No description available.

Sub-micro silica flakes and graphite reinforced polymer composites have been developed in this research. The sub-micro polymer composites are designed to overcome problems associated with nanocomposite technology. Apart from the high cost of production, the other major problem in nanocomposites is reduced efficiency in mechanical reinforcement at high filler loadings. The surface area of nano-fillers in high filler loaded composites is too high resulting in insufficient polymer molecules to wet the filler surface and hence increased filler agglomeration occurs. Increasing filler dimension into the sub-micro range can avoid such difficulties. The dimension of fillers in the sub-micro range also has the advantage of utilising modern polymer processing technologies. In this thesis, sub-micro composite reinforcement has been studied through the identification of suitable fillers, modelling of sub-micro composites and experimental validation of model predictions. The processing methodology, effects of filler loading, aspect ratio and compatibility of filler with matrix are also investigated. In addition, rheology, crystallisation, fracture toughness and the thermodynamic behaviour of submicro filler reinforced polymer composites are studied. The investigation was primarily focused on various combinations of single-layered silica flakes and multi-layered graphite sheets reinforced nylon-6 and polypropylene. The predicted Young's modulus and strengths of developed composites are as high as 3.3 GPa and 81 MPa at 5wt% silica flake content as well as 11.75 GPa and 301MPa at 30wt% flake fraction, respectively. Experimental results show that tensile modulus and strengths of the submicro composites produced agree well with modelling data at low filler loading. They are, however, much lower than the predictions in the high filler loading range due to the breakage of silica flakes during composite processing. In spite of filler breakage, the tensile modulus and strength of 5wt% silica flake reinforced N6 improved 68% and 67% respectively, compared to the N6. The tensile strength is also 38% higher than thatof 5wt% clay/N6 nanocomposites. The tensile modulus of 30wt% silica flake reinforced N6 improved over 240% which is similar to the achievement of 30wt% glass fibre/N6 composites. In addition, the fracture toughness and thermal deflection temperature of a silica flake/N6 composite improved by 17% at 5wt% and 20.3 oC at 10wt% filler content respectively. For multi-layered graphite sheet reinforced polypropylene sub-micro composites, mechanical properties are a function of both filler thickness and filler loading. Over 112% improvement in Young's modulus has been achieved with a composite produced via a multi-extrusion method at 20wt% filler loading, compared with the pure polymer. The crystallinities of all sub-micro composites produced are either similar to or lower than those of pure polymers. This implies that the major contribution to the improvement of mechanical properties is from sub-micro fillers. In addition, the crystallisation rate and crystallisation temperature of sub-micro composites are increased when compared with base polymers due to the nucleation effect of fillers. The introduction of sub-micro fillers into nylon-6 and polypropylene did not result in significant change in rheology. This may be associated with flake breakage and relatively poor interfacial bonding at the filler/polymer interface. In summary, this work demonstrates the potential of sub-micro filler enhanced composites in improving major engineering properties. It also proves that the sub-micro approach can fill the gap between nano and fibre reinforcement.

This thesis describes the production of advanced materials comprising a wide array of polymer-based building blocks. These materials include bio-hybrid polymer-peptide conjugates, based on phenylalanine and poly(ethylene oxide), and polymers with intrinsic microporosity (PIMs). Polymer-peptides conjugates were previously synthesised using click chemistry. Due to the inherent disadvantages of the reported synthesis, a new, simpler, inexpensive protocol was sought. Three synthetic methods based on amidation chemistry were investigated for both oligopeptide and polymerpeptide coupling. The resulting conjugates produced were then assessed by various analytical techniques, and the new synthesis was compared with the established protocol. An investigation was also carried out focussing on polymer-peptide coupling via ester chemistry, involving deprotection of the carboxyl terminus of the peptide. Polymer-peptide conjugates were also assessed for their propensity to self-assemble into thixotropic gels in an array of solvent mixtures. Determination of the rules governing this particular self-assembly (gelation) was required. Initial work suggested that at least four phenylalanine peptide units were necessary for self-assembly, due to favourable hydrogen bond interactions. Quantitative analysis was carried out using three analytical techniques (namely rheology, FTIR, and confocal microscopy) to probe the microstructure of the material and provided further information on the conditions for self-assembly. Several polymers were electrospun in order to produce nanofibres. These included novel materials such as PIMs and the aforementioned bio-hybrid conjugates. An investigation of the parameters governing successful fibre production was carried out for PIMs, polymer-peptide conjugates, and for nanoparticle cages coupled to a polymer scaffold. SEM analysis was carried out on all material produced during these electrospinning experiments.

