Adam Pron

Processible conjugated polymers: from organic semiconductors to organic metals and superconductors

Abstract Since conjugated polymers, i.e. polymers with spatially extended π-bonding system offer unique physical properties, unobtainable for conventional polymers, significant research efforts directed to better understanding of their chemistry, physics and engineering have been undertaken in the past two and half decades. In this paper we critically discuss synthetic routes to principal conjugated polymers such as poly(acetylene), polyheterocyclic polymers, poly( p -phenylene vinylene)s, aromatic poly(azomethine)s and poly(aniline) with special emphasis on the preparation of solution (and in some cases thermally) processible polyconjugated systems. In their neutral (undoped) form conjugated polymers are semiconductors and can be used as active components of ‘plastics electronics’ such as polymer light-emitting diodes, polymer lasers, photovoltaic cells, field-effect transistors, etc. Due to its strongly non-linear I = f ( V ) characteristics in high electric fields, undoped poly(aniline) can be used as stress grading material for high voltage cables. In the next part of the paper we describe redox and acid–base doping of conjugated polymers and its consequences on structural, spectroscopic and electrical transport properties of these materials. Special emphasis is put on dopant engineering, i.e. on the design of the dopants which not only increase electronic conductivity of the polymer but also induce desired properties of the doped polymer system such as improved processibility, special catalytic properties or special optical or spectroscopic properties. Selected examples of technological applications of doped conjugated polymers are presented such as their use as conductive plastics, optical pH sensors, heterogeneous catalysts, gas separation membranes, etc. The paper is completed by the description of the recent discovery of the first organic polymer superconductor.

Conducting blends of polyaniline with conventional polymers

Abstract Phosphoric acid diesters effectively protonate polyaniline and simultaneously plasticize it. This plasticized polyaniline can be processed either by thermal methods or from solution. It can serve, therefore, as a conducting additive to common polymers, such as poly(vinyl chloride), polystyrene, poly(methyl methacrylate) and cellulose derivatives. In each case only few wt. % of polyaniline is needed to render a conductive blend. Microprobe studies and scanning electron microscopy show uniform distribution of polyaniline/diester in the polymer matrix. The conductivity of the blends ranges between 10 −2 and 10 S/cm, and the mechanical properties are those of the non-conducting matrix. Thermally processable blends of PVC, having very good mechanical properties, were prepared by mechanical mixing with plasticized polyaniline, while those of PMMA, PS or cellulose derivatives were cast from solutions.

Processable conducting polymers obtained via protonation of polyaniline with phosphoric acid esters

Abstract The protonation of polyemeraldine base (PANI) by diesters of phosphoric acid was studied. Three esters were investigated, namely bis(2-methylpropyl) hydrogen phosphate (DiBHP), bis(2-ethylhexyl) hydrogen phosphate (DiOHP) and its non-branched analogue, bis(n-octyl) hydrogen phosphate (DnOHP). It has been found that DiOHP, in addition to protonation, plasticizes PANI at very low ester contents i.e. for y ≥0.25 in PANI(DiOHP) y . In PANI(DnOHP) y and PANI(DiBHP) y the plastification starts at y =0.50 and y =0.65 respectively. It is clear that the long and branched substituents facilitate plastification. PANI protonated with DiOHP is soluble in common organic solvents such as toluene, chloroform, THF etc. The soluble fraction is enriched in DiOHP as compared with the parent protonated polymer. The existence of the anions originating from the ester in the soluble part of the polymer was confirmed by FT i.r. and 31 P n.m.r. studies of the soluble fraction. Free-standing films with a conductivity of 10 S cm −1 can be produced by hot pressing PANI(DiOHP) y at temperatures of 80–140°C. At higher temperatures partial degradation of the complex occurs resulting in a decrease of conductivity with a simultaneous increase of Youngs modulus and tensile strength. Blends of DiOHP-plasticized PANI and dioctyl phthalate-plasticized PVC, of excellent mechanical properties, can be prepared by hot pressing at 160°C.

Low polydispersity core/shell nanocrystals of CdSe/ZnSe and CdSe/ZnSe/ZnS type: preparation and optical studies

Abstract A new method for the preparation of core/shell semiconductor nanocrystals of the A(II)B(VI) family is described, which involves the use of safe and cheap reagents at each synthesis stage, namely cadmium oxide as the Cd source and zinc stearate as the Zn source. It enables the fabrication of highly luminescent CdSe/ZnSe core/shell nanocrystals exhibiting a polydispersity

Processable conducting polyaniline

Abstract It has been found that diesters of phosphoric acid with branched alkyl substituents solubilize polyaniline in its protonated (conducting) state. Polyaniline protonated with bis(2-ethylhexyl)phosphate or bis(2-methylpropyl)phosphate is highly soluble in common organic solvents such as toluene, decaline, THF and chlorinated hydrocarbons. Conducting blends with poly(methylmethacrylate) or poly(acylonitrile-butadiene-styrene) (ABS) can be prepared showing high conductivity for very low electroactive component content. Protonation of polyemeraldine base with neat esters results in a heavily plastisized mixture which can be thermally processed to give free standing films with conductivities exceeding 10 S/cm. Depending on the content of the protonating agent, time and temperature of pressing films with different Young modulus, tensile strength and elongation at break can be prepared. Similarly, conducting blends with excellent mechanical properties can be prepared from diester plasticized polyaniline and plasticized polyvinyl chloride.

