Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1988. / Vita. / Includes bibliographical references. / by Mark John Gore. / Ph.D.

Polymer-clay nanocomposites are attracting global interest principally because property enhancements are obtained at low clay particle loadings (1-5 wt%). However there is lack of fundamental understanding of such composites. The aim of this work is to provide an insight into the interaction between polymer and clay. This includes the driving force for intercalation, the reinforcement mechanisms and property-volume fraction relationships. Functionalised poly(ethylene glycol)-clay, poly(c-caprolactone)-clay and thermoplastic starch-clay nanocomposites with a range of polymer molecular weights, clay volume fractions and with different clays were prepared using solution methods, melt-processing methods, and in situ polymerisation. A reliable X-ray diffraction technique for low angle basal plane spacing of clay, the essential parameter for structure determination, was established obtaining ±0.005 Mn between three diffractometers. The basal plane spacing was found to be unaffected by polymer molecular weight and preparation method but was affected by the nature of the polymer and clay. Increasing clay loading could lead to a lower spacing. As a cautionary observation, poly(ethylene glycol) with high molecular weight (2: 10,000) was found to undergo degradation readily during preparation of nanocomposites with and without clay. Competitive sorption experiments for molecular weight showed that high molecular weight fractions of polymer intercalate preferentially into clay during solution preparation. Thermodynamic studies on the intercalation process found that significant enthalpic change occurred during intercalation, which is coincident with the observation that heat-treated clays without interlayer water can intercalate polymer. The calculation of true volume fraction against nominal volume fraction provided reasonable explanation of property enhancement and helps understand the relation between nanocomposites and conventional composites. At a given clay loading, nanocomposites with better dispersion gave more property enhancement than those with lower dispersion or conventional composites. The crystallinity of semicrystalline polymer was also affected by varying extents of dispersion of clay. The use of X-ray diffraction with an internal standard was explored for quantitative analysis of intercalation and exfoliation.

Adhesive bonded applications are used widely in industry because of significant advantages such as uniform stress distribution, and the ability to join different materials. However most epoxy structural adhesives are brittle at room temperature and it is required to improve their toughness. The objective of this work was to understand the fracture of adhesive joints, failure criteria and rubber toughening mechanisms via a series of experiments and FEA modelling. Double lap joints (DLJ) bonded by commercial AV119 adhesive were studied. It was found that local strain and failure path were controlled by adhesive thickness. In order to model adhesive joints accurately and efficiently, systematic fracture tests were implemented to determine the fracture criteria. Mode-I, mode-II and mixed mode fracture energy release rates were obtained by Fixed Arm Peel, 4-point End Notched Flexure (ENF) and Mixed Mode Bending (MMB) tests. Numerical analysis was applied to determine the parameters of the Drucker-Prager material model and Cohesive Zone Model (CZM). The 3D FEA results showed good agreement with experimental results of DLJ and MMB. FEA results successfully demonstrated bonding strength, stress and strain distribution and plastic deformation; and further details were found using sub models. The rubber toughening mechanism was studied by modelling different face-centred micromodels. The stress distributions ahead of the crack tip in global DLJ models were extracted and used as the loading condition for the micromodels, so that a relationship between macromodel and micromodel has been established. It is found that Von Mises and hydrostatic stress play very important roles in the toughening mechanisms and also predicted that rubber particles with multi-layer structure have more potential to toughen epoxy resin than simple rubber particles.

