But keep in mind that the amorph structure of Ultem can cause relaxation under influence of bondings on the long term. And watch out for notch-sensitive geometries.
I aggree with Eliassen. I saw tests at DSM Engineering plastics where UHMWPE was used in combination with liquid Helium at -269 °C. It still had usable impact strenght. You need a PE with very high molecular wight though, because lower weights will decrease ability to withstand low temperatures...