Polyacetylene Films Research Articles

New process for the production of metal-like, stable polyacetylene

An acetylene polymerization reaction catalysed by an aged mixture of Ti(OC 4H 9) 4/Al(C 2H 5) 3 in a silicone oil reaction medium yields a homogeneous, defect-free, polyacetylene film that can be stretched mechanically by up to 550%, corresponding to stretching rates of 6.5. On being doped with iodine, the film displays an electrical conductivity of ≈ 16 000 S/cm per unit weight. Both washed (catalyst-free) and unwashed (catalyst-containing) films were stretched. An aged mixture of Ti(OC 4H 9) 4/Al(C 2H 5) 3 in silicone oil that has been mixed with various quantities of n-butyllithium and allowed to react with acetylene yields polyacetylene that is highly regular, compact and crystalline in well-defined parallel planes. This film can be stretched by up to 400% ( l/ l o = 5) and, after doping, gives a conductivity of >100 000 S/cm per unit weight.

Neutral photo-excitations in oriented polyacetylene

Polarisation-dependent measurements of photo-induced absorption (PA) have been carried out on highly oriented films of polyacetylene prepared by the Durham precursor route. For temperatures below 100 K, the PA spectrum exhibits an absorption peak at 1.36 eV, as observed for Shirakawa polyacetylene. This feature is due to a neutral excited state. The PA peak is independent of probe-beam polarisation and some 200 times weaker than in Shirakawa samples. These results suggest that the neutral excitations are rendered metastable by interactions with existing structural defects in the polymer.

The Cyclic Voltammetry of Pristine, Isomerized, Deuterated, and Partially Hydrogenated “P‐Type” Polyacetylene

The cyclic voltammetry of ultrathin films of polyacetylene comprised of 2–3 nm microfibrils and their aggregate 20 nm fibrils in methylene chloride, acetonitrile, propylene carbonate, and sulfolane shows many similarities, including a background oxidation of electrolyte that leads to serious coulombic inefficiency in rechargeable polyacetylene battery applications. Reversible charging in neutral aqueous solution is limited by the poor separation between the reversible and irreversible oxidations. Comparison of ultrathin and free‐standing 100 μm thick films showed they have the same maximum reversible doping level and thus energy density of 280 Wh/kg. Pristine, isomerized, deuterated, and hydrogenated show similar electrochemistry. Both n and p‐type chemical doping caused isomerization of to .

Polymerization of Acetylene with New Catalytic Systems and Optimization of the Properties of the Polymers

Abstract This paper reports a comparative study of different catalytic systems for the synthesis of novel polyacetylene films. That the morphology could be optimized by the use of a highly viscous complexing solvent together with thermal treatment [1,2] encouraged us to study the catalytic systems with the aid of ESR spectroscopy. The different samples were characterized with the aid of IR and ESR spectroscopic methods, conductivity measurements, and scanning electron microscopy before and after doping.

Kinetics of chemical n-doping of polyacetylene

Polyacetylene (PA) films were doped with tetrahydrofuran and benzene solutions of organo potassium and lithium compounds. The kinetics for doping were determined from the variation of the dopant concentration with time, as a function of film thickness, dopant concentration and doping temperature. The experimental doping rates were compared to computed values deduced from a doping model taking into account the interfibrillar diffusion, separately measured with an inert hydrocarbon. This comparison allows a minimum value (10 −15 cm 2/s) to be determined for the intrafibrillar diffusion coefficient D f of doped sites and cations. With such a value no homogeneous doping can be carried out at intermediate doping levels, for films thicker than about 50 microns, by a simple control of doping time or dopant concentration. This may, however, be achieved by a thermodynamic control using dopants of suitable redox potential.

Effect of pressing on the diffusional properties of polyacetylene

AbstractPressing of polyacetylene films before (or after) sorption of dopant can reduce significantly the rate of sorption (or desorption) due to reduced effective surface area and pore size, as is found in the experiments of vapour sorption and desorption of hexane and iodine. On exposure to air, the pressed films have significantly lower oxidation rates at 120°C than the unpressed ones.

Photoablation of plasma polymerized polyacetylene films

Polyacetylenes are the simplest organic polymers which readily undergo photoablation. We describe and justify the use of a less well-known technique for synthesizing polyacetylenes and characterize the films so formed. This is of interest because polyacetylenes are organic semiconductors. Glow discharge polymerization yields layers around 1 μm thick. Efficient ablative decomposition of polyacetylene films requires multishot exposures. The role of conjugation in the chains and efficiency of the process as opposed to thermal decomposition is discussed. Because the polymer contains only carbon and hydrogen, ablation in vacuum yields readily identifiable products.

Polyacetylene batteries -scaling up and problems

There is no significant difference between the charge-discharge characteristics of the large-scaled polyacetylene battery (27 cm 2-cell x 4) and that of 2 cm 2 battery. However, the doping inhomogeneity and the non-uniformity of the film thickness cause trouble to enlarge the cathode area. The several problems to be improved, such as self-discharge, degradation of polyacetylene film, decomposition of organic solvent and others are described.

