Research On Fullerene-Induced Crystallization Of Poly (Vinylidene Fluoride) Under High Pressure

Poly (vinylidene fluoride)(PVDF) exhibits a pronounced polymorphism, i.e. α, β, γ,δ and ε, transforming between several crystal forms under certain conditions. The successful development of ferroelectric polymer devices depends on the effective fabrication of polar ferroelectric crystalline structures, such as β and γ.α phase is the most common during the melt crystallization of the polymer, and remains the dominant crystalline form versus the β and γ phases. Thus, there have been numerous attempts to stabilize and obtain the β and γ phases. Moreover, the electromechanical coupling factor of PVDF comprising of β-and γ-extended chain crystals is higher than that of β-and γ-folded chain crystals. Thick lamellar crystals with strong piezoelectricity cannot be grown at ordinary pressure due to the absence of the hexagonal phase, which appears only at high pressures and high temperatures, and is crucial to the formation of PVDF extended-chain crystals with β-or γ-form. With potential value of applications, fullerenes, such as C60and C70, have recently attracted considerable attention from the materials research community. The combination of fullerenes with polymers makes it possible to create novel materials that exhibit the unique properties of fullerenes while taking advantages of the easy processability of polymers. However, to the best of our knowledge, no investigation was performed on the physical or chemical interactions between PVDF and fullerenes at high pressure.Herein, two kinds of composite materials, PVDF/C60and PVDF/C70, with overall good fullerene dispersion, were prepared by an easy physical and mechanical route. And then, the high-pressure crystallization of PVDF/C60and PVDF/C70, as well as the high-pressure annealing of PVDF/C60, are well investigated. PVDF/fullerene nanocomposite samples, with different contents of fullerene were observed by TEM, for understanding the dispersion of fullerene particles in the PVDF matrix. The crystalline phases and thermal behaviors of the neat PVDF and its nanocomposites with fullerenes were identified by means of Wide-angle X-ray diffraction (WAXD), Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The sample surfaces were etched, and then coated with gold for scanning electron microscopy (SEM). The main results are as follows:1) When a shear force was applied to PVDF resin, its a phase was easily transformed into β form, i.e., the phase transformation of PVDF was very sensitive to the experimental conditions. To acquire the information of the original high-pressure treated samples as accurate as possible, the following experimental sequence was suggested during the sample characterizations. WAXD and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) data were all collected on the fresh and smooth fracture surfaces of the samples. After the WAXD and ATR-FTIR characterizations, the sample surfaces were etched, and then coated with gold for SEM observations.2) The PVDF/C60composite samples were crystallized by varying temperature, pressure and crystallization time. In general, the melting temperatures of the high-pressure treated composite samples increased with the increase of crystallization temperature. With the increase of crystallization temperature, the intensity of the peaks characteristic of a and β phase decreased and increased respectively, whether in WAXD or IR spectra. The y-form crystals of PVDF began to appear evidently when the crystallization temperature was above260℃. Especially, no a phase was detected in the sample crystallized at280℃. Its crystal forms are totally a mixture of β and y phase. When the crystallization time was extended, the relative contents of y phase increased all the time, but a phase decreased, and then increased. The tendency of intensity change of the bands for β phase was just contrary to that for a phase. This suggested that a phase is more stable than β phase if crystallization time was further increased. When the crystallization pressure was over400MPa, PVDF is easier to form β and y phase.3) Morphological observation of the as-formed PVDF crystals was conducted by SEM on the etched fracture surfaces of FVDF/C60composite samples. The results showed that the molecular orientation of β phase was enhanced, with the increase of crystallization temperature. The DSC implied that the melting point of the β-form PVDF crystals increased with the enhancement of the molecular orientation in such crystals. PVDF were also induced to form novel y phase crystals with holes, and vert ically aligned nanowire arrays with β phase, by C60at the high pressure. This is the first time that vertically-aligned crystalline nanowire arrays of β-form PVDF, with polygonal shape and hollow structure, were fabricated via a template-free approach.4) Well-dispersed PVDF/C60composite sampleswere annealed at high pressure by varying annealing time. The results showed that the content of a and β phase decreased and increased in the as-fabricated samples, respectively, with the increase of annealing time, and no y phase was detected during the experimental range. In addition to featured a spherulites, large scale formation of vertically-aligned crystalline nanowire arrays of β-form PVDF, with polygonal shape and hollow structure, and with folded-chain lamellae as substructure, were observed unexpectedly by delicate etching technique. The diameters of the nanowires increased with the increase of annealing time. However, the inappropriate increasing of annealing time resulted in the abnormal growth of the nanowires, and they lost their size uniformity. The as-fabricated PVDF nanowire arrays, with their unique structures, may diversify niche applications in converting mechanical, vibrational, and/or hydraulic energy into electricity for self-powered nanodevices.5) The PVDF/C70composites were crystallized in a piston-cylinder high-pressure apparatus, and the polymeric crystalline structur es totally with extended-chain piezoelectric β-or γ-form lamellae were successfully achieved in the composite samples by varying temperature, pressure, crystallization time and composite composition. The c-axis thickness of the extended-chain β-form lamellae of PVDF in the composites increased and decreased with the increase of the applied temperature and pressure, respectively, and it increased with the increase of crystallization time. Although C70was found to be negative for the rapid formation of β-form PVDF crystals, it played an important role in the growth of a β-form PVDF nanowire with extended-chain crystalline substructures. The template-free formation of such piezoelectric nanowires was attributed to a C70-induced self-assembly of the polymer, driven by physical interactions at high pressure. The pressure-crystallized PVDF/C60composites, self-reinforced with unique one-dimensional piezoelectric structures, may diversify niche applications in advanced functional polymeric devices.