Intermediate water (IW) has been reported to play an important role in nonthrombogenicity of biomaterials. However, clear insights into the IW in the hydrated polymer are still debated. In this study, a series of molecular dynamics simulations was performed to identify the IW structure in hydrated poly(ω-methoxyalkyl acrylate)s (PMCxAs, where x indicates the number of methylene carbons) with x = 1-6. Through the quantitative comparison with experimental measurements, IW molecules were suggested to mainly come from the water interacting with an oxygen atom of the polymers, while most of the nonfreezing water (NFW) molecules corresponded to the water interacting with two polymer oxygen atoms. In addition, the IW molecules were found to effectively enhance the flexibility of the PMCxA side chains in comparison with the NFW molecules. The variations of the saturated IW content and the side-chain flexibility with the methylene carbon chain length of PMCxA were also found to be correlated with the experimental nonthrombogenicity of PMCxA, suggesting that the polymer with the more saturated IW content and higher chain flexibility possesses better nonthrombogenicity. Furthermore, through the analyses of the interplays between the IW and polymer and between IW and its adjacent water, we found that the presence of the unique interaction between IW and its adjacent water in the hydrated poly(2-methoxyethyl acrylate) (PMEA) is the main factor causing different cold crystallization behaviors of PMEA from the other PMCxAs rather than the interaction between water and the PMCxA matrix. The findings will be useful in the development of new nonthrombogenic materials.
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