The same can also be said of Banners too, I have one I was given that had been outside too long and it's as brittle as. It's the one thing about Plastic/vinyl it breaks down under UV light, and goes very brittle. Heat also affects the material, so it seems there is No long term for these kind of posters.
I wouldn't care the brittle one I have is for the last Indy film, it's painted artwork too; But it's starting to fall to bits on it's own (must get a pic before it's gone).