Polymer rheology creep

Real polymers require more elaborate systems of springs and dash-pots to describe them. This approach of polymer rheology can be developed to provide criteria for design with structural polymers. At present, this is rarely done instead, graphical data (showing the creep extension after time t at stress a and temperature T) are used to provide an estimate of the likely deformation during the life of the structure. 

Galgali and his colleagues [46] have also shown that the typical rheological response in nanocomposites arises from frictional interactions between the silicate layers and not from the immobilization of confined polymer chains between the silicate layers. They have also shown a dramatic decrease in the creep compliance for the PP-based nanocomposite with 9 wt% MMT. They showed a dramatic three orders of magnitude drop in the zero shear viscosity beyond the apparent yield stress, suggesting that the solid-like behavior in the quiescent state is a result of the percolated structure of the layered silicate. 

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. 

Rossi et al. [30] evaluated rheologically mucins of different origin with polyacrylic acid and sodium carboxymethyl cellulose. The same group also reported a novel rheological approach based on a stationary viscoelastic test (creep test) to describe the interaction between mucoadhesive polymers and mucins [31,32]. Jabbari et al. [33] used attenuated total-reflection infrared spectroscopy to investigate the ehain interpenetration of polyaciylic acid in the mucin interface. 

Understanding of the mechanism of creep failure of polymeric fibres is required for the prediction of lifetimes in technical applications (Northolt et al., 2005). For describing the viscoelastic properties of a polymer fibre use is made of a rheological model as depicted in Fig. 13.103. It consists of a series arrangement of an "elastic" spring representing the chain modulus ech and a "shear" spring, yd with viscoelastic and plastic properties... 

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). 

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. 

G 3. Gibbs, D. A., and E. W. Merrill A shear creep viscometer for rheological studies of polymers. Proc. of Fourth Intern. Congr. on Rheology 2, 183—192 (1965). 

FIGURE 2.42 Rheological models (a) viscous body, (b) elastic-viscous body, (c) viscous-elastic body. (Reprinted from Yu. Potapov, O. Figovsky, Yu. Borisov, S. Pinaev, and D. Beilin, Creep of Polymer Concrete at Compressive Loading, J. Scientific Israel Technological Advantages 5, nos. 1-2 (2003) 1-10. With permission.)... 

In Section 24.1 we have defined ways of prediction of long-term behavior from short-term tests. Let us now provide more examples of application of these concepts. Creep and stress relaxation have been determined for PET/ 0.6PHB, where PET is the poly(ethylene terephthalate), PHB, the p-hydroxybenzoic acid, and 0.6 is the mole fraction of the latter in the copolymer [58]. PET/0.6PHB is a polymer liquid crystal, see chapter 41 on PECs in this Handbook. In temperature ranges of interest it forms 4 coexisting phases [60]. Conventional wisdom said that prediction methods work only for so-called rheologically simple materials, practically for one-phase polymers. Therefore, we have decided to apply as severe a test as possible to our prediction methods and a multiphase PLC is a good choice. 

As mentioned above, interfacial films exhibit non-Newtonian flow, which can be treated in the same manner as for dispersions and polymer solutions. The steady-state flow can be described using Bingham plastic models. The viscoelastic behavior can be treated using stress relaxation or strain relaxation (creep) models as well as dynamic (oscillatory) models. The Bingham-fluid model of interfacial rheological behavior (27) assumes the presence of a surface yield stress, cy, i.e.. 

Several methods were suggested for measurement of the non-Newtonian rheological behavior of surfactant and polymer films. For example, Biswas and Haydon (34) constructed a special apparatus for measurement of the two-dimensional creep and stress relaxation of adsorbed protein films at the o/w interface. In creep experiments, a constant torque (in mNm ) was applied and the resulting deformation (in radians) was recorded as a function of time. In the stress relaxation experiments, a certain deformation Y was produced in the film by applying an initial stress, and the deformation was kept constant by gradually decreasing the stress. 

This chapter has illustrated how stress relaxation, creep, and rheology in polymers depend on the rate of molecular motion of the chains and on the presence of entanglements. It must be remembered that aU macroscopic deformations of matter depend ultimately on molecular motion. In the case of high polymers, the chain s radius of gyration is changed during initial deformation or flow. Thermal motions tend to return the polymer to its initial conformation, thus raising its entropy. Clearly, there is a direct relationship between the mechanical or viscous behavior of polymeric materials and their molecular behavior. 

Viscoelastic models are the most convenient rheological models to describe the creep response of polymer concrete because of the comparatively low design stress levels and deformation limits used in the design of polymer concrete members. 

To further illustrate the point of a liquid with both elastic and viscous behavior, the flow of a rheological liquid is shown in Fig. 1.2. Here a polymer liquid is in a clear horizontal (to avoid gravity effects) tube and a dark reference mark has been inserted that moves with the fluid. The liquid is unpressurized in frame 1 but a constant pressure has been applied in frames 2 through 5 where motion can be seen to have taken place as time progresses. In frame 6 the pressure has been removed and in frames 7 and 8 the liquid can be seen to partially recover. No recovery would take place if this were an ordinary viscous liquid. This is known as an elastic after effect and a similar effect or creep recovery is observed in viscoelastic solids and/or all polymers provided the correct temperature is chosen. 

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