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Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

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

Mahato Maeda¹, Dai Taguchi¹, Takaaki Manaka¹

¹Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan.

The Journal of Physical Chemistry Letters

|December 5, 2024

Summary

Mechanical rubbing enhances molecular orientation in polyimide triboelectric generators. This study uses optical second-harmonic generation to analyze molecular alignment and power output, revealing insights into energy conversion efficiency.

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Area of Science:

Materials Science
Triboelectric Nanogenerators
Nonlinear Optics

Background:

Polyimide materials are crucial for triboelectric nanogenerators (TENGs).
Understanding molecular orientation dynamics is key to optimizing TENG performance.
Mechanical rubbing is a common method to induce polar order in polymers.

Purpose of the Study:

To investigate the dynamic changes in polar molecular orientation within polyimide TENGs during mechanical rubbing.
To correlate molecular orientation dynamics with the electrical power output of the TENGs.
To establish in situ optical second-harmonic generation (SHG) as a method for analyzing TENG power generation mechanisms.

Main Methods:

In situ optical second-harmonic generation (SHG) measurements were employed to observe molecular orientation.
Analysis of s- and p-polarized SHG intensity under s-polarized laser incidence.
I-V measurements were conducted to assess the electrical power output.

Main Results:

Mechanical rubbing enhances polar orientational order in polyimide, with relaxation occurring at a time constant of 7.1 s.
Molecules align along the rubbing direction with a tilt angle of 51° from the surface normal.
SHG analysis predicted a theoretical maximum power of 2.0 μW/cm², while experimental I-V measurements yielded 0.26 μW/cm².

Conclusions:

In situ SHG provides valuable insights into the relationship between molecular orientation and power output in TENGs.
The discrepancy between theoretical and experimental power suggests energy loss mechanisms at external loads.
This study highlights the potential of SHG for characterizing and optimizing triboelectric materials.