Polymer Viscoelasticity weight

The reduced expressions of Table 4 form a set of universal viscoelastic functions. Given the polymer molecular weight, material constants [Jg, Je, etc.), and one extremal relaxation/retardation time, one should be able to predict, roughly, the nature of the system response (within the framework of the linear models) from Eqs. (T 1)—(T 6) and Fig. 2—5. 

L. J. Fetters, D. J. Lohse, D. Richter, T. A. Witten, and A. Zirkel, Connection between Polymer Molecular Weight, Density, Chain Dimensions, and Melt Viscoelastic Properties, ... 

The main feature about molten high polymers (molecular weights higher than about 104) concerns the broadness of the relaxation spectrum that characterises the viscoelastic response of these systems. This broad two-dispersion spectrum may spread over a range of relaxation times going from about 10 9 up to several seconds [4]. It is well illustrated from the modulus of relaxation observed after applying a sudden stress to the polymer the resulting sudden deformation of the sample is then kept constant and the applied stress is released in order to avoid the flow of the polymer. For example, the release of the constraint oxy(t) is expressed as a function of the shear modulus of relaxation Gxy(t) ... 

In transient shear flows starting from an isotropic distribution of fiber orientations, considerably higher viscosities will be initially observed, until the fibers become oriented. In Bibbo s experiments, t]r for isotropically oriented fibers is around 3.5 for v = 75. These viscosities can also be predicted reasonably well by semidilute theory and by simulations (Mackaplow and Shaqfeh 1996). Figure 6-25 shows the shear stress as a function of strain for a polyamide 6 melt with 30% by weight glass fibers of various aspect ratios, where the fibers were initially oriented in the flow-gradient direction. Notice the occurrence of a stress overshoot (presumably due to polymer viscoelasticity), followed by a decrease in viscosity, as the fibers are reoriented into the flow direction. 

Jennings et al. (1971) reported that in the usual case of medium permeability and medium polymer molecular weight, siginflcant increases in viscosity due to viscoelasticity were seen only at rates in excess of 1.5 to 3.0 m/d. The velocity range of 1.74 to 3.30 m/d reported by Han et al. (1995) is in line with that of Jennings et al. Han et al. reported that the range increases with increasing permeability of cores in their experiments. 

These equations are often used in terms of complex variables such as the complex dynamic modulus, E = E + E", where E is called the storage modulus and is related to the amount of energy stored by the viscoelastic sample. E" is termed the loss modulus, which is a measure of the energy dissipated because of the internal friction of the polymer chains, commonly as heat due to the sinusoidal stress or strain applied to the material. The ratio between E lE" is called tan 5 and is a measure of the damping of the material. The Maxwell mechanical model provides a useful representation of the expected behavior of a polymer however, because of the large distribution of molecular weights in the polymer chains, it is necessary to combine several Maxwell elements in parallel to obtain a representation that better approximates the true polymer viscoelastic behavior. Thus, the combination of Maxwell elements in parallel at a fixed strain will produce a time-dependent stress that is the sum of all the elements ... 

The reptation idea could account for the effect of polymer molecular weight and solvent concentration on the dissolution rate. However the key concentration identified in this approach [57,59] is one at the surface. This implies independence of the solvent concentration history. This may not be true as the disentanglement of polymer chains does not commence till the local solvent concentration is greater than a critical value at which the local glass transition temperature is lowered below the experimental temperature so that the glassy polymer changes into a gel. Also, all of the efforts discussed so far failed to take into account the effect of the viscoelastic properties of the polymer on the dissolution mechanism. 

Thereby, for chosen experimental objects were determined unknown parameters of developed mathematical model hyperelastic state constant, weighting coefficient and relaxation spectmm properties. Prediction results of thermomechanical curves trend successfully demonstrated prediction abihty of introduced mathematical description of thick cross-linked polymers viscoelastic pliability in all their physical states (Figure). 

Viscoelastic forces, which work against elongation of the jet in the electric field. This depends on the polymer molecular weight, the solvent, and the type of polymer. 

Tack refers to the adhesion of two surfaces of the same rubbery polymer. When two such surfaces are pressed together and subsequently pulled apart, the maximum force necessary to break the junction depends on the initial time of contact and the normal force applied, as well as the rate of separation and the temperature and other variables. " From the dependence on temperature and polymer molecular weight, it can be inferred that the effectiveness of the bond depends partly on the interdiffusion of molecules across the interface and hence on molecular motions which are reflected in viscoelastic properties in the terminal zone. - However, the effectiveness depends also on the ultimate properties of the polymer itself as discussed in Section E below, and the phenomenon is still not fully understood. 

Fetters, L. J., Lohse, D. J., Richter, D., Witten, T. A. Zirkel, A. (1994). Connection between polymer molecular weight, density, chain dimensions, and melt viscoelastic properties, Mflcromo/ecw/es 27(17) 4639-4647. doi 10.1021/ma00095a001. 

Because the ansatz does not invoke a detailed molecular picture, its predictive powers are restricted. The approach yields the functional dependence of various viscoelastic parameters on frequency or shear rate. The price exacted for the simplicity of derivation is that the ansatz is fundamentally unable to predict numerical values for functional parameters, let alone predict the dependence of those parameters on solvent quality, polymer concentration, or molecular weight. The ansatz is thus substantially noncommunicating with treatments of polymer viscoelasticity that invoke detailed microscopic models of polymer dynamics, such as the models of Bird, etal.(4,5), Graessley(6,7), or Raspaud, etal. S). 

Wu, S. Polymer molecular-weight distribution from dynamic melt viscoelasticity. Polym. Eng. Sd. (1985) 25, pp. 122-128... 

These normal stresses are more pronounced for polymers with a very broad molecular weight distribution. Viscosities and viscoelastic behavior decrease with increasing temperature. In some cases a marked viscosity decrease with time is observed in solutions stored at constant temperature and 2ero shear. The decrease may be due to changes in polymer conformation. The rheological behavior of pure polyacrylamides over wide concentration ranges has been reviewed (5). 

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