(103e) Analysis of AFM Results with MD Calculations On Single-Molecule Stretching of Poly(ethylene oxide) In Water and Hexane

Analysis
of AFM results with MD calculations on single-molecule stretching of poly(ethylene oxide) in water and hexane

Mangesh I. Chaudhari and Lawrence
R. Pratt

Department of Chemical and Biomolecular Engineering

Tulane University, New Orleans,
LA, 70118, USA.

ABSTRACT :

Elasticity
of a single polymer in solution is important for the modeling of foams/gels and
can be directly measurable by atomic force microscopy (AFM). The work compares
stretching behavior of poly (ethylene oxide) polymer in water and hexane.
Parallel tempering molecular dynamic simulations on aqueous 11-, 21-, and
31-mer poly (ethylene oxide) solutions differentiates low-, moderate-, and
high- extension regions (FIG.1) and explains that water swells these polymers.  The low extension (low force) behavior
obtained from experimental and simulation data is often complex. Our previous
results, which combined parallel tempering and window sampling, clarified the
low-extension behavior. Here we return to examine the measured high extension behavior
in detail. We compare simulation results for high polymer extensions with
available experimental data, and we compare simulations for different solvents
with one another.  We analyze
simulation results on aqueous solutions from the perspective of cooperative
H-bonding of water molecules to the ether oxygens in
order to clarify the role of solvent H-bond bridges in the conformational
structures of these polymers.

FIG. 1: Parallel tempering results for
T=297 K distinguish low-, moderate-, and high-extension regimes. The
low-extension regime includes the contact peak on the left of these graphs, and
is associated with the analogous contact-minimum in the CH₄ÉCH₄
potential-mean-force (pmf). That pmf
has been studied in numerous previous simulation calculations over several
decades. The solid curve represents short-distance Ôhydrophobic bonds' with
window sampling that is consistent with the direct simulation results. X-axis
is normalized with RMS end-to-end distance and emphasizes that the high extension
region starts after this value. Parabola fit through high extensions is
comparable with experimental data available on stretching of these polymers.