Solution critical temperature through Excess-entropy-Diffusivity lens

Abstract

Poly-N-isopropyl acrylamide (PNIPAM) is a widely studied thermo-responsive polymer with unique properties and numerous applications, including cell engineering and protein attachment. However, quantifying the responsive behavior of such polymers is challenging due to the vast configurational space. Existing methods for quantification are either focused on dynamic properties or structural properties, but the changes in structural parameters are small, while those in dynamic properties are significantly larger. This study proposes a new computational method considering non-equilibrium-activity-driven segregation to quantify responsive behaviors using structure (excess entropy) and dynamic (diffusivity) properties. We have calculated the Lower Critical Solution Temperature (LCST) for the N-Isopropyl propionamide (NIPPAm) -water system for different solute fractions by scaling excess entropy with diffusivity obtained by performing all-atom Molecular Dynamics simulations. Our method can precisely reproduce the LCST temperature in the 0.17–0.25 mol fraction concentration range. This approach can be used to study the responsive behavior of other polymers at different stimuli and improve our understanding of their applications.