I am looking for an explanation about G prime modulus as it relate to solids particularly pharmaceutical polymers.
Hello.
I assume you are talking about a viscoelastic measurement. If so, this is a basic way to look think of it.
The G' refers to the elastic modulus, and reflects elastic behavior of a material when deformed. The G" refers to the viscous modulus, which reflects the flow of a material while it is deformed. (For instance, if you stretch a real material and hold it, then let it go after a while, does the material go back to the original length, or does it recoil some but not all the way to the original length...)
For instance, if you stretch a rubber band, hold it then let it go after a while, it will return to its original length. This is elastic. On the other hand, if you chew gum then stretch it, hold it, and let it go, it will not go back to its original length because molecules have slipped past each other. This is viscous flow.
Many real materials, including many polymers, will exhibit some elastic and some flow behavior. Usually, the longer the stretch is held, the more flow occurs, which means the elastic response gets weaker (weaker pulling back to shorten the sample over time), and the sample will not return to the original length after letting go because it flowed to a longer stable length.
In real materials, the flow and loss of elastic recoil can be separated out by viscoelastic experiments.
Consider, for instance, putting a sample between two parallel plates, one of which is stationary while the other can oscillate (rotate over a small angle, say clockwise, stopping, then counterclockwise with a sinusoidal angular pattern). In this way, the sample stretches, then relaxes, then stretches in the other direction, etc. (I apologize for the poor description.)
While the polymer sample is stretched, it acts as an elastic trying to resist the stretching. At the same time, there is also some slipping of molecules past each other (a flow of sorts) that happens.
The G' relates the stress (force per area applied) to the strain (the fractional stretching, either angular or linear). The G'' relates change in strain over time that occurs when the applied stress is removed.
They can be separated by oscillatory experiments because of the time relationships between the stress and strain. In elastic type behavior, once the strain is reduced, the stress is instantly changed as well. On the other hand, once the stress is removed, the flow is not instant but takes time. Because of this timing difference, oscillation experiments (which stretch and induce flow, then stop stretching, etc.) can distinguish between the elastic and flow behaviors.
Hopefully this helps a bit.
Hello.
I assume you are talking about a viscoelastic measurement. If so, this is a basic way to look think of it.
The G' refers to the elastic modulus, and reflects elastic behavior of a material when deformed. The G" refers to the viscous modulus, which reflects the flow of a material while it is deformed. (For instance, if you stretch a real material and hold it, then let it go after a while, does the material go back to the original length, or does it recoil some but not all the way to the original length...)
For instance, if you stretch a rubber band, hold it then let it go after a while, it will return to its original length. This is elastic. On the other hand, if you chew gum then stretch it, hold it, and let it go, it will not go back to its original length because molecules have slipped past each other. This is viscous flow.
Many real materials, including many polymers, will exhibit some elastic and some flow behavior. Usually, the longer the stretch is held, the more flow occurs, which means the elastic response gets weaker (weaker pulling back to shorten the sample over time), and the sample will not return to the original length after letting go because it flowed to a longer stable length.
In real materials, the flow and loss of elastic recoil can be separated out by viscoelastic experiments.
Consider, for instance, putting a sample between two parallel plates, one of which is stationary while the other can oscillate (rotate over a small angle, say clockwise, stopping, then counterclockwise with a sinusoidal angular pattern). In this way, the sample stretches, then relaxes, then stretches in the other direction, etc. (I apologize for the poor description.)
While the polymer sample is stretched, it acts as an elastic trying to resist the stretching. At the same time, there is also some slipping of molecules past each other (a flow of sorts) that happens.
The G' relates the stress (force per area applied) to the strain (the fractional stretching, either angular or linear). The G'' relates change in strain over time that occurs when the applied stress is removed.
They can be separated by oscillatory experiments because of the time relationships between the stress and strain. In elastic type behavior, once the strain is reduced, the stress is instantly changed as well. On the other hand, once the stress is removed, the flow is not instant but takes time. Because of this timing difference, oscillation experiments (which stretch and induce flow, then stop stretching, etc.) can distinguish between the elastic and flow behaviors.
Hopefully this helps a bit.
G prime is elastic component of the viscoelastic melt, it has been a rather common practice to associate it with the melt elasticity. When a material undergoes oscillatory stress with frequency, the response can be expressed in terms of a storage modulus, G’ (G prime) a loss modulus, G’’ (G double prime) and a complex viscosity, η* (Eta star).
References
Y. P. Khanna, K. R. Slusarz .Dynamic melt rheology. II: Re-examining the relationship of g′ in oscillatory rheometry to the melt elasticity. 33, (2). 1993 ;122–124
Dear Prof. Robert A. Bellantone, Thank you very much for your concise explanation, I really appreciate it and luckily for me this week I was able to see the work of rheometer function where most of the things you mentioned were demonstrated practically.
Please Sir, Could you shed more light on RELAXATION TIME as it relate to rheology of poymers?
Thank you once again.
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