Polymer rheology viscosity ratio

If the rheology of suspensions and emulsions is difficult to describe theoretically and to determine experimentally, in the case of polymer blends these difficulties reach another order of complication. It suffices to note that in blends both phases are viscoelastic, the viscosity ratio varies over a wide range, and morphology can be very complex. To understand the rheological behavior of blends, it is useful to refer to simpler systems that can offer important insight. The following systems (Table 7.2) are commonly considered and will be treated in the following discussion. 

Rameshwaram et al. [6] investigated the structure-property relationships and the effects of a viscosity ratio on the rheological properties of polymer blends using oscillatory and steady shear rheometry and optical microscopy. 

By analyzing data of Figure 6, it was found that PHB forms a continuous matrix in the molten polymer at any ratio. The experimental values of the viscosity are close to the bottom theoretical curve, which corresponds to the calculation for the case of the formation of the matrix of PHB. Thus, during the melting of PHB observed phase inversion phenomenon in accordance with the laws of more fluid melt [4, 6] PHB forms a continuous phase in the entire range of concentrations. Basic rheological parameters... 

The shear viscosity in polymer melts and solutions has been investigated for more than a half century. The blending process of polymers is generally performed in the molten state, particularly in shear flow. It is thus necessary to understand the flow (or better rheological) characteristics of polymer melts [66-70]. In both reactive and non-reactive blends, the shear viscosity of each polymer, and thus viscosity ratio, and bulk viscosity of polymer blend system needs to be understood. 

Interfacial instability in a number of coextruded polymer systems has been experimentally correlated with viscosity ratios and elasticity ratios (25), and a simplified rheology review has been given (26). 

The amount of LC added, processing condition, viscosity ratio of the blend components and the rheological characteristics of the matrix decide the size, shape, and distribution of the LC polymer domains in the matrix. Appropriate shear forces are required to obtain good mixing and extensional flow to deform the dispersed LC domains into fibrillar shape. The orientation of LC polymer microfibrils can be increased by drawing, which in turn, improves the mechanical properties of the blend. Discussion of blends processing will follow with several examples. 

Rheology is a part of continuum mechanics that assumes continuity, homogeneity and isotropy. In multiphase systems, there is a discontinuity of material properties across the interface, a concentration gradient, and inter-dependence between the flow field and morphology. The flow behavior of blends is complex, caused by viscoelasticity of the phases, the viscosity ratio, A (that varies over a wide range), as well as diverse and variable morphology. To understand the flow behavior of polymer blends, it is beneficial to refer to simpler models — for miscible blends to solutions and mixtures of fractions, while for immiscible systems to emulsions, block copolymers, and suspensions [1,24]. 

A considerable amount of experimental results on the free motion of bubbles and drops in quiescent non-Newtonian media is available in the literature. In most cases, the two-parameter power-law model has been used to model the ambient liquid rheology. It is worthwhile to mention here that in the bulk of the work dealing with the free fall of liquid drops in polymer solutions, the ratio of the viscosity of the dispersed phase to that of the continuous phase is in the range lO -lO" and, hence, these drops may effectively be treated as gas bubbles. At this juncture, it is also important to recall that bubbles in the pretransition region (prior to the abrupt change) behave more like solid spheres and, hence, the drag under these conditions is approximated... 

Finally, Persson and Bertilsson [75] studied aluminum borate whiskers dispersed in PE/PIB of PA/SAN polymer blends. The rheological (viscosity) and morphological (SEM) observations led them to conclude that the effects of viscosity ratio dominated the particle distribution when the interfacial tension between the two polymers was not too high. Otherwise, the thermodynamic interactions were predominant [76]. 

Liebscher M, Tzounis L, Pbtschke P, Heinrich G. Influence of the viscosity ratio in PC/SAN blends filled with MWCNTs on the morphological, electrical, and melt rheological properties. Polymer 2013 54 6801-8. 

We feel confident that given the dual benefit of reinforcement and processability improvement, there are high value niches which would be best filled by an LCP blend, particularly a relatively low-cost variant. The potential low cost and unusual rheology of HIQ polymers opens up interesting scientific and commercial possibilities. It would give an opportunity to control viscosity ratio in a novel way. It could be used either alone, or blended with Vectra, since the two seem to be miscible. We would like to see a resurgence of interest in LCP blends and the introduction of commercially successful in situ composite products after all these years. 

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