Coupled effects of water content and polyvinyl alcohol modification on the microstructure and mechanical behavior of montmorillonite

Polyvinyl alcohol (PVA) modification is commonly used to improve clay performance, but its reinforcement efficiency in montmorillonite remains difficult to predict because it is governed by coupled water-polymer-clay interactions. In this study, molecular dynamics simulations of wettability, hydrated aggregation, water diffusion, and tensile deformation were combined with laboratory direct shear tests to link molecular-scale interfacial processes with macroscopic strength behavior. The results show that PVA does not eliminate the intrinsic hydrophilicity of montmorillonite; instead, it regulates wettability by reducing the accessibility of high-energy hydrophilic sites and transforming the interface from direct clay-water hydration to composite clay-PVA-water interactions. This interfacial regulation becomes mechanically beneficial under intermediate hydration, where water promotes platelet rearrangement, pore densification, and stable PVA-clay adhesion. Under such conditions, PVA chains form effective bridges between adjacent platelets and improve load transfer. Inappropriate water content or PVA dosage disrupts this bridging network, weakens PVA-clay contact, and reduces strength. Overall, PVA-modified montmorillonite is controlled by the balance among polymer bridging, interfacial adhesion, and hydration-induced softening. This work provides a multiscale physical framework for optimizing polymer-modified expansive clays through coordinated control of moisture condition and polymer dosage.