Topological Structure of Networks Formed from Symmetric Four-arm Precursors

Compared to gels formed by asymmetrically coupling bifunctional polymer chains and tetrafunctional junctions, gels formed via A-B type end-linking of symmetric tetra-arm star polymer precursors were found to exhibit higher strength and lower spatial heterogeneity. While loops constituting of odd number of junctions are forbidden by precursor chemistry, these materials still contain topological defects which affect their properties. Here, we demonstrate the superposition of the dilution effect and chain-length effect on loop formation, and show that the entire network topology can be characterized by the measurement of just secondary loops. The loop fractions predicted by the topological simulations are in excellent agreement with the experimental measurements of model tetra-PEG hydrogel. In addition, the delay of gel point as a function of secondary loop fraction is quantitatively predicted without any fitting parameter. Furthermore, careful comparison reveals similarity between the behavior of loops consisting of 2n junctions in the symmetric system and that of loops consisting of n junctions in the asymmetrical system, suggesting analogies between the chemically distinct networks.