Structure and dynamics of chain-grafted polymer nanocomposites
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Polymer nanocomposites are of considerable interest in academia and industry, due to the enhanced properties arising from the addition of nanoparticles to a polymer matrix. Recent developments in controlled radical polymerisations have led to the synthesis of well-defined chain-grafted polymer nanocomposites. Research into the changes in chain conformation and dynamics in these systems is crucial for understanding the effect on macroscopic properties. Therefore, this thesis focuses on an extensive study of the structure and dynamics of these chain-grafted polymer nanocomposites. Two types of nanocomposites were investigated: polymer-silica nanocomposites and polystyrene-fullerene stars. The samples were studied primarily by neutron scattering techniques, along with complementary techniques such as dynamic light scattering (DLS) and rheological measurements. Small-angle neutron scattering (SANS) data on 6 arm PS-fullerene samples were analysed using the standard star and core-star models. The model fits have shown that the PS-fullerene stars have slightly extended chains around the fullerene core, leading to the stars being larger than expected compared to pure polymer stars. Differential scanning calorimetry (DSC), quasi-elastic neutron scattering (QENS) and rheological measurements showed that PS-fullerene stars have unusual dynamics compared to typical polymer stars, exhibiting total rather than arm molecular weight dependence. SANS measurements on polymer-silica nanocomposites in solution established that samples prepared with colloidal silica exhibit no change in chain conformation. However, fumed silica nanocomposites show a significant change in the large structure region that could not currently be modelled. QENS measurements on these samples showed that the dynamics of the polymer chains are significantly slowed down by the presence of fumed silica nanoparticles. Using the Time-Temperature Superposition principle (TTS) on the QENS and rheological data on these samples revealed a significant loss of free volume, which is therefore suggested to be the main cause of the decreased chain dynamics. Rheological measurements also confirmed a large increase in viscosity and modulus of dispersed poly(butyl acrylate)-silica nanocomposites. The presence of fumed silica has a greater effect on these properties than colloidal silica. Finally, a preliminary SANS and microscopy study on two polymer blends containing silica nanoparticles was carried out, establishing that the addition of silica can significantly decrease the miscibility of the blend. The effect of silica on the phase separation temperature is complex, and depends on the concentration of nanoparticles.