PVDF—PA-6 blends

The TEM micrographs of the PVDF-PA-6 blends (Figure 3) reveal that these components are phase separated too. The particle sizes in the four times extruded samples vary between O.lpm and 3pm for the PVDF component and 0.05pm and 2pm for the PA-6, depending on the composition. The greater particles have a composite structure since in their turn they contain particles of the matrix phase as a further level of dispersion. With increasing extrusion cycle number, the dispersion becomes finer, and the composite character, where initially present, is occasionally lost. 

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 3. TEM micrographs of PVDF/PA-6 blends. PVDF is the dark phase. Scale bar corresponds to 2pm (a,b), or 5pm (c).

Figure 7. DSC crystallization curves of PVDF/PA-6 blends. Four extrusion cycles parameter blend composition. (Reproduced with permission from ref. 68. Copyright 1988 Steinkopff-Verlag Darmstadt.)...

Discussion/Colncidence of Crystallization Temperatures. Let us first consider the PVDF/PA-6 blend. In view of the nonaltered T of PVDF, we suppose that the PVDF crystallization induces the PA-6 crystallization rather than vice versa. Hence, the just created crystals of the PVDF matrix act as nucleating heterogeneity for the PA-6. The A y-value between PVDF crystals and PA-6 melt, obviously, is smaller than that of all other heterogeneities which are present in PA-6 to a sufficient extent except, possibly, the species "A". Its associated specific undercooling, moreover, must be so small that the PVDF crystals can induce the crystallization of the PA-6 from the instant of their own creation. 

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. 

Erensch and Jungnickel [1989] and French et al. [1989] have investigated PVDF/PBT blends and related their thermal behavior with the blend morphology. Similar to PVDF/PA-6 blends, the PBT droplet crystallization was completely suppressed in a 85/15 blend and finally crystallized coincidentaly with the PVDF matrix. Again this phenomenon could be related to the fine dispersion of PBT droplets, in number exceeding the available nuclei. Shorter melt-mixing cycles caused a coarser dispersion leading only to a... 

Frensch and Jungnickel (1989) and Frensch et al. (1989) tried to elucidate the crystallization behavior of the minor phase in the binary PVDF/PA-6 blends, in relation to the final blend morphology. They reported that the crystallization of the PA-6 droplets was fractionated and/or retarded, depending on the number of mixing cycles and dispersion size. The smaller the PA-6 droplets, the more pronounced the retardation of the crystallization peak (AT 40 °C). Nevertheless, the melting endotherm remained unaffected. They concluded that part or all of the PA-6 phase finally coincidentally crystallized with the PVDF matrix due to the specific mutual nucleating efficiency of both components. 

Discussion/Estimation of Nuclei Concentrations. The average volumes Vjj of the dispersed PA-6 particles in the four times extruded PVDF/PA-6 85/15 and 75/25 blends as estimated from the electron micrographs amount to about 4xl0 pm and axlO pm, respectively. 

Frensch and Jungnickel [1989, 1991] and Frensch et al. [1989] have investigated the thermal behavior of polyvinylidene fluoride, PVDF, in blends with polyamides, in relation to the blend morphology. PA-6 droplets could be finely dispersed into the PVDF matrix. The crystalhzation temperature of the PVDF matrix did not seem to be affected in the blends. A similar behavior was observed in PVDF/PA-66 blends. 

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].

Torsional pendulum analysis exhibits discrete relaxations at the glass transition temperatures of PVDF and PA-6 at -45°C and 50°C, respectively. This also indicates incompatibility of the blend components in the amorphous phase after solidification. 

Despite the occasional fractionation of the crystallization or its suppression at the usual temperature, the DSC heating curves of all blends exhibit in all cases for both polymers a single melting endotherm (PA-6 one for both crystal modifications) at an almost constant temperature (variation range smaller than 6°C) of about 175°C for the PVDF and of about 217°C/223°C for the PA-6. The relative crystallinity of each component also does not change significantly with composition and extrusion cycle number, in fact, WAXS-and IR-analyses show that, in all samples, PA-6 crystallizes mainly in the l-modification at an almost constant i/a-ratio, and PVDF in the a-modification. 

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. 

Mascia and Hashim [1997] have prepared compatibilized blends of PA with PVDF by using car-boxyhc acid-functionalized PVDF. In an example 20 parts PA-6 was combined with 80 parts PVDF-g-MAA (10% MAA) in an internal mixer at 240°C. The graft copolymer-containing blend was characterized by SEM, FTIR, mechanical properties, selective solvent extraction, and rheology. The effects of adding zinc acetate were studied. 

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