Quenched PVDF Blends

Figure 9.15. Glass transition temperatures of quenched PVdF blends. (Noland et al, 1971.)...

Figure 7.2. Glass transition temperatures of quenched PVDF/PMMA blends after adjustment with a shifting factor (+) Martinez-Salazar et al. (A) Nishi and Wang (o) Noland et (+) Morales et al. (A) Roerdink and Challa. ...

Figure 7.5 shows the relative endotherms of PVDF/PMMA blends as a function of PVDF content. A relative endotherm is defined here as the ratio of a melt endotherm at a certain PVDF blend composition to the endotherm of the pure annealed PVDF in DSC measurements. The DSC measurements are at a constant heating rate of lOC/min. For annealed and solution-cast blends, the relative endotherms are in general greater than those of quenched blends of the same composition. Endotherms can be detected at all compositions when the blends are annealed. The quenched PVDF shows only 75% endotherm relative to the annealed PVDF. No endotherm can be detected when the PVDF content is lower than 50 wt % in quenched PVDF/PMMA blends. Obviously the thermal history of the blends has a tremendous effect on the final performance of the material. 

Figure 7.2. Glass transition temperatures of quenched PVDF/PMMA blends after adjustment with a...

Table 11.16 shows the composition of a PVDF blend for foam fabrication. This recipe was mixed in an intensive power blender to produce a homogeneous powder blend which was then melt blended in a high shear twin screw extruder at 200 rpm and a temperature of 200°C. Pellets were produced by extruding the melt into water and chopping the quenched extrudate. The foamable blend was extruded, through a 25 mm extruder, according to the conditions given in Table 11.17. A comparison of the properties of foamed and solid PVDF wire insulation is presented in Table 11.18.

In particular, blends of PVDF with a series of different polymers (polymethylmethacrylate [100-102], polyethylmethacrylate [101], polyvinyl acetate [101]), for suitable compositions, if quenched from the melt and then annealed above the glass transition temperature, yield the piezoelectric [3 form, rather than the normally obtained a form. The change in the location of the glass transition temperature due to the blending, which would produce changes in the nucleation rates, has been suggested as responsible for this behavior. A second factor which was identified as controlling this behavior is the increase of local /rans-planar conformations in the mixed amorphous phase, due to specific interactions between the polymers [102]. 

Figure 7.1. Glass transition temperature of PVDF/PMMA blends after high-speed quenching as a...

Figure 7.3. Glass transition temperatures of annealed and cast PVDF/PMMA blends (+) Mijovic et aiy (o) Nishi and Wang (A.) Paul and Altamirano (-) calculated results for quenched blends.

The kinetics of the crystallization process can be followed using the FT-IR technique 290). A particularly interesting example comes from the study of the polymer blends of PVDF/PMMA where the crystallization of the alpha and beta forms have been followed during heating of the blend samples which had been quenched from the melt and crystallized by heating at 2 °K/min in the spectrometer. Wien the blend has 70 wt % PVDF the beta crystal form is obtained directly but for other compositions the alpha form is dominant or unique. 

F re 16.4 POM images of the miscible crys-talline/crystalline (PVDF/PBA) blend, showing ratio-dependent mesoscale morphologies. Cooling and quenching effects on these... 

Finally, the PVDF/PVAc film was cut into strips and load in the microcompounder at 200 °C and compounded for 5 minutes at 100 rpm. Following mixing in the DACA, extruded strands were cut and made into films using the Carver press with a spacer of 0.3 mm and 2 metric tons of pressure being applied. The films were compressed for 5 minutes, removed in the hot state from the press, and quenched to room temperature in contact with a metallic cooling plate. The films were used as such for all experiments described herein. Blends of different PVDF content were processed 100, 85, 70, 50, 30, 15, 0 weight percent (wt%). 

The miscibility of the PVDF-PVAc blends was demonstrated via DSC. The polymers show complete miscibility at all blending ratios with a single glass transition temperature in quenched samples (Figure 1) as expected from previous study in which PVDFiPVAc blends show lower critical solution temperature above 300 °C for all compositions [10]. Although the Tg of PVDF is around -35 °C, we could not detect it from the DSC signal with a cooling rate of 5 °C/min. 

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