Study On Graphene Induced Phase Transition Of Piezoelectric Polymer Under High Pressure

Polymer/graphene composites have recently attracted considerable attention from the materials research community. The combination of graphene with polymers makes it possible to create novel composites that exhibit the unique properties of graphene while take advantages of the easy processability of polymers. Particularly, appropriate high-pressure treatment for a semicrystalline polymer/graphene composite may not only enhance the crystallization of the polymer but also obtain certain novel crystalline forms or morphologies. Poly (vinylidene fluoride) (PVDF) is a piezoelectric polymer that exhibits a pronounced polymorphism, i.e. a, β, y,8 and ε, transforming between several crystal forms under certain conditions. Nevertheless, the successful development of ferroelectric polymer devices depends on the effective fabrication of polar ferroelectric crystalline structures, such as (3 and γ.In this study, by the incorporation of graphene (Gr), graphene nano platelet (GNP) and graphene oxide (GO) respectively, a series of well dispersed nano carbon based PVDF composites were fabricated through an environmentally friendly approach. The as fabricated binary composites were crystallized at high pressure by varying the temperature, the pressure and the crystallization time. The recovered high-pressure crystallized samples were then investigated using wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (ATR-FTIR) and scanning electron microscopy (SEM). Also, the hydrophilic and hydrophobic properties of PVDF/GNP composites were characterized with a video contact angle measuring instrument. The main work and conclusions are listed as followings:(1) PVDF/Gr compositesThe observation by transmission electron microscopy (TEM) and SEM showed that the lamellar structured part of Gr platelets were homogeneously dispersed in PVDF matrix. DSC showed that both the melting point and crystallinity of the composite samples increased with the increase of the highest crystallization temperature. Also, the intensity of the peaks characteristic of a phase decreased, while that of β and y phase increased, whether in WAXD or IR spectra. With the increase of crystallization time, both the melting point and melting enthalpy increased during 0～10min and remained more or less the same during 10-60min. However, the composite samples crystallized during 10-60min were endowed with high melting points and high crystallinity, and their crystalline structures were totoally with β and y forms. The variations of melting point, melting enthalpy and crystalline form showed just the same tredency when pressure was increased, and the critical point was 400 MPa. In addition to the β form extended-chain crystals, spherulites and oriented structured crystals of PVDF, SEM disclosed unique three dimensional (3D) micro/nano hybrid structures with crystalline β phase in the high-pressure crystallized samples.(2) PVDF/GNP compositesGNPs were well dispersed in a PVDF/GNP composite with 1.0 wt.% loading, and they aggregated when the loading levels were increased. Both the melting point and melting enthalpy increased with the increase of the highest crystallization temperature, but they increased and then decreased with the increase of crystallization time, pressure and GNP loading. Also, the most appropriate crystallization conditions were found to be 400MPa, 260℃ and 30 min for the growth of piezoelectric 拾 and y crystals. SEM revealed ordinary spherulites, fibrous β crystals, extended-chain β crystals and 3D micro/nano hybrid structures on the etched fracture syrfaces of the high pressure crystallized samples. Although the original surfaces PVDF/GNP composites were high hydrophobic, as suggested by the conducted wettability test, they were successfully converted into hydrophilic surfaces through the high pressure treatment followed by appropriate etching process.(3) PVDF/GO compositesAn overall good dispersion of GOs was achieved in PVDF matrix, and no obvious aggregation was observed. Both the melting point and melting enthalpy of the pressure crystallized samples increased when the highest crystallization temperature was increased, with the content of a and β crystals decreased and increased, respectively. Other conditions being the same, y from crystals were detected in a composite sample crystallized at 275℃ With the increase of crystallization time, both melting point and melting enthalpy generally increased and then decreased. Other than a and β crystal, no y form crystal was observed if crystallization time was less than 10min. Nevertheless, only (3 and y crystals were detected in the samples crystallized for more than 30 min. Furthermore, the measurement results for the samples crystallized at different pressure showed that a,β and y crystals coexisted in the sample crystallized at 300 MPa, and a crystals disappeared when the samples were treated at higher or lower pressure. In addition to the ordinary spherulites, fiber-like β crystals, SEM disclosed certain unique crystalline polymeric strctures with β form, such as nanowires and 3D micro/nano hybrid structures.