Persistence length
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The persistence length is a basic mechanical property quantifying the stiffness of a macromolecule of a polymer. It is defined as the length over which correlations in the direction of the tangent are lost. Or in another words persistence length is the distance which we should travel from one end of the chain to bend it 90 degrees. Persistence length has to be calculated differently for different cases such as rod like chain, worm like chain, coil etc.
In polymer science, persistence length is one half of the Kuhn length, the length of hypothetical segments that the chain can be considered as freely joined. The average projection of the end-to-end vector on the tangent to the chain contour at a chain end in the limit of infinite chain length [1].