Strain amplitude sweeps are standard rheological measurements to determine a polymer's linear viscoelastic region. My system of interest is a filled, reactive thermoplastic polymer.
Hello Niloofar Afshari ,
you mentioned your polymer is a reactive TPE and you thus have a phase separated system (A-b-B-b-A). When shearing your polymer (in the melt?), there are two imaginable processes, which could result in your observation.
1) There is a fraction of polymer A in the domain of B and when sheared the polymer A moves to his own domain. This can be considered as the creation of a crosslnk from a formerly dangling polymer chain.
dangling end -> crosslink
2) There may be some loops when the endblocks are in the same domain. Some polymer blocks of A might be forced to migrate through polymer B under shear and find another domain of polymer A, which creates an additional crosslink.
loop -> crosslink
You can see results of this polymer chain movement through the other polymer sort for example in blockcopolymer melts. If you do a DSC measurement on a pristine (e.g. freshly synthesized, died, but not sheared in the melt) sample you may have one broad or not so well separated glass transition temperatures. If you shear the sample in the melt, the glass transition temperatures separate. This is one indication, that the
Maybe you can do DSC experiments with your sample before and after the strain sweep experiment (or a time sweep with a large strain amplitude -> with this experiment you should also be able to see this change without the influence of a changing strain amplitude) and see similar movements of the Tgs.
Hello Niloofar Afshari ,
in my previous reply I didn't consider the reactivity of the TPE matrix or the additive(s). Maybe the polymer chains' possibility to bind to your particles is increased when sheared. This could be explained by breaking some agglomerates of your filler and thus more available surface for the polymer chains to attach to. This may be tested by two time sweep experiments with different strain amplitudes. The increase of the moduli should be lower for the lower strain amplitude, due to a lower ability to break the agglomerates, then.
If the increase of the moduli is caused by some chemical reaction, you could do a short test in the linear mechanical regime, wait some time at the same temperature and repeat the test. If a chemical reaction caused the increase of moduli in your strain sweep experiments, the moduli in the second measurement should be higher as well.
The shearing increases mobility in your system and, among others, promotes the reaction. If the reaction was stopped due to high viscosity (e.g., a lack of mobility), this is the most probable case.
Another option is improving the filler dispersion.
Finally, certain polymers can have this kind of behaviour; it's classified as Type II LAOS behaviour by Hyun, et al. (or Type IV, if it is only an overshoot). You can read more about what causes that here:
Article A Review of Nonlinear Oscillatory Shear Tests: Analysis and ...
Matthias Heck , she said a reactive thermoplastic polymer, I don't think it necessarily means a block copolymer.
Hello Petr Lepcio ,
you're right, I didn't read the question really carefully. In the case of a homopolymer, the network formation can not be explained by different domains. In this case it's just the formation by adsorption to the filler's surface.
I'm sorry that it's taken me so long to thank you for your help.
Since then we have explored some of the possibilities you laid out in your respective responses that pertain to our system. But the work is still going on. Thank you for your insights!
Matthias Heck Petr Lepcio
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