Homogeneous Nucleation and Fractionated Crystallization

The homogeneous nucleation phenomenon was first studied by droplet crystallization experiments performed on metals [57-60], alkanes [61] and polymers [62-66] when dispersed in inert low molecular weight media. The idea was that when the polymer in the bulk is subdivided into a number of droplets that is larger than the number of active heterogeneities present in the polymer, there should be a certain number of droplets without any active heterogeneity. 

The preparation of immiscible polymer blends is another way to disperse a bulk polymer into fine droplets. It has been reported for several polymers that when they are dispersed in immiscible matrices into droplets with average sizes of around 1 im, they usually exhibit multiple crystallization exotherms in a differential scanning calorimetry (DSC) cooling scan from the melt (at a specific rate, e.g., 10 °Cmin 1). Frensch et al. [67] coined the term fractionated crystallization to indicate the difference exhibited by the bulk polymer, which crystallizes into a single exotherm, in comparison with one dispersed in a large number of droplets, whose crystallization is fractionated temperature-wise during cooling from the melt. 

In order to illustrate the fractionated crystallization behavior we will present here previous results on immiscible atactic PS and isotactic polypropylene blends (iPP) [68]. The cooling behavior of PS, iPP and an 80/20 PS/iPP blend is presented in Fig. 1, as well as that of an unmixed blend , labeled 

The melt mixed 80/20 PS/iPP blend displays a set of exotherms, where the amount of the iPP component that was heterogeneously nucleated is substantially reduced as indicated by the decrease of the crystallization enthalpy in the temperature region where the iPP crystallizes in bulk, i.e., at 109-111 °C (exotherm labeled A). This effect is due to the confinement of iPP into a large number of droplets. If the number of droplets of iPP as a dispersed phase is greater than the number of heterogeneities present in the system, fractionated crystallization occurs. The number of droplets for this composition is known (by scanning electron microscopy observations) to be of the order of 1011 particles cm-3 and polarized optical microscopy (POM) experiments have shown that this iPP contains approximately 9 x 106 heterogeneities cm 3. In fact, it can be seen in Fig. 1 that the fractionated crystallization of the iPP compon- 

It should be pointed out that another possible source of nucleation is the interface between the two phases under consideration. In some of the early droplet works, the authors found that the greatest supercooling needed for the crystallization of a certain droplet population was dependent on the superficial characteristics of the droplets [60,62,66]. 

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Muller AJ, Balsamo V, Arnal ML, Jakob T, SchmalzH, Abetz V. Homogeneous nucleation and fractionated crystallization in block copolymers. Macromolecules 2002 35(8) 3048-58. 

This section has shown examples of how through MD isolation and confinement, which typically results from the self-assembly of block copolymers, crystalline phases can change their nucleation behavior from heterogeneous to superficial, or homogeneous nucleation and their crystallization kinetics can also change from a complex process to a simple first-order process dominated by nucleation. Intermediate behavior is common, when percolated and isolated phases coexist. In many cases fractionated crystallization can be found as well as fractionated melting (although this last case has only been documented once for nanometric PEO droplets within PB-/ -PEO or PE-fo-PEO as presented above). 

Early work by Vonnegut and Schaefer demonstrated that water could be undercooled by close to 40 degrees below the equilibrium freezing point. Wood and Walton carried out a careful series of experiments in 1970 that were interpreted as due to homogeneous nucleation and from which the surface free energy and its temperature derivative were extracted. These experiments used the droplet emulsion method with the fraction of droplets crystallized as a function of time measured with a camera through a micro-... 

Fig. 2 Optical microscopy image of a small section of a poly(ethylene oxide) (PEO) droplet dispersion sample, see text (1000-mm wide) obtained at Tc = - 2.6 °C. Amorphous droplets appear dark and semicrystalline droplets appear white under nearly crossed polarizers. The plot shows the fraction of crystallized droplets as a function of temperature upon cooling (0.4 °C min-1) for homogeneous nucleation. (Reprinted with permission from [84], Copyright 2004 by the American Physical Society)...

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