Rheological properties creep measurements

Thus, measuring rheological properties of dispersions of low-molecular-weight (with viscosity about 60 Pa s) and high-molecular-weight poly(isobutilene) (with viscosity about 10s Pa s) with the same content of filler we see that the values of creep viscosity r c of these systems are practically equal, in spite of the difference of the viscosity of the dispersion medium more than 1000 times [3],... 

In this paper we report some rheological studies of aqueous concentrated polystyrene latex dispersions, in the presence of physically adsorbed poly(vinyl alcohol). This system has been chosen in view of its relevance to many practical systems and since many of the parameters needed for interpretation of the rheological results are available (15-18). The viscoelastic properties of a 20% w/w latex dispersion were investigated as a function of polymer coverage, using creep measurements. 

The rheological properties of gum and carbon black compounds of an ethylene-propylene terpolymer elastomer have been investigated at very low shear stresses and shear rates, using a sandwich rheometer [50]. Emphasis was given to measurements of creep and strain recovery at low stresses, at carbon black flller contents ranging between 20 and 50% by volume. The EPDM-carbon black compounds did not exhibit a zero shear rate viscosity, which tended towards in-Anity at zero shear stress or at a finite shear stress (Fig. 13). This was explained... 

If you step back and think about it, the mechanical and rheological properties of many solids and liquids can be modeled fairly well by just two simple laws, Hooke s law and Newton s law. Both of these are what we call linear models, the stress is proportional to the strain or rate of strain. If we examine viscoelastic properties like creep, the variation of strain with time appears decidedly non-linear (see Figure 13-75). Nevertheless, it is possible to model this non-linear time dependence by the assumption of a linear relationship between stress and strain. By this we mean that if, for example, we measure the strain as a function of time in a creep experiment, then for a given time period (say 1 hour) the strain measured when the applied stress is 2o would be twice the strain measured when the stress was o. 

Rayment, R, Ross-Murphy, S. B., and Elhs, P. R. 1998. Rheological properties of guar galactomannan and rice starch mixtures. II. Creep measurements. Carbohydr. Polym. 35 55-63. 

The measurement of rheological properties for non-Newtonian, lipid-based food systems, such as dilatant, pseudoplastic, and plastic, as depicted in Figure 4.1, are much more difficult. There are several measurement methods that may involve the ratio of shear stress and rate of shear, and also the relationship of stress to time under constant strain (i.e., relaxation) and the relationship of strain to time under constant stress (i.e., creep). In relaxation measurements, a material, by principle, is subjected to a sudden deformation, which is held constant and in many food systems structure, the stress will decay with time. The point at which the stress has decayed to some percentage of the original value is called the relaxation time. When the strain is removed at time tg, the stress returns to zero (Figure 4.8). In creep experi-... 

Thus, for a more complete understanding of bone s response to applied loads, it is important to know its rheologic properties. There have been a number of early studies of the viscoelastic properties of various long bones [Sedlin, 1965 Smith and Keiper, 1965 Lugassy, 1968 Black and Korostoff, 1973 Laird and Kingsbury, 1973]. However, none of these was performed over a wide enough range of frequency (or time) to completely define the viscoelastic properties measured, for example, creep or stress relaxation. Thus it is not possible to mathematically transform one property into any other to compare results of three different experiments on different bones [Lakes and Katz, 1974]. 

Some of the viscoelastic or rheological properties that can be measured using this technique include viscosity, modulus tensile compliance, creep-stress relaxation, gel time and gel temperature, tensile compliance, and stress-strain properties. 

Rheological measurements are used to investigate the bulk properties of suspension concentrates (see Chapter 7 for details). Three types of measurements can be applied (1) Steady-state shear stress-shear rate measurements that allow one to obtain the viscosity of the suspensions and its yield value. (2) Constant stress or creep measurements, which allow one to determine the residual or zero shear viscosity (which can predict sedimentation) and the critical stress above which the structure starts to break-down (the true yield stress). (3) Dynamic or oscillatory measurements that allow one to obtain the complex modulus, the storage modulus (the elastic component) and the loss modulus (the viscous component) as a function of applied strain amplitude and frequency. From a knowledge of the storage modulus and the critical strain above which the structure starts to break-down , one can obtain the cohesive energy density of the structure. 

Three different rheological measurements may be applied to study the bulk properties of suspension concentrates [118-120] (i) Steady state shear stress-shear rate measurements (using a controlled shear rate instrument), (ii) Constant stress (creep) measurements (carried out using a constant stress instrument), (iii) Dynamic (oscillatory) measurements (preferably carried out using a constant strain instrument). These... 

The blister tests have also been extended to the evaluation of rheological properties of thin films, in particular, the nature of glass transition in thin film systems. McKenna and coworkers applied the blister test to the evaluation of the time-dependent creep compliance of nanoscale membranes (O Connell and McKenna 2005 O Connell et al. 2008). O Figure 22.4 is an image of the nano blister test, where a thin polystyrene membrane of thickness 17 nm is inflated under a pressure of 1 psi (= 6.895 kPa). By measuring the deflection... 

Gabriel, C., Kaschta, J., Milnstedt, H. Influence of molecular structure on rheological properties of polyethylenes. I. Creep recovery measurements in shear. Rheol Acta (1998) 37, pp. 7-20... 

Four modes of characterization are of interest chemical analyses, ie, quaUtative and quantitative analyses of all components mechanical characterization, ie, tensile and impact testing morphology of the mbber phase and rheology at a range of shear rates. Other properties measured are stress crack resistance, heat distortion temperatures, flammabiUty, creep, etc, depending on the particular appHcation (239). 

A variety of rheological tests can be used to evaluate the nature and properties of different network structures in foods. The strength of bonds in a fat crystal network can be evaluated by stress relaxation and by the decrease in elastic recovery in creep tests as a function of loading time (deMan et al. 1985). Van Kleef et al. (1978) have reported on the determination of the number of crosslinks in a protein gel from its mechanical and swelling properties. Oakenfull (1984) used shear modulus measurements to estimate the size and thermodynamic stability of junction zones in noncovalently cross-linked gels. 

Measurement of the linear viscoelastic properties is the basic rheological characterization of polymer melts. These properties may he evaluated in the time domain (mainly creep and relaxation experiments) or in the frequency domain in this case we will talk about mechanical spectroscopy, where the sample experiences a harmonic stimulus (either stress or strain). 

Finally Dill and Zimm have designed a new rheological instrument that can not only perform steady shear, but also measure transient responses, e.g., both stress relaxation and strain relaxation (creep recovery) experiments for dilute solutions. They have investigated the properties of T2 DNA, and studied, for example, the concentration dependence of Results are consistent with the theory of Muthukumar and Freed, and from the value of r, at infinite dilution the molecular weight of the DNA may be determined. 

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