The influence of the molecular chain length of PVA on the toughening mechanism of calcium silicate hydrates.

In recent years, polymers have been demonstrated to effectively toughen cementitious materials. However, the mechanism of interaction between the polymers and C-S-H at the nanoscale remains unclear, and the quantitative impact of the polymer chain length on toughening effectiveness is lacking in research. This study employs molecular dynamics techniques to examine the impact of the polyvinyl alcohol (PVA) chain length on the tensile performance and toughening mechanism of C-S-H. The toughening effect in both the X and Z directions exhibits an initial enhancement followed by a decline with increasing chain length. The optimal degrees of polymerization are determined to be 8 and 12 in the X and Z directions, respectively, resulting in an improvement of fracture energy by 146.7% and 29.5%, respectively. During the stretching process along the X and Z axes, the chain length of PVA molecules significantly influences the variation in the number of Ca⋯O bonds in the system, leading to different stress responses. Additionally, PVA molecules form C-O-Si bonds with the silicate layers of C-S-H, bridging the adjacent layers in a left-right or up-down manner. The toughening effect of PVA on C-S-H depends on the behavior of PVA molecules with different chain lengths, and there exists an optimal range of chain length for PVA, enabling it to enhance structural uniformity and adjust its own conformation to absorb strain energy. When the length of PVA molecular chains is too short, it can easily cause stress concentration in the system and its connection with silicates is not significant. Conversely, when the length of PVA molecular chains is too long, the large molecular structure restricts its extension in the defects of C-S-H, and as the stretching progresses, PVA molecules break and form numerous small segments, thereby losing the advantage of the chain length. This study provides a theoretical basis for the ability of polymers to toughen cementitious materials.

A toughening mechanism of the strain rate-optimal chain length on polymer-modified calcium silicate hydrates (CSH).

Polymers are known to effectively improve the toughness of inorganic matrices; however, the mechanism at the molecular level is still unclear. In this study, we used molecular dynamics simulations to unravel the effects and mechanisms of different molecular chain lengths of polyacrylic acid (PAA) on toughening calcium silicate hydrate (CSH), which is the basic building block of cement-based materials. Our simulation results indicate that an optimal molecular chain length of polymers contributes to the largest toughening effect on the matrix, leading to up to 60.98% increase in fracture energy. During the uniaxial tensile tests along the x-axis and z-axis direction, the configuration evolution of the PAA molecule determines the toughening effect. As the polymer unfolds and its size matches the defects of CSH, the stress distribution of the system becomes more homogeneous, which favors an increase in toughness. Furthermore, based on our simulation results and a mathematical model, we propose a theory of "strain rate/optimal chain length". This theory suggests that the optimal toughening effect can be achieved when the molecular chain length of the organic component is 1.3-1.5 times the largest defect size of the inorganic matrix. This work provides molecular-scale insights into the toughening mechanisms of an organic/inorganic system and may have practical implications for improving the toughness of cement-based materials.