Pressure-Induced Phase Transitions of Nonionic Polymer Brushes

Abstract

While the temperature responsive behavior of nonionic polymers has been extensively studied and is nowadays one of the key mechanisms of smart materials, the pressure response of thin films remains basically unexplored. We investigate the conformational transition of nonionic brushes and semidilute solutions induced by hydrostatic pressure and temperature variations. Interestingly, the pressure–temperature phase diagram for the coil-to-globule transition of brushes, probed by neutron reflectometry, nearly coincides with that in semidilute solutions. We also show that the phase behavior can be understood and predicted with simple thermodynamic concepts employed so far for the denaturation of proteins. Fully atomistic molecular dynamics simulations provide molecular insight into the pressure-responsive behavior. Combining all three approaches allows us to demonstrate that pressure-induced hydration of nonionic polymers at low pressure is universal as it is dictated by water and is polymer-independent. In contrast, the pressure-induced dehydration at high pressure is strongly polymer-specific. The outcomes apply to a wide class of nonionic polymers and can aid the design of responsive coatings with the desired pressure-responsive behavior.