Contour length fluctuations in polymer melts: A direct molecular proof

In entangled polymer melts, contour length fluctuations are supposed to lift the chain confinement starting from the chain ends. Using neutron spin echo spectroscopy on polyethylene chains of intermediate length, where only the center part is labeled, allows us to separate the contribution of contour length fluctuations to the dynamic structure factor and thereby explicitly prove the effect on a molecular level. While a corresponding fully labeled chain shows the relaxation by contour length fluctuations additional to the primary relaxation, the data of the center-labeled chain, where the visibility of the fluctuations is switched off, is in excellent agreement with results for a nearly infinite long chain, where contour length fluctuations are known to be negligible.