Mechanically robust, flame-retardant poly(lactic acid) biocomposites via combining cellulose nanofibers and ammonium polyphosphate

Expanding the application range of flame-retardant polymer biocomposites remains a huge challenge for a sustainable society. Despite largely enhanced flame retardancy, until now the resultant poly(lactic acid) (PLA) composites still suffer reduced tensile strength and impact toughness due to improper material design strategies. We, herein, demonstrate the design of a green flame retardant additive (ammonium polyphosphate (APP)@cellulose nanofiber (CNF)) via using the cellulose nanofibers (CNFs) as the green multifunctional additives hybridized with ammonium polyphosphate (APP). The results show that PLA composite with 5 wt % loading of APP@CNF can pass the UL-94 V-0 rating, besides a high limited oxygen index of 27.5%, indicative of a significantly enhanced flame retardancy. Moreover, the 5 wt % of APP@CNF enables the impact strength (σi) of the PVA matrix to significantly improve from 7.63 to 11.8 kJ/m2 (increase by 54%), in addition to a high tensile strength of 50.3 MPa for the resultant flame-retardant PLA composite. The enhanced flame retardancy and mechanical strength performances are attributed to the improved dispersion of APP@CNF and its smaller phase size within the PLA matrix along with their synergistic effect between APP and CNF. This work opens up a facile innovative methodology for the design of high-performance ecofriendly flame retardants and their advanced polymeric composites

Flexible pressure sensors via engineering microstructures for wearable human-machine interaction and health monitoring applications

Flexible pressure sensors capable of transducing pressure stimuli into electrical signals have drawn extensive attention owing to their potential applications for human-machine interaction and healthcare monitoring. To meet these application demands, engineering microstructures in the pressure sensors are an efficient way to improve key sensing performances, such as sensitivity, linear sensing range, response time, hysteresis, and durability. In this review, we provide an overview of the recent advances in the fabrication and application of high-performance flexible pressure sensors via engineering microstructures. The implementation mechanisms and fabrication strategies of microstructures including micropatterned, porous, fiber-network, and multiple microstructures are systematically presented. The applications of flexible pressure sensors with microstructures in the fields of wearable human-machine interaction, and ex vivo and in vivo healthcare monitoring are comprehensively discussed. Finally, the outlook and challenges in the future improvement of flexible pressure sensors toward practical applications are presented

Governing effects of melt viscosity on fire performances of polylactide and its fire-retardant systems

Extreme flammability of polylactide (PLA) has restricted its real-world applications. Traditional research only focuses on developing new effective fire retardants for PLA without considering the effect of melt viscosity on its fire performances. To fill the knowledge gap, a series of PLA matrices of varied melt flow index (MFI) with and without fire retardants are chosen to examine how melt viscosity affects its fire performances. Our results show that the MFI has a governing impact on fire performances of pure PLA and its fire-retardant systems if the samples are placed vertically during fire testing. PLA with higher MFI values achieves higher limiting oxygen index (LOI) values, and a lower loading level of fire retardants is required for PLA to pass a UL-94 V-0 rating. This work unveils the correlation between melt viscosity and their fire performance and offers a practical guidance for creating flame retardant PLA to extend its applications

High‐performance polymeric materials through hydrogen‐bond cross‐linking

It has always been critical to develop high‐performance polymeric materials with exceptional mechanical strength and toughness, thermal stability, and even healable properties for meeting performance requirements in industry. Conventional chemical cross‐linking leads to enhanced mechanical strength and thermostability at the expense of extensibility due to mutually exclusive mechanisms. Such major challenges have recently been addressed by using noncovalent cross‐linking of reversible multiple hydrogen‐bonds (H‐bonds) that widely exist in biological materials, such as silk and muscle. Recent decades have witnessed the development of many tailor‐made high‐performance H‐bond cross‐linked polymeric materials. Here, recent advances in H‐bond cross‐linking strategies are reviewed for creating high‐performance polymeric materials. H‐bond cross‐linking of polymers can be realized via i) self‐association of interchain multiple H‐bonding interactions or specific H‐bond cross‐linking motifs, such as 2‐ureido‐4‐pyrimidone units with self‐complementary quadruple H‐bonds and ii) addition of external cross‐linkers, including small molecules, nanoparticles, and polymer aggregates. The resultant cross‐linked polymers normally exhibit tunable high strength, large extensibility, improved thermostability, and healable capability. Such performance portfolios enable these advanced polymers to find many significant cutting‐edge applications. Major challenges facing existing H‐bond cross‐linking strategies are discussed, and some promising approaches for designing H‐bond cross‐linked polymeric materials in the future are also proposed

Flame retarded polymer nanocomposites: development, trend and future perspective

Polymer nanocomposites are a new class of flame retarded materials which have attracted much attention and considered as a revolutionary new flame retardant approach. A very small amount of nano flame retardants (normally < 5 wt%) can significantly reduce the heat release rate (HRR) and smoke emission (SEA) during the combustion of polymer materials. Moreover, the addition of nano flame retardants can also improve the mechanical properties of polymer materials compared with the deterioration of traditional flame retardants. This paper reviews the recent development in the flame retardant field of polymer nanocomposites and also introduces the related research in our lab. The challenges and problems are discussed and the future development of flame retarded polymer nanocomposites is prospected

