Aspects of the disclosure relate to manufacturing a balloon envelope for use in a stratospheric balloon system. For instance, a stream of polyethylene mixture is extruded through an extruder in order to orient molecules of polymer chains of polyethylene and to provide an oriented film. The oriented film is passed through an electron beam and thereby crosslinking the polymer chains to provide a cross-linked film. The cross-linked film is heat sealed to form the balloon envelope.

Aspects of the disclosure relate to manufacturing a balloon envelope for use in a stratospheric balloon system using an improved double-bubble blown-film extrusion process with water quenching and electron beam processing. A stream of polyethylene mixture is extruded through an extruder to orient molecules of polymer chains and provide an oriented film. The oriented film is passed through an electron beam, crosslinking the polymer chains to produce a cross-linked film with enhanced mechanical properties. The cross-linked film is heat sealed to form the balloon envelope comprising multiple gores. The resulting balloon envelope exhibits superior characteristics including crystallinity greater than 50%, strength to weight ratio greater than 1 MPa/(dtg/cm.sup.3), thickness less than 1.5 mil, thermal emissivity less than 0.03 at 193K, and optical clarity greater than 95%. The manufacturing process may include water quenching to enhance optical clarity and dual electron beam treatment for uniform crosslinking throughout the film thickness.

A multilayer piezoelectret film element, a polymeric porous sheet, and a method for preparing them are disclosed. The present disclosure ensures that the gas can be ionized without damaging the polymeric porous sheet by adjusting the gap between pores and the dielectric strength of the polymeric porous sheet in the multilayer piezoelectret film element. The polymeric porous sheet has a high piezoelectric constant after electric polarization and surface metallization treatment, and can generate a significant amount of charge under mechanical pressure, thereby exhibiting high sensitivity in applications such as piezoelectric sensors, and solving the problems of low piezoelectric activity and fast loss of piezoelectric activity of the polarized polymeric piezoelectret film element in the art. Moreover, the production process is simple, which can ensure continuous production of the polymeric piezoelectret film element.