Page 540 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Rheology and Physical Tests 503

GLOSSARY

Barcol impressor: Instrument used to measure the resistance of a polymer to penetration or
indentation.
Biaxially stretching: Stretching in two directions perpendicular to each other.
Bingham, E. C.: Father of rheology.
Bingham plastic: Plastic that does not fl ow until the external stress exceeds a critical thresh-
old value.

Coefficient of expansion: Change in dimensions per degree of temperature.
Compression strength: Resistance to crushing forces.

Creep: Permanent flow of polymer chains.

Dashpot: Model for Newtonian fluids consisting of a piston and a cylinder containing a vis-
cous liquid.
Dilatant: Shear thickening agent; describes a system where the shear rate does not increase as
rapidly as the applied stress.
Elastic range: Area under a stress–strain curve to the yield point.
Flexural strength: Resistance to bending.
Hooke’s law: Stress is proportional to strain.
Impact strength: Measure of toughness.
Isotropic: Having similar properties in all directions.
Maxwell element or model: Model in which an ideal spring and dashpot are connected in
series; used to study the stress relaxation of polymers.
Modulus: Stress per unit strain; measure of the stiffness of a polymer.

Newtonian fluid: Fluid whose viscosity is proportional to the applied viscosity gradient.
Newton’s law: Stress is proportional to fl ow.
Poisson’s ratio: Ratio of the percentage change in length of a sample under tension to its per-
centage change in width.
Pseudoplastic: Shear thinning agent: system where the shear rate increases faster than the
applied stress.
Relaxation time: Time for stress of a polymer under constant strain to decrease to 1/e or 0.37
of its original value.
Retardation time: Time for the stress in a deformed polymer to decrease to 63% of the orig-
inal value.
Rheology: Science of fl ow.
Rheopectic: Liquid whose viscosity increases with time.
Shear: Stress caused by planes sliding by each other.
Shear strength: Resistance to shearing forces.
Stress (s): Force per unit area.
Stress relaxation: Relaxation of a stressed sample with time after the load is removed.
Tensile strength: Resistance to pulling stresses.
Thixotropic: Liquid whose viscosity decreases with time.
Velocity gradient: Flow rate.
Viscoelastic: Having the properties of a liquid and a solid.
Voigt–Kelvin model or element: Model consisting of an ideal spring and dashpot in par-

allel in which the elastic response is retarded by viscous resistance of the fluid in the
dashpot.
Williams, Landel, and Ferry equation (WLF): Used for predicting viscoelastic prop-
erties at temperatures above T when these properties are known for one specifi c
g
temperature.
Yield point: Point on a stress–strain curve below which there is reversible recovery.

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