Investigation of the general properties and field-induced electromechanical response of polymer nanocomposites with surface-functionalised dielectric nanoparticles.

For the past several decades, polymer composite materials have become increasingly popular in various industrial sectors owing to their advantageous properties, such as light weight and high mechanical performance. Most of the failure modes of composite materials are initiated by the coalescence of microcracks in the matrix. Therefore, matrix toughening is one of the most popular approaches to improve the overall fracture toughness of polymer composite materials. The most widely known approach for matrix toughening is the addition of a second phase, such as rigid or/and rubber particles, to dissipate the fracture energy. Several studies have focused on another approach, known as ‘active toughening’, involves introducing a thermal-induced strain from the fillers to its surrounding matrix, but this approach could only deliver slow and irreversible toughening due to the polymer’s poor thermal conductivity. In this study, a new approach is presented that involves an instantaneous extrinsic strain field activated by remote electromagnetic radiation. Quantification of the real-time field-induced strain evolution with microwave radiation is conducted via fibre optic sensing technology (FBGs). Theoretical expressions correlating the field-induced strain with microwave power level and exposure time have been developed, with the theoretically calculated solution validating the experimental data and describing the underlying physics. This study has introduced functionalised ferroelectric barium titanate nanoparticles (BaTiO₃) as a second phase dispersed into an epoxy matrix. The embedded nanoparticles are capable of introducing electro-strains to their surrounding rigid epoxy when subject to an external electric field, which result from the domain wall movements due to polarisation orientation. A diglycidyl ether of bisphenol A epoxy with the hardener Aradur 3487 were modified with the BaTiO₃ nanoparticles embedment. The silane coupling agent for the nanoparticles’ surface functionalisation was 3-glycidoxypropyl trimethoxysilane (3-GPS). Ultrasonication and solvent-aided mixing (ethanol, C2H6O, 99.9%) are employed to facilitate the dispersion of BaTiO₃ nanoparticles. Firstly, the crystal microstructure of the functionalised BaTiO₃ nanoparticles and the mechanical, thermal, and dielectric properties of epoxy nanocomposite materials have been characterised via various conventional techniques. It has been observed that the addition of the nanoparticles only has an insignificant impact on the curing extent of the epoxy. After that, the surface-bonded fibre grating sensors have been employed to investigate the variation of strain and temperature change of the epoxy nanocomposite materials simultaneously in the microwave oven at 2.45GHz with different power levels. The strains developed in the nanocomposite, adhesive used for FBG bonding, and the holding fixture are then studied via FBG sensors to distinguish the strains induced by domain wall movement from thermally induced strains. Repeated compressive strain fields are observed as a decline in the FBGs strain measurements of epoxy nanocomposite samples with negligible temperature change when placed horizontally in the oven cavity. Raman spectra are used in this study to observe the post-microwave effect of the internal stress state. The blueshift of the characterisation peaks of BaTiO₃ has been identified, thus suggesting a residual stress field experienced by the nanoparticles. The multi-functional nanocomposite development and qualifications presented in this study proposed a novel active toughening technology for high-performance composite applications in numerous sectors.