The interpretation and modelling of the dielectric response of thermosetting materials during cure was the main focus of this study. The equivalence of complex permittivity and complex impedance in terms of information content was outlined in a series of case studies covering the separate effects of dipolar movements and charge migration as well as the combined effect of the two polarisation mechanisms. Equivalent electrical circuits were used in order to model the evolution of the complex impedance during cure. A numerical method that can model consecutive spectra throughout the cure was developed. The method is based on Genetic Algorithms and requires only input from the modelling of the initial spectra. Complex impedance spectra were collected during the cure of a commercial epoxy resin formulation under isothermal and dynamic heating conditions. The spectra were analysed and modelled. The modelling was successful over the whole frequency range of the measurements (1 Hz – 1 MHz). The analysis of the estimated model parameters showed that charge migration dominates the dielectric response in a wide frequency range. In addition, the modelling algorithm also distinguished between the effects of electrode polarisation and dipolar movements in the signal. A new equivalent circuit was used in order to map the frequency regions where the each one of the three phenomena that together comprise the dielectric signal can be monitored most effectively. A chemical cure kinetics model was developed for the studied system. A correlation between the maximum point of the imaginary impedance spectrum and the reaction conversion was established. A mathematical model, based on a simple linear dependence of the dielectric signal on conversion and temperature, was built. The model predictions agreed well with the experimental data. The aim of simplifying the interpretation of the dielectric signals led to the development of a new experimental technique. Temperature Modulated Dielectric Analysis employs temperature modulations superimposed on an underlying thermal profile in order to separate the influence on the signal of the temperature alone from that of the cure reaction. The early study carried out here shows that such measurements are feasible and reveals important issues for its further development.

The high functionality epoxy resins tetraglycidyl-4,4’-diaminodiphenyl-methane(TGDDM) and triglycidyl-p-aminophenol (TGPAP) are the main components in most aerospace grade epoxy resin formulations. Owing to their high reactivity and high viscosity, TGDDM and TGPAP pose difficulties when used in wet layup composite manufacturing. As such, these resins are often modified to achieve the desired performance both in the liquid and cured states. The main objective of this thesis is to optimise a low viscosity multi-component epoxy resin formulation suitable for use as an aerospace grade composite matrix. The formulation will allow for the addition of high levels of thermoplastic to improve the fracture toughness of the resin whilst also maintaining resin processability. Through the use of thermal analytical techniques this thesis aims to study the effects of varying the TGDDM/TGPAP ratio, incorporation of a low viscosity bi-functional epoxy resin, the diglycidyl ether of bisphenol F (DGEBF) and changes to the stoichiometric ratio (r)between reactive groups of the epoxy resin and amine hardener (4,4’-diaminodiphenylsulphone, DDS) in multi-component epoxy resin formulations. Resin formulations were optimised using factorial experimental design (FED). Results from two FED’s showed curing multi-component resins at a low stoichiometric ratio significantly increased the processing window whilst also increasing the glass transition temperature (Tg) of the cured resin. No apparent benefit could be assigned to the inclusion of TGDDM owing to its poor processability and a Tg similar to TGPAP. Up to 60% DGEBF was incorporated in a multi-component resin formulation whilst still attaining a Tg greater than 220°C. Its inclusion at 60% had the additional benefit of increasing the processing window by 48 minutes over TGPAP, an increase of 62%. Two optimised resin formulations, 100% TGPAP (100T) and a binary mix of 60% DGEBF and 40% TGPAP (60D) were taken forward to study the effects of adding a thermoplastic toughener (polyethersulphone, PES) in incremental amounts up to 50wt%. SEM images showed all toughened 100T resins had a phase separated morphology whilst all 60D resins were homogenous. The phase separation seen in 100T did not improve the matrix fracture toughness when loaded at 10 wt% and 30 wt% PES. Only when 50 wt% PES was added did fracture toughness increase in comparison to the homogenous 60D resins. Through factorial experimental design two epoxy resin formulations which excluded TGDDM were optimised with a low stoichiometric ratio. The optimum aerospace formulation is dependent on the desired processability and fracture toughness of the resin. High DGEBF-containing formulations give the longest processing windows whilst the 100% TGPAP formulation toughened with 50% PES has the highest fracture toughness.

This dissertation focuses on developing 3D printing as a fabrication method for microfluidic devices. Specifically, I concentrate on the 3D printing approach known as Digital Light Processing stereolithography (DLP-SLA) in which serially projected images are used to sequentially photopolymerize layers to build a microfluidic device. The motivation for this work is to explore a much faster alternative to cleanroom-based microfabrication that additionally offers the opportunity to densely integrate microfluidic elements in compact 3D layouts for dramatic device volume reduction. In the course of my research, an optical approach was used to guide custom resin formulation to help create the interconnected hollow regions that form a microfluidic device. This was based on a new a mathematical model to calculate the optical dose delivered throughout a 3D printed part, which also explains the effect of voids. The model was verified by a series of 3D printed chips fabricated with a commercial 3D printer and a custom resin. Channels as small as 108 µm x 60 µm were repeatably fabricated. Next, highly compact active fluidic components, including valves, pumps, and multiplexers, were fabricated with the same 3D printer and resin. The valves achieved a 10x size reduction compared with previous results, and were the smallest 3D printed valves at the time. Moreover, by adding thermal initiator to thermally cure devices after 3D printing, the durability of 3D printed valves was improved and up to 1 million actuations were demonstrated.To further decrease the 3D printed feature size, I built a custom 3D printer with a 385 nm LED light source and a 7.56 µm pixel pitch in the plane of the projected image. A custom resin was also developed to take advantage of the new 3D printer's features, which necessitated developing a UV absorber screening process which I applied to 20 candidate absorbers. In addition, a new mathematical model was developed to use only the absorber's molar absorptivity measurement to predict the resin optical penetration depth, which is important for determining the z-resolution that can be achieved with a given resin. The final resin formulation uses 2-nitrophenyl phenyl sulfide (NPS) as the UV absorber. With this resin, along with a new channel narrowing technique, I successfully created flow channel cross sections as small as 18 µm x 20 µm.With the custom 3D printer, smaller valves and pumps become possible, which led to the invention of a new method of creating large numbers of high density chip-to-chip microfluidic interconnects based on either simple integrated microgaskets (SIMs) or controlled-compression integrated microgaskets (CCIMs). Since these structures are directly 3D printed as part of a device, they require no additional materials or fabrication steps. As a demonstration of the efficacy of this approach, 121 chip-to-chip interconnects in an 11 x 11 array for both SIMs and CCIMs with an areal density of 53 interconnects per square mm were demonstrated, and tested up to 50 psi without leaking. Finally, these interconnects were used in the development of 3D printed chips with valves having 30x smaller volume than the valves we previously demonstrated. These valves served as a building block for demonstrating the miniaturization potential of an active fluid mixer using our 3D printing tools, materials, and methods. The mixer provided a set of selectable mixing ratios, and was designed in 2 configurations, a linear dilution mixer-pump (LDMP) and a parallelized dilution mixer-pump (PDMP), which occupy volumes of only 1.5 cubic mm and 2.6 cubic mm, respectively.