Heterogeneous polymers as capacitive sensors: Differences between foam cells and closed or open tubular channels

Abstract

Polymer foams and polymer-film systems with internal cavities are highly compressible, at least in certain direction(s). Their Young's moduli can be several orders of magnitude lower than those of the respective non-porous (essentially cavity-free) materials. Thus, polymer foams and polymer systems with cavities easily deform when the external pressure varies. The capacitance changes noticeably as a result of the sample deformation. Here, we study such heterogeneous polymers as capacitive sensors for applied external mechanical pressure or surrounding gas pressure. Both, cellular polypropylene (PP) foams and fluoroethylene propylene (FEP) film systems with tubular channels are investigated. While the former are closed-cell foams, the latter contain tubular channels that are either open or that have been closed via sealing at elevated temperatures. It is found that the sensor performance is strongly dependent on the material structure.