Recent advances in polysaccharide-based carbon aerogels for environmental remediation and sustainable energy

Carbon aerogels (CAs) with controlled micro-nano pore features have been broadly explored as advanced materials for many important applications in industry. Although graphene and carbon nanotubes aerogels have shown excellent performances, their practical applications are severely limited by their high cost, complex preparation process and low production yield. As one class of carbon-rich natural resources, polysaccharides composed of C, H, O, have been considered as one more attractive precursors for the preparation of renewable, cost-effective and eco-friendly CAs due to their universal availability, renewability and low toxicity. As a class of carbon aerogels, polysaccharide-based carbon aerogels (PS-CAs) also possess high porosity, large surface area, excellent conductivity and good mechanical properties. Therefore, PS-CAs are potential carbon materials applied in environmental remediation and the energy fields. This review highlights the fabrication of PS-CAs, including carbon aerogels, activated carbon aerogels, heteroatom-doped carbon aerogels and carbon aerogels composites, and their potential applications for environmental remediation and sustainable energy (conversion and storage). Following this, key challenges and future perspectives for polysaccharide-based carbon aerogel materials are also briefly discussed. This critical review expects to significantly promote the creation of a sustainable future by utilizing renewable and sustainable natural resources

Preparation and characterization of hydrophobically modified polyacrylamide hydrogels by grafting glycidyl methacrylate

A modified polyacrylamide (PAM-g-GMA) has been prepared by ring-opening reaction of glycidyl methacrylate (GMA) monomer grafted onto the -COO - groups of partially hydrolytic polyacrylamide (PAM) chain. The modified polyacrylamide hydrogels were obtained via a free radical polymerization of PAM-g-GMA without adding any a crosslinker using potassium persulfate (KPS), as initiator, and triethanolamine (TEA), as a coupling agent in aqueous solution. The molecular structure of PAM-g-GMA was characterized by FT-IR, 1H-NMR, and the thermal behaviors of hydrogels were studied by DSC. Furthermore, the swelling property and compressive properties of PAM-g-GMA hydrogels were investigated. The results show that the modified polyacrylamide hydrogels exhibit a remarkable hydration-dehydration change in response to pH in aqueous media and also undergo dramatic increase in volume with increasing temperature. So the modified polyacrylamide hydrogels will have promising and wide applications such as pharmaceutical use, water retention, electrophoretic media and so on

Morphology, healing and mechanical performance of nanofibrillated cellulose reinforced poly(ε-caprolactone)/epoxy composites

In this study, nanofibrillated cellulose (NFC) reinforced poly(ε-caprolactone) (PCL)/epoxy composites were fabricated by a combination method of solvent exchange and melt mixing. Effects of NFC incorporation on morphology, healing capability and mechanical properties of PCL/epoxy systems were investigated. Domain size of the separated phases was reduced and the continuity of PCL-rich phase was increased with NFC addition. Both healing efficiency and mechanical properties were improved due to the uniform distribution and bridging effect of NFC within the polymer matrix. Upon the addition of merely 0.2 wt% of NFC, the healing efficiency was increased by about 26% and simultaneously tensile strength, elongating at break and impact strength were improved by about 27%, 38% and 38%, respectively. Additionally, glass transition temperature of the epoxy phase in the composite was increased by about 12.8 °C

Nonisothermal crystallization behavior of polypropylene-C60 nanocomposites

The nonisothermal crystallization behavior of polypropylene (PP) and PP-fullerene (C60) nanocomposites was studied by differential scanning calorimetry (DSC). The kinetic models based on the Jeziorny, Ozawa, and Mo methods were used to analyze the nonisothermal crystallization process. The onset crystallization temperature (Tc), half-time for the crystallization (t1/2), kinetic parameter (F(T)) by the Mo method and activation energy (ΔE) estimated by the Kissinger method showed that C60 accelerates the crystallization of PP, implying a nucleating role of C60. Furthermore, due to the reduced viscosity of PP by adding 5% C60, the parameters of crystallization kinetics for the PP-5%C60 nanocomposites changed remarkably relative to that of neat PP and when lower contents of C60 were added to PP

Morphological structure and mechanical properties of epoxy/ polysulfone/cellulose nanofiber ternary nanocomposites

Despite some work on epoxy resin/cellulose nanofiber (CNF) system, it remains unclear how CNF affects microstructure and mechanical properties of epoxy/polysulfone (PSF) binary blends so far. We herein introduced CNF into the blends via a combination of solvent exchange and melt mixing. Results show that epoxy/PSF binary blends display three distinct types of phase separated structures but slightly affecting mechanical properties because of poor interfacial adhesion. However, only adding small amount of CNF can enhance impact toughness and tensile strength due to improved interfacial adhesion, probably arising from hydrogen bonding interactions between CNF surfaces and matrix polymers, and penetrating and bridging effects of CNF between different phases. For example, compared with the epoxy/PSF blends, adding 0.2 wt% of CNF can increase impact strength by ∼49%. Additionally, 0.3 wt% of CNF can increase the glass transition temperature by ∼18 °C relative to the epoxy resin