According to the US Plastics corporation:
US Plastics wrote:Polypropylene and PVC are both plastics that are brittle at temperatures below freezing. Polystyrene is considered brittle at 68°F and is very brittle at temperatures below freezing. High, medium and low polyethylene on the other hand, remains impact resistant at low temperatures. Polycarbonate and PETG also have low brittleness temperatures.
Platypus says the film portion of their 2L Platy Bottles are made from nylon/polyethylene and the spout and cap from polyethylene.
From the Platypus FAQ:
Platypus FAQ wrote:Can I freeze my Platypus bottle?
Yes. Lay it half-full on its side in freezer with all air purged. Do not over-fill. Water expands as it freezes and this can burst your bottle
I've had a 2L Platypus bottle freeze before without any issue, but it's only been occasional. I find it quite annoying as the narrow spout means you can't easily get the ice out. So I generally don't take them on winter trips.