[newbie!]

help on the following questions based on the FTIR spectra of HDPE and LDPE

1)why do high density polyethene and low density polyethene have different melting points?

2)based on the IR spectra of polyethene what can you say aobut the dependence of the melting point or chain length.

3) why is the melting point of polypropent different from that of polyethene?

[PolymerKnowHow]

The melting points of polyethylene, including both high density and low density, have a lot to do with crystallinty and branching. Take a second to review the macrostructures of both HDPE and LDPE. Hint: HDPE and LDPE are made via two different processes and thus there are different mechanisms at work.

Question: Are you calculating the crystallinity of these materials with FTIR?

[newbie!]

ok.
i'm not sure.
we took the FTIR of the samples and now have that graph thing with %transmitense up the side and wavenumber (cm-1) along the bottom.
(sorry about the crappy explanation!)
it was a lab experiment we did. we have like 2 known samples HDPE and LDPE and have to work out whether 4unknowns are made from them or not.

[marquis]

Hope this helps.

The IR for HDPE and LDPE is very similar.  You should see three major sets of peaks.  One set is at about 2950 recip cm-these don't give much information.

Typically, the polyethylenes have two more "doublets" (a doublet is a set of two very close, very sharp peaks) present that indicate crystallinity.  The first doublet is in the range of 1450 recip cm.  The second doublet is in the range of 800 recip cm (sorry, I'm at home and don't have the spectra in front of me, so the exact wavenumber may be different).

Some polyethylenes have low crystallinity.  In these cases, the IR is very similar to that of mineral oil (usually called nujol).   These peaks are broader.  Some of the nujol like peaks, present because of low crystallinity, can also be in the crystalline polyethylene.  This is where you should start.

LDPE uses free radical polymerization that has as a side-effect, something called "back biting".  This causes a lot of 4 carbon length side chains on the LDPE polymer backbone.

Good luck with your lab.

[doc30]

Look for the Davydov splitting of the C-H bending modes that arises from the material being in a crystalline environment for HDPE. LDPE is amorphous so the IR spectrum shows no such splitting.

[typhoon2028]

DSC would be a better instrument to use to understand crystallinity and melting point.

HDPE is more crystalline than LDPE.  More crystalline = higher latent heat of fusion = higher melting point.

HDPE is more crystalline, because it lacks the branching LDPE has.  The molecules can pack tighter into a crystalline structure.

[PolymerKnowHow]

TREF would be even better.