The development and improvement of biodegradable plastics from renewable resources is a hot topic in chemical research. One example is the ring-opening polymerisation (ROP) of the cyclic ester rac-lactide to give polylactide (PLA). Current industrial metal initiators are non-stereoselective and this thesis investigates a range of catalytic initiators for the ROP of lactide and its copolymerisation with ε-caprolactone. Chapter 1 presents an introduction to the polymerisation of lactide to yield PLA with insights into polymer tacticity and associated polymer analytical techniques. A current review of the literature, with respect to the controlled polymerisation of lactide using metal initiators, is followed by a discussion of lactide copolymerisation and a review of kinetic studies. Chapter 2 consists of a series of imine bis(phenolate) ligands, with variation in phenol substituents, that have been coordinated to aluminium (III) and titanium (IV) metal centres. For the aluminium-based initiators ligand steric bulk was found to influence the coordination motif; whilst with the larger titanium metal centre the experimental methodology was also influential. Trialling as initiators for the ROP of lactide found the initiators to be non-stereoselective but with rate constants an order of magnitude higher than similar aluminium initiators in the literature. Chapter 3 describes the development of a synthetic route to tripodal unsymmetrical amine tris(phenolate) ligands. Complexes with zirconium (IV) and hafnium (IV) were characterised and a monomer-dimer equilibrium is proposed. An in-depth study for the ROP of lactide suggested that dimer dissociation into an active species, and aggregation during propagation, is influential in determining rate and molecular weight control. The unsymmetrical nature resulted in a step-reduction in stereoselectivity providing more evidence of the enhanced chain-end control previously reported for a C3-symmetric zirconium amine tris(phenolate) initiator. Chapter 4 looks to apply the aforementioned group IV initiators in the one-pot copolymerisation of lactide and ε-caprolactone. Microstructure is analysed and quantified by different methods, with tapered block copolymers being produced similar to that previously reported for aluminium isopropoxide. Few examples of zirconium-based initiators for the copolymerisation of lactide are present in the literature. Chapter 5 compiles the procedures followed for the synthesis of ligands, complexes and polymers. Conditions for the polymerisation kinetics experiments and reactivity ratios are also provided, along with details of all analytical techniques used.

In recent years, new research emphasis has been directed towards integrating multiple functions into polymeric materials. Among these new advances in materials science functional polymers with structural designs are intended to produce a specific polymer function. One group of these polymeric materials are the so-called smart hydrogels. Such polymers undergo a discontinuous or continuous large volume change transition in response to a small environmental stimulus such as temperature, pH-value, ionic strength, etc. The swelling characteristics of these networks have been studied extensively with regards to their use in micro devices or drug delivery systems. In order to utilize thin films of responsive hydrogels based on poly(N-isopropyl-acrylamide) (pNIPAAm) as active matrix in sensor applications, detailed knowledge about their structural and dynamical properties as well as their response to external stimuli is required. In the present study such dynamical and structural changes by swelling and collapsing were studied using Surface Plasmon Resonance / Optical Waveguide Spectroscopy (SPR/OWS) with dissipation. In combination with the reversed Wentzel-Kramers-Brillouin (rWKB) and Bruggeman effective medium approximation and by modelling the hydrogel film as a composite of sublayers with individual complex refractive indices, refractive index/volume fraction gradient profiles perpendicular to the surface are accessible simultaneously with information about local inhomogeneities. The imaginary refractive index k of each sublayer can specifically be interpreted as a measure for static and dynamic inhomogeneities, which were found to be highest at the critical collapse temperature in the layer centre. These results indicate that the hydrogel collapse originates rather from the film centre than from its boundaries. Furthermore, the influence of silica nanoparticles, solvent and film thickness to the swelling behaviour and transition temperature was investigated. Moreover time-resolved quantitative studies of protein-functionalized hydrogel films are shown as well as the use of hydrogel supported protein-tethered bilayer lipid membranes as a new approach toward polymer-supported lipid membranes.

The present theses describes the synthesis, self-assembly and applications of ferrocenylsilane-based diblock copolymers.

