Reduction of Kuhn Length upon Chain Extension

Both polymer size and chain elasticity depend on long-range bond correlations, which determine the chain Kuhn length. These correlations are gradually cut off with increasing externally applied force or polymer confinement, thereby decreasing the effective Kuhn length. We develop a theory for the strain-dependent Kuhn length and validate it with simulations. Our model explains why the Kuhn length obtained from single-molecule force spectroscopy experiments is smaller than the Kuhn length determined from scattering measurements of unperturbed chains. Finally, we propose a crossover function for the Kuhn length as a function of applied force, which can be used for the interpretation of force-extension curves.