Metastable miscible polymer blend

Note In polymers, because of the low mobility of polymer chains, particularly in a glassy state, metastable mixtures may exist for indefinite periods of time without phase separation. This has frequently led to confusion when metastable miscible polymer blends are erroneously claimed to be miscible. 

Polymer blend that exhibits metastable miscibility. 

The effect of dissolved CO2 on the miscibility of polymer blends and on phase transitions of block copolymers has been measured with spectroscopy and scattering (40). The shifts in phase diagrams with CO2 pressure can be pronounced. Polymer blends may be trapped kinetically in metastable states before they have time to phase separate. Metastable polymer blends of polycarbonate (PC) and poly(styrene-cn-acrylonitiile) were formed with liquid and supercritical fluid CO2 in the PCA process, without the need for a surfactant. Because of the rapid mass transfer between the CO2 phase and the solution phase, the blends were trapped in a metastable state before they... 

In polymer blends, different phase behaviors can be observed as shown in Figure 27.2. It is possible to calculate the compositions at which the blend will always separate into more than one phase (two-phase region), and also those compositions in which the blend may either form a single phase or will separate into several phases by different mechanisms, either in the metastable or in the spinodal regions. The binodals separate the miscible (one phase) and the metastable regions the metastable region is... 

Most polymer blends phase-separate with LCST thus the miscibility region stretches from the melting point or Tg up to the binodal while the phase-separated region exists above the spinodal temperature, Ts [1, 212]. Within the region between binodal and spinodal the system is metastable, characterized by strong interrelation between the rheology and thermodynamics [211, 213-215]. 

Partial miscibility Both miscible and immiscible polymer blends that also exhibit macroscopically uniform physical properties usually caused by sufficiently strong interactions between the components of the mixture. The partially miscible blends show either a homogeneous single phase or a heterogeneous phase, depending on certain conditions. This is called metastable miscibility. Sometimes, they may be only partially miscible yet may remain unchanged for an indefinite period. In some situations, however, phase separation of partially miscible systems may occur by activated nucleation. 

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.)... 

Miscibility in polymer blends is controlled by thermodynamic factors such as the polymer-polymer interaction parameter [8,9], the combinatorial entropy [10,11], polymer-solvent interactions [12,13] and the "free volume effect [14,15] in addition to kinetic factors such as the blending protocol, including the evaporation rate of the solvent and the drying conditions of the samples. If the blends appear to be miscible under the given preparation conditions, as is the ca.se for the blends dcscibcd here, it is important to investigate the reversibility of phase separation since the apparent one-phase state may be only metastable. To obtain reliable information about miscibility in these blends, the miscibility behavior was studied in the presence and absence of solvents under conditions which included a reversibility of pha.se separation. An equilibrium phase boundary was then obtained for the binary blend systems by extrapolating to zero solvent concentration. 

Miscibility of polymer blends has been often defined as the capability of a mixture to form a single phase over certain ranges of temperature, pressure and composition. Whether or not a single phase exists depends on the chemical structure, molar mass distribution and molecular architecture of the components present. For a two-component mixture, a necessary and sufficient condition for stable or metastable equilibrium of a homogeneous, single-phase is... 

Recently, combinations of polymers meeting the basic concept of molecular composites have been reported. Poly-(p-phenylene terephthalamide) miscibility with PA-6 and PA-66 was reported [Kyu et al., 1989]. The blends were prepared by rapid coagulation of methane sulfonic acid solutions in water. Above 70% of the rigid rod polymer, polyamide crystallization disappears implying a level of intermixing of the blend constituents. However, thermal treatment results in phase separation thus indicating metastability for this combination. 

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