In extensional flows of linear and long-chain-branched polyolefin melts with similar shear rheology, the branched polymers are found to exhibit significantly enhanced strain hardening in transient extensional flows. Why does this enhancement manifest in extensional viscosity but not in shear viscosity?
I am sorry for not having too much knowledge about rheology but you may ask this rheologist (Philip) and hopefully you will get your answer by clicking on the link below:
https://www.researchgate.net/profile/Philip_Rolfe
To imagine the extensional strain hardening in branched polymer melts, it is worthwhile to assume the polymer melts as a bowl of noodles. The noodles are always entangled with each other. Similarly, when the polymer chains in the melt happen to form entanglements with each other.
On application of shear rate, the entanglements start disentangling among themselves, and this is precisely why we get a shear thinning nature of polymer melts.
However, in case of extensional flow, the entanglements begin to form knots and these knots act as nodes for stress concentration points. Eventually, these knots take up more stress than the rest of the bulk and we can see strain hardening phenomena.
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