Quantitatively predicting the mechanical properties of networks, even the relatively simple linear elasticity, is a long-standing challenge with significant technological importance. This challenge arises because real networks are difficult to accurately characterize due to the formation of defects such as dangling ends and loops which are not fully elastically effective and therefore do not contribute to the mechanical properties of the material to the same extent. While recent efforts to incorporate loops into elasticity, swelling, and fracture theories have shown improved agreement with experimental data, one uncertainty underlying these updates is the role chain conformation plays in property prediction. Small angle neutron scattering was used to measure single chain radii of gyration of end-linked polymer gels before and after cross-linking to calculate the pre-strain, which is the ratio of the average chain size in a cross-linked network to that of a free chain in solution. The pre-strain increased from 1.03 ± 0.01 to 1.07 ± 0.02 as gel synthesis concentration decreased near the overlap concentration, indicating that the chains are slightly more stretched in the network than in solution. Dilute gels with higher loop fractions were found to be spatially homogenous. Form factor and lattice scaling analyses independently confirmed that elastic strands stretch by 7 – 23% from Gaussian conformations to create a space-spanning network, with increased stretching as network synthesis concentration decreases. Pre-strain measurements reported here serve as a point of reference for network theories which rely on this parameter for the calculation of mechanical properties.

Conformation of network strands in polymer gels

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