An Improved Method To Obtain Direct Rheological Evidence of Monomer Density Reequilibration for Entangled Polymer Melts

The detection of the monomer density reequilibration mechanism can be achieved with a good signal-to-noise ratio by using the relaxation spectrum instead of the loss modulus. When normalizing both functions, the signal detected by the spectrum is 8-9 times larger than that of the loss modulus. This shows clearly that spectra are a very powerful tool to highlight molecular processes, albeit the fact that spectra are only recalculated linear viscoelastic data. Table 3 shows the comparison between the different values of the monomeric density equilibration time τMDR for PBd-430K, introduced in the discussions around Figures 2 and 3. It is obvious that the spread is relatively large. The location of the peak is at slightly shorter relaxation times than for the loss modulus, but that is an artifact of the broader glass transition peak in the loss modulus than in the spectrum. The determination from the spectrum leads to values which lie about 0.2 decades away from the inverse crossover frequency 1/ωc, which, considering the large time scales of rheological tests and the still weak signal, is quite good. The fact that the subtraction method leads to higher relaxation times than the normalization and the fit method is logical considering that the subtraction method determines the difference in absolute numbers, the other twomethods determine ratios and, thus, tend to find the lower one of two adjacent peaks of distinctly different height closer to the higher peak. Several other architectures were also tested with this method and were found to also fulfill the relations. Themonomer density reequilibration mechanism’s time regime is chemistry and molecular architecture independent relative to the glass transition and thus identified to be identical with the entanglement motion time τe. This indicates that spectra offer a universal approach for the proper detection of weak processes such as the monomer density reequilibration.