I know why Polymethyl methacrylate can be made to have a low or high density but i am unable to illustrate how they look like on the molecular scale. Can someone help me with a sketch please?

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    $\begingroup$ This might help : 123rf.com/… $\endgroup$ Dec 2, 2016 at 12:49
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    $\begingroup$ @NilayGhosh no it doesn't that's just a monomer... $\endgroup$
    – DSVA
    Dec 3, 2016 at 14:34
  • $\begingroup$ @DSVA if you know the monomer unit, you can easily figure out the polymer. $\endgroup$ Dec 3, 2016 at 15:57
  • $\begingroup$ Also, I said "this might help". I did not actually gave the answer. $\endgroup$ Dec 3, 2016 at 15:59
  • $\begingroup$ @NilayGhosh knowing how the (linear) polymer looks still won't explain the difference between LD and HD PMMA. It's not helping at all. This might be more confusing than anything else. $\endgroup$
    – DSVA
    Dec 3, 2016 at 16:55

1 Answer 1


Low density and high density PMMA differ by density, which is given as the ratio of mass and volume.

Given that they are prepared from the same monomer, two polymers with the same number of monomers will have the same mass.

If the polymers differ in density, this must be due to the fact that they occupy different volumes.

Note that we are talking about polymers here and not the ordered packing of molecules in a crystal. Nevertheless, inter- and intramolecular interactions (and thus "packing") is important here as well.

As a rule of thumb, polymers with higher branching will "pack" less tightly and thus occupy a higher volume.

This results in a lower density.


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