Small Strain-Induced Abrupt Drop in the Electrical Breakdown Strength of Polypropylene Insulation

It is commonly acknowledged that the electrical properties of polymer dielectrics remain stable before yielding. However, in this Letter, an abrupt 17.4% decline in the electrical breakdown strength was observed at 4% tensile strain in polypropylene (PP) insulation, which is far below its yielding strain (ε_y, 16%). In situ scanning electron microscopy was conducted, and microvoids were unexpectedly found, forming at 4% strain, which is inconsistent with the understanding that cavitation only happens near ε_y. The microvoids lead to higher free volume, increased carrier mobility, and the abrupt decline in the breakdown strength. Furthermore, such critical strain was identified as the elastic limit strain (ε_e, 3.5%). On that basis, PP samples with suppressed yields and higher ε_e(5.4%) were prepared with more elastomers. Only a 5.1% decrease in the breakdown strength was observed at 6% strain. This work reveals that suppressing yield and increasing ε_ecan improve the electrical performance stability of semicrystalline dielectrics under mechanical stress.