Morphology-driven trade-offs in highly filled polystyrene films: microsilica vs halloysite nanotubes

Abstract

Polymer composites with tunable thermal stability and surface morphology have attracted significant attention in terms of advanced engineering applications. Enhanced thermal resistance ensures long-term durability and safety under demanding conditions, while controlled surface morphology influences interfacial interactions, wettability, adhesion, barrier properties, and overall material performance. In the present study, polystyrene (PS) composite films containing 40 wt.% microspherical silica (average diameter ≈0.5 μm) or halloysite nanotubes (HNT) were fabricated to investigate morphology-driven structure–property relationships at identical filler loadings. Fourier transform infrared (FTIR) spectroscopy confirmed successful filler incorporation through characteristic Si–O–Si vibrations for silica and O–H and aluminosilicate bands for HNT, without evidence of new covalent bonding, indicating physical dispersion within the PS matrix. Thermogravimetric (TGA) and differential thermogravimetric (DTG) analyses revealed clear morphology-dependent degradation behavior. PS–SiO₂ composites showed delayed degradation onset and higher mid-conversion degradation temperatures ( T _initial and T ₅₀ ), consistent with an effective barrier effect that limits volatile and oxygen diffusion. In contrast, PS–HNT composites exhibited the highest peak degradation temperature ( T _max ), suggesting enhanced thermal stability during the main degradation stage, probably due to confinement and char-forming effects associated with the tubular morphology. DTG analysis further indicated slight shifts in peak-degradation temperatures, confirming the influence of filler geometry on decomposition behavior. A minor early mass-loss event observed for HNT-containing films was attributed to hydroxyl-rich surfaces and adsorbed moisture. Atomic force microscopy (AFM) demonstrated that filler geometry governs surface topology to a significant extent. PS–SiO ₂ films retained a relatively smooth surface, whereas PS–HNT films exhibited pronounced roughening and anisotropic features arising from nanotube-rich regions. Overall, the results identify filler morphology as a key parameter controlling thermal degradation pathways and surface architecture in PS composites, providing valuable insights for the morphology-guided design of advanced polymer nanocomposites.