Rheological Experiments

Rheological experiments have shown that the relative viscosity of compositions filled with the above materials is an exponential function of filler content by volume. The impact of an otherwise constant quantity of filler increases in the series FP — LDP — SDP, i.e., symbatically with the probability of particle comminution in the plasticization process. This effect is most clearly apparent for... 

Tamarind seed XG has been used to affect the rheological properties of other hydrocolloids such as starch [296,297]. Rheological experiments and... 

The effect of oxidative irradiation on mechanical properties on the foams of E-plastomers has been investigated. In this study, stress relaxation and dynamic rheological experiments are used to probe the effects of oxidative irradiation on the stmcture and final properties of these polymeric foams. Experiments conducted on irradiated E-plastomer (octene comonomer) foams of two different densities reveal significantly different behavior. Gamma irradiation of the lighter foam causes stmctural degradation due to chain scission reactions. This is manifested in faster stress-relaxation rates and lower values of elastic modulus and gel fraction in the irradiated samples. The incorporation of O2 into the polymer backbone, verified by IR analysis, conftrms the hypothesis of... 

In case of copper some rheological experiments carried out at a given polymer concentration and increasing amoimt of cations indicates that copper/pectin systems in the one-phase domain behave as a viscoelastic liquid rather than a viscoelastic solid referred to as true gel (G (co) = G, when to—>0 with Gg the equilibrium shear modulus)[35]. Despite the lack of experimental data the range in cation and polymer concentration in which true gels may be observed seemed very limited. These results corroborate the strength of the binding of copper by pectins evidenced by the properties of the phase separation curves. 

Thus for undiluted polymers the relaxation behaviour can be examined over a wider range of apparent frequencies. Similar functions can be constructed for other regions of the phase diagram and other rheological experiments. The method of reduced variables has not been widely tested for aqueous crosslinked polymers. Typically these are polyelectrolytes crosslinked by ionic species. Some of these give rise to very simple relaxation behaviour. For example 98% hydrolysed poly(vinyl acetate) can be crosslinked by sodium tetraborate. The crosslink that forms is shown in Figure 5.31. 

Another development underway is to try to locate on the above mentioned sigmoid curves the critical point (t. ). where gelation occurs. Rheological experiments and structural determinations will be used to calculate some critical quantities as a critical fiber density to gelify the system. 

The formation of ordered sphere-packing structures was observed in certain rheological experiments as just described. Due to the extremely uniform size of the particles, an ordered dense packing structure will develop during sedimentation of the Stober silica particles (see Fig. 2.1.12) when the dispersion is either sterically or electrostatically stabilized. The gemstone opal is essentially based on this principle (80-88). A transmission election replica picture is shown in Figure 2.1.13. The uniform... 

Princen [57, 64, 82] and others [84] also noted the presence of wall-slip in rheological experiments on HIPEs and foams. However, instead of attempting to eliminate this phenomenon, Princen [64] employed it to examine the flow properties of the boundary layer between the bulk emulsion and the container walls, and demonstrated the existence of a wall-slip yield stress, below that of the bulk emulsion. This was attributed to roughness of the viscometer walls. Princen and Kiss [57], and others [85], have also showed that wall-slip could be eliminated, up to a certain finite stress value, by roughening the walls of the viscometer. Alternatively [82, 86], it was demonstrated that wall-slip can be corrected for and effectively removed from calculations. Thus, viscometers with smooth walls can be used. This is preferable, as the degree of roughness required to completely eradicate wall-slip is difficult to determine. 

The non-aqueous HIPEs showed similar properties to their water-containing counterparts. Examination by optical microscopy revealed a polyhedral, poly-disperse microstructure. Rheological experiments indicated typical shear rate vs. shear stress behaviour for a pseudo-plastic material, with a yield stress in evidence. The yield value was seen to increase sharply with increasing dispersed phase volume fraction, above about 96%. Finally, addition of water to the continuous phase was studied. This caused a decrease in the rate of decay of the emulsion yield stress over a period of time, and an increase in stability. The added water increased the strength of the interfacial film, providing a more efficient barrier to coalescence. 

For our present purposes, the network theories suffer from an additional defect. They supply no information on the form of the memory function. The memory function must be obtained for each system by rheological experiments, and there is no way at present to predict how it should vary with the molecular structure of the polymer. For example, M(t) can be obtained from the stress relaxation modulus G(t) ... 

Fig. 4.35 Phase angle as a function of reduced frequency, determined from parallel plate rheology experiments, for a PS-PI diblock (Mv = 130kgmol, 50wt% PS) in dioctyl phthalale at a concentration 24.7 wt% polymer (Jin and Lodge 1997).

Rheological experiments with molten polymers are normally arranged to investigate into the homogeneous inertialess extension under isothermal conditions. 

Recently we have conducted rheological experiments wherein the end-tethered PCL nanocomposites were blended with pure PCL homopolymer. Rheological behavior, particularly the terminal zone slopes, obtained for 5% and 10% (obtained by blending equal weight fractions of PCL homopolymer with a 10 weight % PCL and 20 weight % PCL respectively) were found to be similar to those obtained from the as-prepared nanocomposites. 

Acknowledgments The authors gratefully acknowledge Dr. Martin In for fruitful discussions on rheology experiments and Pascale Guiffrey for UV-Vis analysis. Rhodia silicones company is also acknowledged for the gift of the hydroxypropyl PDMS. 

Blend morphology commonly depends on the weight fraction and viscoelastic properties of each component, the interfacial tension between components, the shape and sizes of the discontinuous phase, and the fabrication conditions and setup. Most rheological experiments applied to homogeneous melts can also be similarly applied to these immiscible blends [55,63,88,89]. The viscoelastic properties arising from these studies should be labeled with a subscript apparent since the equations used to translate rheometer transducer responses to properties incorrectly assume that the material is homogeneous. Nevertheless, these apparent properties are often found to be excellent metrics of fabrication performance. 

Rheological measurements must be tailored to the particular process and problem of interest. This is the key to successful solution of rheological and processing problems. Relevant rheological experiments are best made at the same temperatures, flow rates, and deformation modes that prevail in the process of interest. 

Rheological Experiments. Melt viscosity and low-strain oscillatory experiments were performed on a Rheometrics RDS-7700 dynamic spectrometer equipped with a 0.2-2.0-g-cm torque transducer. The samples were mounted on 25-mm-diameter parallel-plate fixtures with a gap of 0.5 mm. Prior to each scan, samples were heated to 50 °C and then cooled slowly to room temperature. Steady-shear... 

Dynamic rheological experiments provide suitable means for monitoring the gelation process of many biopolymers and for obtaining insight into gel/food structure because... 

Figure 3-40 Illustration of Estimation of Critical Stress from a Stress Sweep at a Fixed Frequency Dynamic Rheological Experiment. Alternatively, as described in the text, one may conduct a strain sweep experiment.

Creep-compliance studies conducted in the linear viscoelastic range also provide valuable information on the viscoelastic behavior of foods (Sherman, 1970 Rao, 1992). The existence of linear viscoelastic range may also be determined from torque-sweep dynamic rheological experiments. The creep-compliance curves obtained at all values of applied stresses in linear viscoelastic range should superimpose on each other. In a creep experiment, an undeformed sample is suddenly subjected to a constant shearing stress, Oc. As shown in Figure 3 1, the strain (y) will increase with time and approach a steady state where the strain rate is constant. The data are analyzed in terms of creep-compliance, defined by the relation ... 

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