Film Thickness Dependence of Morphology Evolution in Polymer-grafted Nanoparticle Composites*

Polymer nanocomposites (PNCs) present enhanced materials functionalities crucial for applications like gas separation membranes and polymer electrolytes. These functionalities are intrinsically correlated with the dispersion state of the nanoparticles (NPs), which depends on various materials parameters and processing conditions. Our study delves into the effects of film thickness on the PNC’s surface and bulk structures, using a model system with silica nanoparticles grafted with poly(methyl methacrylate) within a poly(styrene-ran-acrylonitrile) matrix. As a function of annealing time (t), atomic force microscopy (AFM) revealed lateral phase separation in films ranging between 240 nm and 1400 nm post-annealing. However, the thinnest 120 nm films exhibited enhanced NP dispersion and limited lateral phase separation. The domain growth pattern scaled as t^1/2 for 240 nm and t^1/3 for thicker films, indicating a dimensional crossover near 240 nm. Cross-sectional transmission electron microscopy on films ranging from 1400 nm to 4000 nm identified six distinct morphologies, yielding a morphology map dependent on film thickness and annealing duration. Our findings provide valuable insights into optimizing NP assembly in PNCs to enhance their properties.