Cellular polyethylene-naphthalate films for ferroelectret applications: foaming, inflation and stretching, assessment of electromechanically relevant structural features

Peng Fang 1 , Werner Wirges 1 , Michael Wegener 2 , Larissa Zirkel 3 , and Reimund Gerhard 1
  • 1 Applied Condensed-Matter Physics, Institute of Physics, University of Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany; fax: +493319771577
  • 2 Fraunhofer Institut für Angewandte Polymerforschung, FB Funtionale Polymersysteme, Physikalisch aktive Polymere, Geiselbergstrasse 69, 14476 Potsdam-Golm, Germany; fax: +493315683910
  • 3 Institute of Polymer Materials, Friedrich-Alexander University Erlangen-Nürnberg, Martensstrasse 7, 91058 Erlangen, Germany; fax: +4991318528321

Abstract

Cellular polymer films are frequently employed for packaging, insulation, and printing. They can also be used as ferroelectrets in transducer applications. Here, we propose a preparation process for cellular polyethylene-naphthalate (PEN) films with the following steps: (1) foaming by means of supercritical carbon dioxide (CO2), (2) controlled inflation through gas diffusion and expansion, and (3) biaxial stretching. We describe and assess the cellular structure that is formed under suitable processing conditions. For foaming, the PEN films are saturated with supercritical CO2 at room temperature for a few hours, at a pressure as high as 100 bar. The subsequent temperature treatment is very critical for controlling the sample density. Additional inflation can improve the cellular geometry and higher stabilization temperatures during the inflation process cause stronger inflation and thus lower densities. Stretching may be employed in order to achieve a regular cellular structure with lens-like voids. High electromechanical responses, i.e. large piezoelectric thickness coefficients are found only on samples within the proper density range and with optimal cellular structures

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