Polymer metastable blends

Polymer blend that exhibits metastable miscibility. 

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. 

PCA of proteins suspended in an organic solvent with dissolved polymer PCA to form metastable polymer blends Hydrophilic silica dispersions... 

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... 

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. 

Polymer crystallization has been described in the framework of a phase field free energy pertaining to a crystal order parameter in which = 0 defines the melt and assumes finite values close to unity in the metastable crystal phase, but = 1 at the equilibrium limit (23-25). The crystal phase order parameter (xj/) may be defined as the ratio of the lamellar thickness (f) to the lamellar thickness of a perfect polymer crystal (P), i.e., xlr = l/P, and thus it represents the linear crystallinity, that is, the crystallinity in one dimension. The free energy density of a polymer blend containing one crystalline component may be expressed as... 

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... 

In the course of blending polymers, the following systems can be formed one-phase systems, two-phase (colloid) systems, or systems in a metastable state of transition from a one-phase into a two-phase system. The properties of polymer mixtures are determined to a great extent by the phase equilibrium in the system formed and their properties can be changed by controUing the processes of phase separation, which occur hy two mechanisms hy nucleation and growth or by the spinodal mechanism. 

The line on the temperature vs. composition phase diagram for a mixture of two components, which separates the metastable region from the single-phase regions. Hence, it represents the limits of stability in a two-phase system, viz., a polymer solution or polymer blend. 

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]. 

Mawson, S., Kanakia, S., and Johnston, K.P. (1997) Metastable polymer blends by precipitation with a compressed fluid antisolvent. Polymer 38, 2957-2967. 

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