Network Topology Governs Cryogenic Mechanical Response in Isocyanate-Modified Epoxy Networks

High-performance epoxy-based polymers are increasingly deployed in space applications, where cryogenic service conditions impose stringent demands on mechanical performance over a wide temperature range. Modification of epoxy resins is commonly employed to improve fracture toughness and to mitigate brittle behavior. In this study, isocyanate-modified epoxy networks were investigated by combining molecular dynamics simulations with dynamic mechanical thermal analysis. Structural descriptors such as radius of gyration and hydrogen-bond statistics were correlated with elastic and thermal properties to establish molecular-level structure–property relationships. The simulations reveal homogeneous chain extension for systems containing up to 20% isocyanate, whereas further addition contributes little to backbone elongation. Hydrogen bonds provide a secondary stabilization mechanism at low temperatures, but selectively disabling hydrogen bonding demonstrates that covalent network topology dominates stiffness. The β-transition is consistently identified in simulation and experiment and shifts to lower temperature upon isocyanate incorporation. The corresponding transition temperature Tβ marks a change in the dominant stiffening mechanism. Analysis of molecular fluctuations shows that below Tβ stiffness is governed primarily by network compactness, while above Tβ enhanced segmental mobility leads to accelerated softening despite hydrogen-bond formation.