Interactions Affecting the Mechanical Properties of Macromolecular Microsphere Composite Hydrogels

Macromolecular microsphere composite (MMC) hydrogel is a kind of tough hydrogel fabricated by using peroxidized macromolecular microspheres as polyfunctional initiating and cross-linking centers (PFICC). The contribution of chemical cross-linking (covalent bonding) and physical cross-linking (chain entanglement and hydrogen bonding) to the mechanical properties are understood by testing the hydrogels, which were swollen in water or aqueous urea solutions to different water contents. The as-prepared MMC gels exhibited moderate moduli (60–270 kPa), high fracture tensile stresses (up to 0.54 MPa), high extensibilities (up to 2500%), and high fracture energies (270–770 J m^–2). The moduli of the swollen gels decrease dramatically, but there are no significant changes in fracture tensile strength and fracture strain, even slight increases. More interestingly, the swollen gels show much-enhanced fracture energies, higher than 2000 J m^–2. A gradual decrease in the hysteresis ratio and residual strain is also found in the cyclic tensile testing of the hydrogels that were swollen to different water contents. The covalent bonding determines the tensile strength and fracture energy of the MMC gels, whereas the physical entanglement and hydrogen bonding among the polymer chains contributes mainly to the modulus of the MMC gels, and they are also the main reason for the presence of hysteresis in the loading–unloading cycles

Synthesis of Graphene Peroxide and Its Application in Fabricating Super Extensible and Highly Resilient Nanocomposite Hydrogels

Functionalized graphene has been considered as one of the most important materials for preparing polymer nanocomposites due to its unique physical structure and properties. To increase the interfacial interaction between polymer component and graphene oxide (GO) sheets, <i>in situ</i> grafting polymerization initiated by a free radical initiator immobilized on GO sheets is a better choice. We report a facile and effective strategy for preparing graphene peroxide (GPO) <i>via</i> the radiation-induced peroxidation of GO. The formation of peroxides on GO is proven by iodometric measurement and other characterizations. Using GPO as a polyfunctional initiating and cross-linking center, we obtained GO composite hydrogels exhibiting excellent mechanical properties, namely, very high tensile strength (0.2–1.2 MPa), extremely high elongations (2000–5300%), and excellent resilience. This work provides new insight into the fabrication of GO/polymer nanocomposites to fulfill the excellent mechanical properties of graphene

Rheological Behavior of Tough PVP-<i>in Situ</i>-PAAm Hydrogels Physically Cross-Linked by Cooperative Hydrogen Bonding

Rheology studies were performed on tough PVP-<i>in situ</i>-PAAm hydrogels physically cross-linked by cooperative hydrogen bonding to understand their viscoelastic response and, hence, the interactions and microstructure. The viscoelasticity of the PVP-<i>in situ</i>-PAAm hydrogels was strongly affected by the monomer ratio (<i>C</i>_AAm/<i>C</i>_VP). Hydrogels prepared with a high monomer ratio exhibited weak time, temperature and frequency dependence of the viscoelastic properties, similar to those of chemically cross-linked hydrogels. The storage modulus (<i>G</i>′) of the gels was much greater than the loss moduli (<i>G</i>″) and low loss factor (tan δ < ∼ 0.1), which indicated that they were solid-like, and mostly elastic. These supramolecular gels exhibited a strain- and <i>C</i>_AAm/<i>C</i>_VP-dependent reversible gel (solid) to viscoelastic liquid transition due to the dynamic nature of the cooperative hydrogen bonds. That transition also coincided with the onset of nonlinear viscoelastic behavior. The addition of a low molecular weight compound, urea, that competes for hydrogen bonding sites weakens the gel by decreasing the effective cross-link density or weakening the intermolecular hydrogen bonding

Facile Fabrication of Tough Hydrogels Physically Cross-Linked by Strong Cooperative Hydrogen Bonding

Novel hydrogels with excellent mechanical properties have prompted applications in biomedical and other fields. The reported tough hydrogels are usually fabricated by complicated chemical and/or physical methods. To develop more facile fabrication methods is very important for the practical applications of tough hydrogels. We report a very simple yet novel method for fabricating tough hydrogels that are totally physically cross-linked by cooperative hydrogen bonding between a pre-existing polymer and an <i>in situ</i> polymerized polymer. In this work, tough hydrogels are prepared by heating aqueous acrylamide (AAm) solution in the presence of poly­(<i>N</i>-vinylpyrrolidone) (PVP) but without any chemical initiators or covalent bonding cross-linking agents. Mechanical tests of the as-prepared and swollen PVP-<i>in situ</i>-PAAm hydrogels show that they exhibit very high tensile strengths, high tensile extensibility, high compressive strengths, and low moduli. Comparative synthesis experiments, DSC characterization, and molecular modeling indicate that the formation of strong cooperative hydrogen bonding between the pre-existing PVP and the <i>in situ</i> formed PAAm chains contributes to the gel formation and the toughening of the hydrogels. The unique microstructure of the gels with evenly distributed flexible cross-linking sites and long polymer chains attached to them endow the hydrogels with an excellent mechanism of distributing the applied load