DAY 26: POLYMERS Review: Polymers vs. Metals Strength factors: What factors make

polymers strong?1. Crystallinity2. Increase the chain length Here’s another one:3. Decrease chain flexibility. I.e. make the

backbone more rigid. Here’s an example.

ANOTHER COMMON POLYMER: POLYSTYRENE

Basic Mer

Phenyl group

The second most commonly encountered polymer. This is a hard, yet cheap commodity plastic. Yes, it can be foamed.

This is a thermoplastic polymer.

Polymer

density UTS ksi %EL E ksi

PS 1.05 6.5 1.5 413

MDPE 0.936 2.5 750 90

USES OF PS

Cheap, hard commodity plastic. Or foam.

WHY PS IS STRONGER AND LESS DUCTILE THAN PE

Crystallinity is less. The bulky side groups interfere with the formation of cystals.

BUT

The difference is that the backbone is much more rigid. The backbone can’t rotate as easily because of the phenyl groups. Consequently sliding of adjacent chains is inhibited. Strength is increased at the cost of ductility.

TWO TYPES OF PS

TACTICITY, ACTUALLY THERE ARE THREE

SOME PROPERTIES OF A SYNDIOTACTIC PS

Density: 1.11 g/cc UTS: 10.5 Ksi %EL 1.8% Modulus of Elasticity: 700 Ksi

Just a few remarks about the difference:1. Bulky side groups such as phenyl inhibit

crystallinity.2. Sydndiotactic, ie regular placement on alt.

sides promotes crystallinity.3. Crystallinity enhances strength and stiffness.

IMPROVED PS – HIGH IMPACT PS (HIPS)

We form what is called a “graft copolymer.” This is kind of like an alloy.

Polybutadiene rubber chain

Atactic polystyrene

HIPS is strong and tough!!

POLYPROPYLENE, PP

Here’s another simple mer. Look’s kind of like PS except that instead of a phenyl side group, we have a methyl side group.

Polymer

density UTS ksi %EL E ksi

PP 0.950 5.0 150 300

PS 1.05 6.5 1.5 413

MDPE 0.936 2.5 750 90

Side group is not as bulky as the one in PS.

CRYSTALLINITY AND PP

We have either considerable crystallinity or none depending on tacticity.

Irregular side group placement: amorphous

Regular side group placement: up to 60% crystallinity

PP FIBERS

The isotactic form, which has high crystallinity, can be turned into fiber. The crystallinity is associated with chain alignment in the longitudinal direction of the fiber.

Though PE is more crystalline, it is easier to get PP to form these fibers. Yes, PE fibers are great. (Spectra) But not as economically viable as PP fibers. Nylon, which will be discussed soon is well known for forming fibers.

Lots of PP fiber used in clothing. I.e. mountain climbing thermal underwear.

PP ADVANTAGES

1. Easy to color2. Dishwasher safe, compared to PE.3. High strength to weight4. Doesn’t absorb water readily5. Can be readily used as a plastic as well as a

fiber. See specimens.