Desenvolvimento e estudo do efeito de diferentes nanoestruturas no desempenho de dispositivos baseados em hidrogéis contendo polímeros conjugados

Abstract

In the last decades, the study and development of polymers has became one of the most attractive and challenging in the world of science and technology. Advances involving the synthesis of new materials, together with the nanometric structures creation allowing to extend its field of application in the most diverse areas. Examples are polymers dedicated to biomedical applications such as artificial muscles, controlled drug delivery systems, sensors and other devices capable of interacting with living organisms. Areas such as emergy, previously dominated by metals and ceramics are now heavily invested in developing lighter materials that provide good efficiency, low cost and the possibility of miniaturization, such as photovoltaic solar cells, mobile devices screens, batteries and capacitors. In this work we investigated the properties of crosslinked polymers in the form of hydrogels, based on acrylic acid and acrylamide in the production of electro-actuating electroactive devices that can modify their shape by the application of an electric signal. We will use concomitantly to the hydrogels a conjugated polymer in its conductive form called Polyaniline (PANI) because it allows to amplify the electroactive action by reducing the general conductivity of the final product, thus seeking to meet the demands in the literature for devices that work in low voltage electrical ( 1V) and with an angular velocity response of the order of 20 °/ s. In addition, we evaluated some of these materials in the production of a dye-sensitized solar cell known as DSS (Dye Sensitized Solar Cells) in order to further extend its applicability in device construction. As the electrochemical processes involved in the responses of these devices depend on the diffusion of ions, we developed techniques and improvements in the synthesis that allowed the obtaining of these materials in the nanostructured form, so that, with a greater specific area, it was possible to accelerate the necessary interactions, thus reaching higher materials efficient and responsive. In summary, the scope of the experiments consisted primarily in the production of samples in the form of hydrogels films by photopolymerization, in order to obtain an ideal composition between the constituents Acrylamide, Acrylic Acid and the poly (ethylene glycol diacrylate) (PEGDA) crosslinker. it is observed that in these materials the smaller proportions of the crosslinking agent lead to the formation of a hydrogel with less modulus of elasticity, capable of absorbing more water (450%), which demonstrate faster electroactive responses but slightly less able to transmit force due to deformation itself of the actuator, which is largely compensated by the highest actuation speed. The effect observed for the crosslinker is similar to that of the addition of acrylamide, showing the strong influence of the ionizable groups present in the acrylic acid with the intensification of the electroactive response of the actuators, but at the same time identifying the importance of maintaining a minimum of acrylamide that provides greater mechanical stability.From these previous conclusions, we hear the incorporation of nanostructured and selfmanaged PANI. We implemented a new technique with in situ polymerization of aniline, the reaction being controlled by diffusion at the interface of viscous liquids partially miscible and ultraviolet assisted. This technique proved to be able to create nanobastones and PANI nanofibers inside the hydrogel in reduced times (<10min). In a second configuration, such hydrogels were prepared in the form of nanofibers by the technique of electrofying with ultraviolet in situ polymerization, termed reactive electrowinning. This technique is known to be difficult to apply to hydrogels because of the need to lightpolymerize the nanofibers rapidly during the process and for this we use our own source of UV for this purpose. PANI was added to the hydrogel nanofibers in different concentrations, as well as confined to the core of the nanofibers (shell-core structure). The traditional techniques of characterization of hydrogels such as mass expansion, MEV, AFM, DSC and the evaluation of conductivity by the 4-point technique were used. In addition to these techniques, the interconnectivity of the conducting regions was evaluated by AFM with current probe (I-AFM). The swelling behavior in water was verified by AFM in liquid mode along with the mechanical properties evaluated by nanoindentation. As a parameter of electroactive property measurement we obtained the force and the displacement of the samples in aqueous medium. These responses were related to their composition and structure, showing that both the introduction of nanostructured PANI in the films and the formation of nanofibers promote a significant increase in the responses of the materials. It was obtained nanocomposites that provided performance immersed in water with voltage of 1V, consumption of powers in the 50W range, average speeds of 2mm / s or 24 ° / s and forces of the order of 70 N. In addition, nanostructured PANI hydrogels and nanofibre films were used in practical applications from the construction of 2 devices. The first one involved the use of films with nanostructures of PANI as against electrode of the already mentioned solar cells DSSC obtaining efficiencies of 2.1% using a natural dye extracted from shells of Jabuticaba (Myrciaria cauliflora), being this result considered a the best for this dye. The second device was composed of an actuator based on three layers of nanofibers, the two outer layers being formed by conductive hydrogels as a built-in electrode. This actuator was able to respond to electrical stimuli out of the water, being only moist