Miscibility and Phase Behavior of Polymer Blends

In order to investigate miscibility and phase behavior of polymer blends, differential scanning calorimetry (DSC) has been frequently used for determination of glass transition temperature, crystalline melting temperature and other thermal properties. 

Most miscible polymer blends are amorphous [56-58]. A miscible polymer blend possesses a homogeneous amorphous phase and hence will exhibit a single glass transition temperature (T, ) between the T s of the components. 

Several theoretical and empirical equations have been used to describe the Tj,-composition dependence of miscible blends. The two frequently used equations are the Fox equation [equation (4.44)] and the Gordon-Taylor equation [equation (4.45)]  

In an amorphous polymer blend system, we can determine the phase boundary curve, i.e. a temperature versus composition phase diagram, by using DSC to follow the appearance of two separate T s at a certain annealing temperature. The phase diagram can also be obtained simply by optical observation. The temperature at which the first faint opalescence appears on heating is designated the cloud point. 

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For those cases with weak to moderate specific interactions, a guide for assessing the miscibility and phase behavior of polymer blends is also presented based on classical thermodynamics utilizing a method of balancing the specific interactions and the negative dispersive forces [Coleman et al., 1990, 1991]. This approach successfully predicts the window of miscibility of copolymers with... 

Since, ultimately, the properties of a polymer blend will depend on the final morphology, various research groups have recently undertaken extensive studies of the miscibility and phase behavior of polymer blends. In practice, the physical properties of interest are found either by miscible pairs or by a heterogeneous system, depending on the type of application. Generally, polymer blends can be completely miscible, partially miscible or immiscible, depending on the value of AG [4]. 

Other polymers as the copolymer monomer ratio is varied. The specific rejection model also successfully predicts the miscibility and phase behavior of similar blends. Obviously, there is a thread of similarity between these apparently diverse approaches. 

This article reviews the phase behavior of polymer blends with special emphasis on blends of random copolymers. Thermodynamic issues are considered and then experimental results on miscibility and phase separation are summarized. Section 3 deals with characteristic features of both the liquid-liquid phase separation process and the reverse phenomenon of phase dissolution in blends. This also involves morphology control by definite phase decomposition. In Sect. 4 attention will be focused on flow-induced phase changes in polymer blends. Experimental results and theoretical approaches are outlined. 

Zheng S, Lu H (2003) Miscibility and phase behavior in thermosetting blends of polybenzoxazine and poly(ethylene oxide). Polymer 44 4689-4698... 

FIGURE 1.1 Phase behavior of polymer blends with the upper and the lower critical solution temperature UCST and LCST (A) single-phase miscible region between two binodals, (B) two-phase separated regions of immiscibiUty, surrounded by spinodals, (C) four fragmented metastable regions between binodals and spinodals. (Adapted from Ougizawa Toshiaki and Inoue Takashi. Polym. J., 18, no. 7, 521-527, 1986.)... 

Rabeony et al 1998, Effect of pressure on polymer blend miscibility A temperature-pressure superposition. Macromolecules, Vol. 31, No. 19, PP. 6511-6514 Rodgers, 1991, Procedure for predicting lower critical solution temperature behavior in binary blends of polymers. Macromolecules, Vol. 24, No. 14, PP. 4101-4109 Rudolf Cantow, 1995, Description of phase-behavior of polymer blends by different equation-of-state theories. 2. excess volumes and influence of pressure on miscibility. Macromolecules, Vol. 28, No. 19, PP. 6595-6599... 

In the preceding chapter we have, based on the Flory-Huggins theory, discussed the basis for the phase behavior of polymer blends. Miscible polymer blends and polymer solutions have, even in the mixed one-phase system, spatial variations in the polymer concentration. These concentration fluctuations reflect the thermodynamic parameters of the free energy, as described in the Flory-Huggins model. 

Miscibility and phase behavior of poly(D,L-lactide)/poly(p-vinylphenol) blends./. Appl. Polym. Sci., 70, 811-816. 

There are several criteria for determining the miscibility in polymer blends. The thermodynamic criteria date back to the seminal studies of Flory (1941, 1942) and Huggins (1941, 1942) in the 1940s. Since then, other thermodynamic theories have been developed (Flory 1965 Flory et al. 1968 Orwoll and Flory 1967 Sanchez and Lacombe 1978) for investigating the phase behavior of polymer blends. As the discussion of thermodydnamic criteria for miscibility in polymer pairs is beyond the scope of this chapter, here we illustrate briefly different phase behaviors of polymer blends in general terms, using examples that will facilitate our presentation in the rest of this chapter. 

We have presented the rheological behavior of several miscible blend systems. In doing so, we discussed the phase behavior of polymer blends in general as determined primarily from DSC and/or cloud point measurement, although other experimental techniques employed are also mentioned or referred to. Since there are many miscible polymer blend systems, we had to select only representative miscible polymer blend systems for the reason of limited space available. Although we have presented the rheological... 

Ternary Blends. Discussion of polymer blends is typically limited to those containing only two different components. Of course, inclusion of additional components may be useful in formulating commercial products. The recent Hterature describes the theoretical treatment and experimental studies of the phase behavior of ternary blends (10,21). The most commonly studied ternary mixtures are those where two of the binary pairs are miscible, but the third pair is not. There are limited regions where such ternary mixtures exhibit one phase. A few cases have been examined where all three binary pairs are miscible however, theoretically this does not always ensure homogeneous ternary mixtures (10,21). 

Polymer miscibihty has been the subject of numerous studies. Pressure is a thermodynamic parameter that can affect the phase behavior of polymer mixture and that can be used to enhance the miscibility of polymers. This properly may have an important apphcation in controlhng microstructure. The phase behavior of blends and block copolymers under pressure has recently received significant attention 1-3). 

In discussing the elongational flow behavior of polymer blends, the difference between single-phase (homopolymer or miscible blend) and multiphase (antagonistically immiscible PE/PS blend) was demonstrated in Eigures 2.12 and 2.13,... 

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