Inherently Conductive Poly(dimethylsiloxane) Elastomers Synergistically Mediated by Nanocellulose/Carbon Nanotube Nanohybrids toward Highly Sensitive, Stretchable, and Durable Strain Sensors.

With the rapid development of soft electronics, flexible and stretchable strain sensors are highly desirable. However, coupling of high sensitivity and stretchability in a single strain sensor remains a challenge. Herein, a kind of conductive elastomer is constructed with poly(dimethylsiloxane) (PDMS) and silylated cellulose nanocrystal (SCNC)/carbon nanotube (CNT) nanohybrids through a facile one-pot solution-casting method. The hydrophobic SCNCs can effectively facilitate the dispersion of CNTs in PDMS and synergistically improve the interfacial compatibility between CNTs and the PDMS matrix, resulting in favorable stress and electron transfer in the polymer network. Due to the outstanding electrical conductivity of CNTs and the excellent dispersity and high mechanical performance of SCNCs, combined with the good compatibility between SCNC-mediated carbon nanotubes (SCNC-CNTs) and PDMS, the resulting composite elastomer (SCNC-CNT/PDMS) shows high electrical conductivity (∼2.77 S m-1), tensile strength (∼5.72 MPa), and fatigue resistance properties. The strain sensor assembled by SCNC-CNT/PDMS demonstrates a high strain range above 100%, appealing strain sensitivity with a gauge factor of 37.11 at 50-100% strain, and long-term stability and durability, which is capable of monitoring both real-time human motions and acoustic vibrations. This work paves a new way for the design and controllable preparation of flexible and stretchable conductive elastomers, demonstrating promising applications in wearable devices and intelligent electronics.

Surface and Interface Engineering for Nanocellulosic Advanced Materials.

How do trees support their upright massive bodies? The support comes from the incredibly strong and stiff, and highly crystalline nanoscale fibrils of extended cellulose chains, called cellulose nanofibers. Cellulose nanofibers and their crystalline parts-cellulose nanocrystals, collectively nanocelluloses, are therefore the recent hot materials to incorporate in man-made sustainable, environmentally sound, and mechanically strong materials. Nanocelluloses are generally obtained through a top-down process, during or after which the original surface chemistry and interface interactions can be dramatically changed. Therefore, surface and interface engineering are extremely important when nanocellulosic materials with a bottom-up process are fabricated. Herein, the main focus is on promising chemical modification and nonmodification approaches, aiming to prospect this hot topic from novel aspects, including nanocellulose-, chemistry-, and process-oriented surface and interface engineering for advanced nanocellulosic materials. The reinforcement of nanocelluloses in some functional materials, such as structural materials, films, filaments, aerogels, and foams, is discussed, relating to tailored surface and/or interface engineering. Although some of the nanocellulosic products have already reached the industrial arena, it is hoped that more and more nanocellulose-based products will become available in everyday life in the next few years.

A stretchable, self-healing conductive hydrogels based on nanocellulose supported graphene towards wearable monitoring of human motion.

Stretchable, self-healing and conductive hydrogels have attracted much attention for wearable strain sensors, which are highly required in health monitoring, human-machine interaction and robotics. However, the integration of high stretchability, self-healing capacity and enhanced mechanical performance into one single conductive hydrogel is still challenging. In this work, a type of stretchable, self-healing and conductive composite hydrogels are fabricated by uniformly dispersing TEMPO-oxidized cellulose nanofibers (TOCNFs)-graphene (GN) nanocomposites into polyacrylic acid (PAA) hydrogel through an in-situ free radical polymerization. The resulting hydrogels demonstrate a stretchability (∼850 %), viscoelasticity (storage modulus of 32 kPa), mechanical strength (compression strength of 2.54 MPa, tensile strength of 0.32 MPa), electrical conductivity (∼ 2.5 S m-1) and healing efficiency of 96.7 % within 12 h. The hydrogel-based strain sensor shows a high sensitivity with a gauge factor of 5.8, showing great potential in the field of self-healing wearable electronics.

