Exfoliated Graphene Leads to Exceptional Mechanical Properties of Polymer Composite Films

Molecular Orientation-Regulated Bioinspired Multilayer Composites with Largely Enhanced Mechanical Properties

Natural nacre's hierarchical brick-and-mortar architecture motivates intensive studies on inorganic platelet/polymer multilayer composites, targeting mechanical property enhancement only by two strategies: optimizing the size and alignment of inorganic platelets and improving the interfacial interaction between inorganic platelets and polymers. Herein, a new strategy of polymer chain orientation to enhance the property of bioinspired multilayered composites is presented, which facilitates more stress to be transferred from polymer layers to inorganic platelets by simultaneous stiffening of multiple polymer chains. To this end, bioinspired multilayer films consisting of oriented sodium carboxymethyl cellulose chains and alumina platelets are designed and fabricated by three successive steps of water evaporation-induced gelation in glycerol, high-ratio prestretching, and Cu²⁺ infiltration. Regulating the orientation state of sodium carboxymethyl cellulose leads to a large enhancement of mechanical properties, including Young's modulus (2.3 times), tensile strength (3.2 times), and toughness (2.5 times). It is observed experimentally and predicted theoretically that the increased chain orientation induces failure mode transition in the multilayered films from alumina platelet pull-out to alumina platelet fracture because more stress is transferred to the platelets. This strategy opens an avenue toward rational design and manipulation of polymer aggregation states in inorganic platelet/polymer multilayer composites and allows a highly effective increase in modulus, strength, and toughness.

OPPS Fibers with High Temperature Resistance and Excellent Antioxidant Properties by an Oxidation Method

Polyphenylene sulfide (PPS) fiber products have been widely used for separation and filtration in harsh environments due to their excellent chemical resistance and relatively economical price. However, the poor temperature and weak oxidation resistance of PPS significantly shorten its service life under high temperature and strong oxidation environments. Herein, we report a type of oxidation-modified PPS (OPPS) fibers with excellent high temperature and oxidation resistance. This is achieved by oxidizing the thioether sulfide groups in PPS molecular chains into sulfoxide and sulfone groups and cross-linking the intermolecular chains. Both experiments and density functional theory (DFT) calculations indicate that hypochlorous acid (HClO) molecules can rapidly oxidize the PPS fiber surface. In addition, molecular dynamics (MD) simulations prove that there are strong hydrogen bonds and van der Waals interactions between HClO molecules and OPPS molecular chains, which promote the penetration of HClO molecules into the interior of the fiber to complete the layer-by-layer oxidation. The prepared OPPS-20 fibers exhibit excellent structural stability under high temperature and strong oxidant environments. Impressively, the OPPS-20 nonwoven filter still exhibits a high dust filtration efficiency of 99.95% after aging at 320 °C for 12 h, and the corresponding pressure drop is 24 Pa. In addition, the OPPS-20 nonwoven filter also maintains excellent filtration performance after aging in 60% HNO3 solution for 12 h, and the filtration efficiency and pressure drop are 99.96% and 29 Pa, respectively. This work demonstrates that the novel OPPS fibers have excellent application prospects in the field of separation and filtration in harsh environments.

High-Strength Liquid Crystal Polymer-Graphene Oxide Nanocomposites from Water

We report on the morphology and mechanical properties of nanocomposite films derived from aqueous, hybrid liquid crystalline mixtures of rodlike aggregates of a sulfonated, all-aromatic polyamide, poly(2,2'-disulfonyl-4,4'-benzidine terephthalamide) (PBDT), and graphene oxide (GO) platelets. An isothermal step at 200 °C facilitates in situ partial thermal reduction of GO to reduced GO (rGO) in nanocomposite films. X-ray scattering studies reveal that PBDT-rGO nanocomposites exhibit both higher in-plane alignment of PBDT (the order parameter increases from 0.79 to 0.9 at 1.8 vol % rGO) and alignment along the casting direction (from 0.1 to 0.6 at 1.8 vol % rGO). From dynamic mechanical thermal analysis, the interaction between PBDT and rGO causes the β-relaxation activation energy for PBDT to increase with rGO concentration. Modulus mapping of nanocomposites using atomic force microscopy demonstrates enhanced local stiffness, indicating reinforcement. From stress-strain analysis, the average Young's modulus increases from 16 to 37 GPa at 1.8 vol % rGO and the average tensile strength increases from 210 to 640 MPa. Despite polymer alignment along the casting direction, an average transverse tensile strength of 230 MPa is obtained.

