| dc.description.abstract | The response of materials to applied stress and the resulting deformation is a fundamental
cornerstone of condensed matter physics and materials research. From exciting new prospects such
as the conductive nanosheet networks formed from printed two-dimensional material dispersions like
graphene, to more longstanding puzzles such as the fundamental nature of plasticity in the entangled
threads of polymer glasses, there exists a host of poorly understood mechanisms and interactions yet
to be unravelled by the scientific community. On top of this, means for accurate and comprehensive
mechanical exploration of thin film materials has only recently been made possible due to new
nanomechanical advances, opening up further unexplored avenues of investigation.
In this work, I perform explorations into the nanomechanical properties and processing of thin films of
disordered matter ranging from complex printed networks of graphene and MoS2 nanosheets to glassy
polymers. These materials, though differing in fundamental structure, can be examined using a
common nanomechanical framework. Using carefully aligned flat punch indentation of stiffly
supported thin films, I implemented the recently developed layer compression test which allows for in
situ constitutive analysis of compressive stress vs strain behaviour providing close approximation to a
uniform, confined uniaxial strain state to compressive strains well beyond the plastic yield point. A
finite element exploration was performed to examine the degree of fidelity to uniaxial strain as a
function of tip diameter to film thickness aspect ratio, 𝛼���, and film to substrate modulus ratio, 𝑆���. It was
found that utilising a simple analytical substrate correction, variations to within 1% error are
achievable with typical experimental parameters.
The uniform compression imposed by the layer compression test was utilised experimentally to
perform the first explorations of pressure dependent mechanics of thin film polystyrene and sprayed
graphene nanosheet networks. This revealed a 45% stiffening in the regime of elastic compression up
until the yield point for both materials, despite large fundamental morphological differences between
them. Yielding of thin film PMMA was also observed in the layer compression test, in contrast to
previous studies which found that PMMA would not yield in a compressive uniaxial strain geometry.
This was attributed to an increase of shear compared to pure uniaxial strain, introduced by the layer
compression test contact geometry. Micropatterned polystyrene thin films were also prepared via
spherical tip compression to probe densification using β-NMR spectroscopy, which probes the
sidegroup relaxation dynamics via the decay anisotropy of implanted 8Li. A clear reduction in relaxation
rate was observed for micropatterned film in comparison to an unpatterned counterpart.
The layer compression test was further utilised to explore the compressive mechanical nature of
printed nanosheet network thin films of liquid phase exfoliated graphene and MoS2. A viscoelastic
response was observed, owing to the combined sheet bending and slippage modes of deformation
present. Important mechanical properties such as the effective elastic moduli and yield stress and
strain were measured and quantified for networks with a range of parameters, with changes to these
properties from densification also measured. The results were compared favourably to a folding sheet
model adapted from crumpled sheet mechanics. Creep experiments were performed to quantify time
dependent mechanics under applied strain, and the effect of chemical cross linking on the mechanical
nature of MoS2 networks was also explored.
Strain recovery and morphological changes with compression were analysed on the compressed
network regions using focused ion beam cross sections and electron microscope tomography to gauge
the compatibility of the networks with mechanical post processing for morphological improvements.
Significant strain recovery was noted over long timescales, limiting the potential of compressive post
processing. However, recovery was noted to drop significantly with introduction of shear deformation,
and more extremely to near zero magnitude at a sharply defined stress point, dubbed the lock-in point.
This lock-in phenomena was also associated with a distinct change in mechanical response of the
networks, indicating a fundamental change in material behaviour at this point that is maintained in
ambient conditions for graphene networks when pressure is removed. This lock-in point provides
promising avenues for post processing and further exploration.
In summary, this work provides the first nanomechanical exploration of sprayed nanosheet network
thin films for applications in printed electronics and shines light on various processes of deformation
as well as previously unknown pressure induced behavioural changes, with implications for the
manufacture and operation of a range of printed electronic technologies. | en |
