Polymer melt rheology: Advantages of frequency sweeps using randomly selected frequencies*

Abstract

Measurement of the viscoelastic properties of polymer solids, melts, and solutions is broadly practiced. The so‐called “frequency sweep” appears in thousands of publications, with data covering up to five decades of frequency to reveal, for example, the relative molecular size of a polymer. To perform these experiments, the frequency range is typically “swept” from high to low frequency or vice versa. Unfortunately, this confounds frequency with run time. This is more than annoying; time effects can result in serious mistakes in the interpretation of the data. Fortunately, the confounding can be eliminated by simply running the frequencies in random order, which converts the time trend to random error. Described here is how this can be done and shows via modeling that the result can not only show that time effects are present but can be analyzed to show the time rate of change of the sample. This analysis is demonstrated using two real examples: loss of solvent from poly(dimethyl siloxane) gum and gelation of porcine gelatin. This method compares well with the multiwave technique, but does not require any special software or adjustments of amplitude with frequency or harmonic spacing of the frequencies.

Rheological behavior and thermomechanical properties of polyethylene—Glycidyl methacrylate vitrimers utilizing disulfide exchange

“…The work done by Montgomery T. Shaw showed that, for samples with time dependent change, random ordering of the frequencies tested result in time becoming a random error and decouples the affects due to the angular frequency applied from the time change of the sample. [ 30 ] To determine if there was a time dependent change in the PE‐GMA‐V vitrimer samples, the results from the random frequency tests were compared to the traditional frequency test and are shown in Figure 8A. For all PE‐GMA‐V vitrimers, the storage modulus from the random frequency of the samples deviates from the storage modulus as a result of the traditional frequency tests, indicating that some ongoing change occurs during testing.…”