Polyethylene reptation theory

Self-diffusion in the melt of polyethylene (PE) (or hydrogenated polybutadiene) has also been investigated rather extensively. Figure 8-4, taken from Tirrell s review [2], shows typical experimental data. Each set of data points is fitted closely by a straight line of slope -2, consistent with the reptation theory. However, we see again that the NMR data [17, 18, 22, 23] appear above those from tracer diffusion [24, 25] and neutron scattering [26]. The reason for the discrepancy was left unexplained in Tirrell s paper. 

NMR data on poly(dimethylsiloxane) are shown in Figure 8-5. Typical features are that, as P increases, Dtr for a fixed N decreases sharply and tends to be insensitive to P for P somewhat leuger than N, unless N is in the region of oligomers. Those who are in favor of the reptation theory would explain the observed N dependence of in terms of the concept of tube renewal, while Tanner gave it a different interpretation. Klein [28] confirmed the insensitivity of Ptr to P above N with blended polyethylene melts. 

The explanation given for the growth rates of linear polyethylene is highly specific. Based on the successive modifications that have been made, there is a serious question as to whether the reptation concept, as it has been applied, has general applicability to the crystallization kinetics of polymers. If the basic ideas of reptation theory are appropriate, major alterations have to be made so that it explains the molecular weight dependence of all polymers. 

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