Preparation and properties of conductive polymer nanocomposites
Sefadi, Jeremia Shale
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In this study, composites based on polyolefin matrices (EVA, PP and MAPE) filled with nanostructured expanded graphite (EG) were prepared through melt mixing. Functionalized EG was prepared using a non-covalent surfactant functionalization method. Anionic sodium dodecyl sulphate (SDS) surfactant in water was used for surface modification of EG through sonication processing, and composites containing EG with and without surfactant treatment were compared. Electron beam (EB) irradiation treatment was performed on the samples, and its influence on the overall properties of the composites was investigated. For the EG containing samples, the results showed big agglomerations and poor particle dispersion, while SDS treatment reduced the interparticle attraction resulting in better particle dispersion and interaction between the graphite platelets and the polymer matrix. EB irradiation had no influence on the morphology of the samples, since there was no polymer melting at macroscopic level when the electron beam penetrated the polymer, and the particles could not redisperse. The gel contents of the irradiated EVA samples without and with SDS treatment increased with an increase in EG loading. The irradiated EVA/SDS-EG composites had significantly higher gel content values than the irradiated EVA/EG samples due to the improved interaction and dispersion of the EG platelets in the EVA, which enhanced the energy transfer to the EVA chains and thus the crosslinking efficiency. In the case of the PP samples, there was no gel after soxhlet extraction, indicating that the formation of crosslinked material was a minor process during EB irradiation. For the MAPE samples, three different EB irradiation doses were applied, and their gel contents increased with increasing filler content and EB irradiation dose up to maxima at 4 wt% filler and 100 kGy dose respectively. The number average and weight average molar masses of PP increased with increasing EG and SDS-EG contents, but decreased after EB irradiation due to radiation induced degradation. For the EVA/graphite system, the stress and elongation at break values decreased and the tensile modulus values increased with increasing EG content, and SDS-EG containing samples showed slightly higher values due to better dispersion. This was because of the inherent stiffness of the graphite platelets, and the better dispersion of the SDS modified EG nanosheets because of better interaction between the EG and EVA. EB irradiation gave rise to significantly better tensile properties due to the radiation induced formation of a crosslinking network. For PP reinforced with EG particles, the tensile stress and elongation at break values of the composites generally decreased with increasing EG content due to poor wettability of EG by the PP and poor interfacial adhesion. The tensile modulus of the composites increased with increasing filler content, because of the higher modulus of the EG filler. EB irradiation did not significantly influence the maximum stress values due to the crosslinking and degradation effects which balanced out. For the MAPE/graphite composites all the mechanical properties increased up to a maximum of 4 wt% filler, and the SDS-EG containing samples gave better mechanical properties than the EG containing composites. EB irradiation increased both tensile stress at break and tensile modulus values, while the elongation at break values decreased with increasing EB irradiation dose. The composites exhibited a transition from insulator to conductor with an electrical percolation threshold of 5-8 wt%. The EG containing samples generally showed lower percolation thresholds than the SDS-EG containing samples because SDS formed an electrical isolation layer around the EG particles. The EB irradiation increased the electrical percolation threshold due to radiation induced crosslinking which disturbed the formation of electrical percolation networks. The thermal stability of polymer matrices increased with increasing filler content, but the SDS-EG containing samples were more stable because of the better interaction between the graphite platelets and the polymer matrix. EB irradiation increased the thermal stability when crosslinking dominated, and decreased the thermal stability when chain scission dominated. The presence of EG particles did not influence the melting temperature of the matrix, but shifted crystallization towards higher temperatures, indicating that EG acted as a nucleating agent, but SDS-EG showed slightly lower nucleation efficiency. EB irradiation did not have any influence on the melting and crystallization temperatures of the polymers. Both the storage modulus and complex viscosity of the molten polymer observably increased with increasing filler content, but the storage modulus values increased and complex viscosity values decreased with increasing frequency. The SDS-EG containing samples had a more significant effect on both properties at low EG contents, while at high EG loadings the effect was similar. The presence of EB irradiation significantly increased the storage modulus and complex viscosity values of MAPE samples due to radiation induced crosslinking of the polymer chains.