Identifying trends in hydration behavior for modifications to the hydrophobicity of poly(n-isopropylacrylamide)

Poly(n-isopropylacrylamide), PNIPAM, is a thermo-responsive polymer that has been thoroughly studied for its many applications, such as drug delivery and actuators. Below the lower critical solution temperature (LCST), PNIPAM is well hydrated in the elongated conformation. The transition occuring at the LCST results in a less hydrated collapsed state above the LCST. This volume phase transition is dependent upon the hydration of the polymer and its hydrophobicity. Some research has been done on potential modifications of PNIPAM for applications, but until now there has not been a study of the hydration properties as a function of hydrophobicity. The work presented in this paper applies a Voronoi analysis of the hydration of PNIPAM, as well as PNIPAM with other alkyl substituents. We show from classical MD simulations that increasing hydrophobicity can increase the volume phase change, but there is a lower limit to this trend. Additionally, replica exchange molecular dynamics were conducted on PNIPAM showing a fluctuation between elongated and collapsed states near the LCST.