Page 83 - Carrahers_Polymer_Chemistry,_Eighth_Edition

46 Carraher’s Polymer Chemistry

Chain entanglement

Chain entanglement

FIGURE 2.20 Physical cross-linking through chain entanglement.

Amorphous regions

Crystalline region

FIGURE 2.21 Crystalline portions that act as physical cross-links.

Chain entanglement is a physical means of forming cross-links (Figure 2.20). The incidence of
chain entanglement is related to both the particular polymer (dependent on such factors as bond
angles and substituents) and its length. The longer a polymer, the more apt that there is one or more
chain entanglements. For most linear polymers, chain entanglements occur when chains of 100
units and more are present. Chain entanglements cause the material to act as if its molecular weight
is much greater. For instance, if a chain of polyethylene of 100 units is connected to another polyeth-
ylene chain of 100 units, which is again connected to another chain of 100 units, then the effective
chain length is about 300 units.
The third type of cross-linking involves formation of crystalline portions within an amorphous
grouping (Figure 2.21). For vinyl polymers, the temperature that allows segmental chain mobility
to occur is generally well below room temperature, but the temperature to disrupt crystalline for-
mations is typically well above room temperature. Within mixtures of crystalline and amorphous
structures below the T , the crystalline portions act to “tie-in” or connect the surrounding areas
m
acting as cross-links.
In some situations, crystalline formation can occur because of addition of physical stress
such as the stretching of a rubber band. As the rubber band is stretched, the amorphous random
chains become aligned, forming small areas that are crystalline that oppose further stretching.
Thus, rubber bands contain chemical cross-links, and on extension, crystalline domains form
(Figure 2.22).
There are a number of consequences to the presence of cross-links. Cross-links impart to a
material memory with the chains about and those chains involved with the cross-links locked into

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