Effect of polarization switching on piezoelectric and dielectric performance of electrospun nanofabrics of poly(vinylidene fluoride)/Ca–Al LDH nanocomposite

At present, highly flexible, durable, and lightweight piezoelectric nanogenerators with high-power density and energy conversion efficiency are of great interest. The present study reports a new synthetic route for Ca–Al layered double hydroxide (LDH) nanosheets and incorporation of these two-dimensional nanosheets as filler material into poly(vinylidene fluoride) (PVDF) to produce composite nanofabrics by electrospinning. The polymorphism, crystallinity, and the interaction between PVDF and LDH were studied by Fourier transform infrared spectroscopy, X-ray diffraction, and differential scanning calorimetry techniques. The synergetic effect of PVDF–LDH interaction and in situ stretching due to electrospinning facilitates the nucleation of electroactive β phase up to 82.79%, which makes it a suitable material for piezoelectric-based nanogenerators. The piezoelectric performance of PVDF/Ca–Al LDH composite nanofabrics was demonstrated by hand slapping and frequency-dependent mechanical vibration mode, which delivered a maximum open circuit output voltage of 4.1 and 5.72 V, respectively. Moreover, the composite nanofabrics exhibited a high dielectric constant and low dielectric loss due to superior interfacial polarization at low-frequency region with LDH loading, promising its potential applications in electronic devices. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48697.     

Alternative monomers for 4,4′-methylenedianiline in thermosetting epoxy resins

4,4′-Methylenedianiline (MDA) is utilized to produce high temperature epoxy resins as well as other high-performance composites; however, MDA is a known carcinogen and liver toxicant. Novel dianilines derived from both petroleum and biomass with different quantities and types of methyl and methoxy substituents on the aromatic ring were prepared and studied to reduce toxicity and carcinogenic aspects of the resulting material while maintaining thermal and mechanical integrity. These dianilines were primarily prepared by coupling commercial anilines using paraformaldehyde under acidic conditions. Another dianiline was prepared by nitration of bisphenol A and subsequent reduction of the nitrates to amines. Structure–toxicity relationships indicate that multiple substituents on the aromatic ring are necessary to reduce the toxicity of the dianiline. Epoxy-amine resins were prepared by blending the dianilines with 50 wt % Epikure W liquid amine curing agent and mixing with a stoichiometric amount of EPON 828 epoxy resin. Methoxy and methyl substituents increase the melting point of the dianilines by as much as 65 °C, thereby worsening the processing of these monomers as liquid thermoset resins. Structure–property relationships show that the addition of a methoxy group to the aromatic ring and moving the amine from the 4,4′ position on the dianiline reduces the glass transition temperature (T _g) by approximately 10 °C. A single methyl group has little effect on T _g and two methyl groups increase the T _g only when both are ortho to the amine, yet this causes a large 14 °C increase in T _g. Thermal degradation profiles are not significantly affected by the dianiline. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48707.     

Fabrication of self-healable,conductive, and ultra-strong hydrogel from polyvinyl alcohol and grape seed–extracted polymer

Developing hydrogels with excellent mechanical performances and multiple functions is challenging. Herein, polyvinyl alcohol (PVA) and grape seed extracted polymer (GSP) were used to achieve this goal. The hydrogel was firstly cross-linked by crystalline regions of PVA upon freezing–thawing cycles, it was further immersed in ammonium sulfate (AS) solution to induce the hydrophilic to hydrophobic transition of GSP. The relationship between microstructures and mechanical properties of the obtained PVA–GSP hydrogel was revealed. The results showed that the PVA–GSP precursor hydrogels were weak due to the uneven network and lower cross-link density. After soaking in ammonium sulfate solution, the network became more dense and uniform because of the hydrophilic to hydrophobic transition of GSP. Meanwhile, frictions between polymer chains increased. As a result, ultra-strong (tensile stress = 20.5 MPa), antifatigue (>95%), self-healable (healed stress = 10.0 MPa), and conductive hydrogels were obtained. The strength and toughness of our gel are among top values in literature to date. Furthermore, the swollen gel even after soaking in DI water for 36 hr still maintained tensile strength at the megapascal level, superior to most water-rich hydrogels. This tough and functional gel might find potential applications in cartilage repair, artificial skin, and sensors.