Conductivity and spectroscopic characterization of polyacetylene doped with ferric chloride

Abstractcis‐ and trans‐PolyacetyleneSystematic name: (Z)‐ and (E)‐poly(viny1ene). films prepared by a rare‐earth coordination catalyst were doped with ferric chloride in nitromethane solution. The doped polyacetylene films [CH(FeCl4)y]x exhibit high electrical conductivities which change very little when stored in dry air. Infra‐red absorption bands at 900 cm−1, 1300 cm−1 and 1400 cm−1 are typical for cis‐and trans‐polyacetylenes slightly doped with FeCl3. In the samples with dopant mole fractions y < 0,02, two superimposed ESR signals with different g values are observed: one very narrow, the other one extraordinarily broad. 57Fe‐Mössbauer effect measurements reveal that Fe(II) and Fe(III) species coexist in freshly doped cis‐polyacetylene, appearing in the form of FeCl2 · nH2O and FeCl−4, respectively. Fe(II) species do not interact with the polyacetylene chain; they exist at a site separated from Fe(III) species.

Electronic structure of the polyacetylene film oriented by a liquid crystal solvent under magnetic field

The electrical transport properties of the liquid crystal and magnetic field oriented polyacetylene film are studied. FeCl 3, SO 3, iodine and AsF 5 are the dopants. The observed maximum room temperature conductivity is around 6000 Ω −1 cm −1 with SO 3 doped polyacetylene. High anisotropies ( σ 11(RT)/ σ ⊥(RT) ≳ 10) of room temperature conductivity are observed for FeCl 3, AsF 5 and iodine doped polyacetylenes indicating that the charge transfer is one dimensional in these polyacetylene derivatives. However, in the case of SO 3 doped polyacetylene, the anisotropy is only around three suggesting that the interchain coupling is relatively strong. In thermopower data, no anisotropy is observed confirming that thermopower measures the intrinsic property of the polymer. The temperature dependence of conductivity data of the FeCl 3 doped polyacetylene indicates that there might be an electric field effect and/or a structural rearrangement (which could be the result of the one dimensional instability) below 200 K. Quasi-linear temperature dependence of thermopower is observed for all dopants listed above at the high concentration regime showing the metallic nature of these heavily doped polyacetylenes. After careful examination of AsF 5 and SO 3 doped polyacetylene thermopower data in comparison with the data of unoriented-polyacetylene films, we believe that the AsF 5 doped oriented-polyacetylene shows slightly better crystalinity and the SO 3 doped oriented-polyacetylene shows a coupling between the chain and the dopant.

Raman study of alkali-metal doped (CH) x complexes

Polyacetylene films, chemically or electrochemically doped with alkali metals (Li +, K +, Na +, Rb +, Cs +) have been investigated by Raman spectroscopy. At low doping concentrations, Raman spectra exhibit features indicating a shortening of the conjugated trans segments. At a higher doping level (> 4 %), the Raman spectra present different Raman bands peaked at ⋍ 1270 and 1600 cm −1. The 1600 cm −1 band is also generally observed in p-doped (CH) x films, and is somewhat dependant on the excitation wavelength. Starting from a cis -sample, the Raman features of an electrochemically doped film are quite similar to those described in the case of a chemical doping. Moreover, an electrochemical dedoping clearly shows the reversibility of the process, ending up with a trans -(CH) x sample showing a double peak structure in the Raman bands profiles. Contrary to the p-doped films, n-doped samples can be cycled many times without being degradated. This is in agreement with intercalation stages as evidenced recently. The origin of the 1270 cm −1 Raman band is not completely elucidated. Further studies are needed to figure out if it corresponds to a mode appearing in the non-dimerized chain or more likely to i.r. modes induced by doping and allowed in Raman scattering by symmetry breaking. Concerning the 1600 cm −1 Raman band, its assigment to a C=C stretching mode of strongly perturbed segments or chain ends seems the most reasonable interpretation.

Morphology and characterization of polyacetylene films synthesized in a liquid crystal solvent

The synthesis of oriented polyacetylene films was investigated by using a nematic liquid crystal as a polymerization solvent. As a result, when polymerization was carried out on a quiesent surface of the nematic catalyst solution, partially oriented polyacetylene films were obtained. On the other hand, in the polymerization under a flow of the nematic catalyst solution, highly oriented films were obtained. The order parameter of the oriented sample, estimated from polarized absorption spectra, was about 0.57. The maximum value of parallel electrical conductivity of the films doped with iodine was 4600 S/cm, while that of the films doped with arsenic pentafluoride was 11000 S/cm.

Highly Conducting Polyacetylene Films Prepared in a Liquid Crystal Solvent

Polyacetylene films prepared by the liquid crystal polymerization method with gravity flow technique are investigated through measurements of scanning electron micrographs, polarized infrared absorption spectra, and electrical conductivities. It is explored that the films have significant orientation of conjugated polyene chains along fibril alignment, and that upon doping by I2 or AsF5 they exhibit the highest electrical conductivity among those reported so far. By taking account of both morphological structure and activation energy for electrical conduction, the high conductivity is rationalized with preferential increase of parallel fraction of fibril achieved in the highly oriented polyacetylene films.

