Thermoresponsive Polymers under Solvent Flow through Molecular Dynamics

Common computational methods for describing laminar flow of dilute polymer solutions (LFDPS) in computational physical chemistry and engineering, such as continuum fluid dynamics approaches for the solvent description in conjunction with coarse-grained modeling for the solvated polymers, rely on sets of user-provided parameters poorly amenable to reproduce specific molecular characteristics at the atomic scale of the addressed system. In recent years, a flow molecular dynamics methodology has been shown to be a viable approach for simulating flows of molecular solutions. However, cases developed so far for condensed phase modeling based on this approach have been highly scarce. Here, we investigate the suitability of a de novo nonequilibrium molecular dynamics NEMD as adapted through our custom modified OPLS-AA force field and applied to LFDPS considering three solvents of different viscosities, water, a 50:50 water/glycerol mixture, and glycerol, and two thermoresponsive polymer derivatives of polyacrylamide, PNIPAM and PDEA. We show that the strengths of both computational approaches yield a descriptive atomistic perspective of the directed flow applied to dilute low molecular weight (LMW) polymer solutions in all of the three solvents considered, evidencing along 200 ns the spatiotemporal mechanism of energy and polymer structure changes that an applied flow triggers for elongating a globular polymer without modifying the laminar behavior of the flowing solution. We additionally demonstrate that the mechanism for the polymer structure change from globular to extended coil requires that the applied flow velocity should be at or above a threshold value <i>v</i>_th for polymer elongation to occur, thus evidencing a novel simulation parameter. By employing two LMW polymers that are thermoresponsive, easy to synthesize, and commercially reachable, we further demonstrate with pioneering in silico experiments that the LFDPS systems are realizable at temperatures 10-40 K higher than standard thermodynamic conditions. Hence, in silico LFDPS experiments with thermoresponsive polymers have two physics-based parameters, the flow velocity of the solution and the temperature variations around the lower critical solution temperature, making them a desirable selection for several applications including microfluidics for analyses and biosensing where the polymer's ability to stretch and contract is fundamental.