UV–VIS–NIR and Raman spectroelectrochemistry of regioregular poly(3-octylthiophene): comparison with its non-regioregular analogue

Spectroelectrochemical behaviour of regioregular poly(3-octylthiophene) has been investigated using UV–VIS–NIR and Raman spectroscopies. Static and dynamic UV–VIS–NIR spectroelectrochemical experiments combined with cyclic voltammetry show that oxidative doping of the regioregular polymer is a two-step process in which polarons (radical cations) are first created which then recombine to bipolarons (dications). This two-step oxidative doping mechanism is corroborated by FT Raman spectroelectrochemical studies which show significant changes in the positions and intensities of the Raman bands coinciding with the first and second oxidation peaks in cyclic voltammetry. These changes can be interpreted in terms of the doping induced formation of quinoid sequence of bands in the oxidized polymer. Vibrational calculations carried out for undoped and doped poly(3-octylthiophene) gave a very good agreement between the calculated Raman band frequencies and those recorded experimentally for the regioregular polymer.

Physicochemical and catalytic properties of ployaniline protonated with 12-molybdophosphoric acid

Using acid–base-type doping of polyaniline with H3PMo12O40, two series of catalyst samples were prepared. In series Sl the doping and polymerization were carried out simultaneously, leading to a uniform distribution of the dopant over the whole volume of the polymer. In series SII the doping was achieved by protonation of already formed polyemeraldine base with H3PMo12O40. Since this process must involve diffusion in the solid matrix, the surface becomes enriched in heteropolyanions (Keggin units) due to their limited diffusivity. The examination of physicochemical properties of the samples by XP, EPR and FTIR spectroscopies indicated that the Keggin units incorporated into the polymer matrix retain their identity and represent catalytically active centres. The insertion of the dopant species increases the electrical conductivity of the polymer by several orders of magnitude. The catalytic activity was tested in ethyl alcohol conversion and both the products of acid–base [C2H4 and (C2H5)2O] and redox (CH3CHO) type reactions were observed. Owing to the surface enrichment of the SII series of catalysts their catalytic activity was much higher than that of samples of the Sl series nominally doped to the same level. In both types of catalysts the selectivity to the redox reaction product (CH3CHO) was greatly enhanced in comparison with unsupported H3PMo12O40.

Flexible, highly transparent, and conductive polyaniline-cellulose acetate composite films

Highly transparent and conductive polyaniline-cellulose acetate composite films were cast from m-cresol solutions of protonated polyaniline and cellulose acetate. The following polyaniline protonating agents were used: camphor sulfonic acid, phenylphosphonic acid, dibutyl phosphate, and dioctyl phosphate. The films were plasticized with a mixture of phthalic and phosphoric acid esters added to the solution used for the casting. The addition of plasticizers not only improved the flexibility of the composite films but also significantly lowered the percolation threshold, which is 0.05 wt % (expressed as emeraldine base) for phenylphosphonic acid protonated polyaniline in cellulose acetate matrix.

XPS studies of environmental stability of polypyrrole-poly(vinyl alcohol) composites

Abstract In this paper, we report a study on the aging effects of polypyrrole-poly(vinyl alcohol) composites. A decrease of the conductivity is observed when samples are stored in ambient atmosphere, whereas they are stable in inert environment. A careful analysis of the studied samples has been performed by using both scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). It is known that oxidation processes occur, breaking the conjugation of the polymeric unsaturated chains and this is the main cause for the non-stability of the composites in ambient conditions. Consequently, several oxygen-containing groups are formed in the polymer backbone, as deduced from the analysis of the C, O, N and Cl binding energies.

Spectroscopic properties of poly(3-alkylthiophenes) and their ‘head-to-head’, ‘tail-to-tail’ coupled analogues poly(4,4′-dialkyl-2,2′-bithiophenes)

Abstract Electrochemical doping of new soluble conducting polymers, namely, poly(4,4′-dialkyl-2,2′-bithiophenes), has been studied ‘in situ’ and ‘ex situ’ by various spectroscopic methods. These compounds represent the ‘head-to-head’ and ‘tail-to-tail’ analogues of poly(3-alkylthiophenes). The π→π ∗ transition in poly(4,4′-dialkyl-2,2′-bithiophenes) is blue shifted with respect to that characteristic of poly(3-alkylthiophenes) suggesting a higher torsion angle between the adjacent substituted thiophene rings. The doping-induced bipolaronic bands (Vis-NIR) are, however, located in essentially the same positions as the doping-induced ones in poly(3-alkylthiophenes). This observation may suggest that during doping, more twisted poly(4,4′-dialkyl-2,2′-bithiophenes) adopt a structure more similar to that of doped poly(3-alkylthiophenes). The changes in FT-IR spectra are also consistent with the bipolaron model since all the five expected doping-induced modes appear with the onset of the oxidation peak and grow in intensity with increasing potential.