Polyacetylene Films Research Articles

Polarized resonance Raman scattering of iodine-doped polyacetylene with high conductivity

We investigated the structure and alignment of iodine dopant in the highly conducting polyacetylene (HCPA) with different doping concentration by using polarized resonance Raman spectroscopy (PRRS). It was found for the first time that the I 3 − species is predominant in the highly stretched HCPA film even for the highly doping level ((CHI 0.25) x), in contrast with traditional polyacetylene. We also observed that the intensity ratio of I 3 −/I 5 − and the orientation of I 3 − species increase greatly with increasing initial draw ration of the polyacetylene films.

Various factors affecting the synthesis of highly conducting polyacetylene

The preparative conditions of highly conducting polyacetylene films by acetylene polymerization with a Ti(n-OBu) 4Et 3Al catalyst have been investigated in terms of preparative method of the catalyst, polymerization method, purification method of prepared films and drying method of the films. Highly stretchable films were prepared by the use of a catalyst aged at a temperature higher than 80°C and the stretchability increased with increasing concentration of the catalyst, resulting in higher values of parallel conductivity in the stretch-oriented iodine-doped samples. However, these values were limited to (3–4) × 10 4 S cm −1 due to a decrease in conductivity of the as-prepared films with either increasing aging temperature or concentration of catalyst.

Polymerization of acetylenes with rare earth coordination catalysts

AbstractThe polymerization of acetylene and its derivatives by rare earth coordination catalysts and the characterization of the polymers so obtained in our laboratory are reviewed.Because of the metallic conductivity possessed by doped polyacetylene and the unique properties such as conductivity (semiconductivity), paramagnetism, migration and transfer of energy and chemical reactivity and complex formation ability often shown by acetylenic polymers, which seem promising as specialty polymers, there has been considerable interest in the polymers of acetylene and its derivatives. A wide variety of catalyst systems have been developed for the polymerization of acetylenes. But there has been no information concerning the use of rare earth compounds as catalysts in the polymerization of acetylene and its derivatives. We for the first time in 1981 have succeeded in the polymerization of acetylene with rare earth coordination catalysts, which in turn is a development based upon earlier work on the diene polymerization using rare earth coordination catalysts(Ref. 1). Using rare earth catalysts, acetylene can be polymerized conveniently into high cis polyacetylene films with metallic sheen at room temperature and phenylacetylene can also be polymerized into high molecular weight, high cis polyphenylacetylene films at ambient temperature. Thus new varieties of polyacetylenes have been developed and a novel family of coordination catalysts consisting of a rare earth compound plus trialkyl aluminum for the polymerization of acetylenes has been exploited. This article reviews our studies on the polymerization of acetylene and its derivatives with rare earth coordination catalysts and on the characterization of the polyacetylenes prepared.

Pressure dependence of the conductivity and magnetoconductance in oriented iodine-doped polyacetylene.

The temperature dependences of conductivity and magnetoconductance (MC) have been measured, both parallel and perpendicular to the chain axis, in oriented polyacetylene films doped with iodine. The evolution of anisotropy in conductivity as a function of doping time has been studied. Hydrostatic pressure increases the conductivity for pressures up to 4 kbar; at higher pressure the conductivity gradually decreases. The reduced activation-energy function [W=-T[d(ln\ensuremath{\rho})/dT] is temperature independent in the intermediate temperature region (T\ensuremath{\approxeq}180--60 K) suggesting that the material is just on the metallic side of the critical regime of the disorder-induced metal-insulator transition. However, at 8 kbar, W(T) exhibits a positive temperature coefficient from 180 to 1.2 K, implying that the system is more metallic due to enhanced interchain transport. The ${\mathit{T}}^{1/2}$ dependence of conductivity and the enhanced negative MC, at very low temperatures, indicates that the role of electron-electron interactions is significant. MC measurements have been carried out in all the five different combinations of current with respect to the chain axis and field directions. The sign of MC is positive (negative) when the field is perpendicular (parallel) to the chain axis. The anisotropy in MC is attributed to the anisotropic diffusion coefficient. This is shown by the enhanced positive contribution to the MC at 8 kbar, when the field (current) is parallel (parallel) to the chain axis.At 1.2 K, when both the magnetic field and current are perpendicular to the chain axis, the field-induced crossover from positive to negative MC shows the subtle interplay of weak localization and electron-electron interaction contributions to MC. The inelastic scattering lengths, at 1.2 K, in parallel and perpendicular directions to the chain axis are found to be 1163 and 210 \AA{}, respectively. The ${\mathit{T}}^{\mathrm{\ensuremath{-}}3/4}$ dependence of the inelastic scattering length at low temperatures is consistent with the ${\mathit{T}}^{3/4}$ dependence of conductivity. This indicates that the inelastic electron-electron scattering known to be of importance in disordered metals is the dominant scattering mechanism in iodine-doped oriented polyacetylene.

