Roles of Interlayer Diffusion and Confinements in Manipulating Microstructural Evolutions in Multilayer Assembled Polyvinylidene Fluoride/Poly(methyl methacrylate) Films for Tunable Dielectric and Piezoelectric Performances

We present the multilayer assembly of polyvinylidene fluoride (PVDF)/poly(methyl methacrylate) (PMMA) films through a continuous and scalable layer-assembly process, where the effects of interlayer diffusion and confinements are highlighted. It is found that interlayer diffusion and confinements are strongly dependent on the layer compositions and sizes, which significantly govern the evolutions of the layer architecture and microstructure in the assembled films. Interlayer diffusion driven by the entropic advantage stabilizes the layered morphology in spite of the viscoelastic contrast between components. The overall crystallinity in confined PVDF layers is suppressed by the improved geometrical confinement with the reduction in layer thicknesses and/or by the enhanced molecular confinement with the increase in the fraction of PMMA. Meanwhile, an increasingly favorable formation of an electroactive β-phase is identified particularly for PVDF-rich films. Specifically, the crystallinity of the β-phase reaches around 25% for PVDF/PMMA (75/25) composition. The molecular origin for the β-phase is attributed to the intense stretching that induces the chain conformation of all-trans zigzag during layer assembly. In addition, the dielectric constant of films reaches a higher level of ∼30 depending on the fraction of the β-phase and molecular orientations that evolved from the multilayered assembly. Intriguingly, the piezoelectric output voltage and current, respectively, reach 7.4 V and 0.7 μA and demonstrate the tunable energy-harvesting capability. Both dielectric and piezoelectric performances for these films are found to be mainly contributed by the polarizations of β-phase crystals, where the interlayer diffusion, layer compositions, and sizes further influence significantly those electric responses by governing the charge transport across layers. Mechanisms for these electric responses are further clarified. Our work clearly reveals how the interlayer diffusion and confinements manipulate the microstructure evolutions under real micro-/nanolayer assembly processing conditions in order to up-scale PVDF-based multilayer films for lightweight and flexible capacitors and piezoelectric nanogenerators for electronic devices.