polyNisopropylacrylamide

Poly(N-isopropylacrylamide) (PNIPAM) is a synthetic polymer derived from N-isopropylacrylamide. It is notable for its thermoresponsive behavior in water, existing as a hydrated, swollen material at lower temperatures and collapsing to a compact, hydrophobic state above a characteristic temperature.

The thermoresponsive transition, known as the lower critical solution temperature (LCST), occurs around 32 °C in dilute aqueous solutions. Below the LCST, PNIPAM chains are hydrated by hydrogen bonding with water and remain solvated. Above the LCST, hydrophobic interactions dominate, water is expelled, and the polymer chains collapse. The LCST is influenced by molecular weight, copolymer composition, crosslink density, and solution conditions such as salt concentration, pH, and co-solvents; this allows tuning of the temperature at which phase behavior changes.

Synthesis and architectures: PNIPAM is typically prepared by free radical polymerization of the N-isopropylacrylamide monomer, in water or organic solvents, using initiators such as AIBN or redox systems. Controlled radical polymerization methods (e.g., RAFT, ATRP) enable well-defined molecular weights and architectures, including block copolymers, graft copolymers, and star polymers. Crosslinking yields PNIPAM hydrogels and microgels with reversible swelling and deswelling in response to temperature.

Applications: The temperature-responsive properties make PNIPAM-based materials attractive for drug delivery (temperature-triggered release near body temperature), tissue engineering scaffolds, smart coatings with tunable wettability, and bioseparation systems.

Biocompatibility and limitations: PNIPAM is widely studied for biomedical use and is generally considered biocompatible, but residual monomer, initiators, and crosslinkers can pose safety concerns. Its performance is sensitive to salts and additives that can shift the LCST, and mechanical properties depend on crosslinking and architecture.