Peter J.S. Foot

Durham poly acetylene: preparation and properties of the unoriented material

Abstract Polyacetylene prepared by the Shirakawa route is a mat of crystalline fibrills, which are capable of being oxidized or reduced to a highly conducting state. Polyacetylene can also be produced from a soluble precursor polymer by a thermally driven transformation reaction (the ‘Durham’ route) as a solid, coherent film. This transformation consists of the elimination of a substituted benzene unit followed by the isomerization of the largely cis polyacetylene produced to a disordered trans isomer. We have studied the effect of different transformation conditions and from these studies it is possible to gain an insight into these processes. Techniques such as caloremetry, infrared, resonance Raman and optical spectroscopy, e.s.r., X-ray diffraction and electrical conductivity have all been used to probe the transformation and the product. The resultant information gives a consistent picture of the overall transformation processes detailed above but also shows an ordering associated with higher transformation temperatures. A thorough knowledge of these processes enables the orientation of the precursor to produce highly crystalline material.

The effects of composition and processing variables on the properties of thermoplastic polyaniline blends and composites

Pure polyaniline (PANI) has a high electrical conductivity and can be made soluble and thermoplastic, but it still lacks adequate mechanical properties for large-scale commercial use and therefore, it has been blended with other polymers such as poly(methyl methacrylate) (PMMA). In the work described in this paper, the scaled up synthesis of conductive polyaniline by an oxidative chemical method under controlled pH and temperature has been optimised. Re-doping of deprotonated insulating base with excess of the mono-functional organic acids such as p-toluenesulfonic (TSA) or dodecylbenzenesulfonic (DBSA) in aqueous media was successful. A wide range of techniques including TGA, GPC, EA, FTIR, XRD and SEM were employed for the characterisation of PANI powders and blends. Compositions of PANI-HCI, TSA or DBSA and thermoplastic matrix PMMA with or without a plasticiser were melt-processed by compression moulding for 3 min at 210°C to produce plaques. The effectiveness of four different phenolic plasticisers was compared and hydroquinone was found to produce the blends with the highest conductivities. A few preliminary injection-moulded plaques were made and their conductivities were compared with those of the compression-moulded samples.

Stability and degradation of some electrically conducting polymers

Abstract The oxidative degradation reactions of polyacetylene, prepared from a soluble precursor polymer, are described and compared with those of more common polymers and of polyacetylene prepared by the conventional method. Because the band structure of the π-electron system of the polymer allows the formation of charge-transfer complexes with oxygen, the initial process in the pristine polymer is oxygen doping, with increasing conductivity. This is followed by irreversible degradation which is much faster than that of polyolefins or polydienes and faster than that of the crystalline polymer. Doping to low levels with electron acceptors removes the electrons involved in oxygen doping and the polymer becomes much more stable in air. Doping to high levels leads to new instabilities as the polymer reacts slowly with its counter-ions. Studies of polypyrrole and polythiophene show that these polymers are much more stable than polyacetylene but still undergo degradation reactions. The general features of their degradation mechanisms are discussed.

Preparation and characterization of polypyrrole, N-substituted with liquid crystalline moieties

We describe the synthesis and polymerization of an N-substituted pyrrole. The substituent is an organic spacer chain culminating in a mesogenic cyanobiphenyl group. The monomer molecule is symmetrical, and its polymerization produces effectively exclusive head-to-tail coupling, resulting in increased chemical perfection of the polymer backbone. It is therefore not sensitive to head-tail or head-head coupling effects. The liquid crystallinities of the monomer and polymer have both been examined.

Novel inorganic/conjugated polymer nano-composites

Abstract Inorganic/conjugated polymer nano-composites have been synthesized by a two-stage process involving intercalation of aromatic bases into an inorganic host. Oxidation led to in situ polymerization to produce alternating inorganic/polymer layers, and the transport properties of these complexes are compared with those of the original materials. ‘Soft chemistry’ techniques have been used, with some success so far, to isolate the free polymers for separate study.

Electronic properties of conjugated polymers

The nature of the proposed defect states in conjugated polymers (solitons and polarons in polyacetylene, polarons and bipolarons in other materials) is expected to be strongly dependent on the morphology and chain conjugation length of the polymer. Polyacetylene prepared by the Durham ‘precursor’ route has a lower conjugation length than that prepared by the Shirakawa route, and it is expected that polarons or bipolarons rather than solitons should be the defect state introduced during chemical doping. We discuss here a wide range of physical measurements made on Durham polyacetylene. We present data for two other, processible, polymers. Poly(phenylenesulphide) shows strong optical absorption features below the band gap when reversibly doped with AsF5. Poly(diphenylenediphenylvinylene) shows similar behaviour and evidence for photogeneration and chemical generation of polaron-like states in this material is presented.

Annealing behaviour of conductive poly(3-hexylthiophene) films

We demonstrate that annealing conductive, solvent-case poly(3-hexylthiophene) (P3HT) films under reduced vacuum improved their structural properties and induced higher conductivity. These preliminary results suggest that thermal processing of conductive polymers may be a useful step in their future device fabrication.

Theoretical studies of conducting polymers based on substituted polypyrroles

Abstract Side chain polyprrole polymers have the potential to be processable conducting materials. A knowledge of the conformation and packing of these polymers will aid the molecular design of the appropriate side chains. In this work it was decided to study the minimised structure of polypyrrole and differently substituted polypyrroles, the minimisation of several monomers of the conductive polymers under study; and minimised structures of several chains of each polymer. Information on the angles between neighbouring rings in the backbone together with MD simulations of physical properties (Tg, density) of our models are presented and compared with experimental values. Tg values for two new substituted polypyrroles are presented, for which no experimental values are known.

Some observations on the structure of Durham polyacetylene

Abstract The structure of polyacetylene produced from a precursor polymer is compared with that of polymer produced by a soluble Ziegler-Natta catalyst. X-ray diffraction shows that the precursor polyacetylene can be produced with a wide range of ordering from apparently amorphous to crystalline, although the ordering is never as high as that of polymer produced by the Ziegler-Natta route. An increase in ordering, as measured by X-ray peak width, has been observed during isomerization; this contrasts with normal polyacetylene where no change in order is found. Both forms of polyacetylene show a gradual shift in the interchain d -spacing from 395 pm ( cis/trans mixed) to 365 pm ( trans ). Since only a single peak is found in this region the cis - and trans -forms are not phase-separated on a scale of 3 nm or more. Small-angle X-ray scattering also indicates that discrete crystalline and amorphous regions, as found in polyethylene, are not present

A kinetic study of the Durham precursor route to polyacetylene

Abstract The processes involved in the thermal transformation of ‘Durham polyacetylene precursor’, poly[[5,6-bis(trifluoromethyl)-bicyclo[2,2,2]octa-5,7-diene-2,3-diyl]-1,2-ethenediyl], to all- trans polyacetylene have been studied. Above about 100°C the reactions are virtually concurrent, but at 50°C one can distinguish a first-order elimination ( k = 7.6 × 10 −4 s −1 ), effusion of the elimination product ( D = 4 × 10 −9 cm 2 s −1 ), and a first order cis-trans isomerization ( k = 1.5 × 10 −5 s −1 ). The isomerization is very sensitive to traces of oxygen below 80°C, but at higher temperatures this is less pronounced and the reaction becomes approximately second order. Isomerization is generally more facile in the less ordered Durham polymer than in polymers from the Shirakawa route.