Certain polymers with a particular molecular structure undergo phase transition behavior in water at temperatures above and below a specific value known as the lower critical solution temperature (LCST). At temperatures above the LCST, the hydrophobic bonds within and between the molecules strengthen and the polymer chains aggregate (the aggregated polymer chains appear cloudy in water). Conversely, at temperatures lower than the LCST, the polymer chains bind to water molecules and become hydrated (the polymer dissolves in water and is colorless and transparent in solution). This phase transition phenomenon is reversible, and such polymers are attracting considerable attention as switching molecules in numerous fields of research.
The most widely researched temperature-responsive polymer is poly(N-isopropylacrylamide) (PIPAAm), which has both hydrophilic and hydrophobic regions on its side chains, and with a LCST of 32℃. At temperatures above 32℃, because of the hydrophobic isopropyl group on the side chain, the hydrophobic bonds within and between the molecules strengthen and the polymer chains aggregate. When the temperature is below 32℃, the polymer dissolves in water because the amide bonds of the hydrophilic region bind to water molecules.