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High-performance EMI shielding materials are urgently needed for protecting from electromagnetic pollution. Traditional metal-based EMI shielding materials suffer from high density, poor corrosion resistance, and secondary radiation pollution. Here we report a facile strategy to prepare flexible and lightweight porous PEBA/MWCNT nanocomposite films with significantly enhanced EMI shielding performance. A novel biaxial stretching-assisted microcellular foaming technology is developed to refine cellular morphology. With the novel foaming technology, sub-microcellular and nanocellular PEBA/MWCNT nanocomposite films were prepared, which show nearly two orders of magnitude higher electrical conductivities than microcellular and solid PEBA/MWCNT nanocomposite films. EMI shielding tests with a rectangular waveguide structure show that porous PEBA/MWCNT nanocomposite films show remarkably enhanced EMI shielding performance compared with the unfoamed pristine sample, and their EMI shielding performance increases gradually with reducing cell sizes. While the solid PEBA/MWCNT nanocomposite film has a total shielding effectiveness of 26 dB, the nanocellular one has a total shielding effectiveness of 41 dB. More importantly, both sub-microcellular and nanocellular PEBA/MWCNT nanocomposite films show exceptional absorption-dominated EMI shielding behavior. Furthermore, the proposed strategy is shown to be universally effective for various polymer nanocomposites. Our processed porous PEBA/MWCNT nanocomposite films show a promising alternative to conventional EMI shielding materials. [ABSTRACT FROM AUTHOR]

Lamellar splaying is an important non-crystallographic branching phenomenon in polymer spherulite growth. The origin of lamellar splaying is commonly attributed to the repulsive action caused by cilia, but recent studies using solid nuclear magnetic resonance indicate that there are no cilia in the amorphous phase between adjacent lamellae. In order to further understand the origin of lamellar splaying, we reported a method to prepare poly(L -lactic acid) (PLLA) multilayer lamellae by film isothermal crystallization from the melt state in supercritical CO2. We obtained screw terrace crystals with three different surface amorphous conformations assembled from three PLLA samples with different molecular weights. According to the characterization results of atomic force microscopy, transmission electron microscopy, nuclear magnetic resonance and X-ray diffraction, three kinds of intercrystalline phases were determined as no fold, tight fold, and loose fold. It was further found that the no fold and tight fold rarely contribute to lamellar splaying of PLLA, but the loose fold on the lamellar surface obviously contributes to lamellar splaying of PLLA. Furthermore, the molecular weight dependence and film thickness dependence of the PLLA crystal form indicated that the loose fold may be caused by the limit of in-plane entanglements in the lamellar thickening growth. [ABSTRACT FROM AUTHOR]

Lamellar splaying is an important non-crystallographic branching phenomenon in polymer spherulite growth. The origin of lamellar splaying is commonly attributed to the repulsive action caused by cilia, but recent studies using solid nuclear magnetic resonance indicate that there are no cilia in the amorphous phase between adjacent lamellae. In order to further understand the origin of lamellar splaying, we reported a method to prepare poly(L -lactic acid) (PLLA) multilayer lamellae by film isothermal crystallization from the melt state in supercritical CO2. We obtained screw terrace crystals with three different surface amorphous conformations assembled from three PLLA samples with different molecular weights. According to the characterization results of atomic force microscopy, transmission electron microscopy, nuclear magnetic resonance and X-ray diffraction, three kinds of intercrystalline phases were determined as no fold, tight fold, and loose fold. It was further found that the no fold and tight fold rarely contribute to lamellar splaying of PLLA, but the loose fold on the lamellar surface obviously contributes to lamellar splaying of PLLA. Furthermore, the molecular weight dependence and film thickness dependence of the PLLA crystal form indicated that the loose fold may be caused by the limit of in-plane entanglements in the lamellar thickening growth. [ABSTRACT FROM AUTHOR]

Porous carbon materials have a broad range of potential applications, such as in electrochemical energy storage, filtration, catalysis, sensors, hydrogen storage, automobile exhaust treatment and so on. In this work, morphology evolution of metal–organic frameworks on a 3D metal substrate is explored via a simple binary-solvent method. With this rationally designed precursor, a graphene-like N-doped hierarchical porous carbon array on a 3D metal substrate (NPC/Cu) is obtained by a green and one-step vacuum de-metal assisted carbonization process. In situ-produced low boiling-point zinc elements can be evaporated and recycled during this process. The unique architecture where an N-doped porous carbon array is grown on a 3D metal substrate favors the graphitization degree and areal capacities of carbon anodes. When NPC/Cu is applied as a self-standing and binder-free anode for potassium-ion batteries (KIBs) in various electrolytes, an eminent electrochemical performance with a high reversible capacity of 315 mA h g−1 at 50 mA g−1 after 500 cycles, a superior rate performance of 120 mA h g−1 at 21C, and a relatively stable capacity of 129 mA h g−1 at 2000 mA g−1 after 20 000 cycles (corresponding to a capacity decay of only 0.0034% per cycle) can be obtained in a 5 M concentrated ether electrolyte. Furthermore, the superior electrochemical performance, outperforming that of most of the reported carbonaceous anodes for KIBs, can be attributed to the potassium storage mechanism dominated by capacitive-controlled behavior, proven by quantitative kinetics analysis. This work can shed some light on searching for carbon-based materials for KIBs and offer new insight to construct porous materials. [ABSTRACT FROM AUTHOR]

