https://www.cnet.com/news/rumored-galaxy-xs-foldable-phone-screen-specs-7-3-inch-and-4-5-inch/

Samsung wouldn't reveal any further details about the handset's hardware but did concede when I asked if the screens could be made of different materials. My guess is that we'll see Gorilla Glass cover the 4.5-inch screen and a type of plastic cover the interior screen. 

Although several brands are working on thin, foldable glass that can withstand bending hundreds of thousands of times without breaking, there's a strong chance that Samsung could follow in the footsteps of the Royole FlexPai foldable phone and use a type of polymer, a plastic, to cover the display material below. On Tuesday, Royole showed the FlexPai to a group of journalists in San Francisco. This device is on preorder in China and available to developers globally.