Poly(N-isopropylacrylamide)

Poly(N-isopropylacrylamide) (variously abbreviated PNIPA, PNIPAAm, NIPA, PNIPAA or PNIPAm) is a temperature-responsive polymer that was first synthesized in the 1950s. It can be synthesized from N-isopropylacrylamide which is commercially available. It is synthesized via free-radical polymerization and is readily functionalized making it useful in a variety of applications.Aqueous solution of PNIPA before heating. The negative enthalpy of hydrogen bonding dominates and PNIPA is dissolved in the solution.The mixture after heating with a heat gun. The negative entropy of mixing dominates due to the increase in temperature and PNIPA phase separates from the water. Poly(N-isopropylacrylamide) (variously abbreviated PNIPA, PNIPAAm, NIPA, PNIPAA or PNIPAm) is a temperature-responsive polymer that was first synthesized in the 1950s. It can be synthesized from N-isopropylacrylamide which is commercially available. It is synthesized via free-radical polymerization and is readily functionalized making it useful in a variety of applications. It forms a three-dimensional hydrogel when cross-linked with N,N’-methylene-bis-acrylamide (MBAm) or N,N’-cystamine-bis-acrylamide (CBAm). When heated in water above 32 °C (90 °F), it undergoes a reversible lower critical solution temperature (LCST) phase transition from a swollen hydrated state to a shrunken dehydrated state, losing about 90% of its volume. Since PNIPA expels its liquid contents at a temperature near that of the human body, PNIPA has been investigated by many researchers for possible applications in tissue engineering and controlled drug delivery. The synthesis of poly(N-isopropylacrylamide) began with the synthesis of the acrylamide monomer by Sprecht in 1956. In 1957, Shearer patented the first application for what would be later identified as PNIPA for the use as a rodent repellent. Early work was piqued by theoretical curiosity of the material properties of PNIPA. The first report of PNIPA came in 1968, which elucidated the unique thermal behavior in aqueous solutions. The 1980s marked an explosion in interest in PNIPAs with the realization of potential applications due to its unique thermal behavior in aqueous solutions. PNIPA is one of the most studied thermosensitive hydrogels. In dilute solution, it undergoes a coil-to-globule transition. PNIPA possesses an inverse solubility upon heating. It changes hydrophilicity and hydrophobicity abruptly at its LCST. At lower temperatures PNIPA orders itself in solution in order to hydrogen bond with the already arranged water molecules. The water molecules must reorient around the nonpolar regions of PNIPA which results in a decreased entropy. At lower temperatures, such as room temperature, the negative enthalpy term ( Δ H {displaystyle Delta H} ) from hydrogen bonding effects dominates the Gibbs free energy, causing the PNIPA to absorb water and dissolve in solution. At higher temperatures, the entropy term ( Δ S {displaystyle Delta S} ) dominates, causing the PNIPA to release water and phase separate which can be seen in the following demonstration. Homopolymerization Copolymerization Terpolymerization Cross-linked Hydrogel