Abstract
Impact-resistance and toughness in natural composites is achieved due to the size of the constituent materials and their arrangement within the composites. The helicoidal geometry is known to provide a toughening and impact-resistance mechanism in materials. Achieving a helicoidal structural design on the micro-nano scale while adhering to conventional fabrication techniques has been challenging. These limitations of current methods are explored and overcome in this thesis. This thesis develops an unconventional approach that is based on electrospinning to fabricate helicoidal architectures. This thesis addresses the development of the technology and fabrication methodologies, as well as the protocols for their implementation, to bridge the gap between the micro-metric and nano-metric architectures of a material, and the macro-scale constructions. The mechanical properties and deformation behaviour of these materials are investigated. The result of this research is a new class of high-performance hierarchical electrospun three-dimensional (3D) helicoidal fiber-reinforced polymer composites. The helicoidally organized materials that are fabricated in this thesis improve the toughness and tolerance to damage of non-hierarchically organized bulk materials of similar composition and even those with different hierarchical organizations, such as unidirectional, cross-ply, and mesh designs. In the initial approach, the production of fibers with diameters of hundreds of micrometers and helicoidal geometry was achieved using melting-based near-field electrospinning (NFES). These fibers displayed a greatly enhanced toughness of 8 0:8 J mm??3, which represents an improvement of 1:5 and 5:3 times with respect to the unidirectional construction and bulk construction, respectively, of materials with similar composition. This result was improved by the surface treatment of the polycaprolactone (PCL)-based fibers to include amino moieties, which caused a three-fold increase in toughness. The limits of the approach were extended by the development of solution-based NFES, instead of melting-based NFES, which enabled more accurate control of the diameters of the fibers, which successfully produced helicoidal geometries with diameters of tens of micrometers. The improved composites showed an enhanced toughness that reached 17:9 0:8 J mm??3, which represents an improvement of twelve times toughness of the unorganized bulk material. The helicoidal fibrous composites were employed to reinforce a three-dimensional polymeric composite by embedding them in a matrix of polyvinyl alcohol (PVA), which produces a helicoidally aligned synthetic structural composite (HA-SSC). The composites were optimized at a fiber content of 88% and subjected to a surface treatment with surfactant Triton X-100, which yielded an improved specific toughness (21:1 0:4 J g??1) and specific impact-resistance (128 2 mJ cm3 g??1) with respect to the bulk matrix and cross-ply composites. The study of the influence of the angular orientation of the reinforced fibers on the mechanical characteristics of the composite revealed that small angular offsets produce composites with enhanced toughness (3 0:3 J g??1) and specific impact-resistance (47 1:5 mJ cm3 g??1) with respect to those with larger angular offsets. The upscale of the technology to levels relevant for manufacturing was explored with the development of the electrospinning method based on parallel plates and farfields (pp-FFES). This change in the geometry of the collector enabled the production iv of sub-micron and nano-sized 3D helicoidal fiber-reinforced polymer composites. HASSCs produced by pp-FFES show a specific toughness of 2:5 0:4 J g??1, which comprises an improvement of 1:7 and 1:2 times the toughness of the randomly oriented composite and bulk matrix, respectively. Similar results were obtained for the specific impact-resistance. The specific impact-resistance of 8 0:8 mJ cm3 g??1 of the HA-SSCs represents an improvement of 2 and 2:4 times the randomly oriented composite and the bulk matrix, respectively. We believe that the advancement in the understanding and fabrication of the hierarchical composites with helicoidally aligned reinforcements will have a direct implementation in multiple fields, because materials with enhanced mechanical properties are central in applications that require high-performance solutions, such as protective clothing, wearable piezoelectric sensors, impact-resistant photo-voltaic (PV) solar cells and sensitive electronics coverings.