Tricky questions about glass transition and crystalline melting

[ymhiq]

Homework Statement

How do you explain that crystalline melting temperature T_f of PE is greater than the T_f of PEO? How do you explain that the glass transition temperature of PP is lower than the glass transition temperatures of PVC and PS?

Homework Equations

None

The Attempt at a Solution

How do you explain that crystalline melting temperature T_f of PE is greater than the T_f of PEO?

I read something about the higher the molecular weight the higher the crystalline melting temperature, so I found that PEO is also PGE and its molecular weight goes from 1000 to 20000, PE has a molecular weight between 10000 and 40000. It makes sense. I kept reading and I found that PTFE has got a T_f of 330 ºC because of its crystallinity 94% so I found that the crystallinity of PE is about 45% but crystallinity of PGE is 83% and it do not make sense.

I got confused. I decided to answer the next question but...

How do you explain that the glass transition temperature of PP is lower than the glass transition temperatures of PVC and PS?

I also read that glass transition temperature is related to crystalline melting temperature and its a fraction of its value(tow thirds for unsymmetrical polymers and half for symmetrical ones). However PP molecular weight is between 200000 and 700000, PVC molecular weight is between 100000 and 200000 and finally PS is between 50000 and 100000 and it do not make sense.

Any help would be greatly appreciated.

 

[TeethWhitener]

1) You mean PEG (polyethylene glycol) instead of PGE (polyglycerol ester). PEO (polyethylene oxide) is the same as PEG, although they sometimes refer to different molecular weight regimes. In general, yes, the MW of PEO will be significantly less than PE, and this leads directly to the observed higher melting point of PE.
2) For the glass transition temperature, you're no longer looking at the molecular weight, but rather how easily the polymer chains pack in a crystalline configuration versus how easy it is for them to pack amorphously. For PP (polypropylene), each monomer unit has a methyl side group which inhibits efficient packing and lowers the glass transition temperature: the temperature where the polymer transitions from largely crystalline to largely amorphous. This is most apparent when comparing atactic PP (methyl group is "randomly" placed on the polymer backbone) to isotactic PP (methyl group is periodically placed on polymer backbone). The T_g of atactic PP is quite a bit lower than that of isotactic PP. On the other hand, PVC (polyvinyl chloride) packs very efficiently because the chlorides provide enough of a dipole to match up in a crystalline fashion (this doesn't happen with the nonpolar methyl groups). For the case of PS (polystyrene), the aryl side groups can ##\pi-\pi## stack and create crystalline interactions, so it too has a relatively high T_g.