Organic Chemistry

polymer used to make dental impressions

n

O N

NO

O

O

O

O

O

O

DESIGNING A POLYMER

A polymer used for making dental impressions

must be soft enough initially to be molded around

the teeth, but must become hard enough later to maintain a fixed

shape. The polymer commonly used for dental impressions

contains three-membered aziridine rings that react to cross-link

Section 28.7 Physical Properties of Polymers 095

the chains. Because aziridine rings are not very reactive, cross-

linking occurs relatively slowly, so most of the hardening of the

polymer does not occur until the polymer is removed from the

patient’s mouth.

conventional polymer oriented polymer

>Figure 28.5

The creation of an oriented

polymer.

Elastomers
An elastomeris a polymer that stretches and then reverts to its original shape. It is a
randomly oriented amorphous polymer, but it must have some cross-linking so that the
chains do not slip over one another. When elastomers are stretched, the random chains
stretch out. The van der Waals forces are not strong enough to maintain them in that
arrangement; therefore, when the stretching force is removed, the chains go back to
their random shapes. Rubber is an example of an elastomer.

Oriented Polymers
Polymers that are stronger than steel or that conduct electricity almost as well as cop-
per can be made by taking the polymer chains obtained by conventional polymeriza-
tion, stretching them out, and putting them back together in a parallel fashion
(Figure 28.5). Such polymers are called oriented polymers. Converting conventional
polymers into oriented polymers has been compared to “uncooking”spaghetti. The
conventional polymer is disordered cooked spaghetti, whereas the oriented polymer is
ordered raw spaghetti.

A polymer used for making contact lenses must be suffi-

ciently hydrophilic to allow lubrication of the eye. Such a poly-

mer, therefore, has many OH groups. C

CH 3

polymer used to make contact lenses

CH 2 CH CH 2 CH CH 2 CH CH 2 CH

O C

CH 2

O C

CH 3

O C

CH 2

CH 2 OH CH 2 OH

O

Dyneema®, the strongest commercially available fabric, is an oriented polyethyl-
ene polymer. Its molecular weight is 100 times greater than that of high-density
polyethylene. It is lighter than Kevlar®and at least 40% stronger. A rope made of
Dyneema®can lift almost 119,000 pounds, whereas a steel rope of similar size fails
before the weight reaches 13,000 pounds! It is astounding that a chain of carbon
atoms can be stretched and properly oriented to produce a material stronger than
steel. Dyneema is used to make full-face crash helmets, protective fencing suits, and
hang gliders.