TA AR-G2, Getting Started Manual

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Page 18 AR-G2/AR 2000ex/AR 1500ex Getting Started Guide
It was many years before an electrically driven version of Couette's CMT apparatus was developed. The
next major advance in rheological instrumentation was introduced by Weissenberg in the 1940's. Weissen-
berg's intention was to investigate the viscoelasticity of polymer melts and solutions, but the viscometers
that existed at the time were not suitable for this study. This led to the next advance in instrumentation.
The study of elasticity parallels closely the study of viscosity. The first scientific reference to elasticity was
made by Robert Hooke, a correspondent and rival of Newton's, who published his famous anagram “CEI-
INOSSITTUU” in 1676, revealed as “ut tensio sic uis" (as the extension, so the force) in 1679. Hooke's
Law, as it came to be called, was supported by experimental observation, but it was not until the work of
Young in the early nineteenth century that it was realized that the law could be applied to material proper-
ties, rather than simply to extensive sample properties. In modern terminology, we would summarize
Young's findings by saying that the strain is proportional to the stress, and we would refer to the “constant
of proportionality” as the “modulus of the material.” Later in the nineteenth century, the work of Maxwell,
Voigt, Kelvin, Boltzmann and others showed that the distinction between viscous liquids and elastic solids
was not as clear as had previously been thought. Most of the materials listed above as non-Newtonian, are
also viscoelastic, in that they exhibit aspects of both types of behavior. (The names of the scientists who
contributed to the development of rheology reveal its importance: Einstein was also involved, and rheolo-
gists like to say that in their discipline Newton, Maxwell and Einstein did the easy bits.)
To conduct his investigation into polymer viscoelasticity, Weissenberg developed the first modern, electri-
cally driven, rheometer during the early 1940s, the basis of which was a lathe turned on its end. As such it
differed in two very significant ways from the Couette viscometers, firstly in that it was what later became
called a controlled rate rheometer, and secondly in that the actuator and detector were mounted on separate
axes. To adopt the principle of naming used above, this can be called the “separate motor and transducer”
(SMT) design. The principle of operation was that one of the platens of the measuring system was rotated
at a set angular speed, the torque transmitted by the sample being measured at the other platen. Weissen-
berg called his instrument a “Rheogoniometer”, since both the torque and the axial force could be mea-
sured, the latter being used to calculate the normal stress which results from the elasticity of the sample. In
the late 1940s the rheogoniometer was commercialized, but its price was beyond the range of most materi-
als testing laboratories. In 1970 Chris Macosko and Joe Starita formed the Rheometrics company (later
renamed Rheometric Scientific) to produce a lower cost alternative, and launched the first of a long line of
high quality SMT rheometers that led eventually to the modern ARES. Rheometric Scientific was acquired
by TA Instruments in January 2003, and its products continue to be manufactured and developed.
In the meantime, interest revived in CMT instruments, partly because of a desire to perform creep tests,
and partly because of the need to investigate the phenomenon of the yield stress in more detail, for which
the available SMT rheometers lacked the sensitivity. To these ends, Jack Deer, who was employed as a
technician at the London School of Pharmacy, designed a rotational rheometer based on the Couette vis-
cometer, but with the weight replaced, originally by an air-turbine drive, and later by a drag cup motor
(both shown in
Figure 1 ). To reduce the friction in the instrument, an air bearing was introduced. Deer's
first published description of the instrument appeared in 1968 [Davis, Deer and Warburton, J. Sci. Instr. 2,
933-936, 1968]. He began to commercialize it shortly afterwards. In the early 1980's the design was taken
up by the Carri-Med company, at Deer's instigation, and that company launched its first rheometer, the
CSR, in 1984 (show n in
Figure 2 ).