Key Role of Cross-Linking Homogeneity in Polyurethane Mechanical Properties: Insights from Molecular Dynamics.

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Polyurethane (PU) grouting materials play a crucial role in trenchless rehabilitation with their mechanical properties significantly influenced by the cross-linking degree and cross-linking uniformity. This study investigates the effects of inhomogeneous cross-linking on the mechanical behavior of PU, an area that remains underexplored. We developed PU molecular models with varying cross-linking degrees and cross-linking uniformity, quantitatively described by the parameter J. A series-parallel spring network model was proposed to analyze their micromechanical responses under tensile stress. Our results indicate that cross-linking homogeneity, compared to the cross-linking degree, has a more significant impact on the mechanical properties of PU due to weaker parallel effects in specific series directions within the elastic network. Additionally, this defective network leads to larger free volume, stronger nonbonded interactions, smaller elastic variation, and weaker bonding contributions. Interestingly, this influence has an upper limit; under normal circumstances, J increases as the cross-linking degree increases. When J reaches a certain value, its effect diminishes, as indicated by the disappearance of network slippage during tensile testing. Therefore, PU with more uniform cross-linking exhibits a higher yield strength. These findings provide new insights into how cross-linking networks affect the mechanical properties of PU.