Extrusion-based 3D printing is commonly used to produce complex structures from thermoplastics. Conventional fused filament fabrication (FFF) is used widely and forms objects through layer-by-layer deposition. However, there is often a need for thin support structures based on single print layers, for example in the medical fields of hernial meshes and stents. FFF is currently incapable of achieving spatial property variation on this scale. The work in this thesis aimed to explore the formation of complex structures, utilising a conventional FFF 3D printer to manipulate polymer properties during printing.

In conventional macroscopic prints, first layer adhesion is prioritised. However, thin structures consisting of a single or few layers are subsequently difficult to remove from the substrate without damage. Here, consistency of the first layer is crucial to successful printing. In order to study property variation, a technique to produce thin, free-standing polylactic acid (PLLA) layers was developed to overcome the issue of first layer consistency. It was proposed that a sacrificial polyvinyl alcohol (PVA) layer might address issues associated with both damage during removal and print-to-print variability. To evaluate this technique, PVA and PLLA layers were printed at increasing layer separations establishing a linear relationship between programmed layer separation and thickness. This proportionality was found to vary with both extrusion factor and polymer composition. PLLA layers were found to be consistent to within a 6% gradient, translating to a required printer miscalibration in excess of 100 μm before changes in PLLA exceeded experimental error. The free-standing PLLA layers were imaged using micro-CT scanning, which revealed the successful deposition and release of 25 μm thick layers. These currently represent the thinnest reported free-standing structures produced on a conventional FFF printer.

A strategy to achieve intra-layer variation in polymer microstructure was then developed with the aim of controlling polymer microstructure during printing. A model was derived to predict the alignment in an element of extrudate resulting from the relative motion between the nozzle and print surface during printing and two key findings were obtained. Firstly, a reduction in the volume of material deposited per distance traversed, the extrusion factor, was found to increase the strain on the element of material and increased the predicted alignment. Secondly, increasing print speed at a constant extrusion factor was modelled to increase the strain rate experienced and the expected alignment while maintaining consistent deposition.

To address the main aim of this thesis, the potential for within layer property variation was tested by printing thin single layers at a range of extrusion parameters. Parameters including print speed, extrusion factor, layer separation, surface temperature were explored along with the effects of annealing to assess their effect on intra-layer properties. Alignment was confirmed experimentally through birefringence measurements. It was found that, by increasing print speed and reducing layer separation and extrusion factor, a birefringence of up to Δn = 9 x 10-^4 could be achieved in printed single layers. Elevated bed temperatures and annealing times were found to increase sample crystallinity and peak birefringence significantly from ≈ 22% to 42% and from Δn = 9 x 10^-4 to 5.5 x 10^-3. Crystallisation was shown to be localisable through the use of a custom printing stage. These results constitute a novel demonstration of spatially varying intra-layer properties, and successfully demonstrate command over intra-layer polymer microstructure and composition.

The concepts of intra-layer variation were then used for macroscopic multi-layer prints. The induced optical retardation was measured over multiple layers and the effect was found to be cumulative. Speed variations were investigated to control alignment and a novel steganographic printing technique was explored. In multi-layer structures, retardations in the range of 0 to 800 nm were demonstrated. The cumulative effect of multiple layers was evidenced in the printing of wedges, highlighting increasing retardation. The printing of perpendicular layers was utilised for compensation, allowing extinction to be achieved. This effect, coupled with an optimised multi-layer block displaying the range of retardations achievable with variations in print speed, was employed to 3D print a physical Michel Levy chart extending across the first and part of the second order. Finally, spatial variations in retardation were utilised to successfully produce steganographic prints, concealing text and a hidden image.

For more drastic changes in physical property, for example a variation from high stiffness to high toughness, a change in thermoplastic is required. Currently, material transitions require multiple layers to propagate, or offer weak inter-layer bonds. To enable multi-material printing in thin structures, first the inter-layer interface was assessed by 3D printing single layer lap shear and T-peel samples, which were mechanically tested. The interface was found to be significantly stronger when loaded parallel to the bond (lap shear) as opposed to perpendicular (T-peel). Controlled deposition was used to print a multi-material transition from dual PLLA and polyhydroxyalkanoate (PHA)/PLLA blend layers to a single PLLA layer, orientating the interface towards the printing direction. This transition was imaged using polarised light microscopy, and had a total thickness of ≈ 125 μm. The results confirmed that it is possible to control material transitions whilst minimising perpendicular loading of the interface.

This body of work demonstrates successful intra-layer property variation using conventional FFF. Manipulation of printing parameters has allowed the control of intra-layer alignment, percentage crystallinity, and crystallographic texture, and facilitated the continuous transition in layer composition along the printing direction. The results of this thesis offer important insights in the development of complex 3D printed structures that may enable the design of patient specific scaffolding such as surgical meshes and stents. The ability to control spatial variation of optical properties has potential applications in the 3D printing of photonics that may be integrated directly into medical devices to enable patient- or implant monitoring and diagnostics. Further, the novel steganographic printing technique developed may be utilised in manufacturing, allowing printer fingerprinting, part-identification and tracking.