In this study, the mechanical and thermal properties of graphene were systematically investigated using molecular dynamic simulations. The effects of temperature, strain rate and defect on the mechanical properties, including Young’s modulus, fracture strength and fracture strain, were studied. The results indicate that the Young’s modulus, fracture strength and fracture strain of graphene decreased with the increase of temperature, while the fracture strength of graphene along the zigzag direction was more sensitive to the strain rate than that along armchair direction by calculating the strain rate sensitive index. The mechanical properties were significantly reduced with the existence of defect, which was due to more cracks and local stress concentration points. Besides, the thermal conductivity of graphene followed a power law of λ~L^0.28, and decreased monotonously with the increase of defect concentration. Compared with the pristine graphene, the thermal conductivity of defective graphene showed a low temperature-dependent behavior since the phonon scattering caused by defect dominated the thermal properties. In addition, the corresponding underlying mechanisms were analyzed by the stress distribution, fracture structure during the deformation and phonon vibration power spectrum.

In this study, the formability of woven carbon-fiber (CF)-reinforced polyether-ether-ketone (PEEK) composite sheets in the solid-state thermoforming process were investigated, and the failure mechanisms were discussed. The formability of the woven CF/PEEK sheets were analyzed using flexural tests, Erichsen test, and microscopic observation. The results show that the formability of CF/PEEK sheets significantly increases as the temperature rises from 165 to 325 °C, and slightly decreases as the deformation speed rises from 2 to 120 mm/min. The deformation of the sheets is caused by plastic deformation, shear deformation and squeeze deformation, without plastic thinning and fiber slippage, which is due to the restriction of the solid matrix and locked fibers. Moreover, the wrinkles will cause fiber fracture at lower temperatures and delamination at higher temperatures. At higher temperatures, the wrinkles mainly occur at the position with [0°/90°] fibers due to the squeezing of the matrix and fibers.

As the most intensively investigated ferroelectric polymers, poly(vinylidene fluoride) and its co-/ter-polymers enable major breakthroughs in a wide range of applications. Since defects play a vital role in tuning a spectrum of physical properties of poly(vinylidene fluoride)-based ferroelectric polymers, defect engineering has become an ingenious and robust strategy in the design of high-performance ferroelectric polymers. In this Review, we summarize the physical insights into the role of defects induced by various monomers at the molecular level on the physical properties and the structure–property relationship of defect-modified ferroelectric polymers. We focus on the fundamentals of the different structural defects on tailoring the dielectric, ferroelectric, electromechanical, and electrocaloric properties, along with the device performance enhancement in capacitors, actuators, and solid-state cooling. The influence of defects on the electric field dependence of the electrostriction and electrocaloric response is highlighted. The role of chiral defects in driving the emergent relaxor properties and morphotropic phase boundary behavior of ferroelectric polymers is discussed. Finally, we offer insightful perspectives on the challenges and opportunities in this rapidly evolving field. The underlying mechanisms revealed in the article are anticipated to guide future fundamental and applied studies of ferroelectric polymers that capitalize on defect engineering for electronic and energy applications.

Fiber-reinforced polymer composites with high damping performance have been required in diverse applications. The interlayer slip characteristics of the graphene family offer a clear benefit in enhancing the damping performance of materials. In this study, an oriented graphene oxide (GO) structure was designed on the carbon fiber surface to enhance the damping capacity of the composites. The molecular dynamics method was proposed to investigate the damping mechanism of multilayer GO-reinforced polymer composites based on energy dissipation, which is consistent with the results revealed by dynamic mechanical analysis. Under a wide range of loading strain, vibration frequency, and temperature, GO induces a higher loss factor/lower quality factor of the composite materials. The visualization of the atomic displacement field demonstrates that the sliding of multilayer GO during vibration enhances the mutual friction among polymer segments, leading to a pronounced increase in the energy loss. Notably, substantial enhancements in damping properties were observed with thicker GO coatings. This is due to the fact that the uniformly distributed shear stresses are more likely to activate interlayer slip, and higher frictional forces consume more mechanical energy.