Highly conductive polyacetylene film prepared by the liquid crystal polymerization method under magnetic field

Advanced synthetic method of highly oriented polyacetylene film has been developed by applying magnetic force into a polymerization system consisting of nematic liquid crystals. Polymerizations were carried out at 10 – 13°C in order to keep the nematic phase under magnetic field of 2 – 14 kGauss. Polyacetylene films prepared showed high alignment of fibrils, providing not only crucial infrared dichroism in polarized infrared absorption spectra, but also the highest value of electrical conductivity with 10 4 S/cm order upon iodine doping.

Photoexcitation in oriented polyacetylene

We present polarisation-dependent measurements of optical absorption, reflectivity, photoinduced absorption (PA) and photoconductivity (PC) on stretch-oriented films of Durham-route polyacetylene. We have looked, in particular, for evidence that optical absorption of band-gap light polarised with the E vector perpendicular to the chains is due to inter-chain excitation from π to π ∗ states. We find that the spectral dependence of the optical absorption and its temperature derivative is different for the two polarisations, and that there is evidence that the long-lived photoexcited charge states that give rise to PA and PC are preferentially excited with E polarised perpendicular to the chains. At excitation levels sufficiently low to give a linear response we find an anisotropy between the two polarisations of about 4. We find some differences between PA and PC in oriented Durham polyacetylene and as reported in Shirakawa polyacetylene, and we discuss these briefly.

Reversibility of the doping of polyacetylene with iodine in the gas phase

The reversibility of the doping reaction with iodine of Shirakawa polyacetylene (PA) films, carried out in the gas phase, has been studied by means of electrical conductivity, i.r. and ESR measurements. As expected, the level of the doping strongly influences the rate of the back reaction, diffusion processes being involved through the various parts of the film. These processes are characterized by diffusion constants which differ by orders of magnitude. For lightly doped PA samples, it is easily possible to recover the initial characteristics of the electrical conductivity, while the optical i.r. characteristics may be recovered by heating the samples at 50°C, if both cis and trans isomers are present. For lightly doped all trans samples, this heat treatment was not necessary. The heavily doped all trans PA samples also regain their initial characteristics after brief heating, showing that the desorption of I 2 from the interior of the film needs to be thermally activated.

Localization and Interaction Effects in Metallic Polyacetylene

The electrical conductivity σ(T) of heavily iodine-doped films of polyacetylene (CHIy)x, y≃0.25-0.3, has been investigated in the temperature range T = 20 mK -295 K and in magnetic fields up to 6T. We find that σ(T)=σ(0)+σ1(T), where σ(0)≃0-30 (Ωcm)-1 and σ(T)≃C√T below 150 K. The slopes C are close to the quasi-universal value of 6 (Ωcm)-1K1/2 reported for a variety of disordered metals. The observed magnetoresistance can be semi-quantitatively related to contributions from three-dimensional localization and electron-electron interaction effects. First results on (CHK0.12)x also reveal a √T dependence of σ1(T).

Morphology of polyacetylene produced in the presence of the soluble catalyst Ti(OnBu) 4- n-BuLi

When the reaction parameters for the polymerization of acetylene in the presence of Ti(OnBu) 4-n-BuLi as catalyst are altered, the macroscopic morphology of the polymer (CH), is also changed, to an extent that is mainly dependent on the concentration of the catalyst. At higher catalyst concentrations, polymerization occurs on the free surface of the catalyst to form a polyacetylene film which retards further polymerization because diffusion of acetylene monomer through the film is thus restricted. At lower catalyst concentrations, however, successive diffusion and polymerization can occur, and yield a gelatinous mass. At extremely low concentrations, the product is instead a powder. Despite these differences in macroscopic morphology, scanning electron microscopy reveals that all forms of polyacetylene are fibrillar in structure. However, the diameter of the fibrils, which is 60 Å on average, may depend on the polymerization conditions.

Metallic electrical-conductivity of doped-polyacetylene at low temperatures

A metallic temperature dependence down to 1.9 K was observed in the electrical conductivity of heavily AsF 6 −, ClSO 3 − or FeCl 4 −-doped polyacetylene films by means of the voltage-shorted-compaction (VSC) method. The temperature dependence was fitted to a T −2 law over nearly three orders of magnitude of conductivities.

A semiconductive polymer film sensor for glucose

The electrical conductivity of organic polymers such as polyacetylene and its derivatives can be varied over twelve orders of magnitude with small amounts (0–3%) of various dopants, such as iodine, bromine, and perchloric acid. Semiconductive polyacetylene film doped with iodine is sensitive to hydrogen peroxide, and can be used as a quantitative hydrogen peroxide sensor. A rapid, quantitative sensor for glucose, using the flavoprotein glucose oxidase, is described and introduces a novel electro-active material, polyacetylene, as the basis fora new biosensor. A significant increase in the sensitivity of this device has been obtained by mediating the doping reaction through lactoperoxidase and potassium iodide.