Frequency-dependent dielectric measurements, often called frequency-dependent electromagnetic sensing (FDEMS), made over many frequency ranges, Hz-MHz, provide a sensitive, convenient automated means for monitoring in situ the processing properties of thermosets and thermoplastics. Using a planar wafer-thin sensor, measurements can be made in situ in almost any environment. Through the frequency dependence of the impedance, this sensing technique is able to monitor chemical and physical changes throughout the entire cure process. In this presentation we discuss how to use the frequency dependence in the Hz-MHz range to separate and determine parameters governing ionic and dipolar mobility is discussed. In turn, the relationship between these two molecular probes, ionic translational mobility and dipolar rotational mobility, and macroscopic processing properties is also discussed.
The cure of a high-temperature polyimide (PMR-15) and the cure of several epoxy resins were monitored using dielectric sensing. The ionic and dipolar mobilities are quantitatively related to the viscosity, degree of cure and the build up in glass transition temperature. The ability of the change in ionic mobility to detect the gel point through the determination of an inflection point and through the use of a power law (1 Οͺ t/t gel ) x is discussed. Finally the application of in situ sensing to monitor processing properties and to control the cure process intelligently in a complex part in an autoclave is described.