[xshadow]

Hi!!
I have some doubt about the factors which affect the Melting point...please help me !!

My textbook says:
T_m= ΔH_m/ΔS_m
Then My textbook says melting point increases if the polymeric chains has a low flexibility...(less ΔS.T higher)

For example says that:
a)
 C-O-C bond are more flexible than C-C-C
 CF₂-CF₂ are more flexible than CH₂-CH₂

b)

R-C-N-HR
   ||
   O

Gives more rigidity to the polymeric chain than:

R-C-OR
   ||
   O

About this fact says  very quickly (so I don't understand well) :  Hydogenuk bond (N-H I suppose) strenght and resonance effect of the ammide group


Someone can explain these facts, a) and b)  ??
Why  ammide group gives more rigidity to the chain compared to the ester group??
Why C-O-C bond are more flexible than C-C-C???

Thanks very much!!

[Enthalpy]

a) Something is fishy, because PE melts more easily than both POM and PTFE.

And while I'm pleased with rigid molecules harder to melt, it isn't the only factor. Stronger intermolecular bonds also keep a compound solid. So I wouldn't compare C-O-C with C-C-C based on rigidity only. Which leads to

b) I see a hydrogen bond between the H attached to N and the O.

[xshadow]

a) Something is fishy, because PE melts more easily than both POM and PTFE.

And while I'm pleased with rigid molecules harder to melt, it isn't the only factor. Stronger intermolecular bonds also keep a compound solid. So I wouldn't compare C-O-C with C-C-C based on rigidity only. Which leads to

b) I see a hydrogen bond between the H attached to N and the O.

1)But why  there's more rigidity in C-O-C  than C-C-C...what is the rigidity in this contest??
2)What does imply the presence og a hydrogen bond???
The intermolecular force are stronger?? so...less mobility??

[Enthalpy]

- Intermolecular forces are important to keep a compound solid. Stiff molecules are just one other factor.
- Hydrogen bonds are strong intermolecular forces.
- So polyamides use to have a higher melting point than polyethylene.

The contribution of stiff molecules might be seen in aramides, which stay solid (they decompose) at higher temperatures than older polyamides.

Though, melting points are very difficult to explain or predict. No theory works properly. The polyesters you cited have a melting point similar to old polyamides.