  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.
