Summary

Supramolecular polymer networks consist of mono-, oligo-, or polymeric building blocks that are linked by transient, non-covalent bonds. Whereas much research effort has been spent on classical covalent polymer networks regarding their microscopic structure and its influence on the macroscopic material properties, classically with respect to spatial inhomogeneities of the crosslinking density on scales of 10–1000 nm [1] and more recently also with respect to local chain connectivity defects on scales of 1–10 nm [2], such knowledge is largely absent for supramolecular polymer networks. Our own past work has focused on such networks based on a model network platform built by non-covalent linking of monodisperse star-shaped four-arm polymeric building blocks [3]. The building-block connectivity is realized by complexation of terpyridine end groups to transition metal ions. The networks obtained by that approach contain only negligible amounts of inhomogeneities on scales of 10–1000 nm [3], and at appropriate polymer concentration and crosslinker stoichiometry, their elastic moduli even indicate just little misconnectivit [4]. However, if few local connectivity defects are intentionally introduced into these metallo-supramolecular model networks, e.g., by incorporation of a few percent of star polymers in which one of the arms does not carry a terpyridine motif, then there is evidence that the mobility of these defective building blocks through the percolated system is significantly accelerated, even on scales from micro- to millimeters [4]. On micrometer scales, even apparent superdiffusivity of the building blocks is evident [5]. An approach to explain these findings assumes that local connectivity defects around such unsaturated network building blocks allow them to migrate through the network with a fast “walking” mechanism; this conceptual picture, however, is not yet systematically supported and has not been lifted onto a general quantitative level to allow for a generalized modeling of the defect-accelerated network relaxation and permeability. In this project, we intend to systematically deepen our preliminary evidence and to derive a conceptual mechanism for the defect-assisted building block migration in transient model networks that allows for quantitative predictions of their relaxation on micro- and macroscopic time and length scales. For this purpose, we will further develop our model-network platform to hetero-complementary precursor-polymer interconnection to obtain truly model-type defect-free networks as a reference state, and then to purposely impart local connectivity flaws and track the motion of the building blocks that cause them. The perspective of that work is to facilitate the design of transient networks with adjustable local penetration properties and restructuring capabilities.

[1] Seiffert, S. Scattering perspectives on nanostructural inhomogeneity in polymer network gels. Prog. Polym. Sci. 2017, 66, 1-21.

[2] Saalwächter, K.; Seiffert, S. Dynamics-based assessment of nanoscopic polymer-network mesh structures and their defects. Soft Matter 2018, 14 (11), 1976-1991.

[3] Rossow, T.; Habicht, A.; Seiffert, S. Relaxation and dynamics in transient polymer model networks. Macromolecules 2014, 47 (18), 6473-6482.

[4] Habicht, A.; Czarnecki, S.; Rossow, T.; Seiffert, S. Connectivity defects enhance chain dynamics in supramolecular polymer model‐network gels. J. Polym. Sci., Part B: Polym. Phys. 2017, 55 (1), 19- 29.

[5] Tang, S.; Habicht, A.; Li, S.; Seiffert, S.; Olsen, B. D. Self-diffusion of associating star-shaped polymers. Macromolecules 2016, 49 (15), 5599-5608.