Self-assembly is an effective biomimetic technique for surface functionalization and nanostructural synthesis. 3-Aminopropyltriethoxysilane (APTES) is a popular molecule that can assemble over substrates to modify surface properties. Titanium is a structural material with high weight-specific mechanical properties, corrosion-resistance and bioinertness. Here, a systematic investigation was carried out to optimize the self-assembly of an APTES-modified film on an oxidized titanium surface in order to improve its biocompatibility as an implant material and molecular selectivity, e.g. for CO2 capture. A clean TiOx layer was formed on titanium after the treatment in a Piranha solution of H2SO4 : H2O2 = 3:1. The IR spectra confirmed that the formation of the APTES-modified film (called APS film) on the surface by the presence of the Si-O-Ti and Si-O-Si covalent bonds. The ordering of the self-assembled film did not show strong temperature dependence from 30 to 70ºC, although a thicker film was noted at a higher temperature. Anhydrous toluene as the solvent is essential to the formation of a well-ordered and thin film, compared with hydrous toluene. The well-assembled film was formed on the oxidised titanium surface in the anhydrous toluene solution of ~0.2 v% APTES at 30°C for 16 hours. A higher APTES concentration leads to a disorder film with protonated –NH3 + groups, whereas a lower concentration causes end groups of the adsorbed APTES to loop with the -OH groups on the surface. The APS film with the free –NH2 functional groups is more stable in aqueous solution with pH 10, although it is still hydrolyzed according to the intensity of the –Si-O-Si- bond in the IR spectra. The well-ordered APS film with the –NH2 groups cannot induce heterogeneous nucleation in a simulated body fluid (SBF), because the –NH2 groups are neutral in the solution and the –CH2- hydrophobic groups are exposed in the disordered structure of the APS film. In the application of biofuel cells, the laccase from Trametes versicolor as an enzyme was immobilized on titanium and graphite with the APS film by the covalent bond, respectively. Compared with the native laccase, optimum pH of the immobilized laccase decreased to 3 because of the increase of turnover number (Kcat). Further comparison of Michaelis-Menten constant (Km) of the immobilized laccase with the native one clearly shows that the increase of Km value is mainly due to the change of configuration of the active site, further leading to the lower affinity of immobilized laccase towards the substrate. The laccase on graphite shows higher optimum temperature and twice lower the Km value, compared with the laccase on titanium, which results from the surface morphology of graphite after oxidation. For electrochemical behaviour, graphite with the laccase as electrode does not show direct electron transfer (DET), due to the long electron tunnel between the T1 centre and electrode surface. However, the electrode with laccase shows good mediator electron transfer (MET) in the presence of mediator.

Adjoint based optimisation has until now demonstrated a great promise for optimisation in aerodynamics due to its independence of the number of design variables. This is essential in large industrial applications, where hundreds of parameters might be needed so as to describe the geometry. Although the computational cost of the methodology is smaller than that of stochastic optimisation methods, the implementation and related program maintenance time and effort could be particularly high. The aim of the present is to contribute to the effort of redusing the cost above by examining whether programs using the adjoint methodology for optimisation can be automatically generated and maintained via Automatic Differentiation, while presenting comparable performance to hand derived adjoints. This could lead to accurate adjoint based optimisation codes, which would inherit any change or addition to the relative original Computational Fluid Dynamics code. Such a methodology is presented and all the different steps involved are detailed. It is found that although a considerable initial effort is required for preparation of the source code for differentiation, hand assembly of the sensitivity algorithms and scripting for the automation of the entire process, the target of this research program is achieved and fully automatically generated adjoint codes with comparable performance can be acquired. After applying the methodology to a number of aerodynamic shape optimisation examples, the logic is also extended to higher derivatives, which could also be included in the optimisation process for robust design.

There have been several studies on the tearing of rubber materials since the seminal paper on rupture of rubber was published by Rivlin and Thomas (1953).The behaviour is typically characterised using a fracture mechanics approach whereby the rubber has a geometrically independent relationship between crack growth rate during tearing versus strain energy release rate. This approach works well under conditions of steady tearing as the crack growth rate is easy to measure. However, this approach is much harder to interpret under the condition where the rubber exhibits discontinuous crack growth behaviour such as knotty tearing or stick slip tearing. Unfortunately, these are common tearing conditions observed in practice for filled rubbers as well as for some unfilled rubbers, especially those such as natural rubber that are capable of strain-induced crystallisation. Under these conditions it is not clear what the actual crack growth rate is as the value typically given results from the average of a very rapid tearing rate and a zero velocity tearing rate. The aim of this work is to develop a new approach to characterise the unsteady tearing behaviour of rubber in terms of the relationship between the rate of increase in the strain energy at the crack tip just immediately prior to the onset of the tearing which is quantified directly as the time derivative of the strain energy release rate · T , and the critical strain energy release rate T* required to propagate the crack. The approach adopted in this study is then evaluated using a range of different crystallising and non-crystallising rubbers as well as crystallising rubbers that have been modified to alter their crystallisation over a range of different test temperatures. Additionally, a new elastic-viscous transition diagram in association with the rate of change in the strain energy release rate at the tip of the crack is presented.

