Are there any techniques to increase the extensional viscosity and/or decrease the shear viscosity (ideally simultaneously) thereby increasing the Trouton ratio?
The two most common methods are blending with a branched analog or up irradiation to produce long chain branches. Depends on your polymer. Do a search on increasing melt strength.
I have never heard of modifying the Trouton ratio by varying the shear viscosity.
The usual way 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.
Assuming you are talking about Uniaxial Extension, Trouton ratio for near zero (~10-4 - 10-3 1/s) elongation rates is close to 3 for most of the polymer melts. However, you'll see increase in extensional viscosity at higher strain rates.
Primary contribution towards elongational viscosity is due to presence of long-chain branches in your polymer backbone, which is mentioned. In order to simultaneously decrease the shear viscosity you may blend your polymer with a low molecular weight polymer. However, this will adeversely affect your elongational viscosity too.
Like others mentioned, you want a high degree of Long Chain Branching, which will increase extensional stress growth and also will favor shear thinning.
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