How to characterise polymers with molecular simulation?

Shearing a unit cell with ethanol to determine the viscosity.

In the final article of this How To series, we'll be looking at how basic characterisation of a polymer structure. The previous article detailed how to make a polymer, now we've got to get some useful values from it to help guide materials research. If you've enjoyed these articles, follow me here for more molecular dynamics (MD) and materials science content in the future.

Degree of Cure

Different from the lab, degree of cure (DoC) is a simulation variable and not a property. The user has total control over when to stop the polymerisation process, and there are techniques to develop highly cured systems that may prove impractical to replicate in the lab. DoC governs many of the additional properties of the simulation so it is important that the user targets a DoC that best represents their laboratory samples. This is easily done by defining how the monomers bond together and keeping track of how many bonds have been created. Once the simulation has reached the specified number of bonds, and thus the target DoC, it's time to explore the properties of the newly created polymer.

Density & Free Volume

Density is the simplest value to determine for a polymer as it falls out of every MD simulation without additional work when the polymer is relaxed using an NPT ensemble. Density is also a great way to initially validate a simulation; if the polymer has a density to close to empirical values, it implies the forcefield and simulation setup is doing a reasonable job of creating the whole polymer structure. Equally useful are values like free volume, which can be calculated by measuring the empty space within the structure and doesn't require any additional simulation stages.

Further Simulations

For many of the most interesting properties, further characterisation simulations are required. These generally involve taking the relaxed cured polymer and stressing it through various external conditions. For example, the glass transition temperature of the polymer is found by gradually raising the temperature in the simulation and recording how the volume changes. Mechanical properties like the Young's modulus can be found by applying a tensile force to unit cell, and observing how the atoms behave as the box elongates. These simulations can be challenging and time-consuming to get right initially, but are ultimately faster than lab testing offer incredibly valuable insight for materials screening.

There's a large range of different properties, and chemical insights you can discover with molecular simulation. This article has covered some of the key ones to get started, but methods for new properties are being developed all the time. Hopefully you've enjoyed this series, and learnt how you can setup a molecular simulation to drive your own materials science research with digital discovery! Thanks for reading!