Marangoni-Induced\nHoneycomb Structures in Spin-Coated\nPolymer Nanocomposite Films

This study investigates Marangoni\neffect-induced structural changes\nin spin-coated polymer nanocomposite (PNC) films composed of poly(methyl\nmethacrylate)-grafted silica nanoparticles (NPs) and poly(styrene-<i>ran</i>-acrylonitrile). Films cast from methyl isobutyl ketone\n(MIBK) solvent exhibit distinct hexagonal honeycomb cells with thickness\ngradients driven by surface tension variations. Atomic force microscopy\nreveals protruded ridges and junctions at cell intersections, where\nNP concentration is the highest. Upon annealing at 155 °C, NPs\nsegregate to the surface due to their lower surface energy, and the\ninitially protruding features flatten and eventually form depressed\nchannels while maintaining higher NP density than surrounding areas.\nTime-of-flight secondary ion mass spectrometry corroborated these\nfindings, highlighting enhanced surface segregation of NPs in MIBK\nfilms. These defects can be eliminated using methyl isoamyl ketone\n(MIAK) as a solvent that produces homogeneous films of uniform thickness.\nThis study highlights the impact of the Marangoni effect on the microstructure\nand surface properties of PNC films, providing insights for enhancing\nfilm quality and performance.