Molecular Dynamics Study on Interfacial Bonding Mechanism of Polyvinyl Pyrrolidone-co-Polyacrylic Acid Modified Glass Fibers and Calcium Silicate Hydrate

Glass fiber reinforced cement is utilized in construction field. Interfacial modification with coupling agents and functional polymers represents a critical strategy to enhance interfacial bonding. However, molecular mechanisms remain insufficient. In this study, molecular dynamics were used to evaluate interfacial responses of C–S–H/glass fiber modified with KH-570 and poly (vinylpyrrolidone-co-acrylic acid) (PVP–co–PAA). The results demonstrated that KH-570 induced marginal increase, while PVP–co–PAA led to remarkable enhancement. The analyses of coordination number, bond lifetime decay decoded interfacial bonding networks before and after modification. This study could reveal the distinct failure pathways and molecular mechanisms of interfacial bonding.

All the MD simulations were performed based on a package named LAMMPS. Firstly, all the systems were relaxed under NVT ensemble last for 5 ns, with timestep 1 fs and 300 K. Temperature control was achieved by Nose-Hoover thermostat and damping parameter 0.1 ps. The bottom Ca layers of C–S–H were fixed throughout the simulation to prevent model rotation. An additional 2 ns relaxation was conducted and trajectories were recorded every 1 ps. During the shear simulations, spring forces were applied on atoms located at the top of SiO₂ slab.

The molecular dynamics (MD) simulations are employed to evaluate shear mechanical properties of C–S–H/SiO₂, C–S–H/KH-570 and C–S–H/PVP–co–PAA interfaces. The molecular mechanisms of interfacial modification are thoroughly revealed via analyzing the types and dynamic behaviors of interfacial bonding networks. During dynamic failure, PVP–co–PAA exhibits a distinct behavior compared with rigid SiO₂. Flexible polymer networks undergo conformational changes, leading to an increased shear displacement. The density analyses of bonding networks indicates that interfacial bonding networks can reorganize from O_SiO2—Ca—O_C–S–H into O_KH-570/O_{PVP–co–PAA}—Ca—O_C–S–H. Moreover, interfacial failure is decided by ionic bonding of O_SiO2/O_KH-570/O_{PVP–co–PAA}—Ca. The coordination number follows an increasing order of O_SiO2<O_KH-570<O_{PVP–co–PAA}, explaining the differences of interfacial mechanical strength.

The MD simulation revealed that PVP–co–PAA modification showed the most enhancement, with an improvement of 302.7%. The coordination number followed an increasing order of O_SiO2<O_KH-570<O_{PVP–co–PAA}. The time correlation function (TCF) also revealed that interfacial stability was enhanced after modification. Based on the coordination number and the TCF results, the differences of interfacial mechanical strength were explained. These findings could provide fundamental insights to guide the functional design of fiber-cement interfaces.