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Polymer melts show a characteristic molecular weight dependent relaxationtime that can be related to the unentangled and entangled regime. At highmolecular weights, influence of contour length fluctuations and con-straintrelease cease and pure reptation prevails. With the broad frequency rangedielectric spectroscopy can fol-low polymer dynamics over a very broadtemperature range, with the additional advantage of recording the spec-tralshape which contains information on reptation. Here we investigate the apparentdiscrepancy in the molecular weight for the onset of pure reptation from themolecular weight dependence and the spectral shape. We examined the popularderivative method and compared it with a version that includes higher orderterms. Higher order terms lead to a more accurate peak shape and position thanthose determined with the simpler version. This becomes important ifexperimental spectra contain conductivity and polarization contributions.Higher order terms require the introduction of an interpolating function toanalyze experimental spectra, which lets the Havriliak-Negami function appearto be a more robust, yet reliable tool to determine the peak shapes. We reachthe conclusion that molecular weight dependence and spectral shape can be bothstrongly affected by conductivity and polarization contributions. While thisleaves uncertainties on the accurate value of the transition molecular weight,the peak shape points to the existence of reptation and contour lengthfluctuations in polyisoprene with a molecular weight greater than 1000 kg/mol,which would imply a ten times greater threshold molecular weight than expectedfrom previous estimates using the molecular weight dependence.

On the Determination of the Transition to Pure Reptation by Dielectric Spectroscopy