Dynamic Model Metallo‐Supramolecular Dual‐Network Hydrogels with Independently Tunable Network Crosslinks

Abstract

Hybrid polymer networks emerge between chemical and physical crosslinking, where two different modes of chain connectivity control the material behavior. However, rational relations between their microstructural characteristics, supramolecular kinetics, and the resulting network mechanics and dynamics are not well developed. To address this shortcoming, this study introduces a material platform based on a model dual-network hydrogel, comprising independently tunable chemical and physical crosslinks. The idea is realized by a click reaction between a tetra-PEG and a linear-PEG precursor, whereby the linear block also carries a terpyridine ligand at each end that can form additional physical crosslinks by metal ion-bis(terpyridine) complexation. We change the number of chemical crosslinks by varying the molar mass of the tetra-PEG, and we independently tune the metallo-supramolecular bonds by using different metal ions, Mn 2+ , Zn 2+ , Co 2+ , and Ni 2+ . Based on that modular approach, we study the rheological behavior and the diffusivity of fluorescent polymeric tracers. The dissociation of the metallosupramolecular bonds provides a relaxation step, whose timescale and intensity are quantified by a sticky Rouse model. These two characteristics differ not only depending on the metal ion but also according to the chemical network mesh size, which highlights an interplay between the chemical and physical crosslinks.

Bridging Rigidity and Flexibility: Modulation of Supramolecular Hydrogels by Metal Complexation

“…Further on, we investigated the hydrogel formation upon addition of stronger complexing Ni 2+ ions (Figure 4B, ) which form the most kinetically inert complexes with average dissociation times in the range of hours (log( k ass ) = 3.1, log( k dis ) = −7.6, log K = 21.8). [ 55 ] Due to the high association constant, reversible polymer networks cross‐linked by bis(Tpy)‐Ni 2+ complexes show purely elastic behavior at room temperature [ 57 ] and typical network relaxation times measurable by stress relaxation experiments at elevated temperatures (70 °C) comprise more than 15 h. [ 58 ] However, in the present study, a lower plateau modulus but similar LVE regime was observed in comparison to the Fe 2+ and Zn 2+ cross‐linked gels. The nanostructural investigation of the Ni 2+ cross‐linked nanorods via TEM revealed not only a decrease in the average length compared to the other metal ion treated samples (Fe 2+ : L n = 47 ± 20 nm; Zn 2+: L n = 53 ± 24 nm; Ni 2+ : L n = 27 ±10 nm), but also a significantly increased number of clusters (Figure 3E).…”

“…Further on, we investigated the hydrogel formation upon addition of stronger complexing Ni 2+ ions (Figure 4B, ) which form the most kinetically inert complexes with average dissociation times in the range of hours (log(k ass ) = 3.1, log(k dis ) = −7.6, log K = 21.8). [55] Due to the high association constant, reversible polymer networks cross-linked by bis(Tpy)-Ni 2+ complexes show purely elastic behavior at room temperature [57] and typical network relaxation times measurable by stress relaxation experiments at elevated temperatures (70 °C) comprise more than Step strain measurements (same color code as before) (M 2+ :Tpy = 1:2 if not indicated otherwise, 𝜔 = 1 rad s −1 ), H 2 O, 20 °C, G′: full symbols, G″: empty symbols). Note, that all metal ions were added in form of their respective triflate salts.…”

Swelling and Mechanical Properties of Polyacrylamide-Derivative Dual-Crosslink Hydrogels Having Metal–Ligand Coordination Bonds as Transient Crosslinks

“…While the frequency dependence of G’ is clear and qualitatively similar to that of the Ni-based gel, we did not observe a peak in loss modulus, which could have been associated with the lifetime of the physical bonds. This result suggested that the physical bond breaking and chain relaxation occurred at much higher frequencies [ 13 , 18 , 25 ], which indicated a much shorter lifetime for the Zn 2+ –imidazole bond than that for the Ni 2+ –imidazole bond. At low frequencies, the values of G ’ approached those of the chemical gel.…”

Emergence, evidence, and effect of junction clustering in supramolecular polymer materials

“…However, in supramolecular polymers, the enhanced polymeric dynamics of longer chains can overcome the association tendency of supramolecular units and thereby decrease the extent of phase-separation. 90,91 Moreover, in supramolecular polymeric hydrogels, the polar polymer precursor is responsible for assuring the system's solubility, which undermines the phase-segregation affinity of the supramolecular units. 80 Therefore, the effect of the polymer core length and incompatibility on enhancing the aggregation of associative motifs can be compromised by the enhanced polymeric dynamics and the increased solubility, respectively.…”