Temperature-Driven Grafted Nanoparticle Penetration into Polymer Melt: Role of Enthalpic and Entropic Interactions

Understanding the fundamentals of nanoparticle (NP) penetration into soft matter systems is indispensable for numerous applications ranging from targeted nanoparticle-based drug delivery to generating hybrid polymer nanocomposite materials. Hence, it is crucial to identify the parameters which control the extent of NP penetration. Here we study the penetration of polystyrene-grafted Au nanoparticles (PGNPs) into an entropically/enthalpically coupled soft polymer film. The system consists of two layers: ultrathin monolayer of ordered grains of PGNPs on top of a bulk polymer film. To study enthalpic effects on nanoparticle penetration, PGNP monolayer was coupled to two different polymers, polystyrene (PS) and poly­(tert-butyl acrylate) (PtBA). When the temperature of the system is increased toward the glass transition temperature of underlying films, the width and extent of penetration of the PGNP layer depends on the Flory–Huggins parameter between the graft chain of the PGNPs and the underlying matrix polymer. In athermal cases (PGNP/PS) (χ = 0), the initially compact monolayer undergoes structural disordering and individual PGNPs penetrate into PS films to form a broad layer. However, in the second case (PGNP/PtBA) (χ ≈ 0.26), unfavorable enthalpic interactions results in PGNPs penetrating together as a monolayer into PtBA leading to the formation of a narrow layer of PGNP. The extent of PGNP penetration is improved upon increasing the entropic and enthalpic compatibility between PGNPs and underlying bulk layer. The experimental findings are corroborated by molecular dynamics simulation studies, where the time evolution of PGNP penetration into a bottom polymer layer is found to be similar to that in experiments.