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If you've ever taken the plastic from a plastic bag and pulled on it, you might know what I mean. If you pull quickly, the plastic tears relatively easily, but if you pull it slowly, it gets stretched out into an indestructible plastic string. Once stretched, pulling the plastic as hard as you did to tear it before yields no results.

Similarly, adhesive bandages are pulled off the skin (almost) painlessly if pulled quickly, but seem to stick even under the same level of force if it is applied more gradually.

What explains this phenomenon?

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Polymers have a special property called viscoelasticity, which means they may strain elastically (elasticity) then flow (visco). The models below are mechanical representations of this phenomenon using springs and dashpots (dampers).

Maxwell Model Kelvin Voigt Model
$\qquad\qquad\qquad$ Maxwell Model $\qquad\qquad\qquad\qquad\qquad\qquad$ Kelvin–Voigt Model


Time Stress Deformation
$\qquad\qquad\qquad$Time Stress Deformation Plot

This property of viscoelasticity allows plastics, when pulled slowly (low strain rate), time for the polymer chains to rearrange and the plastic to flow which means it is less rigid but more elastic (think stretching silly putty). When the plastic is pulled quickly the plastic is more rigid and breaks (like breaking silly putty). AS I have alluded you can observe this with silly putty which flows when strain is low and will stretch to long lengths while snap into two pieces when pulled rapidly.

So far I have only explained why plastics will stretch or snap, but not why they are hard to tear after stretching. This is caused because as the plastic is stretched the polymer chains align as shown below. As it turns out polymers are hardest to tear perpendicular to the direction of the molecules because you have to break the bonds of the chain rather than some bond breaking and some chains separating. As a result, the plastic becomes hard to tear across the stretched part, but it is now easier to tear along the stretch (parallel) since in is mostly chain separating to do so, and indeed a tear in a stretched bag will follow the direction of a stretch.

enter image description here

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  • $\begingroup$ But what's the typical length of a polymer chain in a plastic bag? If it's short on the macroscopic scale, then even in the last case the main thing that could be happening when you tear a bag isn't that you are breaking carbon-carbon bonds, but rather that you are forcing the chains to separate from each other. In that case, the reason the bag is harder to tear would instead be that the untangled chain arrangement means you have greater inter-chain attraction (from VDW forces?) to overcome. Not disagreeing w/ you, just wondering what's the evidence that supports what you describe. $\endgroup$ – theorist Oct 5 '18 at 4:38

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