The macroscopic properties of polymer networks are dictated by the structure and conformation of individual chains, but current models rely upon the underlying assumption that the single-chain structure is the same in a network as it is in solution. This offers qualitative estimates of material properties such as the linear mechanical behavior or percolation thresholds; however, numerous deviations from this idealization pervade literature. The most prominent example is chain prestrain, where the average chain conformation is extended in a network in comparison to its solution state. This reinforces the elasticity of gels and enables their formation well below the overlap concentration, serving as an additional lever to tune the structure and mechanics of network materials. However, the molecular origin of these behaviors remains unknown. Here, we demonstrate that these deviations are caused by additional long-range bond-vector correlations inherent to the multi-chain connectivity of polymer networks. These effects radially orient bonds way from a junction point, providing a thermodynamic argument for single-chain prestrain that quantitatively matches previous experimental measurements and rationalizes numerous deviations from ideality. To further validate the equilibrium nature of this picture, the structure and dynamics of two transient metallogels was characterized via neutron scattering and spin-echo spectroscopy. These measurements qualitatively validated the picture that high crosslink densities experience significant junction-induced bond orientation while linear-chain statistics are recovered in the long-chain, low crosslink density limit. Overall, this work challenges the preconceived notion that the local structure of a polymer network represents a temporal snapshot of a semi-dilute solution or polymer melt structure. Integrating these molecular details into traditional network theories will enable more accurate predictions of bulk mechanical properties and inspire further investigations into using chain prestrain as an additional lever to reinforce network elasticity.
