92168.pdf

260 Rheology

Owing to the thermal nature of the restoring force, the deformation
of rubber, for a given load, decreases as the temperature is raised.
This contrasts with the elasticity of a metal spring, which is due to
individual atoms being slightly displaced from their local equilibrium
positions, the coil structure greatly multiplying this effect, and which
increases with increasing temperature.
If the degree of cross-linking is not very great, as in crude rubber,
viscous flow can occur, the polymer chains moving permanently into
new equilibrium positions. Excessive cross-linking, on the other
hand, restricts changes in the chain configurations and the rubber
becomes hard and difficult to deform.
Partial crystallisation may take place in polymeric materials,
especially when stretched and/or cooled. From the mechanical
standpoint, the introduction of crystalline regions in a polymer is
equivalent to increasing the degree of cross-linking, and a partial loss
of elasticity results.
Polymers exhibit a glass transition temperature below which the
chain arrangements are frozen. Thermal motion no longer overcomes
the attractive forces between the polymer chains, and the sample
becomes hard and brittle.

Non-finear viscoelasticity

Viscoelasticity is termed linear when the time-dependent compliance

(strain/stress) of a material is independent of the magnitude of the

applied stress. All materials have a linearity limit (see Table 9.2).

Table 9.2 Linear viscoelasticity limits

Material Stress/N mT^2 Percentage strain

Elastomers

Plastics

Fats

c. 106 -10^7

c. 106 -10^7

c. 102

c. 10-100

c. 0.1-1

c. 0.01

The linearity limit of elastomers is large, because their deformation
is of an entropic nature and does not involve bond rupture and re-
formation.
Viscoelastic materials have much lower linearity limits. For the
segments or particles in such systems to move (flow) relative to one