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I was holding a piece of plastic earlier, and I ripped it in half. This caused me to start to think about what was happening at the atomic level. If the plastic is being ripped, clearly bonds are being broken and new bonds are being formed, so is this not a chemical reaction?

I realize that the plastic retains all of its chemical characteristics despite it being ripped, so I am quite confused about how to classify what is occurring.

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When you rip, tear or mechanically deform a polymer (for example a piece of plastic) you are putting energy into the material. The energy from this deformation causes the polymer chains in the vicinity of the deformation to attempt to align. To some degree this partial alignment makes continued deformation easier. To continue tearing the polymer apart you just need to overcome the van der Waals forces holding the chains together.

In addition to the polymer chains being pulled apart, some chemical bonds will be broken. I don't know how many bonds are broken per chain pulled apart, but I suspect it is low. Low or not, some free radicals (or ions) will be generated when the chemical bonds break. If the experiment is performed under ambient conditions, the radicals or ions will quickly react with oxygen or ions in the air. If the experiment is performed in an inert atmosphere, then the radicals will persist (dangling bonds) and can be observed in an esr spectrometer. It has been demonstrated that cross-linked polymers, which are less prone to flow deformation and therefore more likely to undergo bond breaking, do produce larger esr signals.

So most of what is going on is molecular deformation and pulling apart of the polymer chains - but some bond breaking does occur.

It would be an interesting experiment to cut two pieces of plastic and then cut one of them into finer shreds. Place the two samples into separate tubes and then record their esr signals. How much more intense would the signal be in the finely cut sample compared to the bulk sample? Polycarbonate-A, straight out of the reagent bottle, produces an esr signal.

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Agreed: a physical action can cause a chemical reaction. For example, any attempt to manipulate $\ce{NI3}$ physically causes it to decompose (detonate). For that matter, dividing metals into tiny pieces changes chemical behavior, because bonds that would have been hidden in the interior are exposed at the surface. Finely divided iron, for example, becomes pyrophoric, yet bulk iron can be heated in air without rapid oxidation..

The question is one of degree: how much of a chemical change resulted from ripping the plastic. Can you perform any chemical test that would distinguish the shreds from the whole, such as solubility? For example, bulk carbon, whether in graphite or diamond, is not soluble in most liquids at room temperature (though they dissolve in some molten metals), but as fullerene particles, carbon is much more soluble.

This could be an interesting experiment: prepare some bulk samples of a plastic, and grind some others (perhaps in a chilled ball mill?) and compare some chemical characteristics.

See Properties of Nanoparticles and Size matters: why nanomaterials are different.

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    $\begingroup$ If you subject an aqueous solution of polyacrylamide to a lot of mechanical shearing then the polymer chains tend to become shorter. I know that during plastics processing that polymer chains can become shorter due to the shear forces which occur during in extruders. $\endgroup$ – Nuclear Chemist Apr 15 '18 at 9:43

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