EARNSHAW, Jacob Kristian (2022). Molecular Dynamics simulation of polyhedral oligomeric silsesquioxanes (POSS) and their polymeric nanocomposites. Doctoral, Sheffield Hallam University. [Thesis]
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Earnshaw_2023_PhD_MolecularDynamicsSimulation.pdf - Accepted Version
Available under License Creative Commons Attribution Non-commercial No Derivatives.
Earnshaw_2023_PhD_MolecularDynamicsSimulation.pdf - Accepted Version
Available under License Creative Commons Attribution Non-commercial No Derivatives.
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Abstract
Within this thesis, I use atomistic Molecular Dynamics simulation to elucidate the thermomechanical
properties of polyhedral oligomeric silsesquioxanes (POSS). These molecules have
unique and interesting properties due to their nanoscale size and organic-inorganic nature.
POSS species comprise a rigid silica core which is functionalised with organic moieties at the
vertices. These nanoparticles exhibit a wide range of behaviours and properties due to the
scope of viable functionalising groups. I largely focus here on the cubic T8 conformation,
which typically ranges from 1-10 nm in diameter.
For pure POSS systems, I use systematic simulation studies to characterise the sensitivity
of the glass transition temperature (Tg), an important macroscopic property, to molecular
structure. For POSS species functionalised with eight
exible groups, I identify a key
molecular feature, namely, the breathing mode, as the degree of freedom that controls the
macroscopic Tg. Due to their high degree of inter-molecular entanglement, these systems
exhibit very little molecular mobility. Thus, it is the molecular-level movements within the
functionalising groups that underpin the systemic change from a glassy state to rubbery
state. The in
uence of structural morphology in tetra functionalised T8 systems are also
studied. Here, I observe the adoption of richer packing arrangements and shifts in the Tg
values due to alternative arrangements of the functionalising groups about the central core.
I also examine the spontaneous formation of crystalline structures from amorphous starting
configurations by systems of rigid POSS molecules.
Due to the versatility in their structure, POSS are frequently included as a component
in nanocomposite materials. Within this work, I have study POSS-polymer hybrids as both
blended and grafted nanocomposites. I use hydroxyl-terminated polybutadiene (HTPB) for
the polymer matrix and explain the observed changes in Tg as a product of the molecular
behaviour and interactions. When grafted, POSS have a local anchoring effect on the dihedral
rotational freedom available to the C-C single bonds of the HTPB backbone. Thus, I
observe a significant increase in the Tg for these systems. When blended, rigid POSS hybrids
exhibit significant phase separation, whereas
flexible POSS species are much more dispersed
within a HTPB matrix. Both systems impart less specific impact on the dihedral rotation
of the HTPB matrix than is observed in the grafted systems. However, increases in Tg are
consistently observed with increasing POSS concentration. Through the simulation of POSS
blended with a chemically similar but sterically different matrix, I further characterise the
mixing behaviours of POSS species as a function of their functionalising group.
These studies provide insight into the structure-property relationship for a variety of POSS
species as pure entities and nanocomposite hybrids, thus providing understanding for future
programmes of synthesis or nanocomposite design, as well as necessary target properties for
candidate coarse-grained models of POSS systems.
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