Nucleation polymer blend phase separation

Zhang X, Man X, Han CC, Yan D. Nucleation induced by phase separation in the interface of polyolefin blend. Polymer... 

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. 

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).

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].

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. 

Fig. 6.6 Schemes of two possible mechanisms whereby surface chemistry templates can guide the phase separation in polymer blend films. In both cases, the substrate is predominately one stnface chemistry—light green areas—with a second surface chemistry covering a minority of the substrate—black areas. In (a)-(e), the film vertically phase separates as induced by the majority surface chemistry, and then the minority surface chemistry induces dewetting of the lower polymer. In (f)-(j), the minority surface chemistry nucleates phase separation at precise locations before spontaneous nucleation can occur [6, 14]...

The adjoining Fig. 6.11b plots the probability of successful nucleation as a ftmetion of template feature size as compiled from Fig. 6.11a and similar data sets. Several observations can be made from this array of data. First, a minimum template feature size of 50-200 nm is necessary to have a chance of nucleating phase separation, and the probability of successful nucleation is statistical in nature. Second, the probability of successful nucleation and the size of the induced features depend on the polymer blend ratio. While this data set was only acquired for one type of polymer blend film that follows a nucleated phase separation schemed in Fig. 6.6a-e, the size effects and the statistical nature... 

The interplay of phase separation and polymer crystallization in the multi-component systems influences not only the thermodynamics of phase transitions, but also their kinetics. This provides an opportunity to tune the complex morphology of multi-phase structures via the interplay. In the following, we further introduce three aspects of theoretical and simulation progresses enhanced phase separation in the blends containing crystallizable polymers accelerated crystal nucleation separately in the bulk phase of concentrated solutions, at interfaces of immiscible blends and of solutions, and in single-chain systems and interplay in diblock copolymers. In the end, we introduce the implication of interplay in understanding biological systems. 

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. 

The application of the dynamic SCF theory [97] or EPD [29,31,109] to the collective dynamics of concentration fluctuations and the relation between the dynamics of collective concentration fluctuations and the single chain dynamics is an additional, practically important aspect. We have merely illustrated the simplest possible case—the early stages of spontaneous phase separation within purely diffusive dynamics. In applications the hydrodynamic effects [110,111], shear and viscoelasticity [112] might become important. Even deceptively simple situations—like nucleation phenomena in binary polymer blends—still pose challenging questions [113]. Also the assumption of local equilibrium for the chain conformations, which allows us to use the SCE free energy functional, has to be questioned critically. Methods have been devised to incorporate some of these complications [76,96,99, 111, 112] but the development in this area is still in its early stages. 

The area below the spinodal curve is the region of absolute instability of a polymer blend. The phase separation in this region is controlled by a spinodal mechanism. The region between spinodal and binodal curves is called the metastable region. Phase separation in this region is controlled by a nucleation mechanism. 

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