Self-Healable Electro-Conductive Hydrogels Based on Core-Shell Structured Nanocellulose/Carbon Nanotubes Hybrids for Use as Flexible Supercapacitors.

Recently, with the development of personal wearable electronic devices, the demand for portable power is miniaturization and flexibility. Electro-conductive hydrogels (ECHs) are considered to have great application prospects in portable energy-storage devices. However, the synergistic properties of self-healability, viscoelasticity, and ideal electrochemistry are key problems. Herein, a novel ECH was synthesized by combining polyvinyl alcohol-borax (PVA) hydrogel matrix and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-cellulose nanofibers (TOCNFs), carbon nanotubes (CNTs), and polyaniline (PANI). Among them, CNTs provided excellent electrical conductivity; TOCNFs acted as a dispersant to help CNTs form a stable suspension; PANI enhanced electrochemical performance by forming a "core-shell" structural composite. The freeze-standing composite hydrogel with a hierarchical 3D-network structure possessed the compression stress (~152 kPa) and storage modulus (~18.2 kPa). The composite hydrogel also possessed low density (~1.2 g cm-3), high water-content (~95%), excellent flexibility, self-healing capability, electrical conductivity (15.3 S m-1), and specific capacitance of 226.8 F g-1 at 0.4 A g-1. The fabricated solid-state all-in-one supercapacitor device remained capacitance retention (~90%) after 10 cutting/healing cycles and capacitance retention (~85%) after 1000 bending cycles. The novel ECH had potential applications in advanced personalized wearable electronic devices.

Electrospun Core-Shell Nanofibrous Membranes with Nanocellulose-Stabilized Carbon Nanotubes for Use as High-Performance Flexible Supercapacitor Electrodes with Enhanced Water Resistance, Thermal Stability, and Mechanical Toughness.

A high-performance flexible supercapacitor electrode with a core-shell structure is successfully developed from cellulose nanocrystal (CNC)-stabilized carbon nanotubes (CNTs). By incorporating poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA), a cross-linked nanofibrous membrane (CNT-CNC/PVA-PAA) is prepared as the core material via directional electrospinning, followed by a thermal treatment. The flexible supercapacitor electrodes are eventually fabricated via the in situ polymerization of polyaniline (PANI), which was used as the coating shell material, on the aligned electrospun nanofibers. By taking advantage of the thermally induced esterification cross-linking that occurs among PVA, PAA, and the CNT-CNC nanohybrids, the membranes present with enhanced water resistance, mechanical strength, and thermal stability. After the surface coating of the PANI shell, the optimized PANI@CNT-CNC/PVA-PAA nanofibrous membranes exhibit a large porosity, an enhanced specific surface area, a superior tensile strength of ∼54.8 MPa, and a favorable electroconductivity of ∼0.44 S m-1. As expected, the nanofibrous electrodes with a specific capacitance of 164.6 F g-1 can maintain 91% of the original capacitance after 2000 cycles. The symmetrical solid-state supercapacitor assembled by the nanofibrous electrodes shows an excellent capacitance of 155.5 F g-1 and a remarkable capacitance retention of 92, 90, and 89% after 2000 cycles under flat, bending, and twisting deformations, respectively.

Preparation of a highly crosslinked biosafe dental nanocomposite resin with a tetrafunctional methacrylate quaternary ammonium salt monomer.

