Preparation and properties of conductive polymer nanocomposites

[摘要] In this study, composites based on polyolefin matrices (EVA, PP and MAPE) filled with nanostructuredexpanded graphite (EG) were prepared through melt mixing. Functionalized EG wasprepared using a non-covalent surfactant functionalization method. Anionic sodium dodecylsulphate (SDS) surfactant in water was used for surface modification of EG through sonicationprocessing, 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 theoverall properties of the composites was investigated.For the EG containing samples, the results showed big agglomerations and poor particledispersion, while SDS treatment reduced the interparticle attraction resulting in better particledispersion and interaction between the graphite platelets and the polymer matrix. EB irradiationhad no influence on the morphology of the samples, since there was no polymer melting atmacroscopic 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 treatmentincreased with an increase in EG loading. The irradiated EVA/SDS-EG composites hadsignificantly higher gel content values than the irradiated EVA/EG samples due to the improvedinteraction and dispersion of the EG platelets in the EVA, which enhanced the energy transfer tothe EVA chains and thus the crosslinking efficiency. In the case of the PP samples, there was nogel after soxhlet extraction, indicating that the formation of crosslinked material was a minorprocess during EB irradiation. For the MAPE samples, three different EB irradiation doses wereapplied, and their gel contents increased with increasing filler content and EB irradiation dose upto maxima at 4 wt% filler and 100 kGy dose respectively. The number average and weightaverage molar masses of PP increased with increasing EG and SDS-EG contents, but decreasedafter EB irradiation due to radiation induced degradation.For the EVA/graphite system, the stress and elongation at break values decreased and thetensile modulus values increased with increasing EG content, and SDS-EG containing samplesshowed slightly higher values due to better dispersion. This was because of the inherent stiffnessof the graphite platelets, and the better dispersion of the SDS modified EG nanosheets because ofbetter interaction between the EG and EVA. EB irradiation gave rise to significantly better tensileproperties due to the radiation induced formation of a crosslinking network. For PP reinforcedwith 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 poorinterfacial adhesion. The tensile modulus of the composites increased with increasing fillercontent, because of the higher modulus of the EG filler. EB irradiation did not significantlyinfluence the maximum stress values due to the crosslinking and degradation effects whichbalanced out. For the MAPE/graphite composites all the mechanical properties increased up to amaximum of 4 wt% filler, and the SDS-EG containing samples gave better mechanical propertiesthan the EG containing composites. EB irradiation increased both tensile stress at break andtensile modulus values, while the elongation at break values decreased with increasing EBirradiation dose.The composites exhibited a transition from insulator to conductor with an electricalpercolation threshold of 5-8 wt%. The EG containing samples generally showed lowerpercolation thresholds than the SDS-EG containing samples because SDS formed an electricalisolation layer around the EG particles. The EB irradiation increased the electrical percolationthreshold due to radiation induced crosslinking which disturbed the formation of electricalpercolation networks.The thermal stability of polymer matrices increased with increasing filler content, but theSDS-EG containing samples were more stable because of the better interaction between thegraphite platelets and the polymer matrix. EB irradiation increased the thermal stability whencrosslinking dominated, and decreased the thermal stability when chain scission dominated. Thepresence of EG particles did not influence the melting temperature of the matrix, but shiftedcrystallization towards higher temperatures, indicating that EG acted as a nucleating agent, butSDS-EG showed slightly lower nucleation efficiency. EB irradiation did not have any influenceon the melting and crystallization temperatures of the polymers.Both the storage modulus and complex viscosity of the molten polymer observablyincreased with increasing filler content, but the storage modulus values increased and complexviscosity values decreased with increasing frequency. The SDS-EG containing samples had amore significant effect on both properties at low EG contents, while at high EG loadings theeffect was similar. The presence of EB irradiation significantly increased the storage modulusand complex viscosity values of MAPE samples due to radiation induced crosslinking of thepolymer chains.

[发布机构] University of the Free State