[Doze]

Hello Chemistry Community!

I basically have to choose which polymer is better for water piping, though there is some chemistry I am not understanding associated with the relations of tacticity, crystallinity and strength of the polymers. (PVC and HDPE)

I did some research, and found that PVC is atactic (or heterotactic) because of the Chlorine units on the backbone (yet it doesn't say why the chlorine makes it like that?). This makes PVC more amorphous making it harder to arrange into layers , making it less crystalline, unlike HPDE (which is very crystalline because it's non branched and syndiotactic).PVC is known for its strength and durability, apparently it's 'Semi-crystalline' and has much more compression strength than HDPE (9500 psi compared to 3500-3700 psi for HDPE). Is it for this reason ? :


HDPE is far more crystalline, since it's linear and syndiotactic, though is it less stronger than PVC because of it only harnessing intermolecular forces within its chains, is this why? And PVC, which has covalent bonds (cross links?) between its chains because of its chlorine, which makes it stronger than HDPE regardless of
how less crystalline it is? I doubt that is why though... I think it may be the molar mass distribution or something along the lines...maybe?                         

2) I also was wondering if the reason for HDPE's glass transition temperature being 120 to 130 °C (248 to 266 °F) compared to 80°C for PVC, was because HDPE is more crystalline? But aren't PVC's covalent bonds holding chains together? Because apparently glass transition allows 'molecular chains to slide past each other when a force is applied', shouldn't HDPE have a lower Tg temperature than PVC and have its molecules 'slide' easier when it reaches its Tg?


This is really bugging me, I apologize for the rather lengthy question, though help would be much appreciated


Thank you for your time.

[fledarmus]

The answer to your question lies in the structure of the polymer backbone structure and how that allows the polymer chains to pack.

HDPE is essentially very, very long chains of -CH₂- groups:

-CH₂-CH₂-CH₂-CH₂-CH₂-CH₂-CH₂-CH₂-

These chains can lie very nicely alongside each other, like the threads in a length of rope. That allows for significant sections of crystallinity, because large portions of these chains can pack together into linear crystal structures.  The term "high-density" comes directly from this ability to pack close together, as compared to "low-density" polyethylene, in which the chains are more highly branched. Each branch interrupts the crystal packing.

As for PVC, it is the same thing except that every other carbon has a bond to a chlorine atom instead of one of the hydrogens in HDPE:

    Cl         Cl         Cl          Cl
   /           /           /           /         
-CH-CH₂-CH-CH₂-CH-CH₂-CH-

These strands can't pack as tightly together because the chlorine atoms get in the way. That lowers the crystallinity of the material, and makes the resulting structure more amorphous.

In terms of the physical properties you refer to in 2) of your post - the better backing ability of HDPE increases the areas of crystallinity, which increases the glass transition temperature - it takes more energy to convince the chains to move past each other and form a more disordered, amorphous mix. There are no crosslinks in either of these polymers (unless other monomers are added to give crosslinking points).

There are applications where more crystallinity for the polymers is desirable - in general, the more crystalline a polymer is, the harder and more brittle it will be. This allows it to withstand wear better, but can make it weaker to impacts. Amorphous polymers tend to be more flexible, but more malleable and more durable. For your application, my guess is that you are looking for a flexible, durable, tough material, rather than a harder, stiffer, more brittle material.

Hope this helps...