Advanced polymeric materials for applications in technical equipment for snow sports

The thesis is divided in three chapters, each one covering one topic. Initially, the thermo-mechanical and impact properties of materials used for back protectors have been analysed. Dynamical mechanical analysis (DMTA) has shown that materials used for soft-shell protectors present frequency-sensitive properties. Furthermore, through impact tests, the shock absorbing characteristics of the materials have been investigated proving the differences between soft and hard-shell protectors; moreover it has been demonstrated that the materials used for soft-shell protectors maintain their protective properties after multi-impacts. The second chapter covers the effect of the visco-elastic properties of the thermoplastic polymers on the flexural and rebound behaviours of ski boots. DMTA analysis on the materials and flexural and rebound testing on the boots have been performed. A comparison of the results highlighted a correlation between the visco-elastic properties and the flexural and rebound behaviour of ski boots. The same experimental methods have been used to investigate the influence of the design on the flexural and rebound behaviours. Finally in the third chapter the thermoplastic materials employed for the construction of ski boots soles have been characterized in terms of chemical composition, hardness, crystallinity, surface roughness and coefficient of friction (COF). The results showed a relation between material hardness and grip, in particular softer materials provide more grip with respect to harder materials. On the contrary, the surface roughness has a negative effect on friction because of the decrease in contact area. The measure of grip on inclined wet surfaces showed again a relation between hardness and grip. The performance ranking of the different materials has been the same for the COF and for the slip angle tests, indicating that COF can be used as a parameter for the choice of the optimal material to be used for the soles of ski boots.

New functional polythiophene based materials: fine-tuning of photovoltaic or chiroptical properties

There is a remarkable level of interest in the development of π-conjugated polymers (ICPs) which have been employed, thanks to their promising optical and electronic properties, in numerous applications including photovoltaic cells, light emitting diodes and thin-film transistors. Although high power conversion efficiency can be reached using poly(3-alkylthiophenes) (P3ATs) as electron-donating materials in polymeric solar cells of the Bulk-Heterojunction type (BHJ), their relatively large band gap limits the solar spectrum fraction that can be utilized. The research work described in this dissertation thus concerns the synthesis, characterization and study of the optical and photoactivity properties of new organic semiconducting materials based on polythiophenes. In detail, various narrow band gap polymers and copolymers were developed through different approaches and were characterized by several complementary techniques, such as gel permeation chromatography (GPC), NMR spectroscopy, thermal analyses (DSC, TGA), UV-Vis/PL spectroscopy and cyclic voltammetry (CV), in order to investigate their structural and chemical/photophysical properties. Moreover, the polymeric derivatives were tested as active material in air-processed organic solar cells. The activity has also been devoted to investigate the behavior of polythiophenes with chiral side chain, that are fascinating materials capable to assume helix supramolecular structures, exhibiting optical activity in the aggregated state.

Peptide-based low molecular weight gelators for the preparation of self-assembled materials

Low molecular weight gelators (LMWGs) based on pseudo-peptides are here studied for the preparation of supramolecular materials. These compounds can self-assemble through non-covalent interactions such as hydrogen bonds and π-π stacking, forming fibres and gels. A wide variety of materials can be prepared starting from these building blocks, which can be tuned and functionalised depending on the application. In this work, derivatives of the three aromatic amino acids L-Phenylalanine, L-Tyrosine and L-DOPA (3,4-dihydroxiphenylalanine) were synthesised and tested as gelators for water or organic solvents. First, the optimal gelating conditions were studied for each compound, varying concentration, solvent and trigger. Then the materials were characterised in terms of mechanical properties and morphology. Water remediation from dye pollution was the first focus of this work. Organogels were studied as absorbent of dyes from contaminated water. Hydrogels functionalised with TiO2 nanoparticles and graphene platelets were proposed as efficient materials for the photo-degradation of dyes. An efficient method for the incorporation of graphene inside hydrogels using the gelator itself as dispersant was proposed. In these materials a high storage modulus coexists with good self-healing and biocompatibility. The incorporation of a mineral phase inside the gel matrix was then investigated, leading to the preparation of composite organic/inorganic materials. In a first study, the growth of calcium carbonate crystals was achieved inside the hydrogel, which preserved its structure after crystal formation. Then the self-assembled fibres made of LMWGs were used for the first time instead of the polymeric ones as reinforcement inside calcium phosphate cements (CPCs) for bone regeneration. Gel-to-crystal transitions occurring with time in a metastable gel were also examined. The formation of organic crystals in gels can be achieved in multicomponent systems, in which a second gelator constitutes the independent gel network. Finally, some compounds unable to gelate were tested as underwater adhesives.

