Decreasing the shear viscosity of a melt can improve processability. However, a decrease in shear viscosity also result in a decrease in extensional viscosity which is detrimental in the shaping phase and end product properties. Is there a way to increase the extensional viscosity while simultaneously decreasing the shear viscosity of a polymer melt?
The short answer is NO, the shear and extension viscosities are related and at the Newtonian limit (low shear rates) the extensional viscosity is three times the shear viscosity. At higher rates, Trouton's ratio increases and the amount depends on microstructure. I would guess that you could significantly increase the extensional viscosity without varying the shear viscosity (too much) by adding a small concentratiion of low aspect ratio fibers.
The short answer is NO, the shear and extension viscosities are related and at the Newtonian limit (low shear rates) the extensional viscosity is three times the shear viscosity. At higher rates, Trouton's ratio increases and the amount depends on microstructure. I would guess that you could significantly increase the extensional viscosity without varying the shear viscosity (too much) by adding a small concentratiion of low aspect ratio fibers.
I agree with Peter.
However, the more common way is to adjust the molar mass distribution or add long-chain branching somehow.
The usual way to modify the Trouton ratio is to introduce long-chain branching. Methods for this are: 1. blending with a long-chain branched material (not necessarily identical in chemistry, e.g. there are some PP-grades with high molecular LDPE for better strain hardening) 2. reactive extrusion 3. gamma or electron beam irradiation 4. synthesis of the polymer under branchign forming conditions Another way is blending with a very high molecular polymer (in small weight fractions) to create a high molecular shoulder in the molar mass distribution. You should be aware that depending on the desired application, the strain hardening type (how much strain hardening is observed under which condition) needs to be adjusted. This is a very complex manner, which is only understood by relatively few people and would take a lot of space to explain it properly. I would suggest reading the papers of Münstedt's group on this matter.
Agree with Florian.
Most often, functionalized and long chain branched polymers are used to overcome such issue.
For blown film fabrication? The balance of rate of segment aggregation and polymer Mw may be useful when using a block co-polymer.
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