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This study investigates Marangoni effect-induced structural changes in spin-coated polymer nanocomposite (PNC) films composed of poly(methyl methacrylate)-grafted silica nanoparticles (NPs) and poly(styrene-ran-acrylonitrile). Films cast from methyl isobutyl ketone (MIBK) solvent exhibit distinct hexagonal honeycomb cells with thickness gradients driven by surface tension variations. Atomic force microscopy reveals protruded ridges and junctions at cell intersections, where NP concentration is the highest. Upon annealing at 155 degrees C, NPs segregate to the surface due to their lower surface energy, and the initially protruding features flatten and eventually form depressed channels while maintaining higher NP density than surrounding areas. Time-of-flight secondary ion mass spectrometry corroborated these findings, highlighting enhanced surface segregation of NPs in MIBK films. These defects can be eliminated using methyl isoamyl ketone (MIAK) as a solvent that produces homogeneous films of uniform thickness. This study highlights the impact of the Marangoni effect on the microstructure and surface properties of PNC films, providing insights for enhancing film quality and performance. 

This study investigates the interplay between film thickness and the surface and internal morphologies in polymer nanocomposite (PNC) films. The PNC is 25 wt.% poly(methyl methacrylate)-grafted silica nanoparticles (NPs) in poly(styrene-ran-acrylonitrile) annealed in the two-phase region. At greatest confinement (120 nm), NP surface density remains constant and lateral phase separation is inhibited upon annealing. For thicker films (240 nm to 1400 nm), surface density increases with time before approaching ca. 740 NP/μm2, consistent with 2D random close packing. Moreover, lateral domain growth exhibits a dimensional crossover as thickness increases from 𝑡 to , consistent with domain coalescence. Water contact angles 1/2 𝑡1/3 decrease upon annealing in agreement with the lateral domain composition. For thickest films (1400 nm to 4000 nm), a morphology map summarizes the distinct internal arrangements of NPs: disordered aggregates, continuous vertical pillars, discrete vertical pillars, isolated domains, and random networks. This study of PNC films provides guidance for controlling surface and bulk structure which can lead to improved barrier, mechanical and transport properties. 

We show that the polymer-grafted nanoparticles (NPs) initially welldispersed in a polymer matrix segregate to the free surface of a film upon thermal annealing in the one-phase region of the phase diagram because the grafted polymer has a lower surface energy than the matrix polymer. Using a combination of atomic force microscopy, transmission electron microscopy, and Rutherford backscattering spectrometry, the evolution of the poly(methyl methacrylate)-grafted silica NP (PMMA NP) surface excess in 25/75 wt % PMMA NP/poly(styrene-ranacrylonitrile) films is observed as a function of annealing time at 150 °C (T < TLCST). The temporal growth of the surface excess is interpreted as a competition between entropic contributions, surface energy differences of the constituents, and the Flory−Huggins interaction parameter, χ. For the first time in a miscible polymer nanocomposite mixture, quantitative comparisons of NP surface segregation are made with the predictions of theory derived for analogous polymer blends. These studies provide insight for designing polymer nanocomposite films with advantageous surface properties such as wettability and hardness and motivate the need for developing rigorous models that capture complex polymer nanocomposite phase behaviors.