Synthesis and Properties of Polyacetylene Films Prepared by Ti(OEt)4–AlMe3 Catalyst System

Abstract Novel homogeneous catalyst system of composed tetraethoxytitanium–trimethylaluminum, Ti(OEt)4–AlMe3, is employed for synthesis of polyacetylene film. It is confirmed that the present system has a high catalytic activity for acetylene polymerization, and its activity is comparable to that of the representative Ti(O-n-Bu)4–AlEt3 catalyst system. Experimental conditions, especially for preparation and aging of the catalyst, are optimized in view of properties of polyacetylene film such as cis content, mechanical stretchability, and electrical conductivity. The homogeneity of the present catalyst aged at room temperature enables us to synthesize highly conductive non-stretched and stretched films whose electrical conductivities after iodine doping are 2.0 × 103 and 1.2 × 104 S cm−1, respectively. Discussion has also been made in terms of the effect of high-temperature aging for the catalyst, the role of co-catalyst as a reductant, and the dependence of electrical conductivity on film thickness.

Dependence of electrical conductivity on film thickness in polyacetylene — effects of mechanically forced parallel and in-plane alignments of fibrils

We have presented conventional analytical formulae which correlate the electrical conductivity of polyacetylene film with its macroscopic morphology represented by mechanically forced fibril alignment (parallel alignment) and in-plane alignment. Through numerical evaluations based on the present analytical formulae, we have clarified that the in-plane alignment contributes to enhance the parallel conductivity of the film in a cooperative manner with the parallel alignment. In particular, the in-plane alignment becomes crucially dominant when the film thickness is less than 1.0 μm, irrespective of whether the film is as-grown or mechanically stretched. On the other hand, although the parallel alignment expectedly decreases the conductivity perpendicular to the elongation direction along with the mechanical stretching, the decrease is partly com pensated by an increase due to the in-plane alignment.

Novel Morphology of Vertically Aligned Polyacetylene Film

Abstract Liquid crystal polymerization method was modified in order to prepare a vertically aligned polyacetylene film. As a result, a film with a very curious morphology was formed. The film was composed of two layers. The layer on the solvent side was a vertically oriented fibril structure, whereas the layer on the acetylene gas side was a randomly oriented fibril structure. The modified method is a very promising one for visualizing the polymer growth process, and the present work clearly demonstrates that a polyacetylene chain grows in the liquid crystal solvent.

The temperature dependence of conductivity anisotropy in intermediately doped polyacetylene

The temperature dependence of electrical conductivity and conductivity anisotropy of FeCl 3 doped stretch-oriented polyacetylene film at different doping levels were measured by using Montgomery technique. The increase of conductivity anisotropy upon doping is related to the aligned fibrillar structure and the interfibrillar contact effect. Weak temperature dependence, the slight decrease at low temperature and the persistence of anisotropy through aging indicate that the interfibrillar contact barriers limit the quasi-one-dimensional charge transport in heavily doped polyacetylene and play a major role in the conductivity anisotropy.

