Entropic forces act as springs on stretched chains

Figure. 1. The polymer chains with different amounts of stretches

Entropic forces in a chain conformations oppose thermal fluctuations of the chain. When at 0 K, there are no thermal fluctuations and therefor no entropic forces. However, with an increase in temperature, there becomes a greater amount of accessible configurations of a polymer chain from fluctuations. There are also entropic forces that affect the configuration of the chain. As a polymer chain stretches, the number of accessible configurations decreases. Due to the tendency of the system towards higher entropy (more disorder/more configurations), the entropic force will limit chain extension.

For small chain extensions, the end-to-end distance, the entropic force is linear in displacement. From a force vs. displacement curve, the linear portion can be fit to find an effective spring constant. The effective spring constant can be compared to a theoretical spring constant for accuracy.

Increasing end-to-end distance with pull options in GROMACS

To measure the entropic force on the chain, it will require the recording of fluctuations of an equilibrium chain with a number of end-to-end distances. The end-to-end distances will be fixed using the GROMACS pull options to restrain the first and last atom in the chain.

[Page 2: Generating the simulation]