The synthesis and characterisation of novel semiconduction ladder polymers and their use as conductive fillers in composite materials
A series of ladder polymers have been synthesised from polycondensation reactions between phenazinium based monomers such as safranine O and acridine yellow G and benzoquinone type monomers. The same polymeric products were obtained regardless of whether the polymerisations were carried out in polyphosphoric acid, in organic solvent, or in the absence of solvent in the solid phase. However, the degree of completed ladder structure contained within the polymer was found to vary with different synthesis techniques. In general the ladder polymer prepared in PPA was superior in terms of intrinsic viscosity (molecular weight) and thermal stability in comparison to the same polymer prepared in organic solvent or in the solid phase. The polymer solubilities were poor in organic solvents and attempts to increase their solubilities with the introduction of bulky side-groups into the polymer structure was successful only in producing soluble single-strand polymers. The electrical conductivities (o) of the polymers lay in semi-conducting range with a clear trend showing that: 0PPA > 0DMAc/S.P. > 0single-strand > 0two-stagepolymers. The electrical conductivity was found to be stable for at least 30 days at incubation temperatures as high as 250° C. Temperature dependent conductivity studies show that plots of o versus T1/4 are not perfectly linear, although correlation coefficients for plots against '11/4 and T1[beta] show that the data tends to favour Mott's three-dimensional conduction model. Composite samples of thermoplastic PMMA and thermosetting polyester with ladder polymer showed percolation thresholds at 7.9% v/vand 8.6% viv filler loadings respectively with maximum conductivities close to those of the pristine polymer at filler loadings of 11.2% v/vand 13.0% vl» respectively. The temperature dependence of PMMA/ladder composites showed PTC behaviour between the softening point and melting point of PMMA with NTC behaviour thereafter. Thermosetting polyester composites showed little temperature sensitivity up to the point of thermal degradation at which catastrophic, permanent loss in conductivity occurred. The electrical conductivity of ladder/PMMA samples was found to be stable for at least 30 days at 250°C.