Preparation and properties of conductive polymer nanocomposites
Loading...
Date
2015-01
Authors
Sefadi, Jeremia Shale
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State (Qwaqwa Campus)
Abstract
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.
Description
Keywords
Thesis (Ph.D. (Chemistry))--University of the Free State (Qwaqwa Campus), 2015, Polymers, Nanocomposites (Materials), Conductive polymers, Copolymers, Polyolefins