Growth polymer blend phase separation

In general, a block copolymer added to immiscible polymer blend significantly suppresses the growth rate of phase-separated domains due to the reduction of interfacial tension resulting from a preferential localization of block copolymer at the interface. However, the retardation effect by the block copolymer is found to be dependent upon the structure of the block copolymer added, such as the interaction energy, the chain length, and the composition of block copolymer. 

An important question in modern polymer science relates to the mechanism by which polymer-polymer solutions phase separate on crossing their critical solution temperatures or compositions (see also Volume 2 Chapter 4). Blends of two high molecular weight polymers usually produce lower critical solution temperatures, LCST s, which means that combinations of two polymers may be miscible at some lower temperature, but phase separate at a higher temperature." This is a direct result of the very small entropy of mixing of two polymers/ Two types of phase separation are known, nucleation and growth, and spinodal decomposition. 

Conflicting results have been found for the explicit time evolution of the correlation length during isothermal phase separation. A 1/3-power law in the growth of patterns, which is characteristic for the hydrodynamically controlled Lifshitz Slyozov process, was confirmed in Ref. [99] while an exponential increase over a certain period of time was established in Ref. [21]. Nevertheless, it is evidenced that in blends comprising liquid-crystalline polymers spinodal decomposition and subsequent coarsening processes take a course similarly to isotropic liquid mixtures. 

Figure 1.31. The development of the characteristic morphology of polymer blends that have phase-separated by nucleation and growth compared with the case of spinodal decomposition. The local fluctuations in concentration leading to phase separation are also shown. Adapted from Olabisi (1979).

The reduction of the interfacial tension in the melt results in the formation of lower-diameter particles as phase separation occurs. These are also stabilized by the compatibilizer and resist growth during subsequent thermal treatment such as annealing. Thirdly, the enhanced interfacial adhesion results in mechanical continuity such as stress transfer between the phases. This is a fundamental requirement for a useful polymer blend. 

An alternative, and interesting, possibility is to introduce a phase-separating blend as the fluid component (a schematic of this system is shown in Fig. 11). The phase-separating A-B polymer blend will evolve, and phase separate, at its own length- and timescales. However, to minimize the interface between the A and B domains of the polymer blend it may be desirable for the length-scale of phase separation to conform to the wavelength of undulation growth found in... 

The discussion on the crystallization behavior of neat polymers would be expected to be applicable to immiscible polymer blends, where the crystallization takes place within domains of nearly neat component, largely unaffected by the presence of other polymers. However, although both phases are physically separated, they can exert a profound influence on each other. The presence of the second component can disturb the normal crystallization process, thus influencing crystallization kinetics, spherulite growth rate, semicrystalline morphology, etc. 

Figure 17.6. Generalized comparison of phase separated blends comprising an electrically conducting polymer as a minor constituent. Normal nucleation and growth yields non-conductive blends micellar-type morphology yields electrical conductivity [Heeger, 1993].

Phase separation in binary alloys, polymer blends, and fluid mixtures has been studied intensively for many years [148]. It takes place when a two-component mixture is quenched from a disordered state into a two-phase coexistence region. After such a quench, composition fluctuations are created that grow and form domain structures. In the late stages, the domain structure coarsens in time, a process that is driven by the interfacial tension coexisting phases. The growth of the characteristic length scale R t) follows a power-law behavior,... 

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 scientific literature on crystallization in polymer blends clearly indicates that the crystallization behavior and the semicrystalline morphology of a polymer are significantly modified by the presence of the second component even when both phases are physically separated due to their immiscibility. The presence of the second component, either in the molten or solid state, can affect both nucleation and crystal growth of the crystallizing polymer. The effect of blending on the overall crystallization rate is the net combined effect on nucleation and growth. 

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