Introduction to In Situ Forming Hydrogels for Biomedical Applications - In-Situ Gelling Polymers: For Biomedical Applications - page 16

Fig. 7 Graph of an
oscillatory stress sweep

G', G” (Pa)

LVR

G', Storage modulus

G”, Loss modulus

Suggested
measurement
range here

Oscillatory stress ( Pa )

general guide, this section should cover the general rheological characterization
of the different unknown materials, however, it also requires certain amount of
creativity on the part of the rheologist to design the most appropriate protocol. A
point to note is that these recommended conditions could be independently used
to further evaluate a materials' rheological response. In all the experiments, it is
important that the sample is well conditioned to a particular temperature before
proceeding with the measurements.
Most of the time, the rheological properties of a viscoelastic material are strain-
independent up to a critical strain level. When the strain exceeds the critical level, the
storage modulus of the material declines and a non-linear behavior is observed. The
measurement of the strain amplitude dependence of the storage and loss moduli (G′,
G″) is a usually the first step taken to characterize a material's viscoelastic behav-
ior and to determine the pseudo-linear viscoelastic region (LVR) of the material. An
oscillatory stress sweep (OSS) will give a general range of where the LVR is located.
The range of the stress sweep should be tested over the allowable shear stress
(torque ~1-10,000 ʼ N m) range of the instrument. In future experiments, the shear
stress range can be adjusted appropriately to collect only reliable data. As the allow-
able shear stress range is dependent on the geometry used, torque will be used as the
controlled variable. The frequency should be set to a value of about 1 Hz (Fig. 7 ).
After the material's LVR has been defined by a strain sweep, its structure can
be further characterized using a frequency sweep at a strain below the critical
strain. This experiment provides more information about the effect of colloidal
forces and the interactions among particles.
In a frequency sweep, measurements are made over a range of oscillation fre-
quencies at a constant oscillation amplitude and temperature. Below the critical
strain, the elastic modulus G′ is often nearly independent of frequency, which is a
characteristic of a structured or solid-like material. On the other hand, frequency-
dependent elastic modulus is a characteristic of a more fluid-like material.