Two-dimensional MXene nanosheets on nano-scale fibrils in hierarchical porous structure to achieve ultra-high sensitivity.

The complex hybrid nanostructure combining a two-dimensional (2D) conductive material and a hierarchical nanoscale skeleton plays an important role to enhance its piezoresistive sensitivity. To construct such a novel hybrid nanostructure, a piezoresistive sensor was designed with the following strategy to take the full advantages of 2D MXene and nanoscale fibrils: ethylene oxide propylene oxide random copolymer (EOPO) was grafted to ethylene-vinyl alcohol (EVOH) molecular chains and was foamed by an environmentally-friendly supercritical CO2 (scCO2) foaming technology to fabricate abundant nanoscale EVOH fibrils surrounding micropores; MXene featured as a 2D structure of nanoscale size that strongly interacted with this hierarchical nanoscale skeleton, and MXene not only convolved on nanoscale fibrils to generate bumps but also MXene covered the end of broken fibrils to build spots, and furthermore, MXene adhered on the soft EOPO embedded EVOH fibrils to form wrinkles, in which these bumps, spots and wrinkles assembled by highly conductive 2D MXene offered sufficient contacts when the hierarchical nanoscale skeleton was compressed (these contacts would then destruct when the skeleton recovered). Such an elaborated hybrid nanostructural design exploits the full potential of 2D MXene and hence achieves an ultra-high sensitivity of 6895.0 kPa-1 for this fabricated MXene piezoresistive sensor.

Efficient Electromagnetic Wave Absorption and Thermal Infrared Stealth in PVTMS@MWCNT Nano-Aerogel via Abundant Nano-Sized Cavities and Attenuation Interfaces.

Pre-polymerized vinyl trimethoxy silane (PVTMS)@MWCNT nano-aerogel system was constructed via radical polymerization, sol-gel transition and supercritical CO2 drying. The fabricated organic-inorganic hybrid PVTMS@MWCNT aerogel structure shows nano-pore size (30-40 nm), high specific surface area (559 m2 g-1), high void fraction (91.7%) and enhanced mechanical property: (1) the nano-pore size is beneficial for efficiently blocking thermal conduction and thermal convection via Knudsen effect (beneficial for infrared (IR) stealth); (2) the heterogeneous interface was beneficial for IR reflection (beneficial for IR stealth) and MWCNT polarization loss (beneficial for electromagnetic wave (EMW) attenuation); (3) the high void fraction was beneficial for enhancing thermal insulation (beneficial for IR stealth) and EMW impedance match (beneficial for EMW attenuation). Guided by the above theoretical design strategy, PVTMS@MWCNT nano-aerogel shows superior EMW absorption property (cover all Ku-band) and thermal IR stealth property (ΔT reached 60.7 °C). Followed by a facial combination of the above nano-aerogel with graphene film of high electrical conductivity, an extremely high electromagnetic interference shielding material (66.5 dB, 2.06 mm thickness) with superior absorption performance of an average absorption-to-reflection (A/R) coefficient ratio of 25.4 and a low reflection bandwidth of 4.1 GHz (A/R ratio more than 10) was experimentally obtained in this work.

Structural, energetic and dynamic investigation of poly(ethylene oxide) in imidazolium-based ionic liquids with different cationic structures.

In this work, two imidazolium-based ionic liquids (ILs) with different cations including dications (DIL) and monocations (MIL) were blended with poly(ethylene oxide) (PEO). The influence of ILs' structure on the structural and dynamic properties of a PEO/IL system was investigated by molecular dynamics (MD) simulation and density functional theory (DFT) methods. The simulation results show that DIL exhibits weaker interaction with PEO than MIL due to a stronger IL aggregation effect. The intermolecular interaction also makes the PEO chain tend to organize around the imidazolium ring of ILs, which causes the conformational entropy loss. Compared with PEO/MIL, this phenomenon is more significant in PEO/DIL because of the double positive centers of the dication and a longer hydrogen bond lifetime. MD simulation also demonstrates that DIL could act as a "crosslinker" to promote the formation of a physical crosslinking network which has strong dependence on the concentration of IL. The competition between physical crosslinking and plasticizing effects induces non-monotonic variations of relaxation time in PEO/DIL, which is consistent with its unusual change of the glass transition temperature (Tg). Despite stronger hydrogen bonding interactions between PEO and MIL demonstrated by atom-in-molecules (AIM) and reduced density gradient (RDG) analysis, the segmental mobility is slower in PEO/DIL according to the MSD curve. These differences in multiple structural or energetic factors finally lead to different conductive mechanisms and hence obtain different ionic conductivities.

Correction: From micro/nano structured isotactic polypropylene to a multifunctional low-density nanoporous medium.

[This corrects the article DOI: 10.1039/C6RA22607H.].

