Page 281 - Carrahers_Polymer_Chemistry,_Eighth_Edition

244 Carraher’s Polymer Chemistry

increase in the modulus. The cause of this limited extensibility was for many years believed to be
due to the molecular extent of uncoiling of the polymer segments composing the elastomeric mate-
rial. Today we know that while such ultimate chain elongations may contribute to this rapid increase
in modulus, the primary reason for many elastomeric materials involves strain-induced crystalliza-
tion. As a general observation, when the large increase in modulus is mainly due to limiting chain
extension the limit will not be primarily dependent on temperature and presence/absence of a dilu-
ent. Conversely, when the abrupt increase in modulus is dependent on temperature and diluent, the
limiting factor is probably stress-induced crystallization.
Ultimate properties of toughness (energy to rupture), tensile strength, and maximum extensibil-
ity are all affected by strain-induced crystallization. In general, the higher the temperature the lower
the extent of crystallization and consequently the lower these stress–strain-related properties. There
is also a parallel result brought about by the presence of increased amounts of diluent since this also
discourages stress-related crystallization.
There are some so-called noncrystallizable networks where stress–strain behavior is fairly inde-
pendent of diluent and temperature. One such system is formed from reaction of hydroxyl- terminated
polydimethylsiloxane (PDMS), through reaction with tetraethyl orthosilicate. While these materials
are not particularly important, commercially they allow a testing of various effects in particular the
so-called “weakest-link” theory, where it is believed that rupture is initiated by the shortest chains
because of their limiting extensibility. From such studies, it was found that at long extensions that
short chains that should be the “weakest-link” or more aptly put, the “shortest-link,” was not the
limiting factor but rather the system “shared” the distortion throughout the network distributing the
strain. This redistributing continues until no further distributing is possible at which case stress-
induced rupture occurs. Introduction of short or limiting chains has a positive affect on the modulus
related properties because shorter chains are better at distributing induced strain.
We are now able to construct so-called bimodal network systems composed of short and long
(average length between cross-links) chains and multimodel systems containing chains of prede-
termined differing lengths. It has been found that short chains are better at reapportioning applied
stress–strain than longer chains so that greater elongation is required to bring about the “upturn” in
modulus. In general, the stress–strain curve for short chains is steeper than that for long-chain net-
works as expected. Interestingly, a combination of short- and long-chained networks gives a stress–
strain curve that is between the one found for short-chained networks and the one for long-chained
networks such that the area under the stress–strain curve, toughness, is much greater than for either
of the monomodal systems. Products are being developed to take advantage of this fi nding.
Mismatching is important for some applications. Thus, moderately polar materials such as poly-
isoprene are ideal materials for hose construction where the liquid is nonpolar such as gasoline,

flight fuel, lubricants, oils, and so on, while nonpolar materials such as polyethylene-intense copo-
lymers would be suitable for use with transport and containment of polar liquids such as water.
Fillers are often added to polymeric networks. In particular, those elastomers that do not undergo

strain-induced crystallization generally have some reenforcing filler added. The important cases
involve carbon black that is added to many materials, including NR and silica that is added to

siloxane elastomers. These fillers generally give the materials increased modulus, tear and abrasion
resistance, extensibility, tensile strength, and resilience. Counter, they often create other effects such
as giving the materials generally a higher hysteresis (heat build up when the material is exposed to
repeated cycles of stress–strain) and compression set (permanent deformation).

7.14 POLYMER MIXTURES
Today, there exist a number of polymer mixtures, including blends noted in the previous section.
Here we will briefly look at two more mixtures. A plastic or polymer alloy is a physical mixture of

two or more polymers in a melt and is often structurally similar to blends, in fact the terms blends
and alloys are sometimes used to describe the same materials. While some cross-linking may occur,

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