Abbas, Zaid Kahtan (2009) Intrinsically conducting polyaniline blends. (PhD thesis), Kingston University, .
Abstract
The project involved the production of, conductive polyanilines and their blending with elastomers to make new conducting' rubbers. Conductive elastomers already have commercial applications such as antistatic coatings, artificial muscles, sensors and electromagnetic screening. These have been based on carbon black or metal fillers, or more recently conducting polymer powders incorporated into natural or synthetic rubbers. A number of polyaniline-rubber blends have been reported in the literature. A key aim of this project was to improve the compatibility, mechanical, thermal stability and electrical properties of this type of blend through different mixing methods such as solution and thermo-mechanical with more systematic mixing procedures and better optimised mixing conditions. The first part of the work was to optimise the synthesis of conducting polyanilines (P Ani), polyanisidines (poAnis), and copolymers of the latter two, in the presence of a variety of protonating agents under a range of conditions, in order to study the influence of the synthetic conditions on the physical, chemical and morphological characteristics of the polymers. PAnilPoAnis products were prepared via solution polymerisation (simple oxidation at low pH) or via emulsion polymerisation using sulfonic acids (surfactants). Syntheses yielded the emeraldine salts (ES), which were either kept as such, or deprotonated using ammonia solution. There were noticeable differences in the behaviour of the two families of polymers. P Anis display better conductivity and a higher thermal stability than PoAnis due to their less hindered structure. These differences in physical characteristics tend to fade when the compounds are synthesised using a protonic surfactant such as dodecylbenzenesulfonic acid (DBSA). Here, the surfactant-induced order of the final polymeric particles becomes dominant over that of the pendant methoxy-group of PoAnis. Scanning electron microscopy (SEM) revealed the strong influence of the protonic acid used for the synthesis onthe shapes of the final particles. Hence, P Ani-DB SA has a monoclinic appearance and rhombic shape particles while P Ani-ß-naphthalenesulfonic acid (ß-NSA) has a "flakey" morphology. It was shown that a very small quantity of co-monomer could change drastically the physical characteristics of P Ani; hence, when anisidine (Anis) is added to the aniline-acid complex, a different crystalline structure is achieved. The chosen polymer (P Ani-DB SA) was matched with two individual elastomers, one based on an ethylene oxide co-polymer and the other on a nitrile butadiene co-polymer, both of which had suitable solubility parameters. The polymers were solution-blended and studied for compatibility by means of microscopy, electrical conductivity measurements and thermal analysis. Blends of poly (epichlorohydrin-co-ethylene oxide) rubber (Zeon Hydrin" C2000L) and polyaniline doped with dodecylbenzenesulfonic acid (P Ani-DB SA), containing different weight fractions of P Ani, were cast from solution. Solubility parameters for both P Ani¬DBSA and Hydrin® were calculated and found to be comparable, favouring some degree of miscibility. Conductive transparent films were formed by casting onto PTFE substrates. Conductivities of the cast films were found to be in the region of 10-7 Scm" for 1 wt% P Ani¬DBSA. Electrical conductivities increased with the proportion of P Ani-DB SA, showing a percolation threshold as low as I wt%, with highest conductivity achieved at, 50wt%. Decomposition steps of conductive blends were investigated using thermogravimetric analysis (TOA) and differential scanning calorimetry (DSC). The thermal stability of the blends was influenced by the ratio ofPAni-DBSA to Hydrin'". The effect of composition on the glass transition in the blends was determined using thermomechanical analysis (TMA). Thin films of the blends of dodecylbenzenesulfonic acid (DB SA) doped polyaniline (P Ani) and pure grade of Poly[(acrylonitrile-co-butadiene)/poly vinylchloride] (Zeon Nipol® DN171) were prepared via co-dissolution, at varying ratios of conducting filler (0.5-50)wt %. The electrical conductivity correlated with the concentration ofP Ani-DB SA. At a percolation threshold ofO.9% (wt/wt), an increase in electrical conductivity of approximately eight orders of magnitude was observed, with a recorded conductivity of 1.21xlO-4 S.cm-1 for the most conductive blend (consisting of 50wt% PAni). Characterisation by UV-visible spectroscopy indicated the retention of the doped state of the conductive P Ani-DB SA in each elastomer blend. Morphological studies by optical microscopy, showed the presence of P Ani-DB SA agglomerate networks (or interpenetrating networks) at higher concentrations; however, at lower concentrations of P Ani-DB SA, the dispersion and encapsulation of the conductive species was observed. Due to the double phase morphology of Nipol® DN 171, the encapsulating phenomenon was attributed to the presence of the thermoplastic (poly vinylchloride). Solubility parameter calculations confirmed the compatibility ofP Ani-DB SA with the constituents of the Nípol" DN171 elastomer. This was supported by the infrared spectrum, which shows the presence of functional groups specific to the reacting components of each blend. Thermal studies, performed by DSC and TOA, indicated an increased thermal stability with increasing concentration of the elastomer.
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