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

In summary, the glass transition temperature (Tg) is related to the free rotation of units along the chain. When the temperature drops below Tg, the rotation becomes restricted, resulting in a glassy state. This rotation depends on factors such as the size of side groups, the components of the main chain, and the polarity of side groups. In the case of polyethylene (PE) and polyethylene glycol (PEG), PEG has a higher Tg due to the introduction of C-O bonds, which rotate more freely than C-C bonds, and the increased polarity of the side groups. Despite the terminology of "side groups," it is the polarity that plays a major role in determining the Tg.
  • #1
triplej
7
0
Can anyone tell me why poly(ethelyne glycol) has a higher Tg than polyethylene?
 
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  • #2
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.
 
  • #3
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?
 
  • #4
Above Tg the material is elastic.

Below Tg 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?
 
  • #5
Studiot said:
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 Tm which is observed. This also might imply a lowering of Tg 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 Tg.

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?
 
  • #6
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 [tex]\delta[/tex] +ve and the other [tex]\delta[/tex]-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.
 

1. What is the definition of glass transition temperature?

The glass transition temperature (Tg) is the temperature at which an amorphous material, such as glass, transitions from a hard, rigid state to a more flexible, rubbery state. It is the temperature at which the material's molecular structure undergoes a change in behavior.

2. How is glass transition temperature different from melting temperature?

Glass transition temperature is the temperature at which an amorphous material changes from a solid to a rubbery state, while melting temperature is the temperature at which a crystalline material changes from a solid to a liquid state. Glass transition temperature does not involve a change in the material's chemical structure, while melting temperature does.

3. What factors can affect the glass transition temperature of a material?

The chemical composition, molecular weight, and molecular structure of a material can all affect its glass transition temperature. Other factors that can influence Tg include the rate of heating or cooling, the presence of additives or impurities, and the thermal history of the material.

4. Why is glass transition temperature important in materials science?

Glass transition temperature plays a crucial role in determining the properties and applications of materials. It can affect the mechanical, thermal, and electrical properties of a material, as well as its processability and stability. Understanding Tg can help scientists and engineers design and develop new materials with desired properties.

5. How is glass transition temperature measured?

Glass transition temperature can be measured using various techniques, such as differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermomechanical analysis (TMA). These methods involve subjecting a sample of the material to controlled heating or cooling and monitoring its physical and thermal changes, such as expansion, contraction, or energy absorption. The Tg is typically identified as the point of inflection on a plot of temperature versus a measured property.

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