Strain Modulation of Crystallinity, Orientation and Piezoelectric Properties of Poly(vinylidene)—Poly(methyl methacrylate) Blends

Technologically-relevant semicrystalline, electroactive polymers such as poly(vinylidene fluoride) (PVDF) have attracted a lot of attention because of their potential applications in sensors and energy harvesting units. From a viewpoint of basic research, crystallization and crystal phase transitions of PVDF provided an interesting topic to study formation of different crystal polymorphs under the influence of external stimuli such as electric field and mechanical stress. Over four decades of experimental studies on PVDF provided a solid knowledge on relationships between nucleation, crystallization rate, and solid-state recrystallization, which underpinned market development of several high-tech companies in 21^st century.<br/>Imparting piezoelectricity to PVDF requires uniaxially, homogeneously and permanently oriented dipole moments of CH₂-CF₂monomer units in polar crystal polymorphs. There are several technologically-proven strategies to render PVDF piezoelectric. One of the strategies is based on blending of PVDF with poly(methyl methacrylate) (PMMA) and deformation of the blends under the external electric field. PVDF and PMMA are thermodynamically miscible above upper critical solubility temperature. Generally, PMMA reduces the overall crystallinity of PVDF by “trapping” PVDF macromolecules in the PMMA bulk. The “trapping” reduces the crystallization rate. Therefore melt crystallization of PVDF in the PVDF:PMMA blends requires extended annealing at crystallization temperature or slow cooling of the melt. Quenching the molten PVDF:PMMA blends makes them amorphous whereas an appropriate thermal treatment of the fully amorphous blends can cause a preferential formation of polar crystals. Mechanical stress enhances the latter process and causes the crystals to orient along the deformation axis. However, the mechanism of formation of polar crystals after phase separation as well as recrystallization of non-polar polymorphs raise controversies. So does the role of a strain in the case of the blends.<br/>Our study was aimed, therefore, at explaining the role of PMMA on formation of polar polymorphs of PVDF, i.e. the β and γ crystal phases. Our experiments were also supposed to shed light on mechanism of transforming of the isotropic non-polar PVDF polymorph (the a-phase) into oriented polar crystals during deformation in a viscoelastic state. The experimental approach included real-time, in-situ mapping of crystal structure development in the blends of PVDF with four different ratios of PMMA during their uniaxial deformation. PVDF crystal structure was monitored with time-resolved wide angle X-ray diffraction (WAXD) with 30-microns spatial resolution to precisely address the role of local strain on crystal orientation and phase composition. Small-angle X-ray scattering experiments were performed to supplement the WAXD data with variation of the lamellar structure of PVDF upon deformation. Additionally, the ex-situ Fourier transform infrared spectroscopy (FTIR) was used to refine the conclusions from WAXD. The studies performed with the high-spatial resolution enabled finding correlations between the local strain, orientation and the phase composition of the blends, including the γ to β ratios in different samples as a function of blend composition and local strain. In the contribution, we present static phase composition maps of samples under deformation and the evolution of the maps upon varying the strain. We also discuss correlation between Hermans orientation factors and phase compositions. The work can be considered as a fundamental contribution for development of the polymer blends for sensing and energy harvesting applications.<br/><br/>Acknowledgement<br/>WAXS and SAXS experiments were performed at the P03 Beamline at PETRA III/German Electron Synchrotron (DESY) Hamburg, Germany. The work was supported by National Science Centre Poland (NCN) through the grant UMO-2017/25/B/ST5/02869