Transparency enhancement for photoinitiated polymerization (UV curing) through magnetic field alignment in a...

We use a magnetic field to align nickel particles into stringlike assemblies in urethane oligomer mixtures and create a semitransparent UV-curable nickel particle/polymer composite with anisotropic electrical conductivity and piezoresistive properties. When the particles are uniformly distributed in the oligourethane matrix, the mixture is moderately conductive at higher particle fractions but becomes insulating once the fraction is below about 5 vol %. With the particle fraction below this threshold and using an external magnetic field, the particles are aligned into continuous pathways through the oligomer mixtures following the magnetic flux lines. The matrix is subsequently cured by UV light. This results in conductivity and piezoresistivity along the alignment direction, while the material is not conducting perpendicular to the alignment direction. The lower particle fraction results in a lower number of absorbers for UV light: the decrease from 5 to 1 vol % increases optical transmission from 10% to 50% in the UV/vis region. This leads to a shorter photocuring time, typically from tens of seconds to seconds for 300-μm-thick films at a wavelength of 365 nm. We propose that this concept could be applied in areas such as pressure sensors.