Nucleation phase separation

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... 

Figure 1. Representation of a noncovalent network of random coils crosslinked by interchain association (a) or by microcrystalline domains (b) non-covalent network of rod-like polymers or polymer aggregates whose "crosslinking" is a manifestation of a brush-pile of rods (c) or fibers (aggregates of aligned rods) (d), or a result of non-nucleated phase separation kinetics (e).

Strnad Zdenek (1939-) Czech chem., expert in glass, inventor of bioactive dental implants (book, Glassceramics nucleation, phase-separation and crystalization 1986) Strouhal Cenek (Vincenc) (1850-1922) Czech phys., first professor of experimental physics to the Czech Technical Univ., known for work in acoustics (Strouhal s eddy pitch) and thernodynamics (book Thermics 1906) Strutt (baron Rayleigh) John William (1842-1919) Brit, math., theory of sound, dynamics and resonance of elastic bodies, contributor to optics, acoustics and electricity, hydrodynamics, (Rayleigh number named after him)... 

Thermodynamics and kinetics of phase separation of polymer mixtures have benefited greatly from theories of spinodal decomposition and of classical nucleation. In fact, the best documented tests of the theory of spinodal decomposition have been performed on polymer mixtures. 

The validity of mean field theory for N —y oo has striking consequences for the initial stages of phase separation. " In a metastable state slightly inside the coexistence curve, the nucleation free energy barrier is due to spherical droplets with a radius R The free energy excess of a droplet is written in terms of bulk and surface terms " "... 

Keywords PCM phase change latent heat melting heat storage cold storage corrosion phase separation incongruent melting subcooling nucleator products. 

Direct evidence of nucleation during the induction period will also solve a recent argument within the field of polymer science as to whether the mechanism of the induction of polymers is related to the nucleation process or to the phase separation process (including spinodal decomposition). The latter was proposed by Imai et al. based on SAXS observation of so-called cold crystallization from the quenched glass (amorphous state) of polyethylene terephthalate) (PET) [19]. They supposed that the latter mechanism could be expanded to the usual melt crystallization, but there is no experimental support for the supposition. Our results will confirm that the nucleation mechanism is correct, in the case of melt crystallization. 

The answer to the argument as to whether the mechanism of the induction of polymers is related to the nucleation process (as predicted in CNT [1-4]) or to the phase separation process [19,32] is that the nucleation process is correct in the case of melt crystallization. 

On the basis of the concept described above, we propose a model for the homogeneous crystallization mechanism of one component polymers, which is schematically shown in Fig. 31. When the crystallization temperature is in the coexistence region above the binodal temperature Tb, crystal nucleation occurs directly from the melt, which is the well-known mechanism of polymer crystal nucleation. However, the rate of crystallization from the coexistence region is considered to be extremely slow, resulting in single crystals in the melt matrix. Crystallization at a greater rate always involves phase separation the quench below Tb causes phase separations. The most popular case... 

Composition range 30-80% Rh. In this composition range phase separation occurs, and the structure of such Pd-Rh alloy films has been reviewed (Section II). Phase I varied in composition and phase II contained 88 5% Rh. It was proposed that these results could be explained by the preferential nucleation of rhodium so that the crystallites consisted of a phase II kernel surrounded by an outer shell (phase I), the Rh content of which increased with an overall increase in the Rh content of the alloy film. Note the essential difference to the Cu-Ni films (38, 33) discussed in Section IV.A where complete separation into two phases of fixed equilibrium composition is envisaged, and over a wide composition range the crystallite surfaces have the same composition. 

Explicit forms for the stress tensors d1 are deduced from the microscopic expressions for the component stress tensors and from the scheme of the total stress devision between the components [164]. Within this model almost all essential features of the viscoelastic phase separation observable experimentally can be reproduced [165] (see Fig. 20) existence of a frozen period after the quench nucleation of the less viscous phase in a droplet pattern the volume shrinking of the more viscous phase transient formation of the bicontinuous network structure phase inversion in the final stage. 

The decomposition of a solution with composition outside the spinodal region but within the metastable region can be analyzed in a similar way. Let us assume that a sample with composition in this region is cooled to low temperatures. Small fluctuations in composition now initially lead to an increase in the Gibbs energy and the separation of the original homogeneous solution must occur by nucleation of a new phase. The formation of this phase is thermally activated. Two solutions with different composition appear, but in this case the composition of the nucleated phase is well defined at all times and only the relative amount of the two phases varies with time. 

Careful cooling of pure water at atmospheric pressure can result in water that is able to remain liquid to at least 38 °C below its normal freezing point (0 °C) without crystallizing. This supercooled water is metastable and will crystallize rapidly upon being disturbed. The lower the temperature of the supercooled water, the more likely that ice will nucleate. Bulk water can be supercooled to about — 38 °C (Ball, 2001 Chaplin, 2004). By increasing the pressure to about 210 MPa, liquid water may be supercooled to — 92 °C (Chaplin, 2004). A second critical point (C ) has been hypothesized (Tc = 220 K and Pc = 100 MPa), below which the supercooled liquid phase separates into two distinct liquid phases a low-density liquid (LDL) phase and a high-density liquid (HDL) phase (Mishima and Stanley, 1998 Poole et al., 1992 Stanley et al., 2000). Water near the hypothesized second critical point is a fluctuating mixture of LDL and HDL phases. 

A kinetic model for single-phase polymerizations— that is, reactions where because of the similarity of structure the polymer grows as a solid-state solution in the monomer crystal without phase separation—has been proposed by Baughman [294] to explain the experimental behavior observed in the temperature- or light-induced polymerization of substimted diacetylenes R—C=C—C=C—R. The basic feature of the model is that the rate constant for nucleation is assumed to depend on the fraction of converted monomer x(f) and is not constant like it is assumed in the Avrami model discussed above. The rate of the solid-state polymerization is given by... 

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