Real-Time Investigation of PNIPAM Grafting Dynamics and Grafting Density-Dependent Assembly of Gold Nanoparticles

Polymer grafting density is a critical parameter in regulating nanoparticle self-assembly, as precise control over it enables programmable design of nanostructures. In this study, a novel quartz crystal microbalance (QCM)-based method is developed for in situ monitoring of the grafting kinetics of poly(N-isopropylacrylamide) (PNIPAM) onto gold nanoparticles (AuNPs). Experimental results show that during the initial grafting stage (t = 360 s), a grafting rate constant (k₁) of 1.06 × 10^-2 chains nm^{- 2} s^-1 and a diffusion-controlled mechanism can be observed. As grafting proceeds, the polymer chains adopt an extended conformation, resulting in a slower grafting rate. Isothermal titration calorimetry (ITC) measurements further validated the reliability of the QCM approach. Further investigations revealed the significant influence of polymer chain length, interparticle spacing, and temperature on grafting behavior. In situ small-angle X-ray scattering (SAXS) studies demonstrated that grafting density plays a decisive role in determining the assembly structures: long-chain PNIPAM grafted at low density forms random close-packed (RCP) structure, while high grafting density promotes face-centered cubic (FCC) ordering. This work not only establishes an in situ method for monitoring grafting on AuNPs, but also clarifies polymer density-structure relationships for nanomaterial design.