Shear flow experiments

Taking into account the relevance of the range of semi-dilute solutions (in which intermolecular interactions and entanglements are of increasing importance) for industrial applications, a more detailed picture of the interrelationships between the solution structure and the rheological properties of these solutions was needed. The nature of entanglements at concentrations above the critical value c leads to the viscoelastic properties observable in shear flow experiments. The viscous part of the flow behaviour of a polymer in solution is usually represented by the zero-shear viscosity, rj0, which depends on the con-... 

The network structure near the gel point is very fragile and can be broken by the shear flow experiment. 

Applications of optical methods to study dilute colloidal dispersions subject to flow were pioneered by Mason and coworkers. These authors used simple turbidity measurements to follow the orientation dynamics of ellipsoidal particles during transient shear flow experiments [175,176], In addition, the superposition of shear and electric fields were studied. The goal of this work was to verify the predictions of theories predicting the orientation distributions of prolate and oblate particles, such as that discussed in section 7.2.I.2. This simple technique clearly demonstrated the phenomena of particle rotations within Jeffery orbits, as well as the effects of Brownian motion and particle size distributions. The method employed a parallel plate flow cell with the light sent down the velocity gradient axis. 

In the Initiation of steady shear flow and the steady shear flow experiments, the shaft angle encorder was attached to a timing motor and data acquisition proceeds In the same way as In the oscillatory experiments. These tests were performed at shear rates between 5xl0 2 sec and 2000 sec . ... 

A rigid sphere in shear flow experiences a force that moves the particle normal to the flow direction. 

Fig. 2. Shear flow experiments and corresponding material functions,...

Fig. 3. Additional transient shear flow experiments for studying relaxation of structure and orientation.

The transient shear flow experiments described in Figure 2 may provide the most insight into development of orientation and structure in LCP. We first look at stress growth at the start up of shear flow. In this experiment the stress build up at the start up of flow is monitored as a function of time. Some representative data for a 60 mole % PHB/PET copolyester are presented in Figure 16. At this particular temperature we observe two stress peaks. 

Figure 3.50. Portions of a polymer chain in shear flow experience different velocities depending on the distance from the stationary plate.

To explore viscoelasticity in liquids, consider the simple shear flow experiment shown in Figure 33.9. Begin the experiment by applying a shear force to the solution until it reaches steady state. At that time, the total force /, that is being applied to the top plate (divided by the number of molecules) equals the sum of the viscous and elastic forces exerted by the molecule. 

FIG. 13-21. Logarithmic plots of elongational growth viscosity against time for a polyethylene at I S0°C at various rates of elongation as indicated. The shaded area is where is obtained from shear flow experiments. (Bird, after Meissner."") Reproduced, by permission, from Meissner, Rheol. Acta, 10,230 (1971). Steinkopff Verlag, Darmstadt. 

D. E. Riemer, The Shear Flow Experience of Ink During Screen Printing, Proc. Inti. Symp. Microelec., Minneapolis, pp. 335-340,1987. 

The shear viscosity coefficients tJi, t]2, and 773 can separately be determined in shear flow experiments with adequate director orientations [27]. 

The SmC phase is sheared between two parallel plates of infinite dimension. The distance between the plates is so large that the influence of the surface alignment at the plates can be neglected in the bulk. The orientation of the directors, the velocity and the velocity gradient is the same as already discussed for the shear flow experiment (Fig. 18). Furthermore, a rotation of the director c is now allowed. Calculation of the torque according to Eq. (104) and F =0 gives... 

Oscillatory shear flow properties (also referred to as dynamic viscoelastic properties) have long been used to investigate the viscoelastic properties of polymeric materials (Ferry 1980). Oscillatory shear flow measurement requires an instrument that can generate sinusoidal strain as an input to the fluid under test and record the stress resulting from the deformed fluid as an output. For such purposes, a parallel-plates flxture as well as a cone-and-plate flxture can be used the uniform shear rate in the radial direction that is necessary when conducting steady-state shear flow experiments is no longer necessary. 

As we have demonstrated, an erroneous conclusion can be drawn on the sign of first normal stress difference in TLCP if the transient or steady-state shear flow experiments were carried out in the presence of residual normal force that was generated... 

Figure 11.1 Micrographs for the 70/30 PMMA/PS blend, in which the dark areas represent the PS phase and the gray/white areas represent the PMMA phase (a) before being subjected to shear flow, (b) along the flow direction after being subjected to shear flow at y = 0.1 s in a cone-and-plate rheometer, (c) in the flow direction after being snbjected to shear flow at y = 10 s in a capillary die, and (d) in the flow direction after being snbjected to shear flow at y = 100 s l in a capillary die. The shear flow experiments were carried ont at 210 °C. (Reprinted from Han et al., Polymer 36 2451. Copyright 1995, with permission from Elsevier.)...

When presenting the rheological behavior of homopolymers in Chapters 3-6, we compared (1) log G versus logw plots obtained from oscillatory shear flow experiments with logTVj versus logy plots obtained from steady-state shear flow experiments,... 

Straightforward evaluation of Yi from measurements of the torque exerted by a rotating director on the sample holder walls was pioneered by Tsvetkov in the late 1930s [8,9]. The alignment can be measured by optical methods, but it can also be found indirectly from measurements of the viscosity coefficient, tIo, in the simple shear flow experiment in the absence of a locking external field, when the director is aligned (and immobilised) by the flow. 

The various idealized shear-flow experiments are shown in Figure 2. These experiments can be reasonably well approximated in a cone-and-plate viscometer. This device can be programmed to give a wide variety of time-dependent shear flows, and the velocity gradient is known to be very nearly constant in the gap between the cone and plate. We now discuss some of the material functions that can be measured. 

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