Poly(Alkylthiophene) Blends Processing, Morphology and Conductivity
Intrinsically conducting polymers may be an attractive alternative instead to polymers with conducting fillers. The aim of the present work was to study the potential of the processable conducting polymer poly(3-alkylthiophene) as well as possible problems related to its processing.
One way to improve the properties of polymers is to make polymer blends. In this work, undoped blends of poly(3-octylthiophene) (POT) were processed with common polymers. The influence of the matrix polymer on morphology, miscibility and conductivity was studied. It was found that these blends show two phase behaviors and that the viscosity ratio is of great importance for morphology and hence for conductivity. Melt-mixed PVC/POT blends form a co-continuous structure with relatively low amounts of POT and show high conductivities (~3 S/cm with 23 wt % of POT). Even higher conductivities are obtained when the plasticizer di-iso-octylphtalate (DOP) is added. POT blends with PS and PE are phase-separated but the addition of copolymers consisting of PS or PE main chains, respectively, grafted with polyethylenoxide side chains, decreases the size of the POT phases. This addition increases the conductivities of doped blends up to several orders of magnitude. In a PS/POT blend with 20 wt % POT, an amount of only 5 wt % of the copolymer increases the conductivity from 0.0051 to 0.46 S/cm. Co-continuous PE/PS blends containing POT have high conductivities, indicating that POT is also continuous.
The influence of a non-conducting filler on melt-mixed conducting polymer blends of POT was examined with respect to their morphology, rheology, conductivity and acid-base properties. It was found that the adhesion at the polymer-filler interface in combination with the viscosity ratios between the polymers exerts a considerable influence on the morphology and hence, on the conductivity. For e.g. blends of LDPE and POT, the addition of whiskers changed the morphology and increased the conductivity by several orders of magnitude.
Thermal oxidation of POT films was carried out at conditions relevant to processing (180-220 °C). It was found that molecular enlargement occurred at high temperatures, which, in the presence of air, led to the formation of insoluble gel. Identification of the key products showed that the major oxygen-containing group is ketone in the alpha carbon position of the alkyl side chain. Hence, the mechanism for thermal oxidation at processing temperatures indicated mainly a side chain degradation, which is contrary to the mechanism found for photo-oxidation of poly(alkylthiophenes).