Effects of moisture on the dielectric properties of polyoxymethylene (POM)

The effects of absorbed water on the gamma-relaxation process of polyoxymethylene (POM) were studied using dielectric spectroscopy and thermogravimetric analysis. The spectra were measured isothermally over the frequency range of 10 MHz to 2 GHz, and the water content was varied from dried to 1.65 wt% (saturated). The strength and rate of the gamma-relaxation process increased with increasing water content, contradicting earlier studies that found this process to be relatively insensitive to changes in the molecular environment. Two different formalisms that express the fraction of contributing water dipoles to the measured spectra were compared using the experimental data. The results of the two expressions differed by approximately 20% in their estimation of unbound, participative water and both suggest that the water relaxation process is slowed for the absorbed water in comparison to free, or bulk, water. The shift in the measured spectra for the water/POM system is attributed to the steric hindrances imposed on the water molecules by the hydrogen bonds formed with the polymer molecules. The effects of water sorption by POM were used to derive the associative effects on antenna performance for a hypothetical microstrip antenna comprised of a dielectric layer of POM. It was found that the resonant frequency decreased by 7.5% for a water uptake of 1.65 weight percent.