Water hydration behavior on the surface of materials is thought to be the fundamental factor determining subsequent biological responses and bio/blood compatibility of materials. Poly(sulfobetaine methacrylate) (polySBMA), a zwitterionic sulfobetaine polymer, is one of the biomembrane mimetic materials that exhibits excellent bio/blood compatibility. It has been reported that the zwitterionic polySBMA exhibits “antipolyelectrolyte” behavior, which changes polymer conformations and antifouling properties, in solutions containing salt ions. Here, we studied the hydration behavior of polySBMA by thermal analysis of differential scanning calorimetry (DSC) and identified various water hydrated states in different salt concentrations. We clarified the salt effects by adding monovalent (NaCl) /divalent (MgCl2) ions in polySBMA aqueous solutions. The results indicated that with the addition of miniscule salts, ions are attracted to the zwitterionic groups and the hydrated states are significantly changed. The attracted ions promote the incorporation of water molecules into polymer chains and enhance the solubility of polySBMA. Nevertheless, when large amounts of salts are added into the system, the excess ions tend to bind with free water molecules, which may impose an osmotic pressure on the hydrated polySBMA molecules and causes the shrinkage of the molecules. These results demonstrated that salt has a significant effect on polySBMA-water interactions, which may affect the biocompatibility of the material.
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