One often uses Frequency sweeps at constant temperature to analyse the rheological characteristics of polymer melts. But, I suggest, it would be preferable to popularize Temperature sweeps at constant frequency w instead.
Why?
Because frequency plays two roles and it would be easier to deconvolute their respective influence:
- Frequency increases the value of Tg (or Talpha) and therefore decreases free volume when T remains constant; this increases viscosity.
- Frequency plays a significant shear-thinning role in reducing viscosity.
By working at (T-Tg(w)) constant one can study how the rheological parameters vary at constant free volume. There are surprising and interesting results from such an analysis, but it requires to correct the data obtained at constant T and to know the variation of Tg(w), i.e. the frequency map for the Talpha relaxation.
By working at constant w, Tg(w) is fixed, and one can obtain shear-thinning results by simply sweeping the temperature (cooling or heating) at a given rate.
In the video-lecture available from the following link, we study the incidence of working at constant free volume on the classical relationships found in rheology, such as the Maxwell fits at low w of G'(w) and G"(w), or the evaluation of the terminal time from the maximum of G'/w vs logw etc.
http://www.eknetcampus.com/videopresentation/On the incidence of Tg and Talpha on the formulation of rheological equations2.avi
This work suggests the importance of incorporating the influence of several parameters: w Frequency, T temperature, P Pressure, M molecular weight, L lambda, the elongational stretch ratio, on the value of Tg, i.e.
Talpha (w,T,P,M,lamda)
