Microrheology of Polymer Blends

The structure and morphology of immiscible blends depends on many factors among which the flow history and the interfacial properties are the most important. At high dilution, and at low flow rates the morphology of polymer blends is controlled by three dimensionless microrheologi-cal parameters (i) the viscosity ratio, where r j is the viscosity of the dispersed liquid and r 2 that of the matrix (ii) the capillarity number, k = d / Vj2, where d... 

Flow of emulsions provides the best model for polymer blends, where the viscosity of both polymers is comparable. The microrheology of emulsions provides the best, predictive approach to morphological changes that take place during flow of polymer blends. The effect of emulsifiers on the drop size and its stability in emulsions has direct equivalence in the compatibilization effects in polymer blends. 

Table 7.6 provides a partial reference to studies on the effects of flow on the morphology of polymer blends [Lohfink, 1990 Walling, 1995]. Dispersed phase morphology development has been mainly studied in a capillary flow. To explain the fibrillation processes, not only the viscosity ratio, but also the elasticity effects and the interfacial properties had to be considered. In agreement with the microrheology of Newtonian systems, an upper bound for the viscosity ratio, X, has also been reported for polymer blends — above certain value of X (which could be significantly larger than the... 

To support these new tendencies the research community has been asked to provide better predictive methods for the multicomponent blends as well as improved sensors for the closed-loop process control. In particular, the evolution of morphology during the compounding and processing steps is of paramount importance. Microrheology and coalescence are the keys to describing the structure evolution of polymer blends. 

The orientation of particles in flow is of particular interest to microrheology. To predict the macroscopic rheological properties of a multiphase system, a detailed description of each phase behavior is required. In this field, contributions from the Pulp and Paper Research Institute of Canada by Mason et al. and later by van de Ven and his coauthors are particularly valuable. The earlier results were summarized by Goldsmith and Mason (1967), the latter by Van de Ven (1989). The microrheology has been particularly well developed for infinitely dilute systems in Newtonian matrix - either solid particles or liquid drops. In the present part, only the former system will be summarized. More extensive discussion of microrheology of the liquid-liquid systems will be presented later, while considering the rheological behavior of polymer blends. 

The microrheology discussed in Section 2.1.2.3 describes the breakup of isolated drops in a Newtonian system. The mechanisms leading to deformation and breakup take into account the three principal variables the viscosity ratio (X), critical capillary number (Kcni), and the reduced time (f ), defined in Eq. (2.19). For application of microrheology to polymer blends the theories developed for Newtonian emulsions need to be extended to viscoelastic systems in the fidl range of composition, that is, they must take into account coalescence. Since the microrheology evolution up to about the year 2000 has been summarized by Utradri and Kamal [3] the following text win focus on more recent developments. 

The dynamic behavior of polymer blends under low strain has been theoretically treated from the perspective of microrheology. Table 2.3 lists a summary of this approach. These models well describe the experimental data within the range of stresses and concentrations where neither drop-breakup nor coalescence takes place. The two latter models yield similar predictions as that of Palierne. The last entry in the Table 2.3 is an empirical modification of Palieme s model by replacement of the volume fraction of dispersed phase by its efiective quantity (Eq. (2.24)), which extends the applicability of the relation up to 0 < 0.449. However, at these high concentrations the drop-drop interactions absent in the Palierne model must complicate the flow and coalescence is expected. The practical solution to the latter problem is compatibilization, but the presence of the third component in blends has not been treated theoretically. 

At low concentration of a second polymer, blends have dispersed-phase morphology of a matrix and discrete second phase. As the concentration increases, at the percolation threshold volume fraction of the dispersed phase, (f>c 0.16, the blends structure changes into co-continuous. Maximum co-continuity is achieved at the phase inversion concentration, (py. The morphology as well as the level of stress leads to different viscosity-composition dependencies. The deformation and dispersion processes are best described by microrheology, using the three dimensionless parameters the viscosity ratio (2), the capillarity number (k), and the reduced time (f ), respectively (Taylor 1932) ... 

Reigden-Stolk C V D and Sara A (1986) A study on polymer blending microrheology Part 3 Deformation of newtonian drops submerged in another newtonian fluid flowing through a converging cone, Polym Eng Sci 26 1229-1239. 

Van der Reijden-Stolk, C. and A. Sara. 1986. A Study on Polymer Blending Microrheology Part 3 Deformation of Newtonian Drops Submerged in Another Newtonian Fluid Flowing Through a Converging Cone. Polym. Eng. ScL, 26(18), 1229-1239. 

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