Chain stiffness intensifies the reptation characteristics of polymer dynamics in the melt

The reptation concept in polymer dynamics is studied for model chains with added stiffness. The main idea of a chain diffusing inside a tube can be transferred from fully flexible chains. However, the picture, which renormalizes the chain of locally stiff Kuhn segments onto a flexible chain of fewer segments fails. The entanglement length shrinks with increasing persistence length. If entanglement length and persistence length come to the same order of magnitude the picture of a tight tube is better suited, in which chain segments can move only along the contour, any transverse motion being much reduced. Thus, as stiffness increases, the monomers loose their freedom to perform random walks inside the tube, the ``Rouse-like'' part of their dynamics. As a result, for persistence lengths of more than five monomers no length scale can be described by the Rouse model.