Viscoelastic pressure-driven

Piston Cylinder (Extrusion). Pressure-driven piston cylinder capillary viscometers, ie, extmsion rheometers (Fig. 25), are used primarily to measure the melt viscosity of polymers and other viscous materials (21,47,49,50). A reservoir is connected to a capillary tube, and molten polymer or another material is extmded through the capillary by means of a piston to which a constant force is appHed. Viscosity can be determined from the volumetric flow rate and the pressure drop along the capillary. The basic method and test conditions for a number of thermoplastics are described in ASTM D1238. Melt viscoelasticity can influence the results (160). 

The most complete model to date for describing Case II diffusion is that of Thomas and Windle (13-16). They envision the process as a coupled swelling-diffusion problem in which the swelling rate is treated as a linear viscoelastic deformation driven by osmotic pressure. This model leads to the idea of a precursor phase propagating ahead of a moving boundary, as we have depicted in Figure 4. While Thomas and Windle have used numerical methods to examine in detail the predictions of their model, this model is difficult to test with the data obtained here. 

Turgor-driven cell expansion is a consequence of three superimposed but independent phenomena viscoelastic deformation, wall stress relaxation, and wall synthesis. Viscoelastic effects are reversible and occur whenever walls are placed under tension by turgor pressure. Viscoelasticity is a intrinsic mechanical property of the primary wall that is not mediated by enzymic or chemical reactions [76, 77]. In contrast, wall stress relaxation which is associated with expansive growth is believed to involve the chemical or enzymic irreversible loosening of load-bearing bonds in the wall. As a consequence, turgor pressure and water potential are reduced. Water is then taken up by the cell with the result that turgor is restored and there is a net increase in cell volume. 

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