Page 561 - Carrahers_Polymer_Chemistry,_Eighth_Edition

524 Carraher’s Polymer Chemistry

followed by a combination of further chain degradation to give finally small products and formation

of complex cyclic products. Elimination of HCl further accelerates additional HCl elimination and
increased property loss. PVC degradation is decreased by addition of agents that impede degrada-
tion such as those that neutralize HCl, trap free radicals, and/or that react with the forming double
bonds to impede further depolymerization. Commercial PVC often contains organotin or antimony
mercaptide compounds that act as stabilizers.
In general, for vinyl polymers thermal degradation in air (combustion) produces the expected
products of water, carbon dioxide, and char along with numerous hydrocarbon products. Thermally,
simple combustion of polymeric materials gives a complex of compounds that varies according
to the particular reaction conditions. Application of heat under controlled conditions can result in
true depolymerization generally occurring via an unzipping. Such depolymerization may be related
to the ceiling temperature of the particular polymer. Polymers such as poly(methyl methacrylate)
PMMA and poly(alpha-methylstyrene) depolymerize to give large amounts of monomer when
heated under the appropriate conditions. Thermal depolymerization generally results in some char
and formation of smaller molecules, including water, methanol, and carbon dioxide.
Most polymers are susceptible to degradation under natural radiation, sunlight, and high tem-
peratures even in the presence of antioxidants. Thus, low-density polyethylene (LDPE) sheets,
impregnated with carbon black, become brittle after exposure to 1 year’s elements in South Florida.
High-density polyethylene (HDPE), while more costly, does stand up better to these elements, but
again after several seasons, the elements win and the HDPE sheets become brittle and break. Long-
term degradation is often indicated in clear polymers by a yellowing and a decrease in mechanical
properties.
Most polymers are subject to oxidative degradation, particularly in the presence of other “entic-
ers” such as heat, a good supply of air, various catalysts, high-energy radiation, including ultraviolet
(UV) and higher energy visible light, and mechanical stressing that not only exposes additional
polymer to the “elements” but also brings about the actual breakage of bonds subsequently leading
to additional breakdown.
While polymers that contain sites of unsaturation, such as polyisoprene and the polybutadienes,
are most susceptible to oxygen and ozone oxidation, most other polymers also show some suscep-
tibility to such degradation, including natural rubber, PS, PP, nylons, PEs, and most natural and
naturally derived polymers.
Because of the prevalence of degradation by oxidation, antioxidants are generally added. These
antioxidants are generally compounds that readily react with free radicals or those that may act to
lessen the effects of “enticers” such as UV radiation.
Mechanical degradation, while applied on a macrolevel, can result in not only chain rearrange-
ment but also in chain degradation. Such forces may be repetitive or abrupt and may act on the
polymer while it is in solution, melt, elastic, or below its T . Passage of polymer melts through a
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tiny orifice for fabrication purposes can result in both chain alignment and chain breakage. In the
case of rubber, mastication of the elastomer, breaking polymer chains, is intentional allowing easier
deformation and processability. While shearing itself can result in chain breakage, chain breakage
is often associated with localized heat buildup that is a consequence of chains “rubbing” together,
and so on (molecular friction).
Most heterochained polymers, including condensation polymers, are susceptible to aqueous-as-
sociated acid or base degradation. This mode of degradation is referred to as hydrolysis. This sus-
ceptibility is due to a combination of the chemical reactivity of heteroatom sites and to the materials
being at least wetted by the aqueous solution allowing contact between the proton or hydroxide ion
to occur. Both of these factors are related to the difference in the electronegatives of the two dif-
ferent atoms resulting in the formation of a dipole that acts as a site for nucleophilic/electrophilic
chemical attack and that allows polar materials to come in contact with it. Such polymers can be

partially protected by application of a thin film of hydrocarbon polymer that acts to repel the aque-
ous solutions.

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