Facile methods for enhancement of conductivity in polyaniline

More than three decades ago polymers were considered to be insulators. Polymers could be made more or less conductive by the addition of fillers such as carbon black. This notion changed after the ‘experimental accidental discovery’ of extremely high conductivities in doped polyacetylene. This discovery sparked a strong interest in intrinsically conducting polymers, which are characterised by a conjugated backbone structure. However, the commercial use of these ‘conducting polymers’ was hampered because of their intractability (processing) and instability in air. The development of a new generation of conducting polymers, next to polyacetylene, is now on the verge of playing a big role in future electronic devices e.g. light emitting diodes, photovoltaic cells, flexible displays, sensors to list a few amongst the many. Amongst this new generation of intrinsically conducting polymers, polyaniline (PANI) is one of the most promising candidates and a lot of studies have been devoted to this polymer. The reason is that PANI is easy to synthesize and is rather environmental stable combined with a high conductivity. However, PANI in its conducting form is also intractable in nature. Subsequent developments to modify PANI into a processable form by means of counter ion induced processing, backbone substitution with alkyl chains etc. led to the use of PANI in a number of applications, usually in (nano)composites where PANI in dispersed form acts as a conductive filler. Upon mixing PANI with conventional polymers, the strong electrostatic interactions between PANI molecules results in a phase segregated morphology, which ultimately results in a loss of connectivity between the dispersed PANI particles and hence poor conductivity.. Thus, a high(er) filler content is a prerequisite to achieve the necessary so-called percolation threshold. The downside of increasing the PANI concentration is loss in physical and mechanical properties of the composite/blend. The conductivity of PANI composites can be significantly improved by addition of nanofillers such as carbon nanotubes (CNTs) either multiwall or single wall connecting the dispersed PANI particles and that is one of the goals in the thesis. CNTs are attractive fillers because of their extremely high aspect ratio and exceptionally high electrical conductivity. However, the combined processing of PANI with CNTs as dispersed phases in a polymer matrix remains a considerable challenge because CNTs are also intractable in nature, like PANI. Though the enhancement in conductivities is reported in PANI-SWNT by in-situ synthesis of grafting of PANI chains on functionalized SWNTs, the reported methods do not elaborate on the processing of CNTs and PANI simultaneously. The thesis addresses the issue of co-dispersing CNTs and PANI s based on novel methods developed to a facile and combined processing of PANI with SWNTs from common organic solvent or strong acids. In Chapter 2 the synthesis of PANI is described using interfacial polycondensation at room temperature. Aniline in xylene was the organic phase on top of 1M HCl/water (bottom phase) and the polymerisation was initiated by ammonium persulphate resulting in agglomerated nano-fibrillar PANI at the interface. Subsequently, an anionic surfactant was added and after stirring and subsequently leaving the system at rest, the PANI was completely dispersed in the organic phase. In sequential experiments, single-wall CNTs, hereafter referred to as SWNT, were pre-dispersed in the aqueous phase using an anionic surfactant to obtain exfoliated CNTs in the aqueous phase and the same experimental procedure was adopted as described above. Due to the use of a surfactant, prior to polymerization, the interfacial polymerization occurred throughout the system but after stirring and leaving the system at rest, both the PANI and CNTs were completely dispersed in the organic (top phase). It is postulated that the strong interactions between the PANI backbone with the surfactant head groups as well as the hydrophobic interactions between the surfactant alkyl chains with the SWNTs are collectively responsible for the stabilization of three component system within the organic solvent such as, xylene. In Chapter 3, the use of organic PANI dispersions as described in chapter 2 is made to determine the percolation threshold, the minimum concentration required to obtain connectivity and hence conductivity between the dispersed PANI particles, within polar (polyamide 6= PA6) and apolar polymers (polyethylene). The feasibility of using organic dispersions of PANI is demonstrated by a solution-cast and melt-processing route for PA6 and polyethylene respectively. The percolation threshold of PANI within PA6 and ultra-high-molecular weight (UHMW)-PE nanocomposites is observed at 4.8 weight % and ~1.5 to 1.7weight %, respectively. The effect of the molar mass is examined further using low molar mass polyethylene and the percolation threshold is not achieved until 6 weight % of PANI. In the case of PA6-PANI nanocomposites, the presence of PANI significantly affected the melting and crystallization behaviour. Further, the conductivity in the annealed PA6-PANI nanocomposite decreased owing to reorganisation of the crystalline component which facilitated phase separation of the dispersed PANI leading into the formation of segregated domains – thus ultimately reducing the conductivity. In comparison, UHMWPE-PANI nanocomposites did not show any influence of PANI on the melting behaviour as well as annealed films did not show significant decrease in the conductivities. In chapter 4 the effect of addition of CNTs on the percolation threshold was studied. In the case of PA6- PANI+ SWNT based nanocomposites the percolation threshold is decreased by nearly 20% in the presence of only 0.04 weight % SWNTs. For comparison, PA6-SWNT films are prepared but lacklustre dispersion of SWNT is observed owing to strong SWNT agglomeration within PA6. Based on the experimental evidence, it is concluded that surfactant modified PANI acts as the effective dispersant for SWNTs within low molar mass PA6. As a consequence, SWNTs acted as nanobridges amongst conducting PANI and modified the percolation threshold. In addition nearly an order of magnitude higher conductivity is achieved at comparable PANI concentration. The reduction in the percolation threshold is also achieved in UHMWPE based nanocomposite films at only 0.02 weight % SWNTs. In addition, it is also observed that SWNT alone showed very low percolation thresholds (~0.28 weight %) with higher conductivity values compared to UHMWPE-PANI nanocomposites. Extending the influence of counter ions, Chapter 5 deals with the interactions of PANI with superheated water, 1 M HCl in water and 1 M HNO3 in water. The corresponding changes are followed in-situ using wide-angle x-ray diffraction and subsequently modelled using Materials Studio Software (Accelrys Software Inc.). Based on the information, the unit cell for emeraldine salt form of PANI is proposed. Expansion of the crystallographic lattice is observed upon treatment with superheated solvents that is found to be related to the presence and size of the anions in the solvent. All superheated solvents perpetuated disruption of crystallinity within pristine PANI. The corresponding changes on PANI backbone interactions are supported by the method described in Chapter 6. In Chapter 6, the issue of tailoring the percolation threshold was revisited but within the high performance polymer, polyparaphenylene terepthalamide (PPTA). For this purpose a superacid based route is followed. In the first part, using 98% chlorosulfonic acid, individual processing of either PANI or SWNT or a combination of PANI-SWNT is demonstrated. In subsequent part, modification of the percolation threshold in PPTA is demonstrated. The corresponding conductivity measurements exhibit that SWNTs alone formed a rather low percolation threshold (~0.6 weight %), while PANI required significantly higher amount to percolate within oriented PPTA films. This result can be attributed to the corresponding morphologies observed after treatment with superacid. The overall percolation threshold in PPTA-PANI nanocomposites is greatly reduced by addition of the minute quantity of the SWNTs (~0.3 weight %). The preliminary experiments suggested an order of magnitude lower value for the conductivity compared to pristine PANI but reduction in the percolation is achieved at nearly 25% and 50% less amount of PANI and SWNTs respectively. This experimental evidence clearly suggests that SWNTs successfully formed the nanobridges between PANI in oriented films based on PPTA