transition in which fluctuations dominate the behaviour of the system. Analysis of new X-ray crystallographic data gives some support to a structural model in which the transition would have a substantial two dimensional character.

A Spectroscopic Study of the Second Order Phase Transition in Bis (p-toluene sulfonate) Diacetylene Polymer Crystals

The pressure dependence of reflection, absorption and resonant Raman spectra of single crystals of toluene sulphonate dicetylene polymer have been measured under hydrostatic conditions to 5 kbars. The peak of the reflection spectrum near 2 eV shifted to lower energies with increasing pressure at a rate of 7.3+or-0.3 meV per kbar. This result is shown to be in qualitative agreement with the prediction of molecular orbital calculations. The temperature and pressure dependence of the resonant Raman spectra are discussed using a simple point mass and anharmonic spring constant model.

A. C. Cottle

Papers

A Spectroscopic Study of the Second Order Phase Transition in Bis (p-toluene sulfonate) Diacetylene Polymer Crystals

Pressure and temperature dependence of the electronic and vibrational excitations of a conjugated polymer crystal