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Suppose I am running a molecular dynamics simulation of a polymer solution that exhibits upper critical solution temperature. From standard solution thermodynamics, we expect to see a polymer-rich zone and a polymer-lean zone. How do we spot these in a simulation cell?

Is it a visual check, or is there analysis one can perform to see this? What are some key methods one employs to check solution phase behavior?

For example, I am told that PNIPAm exhibits lower critical solution temperature (LCST) above 305 K. I expect the PNIPAm chains to congregate and so on. But my question is, what thermodynamic quantity or correlation should I be on the look out for after I have run this simulation to prove that I am seeing LCST-type phase separation?

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  • $\begingroup$ how short are the polymers? if short, for polymers, you may be able to run Gibbs Ensemble Monte Carlo $\endgroup$
    – B. Kelly
    Commented Jan 12, 2022 at 1:22
  • $\begingroup$ @B.Kelly I expect the simulation cell to have something like 10 40-mers? $\endgroup$
    – megamence
    Commented Jan 12, 2022 at 1:36
  • $\begingroup$ David kofke or Peter Cummings may be names worth exploring. I am not sure if they do polymers, but they sure do phase separation and miscibility $\endgroup$
    – B. Kelly
    Commented Jan 12, 2022 at 2:03
  • $\begingroup$ Hi megamance! Were those suggestions by BKelly useful? How are things going with this almost 8-month old question? Please update us! $\endgroup$
    – Nike Dattani - No Free Time
    Commented Sep 6, 2022 at 20:19

1 Answer 1

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Turns out the best way to spot phase transitions in polymers is to run the extended cell simulations and calculate surface tension, as seen in this paper: http://dx.doi.org/10.1063/1.3227905.

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