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In the last decade, a great deal of research has been devoted to the design and development of graphene-based polymer nanocomposites characterized by a prescribed arrangement of the graphene-based nanosheets into spatially segregated 3D architectures. The formation of a continuous filler network obtained by confining the nanosheets into a constrained volume of the polymeric matrix is particularly attractive from a technological point of view. The preparation of segregated 3D graphene-based architectures facilitates the proper tailoring of the overall performance of the resulting polymer nanocomposites, providing significant improvements in terms of structural (i.e., mechanical properties) and functional (electrical properties, sensing ability, and adsorption/absorption capacity) features. This review focuses on polymer-based nanocomposites in two categories, namely bulk and porous (foam and aerogels) systems. These all share the common distinctive feature that relies on the peculiar arrangement of the graphene-based nanosheets in the form of a segregated yet continuous 3D assembly. For each class of materials, the main preparation strategies are presented and the resulting structure-property correlations are highlighted and discussed, together with the technological implications, and possible future directions.

Anisotropic aerogel-foam composites were developed by embedding a reduced graphene oxide (rGO)/chitosan aerogel directly into an open-cell polyurethane foam through an in situ bidirectional freeze-drying process. The resulting aerogel-foam composites possess both excellent compression-resilience performance and stable piezo-resistive properties due, respectively, to the excellent mechanical properties of polyurethane foams and to the presence of a chitosan-based aerogel loaded with rGO. The latter, indeed, provides outstanding electrical properties due to its conductive and parallel flat lamellar structure. It has been proven that both mechanical and piezo-resistive properties are stable even after 1000 loading/unloading cycles and a reduction of the electrical resistance of about 86% is observed upon the application of a 60% strain. The high sensitivity, long cycling life, and reliable performance over a wide strain range make this unique anisotropic aerogel-foam composite a highly promising candidate for the production of wearable sensors and healthcare monitoring devices.

Blends of poly(lactic acid)/polycaprolactone (PLA/PCL) were electrospun under various conditions to study the influence of solution concentration, feed rate and voltage supply on the morphology of the nanofibers. To improve compatibility and to help produce fine electrospun nanofibers, an L-lactide/caprolactone (LACL) copolymer was introduced as a compatibilizer in the PLA/PCL blends. It was found that the solution concentration was a principal governing factor. The mean diameter of the fibers increased with the solution concentration, feed rate and voltage. Too high of a concentration and feed rate caused the fibers to stick to each other. A slow feed rate, 10% solution concentration, and 20 kV voltage were capable of producing thin, smooth and uniform fibers. Preliminary biocompatibility assays of the nanofibers were conducted with NIH 3T3 cells. The cells grown on the nanofiber blend exhibited spindle-like morphologies. The addition of PCL and LACL copolymer was found to improve the biocompatibility of PLA nanofibers, suggesting their potential application as cell culture scaffolds.

Crosslinked poly(vinyl alcohol) (PVA)/cellulose nanofibril (CNF) hybrid aerogel micro-spheres with two different particle sizes were fabricated via a combination of the water-in-oil (W/O) emulsification process and the freeze-drying process. The aerogel micro-spheres were highly porous with a bulk density as low as 0.0047 g/cm3 for the large microspheres. The pore size of the microspheres ranged from nano- to micro-meters. Preliminary biocompatibility assays of the aerogel microspheres were investigated with NIH 3T3 cells to explore their potential application as cell culture scaffolds. The highly crosslinked aerogel microspheres were robust and were able to maintain their shape during the cell culture process. The live/dead assay showed that the cells could be seeded, attached, and proliferated on the surface of PVA/CNF aerogel microspheres. The fluorescence images showed that some of the cells migrated into the inner pores of the microspheres. Moreover, the large microspheres with larger average pore sizes had a higher cell count than that of the small microspheres. This study confirms that the PVA/CNF aerogel microspheres fabricated in this work are nontoxic and biocompatible. Furthermore, the interconnected, highly porous nanofibrous structure of the microspheres can successfully facilitate cell attachment, differentiation, and proliferation.

We present the polarization-dependent highly absorptive in Ka-band composition of conventional polyurethane foam filled with in situ synthesized aerogel coated by reduced graphene oxide (rGO). The rGO-based aerogel was in situ prepared into the open-cell polyurethane foam (PUF) skeleton through a bidirectional freeze-drying process. The aerogel is composed of the flat lamellas stacks, possessing the anisotropic structure and unique electromagnetic properties. Further improvement of the electromagnetic shielding ability was possible by the rGO coating introduction as a coupling layer between PUF and rGO-based aerogel. This enhances the overall conductivity of the resulting composites: 1.41 + 3.33i S/m vs. 0.9 + 2.45i S/m for PUF loaded with in situ synthesized aerogel without rGO coating.With this mechanically robust plane easy to process coating one could achieve &minus, 20 dB by power with the record light structure (0.0462 g/cm2). That could compete in view of the weight per cm2 even with graphene-based absorbers comprising either dielectric matching elements or back metal reflectors, or both.

