Enhancing the cellular structure and the electromechanical response of ferroelectrets - gas diffusion expansion of voided polypropylene films

Cellular electromechanically active polymers are heterogeneous materials, in which lens-shaped and usually air-filled voids are surrounded by a polymer matrix. Electrical charging leads to charge separation and trapping at the inner void surfaces and thus to macroscopic dipoles with the dimensions of the voids. The polarization can be switched by suitable electric fields. Here, a hysteresis behavior of the piezoelectric activity as well as of the dielectric displacement as functions of the poling field is observed. Therefore, in analogy to ferroelectrics, cellular electromechanically active space-charge electrets are described as ferroelectrets. Based on their heterogeneity, cellular polymers have the feature, that the dipole moments can be changed by modifying the thicknesses of the inner voids. Originally 37 μm thick cellular (voided) polypropylene films were expanded by means of a nitrogen gas-pressure treatment, which results in a change of the film stiffness. The elastic modulus decreases from 4 MPa for not expanded films up to 1.3 MPa for the films expanded with a gas pressure of 2.0 MPa. The expansion may also lead to an increase of the transducer coefficient from 16 pC/N up to 306 pC/N. In addition to the generation of highly sensitive transducer films, gas diffusion expansion is also a suitable technique for optimizing dynamical sensor properties by adjusting the thickness-extension resonance frequency.