A new approach to thermal responsivity and hydrodynamics of poly(N-isopropylacrylamide) in aqueous solution: all-atom and mesoscopic simulations

Abstract

In this study, the impact of chemical and physical characteristics of poly(N-isopropylacrylamide) (PNIPAm) in water on its thermoresponsive behavior and aggregation was investigated via all-atom and mesoscopic simulations. The calculated dimensional and spatial parameters of the PNIPAm proved the existence of a critical chain length consisting of 15 repeating units (1697 g mol−1) which beyond it the PNIPAm conformations depend on the temperature. The influence of chain length on the interfacial behaviors and non-bonded interactions of PNIPAm in water indicated that the thermoresponsive behavior of the PNIPAm chain is independent of the chain length, from interactional aspect. Interestingly, the interactional barriers against the PNIPAm temperature dependency was correlated to the polymer intramolecular interactions and their contribution to heat capacity (C p-intra ). The simulation results indicated the highest barrier energy against the polymer bending at the critical chain length with C p-intra = 1.0189 kcal mol−1. Additionally, the influence of the chain length on its aggregation mechanism in water was evaluated using DPD simulation to correlate the PNIPAm spatial shape to dynamics and frictional solvent forces acting on the solute. According to the simulation results, the PNIPAm has an innate ability for chain aggregation. The higher and less aggregation intensity was occurred for the chain with shorter and longer length than the critical chain length, respectively. The presented studies can be used to design PNIPAm block with a proper molecular weight for preparing an efficient copolymer with chain network formation ability in water.