Structural changes of PMMA substrates with different electrolyte solutions: A molecular dynamics study

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The structure of polyelectrolyte substrates could be changed from swelling to collapse by exchanging different electrolyte solutions. However, few investigations explained the phenomenon at the molecular level. In this paper, we employed a molecular dynamics method to simulate the structural changes of poly(methacrylic acid sodium) (PMMA) substrates with the presence of pure water, NH4Cl, tetramethylammonium bromide (TMAB) and tetraethylammonium bromide (TEAB) solution. The simulated results show that the percentage of water molecules penetrated into the substrate and the height of PMMA substrates gradually decrease following the order pure water>NH4+>TMA+>TEA+. This may be ascribe to the hydrophobic interaction between the solute cations and water molecules, which can exclude water molecules from the substrates and further influence the height of PMMA substrates. In particular, through the observation at the molecular level, it is obvious that the existence of water bridge structure and salt bridge structure formed between penetrated particles and PMMA chains play a key role in the structural change of PMMA substrates. Water bridges can induce the swelling of PMMA substrates and salt bridges will result in collapse happening. This work may promote a further understanding of the cationic effects on the structure of PMMA substrates at the molecular level and provide a new insight to study the interfacial micro-structure of other biological substrates in future.