Many of our modern technologies require materials with unusual combinations of properties that cannot be met by the conventional metal alloys, ceramics, and polymeric materials. This is especially true for materials that are needed for aerospace, underwater, and transportation applications. For example, researchers, engineers and material scientists are increasingly searching for structural materials that have low densities, are strong, stiff, as well as abrasion and impact resistant, in addition not to be easily corroded. Material property combinations and ranges have been, and are yet being, extended by the development of composite materials. Polymer-based composites are the combinations of two or more organic and inorganic materials, mixed together to create a new material, the composite, with enhanced physical cumulative properties of the constituents. The polymer acts as the matrix, while the filler is dispersed in order to improve the physical properties of the final composite. Polymer matrices and fillers are chosen to create composites with tailored properties; e.g., high-modulus but brittle carbon fibres are added to low-modulus polymers to create a stiff, lightweight composite with increased toughness compared to the bare polymer. Recently, however, researchers have reached the limits of optimizing composite properties of traditional micrometre-scale composite fillers, because the properties achieved usually involve compromises: stiffness is traded for toughness, or toughness is obtained at the cost of optical transparency. In addition, macroscopic defects due to regions of the high or low volume fraction of filler often lead to breakdown or failure. Recently, a large window of opportunities has opened to overcome the limitations of traditional micrometre-scale polymer composites: nanoscale filled polymer composites, in which the filler is <100 nm in at least one dimension. Examples of nanoscale fillers are carbon black, carbon nanotubes (CNTs), exfoliated clays, and two-dimensional (2D) crystals, such as graphene. In particular, graphene has a Young’s Modulus of 1 TPa and intrinsic strength of 130 GPa, electrical conductivity, σ, of up to 108 S m−1, thermal conductivity of ~ 5×103 W m−1·K−1, and a specific surface area of 2630 m2 g–1, being, therefore, a promising filler for polymer matrices. Graphene/polymer composites possess not only increased stiffness and strength, compared to the pristine matrices, but can be useful for multi-functional applications such as in the electronic field, as wearable strain sensors, printed electrodes, conductive adhesives, and supercapacitors.Other 2D crystals, with their own and peculiar properties, can be used as fillers for different kind of applications. As an example, hexagonal-boron nitride (h-BN) has similar mechanical and thermal properties compared to those of graphene, but it is an electrical insulator. Therefore, h-BN/polymer composites can cover applications in which high electrical conductivity is undesirable, e.g. for thermal management or food packaging. Whereas, 2D semiconductors, such as Black Phosphorous (BP) and transition metal dichalcogenides (TMDs), can be exploited as fillers for developing composites useful for optoelectronic applications, such as pulsed fibre lasers and photo-actuators. In spite of the recent development of 2D crystals/polymer composites, there are many questions yet to be answered. In particular, many technical barriers involving structure control, dispersion of 2D crystals in the matrix, the interfacial interaction between 2D crystals and matrix, and re-aggregation issues between 2D crystal flakes must be taken into account to target the wide applications of these advanced composites. The aim of my PhD work, presented in this Thesis, was indeed to investigate 2D crystals as potential fillers for the development of future polymer-based composites, trying to meet the requirements set by the aforementioned open-questions, linking the morphology of the 2D crystals and their dispersion in the polymer matrix to the final properties of the as-produced composites. In my PhD work, I focused my attention, particularly on graphene, h-BN, and WS2. The 2D crystals used in this work have been synthesized exploiting sonication-assisted LPE and wet-jet milling (WJM)-assisted LPE, the latter being a completely novel approach developed in our group that allows an industrial rate production of 2D crystals, meeting the demand of large-scale filler production required for the composite field. The morphology of the as-synthesized 2D crystals, in terms of surface area (A), lateral size (l) and thickness (t), has been tuned by exploiting sedimentation-based separation (SBS). The size-sorted 2D crystals are subsequently used as fillers in polymer composites, investigating several polymer matrices: polycarbonate (PC), acrylonitrile butadiene styrene (ABS), and polylactic acid (PLA). Composites are produced by means of solution blending technique, allowing a thorough dispersion of the fillers inside the matrix, and then coagulated or cast in order to obtain composite pellets or composite films. Mechanical, thermal and electrical characterizations of composite films have been performed and the obtained results are linked to the morphological properties of the fillers.
|Titolo della tesi:||Science and technology of graphene-based inks for polymer-composite applications|
|Data di discussione:||14-mar-2019|
|Appare nelle tipologie:||Tesi di dottorato|