Superhydrophobic and crosslinked poly(vinyl alcohol) (PVA)/cellulose nanofibril (CNF) aerogel microspheres were prepared via a combination of the water-in-oil (W/O) emulsification process with the freeze-drying process, followed by thermal chemical vapor deposition of methyltrichlorosilane. The oil phase and the cooling agent were judiciously selected to ensure that the frozen ice microspheres can be easily separated from the emulsion system. The silanized microspheres were highly porous with a bulk density ranging from 4.66 to 16.54mg/cm(3). The effects of the solution pH, stirring rate, and emulsifier concentration on the morphology and microstructure of the aerogel microspheres were studied. The highly porous structure of the ultralight aerogel microspheres demonstrated an ultrahigh crude oil absorption capacity (up to 116 times its own weight). This study provides a novel approach for the large-scale preparation of polymeric aerogel microspheres with well-controlled particle sizes that can be used for various applications including oil and chemical spill/leak clean-up.

In this study, a new method of fabricating conductive NR nanocomposites was proposed. Graphene oxide (GO) was obtained through oxidization and exfoliation of graphite and was employed as a reinforcing nanofiller for natural rubber (NR). GO/NR nanocomposites with different concentrations of GO were prepared by a latex mixing and casting method. The nanocomposites were characterized by Raman spectroscopy and TEM, and the thermal and mechanical properties were evaluated by TGA, DSC, tensile testing, and DMA. The results showed that the nanocomposite showed a segregated “weblike” morphology. The addition of GO slightly decreased the glass transition temperature of NR and improved the thermal stability of the material. The tensile strength and storage modulus of the nanocomposites significantly increased with the GO contents, indicating GO had a strong reinforcing tendency on NR. The electrically conductive reduced graphene oxide (r-GO)/NR nanocomposites were obtained by immersing GO/NR nanocomposites in a hydroiodic (HI) acid aqueous solution for reduction. HI acid can efficiently reduce the GO into graphene in 2 h at 60°C. The volumetric electrical conductivity of the nanocomposites increased with the r-GO contents, and the conductivity of the 5% r-GO/NR nanocomposite was as high as 1 S/m, which showed its promising application as stretchable electrical conductors and elastomers. POLYM. COMPOS., 38:E199–E207, 2017. © 2016 Society of Plastics Engineers

The graphene oxide aerogel microspheres were prepared by a novel approach, i.e. a combination of electrospraying and freeze-casting. The unique well-defined “center-diverging microchannel” dandelion-like structures were observed in the aerogel microspheres and a formation mechanism of the structure, i.e. radial-directional freezing–thawing, was proposed. After further thermal reduction, the reduced graphene oxide aerogel microspheres were obtained and the center-diverging microchannel structures can be remained. Due to their highly porous, hierarchical and hydrophobic structure as well as a random packing effect, the aerogel microspheres possess excellent adsorption ability for various organic solvents and oils, showing their potential applications in environmental pollutant treatment.

The graphene nanosheets (GNS)/polydimethylsiloxane (PDMS) composite films with out-of-plane dielectric actuation behavior were prepared through a layer-by-layer spin coating process. The GNS-PDMS/PDMS composite films with 1~3 layers of GNS-PDMS films were spin coated on top of the PDMS film. The dielectric, mechanical, and electromechanical actuation properties of the composite films were investigated. The dielectric constant of the GNS-PDMS3/PDMS composite film at 1 kHz is 5.52, which is 1.7 times that of the GNS-PDMS1/PDMS composite film. The actuated displacement of the GNS-PDMS/PDMS composite films is greatly enhanced by increasing the number of GNS-PDMS layers. This study provides a novel alternative approach for fabricating high-performance actuators with out-of-plane actuation behavior.

l-Lactide/caprolactone copolymer (LACL) was employed as a compatibilizer in the immiscible blends of polylactide (PLA) and polycaprolactone (PCL). Films of neat PLA, PLA/PCL blend, and PLA/PCL/LACL blends were prepared by solution casting and conditioning compression. The effects of LACL on the morphology, mechanical properties, and crystallization behaviors of the blends were investigated. The addition of LACL decreased the dimensions of the dispersed PCL domains in the ternary blends. The ultimate strain of the blend with 5% LACL was significantly improved and was accompanied by a small drop in tensile strength and modulus. Both the nonisothermal and isothermal cold crystallization of PLA were enhanced by PCL, and further improved by incorporating LACL. Polarized optical microscopy observation showed that the size of crystals decreased with the addition of PCL and LACL.

