Louie, how can I tell for sure that it's not made of PVC? It's pretty thin material, and w some relief.
Mylar is a Dupont trade name for a strong type of polyester.
To tell the difference between Mylar and PVC, try to tear it (you may need to start it with a cut). Mylar should tear apart fairly cleanly. PVC will stretch and deform before it tears whereas Mylar will not.
If you can push a finger into it and form a finger shaped protrusion in it, it is most likely PVC.
You could also do another test to check between Mylar and polycarbonate. If it is polycarbonate, creasing and folding it back on itself a few times will cause it to crack along the crease. Mylar is much more resistant to this.
I doubt that these sleeves are made from polycarbonate though.
Steve.