Isn't room temperature PC already in its glass state? Would cooling it further really affect it much?
A better question, and another interesting idea - how does it flow with a heat gun? Can you make cool sculptures?
Just a thought (from my real life as a plastics engineer) Polycarbonate is the material that CD's are made from. its the same stuff thats used in riot shields and safety goggles, etc, so its designed not to break,
but it has two achillies heels,
it is sensitive to scratching, so if you score the CD before you break it, with a sharp point, an awl or nail or something like that, and its also sensitive to temperature. If you make it really cold it should be easier to break (at least thats how the theory goes, some of my students tried this out one frost night with a telephone kiosk, but thats a whole other story)
so scratch it and then put it in the freezer overnight, at the bottom of the freezer so its colder, then smash it with a hammer while still very cold...
let me know how you get on as I havent actually tried this myself!!