Molecular Dynamics simulation of polyhedral oligomeric silsesquioxanes (POSS) and their polymeric nanocomposites

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.