Vehicular and Core–Shell Nanoparticle Diffusion in Attractive Entangled Polymer Melts

This work studies nanoparticle (NP) diffusion in attractive polymer melts and reveals two distinct dynamic modes: vehicular and core–shell. By diffusing alumina NPs (RNP = 6.5 nm) and silica NPs (RNP = 8.3 and 26.2 nm) into poly(2-vinylpyridine) melts of various molecular weights (14–1220 kDa), we examine the impact of the RNP, polymer size (Rg), and surface chemistry on NP diffusion. Using time-of-flight secondary ion mass spectrometry and trilayer samples, we measure cross-sectional nanoparticle concentration profiles as a function of the annealing time and extract nanoparticle diffusion coefficients. Both small and large silica NPs (Rg/RNP = 0.12–3.6) display core–shell behavior, while alumina NPs (Rg/RNP = 0.50–4.6) diverge sharply with increasing polymer molecular weight, aligning with theoretically predicted vehicular diffusion. The transition from core–shell to vehicular diffusion is the result of both increasing molecular weight and weaker NP/polymer attractions and facilitates an estimate of the monomer desorption time.