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Relationship between segmental and chain dynamics in polymer melts as studied by dielectric spectroscopy

Broadband dielectric spectroscopy in the frequency range from 10−5 Hz to 106 Hz is employed to study the relation between α-relaxation and normal mode process for oligomeric polyisoprene and polypropylene glycol samples of different molecular weight. It is shown that relaxation time of the α-relaxation has a much stronger temperature dependence than that of the normal mode process. This means that applicability of the Rouse model to the normal mode relaxation breaks down near the glass transition. This result is further discussed in terms of the different temperature dependence of the two relevant length scales, the correlation length for the α-relaxation ξ and the end-to-end vector of the polymer chain. A quantitative evaluation gives more evidence that there exists a characteristic length scale for the α-relaxation which is in the order of magnitude of 2 to 3 nm at the glass transition temperature.