Thermoplastic polyurethane (TPU)/multiwalled carbon nanotubes (MWCNTs) composites were prepared through twin extrusion and microinjection moulding. The effect of processing temperature on the structure and properties of composites were studied. Scanning electron microscopy and rheological test reveal the realisation of well dispersion and network formation of carbon nanotube (CNT) in TPU. Enhanced electrical conductivity has been found with the increasing mould temperature from 25 to 150°C, post-moulding thermal treatment under 180°C for 1 h leads to a significant increase in the electrical conductivity by over three orders of magnitude for 5 wt-%MWCNT filled TPU composites. Images (SEM and TEM) characterise the morphology of CNT network under low and high temperatures and the microcontact reconstruction of CNT network is proposed to explain the enhancement of electrical conductivity. The increased crystallinity of TPU leads to the enhancement of tensile strength and Young's modulus of composites ...

The in-situ bulk polycondensation process in combination with a ball milling dispersion process was used to prepare the water blown multiwall carbon nanotubes (CNT)/polyurethane (PU) composite foam. The mechanical properties, piezoresistive properties, strain sensitivity, stress and resistance relaxation behaviors of the composite foams were investigated. The results show that the CNT/PU composite foam has a better compression strength than the unfilled polyurethane foams and a negative pressure coefficient behavior under uniaxial compression. The resistance response of CNT/PU nanocomposites foam under cyclic compressive loading was quite stable. The nanocomposite foam containing a weight fraction of carbon nanotubes close to the percolation threshold presents the largest strain sensitivity for the resistance. The characteristic of resistance relaxation of CNT/PU composite foam is different from the stress relaxation due to the different relaxation mechanism. During compressive stress relaxation, the CNT/PU foam composites have excellent resistance recoverability while poor stress recoverability.

Superhydrophobic poly(vinyl alcohol) (PVA)/cellulose nanofibril (CNF) aerogels with a unidirectionally aligned microtubular porous structure were prepared using a unidirectional freeze-drying process, followed by the thermal chemical vapor deposition of methyltrichlorosilane. The silanized aerogels were characterized using various techniques including scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, and contact angle measurements. The structure of the aerogels fully filled with polydimethylsiloxane (PDMS) was confirmed by SEM and optical microscopy. The mechanical properties of the resulting PDMS/aerogel composites were examined using both compressive and tensile tests. The compressive and tensile Young's moduli of the fully filled PDMS/aerogel composites were more than 2-fold and 15-fold higher than those of pure PDMS. This study provides a novel alternative approach for preparing high performance polymer nanocomposites with a bicontinuous structure.

Reinforced natural rubber (NR) nanocomposites were prepared by solution mixing. casting. and evaporation of pre-vulcanized natural rubber latex and an aqueous suspension of cellulose nanofibrils (CNFs) extracted from bleached eucalyptus kraft pulp. Scanning electron microscopy (SEM) images showed that there were no micro-scaled aggregates observed in the nanocomposites. The addition of CNFs changed the mechanical behavior of the nanocomposites as observed in tensile tests. The tensile strength was significantly enhanced with CNFs, but the material became brittle at higher CNFs loading levels. Dynamic mechanical analysis (DMA) proved the strong reinforcing tendency of CNFs in the NR matrix as the storage modulus of the nanocomposites increased with CNFs content across the entire temperature range. Thermal gravimetric analysis (TGA) showed that CNFs had a slightly negative impact on the thermal stability of NR nanocomposites. There was no change in the glass transition temperature of the rubber matrix with the addition of cellulose nanofibrils, which was characterized by differential scanning calorimetry (DSC) measurements.

Correction for ‘High piezo-resistive performances of anisotropic composites realized by embedding rGO-based chitosan aerogels into open cell polyurethane foams’ by Tianliang Zhai et al., Nanoscale, 2019, 11, 8835–8844.

Graphene oxide (GO) was incorporated into poly(lactic acid) (PLA) as a reinforcing nanofiller to produce composite nanofibrous scaffolds using the electrospinning technique. To improve the dispersion of GO in PLA and the interfacial adhesion between the filler and matrix, GO was surface-grafted with poly(ethylene glycol) (PEG). Morphological, thermal, mechanical, and wettability properties, as well as preliminary cytocompatibility with Swiss mouse NIH 3T3 cells of PLA, PLA/GO, and PLA/GO-g-PEG electrospun nanofibers, were characterized. Results showed that the average diameter of PLA/GO-g-PEG electrospun nanofibers decreased with filler content. Both GO and GO-g-PEG improved the thermal stability of PLA, but GO-g-PEG was more effective. The water contact angle test of the nanofiber mats showed that the addition of GO in PLA did not change the surface wettability of the materials, but PLA/GO-g-PEG samples exhibited improved wettability with lower water contact angles. The tensile strength of the composite nanofiber mats was improved with the addition of GO, and it was further enhanced when GO was surface grafted with PEG. This suggested that improved interfacial adhesion between GO and PLA was achieved by grafting PEG onto the GO. The cell viability and proliferation results showed that the cytocompatibility of PLA was not compromised with the addition of GO and GO-g-PEG. With enhanced mechanical properties as well as good wettability and cytocompatibility, PLA/GO-g-PEG composite nanofibers have the potential to be used as scaffolds in tissue engineering.