Download - Day 26: Polymers
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.