Direct probing of solvent-induced charge degradation in polypropylene electret fibres via electrostatic force microscopy.

Electrostatic force microscopy was used to directly probe solvent-induced charge degradation in electret filter media. Electrostatic force gradient images of individual polypropylene electret fibres were used to quantify the extent of charge degradation caused by the immersion of the fibres into isopropanol. Electrostatic force gradient images were obtained by monitoring the shifts in phase and frequency between the oscillations of the biased atomic force microscopy (AFM) cantilever and those of the piezoelectric driver. Electrostatic force microscopy measurements were performed using non-contact scans at a constant tip-sample separation of 75 nm with varied bias voltages applied to the cantilever. Mathematical expressions, based on the capacitance of the tip-sample system, were used to model the phase and frequency shifts as functions of the applied bias voltage to the tip and the offset voltage due to the fibre's charge. Quantitative agreement between the experimental data and the simplified model was observed.