Page 562 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Reactions on Polymers 525

Enzymatic degradation is complex and not totally agreed upon. Microbes have enzymes, some
of which are capable of breaking selected bonds such as those that appear naturally—including
amide, ester, and ether linkages—and including natural, naturally derived and synthetic materials.
While a purpose of these enzymes is to digest nutrients for the host, when polymers with susceptible
linkages come in contact with a microbe that contains appropriate enzymes, polymer degradation
can occur. While often similar to acid- and base-associated degradations, enzymatic degradations

are more specific bringing about only specific reactions. Even so, it is often difficult to differentiate
between the two and both may occur together.
Some polymer deterioration reactions occur without loss in molecular weight. These include a
wide variety of reactions where free radicals (most typical) or ions are formed and cross-linking
or other nonchain session reaction occurs. Cross-linking discourages chain and segmental chain
movement. At times this cross-link is desired such as in permanent press fabric and in elastomeric
materials. Often the cross-links bring about an increased brittleness beyond that desired.
Some degradation reactions occur without an increase in cross-linking or a lessening in chain
length. Thus, minute amounts of HCl, water, ester, and so on elimination can occur with vinyl
polymers giving localized sites of double bond formation. Because such sites are less fl exible and
because such sites are more susceptible to further degradation, these reactions are generally con-
sidered as unwanted.
Cross-linking reactions can give products with desired increased strength, memory retention, and
so on but accompanying such cross-inking can be unwanted increases in brittleness. Throughout the
text, cross-linking is an important reaction that allows the introduction of desirable properties. For
instance, cross-linking is the basis for many of the elastomeric materials.
Stabilizers are often added to prevent some forms of degradation. Hindered-amine light stabiliz-
ers are added to scavenge free radicals that are light produced in many polymers. UV stabilizers are
added to absorb UV radiation converting some of it into heat. Antioxidants are added to stop free
radical reactions caused by UV radiation.

16.4 CROSS-LINKING

Cross-linking reactions are common for both natural and synthetic and vinyl and condensation
polymers. These cross-links can act to lock in “memory” preventing free-chain movement. Cross-
linking can be chemical or physical. Physical cross-linking occurs in two major modes. First, chain
entanglement acts to cause the tangled chains to act as a whole. Second, crystalline formations,
large scale or small scale, act to lock in particular structures. This crystalline formation typically
increases the strength of a material as well as acting to reduce wholesale chain movement.
Chemical cross-linking often occurs through use of double bonds that are exploited to be the sites
of cross-linking. Cross-linking can be effected either through use of such preferential sites as double
bonds, or through the use of other especially susceptible sites such as tertiary hydrogens. It can occur
without the addition of an external chemical agent or, as in the case of vulcanization, an external
agent, a cross-linking agent as sulfur, is added. Cross-linking can be effected through application of
heat, mechanically, though exposure to ionizing radiation and nonionizing (such as microwave) radi-
ation, through exposure to active chemical agents, or though any combination of these.
Cross-linking can be positive or negative depending on the extent and the intended result.
Chemical cross-linking generally renders the material insoluble. It often increases the strength of

the cross-linked material but decreases its flexibility and increases its brittleness. Most chemical
cross-linking is not easily reversible. The progress of formation of a network polymer has been
described in a variety of ways. As the extent of cross-linking increases, there is a steady increase
in the viscosity of the melt. At some point, there is a rapid increase in viscosity and the mixture
becomes elastic and begins to feel like a rubber. At this point, the mixture is said to be “gelled.”
Beyond this point the polymer is insoluble. The extent of cross-linking can continue beyond this
gel point.

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