Decoupling between macroscopic densification and molecular structural evolution across gelation in cross-linked polymer networks

Abstract Cure-induced shrinkage in thermosetting polymers is generally assumed to reflect progressive densification during network formation. Here, we show that this assumption breaks down at gelation, which marks a transition in structural evolution. Using a model epoxy system, direct shrinkage measurements, wide-angle X-ray scattering, and molecular dynamics (MD) simulations are combined to link macroscopic densification with molecular-scale structural evolution. We find that macroscopic densification occurs predominantly before the gel point and becomes strongly suppressed after the formation of a percolated network. Conversely, characteristic intermolecular distances associated with phenyl-rich and hydroxyl-containing correlations continue to increase, with narrowing distributions, indicating constrained local reorganization within the network. MD simulations reveal that this behavior is accompanied by coarsening of free space, with both its fraction and characteristic size increasing despite an approximately constant density. These results establish gelation as a fundamental boundary separating distinct regimes of structural evolution, providing new principles for controlling shrinkage and internal stress.