Look up the Tg of the plastic in question here:http://www.ptonline.com/
Basically the lower the Tg and th thinner the plastic, the softer it will be at colder temps.
Tg stands for the Temperature of teh glass transition state. Bascially at what temperature the plastic goes from being hard and ridged like to being more leathery.
Also most boots have a two or three letter abreviation on the inside of the shell that will tell you what kind of plastic that they are made off. This will be found on multiple parts of the shell. It can tell you if the boots has a differnt upper and loer material as well.