Rheology of Emulsions and Immiscible Blends

In Sections 9.2.1 and 9.2.2, we considered two basic processes by which emulsions and immiscible polymer blends are formulated—namely, by quenching from a one-phase state 

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Rheology of Emulsions and Immiscible Blends 413 9.3.2 Linear Viscoelasticity... 

Fang et al. (2005) studied the thermal and rheological properties of two types of m-LLDPEs, two LDPEs, and their blends. The C2+6 m-LLDPE-1 was immiscible, whereas the C2+8 m-LLDPE-2 was miscible with the LDPEs, indicating that increasing the length of SCB in m-LLDPEs promoted miscibility with LDPE. The Palieme (1990, 1991) emulsion model provided good predictions of the linear viscoelastic behavior for both miscible and immiscible blends. The low-frequency data showed an influence of the interfacial tension on the elastic modulus of the blends for the immiscible blends. 

Most polymer blends are immiscible. Their flow is complex not only due to the presence of several phases having different rheological properties (as it will be demonstrated later, even in blends of two polymers the third phase, the interphase, must be taken into account), but also due to strain sensitivity of blend morphology. Such complexity of flow behavior can be best put in perspective by comparing it to flow of better understood systems, suspensions, emulsions, and block copolymers. 

Effects of addition of a compatibilizing block copolymer, poly(styrene-b-methyl methacrylate), P(S-b-MMA) on the rheological behavior of an immiscible blend of PS with SAN were studied by dynamic mechanical spectroscopy [Gleisner et al., 1994]. Upon addition of the compatibilizer, the average diameter of PS particles decreased from d = 400 to 120 nm. The data were analyzed using weighted relaxation-time spectra. A modified emulsion model, originally proposed by Choi and Schowalter [1975], made it possible to correlate the particle size and the interfacial tension coefficient with the compatibilizer concentration. It was reported that the particle size reduction and the reduction of occur at different block-copolymer concentrations. 

The rheology of pol5rmer blends is discussed in detail in Chap. 7, Rheology of Pol5rmer Alloys and Blends . Here only an outline will be given. Since the flow of blends is complex, it is useful to refer to a simpler system, e.g., for miscible blends to solutions or a mixture of polymer fractions, for immiscible blends to suspensions or emulsions, and to compatibihzed blends to block copolymers (Utracki 1995 Utracki 2011). It is important to remember that the flow behavior of a multiphase system should be determined at a constant stress, not at a constant deformation rate. 

Structure and properties for binary blends of PLA and PBS are studied both in the solid and molten states. It is foimd that PLA and PBS are immiscible in the molten state and the blends exhibit phase-separated structure. The interfacial tension between PLA and PBS is estimated using a rheological emulsion model proposed by Palieme and foimd to be 3.5 mN/m as shown in Figs. 4.29,4.30 and 4.31. Basic theological parameters are also evaluated for PLA and PBS. It is suggested that the entanglement molecular weight of PLA is lower than that of PBS. 

The results reported in this work on some selected polymer blends demonstrate that the melt rheology of multiphase systems is very complex, even in the linear viscoelastic domain. The main features exhibited by molten immiscible polymer blends are an increase of elasticity at low frequencies and longer relaxation times compared with that of the matrix. The linear viscoelastic properties of blends are satisfactorily described by the Palierne emulsion model and the enhancement of elasticity is ascribed to the deformability of the minor phase s droplets. 

Blends of immiscible polymers form liquid phases if the temperature is higher than the glass transition points (and/or melting points) of the components. Such blends behave as emulsions, and the thermodynamic interaction between the components is often quantified by a phenomenological parameter, the interfacial tension T, given that the components are isotropic and the interface is sharp compared with the phases. (Blends of lightly crosslinked mbbers in liquid matrices can be also classified as emulsions.) The stress Oim due to the interface between different phases is directly related to T (cf. eqn [19]) and the relaxation of Oint is slower than the relaxation of individual component chains. Thus, extensive rheological studies have been made for such emulsion-type blends,... 

With the knowledge of the flow behavior of simpler systems, viz. suspensions, emulsions, block copolymers, as well as that of the mumal interactions between the rheology and thermodynamics near the phase separation, one may consider the flow of more complex systems where all these elements may play a role. Evidently, any constitutive equation that may attempt to describe flow of immiscible polymer blends should combine three elements the stress-induced effects on the concentration gradient an orientation function and the stress-strain description of the systems, including the flow-generated morphology. Such a comprehensive description stiU remains to be formulated. 

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]. 

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