Viscosity in Glass Transitions
Glass transition is associated with a dramatic change in a material's viscosity. In the literature, there are plots of the viscosity versus ratios () of several materials, all converging to a single point where and the viscosity is Pa s. Such plots can show the viscosity-temperature relationships of fragile and strong materials together for comparison. Since the glass transition temperature in such plots has been defined as the temperature at which the viscosity reachesthat level, the convergence to a single point does not reveal any new information on the glass transition phenomenon itself. This is shown by plotting viscosity versus relationships governed by the WLF equation with different coefficients and values with the assigned viscosity at varying from to Pa s. The Arrhenius equation is also included, but only to show that the choice of the viscosity-temperature model does not affect the convergence.
Snapshot 1: viscosity versus plots where corresponds to viscosity of Pa s, generated with the WLF model having the same parameters used to generate the thumbnail
Snapshot 2: viscosity versus plots where corresponds to viscosity of Pa s, generated with the WLF model with different parameters than those used to generate the thumbnail and Snapshot 1
Snapshot 3: viscosity versus plots where is defined by the viscosity of Pa s, generated with the Arrhenius model
Snapshot 4: viscosity versus plots where corresponds to viscosity of Pa s, generated with the Arrhenius model with different parameters than those used to generate Snapshot 3
The object of this Demonstration is to show that the convergence of such curves occurs regardless of the chosen viscosity-temperature model, the magnitudes of its parameters, the assigned to the materials and the chosen viscosity level that defines it.
Perhaps the best-known viscosity-temperature model is the two-parameter WLF model , which for our purpose can be written in the form
where and are the viscosities at temperatures and respectively, both in , and , are constants. This model has been widely used to replace the single-parameter Arrhenius equation, which may only apply at higher temperatures. For our purpose, the Arrhenius equation can be written in the form
where the temperatures are in , is the "energy of activation" and is the universal gas constant in commensurate units. It can be shown that if the absolute temperatures in the Arrhenius equation, and , are replaced by and where is an adjustable constant (all in ), then the resulting expanded model becomes the same as the WLF model and thus also the VTF (or VFT) model, which has long been known as its equivalent .
For what follows we will use the original version of the Arrhenius model. While it is usually not appropriate for temperatures in the neighborhood of the glass transition, it can still show that the viscosity-temperature curve's convergence to a single point is independent of the chosen model.
To run the Demonstration, select the viscosity-temperature model, WLF or Arrhenius, with its setter bar and then choose the viscosity level that defines in the range from to Pa s. Then choose the characteristics of three hypothetical or actual different materials, 1, 2 and 3, whose curves will be shown in red, green and blue, respectively. Use their sliders to enter or vary their settings in , the and parameters of the WLF model, or the of the Arrhenius model.
As expected, regardless of the particular settings, the three curves will always converge to the point where and the viscosity is that which is used to define the glass transition temperature.
 Wikipedia. "Glass Transition." (Jun 6, 2019) en.wikipedia.org/wiki/Glass_transition.
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