PVDF-PBTP blends

SEM of fracture surfaces reveal phase separation in the PVDF-PBTP blends too (Figure 4). The particle sizes (PVDF between 1pm and 20pm, PBTP between 0.1pm and 2pm)) are considerably larger than those in the PVDF-PA-6 blends of comparable composition. The particle diameters again decrease with increasing extrusion cycle number. 

Figure 4. SEM micrographs of PVDF/PBTP blends. Scale bar corresponds to 2um (a,b), or 10pm (c).

Results/Crystallization Behaviour/PVDF-PBTP Blends. The pure PBTP (Figure 9) crystallizes at about IBO C. The crystallization temperature T rises remarkably to 189 C and 194°C for the one and for the four times extruded blends, respectively, after adding 15 vol.-% PVDF. In a similar way, the pure PVDF crystallizes at 140°C whereas the corresponding T in the blends is between 142°C and 148°C. The PVDF in the 15/85 blend, e.g., crystallizes at 147°C after one extrusion cycle whereas it crystallizes at 143°C after four ones. The PBTP crystallization in the four times extruded 85/15 blend is suppressed in a similar manner as already described for the PA-6. In place of that, the PBTP crystallizes at 147 C simultaneously with the PVDF as derived from comparison of the exotherm and the endotherm DSC peaks of the cooling and reheating runs. 

Figure 9. DSC crystallization curves of PVDF/PBTP blends. Parameters blend composition and number of extrusion cycles Z.(Reproduced with permission from ref. 68. Copyright 1988 Steinkopff-Verlag Darmstadt.)...

The average volume of dispersed PBTP particles in the PVDF/PBTP = 85/15 blend after the first and the fourth extrusion cycles amounts to about 4pm and 5x10 pm, respectively. By the same arguments as above one has to conclude from the fractionation of the crystallization of the dispersed PBTP droplets even in the one time extruded blend that the number density of the heterogeneities which nucleate PBTP above 148°C is below about 0.2pm . ... 

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. 

Basicly, the effects are caused by the nucleating activity of different inhomogeneities. They can become more complicated if the second component, in particular their just created crystals, acts as a crystallization nucleating inhomogeneity. Such a - in some cases mutual - nucleating activity is hidden under usual conditions. Finally, some blends, e.g. that of PBTP and PVDF, exhibit a most complicated mutual nucleation behaviour the molten first component acts as nucleating substrate for the second one and becomes then itself nucleated by the newly created crystals of that component. 

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