Environmental Stability of Polymers
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Environmental Stability of Polymers Thomas Atkins and Chris Batich Introduction Polymers, the main ingredient of plastics, constitute the fastest-growing sector of materials—having been introduced only in this century. Their application, however, is plagued by inappropriate use and resulting premature failure. Conversely, polymers are also being attacked as indestructible contributors to the waste stream. Although objective discussion of their merits is complicated by the enormous financial interests involved, it is clear that their rapid use with a short-term experience base has led to many problems. On the positive side, they are amenable to good theoretical and experiential understanding because of the excellent organic and physical chemistry background underlying the field. Hence, we can expect to see even more widespread use of polymers and hopefully fewer problems, as our knowledge base grows, is disseminated, and is applied. This article will address the current field of polymer environmental stability and summarize a few select areas where major technical efforts are being directed. Temperature Stability The fundamental difference between polymers and other classes of materials (ceramics and metals) derives from their linear nature. Although polymers may be connected by cross-links, their basic structure consists of a linear repeating pattern of strong bonds, in contrast to the threedimensional structures associated with metals and ceramics. Only at extremely high cross-link density do polymer structures exhibit the same type of highmodulus behavior as ceramics. Highly cross-linked structures behave more like diamond and may be classified as ceramics. The consequence of this fundamental difference in bonding is that many of the mechanical properties depend on weaker forces between the chains, in addition
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to the primary covalent bonds along the main chain axis. Consequently, polymers are susceptible to thermal effects at relatively low temperatures. Their properties largely reflect the lower magnitude of these secondary forces holding the polymer chains together. This constitutes a great materials advantage because it allows processing at modest temperatures, and has led to the rapid increase in the use of polymeric materials despite their frequently higher raw materials cost. It has also led
Table 1: Types of Bonds and their Strength in Polymers.* Main Chain Covalent Bonds (e.g., C—C, N—C, C — 0 )
60-100 kcal/mole
Secondary (interchain) Hydrogen Bond
*
?
~5 kcal/mole
N—H-.-O—C
Dipole Interaction 8 \ .
/^
O
C8 +
II
II
C8+
~2 kcal/mole
08
van der Waals CH 2
CH 2
CH 2
CH 2
-0.5 kcal/mole
*From Reference 1 and other general sources.
Molecular Structure and Mechanical Properties The useful mechanical properties of polymers result from the high molecular weight of the chains. (See Reference 1, p. 259.) Without entanglements or some other force, beyond simple secondary forces, holding chains together, few useful mechanical properties would develop. For instance, the difference between a
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