(356g) Structure-Property-Performance of Electrically Conductive Nanocomposites

Designing electrically-conductive polymer nanocomposites via distributing conductive fillers throughout the insulating polymers have been considered as a good candidate for exploitation in a wide variety of applications such as electrostatic dissipation and electromagnetic radiation shielding. The increase in contact points among conductive nanoparticles is essential for providing efficient electron transfer and reducing the electrical percolation threshold which can result in proper mechanical properties as well. In this regard, the strategies such as the chemical treatment of components toward modifying polymer/nanoparticle compatibility, embedding nanoparticles in the confined phase of polymer blends, and using high aspect ratio fillers have been proposed.However, the suggested routes for manipulating the assembly of nanoparticles not only increase the cost of production, but also complicate the design of systems due to the concomitant roles of dynamic and thermodynamic parameters.

Powdered carbon black (CB) with different structures has been used as a widely available electrically conductive filler in various industries. However, reaching the percolation threshold of low-structure CB, particularly in a non-polar homopolymer matrix such as polyethylene (PE), is not simple. An important parameter in designing conductive nanocomposites is the rheological behavior of polymer matrix both in low and high shear rates. In this work, it is shown how tuning the rheological behavior of PE via the chain architecture influences the dispersion of low-structure CB particles and consequently the electrical percolation threshold. To do so, the performance of the two grades of tubular and autoclave low-density PE (branched chains) versus the high-density PE (linear chains) all in the MFI range of 0.4-0.8 (g/10min, 2.16 kg, 190 °C) are compared. The results show the electrical percolation threshold of 7.5 vol.% in LDPE matrices while no electrical conductivity is measured up to 10 vol.% in HDPE matrix. These results are assigned to the difference between the viscoelastic behavior of LDPE and HDPE. Compared to HDPE, the presence of side branches makes LDPE more elastic (storage modulus>loss modulus) in linear viscoelastic regime with high shear thinning behavior in high shear rates. Consequently, during the melt-mixing process, CB particles can be much better dispersed in LDPE matrix compared to HDPE. Also, during the molding process when there is no shear, the higher elasticity of LDPE chains slowly forces the contact between the CB particles. Generally, adding fillers increases the young modulus and yield stress of polymers but decrease the elongation at break. The tensile properties of conductive composite of LDPE/CB show less than 30% change in elongation at break compered to the LDPE. This is well beyond the values reported in the literature even those that used high-structure CB grades.

Hydrophilic Ti₃C₂T_x MXene (T = OH, O, F) nanosheets is a metal carbide with excellent performance in electronic applications. The metal core of the MXene nanosheet allows for its excellent electrical conductivity, and the abundant dissociated acidic groups in water facilitate the complexation of MXene with other materials. These features can be applied in designing MXene-based assemblies with controllable morphologies and electrical conductivities. In this work, the electrostatic interaction between negatively charged MXene nanosheets and positively charged polyelectrolytes is successfully utilized to design integrated MXene/polyelectrolyte hybrid thin films, fibers, and aerogels with distinguished internal morphologies. Generally, the direct mixing of positively and negatively charged materials leads to discrete suspended nanoparticles aggregates. Therefore, to prevent this, I apply the diffusion-driven mixing method. In this method, by the diffusion of positively charged poly(allylamine hydrochloride) (PAH) into the single-layer MXene suspension, a range of free-standing hybrid structures are formed. The assemblies are developed by the adsorption of PAH chains from an aqueous solution on the MXene surfaces at the interface and their subsequent diffusion into the bulk of MXene suspension. Also, tuning the physical interaction of MXene/polyelectrolyte controls the morphology and significantly enhances the chemical stability of MXene. My results also show that the molecular and physical characteristics of the polyelectrolyte, such as molecular weight and concentration as well as the size of MXene nanosheets determine the porosity and electrical conduction of the hybrids. Consequently, a general map showing the relation between morphology, electrical conductivity and electromagnetic interference shielding of these nanocomposites is provided.