Abstract
Existing theories cannot predict the mechanical property changes of polyacrylamide hydrogels with different water content because of the absence of side chains. In this study, polyacrylamide hydrogels are prepared and tested to investigate the side chain effect on their mechanical properties. First, the comparison between the effective chain density and total chain density provides proof of the large amount of side chains in the polymer network of PAAm hydrogel. We propose a practical chain density fraction to measure the side chain fraction. Then, the abnormal Young’s moduli-polymer volume fraction relationship reveals that side chains affect the mechanical properties of hydrogel through the insufficient lubrication of water. Water confined in narrow space within a molecular-level size can bear shear force to provide extra deformation resistance. A constitutive mode considering the effect of the insufficient lubrication of water is proposed. Combining this constitutive model with experimental results, we find that this insufficient lubrication of water exists even in equilibrium PAAm hydrogel. Molecular dynamics simulations reveal that this insufficient lubrication of water comes from the constraint of polymer chains. It also demonstrates that when there is insufficient lubrication, the rearrangement of water molecules leads to the persistent energy dissipation in the Mullins effect of PAAm hydrogel.
Highlights
Monomers solved in water form hydrogels that bond through co-polymerization to the polymeric network with crosslinker molecules using more than two crosslinking sites
Combining this constitutive model with experimental results, we find that this insufficient lubrication of water exists even in equilibrium PAAm hydrogel
In addition to the crosslinking polymer network and the polymer network-water mixture, we propose that side chains play quite an important role in the mechanical behaviors of PAAm hydrogels due to the insufficient lubrication of water, which was ignored in previous research
Summary
Monomers solved in water form hydrogels that bond through co-polymerization to the polymeric network with crosslinker molecules using more than two crosslinking sites. The semi-empirical constitutive models mentioned above characterize the deformation behavior of the swelling-deswelling process in hydrogels well, whereas they fail to predict the change of mechanical properties along with the change of water content (or polymer fraction). Pervious experimental results and our experimental results (presented below in this paper) on the E − φnp relationship demonstrate that the exponent n is much higher than 13 in theory One interpretation of this phenomenon is that the increasing concentration of the polymer network makes hydrogel change from a diluted solution to a semi-diluted or even a fully concentrated solution, leading to the polymer chain conformation distribution gradually deviating from a Gaussian distribution or Langevin distribution. We hope this study can give insights into the microstructure of hydrogels and its impact on the mechanical properties
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
