Decoding Elasticity Build-Up and Network Topology in Free-Radical Cross-Linking Polymerization: A Combined Experimental and Atomistic Approach

The range of applications involving free-radical cross-linking processes has grown impressively over the last decades. However, in numerous fields where tightly cross-linked materials are required, the network development and its relation to the elastic properties are still a gray zone, as it is particularly challenging to design experiments that would allow validating the predictions from (often phenomenological) theoretical models or numerical simulations. Here, we report on a successful attempt to align time-resolved infrared-rheology measurements with fully atomistic simulations over the whole conversion range of an important acrylic free-radical cross-linking polymerization, unveiling the different regimes behind the elasticity build-up upon double bond conversion. Our combined experimental-theoretical approach provides an original insight into the various stages of the polymer network growth, from the earliest initiation up to final conversion. More specifically, we show that the pregel and gel formation stages are driven by the formation of branched polyfunctional polymers, which link together toward a sample-spanning network at the gel point. This regime proceeds in the postgel stage until the spatial heterogeneity in the cross-link density vanishes, leaving dangling ends as residual structural defects that then gradually connect to close the network. Following a steep transition at ultimate conversion, the elastic modulus of the network reaches the value predicted by the rubber elasticity theory in the affine limit.