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Title: Polymer-fullerene mixtures : structure, dynamics and engineering applications in bulk and thin films
Author: Wong, Him Cheng
ISNI:       0000 0004 2710 2218
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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This thesis reports an experimental investigation of the structural, dynamical and glass formation properties of model polymer-nanoparticle mixtures, focusing in particular on polystyrene (PS)-fullerene (C60) nanocomposites, both in the bulk and in thin films. We show that the addition of C60 alters the glass formation and dynamics of PS in a non-trivial manner. Combining present differential scanning calorimetry (DSC), dielectric spectroscopy (DS) and previous inelastic neutron scattering (INS) experiments, we find that C60 slows down the chain segmental ( α) relaxation of PS, causing an increase of the glass transition temperature (Tg), dynamic fragility (m), and α relaxation time (Tα ), while also increasing the amplitude of atomic vibrations in deep glassy state, as seen by an increase in mean-square displacement of hydrogen motion. These findings are interpreted as disruption to molecular packing and an increase of free volume in the glass state. General trends in dynamics and glass formation induced by different classes of nanoparticles are compiled and critically interpreted. Specifically, changes to Tg and fragility appear to result from the interplay between the bulk molecular packing state of the nanocomposite glass and both the polymer-nanoparticle interaction strength and interfacial area. Nanoparticle size and dispersion are therefore of paramount importance and a systematic C60 aggregation study using small angle neutron scattering (SANS) and wide angle X-ray scattering (WAXS) was thus carried out. Conditions and limits for miscibility of PS-C60 nanocomposites, at relevant processing steps, were investigated and relevant miscibility and dispersibility thresholds established. The C60 fullerenes are found to associate into fractal-like objects in bulk nanocomposite mixtures, upon annealing above the miscibility concentration and temperature, following asymptotic kinetics. In thin films, however, C60 association is bound by 2D film confinement and the resulting nanocomposite thin film structure changes qualitatively. At low nanoparticle loading, we observe sparse C60 nucleation, accompanied by crystallisation, which is well described by Avrami relation. At increasing C60 concentration, up to the dispersibility limit, a novel nanoparticle self-association mechanism is observed, coined "spinodal clustering". This process yields remarkably regular spinodal-like morphologies of C60 clusters with tuneable characteristic spatial frequency and amplitude, which coarsen with time following well-defined scaling laws, analogous to those of 2D phase separation of binary mixtures. Mapping of this self-assembly process in thin films utilised a combination of optical microscopy (OM), atomic force microscopy (AFM) and neutron reflectivity (NR) techniques. Unexpectedly, photo-illumination is found to affect thin film stability and morphology network. Combined, these allow further tuneability of nanocomposite thin film morphology and yield ultrathin films with unprecedented mechanical integrity and stability at elevated temperatures. Coupling the fundamental processes presented in this thesis, namely the photo-chemical transformation of C60, the spinodal clustering and thin film dewetting of nanocomposite thin films, we introduce a novel self assembly photopatterning approach which is both cheap and procedurally simple. Various technological applications are envisaged in the fields of organic photovoltaics (bulk heterojunctions morphology), rapid pattern assembly (fabrication of polymer-based plastic electronics) and functional hierarchical coatings (ultrathin stable lms). A prototype "circuit" device has been fabricated as a proof of principle and is shown on the cover image.
Supervisor: Cabral, Joao Sponsor: Engineering and Physical Sciences Research Council ; Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral