Carbon Nanofibrous Aerogels Derived from Electrospun Polyimide for Multifunctional Piezoresistive Sensors.

The fabrication of carbon aerogels with ultralow density, high electrical conductivity, and ultraelasticity still remains substantial challenges. This study utilizes electrospun polyimide aerogel as the source to fabricate flexible carbon nanofibrous aerogel (PI-CNA) capable of multifunctional applications. The lightweight PI-CNA based piezoresistive sensor shows a wide linear range (0-217 kPa), rapid response/recovery time, and fatigue resistance (12,000 cycles). More importantly, the superior pressure sensing enables the PI-CNA for all-range healthcare sensing, including pulse monitoring, physiological activity detection, speech recognition, and gait recognition. Moreover, the EMI SE and the A coefficient of the PI-CNA reach 45 dB and 0.62, respectively, indicating the outstanding absorption dominated EMI shielding effects due to the multiple reflections and absorption. Furthermore, PI-CNA exhibits satisfying Joule heating performance up to 120 °C with rapid response time (10-30 s) under low supply voltages (1.5-5 V) and possesses sufficient heating reliability and repeatability in long-term repeated heating/cooling cycles. The fabricated PI-CNA shows significant potential applications in wearable technologies, energy conversion, electronic skin, and artificial intelligence.

Polyvinyl alcohol flame retardant film based on halloysite nanotubes, chitosan and phytic acid with strong mechanical and anti-ultraviolet properties.

The research of additive biomass flame retardants is becoming more and more popular. In this work, amino modified halloysite nanotubes (A-HNTs), chitosan (CS) and phytic acid (PA) were introduced into polyvinyl alcohol (PVA) matrix to construct PA/A-HNT/CS/PVA organic-inorganic composite film with hydrogen bond and covalent bond cross-linking network structure. Adding PA/A-HNT/CS can remarkably improve the mechanical strength, UV resistance and thermal stability of PVA film. Compared with control PVA film, the transmittance of composite film in ultraviolet region decreases from 90 % to <15 %, and the tensile strength raises from 19.8 MPa to 31.0 MPa. The thermal decomposition temperature of the composite film increases, the weight loss rate decreases obviously, and the carbon residue can reach 26 wt% at 700 °C. The limiting oxygen index increases from 18.5 % to 32.2 %. Furthermore, the addition of this flame-retardant system can obviously reduce the combustion intensity of PVA, and its flame-retardant grade can reach V-0. It is of great significance to expand the application of PVA and the development of biomass flame retardant.

Reactions between hydroxyl-substituted alkylperoxy radicals and Criegee intermediates: correlations of the electronic characteristics of methyl substituents and the reactivity.

The reactions of four hydroxyl-substituted alkylperoxy (RO2) radicals with four stabilized Criegee intermediates (SCIs) were investigated. Due to the existence of various reactive sites in both RO2 and SCI, various reaction modes were obtained. By adjusting the relative orientation of the two components of the reactants, different pathways were predicted. The addition of RO2 radical terminal oxygen atom to SCI carbonyl carbon is a favorable reaction mode. For RO2 radicals, increasing the number of methyl substituents in ß-carbon will promote the addition reaction. Carbonyl oxides with hydrogen atoms as substituents in the anti-position react faster than the corresponding carbonyl oxides with hydrogen atom substituents in the syn-position. Thus, the reaction barrier can be "tuned" by the substitution of alkyl groups. The analysis of the transition states have revealed that there were correlations between the reaction barrier heights, NPA charges and electron spin population of the terminal oxygen of RO2, as well as between the interatomic distances of O-C in transition states. As the oligomers formed by the sequential addition of SCIs to RO2 radicals are the common components of secondary organic aerosols, the investigation was able to contribute to understand the formation of SOA.

Understanding the microstructure of particle dispersion in confined copolymer nanocomposites.

The microstructures of diblock copolymer/particle composites confined in slitpores have been investigated using a density functional theory approach. It has been shown that, under the condition of confinement, particles display different distributions near the solid surfaces, in the microdomains of two blocks, and at the microphase interfaces. The final dispersion depends on the balance between the enthalpic contribution arising from the particle-segment attraction as well as the entropy-driven depletion attraction induced by the polymer conformation and the confinement environment. For the systems in which particles weakly attract one block but repel another block, particle dispersion can be enhanced by the increasing confinement effect, and the enhancement becomes more obvious as the size asymmetries of particles and two blocks increase. If the attraction increases, however, particle dispersion declines as the confinement effect increases.

Effect of chain topology of polyethylenimine on physisorption and chemisorption of carbon dioxide.

Polyethylenimine (PEI) is a promising candidate for CO2 capture. In this work, the physisorption and chemisorption of CO2 on various low-molecular-weight PEIs are investigated to identify the effect of chain architecture on sorption. The reliability of theoretical calculations are partially supported by our experimental measurements. Physisorption is calculated independently by the reference interaction-site model integral equation theory; chemisorption is distinguished from the total sorption given by the quantum density functional theory. It is shown that, as the chain length increases, both chemisorption and physisorption drop off nonlinearly, but the decay amplitude of chemisorption is more apparent. Conversely, as the amine group approaches the central triamine unit of each oligomer, the sorption capacity decreases, affecting the sorption equilibrium in a complex way. This arises from the cooperative contribution of an increased steric effect and renormalized electronic distribution.

Role of block copolymer morphology on particle percolation of polymer nanocomposites.

In this study, the effects of nanoparticle volume fraction, block stiffness, and diblock composition on the microstructure and electrical properties of composites are investigated using molecular dynamics simulation. It is shown that selective localization of conductive nanoparticles in a continuous block of diblock copolymer can dramatically reduce the percolation threshold. In the flexible-flexible copolymer systems with a relatively low particle loading, as the ratio of two blocks varies, one sees four kinds of phase structure: signal continuous, lamellar, co-continuous, and dispersed, corresponding to the order-disorder and continuity-dispersion transitions. In consideration of particle connectivity, the best electrical performance can be achieved with a special tri-continuous microstructure. While in the semi-flexible systems, the existence of rigid blocks can destroy the lamellar structure. If particles are located in the flexible block, a moderate stiffness of the rigid block can extensively enlarge the tri-continuous region, and high conductivity can be realized over a wide range of diblock compositions. If particles are located in the rigid block, however, high conductivity only emerges in a narrow composition range. In addition, the block should be prevented from becoming overstiff because this will cause direct particle aggregation.