Phononics of Graphene and Related Materials

In this Perspective, I present a concise account concerning the emergence of the research field investigating the phononic and thermal properties of graphene and related materials, covering the refinement of our understanding of phonon transport in two-dimensional material systems. The initial interest in graphene originated from its unique linear energy dispersion for electrons, revealed in exceptionally high electron mobility, and other exotic electronic and optical properties. Electrons are not the only elemental excitations influenced by a reduction in dimensionality. Phonons-quanta of crystal lattice vibrations-also demonstrate an extreme sensitivity to the number of atomic planes in the few-layer graphene, resulting in unusual heat conduction properties. I outline recent theoretical and experimental developments in the field and discuss how the prospects for the mainstream electronic application of graphene, enabled by its high electron mobility, gradually gave way to emerging real-life products based on few-layer graphene, which utilize its unique heat conduction rather than its electrical conduction properties.

Creation of Highly Ordered "Nano-Mille-Feuille" Hard/Soft Nanoparticle Multilayers with Interparticle Cross-Linking by Diacetylene-Containing Chains

In this study, we attempted to create a precise nanostructure in which hard and soft layers were alternately stacked with a period of 5 nm. Such a periodically stacked structure can be used as a model material to elucidate the origin of the innovative mechanical property improvement proposed for Mg alloy systems. The soft layer was a single layer of nanoparticles made of a ternary copolymer containing a carbazole ring and both hydrocarbon and fluorocarbon chains. The hard layer was a single-particle layer made of magnetite nanoparticles; the surface of these particles was modified with long-chain diynoic acid. The multilayers of each single-particle layer were 54% crystalline and ordered with a D₀₀₁ crystallite size of approximately 20 nm. The nano-mille-feuille structure, in which the hard and soft layers are repeated in a single-digit nanoperiod, was 56% crystalline and ordered with a D₀₀₁ crystallite size of approximately 15 nm. Infrared absorption spectroscopy and X-ray photoelectron spectroscopy did not show any loss of both hard and soft layers. Even when the thickness/number of layers was changed variously, no remarkable decrease was observed in the order. Cross-linking between particles by ultraviolet irradiation polymerization was performed for the direct evaluation of the mechanical properties of this structure in the future. Even after topochemical polymerization between the modified chains, the order of the particle stacking structure was maintained.

Migration of Quaternary Ammonium Cations from Exfoliated Clay/Low-Density Polyethylene Nanocomposites into Food Simulants

Clay/polymer nanocomposites (CPNs) are polymers incorporating refined clay particles that are frequently functionalized with quaternary ammonium cations (QACs) as dispersion aids. There is interest in commercializing CPNs for food contact applications because they have improved strength and barrier properties, but there are few studies on the potential for QACs in CPNs to transfer to foods under conditions of intended use. In this study, we manufactured low-density poly(ethylene) (LDPE)-based CPNs and assessed whether QACs can migrate into several food simulants under accelerated storage conditions. QACs were found to migrate to a fatty food simulant (ethanol) at levels of ∼1.1 μg mg^-1 CPN mass after 10 days at 40 °C, constituting about 4% total migration (proportion of the initial QAC content in the CPN that migrated to the simulant). QAC migration into ethanol was ∼16× higher from LDPE containing approximately the same concentration of QACs but no clay, suggesting that most QACs in the CPN are tightly bound to clay particles and are immobile. Negligible QACs were found to migrate into aqueous, alcoholic, or acidic simulants from CPNs, and the amount of migrated QACs was also found to scale with the temperature and the initial clay concentration. The migration data were compared to a theoretical diffusion model, and it was found that the diffusion constant for QACs in the CPN was several orders of magnitude slower than predicted, which we attributed to the potential for QACs to migrate as dimers or other aggregates rather than as individual ions. Nevertheless, the use of the migration model resulted in a conservative estimate of the mass transfer of QAC from the CPN test specimens.