According to the temperature, the polymer can have a hard and brittle ‘Glassy state’ or soft and flexible ‘Rubbery state.’ The temperature at which the phase transition occurs is called ‘Glass transition temperature, Tg.’ The glass transition temperature only can be applied to amorphous and semi-crystalline polymers. We can predict information about which polymer will exhibit mechanical properties at a specific temperature through the glass transition temperature.
For the detail of the theoretical part, please refer to the Glass transition temperature in Polymers.
In this module tip, we’ll learn about ‘How to obtain the Tg of the polymer system via LAMMPS (Tg/CTE) module.’
1. Prepare MD Model
First, you need to prepare the fully optimized MD model by the thermalization step.
In the example of this module tip, we'll start the simualtion using the Nylon 6 (n=20) polymer system.
The process of structural modeling can be found in the previous post: How to perform the polymer simulation – Polymer MD Modeling.
And the process of thermalization can be found in the previous post: The Essential Step for Polymer Simulation – Thermalization.
2. Perform the Tg Calculation
After adding the LAMMPS (Tg/CTE) module, set the initial conditions. Next, start the job.
Since the LAMMPS (Tg/CTE) module can only be connected to the Thermalization module. Therefore, it is essential to perform the Thermalization calculation before adding the module.
For the example video, we performed the thermalization calculation at the room temperature (300 K) for the Nylon 6 (n-20) system.
In general, the calculation running well with the default settings. Change the start/final temperature to the range which contains Tg of the system. Also, it is more accurate to perform the calculation while reducing the temperature (Tstart > Tfinal).
The example used the temperature range 450 ~ 200 K.
3. Calculation Results
The estimated glass transition temperature of the Nylon 6 (n=20) system is similar to the known value 320 K.
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LAMMPS : Molecular Dynamics Simulation for Polymer – Polymer MD
LAMMPS : How to perform the polymer simulation – Polymer MD Modeling
LAMMPS : The Essential Step for Polymer Simulation – Thermalization