Temperature Dependence for Amorphous Polymers

For amorphous polymers which melt above their glass transition temperature Tg, the WLF equation (according to Williams, Landel, Ferry, Eq. 3.15) with two material-specific parameters q and c2 gives a better description for the shift factors aT than the Arrhenius function according to Eq. 3.14. 

The approximated lines at the right side of Fig. 3.14 correspond to an Arrhenius activation energy of approximately 120 kj/mol, which is significantly higher than the flow activation energy of the semi-crystalline polyolefin melts shown in Fig. 3.13. The temperature dependence of the melt viscosity for amorphous thermoplastics is substantially higher than that of semi-crystalline polymers and increases dramatically as the temperature approaches the glass transition temperature. 

5 Influence of Molecular Parameters on Rheological Properties of Polymer Melts 

The significant influence of molecular weight on viscosity becomes evident during processing if, for example, polymers degrade and the molar mass decreases, the viscosity and therefore the pressure build-up in the extruder can decrease sharply. 

The influence of molar mass distribution on the viscosity function is shown in Fig. 3.16 on the basis of dynamic viscosities of different polystyrenes (PS), which were normalized with respect to their zero shear viscosity. A wider molar mass distribution results in a higher shear thinning in the normalized viscosity function, i. e., the drop in viscosity starts at lower normalized angle frequencies and/or shear rates. 

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