You are probably familiar with the concept that heat helps things dissolve, but that is not always true for polymers. Some polymers, like poly(N-isopropylacrylamide), show a phenomenon known as LCST behavior. At room temperature, the polymer is perfectly soluble in water. If you heat the solution up to around body temperature, though, the polymer will precipitate! This phase transition is completely reversible, and cooling the mixture back down will let the polymer dissolve again. We call polymers like this thermoresponsive, and they have found applications ranging from drug delivery to water purification.
