Page 270 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Copolymerization 233

The graft copolymers of acrylamide, acrylic acid, and cellulose or starch are used as water absor-
bents and in enhanced oil recovery systems.

7.6 ELASTOMERS

Elastomers typically contain chemical and/or physical cross-links. While there are thermoplas-
tic elastomers such as styrene–butadiene–styrene (SBS) and block copolymers, most elastomers
are thermosets. Elastomers are characterized by a disorganized (high-entropy) structure in the
resting or nonstressed state. Application of stress is accompanied by a ready distortion requiring
(relative to plastics and fibers) little stress to effect the distortion. This distortion brings about an

aligning of the chains forming a structure with greater order. The driving force for such a material
to return to its original shape is largely a return to the original less-organized state. While entropy
is the primary driving force for elastomers to return to the original resting state, the cross-links
allow the material to return to its original shape giving the materials a type of memory. Materials
that allow easy distortion generally have minimal interactions between the same or different

chains. This qualification is fulfilled by materials that do not bond through the use of dipolar (or
polar) or hydrogen bonding. Thus, the intermolecular and intramolecular forces of attraction are
small relative to those present in fibers and plastics. Hydrocarbon-intense polymers are examples

of materials that meet his qualification. Production generally requires the initial production of
linear polymers, followed by the insertion of cross-links through a process called vulcanization
or curing. Addition of fillers and other additives such as carbon black also is typical. Table 7.3

contains a listing of important elastomers.
The introduction of cross-links to inhibit chain slippage was discovered by Goodyear in 1839. He
accomplished this through addition of sulfur to natural rubber (NR). Shortly after this, an accelera-
tor, zinc (II) oxide, was used to speed up the process. Other additives were discovered often through
observation and trial-and-error so that today’s elastomers often have a number of important addi-
tives that allow them to perform demanding tasks.
Around 1915, Mote found that a superior abrasion-resistant elastomer was produced through the
use of carbon black. Today, it is recognized that factors such as surface area, structure, and aggre-
gate size are important features in the production of superior elastomers. For instance, high surface
areas (small particle size) increase the reenforcement and consequently the tensile strength and
improve the resistance to tearing and abrasion. Large aggregates give elastomers that have improved
strength before curing, high modulus, and an improved extrusion behavior.
Rubbers typically have low hysteresis. Hysteresis is a measure of the energy absorbed when
the elastomer is deformed. A rubber that absorbs a great amount of energy as it is deformed (such
as a tire hitting bumps on the roadway) is said to have a high hysteresis. The absorbed energy
is equivalent to the reciprocal of resilience such that a material with a low hysteresis has a high
resilience. Rubbers with a particularly high hysteresis are used where heat buildup is desirable
such as in tires to give the tread a better grip on the road and the tire a smoother ride. Thus, tires
with high hysteresis are often preferred in drag-race tires where heat buildup allows better grip
of the track.
The hard/soft segment scenario is utilized in the formation of a number of industrially important
thermoplastic elastomers. Representative examples of such thermoplastic elastomers based on block
copolymers are given in Table 7.4.
Such hard/soft scenarios can also be achieved through employing grafts (Table 7.5), where the
pendant group typically acts as the hard segment with the backbone acting as the soft segment.
Below, 7.36, is a representation of a typical graft copolymer chain. For an effective network to be
formed each “A” chain needs to have at least two “B” grafts to allow for formation of a continuous

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