Miscible versus Immiscible Blends

The phase diagrams for polymer mixtures can take many forms, as noted in Fig. 2.6. where single phase and phase separated regions exist in the composition-temperature range depicted. Many miscible systems (a) exhibit miscibihty over the entire composition-temperature range where both polymers exhibit thermal stabihty. Miscible systems where the level of miscibility is borderline will often show phase separation within the experimentally determined 

Li salt of sulfonated PS Methylated polyamide (poly(N,N -dimethyl sebacamide) lest 62 

To obtain spinodal decomposition exclusively, a minor temperatiue change going through the critical point is required. Spinodal decomposition may occiu if the metastable region is traversed rapidly into the unstable region. One question not well-answered in the hteratiue 

At the early stages of spinodai decomposition, the kinetics of phase separation can be assessed by the formalism of Cahn-Hilliard [74]. The change in the chemical potential in the early stages of spinodai decomposition is expressed by  

At the latter stages of spinodal decomposition (and also nucleation and growth), coarsening of the structure occurs due to interfacial forces(often referred to as Ostwald ripening). The expression of the droplet size-time relationship (Lifshitz-Slyozov expression [77]) is  

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The viscoelastic properties of polymer blends determined by dynamic mechanical analysis to yield E, E" and tand has been reviewed in Section 5.2. The modulus-temperature behavior of polymer blends is a strong function of the phase behavior. In Fig. 6.2, the generalized modulus-temperature behavior of miscible versus immiscible blends is compared for the case of two amorphous polymers with different glass transition temperatures. The phase separated blend exhibits a modulus plateau between the TgS of the components with the plateau position dependent upon the composition. The miscible blends show single Tg behavior, with the Tg position dependent upon the composition. 

Chuang and Han (1984) reported that, for miscible and immiscible blends at constant composition, the plots of Nj versus G12 and G versus G" are independent of T. However, while for single phase systems, the two dependencies are approximately parallel, the steady-state relation may be quite different from the dynamic one for immiscible blends, such as PS/PMMA. 

This chapter has provided a brief presentation of the underlying theories, and the analytical tools and techniques used to characterize polymer blends by X-ray scattering (SAXS and WAXS). The text was not aimed at reviewing the studies conducted with all types of polymer blends - that is, miscible versus immiscible... 

Figure 9.31 Yield energy of the blends versus the propylene-unit content (mol%) in EP copolymer ( ) iPP ( ) miscible blends such as 1PP/EP92 (80/20) blend, iPP/EP89 (80/20) blend, iPP/EP84 (80/ 20) blend (O) immiscible blends such as iPP/EP77 (80/20) blend, iPP/EP70 (80/20) blend, iPP/ EP53 (80/20) blend. (From Reference 37 with permission from Elsevier Ltd.)...

NPT MD simulations are often performed to calculate the volume-temperature properties of the studied system. Figures 6.4 and 6.5 show the specific volume versus temperature plots of miscible poly(hydroxybutyrate) (PHB)/PEO blend (1 2 blends in terms of repeated units) and PHB/PE blend (1 2 blends in terms of repeated units) from the literature [34,36]. The intersect point of the two straight lines segments marks the Tg value. For the miscible PHB/PEO blend, only one Tg (about 258) could be found, and it is a little higher than the experimental values, 230 to 252 K. For the immiscible PHB/PE blend, two Tg (249 K and 187 K) were found. Comparing to the Tg of pure PHB (268 to 278 K) and PE (140 to 170 K), the two transition temperatures correspond to PHB-rich (249 K) and PE-rich domains (187 K) Tg, respectively. 

HDPE immiscibly blends with hutyl ruhher [14] to provide improved chemical resistance, compression set and high-temperatnre mechanical properties versus nnvnlcanised hlends. LDPE and HDPE blend immiscibly with ethylene copolymers to improve environmental stress crack resistance, tonghness, filler acceptance, film tear resistance, improved flexibility and so on. In polyolefin, polybntene-1 forms miscible blends with PP [15,16]. The addition of PP to polybutene-1 increases the crystallisation rate of polyhntene-1 and would have utility as a nucleation additive. 

Figure 8.2 shows a temperature opolymer composition phase diagram for the IR/BR blend, where the 1,2-unit content of BR is regarded as a copolymer composition. In a plot of temperature versus 1,2-unit content of BR, we find a phase boundary between miscible and immiscible states. IR is immiscible with... 

The compressive ultrasonic velocity (6 MHz, at room temperature) was used to study cast blend films of PMMA/PVAc, PMMA/PS, PVC/CR, and PS/EPDM (Singh and Singh 1983 Shaw and Singh 1987). A linear correlation between the sound velocity and the composition was observed for miscible blends, whereas immiscibility, viz., in PMMA/PS blends, the same dependence was irregular. Phase separation in PVC/CR was detected at w = 70 wt% of CR, indicated by a sudden departure from linear correlation. The ultrasonic absorption versus composition gave even stronger evidence of immiscibility. Ultrasonics have been also successfully used to smdy the phase behavior in polyurethanes (Volkova 1981). 

The second dependence is valid when all fractions are either entangled or not. Thus, Hq x) of a miscible blend is a linear combination of the component relaxation spectra and their weight fractions, w, ergo deviation from linearity in plots of log Hq versus Mw/M and log Wmax versus log rjo indicates immiscibility. 

The second dependence in Eq. (2.32) is valid when all fractions are either entangled or not. In consequence, the relaxation spectrum of a miscible polymer blend is a linear combination of the component relaxation spectra and their weight fractions, W(. A strong deviation from linearity in plots of log Hq versus Mw/Afn and log Wmax versus log r]o indicates immiscibility [87, 88]. The principle that in miscible blends polydispersity can be calculated and used to test for system miscibility was extended to other rheological functions sensitive to polydispersity, namely, the power-law exponent (n), the cross-point coordinates (G, o) ), the free volume gradient of viscosity, the initial slope of stress growth function, and so on [3]. 

Several factors are found responsible for why numerous blend systems are not successful. First, the component polymers are usually not miscible with each other due to thermodynamic constraints, for example, lack of solubility and finite inter-fadal tension. Second, immiscible polymer blend preparation is often affected by kinetic constraints, for example, slower rate of deformation of the dispersed polymer and faster rate of coalescence of the droplets. In turn, these rates are directly influenced by the type of flow field, for example, shear versus extensional, strain history, chemical reactions, for example, grafting reactions at polymer-polymer interfaces or polymerization-induced phase separation, and polymer properties, such as viscosity and interfacial tension. Accordingly, the multidisciplinary efforts to analyze, understand, and design polymer blends with improved properties extend from synthesis and characterization to processing and manufacturing. Such efforts... 

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