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Force versus extension curves measure entropic elasticity of single polymerchain in force spectroscopy experiments. A Worm-like Chain model is used todescribe force extension experiments with an intrinsic chain parameter calledpersistence length, which is a measure of local bending flexibility. Forflexible polymers, there is a discrepancy in estimates of persistence length invarious force regimes. For instance, Atomic Force Microscopy (AFM) basedpulling experiments report anomaly low values which are also inconsistent withmagnetic tweezers experiments. To understand this, we investigate the role ofcoupling between microscopic force probe and intrinsic elasticity ofpolyethylene glycol chain in AFM-based experiments. We perform experimentsusing oscillatory rheology by providing an external excitation of fixedfrequency to the probe. We show that a proper quantification of elasticresponse measured directly by oscillatory technique deviates significantly fromconventional force-extension curves. The persistence length obtained by fittingWLC to stiffness extension data matches well with equilibrium tweezersexperiments. In addition, for polystyrene chain in poor solvent no deviation inelastic response is observed between oscillatory and constant velocity pullingexperiments. However, such deviation is seen for polystyrene in good solvent.We attribute this to hydrophobic interaction between monomers of polystyrene inwater. Our results suggest that oscillatory rheology on single polymer chainsprovide quantitative estimate of its elastic response. The consistency invalues of persistence length using magnetic tweezers experiments in low forceregime and the AFM experiments in high force regime suggests that WLC issuccessful in describing the polymer elasticity in the force range typicallyprobed in AFM experiments.

Elasticity of single flexible polymer chains in good and poor solvents