Page 213 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.5

segments of the linear polymer chain. Such motions, however, are suppressed by intermo-
lecular attractions and, to some extent, by the presence of side chains and physical molec-
ular entanglements. Effects of such factors determine the glass transition temperature (T )
g
of the polymer. Below this temperature, a noncrystalline plastic or elastomeric polymer is
a supercooled liquid and behaves in many ways like a rigid glass. Above this temperature,
an uncross-linked noncrystalline linear polymer can flow and be processed and fabricated
into final shapes and forms. Above this temperature, a cross-linked polymer has rubber-
like properties unless it contains significant crystallinity. Thus, a crystalline melting point
(T ) can also be very important determinant for the behavior of polymeric materials.
m
The behavior of a thermoplastic material above its glass transition temperature depends
on its level of crystallinity. As a noncrystalline (amorphous) polymer is slowly heated
from a temperature below its T , it displays a large decrease in modulus as the glass transi-
g
tion temperature is reached. As one heats a semicrystalline plastic from a temperature be-
low its T , it displays a relatively small modulus change at the glass transition temperature,
g
followed by a plateau and then a decreasing modulus as the temperature increases and ap-
proaches the crystalline melting point.
Elastomers have glass transition temperatures well below room temperature. Rigid
thermoplastics have glass transition temperatures that fall in a wide temperature range,
ranging from below to above room temperature. However, if the T is below room temper-
g
ature, the polymer must be semicrystalline to be rigid. If the T is well above room temper-
g
ature, the noncrystalline polymer will be a rigid glassy polymer.
The presence of cross-links in a cured elastomer gives it elasticity and prevents it from
becoming molten and flowing above its glass transition temperature. Elastomers can, and
some do, contain small amounts of crystallinity while still being rubbery. Also, some non-
crystalline elastomers can partially crystallize during stretching, and this can be a strength-
ening, toughening, or tear-strength-increasing mechanism.
The changes in stiffness as a function of temperature, vis à vis T and T are illustrated
g
m
by Fig. 4.4. Table 4.1 gives values of T and T for selected polymers. Viewing Fig. 4.4
m
g
with Table 4.1 in mind, one obtains a perspective with respect to how these polymers be-
have as a function of temperature.
4.2.1 Differentiating Elastomers and Plastics by Measuring Dynamic
Mechanical Properties
A very good way to characterize and differentiate between elastomers and rigid plastics is
by the measurement of dynamic mechanical properties. A most convenient method to
study dynamic mechanical properties is to impose a small, sinusoidal shear or tensile
strain and measure the resulting stress. Dynamic mechanical properties are most simply
determined for a small sinusoidally varying strain, for which the response is a sinusoidally
varying stress. An increase in frequency of the sinusoidal deformation is equivalent to an
increase in strain rate.
The shearing deformation of a sample confined between two parallel plates, as illus-
trated in Fig. 4.5, is described by the strain, γ. Strain is defined as the displacement of the
top surface divided by the height of the sample. The stress, σ, is the tangential force per
unit area producing the deformation. When a sample is subjected to oscillatory shear de-
formations, the strain γ varies sinusoidally with time as

γ t() = γ sin ωt (4.1)
0
where γ is the strain amplitude (peak strain), ω the angular frequency, (2π times the fre-
0
quency in hertz), and t the time. The stress, σ, will also oscillate sinusoidally with the an-

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