The design of antimicrobial dental nanocomposite resin to prevent secondary dental caries and minimize biosafety problems is an important endeavor with both fundamental and practical implications. In the present work, a novel tetrafunctional methacrylate-based polymerizable quaternary ammonium monomer (TMQA) was synthesized with the aim of using it as an immobilized antibacterial agent in methacrylate dental composites, and its structure was characterized. The antibacterial action of TMQA and polymerized resin specimens against suspected cariogenic bacteria Streptococcus mutans were evaluated. Furthermore, the double bond conversion, contact angle, water sorption, solubility, heterogeneity, and crosslink density of the experimental resins with different concentrations of TMQA were investigated. CCK-8 and real-time cell analyses were used to evaluate the cytotoxicity of the experimental resins. The results showed that TMQA was successfully synthesized and had strong antibacterial properties against Streptococcus mutans. The experimental resins with different concentrations of TMQA had a similar degree of conversion and contact angle to the neat resin. With the addition of 4% TMQA to the resins, water absorption and solubility were reduced while their heterogeneity and crosslink density increased. The cell viability of each experimental group was similar to that of the neat resin group and was higher than that of the commercial adhesive single bond 2 group. Therefore, TMQA can be used to impart antibacterial properties to resins and increase the crosslink density of dental resin composites.

Antibacterial and thermomechanical properties of experimental dental resins containing quaternary ammonium monomers with two or four methacrylate groups.

Resins with strong antibacterial and thermomechanical properties are critical for application in oral cavities. In this study, we first evaluated the antibacterial effect of an unfilled resin incorporating 1, 4, and 7 mass% of quaternary ammonium salt (QAS) monomers containing two methacrylate groups (MAE-DB) and four methacrylate groups (TMH-DB) against Streptococcus mutans, and tested the cytotoxicity and thermomechanical properties of the 4 mass% MAE-DB and TMH-DB modified resin materials. A neat resin without a QAS monomer served as the control. As the concentration of both QAS monomers increases, the formation of a Streptococcus mutans biofilm on the experimental material is increasingly inhibited. The results of colony forming unit counts and the metabolic activity showed that both the MAE-DB and TMH-DB modified resins have a strong bactericidal effect on the bacteria in a biofilm, but no bactericidal effect on the bacteria in a solution. The viability-staining and morphology results also demonstrate that the bacteria deform, lyse, shrink, and die on the surface of the two QAS-modified resins. Cytotoxicity results show that the addition of TMH-DB can reduce the cytotoxicity of the resin, while the addition of MAE-DB increases the cytotoxicity of the resin. DMA results show that a TMH-DB modified resin has a higher storage modulus than a MAE-DB modified resin owing to its better crosslink density. The two groups of experimental resins showed a similar glass transition temperature. These data indicate that the two QAS monomers can impart similar antibacterial properties upon contact with a dental resin, whereas TMH-DB can endow the resin with a higher crosslink density and storage modulus than MAE-DB because it has more polymerizable groups.

Isolation and properties of cellulose nanofibrils from coconut palm petioles by different mechanical process.

In this study, cellulose nanofibrils (CNFs) were successfully isolated from coconut palm petiole residues falling off naturally with chemical pretreatments and mechanical treatments by a grinder and a homogenizor. FTIR spectra analysis showed that most of hemicellulose and lignin were removed from the fiber after chemical pretreatments. The compositions of CNFS indicated that high purity of nanofibrils with cellulose contain more than 95% was obtained. X-ray diffractogram demonstrated that chemical pretreatments significantly increased the crystallinity of CNFs from 38.00% to 70.36%; however, 10-15 times of grinding operation followed by homogenizing treatment after the chemical pretreatments did not significantly improve the crystallinity of CNFs. On the contrary, further grinding operation could destroy crystalline regions of the cellulose. SEM image indicated that high quality of CNFs could be isolated from coconut palm petiole residues with chemical treatments in combination of 15 times of grinding followed by 10 times of homogenization and the aspect ratio of the obtained CNFs ranged from 320 to 640. The result of TGA-DTG revealed that the chemical-mechanical treatments improved thermal stability of fiber samples, and the CNFs with 15 grinding passing times had the best thermal stability. This work suggests that the CNFs can be successfully extracted from coconut palm petiole residues and it may be a potential feedstock for nanofiber reinforced composites due to its high aspect ratio and crystallinity.