Intrinsic non-solvent polymerization method for synthesis of polyacetylene films

We have developed a novel solvent-free polymerization method for synthesis of highly stretchable and highly conducting polyacetylene film. In this procedure, we prepare a catalyst by mixing tetraalkoxytitanium, Ti(OR) 4, with trialkylaluminum, AIR 3, without solvent. The catalyst is aged at room temperature for 1 hour followed by an aging at high temperature of 150°C for 1 hour, and then it is subjected to an acetylene polymerization at −78°C. Since the procedure is completely free from solvent until the end of polymerization, it can be called an ‘intrinsic non-solvent’ polymerization method. The method enables us to prepare an ultimately concentrated catalyst that is promising to synthesize polyacetylene films with high density and high stretchability. A prefereble Al/Ti ratio is found to be 4 – 5, but not 2 that has been optimized for high-temperature aged catalyst. This result suggests that the co-catalyst, AIR 3, might play a role of solvent towards catalytically active complexes formed between Ti(OR) 4 and AIR 3.

Comparative high resolution NMR studies of n- and p-type doped polyacetylene films

High resolution NMR provides unique abilities to study either properties of the polymer chains or behavior of the doping agent in the conducting polymers. Magic angle spinning proton decoupled 13C NMR was applied to n-type sodium doped, (CHNa y) x (y ≤ 0.3), and p-type iodine doped, [CH(I 3) y] x (y ≤ 0.19), polyacetylene films prepared by Shirakawa's method. Also 2-dimensional nutation spectra of half-integer spin ( I = 3 2 ) quadrupolar 23Na nuclei were measured. All the spectra were studied as a function of the dopant content, y, determined by weight uptake after doping. The films were controlled by measurement of the conductivity which was about 10 and 50 Ω −1 cm −1 for heavily iodine and sodium doped material, respectively. We discuss our results on: (i) formation of the extended polycarbonium ions (charged solitons), (ii) doping induced cis/trans isomerization, (iii) partial compensation of (CHNa) x films by oxygen and moisture, (iv) intersite electron-electron correlations on the polymer chain, and (v) ordering of the sodium ions and occurrence of at least two different crystal phases of the polyacetylene-sodium system.

Spectral characteristic of metallic state of polyacetylene

The absorption, reflection and ultraviolet photoemission spectra (UPS) of thin films of polyacetylene doped with perchlorate ion or potassium are studied to find the mechanism of insulator to metal transition. The transition occurs by two steps, first the charged soliton chain showing the mid hap absorption is formed in the lightly doped film, and then the metallic state is found in the heavily doped film, which is composed of polson units. The strong far infrared absorption is spectral characteristic band of polson unit. The polson chain gives a metallic far infrared reflectance along the chain direction in the heavily doped film. In the heavily doped film a genuine metallic Fermi level is found in the UPS spectra. The results are interpretted by the molecular orbital calculation of doped polyacetylene which is presented in the accompanied paper.

Mechanism of acetylene polymerization on different types of catalysts. synthesis of high stable polyacetylene

The poymerization of acetylene with different types of catalysts was investigated. Most of them have been used for this purpose first time. These are new Re catalysts and new class sterically straitened catalysts. Use of different catalysts has permeated to regulate the ratio between the rate of polymer chain grows and rate of the solid state formation. As a result the films of polyacetylene (PA) and PA - polyvinylbutyral (PVB) blends with 40 – 50% trans form and very low content defects were obtained. Spectral parameters of obtained PA samples were the same as in PA obtained the other methods. However the rate of their oxidation was considerable lower. For example the process of cis-trans isomerization of PA-PVB thin films could be produced in air atmosphere without oxidation. The most stable films could be kept in chemical laboratory more then two years without change.

Structure and optical properties of polyacetylene prepared via an improved Durham-route precursor polymer

Unoriented and stretch-oriented polyacetylene samples prepared via an improved Durham precursor polymer have been investigated. The standard catalysts for the Durham precursor polymerization have been replaced by a Schrock-type molybdenum-based initiator, which gives a narrower distribution in molecular weight and better stereoregularity in the precursor polymer. We have looked at the process of orientation achieved by stretching free-standing films during thermal conversion. Mixtures of high and lower molecular weight precursor polymers, giving a bimodal distribution, are more readily stretched than polymers with a monomodal molecular weight distribution, and we find that these oriented films show significantly higher levels of orientation and larger crystallite sizes than films prepared from the standard Durham-route precursor. Unoriented films of polyacetylene show optical absorption similar to those of the standard Durham polyacetylene, though there are significant differences in the photoinduced absorption spectra.

