Magnetically switchable micropillars from thiol-acrylate magnetite nanocomposites

Switchable surfaces are desirable since they enable tailoring of various material properties, e.g., wettability and adhesion. Several external stimuli can be used to induce a property change such as temperature, light, pH or force. However, as the use of mechanical force to program shapes can lead to irreversible defects, a non-contact method is advantageous. To circumvent mechanical defects, magnetic fields can be used as an external trigger. Herein, we incorporated iron oxide nanoparticles into photo-curable thiol-acrylate based resins that can be conveniently patterned by nanoimprint lithography. First, iron oxide nanoparticles were synthesized using a coprecipitation process. Subsequently, the particles were functionalized with vinyl- and thiol-containing siloxanes to enable good dispersion and covalent bonding of the particles to the photopolymer matrix. The size of these nanoparticles was characterized by small-angle X-ray scattering and amounted to 5.6 nm for non-functionalized nanoparticles, 5.2 nm for thiol-functionalized and 6.5 nm for vinyl-functionalized ones. To validate the silanization reaction, X-ray photoelectron spectroscopy measurements were carried out in which both Si and S atoms could be detected on the respective particle surfaces. The synthesized nanoparticles were incorporated into a thiol-acrylate resin and the glass transition temperature, viscosity, and curing kinetics of the resulting composites were determined. In a final step, nanoimprints with different aspect ratios (1:3 and 1:6) were produced and the quality of replication was quantified by optical and digital 3D microscopy. Digital laser scanning microscopy was used to detect the tilting of the micropillars under an external magnetic field.