Amphiphilic Gold Nanoparticles Grafted with Poly(N-isopropylacrylamide) and Polystyrene

Abstract

Two types of amphiphilic gold nanoparticles (AuNP-1 and -2) grafted with a mixture of poly(N-isopropylacrylamide) (PNIPAM) and polystyrene (PS) chains in two different compositions have been successfully prepared with the “grafting-to” method in a homogeneous THF phase. These AuNPs were thoroughly characterized by FTIR, 1H NMR, UV−vis, high-resolution transmission electron microscopy, thermogravimetric analysis, and dynamic light scattering to determine the total number of polymer chains bound to the gold nanoparticles, the ratio between PNIPAM and PS chains, and the size of the gold core. Langmuir monolayer experiments at the air−water interface of the two types of AuNPs revealed different compression isotherms of the surface pressure vs particle area (π−A curve) conducted at 20 °C. These amphiphilic gold nanoparticles can be regarded as analogues of amphiphilic diblock copolymers at the air−water interface. The compression isotherm of AuNP-2 with a PNIPAM:PS ratio of 2:1 showed several characteristic regions that can be attributed to the polymer conformational transitions from the pancake, the pancake to brush transition, to the brush. However, the monolayer of AuNP-1, with a ratio of 5:1 of PNIPAM:PS, never reaches a brush stage but showed an extension of the pseudoplateau region upon compression. These differences may be due to the more hydrophilic nature and the more stretched PNIPAM chains. Furthermore, the sessile drop contact angle measurements, conducted at room temperature on both upper and lower surfaces of the AuNP-2 monolayer transferred at 35 mN/m onto either hydrophilic or hydrophobic substrates, are slightly different, 82 ± 2° and 77 ± 2°, respectively. After comparing with the literature data of the contact angles of water on either the pure PS film or the PNIPAM brush, we concluded that the chemically different PNIPAM and PS chains grafted on the surface of the gold core tend to be phase-separated.