The rheology of two different semi-rigid

1
Steady shear, small amplitude oscillatory shear
and capillary break-up extensional rheology
measurements of rod-like polymers
Azuraien Japper_at_Jaafar, Robert J. Poole
Department of Engineering, University of
Liverpool, Brownlow Street, Liverpool, L69 3GH
United Kingdom

• Steady and oscillatory shear measurements
• Plotting ?o versus concentration provides a
  convenient way of estimating c, the so-called
  critical overlap concentration, for both
  polymers. Below c, which for XG is approximately
  0.067 (670 ppm) and for SG, 0.019 (190 ppm),
  both solutions are dilute and ?o scales
  approximately as c1.44, above the critical
  overlap concentration, interactions between the
  molecules occur and ?o increases much more
  rapidly with concentration.

Introduction The rheology of two different
semi-rigid rod-like polysaccharides in aqueous
solutions, Xanthan gum (XG, Ketrol TF from Kelco)
and Scleroglucan (SG, Tinocare GL from Ciba) were
measured over a wide range of concentration
(0.01 - 0.75 w/w). Xanthan gum is a
polyelectrolyte produced using the bacterium
Xanthomonas campestris. Scleroglucan is a
non-ionic polysaccharide produced by the fungi of
genus Sclerotium. The molecular weights of the
polymers are reported by the suppliers to be in
excess of 106 g/mol.

• Methods
• Steady and oscillatory shear measurements
• TA Instrument Rheolyst AR 1000N controlled stress
  rheometer
• Small amplitude oscillatory shear (SAOS)
  measurements only possible for higher
  concentration solutions
• All SAOS measurements were conducted in linear
  viscoelastic region
• Capillary break-up measurements
• ThermoHaake capillary break-up extensional
  rheometer (CaBER) with laser micrometer
  (resolution10?m)
• High-speed digital imaging using Dantec Dynamics
  Nano Sense MKIII high-speed camera at 2000 frames
  per second with Nikon 60mm f/2.8 lens

Capillary break-up measurements

• In this simple technique a cylindrical liquid
  bridge of the test liquid is formed between two
  circular plates 4 mm in diameter. An axial step
  strain is then applied (i.e. the end plates are
  rapidly pulled apart to a fixed separation) which
  results in the formation of an elongated liquid
  thread. The thread diameter reduces due to
  surface tension and information about the
  extensional properties of the liquid can be
  deduced from the evolution of the filament
  midpoint diameter which is monitored using a
  laser micrometer.
• A simple one-dimensional analysis, neglecting
  axial curvature and assuming that the filament is
  axially uniform, shows that the filament can be
  characterised simply by its midpoint diameter
• Alternatively you may calculate a Hencky strain
  at the midpoint, the strain rate and estimate an
  apparent extensional viscosity
• Trouton ratio is calculated from
• Newtonian-like linear thinning of the filament
  was observed sometime later after the initial
  step strain
• Steady uniaxial extensional viscosity increases
  with concentration
• The magnitude of the Trouton ratio confirms the
  non-Newtonian behaviour of both polymers as Tr
  gtgt3. However, the Trouton ratio exhibits a
  decrease in magnitude with increasing
  concentration

0.5 Xanthan gum at 20?C (?i,f0.5, 2.2)
0.5 Scleroglucan at 20?C (?i,f0.5, 2.2)

• Conclusions
• G?? is greater than G? until the crossover
  frequency, which increases as the concentration
  decreases indicating that the behaviour
  corresponds to semi-rigid polymer chains and some
  entanglements still exist, as suggested by Lee
  (2001).
• Due to the semi-rigid nature of the molecules,
  Newtonian-like linear filament thinning behaviour
  was observed in capillary break-up experiments.
• Steady uniaxial extensional viscosity increases
  almost linearly with concentration. These results
  suggest that as concentration increases, the
  polymer exhibits a more ordered molecular
  structure resulting in increase molecular contact
  between molecules which subsequently leads to
  stronger molecular interactions and hence greater
  extensional behaviour.
• The magnitude of the Trouton ratio (gtgt3) confirms
  the non-Newtonian behaviour of these polymer even
  though Newtonian-like linear thinning was
  observed in the capillary break-up experiments.