Samsung is working on its next foldable phone, which according to leaks will be called the Samsung Galaxy Z Flip. According to XDA's Max Weinbach, the new phone will be use ultra-thin flexible glass, instead of polyimide.
In November 2019 Samsung indeed signed an exclusive deal with Korea's Dowoo Insys to supply ultra-thin glass (UTG) for future foldable OLED devices. Samsung is aiming to switch to glass as its current foldable OLEDs which are protected by a plastic cover are highly prone for scratching and are very fragile.
It is likely that Samsung's new foldable OLEDs will still be based on polyimide substrates, and the flexible glass will be used as a cover material to protect the outer part of the display.