The point where the dielectric loss factor (or dissipation factor) turns up
and begins runaway is normally around the glass transistion temperature of
the polymer. (is that for Thermoplastic or Thermosets - I can never
remember?).
If you study a plot of Loss tangent vesus frequency, there is also a
frequency where it it the worst. Sometimes in the UHF or microwave bands.
PTFE ( the original DuPont Teflon) is flat to way out in mm waves.
The best book on the subject of dielectrics was done by Von Hipple for MIT,
in several versions in the late 1940-50 time frame. "Dielelctric Materials"
or something like that. It was out of print for years, but someone has
picked it up and it is being published again. It goes over using various
bridges and also Q meters to measure the losses and the dielectric
constant.
In the appendix is a rather extensive listing of measurements of plastics,
natural materials, liquids, etc for dielectric properties up to S band or
so.
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