Coincident crystallization

The coincident crystallization of the PVDF matrix and dispersed PBTP particles in the 85/15 blend (z = 4) takes place at (142...148)°C, that is, above the T of pure PVDF. It is not clear whether the PVDF or the PBTp crystallizes first. In either case, the nucleation of the first crystallizing component may be induced either by a species of nucleating heterogeneities or by the molten second blend component. The newly created crystals of the one component, then, act immediately as nuclei for the crystallization of the other in the same manner as already described for the PVDF/PA-6 blends. 

When two crystallizable components are blended, a more complex behavior due to the influence of both phases on each other is expected. In general, the discussion for matrix crystallization and droplet crystallization can be combined. However, crystallization of one of the phases can sometimes directly induce crystallization in the second phase. As a consequence, the discussion of blends of this type has been subdivided with respect to the physical state of the second phase during crystallization. The special case of coincident crystallization , in which the two phases crystallize at the same time, is discussed. Finally, the effect of compatibilization of crystalline/crystalline polymer blends is briefly reviewed. 

Sometimes coincident crystallization occurs in finely dispersed morphology... 

Pukanszky et al. [1989], and recently by Muller et al. [1995], and Morales et al., [1995]. The latter authors even mentioned that the retarded crystallization of PP droplets in some cases finally resulted in the coincident crystallization of PP... 

A few authors have observed coincident crystallization of both phases in crystalline/crystaUine immiscible blends. This phenomenon was reported for blends in which the minor phase exhibits a higher degree of undercooling for crystallization due to its fine dispersion (see Section 3.4.4.) and the matrix phase crystallizes at its bulk T that is lower than that of the minor phase. An additional factor that should be taken into account is that a heterogeneous nucleation is promoted on surfaces with a high interfacial tension [Helfand and Sapse, 1975] (i.e., a crystal-... 

Principle of Coincident Crystallization It has been observed that this phenomenon is connected with the phase dispersion of the minor component, and is enhanced when the dispersion becomes finer. Upon cooling from the melt, a finely dispersed phase can exhibit fractionated crystallization, what implies that none, or only part of the dispersed droplets crystallize at their bulk r. This type of crystallization is related to the lack of heterogeneities in the droplets, required for nucleation at the bulk T. 

When the blend is now further cooled, two possible ways of primary nucleation are possible. In a first case, the matrix phase is nucleated by heterogeneous species present in this phase and instantly, newly created crystals appear. Hence, the crystallization temperature of the matrix will be situated at its bulk T. A second possibility for coincident crystallization occurs in the case one finds again a single crystallization peak for the matrix phase, which however takes place above its bulk T. Some novel mutual nucleating mechanism was suggested in such blends a molten component (minor phase) acts as nucleating substrate for the matrix, which instantaneously crystallizes [Erensch and Jungnickel, 1989]. 

A second system investigated by the authors was the PVDF/PBT blend. Similar effects could be observed. However, coincident crystallization in the PVDF/PBT 85/15 blend occurred at a somewhat higher temperature than the bulk Tpyp,p. It could be concluded that in this case, the PBT melt induced the crystallization of the PVDF matrix phase. 

Besides the cases of coincident crystallization reported previously, recent investigations on PP/PA-6 blends in which a compatibihzing agent had been used to obtain a finer and more homogeneous dispersed phase morphology also mentioned coincident crystallization of the PA-6 droplets with the PP matrix [Ikkala et al., 1993 Moon et al., 1994]. However, this has not been observed in the binary blend. 

Figure 3.47. Retarded and/or fractionated crystallization causing coincident crystallization in PVDF/PA-6 and PVDF/PBT blends. Influence of the blend composition (a) and the number of extrusion cycles Z (b) [Frensch and Jungnickel, 1989].

MAH-g-PP +FA-g-EBA +GMA-g-EEA 80/20 60/40 of binary blend and MAH-g-PP PA-6 dispersed coincident crystallization with PP SEES, EBA and E EA form immiscible interlayer between PA-6 and PP => no nucleation =>T =T c,PP c,bulk Compatibilization => serious reduction of PA-6 droplet size => lack of nucleating species => retarded cryst. [Ikkala et al, 1993]... 

In the case of the crystaUization of the matrix in the presence of already solidified or crystallizins particles, migration of nuclei still can play an important role. However, several other phenomena have to be taken into account. First of all, the solidified domains can act as efficient nucleators. Furthermore, retarded crystallization of finely dispersed droplets can nucleate the matrix and leads to coincident crystallization of both phases. Finally, it has been reported that epitaxial crystallization at the interfaces sporadically occurs. All these phenomena lead to an increased heterogeneous nucleation of the matrix phase. 

The morphology of a polyblend consisting of two crystallizable polymers can vary depending on the processing conditions and the relative rates and temperature of crystallization of the constituent polymers. These can either ciystallize at the same time (coincident crystallization, see further) or separately in a sequential... 

S Coincident Crystallization in Crystalline/Crystalline Polymer Blends... 

It is clear that this phenomenon is phase morphology-dependent. Only in those blends where the minor phase is dispersed into sufficiently fine droplets, this phase has the opportunity to exhibit fractirMiated crystallization. Hence, only at low blend compositions and/or good matching viscosities of both phases (where the capillary number C predicts droplet breakup being dominant above coalescence) the occurrence of coincident crystallization is possible. 

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