New evidence supporting the interfacial model of electrically conductive polymer composites. 2. Indication of phase separation during ageing of polyaniline-poly(ethylene-co-vinylacetate) composites
Phase separation in conductive polyaniline-poly(ethylene-co-vinyl acetate) (PANI-EVA) composite films, resulting in formation of PANI enriched (lower side) and PANI deficient (upper side) layers was found during storage of these composites. A distinct difference in the conductivity of the two sides of the films reaching a half order of magnitude was detected. It was shown that phase separation and conductive pathways formation resulted from flocculation of PANI particles during ageing of the films. Subsequently, the total electrical conductivity of the composites of lower PANI content (up to 2.5 wt.%) was substantially enhanced (ca. five orders of magnitude). The observed conductivity jump and phase separation phenomena have been discussed using mathematical terms. Moreover, for the first time a mathematical interpretation of the evolution in percolation behaviour of PANI-EVA composites over time is presented. Coefficients of correlation (r xy ) between average conductivity, σ, (y) and PANI content (x) in the composites were calculated to be ca. 0.85 - 0.99. Besides, the shape of percolation curves (σ vs. PANI content) was found to change during storage of the films. The unstable conducting properties (both conductivity jump and phase separation phenomenal of PANI-EVA composites over time revealed their dissipative nature, and have to be regarded as evidence supporting the dynamic interfacial model of conductive polymer composites.