Molecular structure and properties of click hydrogels with controlled dangling end defect

Research Progress of the Ion Activity Coefficient of Polyelectrolytes: A Review

Polyelectrolyte has wide applications in biomedicine, agriculture and soft robotics. However, it is among one of the least understood physical systems because of the complex interplay of electrostatics and polymer nature. In this review, a comprehensive description is presented on experimental and theoretical studies of the activity coefficient, one of the most important thermodynamic properties of polyelectrolyte. Experimental methods to measure the activity coefficient were introduced, including direct potentiometric measurement and indirect methods such as isopiestic measurement and solubility measurement. Next, progress on the various theoretical approaches was presented, ranging from analytical, empirical and simulation methods. Finally, challenges for future development are proposed on this field.

3D Interlayer Slidable Multilayer Nano-Graphene Oxide Acrylate Crosslinked Tough Hydrogel

The design of three-dimensional crosslinked units with a spatial structure is of great significance for improving the mechanical properties of hydrogels. However, almost all the nanocomposites incorporated in hydrogels were defined as rigid nanofillers without further discussion on the potential contribution from the spatial structure change. In this work, the 3D nano chemical crosslinker multilayer graphene oxide acrylate (mGOa) was developed as a pressure-responsive crosslinker to achieve both low elastic modulus and high compression stress by synergizing more polymer chains against the loading force through interlayer sliding. Results showed that the hydrogel crosslinked by only 2 mg/mL mGOa nano chemical crosslinker in the poly(2-hydroxyethyl methacrylate-co-acrylamide) hydrogel (molar ratio: 1:1) can effectively enhance the mechanical strength up to 14.1 ± 2.1 MPa at a high compressive strain (90.6%) with an elastic modulus of less than 0.03 MPa at the initial 5% compression, whereas the hydrogel crosslinked by methacrylated single-layer graphene oxide (sGOa) or a small-molecule chemical crosslinker, N,N'-methylene bisacrylamide, can only reach 2.3 ± 0.8 MPa and 1.4 ± 0.4 MPa, respectively. In addition, the instantaneous modulus of the mGOa crosslinked hydrogel rapidly increased to the peak value with the increase of strain. The repeated compression test of HcA-mGOa hydrogels showed the responsive increase of the modulus, which was promoted by the synergism of polymer chains under compression. This indicated that the interlayer sliding of mGOa is the key contributor to mechanical strength enhancement, which provides a new rationale to design tough hydrogels.

Relative contributions of chain density and topology to the elasticity of two-dimensional polymer networks

Understanding the relationships between the structure of polymer networks and their mechanical properties is important for the design of advanced soft materials with optimal properties. However, classical rubber elasticity theories often fall short in their description of the network structure, while simulation techniques at molecular scale remain impractical at that length scale. Here we develop a computational approach based on random discrete networks, in which the polymer network is represented as an assembly of non-linear springs connected at crosslinking points. The density of elastically-effective chains, average network coordination and chain contour lengths are varied independently in order to identify their respective contributions to the network elasticity. Numerical results suggest scaling relations between network parameters and elastic properties that are markedly different from the predictions of classical rubber elasticity theories. In particular, the elastic modulus of 2D random networks is found to be independent of density at constant topology, and proportional to the average coordination at constant density. The discrepancy is due to the pre-straining of the chains in the discrete network, which is not accounted for in classical models of rubber elasticity. Our results have implications for the interpretation of experimental data for ideal network gels that are formed by the cross-coupling of macromolecular building blocks in solution.