# Thermoresponsive Polymers under Solvent Flow through Molecular Dynamics

## TL;DR

This paper explores how thermoresponsive polymers behave under solvent flow using molecular dynamics simulations, revealing new insights into polymer elongation and flow behavior.

## Contribution

The study introduces a novel simulation parameter for polymer elongation and demonstrates thermoresponsive polymer behavior under non-standard temperatures.

## Key findings

- A threshold flow velocity is required for polymer elongation in dilute solutions.
- Thermoresponsive polymers can be simulated at temperatures 10–40 K above standard conditions.
- The applied flow triggers spatiotemporal changes in polymer structure without disrupting laminar flow.

## Abstract

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 v
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.