PNIPAm

PNIPAm, or poly(N-isopropylacrylamide), is a synthetic thermo-responsive polymer widely studied for its temperature-induced phase behavior in aqueous solutions. It consists of a polyacrylamide backbone with isopropyl side chains. In water, PNIPAm exhibits a lower critical solution temperature (LCST) around 32°C. Below this temperature the polymer is hydrated and swollen; above it, hydrophobic interactions among the isopropyl groups dominate, causing the polymer to collapse and expel water. This volume-phase transition enables PNIPAm to exist as a hydrogel or microgel that responds to temperature changes.

The LCST can be tuned by copolymerization with other monomers, crosslinking density, ionic strength, and pH. Common crosslinkers create thermo-responsive hydrogels used in drug delivery and tissue engineering, where a temperature trigger can regulate release or cell behavior. PNIPAm networks are usually formed by free radical polymerization of N-isopropylacrylamide, often with crosslinkers such as N,N'-methylene-bis(acrylamide). Controlled architectures can be achieved through reversible-deactivation polymerization methods such as RAFT or ATRP.

Applications include controlled release systems, injectable hydrogels, sensors, actuators, and smart coatings for textiles or ophthalmic devices. Biocompatibility is a consideration; PNIPAm is often regarded as relatively biocompatible, though residual monomer and impurities can affect safety. In physiological environments, the presence of salts and proteins can influence the LCST and swelling behavior, requiring design adjustments.

PNIPAm remains a standard model polymer for studying thermo-responsive hydrogels and smart materials, due to its sharp LCST near physiological temperature and versatile tunability through copolymerization and crosslinking.