Free-Volume and Void Effects

Field-cycling NMR relaxometry is an excellent monitor of chain modes. As illustrated in Fig. 5, it specifically and predominantly probes component B. The other components, A and C, matter only at low temperatures/high frequencies and high temperatures/low molecular weights, respectively. That is. 

Rouse dynamics is expected to apply to molecular weights below the critical value where entanglement effects are not yet effective. Experimental data sets for the proton spin-lattice relaxation dispersion [47, 49, 153] are shown in Fig. 27 in comparison to the theoretical frequency dependence predicted by Eq. 64. Very interestingly, the values for the segment fluctuation time Tj fitted to the Ti data coincide with those derived from the Ti minima (see Fig. 14) corrected for the temperature of the field-cycling measurements. That is, the two independent determination methods lead to consistent results. 

The logarithmic Rouse formula given by Eq. 64 is valid for ft) -C. Deviations of the theoretical curves from the experimental data points are therefore only expected at frequencies close to the minimum condition coTs 1. where simultaneously component A starts to become perceptible (compare Fig. 16). This explains why the PDMS data fit better to the model than poly- 

Entanglement effects can be reduced by dissolving the polymer even if its molecular weight is well above the critical value in the melt. This is demonstrated in Fig. 27c for PDMS dissolved in CCI4. The data can again be well described by the Rouse model. 

The conclusion from these findings is that the Rouse model is perfectly corroborated by NMR relaxometry experiments provided that entanglement effects are excluded or not effective on the time/frequency scale of the exper- 

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