Olympic gels are polymer networks that are connected only by permanent entanglements between ring molecules. These materials are a challenging test of models of polymer entanglements and have the potential to create elastic materials with novel properties.

The swelling equilibrium of Olympic gels was recently studied by Monte Carlo Simulations [1]. In contrast to chemically cross-linked polymer networks, it was observed that Olympic gels made of chains with a larger degree of polymerization, N, exhibit a smaller equilibrium swelling degree, Q ∝ N-0.28φ0-0.72, at the same polymer volume fraction φ0 at network preparation. This observation could be explained by a des-interspersion process of overlapping non-concatenated rings upon swelling. Our results demonstrate that entangled chains swell differently to cross-linked chains. Therefore, our work has the potential to improve significantly our current understanding of the swelling process of conventional chemically linked gels.

The data on concatenation as determined from our ideal Olympic gels is a critical test of previous experimental studies on the trapping of cyclic molecules inside a network [2] and is relevant for understanding trapping of complex molecules in a gel-like environment. This data is also fundamental for understanding the network structure of Olympic gels. In a third work we could show that the network structure is well approximated by mean field models based upon a Poisson distributed number of pairwise concatenations per cyclic molecule [3]. With these results, we could test the efficiency of three different routes (DNA linked by cuts with topo-isomerase II, enhanced cycle formation by selective end-groups (“DNA-Origami”), and the “progressive construction” starting from a melt of non-connected rings) to create Olympic gels. The first route is limited to cyclic DNA strands and provides rather perfect model networks at a sufficiently large average number of concatenations per cyclic DNA molecule. The Origami method leads to networks with increasing defects and a broader distribution of weights of the cyclic strands for increasing overlap number per selectively reacting batch of chains. The progressive construction method allows for the creation of gels with a low amount of defects but suffers from the requirement of extremely large degrees of polymerization of the non-concatenated cyclic precursor molecules. However, the last two strategies can be combined to optimize the synthesis of Olympic gels from standard synthetic polymers.

Publications:

1. M. Lang and J.-U. Sommer,
   Swelling of Olympic gels
   Phys. Rev. Lett. 112, (2014) 238001.
2. M. Lang, J. Fischer, M. Werner, and J.-U. Sommer,
   Olympic gels: Concatenation and swelling
   Macromol. Symp. 358 (2015) 140-147.
3. J. Fischer, M. Lang, and J.-U. Sommer,
   The formation and structure of olympic gels
   J. Chem. Phys. 143 (2015) 243114.