Page 70 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Polymer Structure (Morphology) 33

Chain entanglement allows long chains to act as though they were even longer because entan-
glement causes the entangled chains to act together. Many of the physical properties, such as tensile
strength, of polymers increase dramatically when chain entanglement occurs. The critical chain
length (z) required for the onset of entanglement is dependent on the polarity and shape of the poly-
mer. The number of atoms in the critical chain lengths of PMMA, PS, and polyisobutylene are 208,
730, and 610, respectively.
Viscosity is a measure of the resistance to flow. The latter is a result of cooperative movement of

the polymer segments from vacate location, hole, to another vacate location in a melted state. This
movement is impeded by chain entanglement, high intermolecular forces, the presence of reinforc-
ing agents, and cross-links. The melt viscosity (η) is often found to be proportional to the 3.4 power
of the critical chain length, as shown in Equation 2.6, regardless of the polymer. The constant K is
temperature dependent.

log η = 3.4 log z + log K (2.6)

The flexibility of amorphous polymers above the glassy state, where segmental mobility is pos-

sible, is governed by the same forces as melt viscosity and is dependent on a wriggling type of

segment motion in the polymer chains. This flexibility is increased when many methylene groups

(–CH –) or oxygen atoms (–O–) are present. Thus, the flexibility of aliphatic polyesters usually
2
increases as “m” is increased (2.7).
O O

) —O—C—(CH ) —C—O—)—
—(—(CH 2 m 2 m n (2.7)
Aliphatic polyester

Flexibilizing groups include methylene and ethylene oxides, dimethylsiloxanes, and methylene
groups.
In contrast, the fl exibility of amorphous polymers above the glass state is decreased when stiff-
ening groups such as

O O O
S
R NH R R
O
R
1,4-Phenylene (2.8) Amide (2.9) Sulfone (2.10) Carboxyl (2.11)

are present in the polymer backbone. Thus, poly(ethylene terephthalate) (PET; 2.12) is stiffer and with
higher melting point than poly(ethylene adipate; 2.13), and the former is stiffer than poly(butylene
terephthalate) because of the presence of fewer methylene groups between the stiffening groups.

R
O
O O
O R
R O
O O R O

Poly(ethylene terephthalate) (2.12) Poly(ethylene adipate) (2.13)

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