Improved Mechanical Properties of Zwitterionic Hydrogels with Hydroxyl Groups

Molecular dynamics simulations were performed to examine the mechanical properties of poly(carboxybetaine methacrylate) (pCBMA) hydrogels. pCBMA hydrogels with additional hydroxyl groups as physical cross-linkers (OH-pCBMA hydrogels) were studied for comparison. Results show that OH-pCBMA hydrogels have higher elastic modulus than pCBMA hydrogels. This improvement can be explained by hydrogen bond formation between hydroxyl groups and carboxylate groups in the OH-pCBMA hydrogel, which result in an enhanced polymer network within the hydrogel. The enhancement of the polymer work is also suggested by a smaller value of root-mean-square deviation of zwitterionic side chain pairs in the OH-pCBMA hydrogel than that in the pCBMA hydrogel. Additionally, the presence of hydrogen bonds leads to a lower equilibrium water content for the OH-pCBMA hydrogel compared to the pCBMA hydrogel, which is another reason for the greater mechanical modulus of the OH-pCBMA hydrogel.