Malleability of Polypropylene
Location: Outside U.S.
Date: Spring 2011
Why is polypropylene malleable?
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)
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.
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Update: June 2012