Poly(vinylidene fluoride) (PVDF) has excellent pyro- and piezoelectric properties which are related to the β-crystal of PVDF. Most of the β-crystal of PVDF can be transformed from the α-crystal by mechanical stretching. The effects of stretching conditions on the transformation from α to β phase of PVDF were studied in this paper. In addition, changes of crystalline structure of PVDF during the stretching process were observed in situ under an optical tensile stress microscopy tester, and the samples after being stretched were investigated using a 3D digital microscope and an infrared microscope. The results show that stretching temperature (T_s) and drawing ratio (λ) are critical to the transformation from α-crystal to β-crystal of PVDF. The T_s around 100 °C and λ above 3 is recommended. The in situ observation indicates that the deformation of crystalline structure began from the middle of α-spherulite and extended to another spherulite, and caused the large-scale transformation. Scanning results of infrared microspectroscopic and 3D digital microscopy revealed that α phase of PVDF would change into β phase with the deformation of the spherulite, and the distribution of the different crystalline phase in PVDF after being stretched was also given. It is believed that this work is instructive and meaningful to studies on phase transformation and production of the piezoelectric film of PVDF.

Boron-cross-linked graphene oxide/polyvinyl alcohol (B-GO/PVA) hydrogels with high-water-content and excellent mechanical properties are prepared by freeze/thaw and boron cross-linking methods.

Solid-state nanocomposite polymer electrolytes based on poly(vinyl alcohol)(PVA) incorporating hyperbranched poly(amine-ester) (HBPAE) grafted nano-silica (denoted as SiO₂-g-HBPAE) have been prepared and investigated. Through surface pretreatment of nanoparticles, followed by Michael-addition and a self-condensation process, hyperbranched poly(amine-ester) was directly polymerized from the surface of nano-silica. Then the hypergrafted nanoparticles were added to PVA matrix, and blended with lithium perchlorate via mold casting method to fabricate nanocomposite polymer electrolytes. By introducing hypergrafted nanoparticles, ionic conductivity of solid composite is improved significantly at the testing temperature. Hypergrafted nano-silica may act as solid plasticizer, promoting lithium salt dissociation in the matrix as well as improving segmental motion of matrix. In addition, tensile testing shows that such materials are soft and tough even at room temperature. From the dielectric spectra of nanocomposite polymer electrolyte as the function of temperature, it can be deduced that Arrhenius behavior appears depending on the content of hypergrafted nano-silica and concentration of lithium perchlorate. At a loading of 15 wt% hypergrafted nano-silica and 54 wt% lithium perchlorate, promising ionic conductivities of PVA nanocomposite polymer electrolyte are achieved, about 1.51 × 10⁻⁴ S cm⁻¹ at 25 °C and 1.36 × 10⁻³ S cm⁻¹ at 100 °C.