Electrospun hierarchical artificial muscle for soft robotics applications

Nowadays, one of the most ambitious challenges in soft robotics is the development of actuators capable to achieve performance comparable to skeletal muscles. Scientists have been working for decades, inspired by Nature, to mimic both their complex structure and their perfectly balanced features in terms of linear contraction, force-to-weight ratio, scalability and flexibility. The present Thesis, contextualized within the FET open Horizon 2020 project MAGNIFY, aims to develop a new family of innovative flexible actuators in the field of soft-robotics. For the realization of this actuator, a biomimetic approach has been chosen, drawing inspiration from skeletal muscle. Their hierarchical fibrous structure was mimicked employing the electrospinning technique, while the contraction of sarcomeres was designed employing chains of molecular machines, supramolecular systems capable of performing movements useful to execute specific tasks. The first part deals with the design and production of the basic unit of the artificial muscle, the artificial myofibril, consisting in a novel electrospun core-shell nanofiber, with elastomeric shell and electrically conductive core, coupled with a conductive coating, for the realization of which numerous strategies have been investigated. The second part deals instead with the integration of molecular machines (provided by the project partners) inside these artificial myofibrils, preceded by the study of several model molecules, aimed at simulating the presence of these molecular machines during the initial phases of the project. The last part concerns the realization of an electrospun multiscale hierarchical structure, aimed at reproducing the entire muscle morphology and fibrous organization. These research will be joined together in the near future like the pieces of a puzzle, recreating the artificial actuator most similar to biological muscle ever made, composed of millions of artificial myofibrils, electrically activated in which the nano-scale movement of molecular machines will be incrementally amplified to the macro-scale contraction of the artificial muscle.

Nanostructured smart materials as functional components of medical devices

The field of medical devices has experienced, more than others, technological advances, developments and innovations, thanks to the rapidly expanding scientific knowledge and collaboration between different disciplines such as biology, engineering and materials science. The design of functional components can be achieved by exploiting composite materials based on nanostructured smart materials, that due to the inherent characteristics of single constituents develop unique properties that make them suitable for different applications preserving excellent mechanical proprieties. For instance, recent developments have focused on the fabrication of piezoelectric devices with multiple biomedical functions, as actuation and sensing functions in one component for monitoring pressure signals. The present Ph.D. Thesis aims at investigating nanostructured smart materials embedded into a polymeric matrix to obtain a composite material that can be used as a functional component for medical devices. (i) Nanostructured piezoelectric material with self-sensing capability was successfully manufactured by using ceramic (i.e. lead zirconate titanate (PZT)) and (ii) polymeric (i.e. poly(vinylidene fluoride-trifluoro ethylene (PVDF-TRFE)) piezoelectric materials. PZT nanofibers were obtained by sol-gel electrospinning starting from synthetized PZT precursor solution. Synthesis, sol-gel electrospinning process, and thermal treatment were accurately controlled to obtain PZT nanofibers dimensionally stable with densely packed grains in the perovskite phase. To guarantee the impact resistance of the laminate, the morphology and size of the hosting filler were accurately designed by increasing the surface area to volume ratio. Moreover, to solve the issue relative to the mechanical discrepancy between rigid electronic materials/soft human tissues/different material of the device (iii) a nanostructured flexible composite material based on a network of Poly-L-lactic acid (PLLA) made of curled nanofibers that present a tuneable mechanical response as a function of the applied stress was successful fabricated.

Diazene photoswitches in artificial molecular machines and light-effected materials

The research developed in this thesis focused on the spectroscopic and photochemical characterization of molecular diazene photoswitches, both as individual species and as functional components of mechanically interlocked molecules, molecular-based materials and artificial molecular machines and motors. Among the plethora of photochromes reported so far, azobenzene is the most versatile photoswitch due to its reproducible and well-established photochemical properties. Part I of this thesis work focuses on the characterization of light-responsive supramolecular systems based on azobenzene: a photochemically-driven rotary motor, a light-responsive supramolecular polymeric material and a supramolecular system capable of photoinduced entantiodiscrimination. Despite the wide success of azobenzene photoswitches, the tunability of their photochemical properties as a function of the diversified substitution pattern on its aryl ring presents intrinsic limitations. To overcome this issue, in the last decade heteroaryl azoswitches (i.e., azobenzene having heterocyclic rings in place of one or both phenyl groups) have attracted a great deal of attention. Hence, Part II of this thesis work treats the photochemical characterization of two different families of azoheteroarenes embedding imidazolium and thienyl functionalities in their structures. Their potential implementation in water-soluble artificial molecular machines and light-effected semiconductor materials is also assessed.

Advanced X-ray techniques for the structural characterisation of Prussian Blue Analogue cathode materials

The thesis is dedicated to the implementation of advanced x-ray-based techniques for the investigation of the battery systems, more predominantly, the cathode materials. The implemented characterisation methods include synchrotron based x-ray absorption spectroscopy, powder x-ray diffraction, 2-dimensional x-ray fluorescence, full field transmission soft x-ray microscopy, and laboratory x-ray photoelectron spectroscopy. The research highlights the different areas of expertise for each described method, in terms of material characterisation, exploring their complementarities and intersections. The results are focused over manganese hexacyanoferrate and partially Ni substituted manganese hexacyanoferrate, through both organic and aqueous battery systems. In aqueous system, the modification of cathode composition has been observed with various techniques, indicating to the processes occurring in bulk, surface, locally or in long-range, including with the speciation by 2-dimensional scanning, and the time-resolution, by the implementation of the operando measurements. In organic media, the inhomogenisation of the cathode material during the aging process was investigated by the development of the special image treatment procedure for the maps, obtained from the transmission soft x-ray microscopy. It worth mentioning, that apart from the combination of the outcomes from the various x-ray measurements, the exploration of the new capabilities was also conducted, namely, probing the oxidation state of the element with the synchrotron-based 2-dimensional x-ray fluorescence technique, which, generally, with conventional set up, is not possible to achieve. The results and methodology from this thesis can, of course, be generalised on the characterisation of the other battery systems, and not only, as the x-ray techniques are one of the most informative and sophisticated methods for advanced structural investigation of the materials.