Elastomer Structured Composites with Anisotropic Electric and Magnetic Properties. Application to the Development of Physical Sensors based on Reversible Piezo and Magneto Resistivity

NEGRI, RICARDO MARTÍN; G. A. JORGE; MIETTA, J.L.

Conferencia; XII Congreso Iberoamericano de Polímeros/XIV SLAP; 2014

Structured elastomer composites based on dispersions of magnetic/conductive filler particles in a polymer, curing or evaporating the solven in the presence of a magnetic field are presented. Different fillers were used (cobalt-ferrites, nickel nanochains and nanotubes, magnetites covered with silver) and different polymers as well (PDMS, SBR). The mentioned procedure induce generation of pseudo-chains (needles) inside the elastomer matrix, whci are aligned in the direction of the magnetic field applied dueurng preparation. Thus, the final composites have anisotropic elastic, electric and magnetic properties. For example, electrical conduction is obtained mainly or exclusively in the direction of the aligned pseudo-chains. The Young´s modulus and the magnetization are larger in that direction as well. In addition, these properties are inter-dependent. For instance, the electrical conductivity and the Young modulus changes if a magnetic field is applied on the composite. Large magneto resistivity effects (changes of electrical resistivity larger than 50% with moderated magnetic fields) are obtained using silver-covered magnetite particles (Fe3O4@Ag) in PDMS. Application of an external stress also produces increase of the conductivity though a percolating mechanism. When SBR is used, the addition of diethylene glycol (DEG) prevents formation of a bound rubber layer on the pseudo-chains, allowing percolation and thus conduction. Sensors for stress and magnetic field were developed based on the mentioned properties. As an example, a flexible, anisotropic and portable stress sensor (logarithmic reversible response between 40-350 kPa) was fabricated, where i) the sensing material, ii) electrical contacts and iii) encapsulating material, are based on polydimethylsiloxane (PDMS) composites. The sensing material is a slide of an anisotropic elastomer formed by Fe3O4@Ag particles in PDMS and curing in the presence of a uniform magnetic field. Due to the induced structure, the electrical conductivity appears only between contact points which face each other at both sides of the slide. An array of electrical contacts was implemented based on PDMS-silver paint metallic composites, encapsulated with PDMS also (full-polymeric sensor). The same material can be used as magnetic field sensor, since using Fe3O4 superparamagnetic nanoparticles opens possibilities for reversible magnetoresistance effects.