Direct measurement of the heat of mixing (interaction) of polymers is not feasible due to their high viscosity. The indirect approach described here is a heat of solution method, in which a Hess's Law cycle is used to determine the heat of mixing from the individual heats of solution of the alloy and its constituent polymers in a common solvent. The main limitation of this approach lies in obtaining adequate precision on the experimentally determined heats of solution, as these are large compared to the calculated heat of mixing.With this method the heat of mixing of a 75/25 (%by wt) poly(dimethyl) phenylene oxide/polystyrene alloy has been determined by Setaram C-80 microcalorimeter as 4.9 + 0.2 jg-1. The precision of the method shows an improvement compared with published data.Polymer have become increasingly important with the growth in the world market for performance polymers [1,2]. There is a need for polymeric materials that posses a variety of special physical properties. This is not always possible to achieve with a single polymer, even when reinforced with glass or carbon fibre. The combination of properties offered by polymer alloys, sometimes exceeding that of either component polymer (e.g. high impact polystyrene [3,4], in an attractive solution.Metals form alloys because of an increase in entropy on mixing. In a system containing two polymers the effect of entropy is negligible and consequently most polymer pairs do not mix. The important factor in producing polymer alloys, is the degree of interaction between the polymers. In only a few cases, notably poly(dimethyl phenylene oxide)/polystyrene (PPO/PS) is this interaction sufficiently exothermic to promote miscibility. Direct measurement of the enthalpy of mixing (AHm) is not feasible due to the high viscosity of polymers. Various indirect methods have, however, been published [5]. These include melting point depression, heats of solution, vapour sorption, inverse gas chromatography, small angle X-ray scattering~ neutron scattering, and heat of mixing of low molecular weight analogues. The heat of solution method is the most promising [6] and is the subject of this paper.
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