Piezoelectric behavior of three-dimensionally printed acrylate polymer without filler or poling

We report the piezoelectric behavior of three-dimensionally layer-by-layer printed (bottom-up stereolithography, 21-46-A mu m layer thickness) polymer without filler or poling, using unmodified ultraviolet-curable resins (acrylate ester for Resin 1, and methacrylated monomers/oligomers for Resin 2) that are not known to be piezoelectric. The smaller is the layer thickness, the greater is the shear stress in printing, the more is the molecular alignment, the higher is the out-of-plane electric permittivity, and the stronger is the out-of-plane direct piezoelectric effect, as shown by the electric field output decreasing reversibly and the capacitance increasing reversibly upon out-of-plane compression (stress >= 0.3 kPa). The piezoelectric effect is not merely due to the applied stress increasing the permittivity. The decrease in the electric field output is quite linear up to a stress of 33 kPa. Decrease in the layer thickness from 46 to 26 A mu m increases the relative permittivity from 5.3 to 6.1 and increases the piezoelectric coupling coefficient d from 0.13 to 0.24 pC/N. Resin 2 gives a higher d than Resin 1 (0.43 vs. 0.24 pC/N), probably due to the higher viscosity and consequent higher shear stress during printing. The fractional change in capacitance due to the applied stress increases with decreasing layer thickness, is greater for Resin 2 than Resin 1, and increases with decreasing applied electric field, which causes a converse piezoelectric effect. The capacitance increases with increasing applied electric field used to measure the capacitance. The fractional change in capacitance due to the applied electric field decreases with increasing stress.