Thickness-Dependent Macroscopic Properties of Highly Filled Composite Elastomers: Role of Hierarchical Filler Network and Viscoelastic Behavior

Highly filled thin composite elastomers are crucial for thermomechanical applications in electronics packaging and engineered interfaces. However, most current research focuses on bulk-like materials and performance optimization. This work is the first to investigate the thickness-dependent macroscopic properties of such composites, establishing a distinction between film-like and bulk-like behaviors based on specific properties. Using polydimethylsiloxane-based composite elastomers containing 90 wt % aluminum as a model, we identify two critical thicknesses (200 and 800 μm). Above 800 μm, the material exhibits bulk-like behavior, with mechanical and viscoelastic properties becoming thickness-independent. Below 800 μm, these properties show a pronounced thickness dependence. In contrast, the out-of-plane thermal conductivity exhibits enhancement only below 200 μm, which can be attributed to the anisotropic characteristics of the polymer-mediated hierarchical filler networks. This regime exhibits film-like characteristics, with thermomechanical properties diverging from bulk-like behavior. These findings bridge the gap between basic research and practical applications in thickness-constrained packaging applications.