Relaxation time salinity effects

For polyacrylamide there are two rheological effects which can be explained in terms of its random coil structure. Firstly, it was discussed above that polyacrylamide is much more sensitive than xanthan to solution salinity and hardness. This is explained by the fact that the salinity causes the molecular chain to collapse, which results in a much smaller molecule and hence in a lower viscosity solution. The second effect which can be explained in terms of the polyacrylamide random coil structure is the viscoelastic behaviour of this polymer. This is shown both in the dynamic oscillatory measurements and in the flow through the stepped capillaries (Chauveteau, 1981). When simple models of random chains are constructed, such as the Rouse model (Rouse, 1953 Bird et al, 1987), the internal structure of these bead and spring models gives rise to a spectrum of relaxation times, Analysis of this situation shows that these relaxation times define response times for the molecule, as indicated in the simple Maxwell model for a viscoelastic fluid discussed above. Thus, because of the internal structure of a flexible coil molecule, one would expect to observe some viscoelastic behaviour. This phenomenon is discussed in much more detail by Bird et al (1987b), in which a range of possible molecular models are discussed and the significance of these to the constitutive relationship between stress and deformation rate and deformation history is elaborated. 

Comparison of GT-3 and GT-4. Time dependent assays of the ether extracted GT-3 reported by Miller et al. (16) indicated that it had the same time course of action and irreversible effects as described for GT-4. When the ileal preparation was exposed to 10 ng/ml of GT-3 for 15 minutes a 50% inhibition from control response was observed. Upon contact with the preparation, the toxin caused a slow tonic contraction. Washing the segment with clean saline following the 15 minute exposure period caused a gradual relaxation of the preparation to near the base line levels observed in the control. However, when challenged by agonist at any time following the latency period, and irreversible loss of activity was evident. 

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