The aggregation and phase separation behavior of a hydrophobically modified poly(N-isopropylacrylamide)

To study the water solution properties of a hydrophobically modified poly(N-isopropylacrylamide) (PNIPAM) that is temperature-sensitive, the copolymer of N-isopropylacrylamide (NIPAM) and octadecylacrylate (ODA) was synthesized. The aggregation behavior of the copolymer was studied by surface tension and fluorescent probe methods. Simultaneously, the phenomenon of LCST (Lower Critical Solution Temperature) of the copolymer in aqueous solution with the increase of temperature was also studied by using the fluorescent probe method. Results showed that phase separation occurred in aqueous solution of the copolymer when the temperature was increased to its LCST. The π–A isotherms for the copolymer molecules, as an insoluble monolayer on the water–air interface, was determined by the Langmuir–Blodgett method. The abnormal phenomenon, which the monolayer of the copolymer molecules became more condensed and more condensed with the increase of temperature, was observed. It further indicated that phase separation of the copolymer occurred from another angle. In addition, to prove the thermo-sensitive effect of the copolymer on the release behavior of liposomes, small unilamellar vesicles entrapped with 5(6)-carboxyfluorescein (5(6)-CF) were coated with the copolymer. We found that the coating of the copolymer resulted in reduction of the release below 30°C and enhancement of the release above 30°C. It indicates that there are obvious interactions between the copolymer and liposomes.