Entropy Rules: Molecular Dynamics Simulations of Model Oligomers for Thermoresponsive Polymers.

Alexander Kantardjiev,Petko M Ivanov

doi:10.3390/e22101187

Abstract

We attempted to attain atomic-scale insights into the mechanism of the heat-induced phase transition of two thermoresponsive polymers containing amide groups, poly(N-isopropylacrylamide) (PNIPAM) and poly(2-isopropyl-2-oxazoline) (PIPOZ), and we succeeded in reproducing the existence of lower critical solution temperature (LCST). The simulation data are in accord with experimental findings. We found out that the entropy has an important contribution to the thermodynamics of the phase separation transition. Moreover, after decomposing further the entropy change to contributions from the solutes and from the solvent, it appeared out that the entropy of the solvent has the decisive share for the lowering of the free energy of the system when increasing the temperature above the LCST. Our conclusion is that the thermoresponsive behavior is driven by the entropy of the solvent. The water molecules structured around the functional groups of the polymer that are exposed to contact with the solvent in the extended conformation lower the enthalpy of the system, but at certain temperature the extended conformation of the polymer collapses as a result of dominating entropy gain from “released” water molecules. We stress also on the importance of using more than one reference molecule in the simulation box at the setup of the simulation.

Highlights

IntroductionThere are water-soluble polymers that separate from solution upon heating (inverse temperature-dependent solubility) above the phase transition temperature (lower critical solution temperature, LCST)
Most synthetic macromolecules become more soluble when heated
We found out that the entropy has an important contribution to the thermodynamics of the phase separation transition

Summary

Introduction

There are water-soluble polymers that separate from solution upon heating (inverse temperature-dependent solubility) above the phase transition temperature (lower critical solution temperature, LCST). Such polymers are referred to as thermoresponsive polymers. This phenomenon is explained to result from the balance between the enthalpy contribution from the energy stabilization due to hydrogen bonding of the polymer with the water molecules and the entropy gain of the system at higher temperature that outweighs the enthalpy preference at lower temperatures. Hydrogen bonding between the polymer and the water molecules lowers the free energy of dissolution This effect becomes less important at higher temperature and, entropy effects prevail [1]

Methods

Results

Conclusion