Chemistry Reference
In-Depth Information
black adsorption sites. Entanglements will be more labile at higher temperatures,
where the molecular chains are more flexible, with a net decrease in the number
of effective intermolecular anchor points, an increase in
M
c
, and a decrease in the
retractive force, according to
Eq. (4-31)
.
4.6
Rodlike Macromolecules
Very rigid macromolecules are at the opposite end of the spectrum of proper-
ties from elastomers, which are characterized by weak intermolecular forces, a
high degree of molecular flexibility, and an absence of regular intermolecular
order.
Aromatic polyamides and polyesters are examples of stiff chain polymers.
Poly(
p
-phenylene terephthalamide) (Kevlar
, 1-23) can be made by reaction
(4-33) in a mixture of hexamethylphosphoramide and
N
-methylpyrrolidone:
t
H
H
amide
solvent
NH
2
NH
2
+
Cl
C
O
C
O
Cl
N
NC
O
C
O
+
HCl
x
(1-23) (4-33)
Polyamide-hydrazides
amide
solvent
NH
2
C
O
NH
NH
2
C
O
Cl
+
Cl
C
O
NH
C
O
NH
NH
C
O
C
O
+
HCl
(4-3) (4-34)
and aromatic polyesters like poly(
p
-hydroxybenzoic acid)
H
HO
C
O
O
HO
N
O
+
OH
x
(4-4) (4-35)
also provide rodlike species. These polymers behave in solution like logs on a pond
rather than like random coils. They exhibit liquid crystalline properties where they
have the short-range order of nematic mesophases. The liquid crystals are readily
oriented under shear and can be used to produce very highly oriented ultrastrong
fibers. On a specific weight basis they are stronger and stiffer than steel or glass
and are used to reinforce flexible and rigid composites like tires, conveyor belts,
and body armor as well as to make industrial and military protective clothing.
