Role of nanostructured polymers on the improvement of electrical response-based relative humidity sensors

Nanostructured polymers, with different morphologies and different spatial organizations, have found, in last few years, a wide range of uses in humidity sensors, as a viable alternative to ceramic or semiconducting materials and to oxides (perovskite) compounds. Their enhanced sensitivity towards external stimuli has made them ideal candidates in the design of humidity sensors. This is mainly due to the fact that nanostructured polymers, when embedded with different content of water, represent a heterogeneous system whose dielectric and conductometric properties varies over a wide range, resulting in advantageous sensitive material in resistive-type or capacitive-type humidity sensors. The most notable property of nanostructured polymers is their inherent electrical behavior which is closely connected to the presence of heterogeneities at a nano- or micro-scale level. In the first part of this paper, we discuss and justify, on the basis of the dielectric theory of heterogeneous systems, how the dielectric properties of nanostructured polymers, independently of their chemical nature, could vary as a consequence of a different amount of water adsorption, meeting the basic requirement of any humidity sensor device. In particular, we have analyzed systems with different porosity and different pore interconnections to cover the most part of the structural arrangements and morphologies nanostructured polymers give rise. In the second part of the paper, we present some remarkable examples of nanostructured polymers employed in the fabrication of humidity sensors based on changes in the electrical properties (permittivity and electrical conductivity) upon exposure to moisture, highlighting the main features that make them suitable for sensors, with specific emphasis to characteristic parameters, such as sensitivity, response time, hysteresis and durability.