Mössbauer spectroscopy of highly-oriented polyacetylene doped with 129I

The Mössbauer spectroscopy has been applied to high density polyacetylene films doped with 129I. In addition to the well-known polyiodide anions, I 3 and I 5 −, a new iodine species I α has been found in the present study. The new species I α has been assigned to the iodine atom covalently bound to the chain carbon. Although the iodine Iα introduces sp 3-carbon defects interrupting the alternation of single and double bonds of the chain, the amount of I α has been estimated to be lower than a critical sp 3-carbon concentration which is a theoretically predicted value to break the highly conductive phase of iodine-doped polyacetylene. From the angular dependent Mössbauer spectra obtained under the condition that the oriented direction of the polymer is parallel and perpendicular to the propagating direction of the incident γ-rays, it has been revealed that the molecular axes of I 3 − and I 5 − are aligne along the oriented direction of the polymer.

Picosecond transient resonant Raman scattering in partially isomerized polyacetylene films

Photoexcitation dynamics in the picosecond time domain in partially isomerized (CH) x films has been measured at room temperature, using the technique of transient photoinduced resonant Raman scattering (TPRRS). We show that TPRRS gives considerably more information than transient photoinduced absorption spectroscopy. Following the distinctively different decay kinetics of cis and trans resonantly enhanced vibrations, we have measured the time evolution of photoexcitations in trans and cis segments, separately. We observed a migration of excitons from cis to trans segments, with a time constant between 110 and 215 ps depending on the cis/trans ratio in the films.

Electron spin resonance studies of FeCl 3-doped and K + ion-implanted polyacetylene films

The mechanism of K + ions implanted in both pristine polyacetylene and FeCl 3-doped polyacetylene has been studied using an electron spin resonance (ESR) spectrometer and a four-probe technique. The temperature dependence of ESR characteristics (spin susceptibility, peak-to-peak linewidth) for the samples has been analysed within the soliton model. The ESR results are consistent with our previous studies.

Synthesis and properties of highly conductive polyacetylene films with high mechanical strength

Synthesis and properties of highly conductive polyacetylene films with high mechanical strength

Surface photodegradation and modification of some substituted polyacetylene films

Surface photodegradation and surface modification by graft copolymerization have been carried out on films of two substituted polyacetylenes, viz. poly(( o-(trimethylsilyl)phenyl)acetylene) or poly( o-Me 3SiPA) and poly(1-chloro-2-phenylacetylene) or PCPA. X-ray photoelectron spectroscopy (XPS) data indicate that surface degradation induced by near-UV light in the former involves oxidation of the Si hetero-atoms to the Si-O species, whereas the latter polymer photodegrades via the loss of the Cl hetero-atoms. The surface photodegradation processes in both polymers are facilitated by the presence of O 2 and H 2O. Both polymer films undergo near-UV induced graft copolymerization with acrylamide (AAm), acrylic acid (AAc), Na salt of styrene sulfonic acid (NaSS) and 3-dimethyl(methacryloyloxyethyl) ammonium propane sulfonate (DMAPS) to give more hydrophilic surfaces. The graft density in each case is substantially enhanced for polymer films pre-treated with Ar plasma. Near-UV light induced graft copolymerization in all cases also results in the surface photodegradation of the two acetylene polymer films.

Synthesis of Highly Conducting Polyacetylene Thin Films

Preparative condition of a Ti(O-n-Bu) 4 -AlEt 3 catalyst and polymerization methods were investigated for synthesis of highly conducting polyacetylene films. One of the most important factors is thermal aging of the catalyst. Those films prepared by a catalyst aged over 80°C have good stretchability, high bulk density and high electrical conductivity. On the contrary, those films synthesized by a catalyst aged at room temperature below 80°C are porous and not stretchable. An aging at 150°C for 5 minutes is enough to produce a stretchable highly conducting film. Composition of the catalyst system is also an important factor determining properties of the films

Intrinsic nonsolvent polymerization method for synthesis of highly stretchable and highly conductive polyacetylene films

Intrinsic nonsolvent polymerization method for synthesis of highly stretchable and highly conductive polyacetylene films