In Situ Observation of Orientational Ordering in Polyimide Triboelectric Generators by Using Optical Second-Harmonic Generation Measurement.

By using in situ optical second-harmonic generation (SHG) measurement, dynamical changing of polar molecular orientations caused by mechanical rubbing of polyimide triboelectric generators is observed. The polar orientational order is enhanced during rubbing, which relaxes with the time constant τ = 7.1 s. Analysis of s- and p-polarized SHG intensity under s-polarized laser incidence showed that molecules align along the rubbing direction with the tilt angle θ = 51° from the surface normal. By using the obtained time constant and the tilt angle, the output power in the - measurement is discussed. That is, the SHG result gives a theoretical maximum power of 2.0 μW/cm, while the effective power observed in the - measurement is 0.26 μW/cm, suggesting that merely part of produced power is received at external loads. The in situ SHG measurement gives helpful ways to discuss the power output of triboelectric generators using polar materials.