A Novel Hydrogel with High Mechanical Strength: A Macromolecular Microsphere Composite Hydrogel

TLDR: Gong et al. as discussed by the authors reported a new way of synthesizing hydrogels with a well-defined network structure and high mechanical strength, where a peroxidized MMS acts as both an initiator and a crosslinker.

Abstract: The industrial and biomedical applications of hydrogels made from either natural or synthetic sources are strongly limited by their poor mechanical properties. A normal structure (NS) hydrogel breaks under low stress because there are very few energy dissipation mechanisms to slow crack propagation. In addition, as their crosslinking points are distributed irregularly and the polymer chains between the crosslinking points have different lengths, the stress cannot be evenly distributed between the polymer chains, and crack initiation is facile. Many efforts have been focused on increasing the mechanical strength of hydrogels, but the robustness still remains unsatisfactory. In recent years, three kinds of novel hydrogels with unique structures and high mechanical strength have been developed. Topological (TP) gels have figure-ofeight crosslinkers that can slide along polymer chains. The gel swells to about 500 times its original weight and can be stretched to nearly 20 times its original length. The nanocomposite (NC) hydrogel is made from specific polymers with a water-swellable inorganic clay. Most of the macromolecules are grafted onto nanoparticles, indicating that the nanoparticle clay acts as a highly multifunctional crosslinking agent. We believe that the high mechanical strength of this material has its origin in the very high functionality of the rigid crosslinked points and the lack of short chains between crosslinked components, as every active chain has to stretch between nanoparticles. The extension degree of a chain before breakage is controlled by the relationship between its relaxed end-to-end distance and its contour length, which is low for short chains. When a short chain in an NS hydrogel breaks, its load is thrown onto just one or two other adjacent chains, which dramatically increases their load. Hence, multiple chain fractures occur, causing voids and microcracks. However, in an NC hydrogel with large, rigid crosslinking points, the load from a single broken chain will be spread over many other chains, and the material is less likely to form the microcracks and voids responsible for initiating bulk failure. Gong et al. have reported a new method of obtaining strong and tough hydrogels by making double-network (DN) materials with a high molar ratio of the second network to the first network. In this case, the first network is highly crosslinked and the second network is loosely crosslinked. These DN hydrogels demonstrate extremely high mechanical strength. By adding a third component to a DN gel, either a weakly crosslinked network or noncrosslinked linear chains, gels with high-strength and low-frictional coefficients were obtained. Macromolecular microspheres (MMSs) have become an important structure in polymeric materials. The hydrogel microspheres on the microor nanoscale are known as microgels or nanogels, respectively. They are usually environmentally sensitive and are mainly used in drug delivery and other biomedical applications. However, it is difficult to form bulk hydrogels (macrogels) with these microgels, and when formed, the macrogels do not exhibit high mechanical strength. Very little work has been done on incorporating other kinds of microspheres into bulk hydrogel structures, and the improvement in mechanical strength is far less than for the three hydrogels mentioned above. Here, we report a new way of synthesizing hydrogels with a novel, well-defined network structure and high mechanical strength. In this method, a peroxidized MMS acts as both an initiator and a crosslinker. The mechanism for the formation of the peroxide and the initiation of polymerization, as well as for the formation of a hydrogel, are proposed in Scheme 1. The new hydrogel is a macromolecular microsphere composite (MMC) hydrogel. When the MMS emulsion is irradiated with Co c-rays in oxygen, peroxides (POOR and POOH; here P is the macromolecule that comprises the MMS, and R is a short alkyl group) are formed on the surface and possibly, to a certain extent, in the inner part of the MMS. The formation of peroxides on the MMS was proven with iodometry, which is the common method used to verify their formation and determine the amount formed in the polymers. Potassium iodide and isopropyl alcohol were added to the irradiated MMS emulsion, and as the solution was heated and refluxed for 30 min, it gradually became yellow, which indicates the formation of I2 and further establishes the presence of peroxides on the MMS. The peroxides decomposed under heat to form the free radicals PO , OR , and OH . PO initiated the grafting of C O M M U N IC A TI O N