Link length and energy fluctuations in extensible freely jointed chains

TL;DR

This paper analyzes the fluctuations in link length and energy in an extensible freely jointed chain model, providing analytic relations and verifying their accuracy for understanding polymer chain thermodynamics.

Contribution

It introduces asymptotically correct analytic relations for fluctuations in link length and energy, enhancing the modeling of polymer chain thermodynamics.

Findings

Fluctuations are quantified via average, standard deviation, and distributions.

Asymptotic relations are verified to be accurate and correct within small error terms.

In some cases, fluctuations follow an approximately normal distribution.

Abstract

The freely jointed chain is often applied to model the thermodynamics of single polymer chains, but the traditional formulation of the model lacks internal energy changes due to bond stretching. For this reason, the extensible freely jointed chain model includes a potential energy function, typically harmonic, that governs the length of each link in the chain. Among the other quantities of interest that are subject to thermal fluctuations, these link lengths and energies too fluctuate about their ensemble average values. Since a plethora of models for polymer chains and networks incorporate chain dissociation as a function of either link length or energy, these fluctuations are crucial to understand and quantify. Motivated by this fact, fluctuations in link length and energy are analyzed within a freely jointed chain under an applied force. These fluctuations are quantified through their average values, standard deviations, and probability distributions. Across all values, asymptotically correct analytic relations and their less ergonomic exact counterparts are introduced. The asymptotic relations are verified to be accurate through direct comparison and to be correct within transcendentally small terms through error analysis. In certain cases, the fluctuations are shown to be approximately normally distributed. Hereafter, model components predicated on link length or energy ought to account for these fluctuations.