Synergistic Manipulation of Zero-Dimension and One-Dimension Hybrid Nanofillers in Multi-Layer Two-Dimension Thin Films to Construct Light Weight Electromagnetic Interference Material.

Nanocomposite foam with a large expansion ratio and thin cell walls is promising for electromagnetic interference (EMI) shielding materials, due to the low electromagnetic (EM) reflection and high EM absorption. To overcome the dimensional limitation from two-dimension (2D) thin walls on the construction of conductive network, a strategy combining hybrid conductive nanofillers in semi-crystalline matrix together with supercritical CO2 (scCO2) foaming was applied: (1) one-dimension (1D) CNTs with moderate aspect ratio was used to minimize the dimensional confinement from 2D thin walls while constructing the main EM absorbing network; (2) zero-dimension (0D) carbon black (CB) with no dimensional confinement was used to connect the separated CNTs in thin walls and to expand the EM absorbing network; (3) scCO2 foaming was applied to obtain a cellular structure with multi-layer thin walls and a large amount of air cells to reduce the reflected EM; (4) semi-crystalline polymer was selected so that the rheological behavior could be adjusted by optimizing crystallization and filler content to regulate the cellular structure. Consequently, an advanced material featured as lightweight, high EM absorption and low EM reflection was obtained at 0.48 vol.% hybrid nanofillers and a density of 0.067 g/cm3, whose specific EMI shielding performance was 183 dB cm3/g.

Wrong expectation of superinsulation behavior from largely-expanded nanocellular foams.

This work aims to predict the thermal conductivity of microcellular and nanocellular thermal insulation foams to explore the correlation between the cellular structure and the thermal insulating properties. Closed-cell foam consisting of cell walls and struts was used as the base geometry for modeling. The mathematical correlations to calculate the thickness of cell walls and the diameter of struts for a given cell size, the void fraction and the volume fraction of polymer located in struts were investigated. Then, a mathematical model for the conductive thermal conductivity including the dependency on the void fraction, the strut fraction and the Knudsen effect for gas was introduced. The radiative thermal conductivity was determined by analyzing the attenuation of radiative energy by absorption and scattering based on Mie's theory together with electromagnetic wave interference, as well as interference of propagating waves and tunneling of the radiative energy by evanescent waves in the cells. The thermal conductivity model was validated by experimental data and used to predict the thermal conductivity of polystyrene (PS) and poly(methyl methacrylate) (PMMA) foams at various cell sizes and volume expansion ratios. It was found that the radiative thermal conductivity plays a crucial role in nanocellular foam. The trade-off between the cell size and cell wall thickness when cell walls become thinner and highly transparent to thermal radiation was demonstrated, leading to the optimal volume expansion ratio at which the thermal conductivities were minimized. Perspectives for the manufacture of high-performance thermal insulation foams are also discussed.

Thermally Reversible and Irreversible Phase Transition Behaviors in Poly(ethylene oxide)/Ionic Liquid Mixtures.

The irreversible and reversible phase transition behaviors during phase separation-recovery (heating-cooling) cycles for poly(ethylene oxide)/1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid (PEO/[EMIM][BF4 ]) mixtures with a lower critical solution temperature phase diagram are reported for the first time. The evident differential scanning calorimetry endothermic and exothermic peaks are observed during the heating-cooling scan cycles near the phase boundary, in which the large heat loss for samples below the critical composition (60 wt% PEO) and obvious downward shift of phase transition temperature for all the compositions between the first and second cycles are particularly attractive. After the first recovery process, a reversible behavior during the next cycles is expected. These interesting phenomena are further confirmed by optical microscopy and Fourier-transform infrared measurements. It is demonstrated that the disruption and partial recovery of the hydrogen bonds, combined with the conformational change of PEO chains, can contribute to this irreversible behavior as well as a conversion to reversible phase transition behavior.

Development of high-porosity resorcinol formaldehyde aerogels with enhanced mechanical properties through improved particle necking under CO2 supercritical conditions.

A new high porosity resorcinol-formaldehyde (RF) aerogel with improved particle necking is presented in this work. This RF aerogel was developed under CO2 supercritical drying conditions without any structural shrinkage. The water content and the catalyst percentage were varied to modify the particles' nucleation and growth mechanisms and to control particle-particle connections. The nucleation mechanism solely dependent on the initial catalyst percentage; the number of nuclei increased with the catalyst percentage. However, the growth and connection of the particles dependent on both the water content and the catalyst percentage through their effect on the pH value. As the water content increased to have a larger void fraction, the pH value decreased. Consequently, the spherical growth of the particles became dominant and, thereby, the connection of the particles became more difficult. But as the catalyst percentage increased, the pH value increased, and the connection of the particles became facilitated with the formation of necks around the particles. As a result, the semi-fibril-like structure was developed with a high void fraction. A 30% increase in the structural elasticity and a very low thermal conductivity of 0.0249W/mK were obtained.