D

AY

26: P

OLYMERS

 Review: Polymers vs. Metals  1.

2.

 3.

Strength factors: What factors make polymers strong?

Crystallinity Increase the chain length Here’s another one: Decrease chain flexibility. I.e. make the backbone more rigid. Here’s an example.

A

NOTHER

C

OMMON

P

OLY

S

TYRENE

P

OLYMER

:

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

Phenyl group This is a thermoplastic polymer.

Polymer density

PS 1.05

MDPE 0.936

UTS ksi

6.5

2.5

%EL

1.5

750

E ksi

413 90

U

SES OF

PS

 Cheap, hard commodity plastic. Or foam.

W

HY THAN

PS

IS STRONGER AND LESS DUCTILE

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.

T

WO

T

YPES OF

PS

T

ACTICITY

,

ACTUALLY THERE ARE THREE

S

OME

P

ROPERTIES OF A

S

YNDIOTACTIC

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.

2.

3.

Bulky side groups such as phenyl inhibit crystallinity.

Sydndiotactic, ie regular placement on alt. sides promotes crystallinity.

Crystallinity enhances strength and stiffness.

I

MPROVED

PS – H

IGH

I

MPACT

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!!

P

OLYPROPYLENE

, 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.

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

Polymer density

PP 0.950

PS MDPE 1.05

0.936

UTS ksi

5.0

6.5

2.5

%EL

150 1.5

750

E ksi

300 413 90

C

RYSTALLINITY AND

PP

 We have either considerable crystallinity or none depending on tacticity.

Regular side group placement: up to 60% crystallinity Irregular side group placement: amorphous

PP F

IBERS  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 A

DVANTAGES 1.

2.

3.

4.

5.

Easy to color Dishwasher safe, compared to PE.

High strength to weight Doesn’t absorb water readily Can be readily used as a plastic as well as a fiber. See specimens.