The development of remotely addressable tools to encapsulate, store and deliver active materials to living cells is a particularly challenging topic of material science. As drug delivery agents, microcontainers not only require high mechanical stability or to be delivered at target cells, but they should also possess efficient remotely addressable release mechanisms. Light responsive polyelectrolyte capsules are well suited for such purposes. Capsules are constructed using the Layer‐by‐Layer technique where oppositely charged polymers are alternatively deposited on a sacrificial template. The interest for such microcapsules lays in their versatile composition and stimuli‐responsive properties, which can be altered to suit specific needs. The primary aim of this work was to develop polymeric capsules with efficient optically addressable release mechanisms. Previous work on this topic revealed severe flaws in biological environments, especially with regards to the high energy requirements necessary for laser‐induced release and in the very limited knowledge of the fate of microcapsules in living cells. These issues were addressed by developing alternative types of light‐responsive capsules and gaining better understanding of existing ones. Three types of materials were used to sensitize microcapsules to the near‐UV, visible and near‐IR spectral regions: (1) azobenzene‐substituted polymers, (2) gold nanoparticles and (3) photocatalytic porphyrinoid dyes. Various methods were used for the characterization of microcapsules, including laser scanning confocal microscopy, colloidal probe and standard atomic force microscopy, electron microscopy, fluorescence spectrophotometry, UV‐visible spectroscopy and differential scanning calorimetry. Shells were probed for their mechanical stability as well as encapsulation and release behavior based on parameters such as: assembly strategies, shell deformability, permeability, thermal response and response to laser irradiation. This thesis begins with a brief introduction followed by an extensive literature review summarizing the various topics relevant to the work. The materials and methods used in the investigations are catalogued in Chapter 3 . Chapter 4 presents the destructive effects of pulsed UV lasing on polymeric microcapsules and introduces azobenzene‐functionalized capsules with the ability to encapsulate macromolecules by exposure to continuous wave UV light. Chapter 5 looks at the mechanical properties of capsules functionalized with gold nanoparticles as well as their remote release capabilities under near‐IR irradiation. While most of these studies were conducted ex vivo, Chapter 5 concludes with a summary of studies performed in vitro, which demonstrates that it is not only possible to release substances in living cells by light but that the latter also survive in the process. Finally, in Chapter 6, the assembly and light induced destabilization of microcapsules containing porphyrinoid dyes is presented.

TiO2 has been extensively studied in the last decades due to its interesting optical and electronic properties, which, combined with low fabrication costs, renders this material very attractive for applications in photovoltaic and photocatalysis. However, the performances of titania in specific device applications were found to be strongly dependent on the synthetic methods selected for its production. The majority of such synthetic procedures rely on the hydrolysis of suitable precursors and often produce an amorphous solid, generally referred as the “amorphous” titanium oxide beside the crystalline titania. In this thesis, we thus set out to investigate amorphous materials produced by the hydrolysis of titanium tetrachlorides and tetraisopropoxide. We show that these amorphous products consists of titanium oxide hydrates, which are relatively stable at room temperature and fully convert into crystalline titania only after extended temperature treatments. We also find that titanium oxide hydrates may display highly desirable characteristic such as a strong photochromic response – especially when placed in a suitable chemical environment. In the following chapter, we then show 3 that hybrid systems can be readily prepared of titanium oxide hydrates with, for instance, macromolecular materials such as poly(vinylalcohol). The amorphous nature of the titanium oxide hydrates allows to introduce more than 90 vol.% of the inorganic species into such systems – compared to 15 vol.% or less when producing hybrids comprising, e.g., crystalline nanoparticles of TiO2. Therefore, materials can be realized that display a refractive index n of at least 2.1, without compromising transparency of the resulting structures. Remarkably, n can not only be adjusted by varying the content of the inorganic species, but also through suitable heat treatments and/or irradiation with UV-light. Potential applications for such new, versatile and tunable optical systems are also discussed in this thesis.

The research conducted is in the field of innovation and focuses on the UK energy sector. The key theme of the study is the transition towards a hydrogen economy with fuel cell technologies at the epicentre and takes into account the relevant scientific, technological, economic and policy issues. In order to provide an understanding of the factors that affect techno-economic transitions to alternative energy systems, the thesis investigates the historical transition processes such as the transition to electrification in the early 1900s and recent transitions to CCGT and renewable energy systems (wind, biofuels and solar) that have taken place since the late 1980s. As the developmental status of hydrogen technologies lay at the heart of these transitions, a thorough analysis of the hydrogen and fuel cell technologies, the R&D requirements, and innovations required in different scientific fields (including materials science) to develop these technologies is conducted. At the same time, as other factors such as sustainability, climate change and security of supply concerns can greatly affect the direction of the transition processes, that includes R&D activities and investment in alternative energy technologies, an overview of these factors is also provided. The analysis employs a new theoretical framework that combines two well established theories in the literature, Techno-economic Transitions and Large Technological Systems. By using this new framework, the technological transition towards a hydrogen energy system can be analysed at three levels, (global, national and local). The analysis is narrowed down to the local level in order to determine the timing of a transition in London and how it can form the foundation for a wider a transition at the national level based on alternative technologies.

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