Persistent Homology Analysis of Molecular Structural Origins of the Onset of Local Plasticity and Cavitation in Polycarbonate: A Coarse-Grained Molecular Dynamics Study

Abstract

Identifying the structural precursors that govern plastic deformation in amorphous polymers remains a longstanding challenge due to the absence of well-defined defect structures common in crystalline solids. In this study, we investigate the molecular origins of stage transitions in amorphous polycarbonate by integrating coarse-grained molecular dynamics with persistent homology, a topological data analysis framework that captures intrinsic structural organization without arbitrary parameters. While conventional Gaussian-surface-based void detection effectively captures cavitation and void coalescence associated with the upper yielding process, it fails to detect the earlier, more subtle onset of structural heterogeneity. By analyzing second-order homology features and introducing a physically grounded birth-radius criterion linked to the first-shell coordination distance, we successfully isolate mechanically significant cavity-like structures at a sub-nanoscale (≈1 nm). The emergence of these persistent features provides a distinct structural precursor to the transition from homogeneous deformation to incipient plasticity, significantly preceding visible nanovoid formation. Spatial correlation and local strain analyses reveal that the vertices of these topological cavities exhibit pronounced non-affine strain localization and serve as preferential sites for subsequent cavitation. These findings establish persistent homology as a framework for detecting the localization of plasticity in disordered materials.