Interfacial structure and chain anisotropy in PVC/SiO2nanocomposites: a layer-resolved static analysis by molecular dynamics.

The interphase formed between polymers and inorganic nanofillers plays a central role in determining the macroscopic properties of polymer nanocomposites, yet its molecular structure remains difficult to characterise in a systematic and physically meaningful way. In this work, we investigate the interfacial structuring of an amorphous polyvinyl chloride matrix confined by a rigid, SiO2slab using all-atom molecular dynamics simulations. To resolve the gradual transition from the inorganic surface to the bulk polymer, we introduce and compare three chain-based layer-assignment schemes that progressively incorporate geometric proximity, conformational deformation, and explicit polymer-surface contacts. By combining layer-resolved real-space correlation functions, static structure factors, and gyration-tensor analysis, we obtain a comprehensive multiscale description of the polymer interphase. Density profiles and one-dimensional correlation functions reveal pronounced layering and anisotropic organisation normal to the interface, whereas lateral correlations remain short-ranged and melt-like, indicating that interfacial effects are strongly directional. Conformational analysis shows that interfacial chains are not primarily distinguished by large changes in coil size but rather by increased shape anisotropy and a preferential alignment parallel to the surface, with these distortions propagating several radii of gyration into the polymer matrix. Among the proposed schemes, the contact-aware model provides the sharpest discrimination of interfacial chains and yields the most consistent description of anisotropic correlations and conformational response, demonstrating the importance of explicitly accounting for polymer-surface interactions in defining interfacial layers. Overall, this work proposes a transferable, physically grounded framework for identifying and characterising polymer interphases in nanocomposites, linking local interfacial chemistry to mesoscopic structural organisation and collective density fluctuations.