Relaxation Processes in Supramolecular Metallogels Based on Histidine–Nickel Coordination Bonds

Understanding the quantitative relationship between the dynamic mechanical properties of associating polymer networks and the dynamics of sticker bonds represents an important problem in polymer science because materials mechanics is affected by not only the sticker bond chemistry but also the sticker position, the polymer structure, and the physical environment of the associating polymers such as concentration and solvent quality. In this study, associating networks formed by structurally well-defined linear poly­(<i>N</i>,<i>N</i>-dimethyl­acrylamide) polymers with histidine side groups in complexation with Ni²⁺ ions are chosen as a model system. “Sticker diffusion and dissociation spectrometry” is developed as a new method to quantify the dissociation dynamics of stickers within the network environment where the stickers are covalently attached to polymers above their overlap concentration. The estimated time constants for junction dissociation in gels are shown to be substantially different than the ones measured by metal exchange experiments on small-molecule junctions in the dilute solution limit. Additionally, the in-gel dissociation time constants exhibit the same temperature dependence as the network relaxation times inferred from rheological characterization, which serves as the basis for time–temperature superposition, provided that the network relaxation is governed by the dissociation kinetics of stickers. Furthermore, self-diffusion of these associating polymers is probed by forced Rayleigh scattering, and pure Fickian diffusive behavior is revealed. The characteristic time constants for all the explored dynamic processes are finally viewed in the superimposed frequency sweep spectrum, demonstrating the inherent hierarchical relaxation in associating polymer networks even with only a single type of junction functionality

Anomalous Self-Diffusion and Sticky Rouse Dynamics in Associative Protein Hydrogels

Natural and synthetic materials based on associating polymers possess diverse mechanical behavior, transport properties and responsiveness to external stimuli. Although much is known about their dynamics on the molecular and macroscopic level, knowledge of self-diffusive dynamics of the network-forming constituents remains limited. Using forced Rayleigh scattering, anomalous self-diffusion is observed in model associating protein hydrogels originating from the interconversion between species that diffuse in both the molecular and associated state. The diffusion can be quantitatively modeled using a two-state model for polymers in the gel, where diffusivity in the associated state is critical to the super diffusive behavior. The dissociation time from bulk rheology measurements was 2–3 orders of magnitude smaller than the one measured by diffusion, because the former characterizes submolecular dissociation dynamics, whereas the latter depicts single protein molecules completely disengaging from the network. Rheological data also show a sticky Rouse-like relaxation at long times due to collective relaxation of large groups of proteins, suggesting mobility of associated molecules. This study experimentally demonstrates a hierarchy of relaxation processes in associating polymer networks, and it is anticipated that the results can be generalized to other associative systems to better understand the relationship of dynamics among sticky bonds, single molecules, and the entire network

Responsive Block Copolymer Photonics Triggered by Protein–Polyelectrolyte Coacervation

Ionic interactions between proteins and polyelectrolytes are demonstrated as a method to trigger responsive transitions in block copolymer (BCP) photonic gels containing one neutral hydrophobic block and one cationic hydrophilic block. Poly(2-vinylpyridine) (P2VP) blocks in lamellar poly(styrene-<i>b</i>-2-vinylpyridine) block copolymer thin films are quaternized with primary bromides to yield swollen gels that show strong reflectivity peaks in the visible range; exposure to aqueous solutions of various proteins alters the swelling ratios of the quaternized P2VP (QP2VP) gel layers in the PS-QP2VP materials due to the ionic interactions between proteins and the polyelectrolyte. Parameters such as charge density, hydrophobicity, and cross-link density of the QP2VP gel layers as well as the charge and size of the proteins play significant roles on the photonic responses of the BCP gels. Differences in the size and pH-dependent charge of proteins provide a basis for fingerprinting proteins based on their temporal and equilibrium photonic response. The results demonstrate that the BCP gels and their photonic effect provide a robust and visually interpretable method to differentiate different proteins

Oxidatively Responsive Chain Extension to Entangle Engineered Protein Hydrogels

Engineering artificial protein hydrogels for medical applications requires precise control over their mechanical properties, including stiffness, toughness, extensibility, and stability in the physiological environment. Here we demonstrate topological entanglement as an effective strategy to robustly increase the mechanical tunability of a transient hydrogel network based on coiled-coil interactions. Chain extension and entanglement are achieved by coupling the cysteine residues near the N- and C-termini, and the resulting chain distribution is found to agree with the Jacobson–Stockmayer theory. By exploiting the reversible nature of the disulfide bonds, the entanglement effect can be switched on and off by redox stimuli. With the presence of entanglements, hydrogels exhibit a 7.2-fold enhanced creep resistance and a suppressed erosion rate by a factor of 5.8, making the gels more mechanically stable in a physiologically relevant open system. While hardly affecting material stiffness (only resulting in a 1.5-fold increase in the plateau modulus), the entanglements remarkably lead to hydrogels with a toughness of 65 000 J m^–3 and extensibility to approximately 3000% engineering strain, which enables the preparation of tough yet soft tissue simulants. This improvement in mechanical properties resembles that from double-network hydrogels but is achieved with the use of a single associating network and topological entanglement. Therefore, redox-triggered chain entanglement offers an effective approach for constructing mechanically enhanced and responsive injectable hydrogels

Defects, Solvent Quality, and Photonic Response in Lamellar Block Copolymer Gels

Stimuli-responsive photonic gels are made from the lamellar block copolymer (BCP) poly­(styrene-<i>b</i>-2-vinylpyridine) (PS–P2VP), where the photonic responses are triggered by swelling/deswelling of the P2VP block with a selective solvent. When compared to isotropic swelling in chemically cross-linked homopolymer gels, the P2VP block in the lamellar BCP shows significantly lower degrees of swelling in alcohol–water cosolvents. The glassy PS layers completely constrain the lateral expansion of the P2VP gel layers and the dislocation defect network that develops during BCP self-assembly provides a counter force to vertical swelling. A model based on Flory–Huggins mixing and dislocation network strain energy is proposed to capture the swelling behavior of the BCP and is then used to estimate the dislocation network density in the lamellar BCP. This work establishes the quantitative relationship between the reflective color of the photonic gel, the effective χ parameter of the swellable block and the solvent, and the defect density of the BCP film and demonstrates the potential utility of these photonic materials as a quick means to measure solvent quality or defect density

Antiviral Agents from Multivalent Presentation of Sialyl Oligosaccharides on Brush Polymers

Bioinspired brush polymers containing α-2,6-linked sialic acids at the side chain termini were synthesized by protection-group-free, ring-opening metathesis polymerization. Polymers showed strain-selective antiviral activity through multivalent presentation of the sialosides. The multivalent effect was further controlled by independently varying the degree of polymerization, the number density of sialic acids, and the length of side chains in the brush polymers. Optimizing the three-dimensional sialoside spacing for better binding to hemagglutinin trimers was of critical importance to enhance the multivalent effect and the antiviral activity determined by hemagglutination inhibition assays and in vitro infection assays. By taking advantage of their structural similarities with native mucins, these brush polymers can be used as model systems to dissect the intricate design principles in natural mucins