Plexiglass is not UV stable. You will have to store the work in total darkness, which is pointless if your purpose is to display it. UV causes embrittlement and opaqueness or a milky discolouration of plexiglass (aka perspex or acrylic). For industrial use, such materials are usually coated with a UV screen of some sorts, but even then it has a limited practical lifespan. So in short, no acrylic materials are archival, and they are much more prone to chemical attack than many other plastics, too.