Why is PVC brittle ?

My textbook states that PVC is brittle because :

"...the larger Cl atoms tend to catch on each other when the polymer chains are pulled apart."

To be honest, I am not too satisfied with this explanation nor do I fully understand what the size of the Cl atom has to with the material being brittle.

In terms of chemical bonds what are the pre-requisites for a brittle material ?

Reply 1

Ari Ben Canaan

Apparently very cold temperatures make PVC even more brittle.... Why is this so ?

Original post by EierVonSatan

(edited 12 years ago)

Reply 2

EierVonSatan

You're asking the wrong person if you want to know anything about material science/polymers :p:

Reply 3

liquid394

Original post by Ari Ben Canaan

Sometimes PVC can become brittle due to sunlight (U.V radiation) on PVC cause free readicals of Cl to be produced.
I beleive it forms a covalent bond with Carbon, take a look at this http://upload.wikimedia.org/wikipedia/commons/thumb/e/e3/PVC-3D-vdW.png/220px-PVC-3D-vdW.png then when you pull one chain from one side and the other from the other side the big hlorine molecules are going to lock into one another hance what the above quote said. hope this helps if u need anymore help just ask.

Reply 4

Bradshaw

A material is described as brittle if it easy to fracture. Fracture involves the breaking of chemical bonds, and so most crystalline materials (e.g. metals) are brittle. The only way the material can respond to a stress is through stretching of chemical bonds, which then break.

Some polymers are much less brittle because they can respond to an applied stress in a different way, by sliding the chains past one another. This involves no breaking of bonds, and so the material will not fracture. The large CL atoms (amongst other things) prevent this sliding and so the material becomes more brittle.

Reply 5

Ari Ben Canaan

Original post by Bradshaw

So the presence of more amorphous regions in certain spider silks means that the polymer chains can slide past each other providing the elasticity we observe in such silk ?

Also, normal silk is not as elastic because of the more Beta pleated sheets (crystalline regions) which are more rigid ?

Reply 6

mytutoring

Original post by Ari Ben Canaan

OK - from a materials chemist - PVC is a brittle material, most of the PVC we see in use appears more plastic becase it has small molecules called pastisisers in them.

Most polymers have a degree of randomness to them, like spagetti in a bowl. this mean that there is extra space between the molecules. If a polymer/plastic is too be maluble - not brittle, then the chains need to be able to move past each other and therefore there needs to be sufficient space for this to happen. As we warm a plastic the chains move more and have more fee space, at some point there is enought space for the chains to pass eachother and the material goes from being brittle to being plastic.

If we compare polyethylene and PVC they are similar polymers but PE is plastic at room temp and PVC is not, this is because the chlorines on the PVC chain are large and inflexable and therefore require more free space in the polymer for the chains to pass eachother, therefore a higher temperature before it becomes plastic.

The pasticisers help this process by acting to increase the free volume in the polymer and therefore lowering the temperature at which the material becomes plastic.

To learn more about this try reading up on glass transision temperatures.

(edited 12 years ago)

Reply 7

mytutoring

Original post by Bradshaw

Fracture of a material does not necessarly require the breaking of chemical bonds, and ususally required the overcomming of intermolecular forces between the molecules in a structure. Metals often ductile due to the nature of the bonding, and are rarely crystalline, but polycrystalline. The defomation and fracture of such materials is complex and dependent on the thermal history of the material. ie- anealed metals are brittle, unanealed metals are often not. check out copper tubes at your local DIY, the long strait ones are anealled copper and more brittle than the rolls of tube.

Reply 8

Bradshaw

Original post by mytutoring

Indeed I was presenting a rather simplified picture of things, and fracture mechanics are a lot more complicated than that! I'm pretty sure you've got it the wrong way around with annealed / non-annealed metals though? Annealing will remove dislocations and lead to a more ductile material than something that has been worked.

Reply 9

mytutoring

Original post by Bradshaw

Yep your right, I shouln't post so late at night!

Reply 10

SimpleGirl

Original post by Ari Ben Canaan

Apparently very cold temperatures make PVC even more brittle.... Why is this so ?

At lower temperatures, below the Tg temperature of the polymer, the chains don't have enough energy to move/slide over one another, so effectively they become 'frozen'. This makes them more brittle; therefore, if force is applied to the polymer, it will cause the 'frozen' chains to break.

This is what my answer would be to this question based on what I've already been taught.