Large Scale Production of Porous and Non-Porous PVDF/GNPs Nanocomposites for Electrical and Electromechanical Applications

In this thesis work, PVDF/GNP nanocomposites have been investigated, focusing on electrical, electromechanical, and electromagnetic applications. PVDF/GNP nanocomposites comprise a new generation of multifunctional materials that combine the properties of PVDF and of GNPs. In particular, different nanocomposites made of PVDF filled with different weight concentrations of GNPs were fabricated, without any chemical modification or functionalization, either on GNPs or on polymer chains. Thus, this work can open new perspectives in the use of graphene-based nanofillers in polymer composites, since no chemical modification or functionalization of graphene is needed. Furthermore, the effect of GNPs on morphology, electrical, electromagnetic, mechanical, and electromechanical properties of PVDF/GNP nanocomposites have been studied. This thesis is organized into two parts. The first one consists of five chapters and deals with the PVDF/GNP nanocomposite film production and characterization. In the first Chapter, short overviews of GNPs and PVDF are provided, focusing on their structure, main properties, and synthesis techniques. In Chapter 2 the best time-temperature combination in the PVDF-film preparation process is discussed. This combination is very important for the PVDF film structure. Then, GNP/PVDF nanocomposites were fabricated via the solution mixing method. It was found that the addition of GNP in PVDF has a strong effect on the conductivity of the nanocomposite. In particular, when 2wt % GNP is added in the PVDF polymer matrix, the electrical conductivity of nanocomposites is around 16 orders of magnitude greater than the one of pure PVDF. In Chapter 3, the nucleation effect of unmodified GNPs on PVDF/GNP composite films was investigated. To the best of our knowledge, this is the first study focused on the use of GNPs without any chemical modification or functionalization as nucleation agents for β-phase formation enhancement. Furthermore, the morphological, electrical, mechanical and electromechanical properties of film nanocomposites were significantly affected by the nucleation effect of GNPs on polymer chains. Chapter 4 deals with the evaluation of the piezo-resistive properties of PVDF composite films filled with GNPs. The samples have thickness in the range of 20-30 µm and they are characterized by high flexibility and stability, and by remarkable chemical and physical resistances. The piezo-resistive behavior of the PVDF/GNP composite films filled at 1.5% and 2% wt has been studied under quasi static and cyclic flexural loadings. In both cases, the produced films show a stable and repeatable response to the applied flexural strain. In particular, the computed sensitivity at a strain of 1.5% is nearly 15 for the PVDF/GNP film loaded at 1.5% wt. On the other hand, Chapter 5 deals with the piezoelectric response, measured through the piezoresponse force microscopy (PFM). PFM investigations have been adopted to assess the piezoelectric properties of the PVDF/GNP nanocomposites at the nanoscale. The piezoelectric responses of the different samples were compared: neat PVDF, PVDF nanocomposite filled with GNP at 0.3 wt%, 0.5 wt% and 0.7 wt%. The enhancement of the piezoelectric response of the PVDF / GNP nanocomposite can be explained assuming that GNPs induce the formation of the β-phase in PVDF, as shown elsewhere. The results show a qualitative correlation between induced β-phase, as assessed through FT-IR measurements, and intensity of the measured piezoelectric response, resulting from the PFM analysis. The second part of the thesis consists of two chapters and is focused on PVDF/GNP nanocomposites for electromagnetic and power generation applications. Chapter 6 deals with the synthesis and characterization of 3D porous graphene nanocomposite aerogels for electromagnetic applications. The produced nanocomposites are morphologically and electrically characterized, and their relative complex permittivity is measured in the frequency range of 8-18 GHz. Finally, in Chapter 8, a novel flexible and washable membrane for renewable energy production is investigated. In particular, an aluminum-PVDF/GNP membrane saline battery is designed, fabricated, and characterized. It is noticed that the voltage generated is quite stable with the time since voltage variations are visible only at the beginning of the measurements. An almost constant voltage of 0.8 V was measured in the matching condition, i.e. when the cell is loaded on a 470 kΩ resistance