Natural fibre-reinforced poly(lactic acid) (PLA) laminates were prepared by a conventional film stacking method from PLA films and natural fabrics with a cross ply layup of [0/90/0/90/0/90], followed by hot compression. Natural fibre (NF) nano-hydroxyapatite (nHA) filled composites were produced by the same manufacturing technique with matrix films that had varying concentrations of nHA in the PLA. Their flammability, thermal, moisture absorption and mechanical properties were analysed in terms of the amount of nHA. The flame behavior of neat PLA and composites evaluated by the UL-94 test demonstrated that only the composite containing the highest quantity of nHA (i.e., 40 wt% nHA in matrix) was found to achieve an FH-1 rating and exhibited no recorded burn rate, whereas other composites obtained only an FH-3. The thermal degradation temperature and mass residue were also observed, via thermogravimetric analysis, to increase when increasing concentrations of nHA were added to the NF composite. The tensile strength, tensile modulus and flexural modulus of the neat resin were found to increase significantly with the introduction of flax fibre. Conversely, moisture absorption was found to increase and mechanical properties to decrease with both the presence of NF and increasing concentrations of nHA, and subsequent mechanical properties experienced an obvious reduction.
The contact electrification of ferroelectric polymer can be more complicated due to its ordered permanent molecular dipoles and dipole–dipole interactions. Herein, the polyvinylidene fluoride (PVDF)‐Cu is taken as an example to investigate the mechanism of ferroelectric polymer‐metal contact electrification via first‐principles calculations. It is revealed that different from non‐ferroelectric polymers, when ferroelectric polymers are in contact with metals, the charge transfer occurs not only at the interface but also inside the polymer due to the existence of polar phases. Specifically, the polar phases in the crystallization region can effectively enhance the charge transfer between the ferroelectric polymer and metal because the polar molecules in PVDF possess the stronger electrostatic potential, more delocalized lowest unoccupied molecular orbital, and additional dipole–dipole interactions compared with nonpolar molecules. In addition, the coupling mechanism of piezoelectricity and triboelectricity in ferroelectric polymer‐metal contact electrification under compression is also investigated. It is demonstrated that the deformation increases the degree of noncoincidence between positive and negative charge centers in polar phases and causes charge transfer between the polar molecular chains of PVDF, thus producing the extra charge transfer between the ferroelectric polymer and metal. This study provides a theoretical basis for the material design of triboelectric nanogenerators based on ferroelectric polymers.
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