Conducting elastomer blends
There are many existing and developing commercial applications for electrically-conductive elastomeric blends. 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, often with poor electrical conductivities have been reported in the literature. Interest was found in this project to improve the compatibility, mechanical and electrical properties of this type of blend through different mixing methods (i.e. solution and thermo-mechanical) with more systematic mixing procedures and better optimised mixing conditions. Poly(butadiene-co-acrylonitrile) rubber (NBR) and polyaniline dodecylbenzenesulfonate (PAni.DBSA) were chosen for study as blends in the present work, because their solubility parameters were calculated to be compatible (NBR with 48.2 wt% of acrylonitrile content had the best compatibility with PAni.DBSA) and also because the polymers were thermally stable and readily available. No literature work was found relating to the electrical properties of natural rubber-PAni.DBSA blend. Therefore, epoxidised natural rubber (with 50 mole% of epoxide level) was also selected in this work in order to investigate a novel usage of this natural resource. Non-vulcanised NBR-PAni.DBSA and non-vulcanised ENR-PAni.DBSA blends with useful electrical conductivities (up to about 10[sup]-2 and 10[sup]-3S.cm[sup]-1 respectively) were successfully prepared by solution mixing for the first time in this work. Peroxide-vulcanised NBR-PAni.DBSA blends with similar electrical conductivities (up to x 10[sup]-2S.cm[sup]-1) were also successfully prepared for the first time by thermo-mechanical mixing (with an internal mixer). The conductivities of all these blends were much higher than those of comparable materials reported in the literature due to the reasonably good compatibility between the two polymers and the high mixing efficiency of the two chosen mixing methods (i.e. solution and thermo-mechanical mixing). The electrical conductivities of the peroxide-vulcanised blends were found to be unaffected by the presence of dicumyl peroxide vulcanising agent. The FT-IR spectra of both non-vulcanised and peroxide-vulcanised NBR-Pani.DBSA blends (either prepared by solution or thermo-mechanical mixing) resembled a superposition of the spectra of the constituent materials. However, some notable peak shifts were observed, providing evidence of the bonding interaction between the two polymers. For ENR-PAni.DBSA blends, the ring opening of ENR with increasing PAni.DBSA content was successfully quantified by NMR and FT-IR spectroscopic (based on the intensities of all spectroscopy bands assigned to the functional groups relating to ENR ring opening). The non-vulcanised and peroxide-vulcanised NBR-PAni.DBSA blends with up to 30 wt% of PAni.DBSA, (produced either by solution or thermo-mechanical mixing) showed the largest temperature shifts for their thermal events in both the sub- and above-ambient temperature DSC thermograms, strengthening the evidence of interaction between the two polymers. Progressive glass transition temperature (Tg) shifts with increasing PAni.DBSA content were observed for the ENR-PAni.DBSA blends from both the sub- and the above-ambient temperature DSC thermograms, supporting the proposition that there was further ring opening of the ENR. Optical and electron micrographs of all NBR- and ENR-PAni.DBSA blends showed that the higher the content of PAni.DBSA, the greater the phase separation (dark-green regions with larger proportion of PAni.DBSA) between the rubber and conductive filler. However, the thermo-mechanically mixed NBR-PAni.DBSA blends (made by high temperature-processing, [tilde]140[sup]oC), produced more well-blended regions (seen as rubber-rich pale-green regions) between the two polymers, even with a high level of PAni.DBSA content, i.e. [greater than or equal to]40 wt%. Transmission electron micrographs of all NBR- and ENR-PAni.DBSA blends prepared in this work revealed numerous small PAni.DBSA particles (of colloidal dimensions, 50-1000nm), which were also likely to have contributed to the electrical conductivities of all these blends (especially for those that were below their percolation thresholds). The effect of PAni.DBSA. polymer chain alignment on the peroxide-vulcanised NBR-PAni.DBSA blends prepared by thermo-mechanical mixing was also studied in this work. It was found that both the mechanical properties (i.e. tensile properties and tear strength) and the electrical conductivities of the blends were enhanced, when they were strained along the orientation axis of elongated PAni.DBSA polymer chains (the orientation tended to be parallel to the flow direction when the blends were passing through a two roll-mill). Both FT-IR spectroscopy and optical microscopy successfully revealed a higher level of inter-chain interaction among the elongated PAni.DBSA particles for the blends strained along this particular orientation axis. All the peroxide-vulcanised blends also exhibited a good historical memory in terms of their electrical conductivities (99% retention of the original non-strained sample's value) during the multi-cycles of strain loading and unloading. This was attributed to the high proportion of well-blended regions in all the blends prepared by thermo-mechanical mixing.