All polymer melts are viscoelastic materials; that is, their response to external stress lies in varying extent between that of a viscous liquid and an elastic solid. In an ideal viscous liquid, the energy of deformation is dissipated in the form of heat and cannot be recovered just by releasing the external forces; whereas, in an ideal elastic solid, the deformation is fully recovered when the stresses are released. By subjecting the polymer melt to small-amplitude oscillatory shear, the two linear viscoelastic material functions G” (loss modulus) and G’ (storage modulus) are obtained. The ratio of the loss modulus to the storage modulus is defined as the loss tangent. Thus, tan delta which is equal to G”/G’ gives a measure of the viscous portion to the elastic portion. For more details on the rheological parameters of importance and how and why they are measured, you may refer to the following book from where the above information was extracted.
A. V. Shenoy and D. R. Saini, Thermoplastic Melt Rheology and Processing, Marcel Dekker Inc., New York (1996).
All polymer melts are viscoelastic materials; that is, their response to external stress lies in varying extent between that of a viscous liquid and an elastic solid. In an ideal viscous liquid, the energy of deformation is dissipated in the form of heat and cannot be recovered just by releasing the external forces; whereas, in an ideal elastic solid, the deformation is fully recovered when the stresses are released. By subjecting the polymer melt to small-amplitude oscillatory shear, the two linear viscoelastic material functions G” (loss modulus) and G’ (storage modulus) are obtained. The ratio of the loss modulus to the storage modulus is defined as the loss tangent. Thus, tan delta which is equal to G”/G’ gives a measure of the viscous portion to the elastic portion. For more details on the rheological parameters of importance and how and why they are measured, you may refer to the following book from where the above information was extracted.
A. V. Shenoy and D. R. Saini, Thermoplastic Melt Rheology and Processing, Marcel Dekker Inc., New York (1996).
Tan delta gives very important information regarding rheological calculations. In short, it gives the ratio of viscous portion to elastic one in polymers.
Adding to the above answers:
Curves produced from the Dynamic Mechanical Analysis (DMA) for tan delta are very important in detecting the glass transition temperature (Tg) of the polymer (Tg is usually located at the peak of the curve of tandelta vs. temperature). DMA is generally more sensitive in measuring Tg than other methods such as the Differential Scanning Calorimetry (DSC). Also, the height and the shape of the tandelta curve are dependent on the amount of amorphous regions in the structure.
tan delta is an important analysis criteria in deciding the material properties. The material behavior can directly be extracted from the tan(delta) value.
if Tan(delta) > 1 (G" > G') (liquid or 'sol' )
Tan(delta) = 1 (G' = G") (viscoelastic or 'gel point')
Tan(delta) < 1 (G"
tan delta looks at the ratio of the "flow" (loss modulus G") to the "stiffness" (storage modulus) as mentioned above. as the ratio of G" to G' changes you can learn how the material is changing from a flowable liquid to a non flowable solid and vise versa.
For cure kinetics, for example, the location and height of the tan peak corresponds to the onset of curing. This can be used to study, or to even to qualify materials for applications where curing is important.
For melting, the temperature dependence indicates when melting begins to occur such that the material becomes flowable.
For shear rate dependent measurements, the change in tan delta indicates at what shear rates the material can flow or not (think Catsup).
Also, because the G" and G' curves are fundamentally based on molecular motion mechanics, the shape of these curves and the tan delta peak can inform you on the type of mechanisms causing your viscoelastic properties to change.
See for instance Kevin Menard, Dynamic Mechanical Analysis: a practical introduction, 2nd ed. Or for more mathematics see N.G. McCrum, B.E. Read and G. Williams Anelastic and Dielectric Effects in Polymer Solids.
@Sharheel abid
You can find these in any standard book on rheology.
https://books.google.co.in/books?id=NLabiVZSE7QC&pg=PA315&dq=tan+delta+and+viscoelasticity&hl=en&sa=X&ved=0ahUKEwiowJCL2-jhAhXNinAKHVJlAscQ6AEILzAB#v=onepage&q=tan%20delta%20and%20viscoelasticity&f=false
Or see this technical note by Anton paar
https://wiki.anton-paar.com/en/basics-of-rheology/
Besides rheological behavior, this parameter is helpful for characterisation of multiphase systems (thermoplastics, plasticized systems, blends, composites) i.e. mutual interactions, chains mobility or its limitation, miscibility of phases.
You can see Barzic, A.I., Ioan, S., 2017. Multiphase polymer systems: Micro- to nanostructured evolution in advanced technologies, CRC Press, Taylor & Francis Group. (some chapters are availble on Google). BR
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