In this study, we used a high-pressure foam injection molding process to fabricate microcellular high-impact polystyrene (HIPS) foams with a tailored cellular structure. The process is cost-effective, highly efficient and flexible, and can be easily scaled up to complex components. The cellular structure of HIPS foam can be tuned over a wide range by manipulating packing time, cooling time, mold temperature, and mold-opening distance. Microcellular HIPS foam with a weight reduction of up to 60% was prepared, which possesses a low thermal conductivity of 60 mW m−1 K−1 and an ultra-low dielectric constant of 1.25. Both the thermal conductivity and the dielectric constant can be tailored by regulating the expansion ratio of HIPS foam. Mathematical models based on the mixing rule were developed to clarify the dependence of thermal conductivity and dielectric constant on the cellular structure of the foam. The outstanding thermally and electrically insulating properties of HIPS foams come from a large amount of air in the microcellular structure. These lightweight, thermally insulating, and ultralow dielectric microcellular HIPS foams hold great promise as an ultra-efficient insulating material for future use in many applications such as microelectronics and microelectromechanical systems (MEMSs). [ABSTRACT FROM AUTHOR]

High-performance EMI shielding materials with renewable characteristics are needed to address the issue of electromagnetic radiation pollution. The use of traditional metal-based EMI shielding materials is limited by their high density, corrosiveness, and expensive processing costs. At the same time, using regular fossil-fuel-driven conductive polymer composite-based EMI shielding materials creates environmental pollution, exacerbates resource consumption, and offers poor electromagnetic shielding effectiveness. Moreover, most of the processing methods used for conductive polymer composite-based EMI shielding materials are focused on a batch-scale process, which cannot easily be scaled up. We studied a sustainable foam injection molding-based method to efficiently fabricate renewable microcellular PLA/graphite nanocomposite foams, with improved mechanical and electrical properties for ultra-efficient EMI shielding applications. A microcellular PLA/graphite nanocomposite foam, with a density of 0.7 g cm−3 and a thickness of 2.0 mm, exhibits an outstanding EMI shielding performance with a total electromagnetic interference shielding effectiveness (EMI SE) of up to 45 dB. More importantly, thanks to the reduced reflection, which resulted from the strong thin-film interference effect, this lightweight porous nanocomposite foam has an absorption-dominated EMI shielding feature with a radiation energy reflection of less than 15%. The nanographite reorientation, which resulted from foaming, led to the microcellular PLA/graphite nanocomposite foam's electrical conductivity being dramatically increased by almost six orders of magnitude, in relation to the unfoamed sample. Furthermore, the microcellular PLA/graphite nanocomposite foam also exhibited outstanding mechanical properties. These were characterized by a strong specific strength and modulus, and super-ductile fracture behavior. Thus, this lightweight sustainable nanocomposite foam demonstrated great promise as an ultra-efficient EMI shielding material for future use in many applications such as aerospace and electronics. [ABSTRACT FROM AUTHOR]

Since CO2 is a kind of nontoxic, non-flammable and biocompatible fluid, introducing CO2 in the PLLA formation process has been regarded as a green way to the manufacture of biological products or medical supplies. However, it is still a challenge to understand the influence of CO2 on the crystal growth behavior of PLLA. Here, we developed an in situ high-pressure observation system, composed of optics, polarization optics and a small angle laser scattering system, to record the growth process of PLLA crystals in a pressurized CO2 environment. It is found that, at a low temperature (near Tg), low pressure CO2 (0.5 MPa in this work) can still induce the formation of numerous micron-sized spherulites of PLLA. Therefore, the introduction of CO2 can significantly enhance the crystallization ability of PLLA and decrease the crystallization temperature, which is helpful in improving the mechanical properties of PLLA products. We also found that a snowflake-shaped crystal was assembled by rhombic lamellae under pressurized CO2. There is a melt accumulation zone surrounding the growth front of the snowflake-shaped crystal, indicating that the growth front nucleation is limited by the pressurized CO2. This melt accumulation zone is quite different from the melt depletion zone existing ahead of the reported dendritic crystal front. Interestingly, in a high-pressure CO2 environment, a kind of bamboo-like branch is formed in a rhythmic growth mode. The repeating unit of the bamboo-like branch is constructed by an asymmetric terrace crystal originated from screw dislocation in the melt accumulation zone. These results demonstrated that CO2 has a remarkable tunability on the polymer crystal morphology.