Solvent-free extrusion emulsification (SFEE) is new top-down technique specially suited to high viscosity polymers (100-1000 Pa.s) for producing sub-micron (100-500 nm) particles inside a twin screw extruder (TSE) without the use of hazardous solvents. SFEE has been difficult to implement in industry due to process sensitivities and a lack of mechanistic knowledge on how the polymer-water morphology must develop prior to inversion. To devise a mechanistic explanation of the critical stages of the process, an inline orifice-plate type viscometer was developed to monitor rheological changes previously witnessed in early batch studies. The general variables of study throughout the thesis included the manner by which sodium hydroxide (NaOH) can be added as well as the NaOH content necessary, resin-to-water (R/W) ratio, and surfactant content. The last study in the thesis explores the influence of matrix viscosity, which was accomplished by crosslinking the polyester. The striated lamellae morphology of the polyester-water system, critically controlling the final particle size, depended on two factors, specifically surface energy (determined by endgroup conversion and added surfactant) and matrix viscosity. Analysis of the rheological response indicated that a higher polar surface energy contribution had the greatest influence on the morphological state, demonstrating a steeper viscosity transition due to more favourable and more rapid incorporation of water within the polyester matrix. A strong correlation was repeatedly found between particle size and this viscosity transition, which has been related to the thickness of striated lamellae through a theory of lamellae coarsening (or thinning as is more relevant to the current process). The reported lamellae coarsening model in the literature, which shows the predominant effects of interfacial energy and viscosity on lamellae thickness in a mixed phase system showed excellent correspondence to the results in this thesis. Among the variables of study in this thesis, the dissolution of the sodium hydroxide species (when added as a solid particle) and the kinetics of end-groups conversion proved to be rate-limiting phenomena to generating thinner striated lamellae. The ionic strength of the system was notably important to the viscosity change occurring in the process as water was added for the first time and subsequently influenced the particle size produced, particularly when additional surfactant was not added and the system relied exclusively on the carboxylate endgroups present. Finally, with mounting evidence that SFEE showed significant sensitivity to the matrix viscosity, a final study examined the effectiveness of SFEE in the face of ever increasing viscous force by blending a crosslinked polyester into the neat resin at different weight fractions. With higher viscosity there was a corresponding decrease in interfacial area growth between the polyester and water, resulting in increased particle size but even with a viscosity near 800 Pa.s, far above a traditional oil-in-water system, it was still found possible in this study to create nano-sized particles by SFEE. / Thesis / Doctor of Philosophy (PhD)