Why does poly(ethelyne glycol) have a higher Tg than polyethylene?

[triplej]

Can anyone tell me why poly(ethelyne glycol) has a higher Tg than polyethylene?

 

[Studiot]

The glass transition temperature is all about free rotation of the units along the chain.
Below some temperature they can no longer rotate - this is called the glass transition temperature.

The rotation depends upon many things

The size of the side groups
The components of the main chain
The polarity of side groups

In the case of polyethyene v polyethylene glycol

Polyethylene has all C-C bonds

Polyehtylene glycol has a C-0-C every so often. This spaces out the C-C repeat units.

 

[triplej]

Does that mean C-O bonds don't rotate as freely as C-C bonds, maybe an inequality in the electronic character of the C and O atoms that causes stiffness?

 

[Studiot]

Above T_g the material is elastic.

Below T_g the material is glassy (rigid).

Below some temperature they can no longer rotate

The glassy state occurs when the chains loose their ability to rotate about their bonds.

Does that mean C-O bonds don't rotate as freely as C-C bonds, maybe an inequality in the electronic character of the C and O atoms that causes stiffness?

So would you like to reconsider this?

 

[triplej]

The glass transition temperature is all about free rotation of the units along the chain.
Below some temperature they can no longer rotate - this is called the glass transition temperature.

The rotation depends upon many things

The size of the side groups
The components of the main chain
The polarity of side groups

I'll consider these three things you've mentioned.

Size of side groups:

PE has very small side groups in the H atoms, while PEG has no side groups on the O. This would imply that PE might have slightly more hindered rotation, meaning a higher glass transition than PEG, however this is not the case, so the steric effect of the H groups must be negligible.

Components of the main chain:

C-O bonds rotate more freely than C-C bonds, leading to a lowering of PEG's T_m which is observed. This also might imply a lowering of T_g however this is not observed so it is not true.

Polarity of side groups:

There are no side groups to speak of in either case, besides the H of PE, which are extremely non polar. If we were to consider the polarity of not only side groups, but of entire monomers, we would find PEG to be quite a bit more polar than PE. However, since the wording is "polarity of side groups" I am hesitant to do this. Still, if we were to consider this the main factor, it would mean increased viscosity in PEG and explain the lowered T_g.

So despite the wording, it must be a polarity issue. So, why is it explicitly defined as "side groups" and not "monomers"? (it is written the same in one of my textbooks) Or have i made some other mistake?

 

[Studiot]

The polarity issue arises when we introduce highly electronegative groups or molecules such as chlorine or oxygen. This leads to a partial electron shift with the carbon \delta +ve and the other \delta-ve.

In the case under discussion oxygen has two (negative) lone pairs sticking out instead of side groups.
The geometry of C-O-C is different from C-C-C because of this.