The incorporation of nanoparticles into polydimethylsiloxane polymers either in the form of physical blending or chemical crosslinking has long been studied as it can improve the properties of composite materials. Interactions between the host polymer and the filler particle, filler concentration and conformation of each component are the key factors that influence these properties. Understanding the effect of these factors is of fundamental importance in all practical applications of composite materials. This thesis describes the study of a range of PDMS composites by using a variety of experimental techniques. The main techniques used were spin-spin (T2) relaxation and diffusion NMR spectroscopy, rheology and small-angle neutron scattering (SANS). The molecular mobility of a series of PDMS melts has been studied for both unentangled and entangled molecular weight regimes separated by the critical entanglement molecular weight (Mc) of the polymer. The experimental results revealed the effect of molecular weight and polydispersity of the polymers on their segmental mobility. The dramatic decrease of chain mobility observed at molecular weight above Mc was attributed to the effect of chain entanglements. The effect of nano-sized trimethylsilylated polysilicate resin (R2) on the chain mobility of PDMS in the form of physically blended was also examined. Two different concentrations (17 and 30 vol%) of R2 were incorporated into a wide range molecular weight of PDMS melts. Below Mc, the R2 particle was found to reinforce the PDMS at all particle loadings, whereas a plasticisation effect was observed for high molecular weight PDMS above Mc. This was attributed to a reduction of the degree of the entanglements when polymer chains adsorbed on particles. Chemically bonded composites of PDMS and polyhedral oligomeric silsesquioxane (POSS) were successfully synthesised via hydrosilylation. The length of the PDMS central block was found to affect both the size and the molecular mobility of the triblock polymers. The weight fraction of POSS and substituted groups on POSS were also seen to affect the molecular mobility. Finally, a series ofrandom crosslink polymer films ofPDMS and phenylsilsesquioxane (TPh) was studied by AFM, TEM, SAXS and SANS techniques to investigate the factors influencing the optical clarity of the samples. The degree of swelling and the segmental mobility of the sample films swollen in good and poor solvents were also studied.

This work examines the thermal degradation of polybenzoxazines by monitoring the thermal degradation and evaluating the physical properties of the materials during the process. Polymers from five chemically similar bis-benzoxazine monomers have been prepared and characterised. One of the major successes of this project has been developing a reliable method to produce polybenzoxazines that are free of voids. DSC and DMA have been used to study the thermal and mechanical properties of the polybenzoxazines revealing that the phenolphthalein based bisphenol linkage (PBP-a) gives rise to the highest glass transition temperature at 200 QC with the dicyclopentadiene linkage (PBD-a) giving the lowest (150 QC), a trend which has been shown to relate to crosslink density. Thermal degradation of the materials has been studied primarily via TGA to reveal that under nitrogen polybenzoxazines (generally) undergo three stages of thermal degradation. These were identified by pyrolysis-GCIMS as: breakdown of Mannich bridges (releasing aniline), collapse of bisphenol linkages (releasing phenolic specie~) and finally degradation of the remaining char (releasing more complex structures). Char yield (at 800 QC) was greatest for PBP-a and least for PBD-a. Experimental parameters such as particle size, heating rate and atmosphere have also been found to influence thermal degradation. Pyrolysis-GC/MS has, in addition demonstrated how degradation processes vary with polybenzoxazirie backbone, atmosphere and temperature. Some polybenzoxazines have been found to expand dramatically as they are heated with some forming irregular shapes (PBP-a) whilst others (PBT -a) retain their shape well. As they char, polybenzoxazines have been found via X-ray micro-CT scanning to form porous networks dependent on backbone structure. Nanoindentation has also revealed that these materials become harder as they are charred at increasing temperatures. Computer models have been developed to simulate and predict Tg, onset of thermal degradation temperature and volumetric changes of the polybenzoxazines used in this work.

This thesis reports the formulation, characterisation and investigation of the ageing behaviour using a combination of dielectric spectroscopy and thermal analysis techniques of a series of novel polysiloxane nanocomposite elastomers which incorporate the organically modified Montmorillonite nano-clay Cloisite 6A and polyhedral oligomeric silsequioxane (POSS) as nano-scale fillers.

Poly(vinyl alcohol) (PV A) is a polymer with unique properties such as biodegradablility, biocompatibility, non-toxicity and strong hydrogen bonding capability. These positive properties are due to the pendant hydroxyl groups which dominate the chemistry of the polymer but they can also lead to one major disadvantage, the polymer in water will show aging. The aging or syneresis is due to the polymer chains coming together to form crystalline regions and forcing the weakly bonded water molecules between the chains out. This effect can be countered upon the addition of solvents that interact with the polymer chains more strongly which helps to maintain the polymer network in a more amorphous state. Therefore the effect of solvents was investigated in various poly(vinyl alcohol) network states, from solution to gel and film. The solvents could be divided based on their ability to donate labile hydrogen atoms, protic or aprotic, or on their relative size. The protic solvents were propylene glycol (pG) and dipropylene glycol (DPG) and the aprotic were dimethyl sulphoxide (DMSO), 2-pyrrolidone, N-methyl pyrrolidone (NMP), and N-ethyl pyrrolidone (NEP). This knowledge was applied when methods, such as the addition of small cross-linkers and polymeric blends, were used to engineer the properties of PV A gels and films. The size of the additive and the solvent system it was in manufactured in played an important role in the outcome of the network. Two dicarboxylic acid cross-linkers were studied, glutaric acid and sebacic acid as well as ionic polymers with carboxylic acid functionalities were also investigated. The polyelectrolytes were poly( acrylic acid-eo-maleic acid) (P AM) and poly(2-acrylamido-2-methylpropanesulfonic acid) (P AMPS).