Elastic response of poly(ethylene glycol) polymer chains studied using dynamic atomic force microscopy

TL;DR

This study uses advanced atomic force microscopy to measure the elastic response of poly(ethylene glycol) chains, revealing detailed thermodynamics and conformational transitions under force.

Contribution

It introduces a lock-in based amplitude modulation-AFM technique to accurately measure polymer stiffness and analyze conformational changes at the nanoscale.

Findings

Stiffness data correlates linearly with lock-in amplifier signal.

Persistence length derived from WLC model is physically meaningful.

Entropy-driven conformational transition observed at around 250 pN.

Abstract

Stretching response of polymer chains under external force is crucial in understanding polymer dynamics under equilibrium and non-equilibrium conditions. Here we measure the elastic response of poly(ethylene glycol) using a lock-in based amplitude modulation-AFM with sub-angstrom amplitude in both low and high frequency regime. Appropriate analysis that takes into account the cantilever geometry and hydrodynamic loading effects of oscillating cantilever in liquid, relates X signal of lock-in amplifier linearly to stiffness.Stiffness data extracted from X signal was compared with stiffness from derivative of conventional "static" force extension curves. For stiffness data from X signal, fitting to standard entropic model of WLC gives a physically meaningful value of persistense length and also follows scaling behaviour of WLC. Entropy dominated conformational transition with its characteristic V-shaped signature was observed at arount 250 pN. Accurate measurement of stiffness enabled us in understanding the thermodynamics of conformational changes.