Electromechanical properties of melamine foams covered by polypyrrole nanotubes and carbonaceous fillers
Abstract
Melamine foam became a popular flexible material, which can be used as a free-standing scaffold of electrically conductive fillers for development of experimental low-pressure sensors. Here, melamine foam was covered with conducting polymer polypyrrole of nanotubular morphology (PPy-NT) to obtain a material with high conductivity and mechanical flexibility.
A surfactant polyvinylpyrrolidone and two carbonaceous fillers (carbon black and expanded graphite) serving as modifying agents of electrical and mechanical properties were used as composite components during in situ polymerization and deposition of PPy-NT on the surface of melamine foam. Electrical, mechanical, and structural properties of the fillers and their melamine composites were assessed and discussed.
Effects of the surfactant and the fillers on pressure sensing properties of melamine composites were studied using low pressure testing (Delta p < 160 kPa) in terms of a characteristic curve, stability, repeatability, hysteresis and memory effect. Impedance spectroscopy in the range of 4 Hz to 5 MHz was employed as a tool for better understanding of electrical behavior of a melamine foam under the stress.
An equivalent electrical circuit of foam samples consisting of parallel RC and distributed elements was estimated, modelled and compared with all measured data. Overall, prepared melamine composites exhibit practically applicable piezoresistive effect (impedance Z changes approx. in four orders of magnitude in the measured range, which can be modified by addition of carbonaceous components), DC electrical resistivity, volt-ampere characteristics as well as interesting AC electrical properties.
On the other side, they suffer from memory effect (up to 40%) and hysteresis (30 - 45%) appearing especially when a testing pressure was too high. Hence, the goal of this work was to assess and present strengths and weaknesses of electromechanical properties of melamine foams, which have substantial influence on their application in highly sensitive piezoresistive pressure sensors or flexible supercapacitors and batteries.