Strong Means Slow: Dynamic Contributions to the Bulk Mechanical Properties of Supramolecular Networks

Abstract

The desire to control rationally, through small-molecule synthesis, the properties of bulk materials has led to supramolecular approaches to, for example, polymers, [1,2] strong and weak organogels, [3] amphiphilic assemblies, [4] and liquid crystals.[5] In these materials, specific and well-defined interactions between molecules contribute to bulk material properties on two levels: structure and dynamics. Although the dynamic nature of the defining interactions is often the attribute that distinguishes supramolecular materials from their covalent counterparts, [6] direct mechanistic studies of the molecular contributions to the dynamic properties of materials are less abundant than structural studies.[7] Herein, we report that a simple structural variation in the networks of supramolecular sols and organogels provides a direct and quantitative measure of the relationship between molecular dynamics and macroscopic rheological properties. The approach is akin to a macromolecular "kinetic isotope effect", in that kinetic contributions to rate-determining processes are revealed by the differences in two isostructural systems. We find that it is the dynamics of molecular crosslinks, much more so than their thermodynamics, which are specifically and quantitatively responsible for the bulk viscoelastic properties of the supramolecular networks.The system under consideration is poly(4-vinylpyridine) (PVP) that is cross-linked by bis(M ii -pincer) compounds 1 (see Figure 2) derived from the work of van Koten [8] (M = Pd or Pt, Figure 1). [9] We recently reported that simple steric effects in the alkylamino ligands of 1 and related compounds change the rate of ligand exchange by approximately two orders of magnitude while leaving the thermodynamics of association effectively constant. [10] This independent control of dynamics is particularly significant; cross-linkers 1 a and 1 b (Figure 2) are structurally identical components within the network, and so their similar thermodynamics ensure that the extent and nature of cross-linking is essentially the same in the samples prepared from them. By extending the family of cross-linkers to the Pt II -pincer molecules 1 c and 1 d (Figure 2) we are able to probe the effect of a wide range of molecular dynamics.The addition of 2 % (by functional group) 1 b to a 100 mg mL À1 DMSO solution of PVP (Aldrich, M r = 60 000) gives rise to a clear, thick, deep yellow solution whose viscosity is approximately 2000-times greater than that of PVP alone (33 Pa s vs. 0.016 Pa s). Two control experiments confirm that the viscosity increase is due to coordinative cross-linking of the PVP by 1 b. First, the viscosity does not increase upon the addition of the same quantity of monomeric À .

Model Transient Networks from Strongly Hydrogen-Bonded Polymers

“…Presumably as the distance between two stickers along the chain is decreased, the relaxation of stress will require their concurrent unbinding. As a result, increasing the local concentration of UPy's leads to a correlation of their dynamics (also reflected in the flow activation energy, Figure 11), and as previously demonstrated, 27 it is the dynamics of unbinding and binding that dominate the macroscopic response.…”

“…In an early series of papers, Stadler and Freitas 11,[22][23][24] studied the rheological behavior of polybutadienes lightly modified with double-hydrogen-bonding phenylurazole groups, finding that even with light substitution along the backbone the storage modulus plateau was broadened and shifted to lower frequencies (although the overall network properties were primarily dictated by the highly entangled polymer backbone and the crystallization of the phenyl urazole side chains). More recently, reversible networks based on metal-ligand coordination have been synthesized and the macroscopic material properties directly related to the coordination dynamics; [25][26][27][28] in this case it was found that formation of a strong network required not just a high association constant but also slow dynamics of the reversible bond. Additionally, supramolecular gels based on hydrogen bonding in ionic liquids have been demonstrated and their rheological properties related to the strength and number of hydrogen bonds as well as stoichiometry of donor and acceptor units.…”

An Alternative Route Towards Metal–Polymer Hybrid Materials Prepared by Vapor‐Phase Processing

“…a) Schematic illustration of the phase diagram of polytetrafl uoroethylene (PTFE). Molecular packing and chain conformations of phase I, II, III, and IV are the disordered pseudohexagonal 15 7 helix, the triclinic 13 6 helix, the orthorhombic planar zigzag and the hexagonal 15 7 helix, respectively. The fi gure is redrawn from a previously published one.…”

Metallo-supramolecular modules as a paradigm for materials science

“…Note that this work shows how dynamics and mechanism of metal-ligand exchange reactions are not only a qualitative but also a quantitative representation of the viscoelastic properties of bulk materials [78]. Although thermodynamics are a primary design consideration in supramolecular chemistry, the dynamics of the interactions are particularly important under nonequilibrium conditions, such as those imposed under mechanical stress [76,77,79]. Recently, Craig and coworkers used single-molecule force spectroscopy to study the mechanical activation of the ligand substitution reaction.…”

“…Steric effects in the spectator ligands of the complex, therefore, profoundly affect the rate of ligand exchange while having little effect on the stability of the complex. Pincer motifs have proven to be attractive in this regard due to the ease of steric manipulation of the spectator ligands and the added advantage of increased stability relative to the moderate stability of aryl Pd(II) complexes [76,77]. Using this approach, the authors studied supramolecular networks with a continuous 3D topology based on polypyridine and a ditopic pincer ligand (crosslinker) in DMSO.…”