Molecular Zero-shear viscosity, effect

The effect of overall molecular weight or the number of blocks on rheological properties for the samples from the second fractionation can be illustrated as a plot of reduced viscosity vs. a function proportional to the principal molecular relaxation time (Figure 2). This function includes the variables of zero shear viscosity, shear rate, y, and absolute temperature, T, in addition to molecular weight, and allows the data to be expressed as a single master curve (10). All but one of the fractions from the copolymer containing 50% polystyrene fall on this... 

As shown in Fig. 10.9, the viscosity data points of the F40 and NBS samples as well as those of the other samples are closely on the theoretical curve, without showing a reduction of 30% which occurs to the K values (Fig. 10.5) obtained from the analyses of the G t) curves of the two samples. Due to the modification of the zero-shear viscosity by a narrow molecular-weight distribution as discussed above, the effect on the viscosity from the 30% reduction in K is largely cancelled out by the artificial broadening of the molecular-weight distribution extracted from the G t) line-shape analysis. 

The zero shear viscosity of flexible linear polymers varies experimentally with and theoretically with [20]. Due to the highly restricted rotational diffusion, the viscosity of TLCPs is much more sensitive to the molecular weight than that of ordinary thermoplastics as discussed in section 3. Doi and Edwards predicted that the viscosity of rod-like polymers in semi-dilute solutions scales with A/ [see Equation (12)] [2]. Such a high power dependence of viscosity on the molecular weight has been experimentally observed both for lyotropic LCPs [14,15] and for TLCPs [16-18]. The experimental values of the exponent range from 4 to 7 depending on the chemical structure, the chain stiffness, and the domain or defect structure of the liquid crystalline solution or melt. The anisotropicity of the liquid seems to have little effect on the exponent. A slightly smaller exponent for the nematic phase than for the isotropic phase (6 in the nematic phase versus 6.5 in the isotropic... 

Figure 8.9 The effect of molecular weight on the zero shear viscosity (ZSV) of Thermx LNOOl [5]. 

Similar results are obtained for linear, four and six-branched polyisoprenes at a concentration of 0.145 g/ml. In this case, however, at hi r concentrations more serious deviations occur from theory. The higher molecular weight samples can have zero shear viscosities higher than linear polymers of the same molecular wei t Such behavior was first noted in a study of melt viscosity of regular star-branched polybutadienesViscosities of the order of one hundred times that of a linear equivalent could be observed, but the effect decreased rapidly on dilution with solvents i.e. the viscosities of branched polymers were more sensitive to concentration than those of linear polymers. Star-branched polyisoprenes show viscosity enhance-... 

Figure 6.4 Effect of molecular weight on zero-shear viscosity and elasticity coefficient for PLLA at 200°C (Ferry 1980).

Dutta [27] provided a comparison between experimental and calculated zero-shear viscosities from Eq. (4.12) for 35 different polymer grades covering several polymeric species of widely varying melt flow indices. However, the given plot of % (calculated) versus 110 (measured) on log-log scales of 3 X 3 cycles masks a lot of error in the estimate. MFI is rather insensitive to changes in molecular-weight distribution (MWD) as can be seen from Fig. 4.3 (from Hanson [9]). However, MWD has profound effects on low-shear viscosity behavior. 

In this section, we present the molecular theory for the linear dynamic viscoelasticity of miscible polymer blends by Han and Kim (1989a, 1989b), which is based on the concept of the tube model presented in Chapter 4. Specifically, the reptation of two primitive chains of dissimilar chemical structures under an external potential will be considered, and the expressions for the linear viscoelastic properties of miscible polymer blends will be presented. We will first present the expressions for zero-shear viscosity ob. dynamic storage and loss moduli G co) and G " co), and steady-state compliance J° for binary miscible blends of monodisperse, entangled flexible homopolymers and then consider the effect of polydispersity. There are a few other molecular theories reported... 

Effect of Molecular Weight on the Zero-Shear Viscosity of Disordered Diblock Copolymers... 

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