Thermoresponsive Properties of N-Isopropylacrylamide Oligomer Brushes Grafted to Gold Nanoparticles: Effects of Molar Mass and Gold Core Size

Abstract

To explore the thermal response of the densely packed inner regime of poly(N-isopropylacrylamide) (PNIPAM) brushes grafted to gold nanoparticles (AuNPs), we systematically studied the thermoresponsive properties of NIPAM oligomeric brushes affected both by oligomer molar mass and Au core size. A series of NIPAM oligomers with various molar masses ranging from ca. 600 to 3400 g/mol were obtained by RAFT polymerization and fractionated with HPLC. The AuNPs stabilized with various NIPAM oligomers were prepared by a one-pot reaction and further fractionated to achieve three pairs of AuNP fractions with narrow size distributions. When decreasing the molar mass of brush chains from ca. 3300 to 700 g/mol, a significant molar mass effect on the thermal transition was found, i.e., the phase transition temperature (defined as the endothermic peak temperature Tp by DSC) shifted from ca. 31 to 15 °C and the endothermic peak became broadened. As a comparison, we also studied the aqueous solutions of free NIPAM oligomers (molar mass from ca. 3400 to 600 g/mol) by turbidity measurements. They showed a completely opposite trend of the thermally induced phase transitions; i.e., the transition shifted to higher temperature with decreasing molar mass. The Au core size also affected the thermal response of NIPAM oligomer brushes, especially in the case of the shortest oligomers. Large Au cores caused the thermal transition of NIPAM oligomer brush to occur at lower temperatures compared to the small Au cores. This was attributed to the hydrophobic nature of Au nanocrystal surfaces. Enthalpy changes (ΔH) associated with the thermal transitions of the oligomer brushes are indicative of strong interchain interactions in the brushes, especially on large Au cores.