Free energy calculations in associative polymers using molecular dynamics

ABSTRACT

In this work, equilibrium molecular dynamics is used to predict the change in the Helmholtz free energy ($\mathrm{\Delta }A$) for solutions of associative polymers of the telechelic type. $\mathrm{\Delta }A$ is calculated using the thermodynamic integration method, wherein molecular chains are simulated in a coarse-grain scale within an NVT ensemble. Simulations take place in the concentration interval $0.03\le c\le 0.25$, such that aggregate formation is possible. The initial configuration is that of dispersed free chains; the increase of interactions among stickers signals the onset of the micelle formation process. The solution with high concentration experiences substantial micelle aggregation despite little conformation changes. Therein, transient networks among micellar flowers are formed. The decrease in free energy is related to the formation of aggregates to achieve stability. When solutions are thermodynamically stable, the free energy and configurational entropy decrease and increase non-linearly with concentration, respectively. Below critical micellar concentration, this behaviour is associated with a step-by-step process along which the sticker groups gather to form the first thermodynamically stable aggregate. Above critical micellar concentration, the most probable distribution increases with concentration, therefore, free energy decreases with increasing aggregates that form the network.

KEYWORDS:

• molecular dynamics
• associative polymers
• free energy

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

We acknowledge the support received by project DGAPA-UNAM (grant number IN100620).