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Name: Jolynn
Status: student
Grade: 9-12
Location: Outside U.S.
Country: Singapore
Date: Spring 2011


Question:
Why is polypropylene malleable?



Replies:
Jolynn,

In order for any substance to be malleable, the particles that make up the substance must be able to do two things: they must be able to move (change position, translate), and they must be able to hang on to other particles as they are moving.

In metals, one of the models (electron sea model) is that the electrons of the metal atoms tend to be shared across many atoms. These electrons are able to move from atom to atom, and it is these electrons that hold atoms together. So when force is applied to a metal sample, the force allows atoms to move (translate), but since the electrons are all over the place and hanging on to the atoms, the atoms do not fly apart, they just change locations. This then is malleability: the ability of atoms to move into different locations when force is applied while still remaining in the solid phase and not breaking the bulk sample apart.

In plastics such as polypropylene, the particles are very long chain molecules that are entangled with each other (think cooked long noodles). So when force is applied the intermolecular forces (in this case: London Forces) which are all over the place, hold the long molecules together, so too the entanglements (like a mess of string). So the molecules can move, but they don't fly apart.

Greg (Roberto Gregorius) Canisius College


A simple model for a high molecular weight polymer, such as polypropylene, is a bowl of entangled very, very long strands of spaghetti. If you grasp a "lump" of the strands and pull on the lump slowly, the strands will respond to the applied force by stretching out. That is, the strands are malleable. At the other extremes, if you do not pull on the strands sufficiently, the strands will "relax" and behave like an elastic "lump". At the other extreme, if you pull on the "lump" very rapidly two things well happen. Either you will lift the entire " lump" as a solid, or the strands will fracture. In between these extremes is a region of leather-like behavior. That is the spaghetti will absorb a lot of the stretching energy by changing the local configuration causing the strands to slip past one another. That set of conditions corresponds to the polymer being malleable -- deforming, but not on a time scale sufficient to cause the polymer to fracture. This process is complicated. It depends upon the molecular weight (chain length), which further depends upon the temperature, bonds between the various strands, and a number of other structural parameters. Nonetheless, thinking about polymers as strands of spaghetti, does provide a qualitative picture of polymer dynamics.

Vince Calder


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