There is a paper with an interesting title. but sadly it's locked in at researchgate
Sadly. much good research ends up there, unvaliable for most people:
https://www.researchgate.net/publication/286480769_Dielectric_Properties_of_Polymer_at_Microwave_Frequencies

RF Cafe has some dielectric data here

http://www.rfcafe.com/references/electrical/dielectric-constants-strengths.htm

unfortunately, you know how these free web sites are... no reference provided!

I used to know an antenna guy that kept a loose-leaf notebook with every pages of stuff he had printed out on any material he ever found. I regret that I did not borrow it and spend an evening at the xerox machine...

SM4RZW,
That Researchgate link seems to include the full text of that paper now?

Weird that it was there for free

I downloaded a copy. One of these days I will try to organize some more material properties stuff and post...

Steve