Hydrogels are described as cross-linked polymer chains swollen in water. Owing to their deformability, biocompatibility and permeability, they are majorly applied in the biomedical field in forms of films including wound dressings, nanoparticles for drug delivery systems, and coacervates as tissue matrices. Therefore, improvement and control of rheological properties of hydrogels are necessary for improved performance of the above applications.
There are two broad categories of the hydrogels, namely; physical and chemical hydrogels. Chemical gels are networks synthesized by covalent bonds. On the other side, physical gels are synthesized by noncovalent physical bonds i.e. metal-ligand bond, hydrogen bond, electrostatic interactions and hydrophobic interaction. A lot of research efforts have recently focused on these physical cross-linking considering that these transient cross-links can reversibly break and reform dissipating a lot of energy and enable self-healing under deformation.
In order to come up with an adequate design of self-healing hydrogels based on supramolecular cross-linking, sufficient understanding of the dynamic attributes of the supramolecular cross-linking is important. Therefore, researchers led by Dr Tetsuharu Narita at ESPCI Paris (PSL Research University) in France investigated the dynamic properties of the physical gels with a metal-ligand bond around the physical gel point implementing dynamic light scattering as well as microrheology based on diffusing-wave spectroscopy. The adopted supramolecular physical gel consisted of polyacrylamide-derivative associating polymers which contained imidazole group that could form metal-ligand bonds in the presence of transition metal ions such as nickel ions. Their work is now published in Polymer.
The authors prepared poly(acrylamide-co-1-vinylimidazole) through free radical polymerization in water. In a bid to curtail secondary reactions, they performed polymerization in the presence of a chain—transfer agent (sodium formate) at 30 °C. The researchers added to flask of water, 1-vinylimidazole, acrylamide and sodium formate and the solution stirred. A solution of the initiator was prepared separately and added to the flask. The polymerization reaction proceeded for 5 h, then after dialysis against water for a week, a dry copolymer obtained by lyophilization of the viscous copolymer solution.
The authors then performed diffusing-wave spectroscopy microrheology of the copolymer containing nickel ions at different concentrations. For microrheological measurements, they dispersed polystyrene microspheres as probe. By analyzing the multiple dynamic light scattering signals from the probe microspheres, viscoelastic properties of the transiently crosslinked polymer solutions are measured.
Through the diffusive-wave spectroscopy measurements, the authors microrheologically determined the physical gel point where the nickel ion concentration was about 1.1mM, by observing the high frequency Winter-Chambon power-law behavior, which cannot be easily measured by classical macroscopic shear rheometry.
By dynamic light scattering they observed three autocorrelation modes, namely; fast, slow, and intermediate modes. The fast and slow modes were found to be diffusive in the entire nickel ion concentration regime studied. The fast mode corresponded to the collective diffusion of the network of cross-linked or entangled chains. However, the slow mode corresponded to the Brownian diffusion of the polymer clusters.
For the intermediate mode, the shift from a diffusive mode to a relaxation mode was noted to increase with nickel ion concentration. The nickel ion concentration, where the dynamical properties changed, corresponded to microrheologically determine physical gel point. In the pregel regime, the authors observed diffusion of cluster in the dynamic light scattering measurement as intermediate mode. In the developed system, it was possible to tune the transient cross-linking dynamics within a range of varying temperature or transition metal species.
Hiroto Ozaki, Tsutomu Indei, Tsuyoshi Koga, Tetsuharu Narita. Physical gelation of supramolecular hydrogels cross-linked by metal ligand interactions: Dynamic light scattering and microrheological studies. Polymer, Available online 15 February 2017.Go To Polymer