Microstructure and characterization of electrospun poly(vinyl alcohol) nanofiber scaffolds filled with graphene nanosheets

Graphene nanosheets (GNSs) have attracted significant scientific attention because of their remarkable features, including exceptional electron transport, excellent mechanical properties, high surface area, and antibacterial functions. Poly(vinyl alcohol) (PVA) solutions filled with GNSs were prepared for electrospinning, and their spinnability was correlated with their solution properties. The effects of GNS addition on solution rheology and conductivity were investigated. The as‐spun fibers were characterized via scanning electron microscopy (SEM), transmission electron microscopy (TEM), wide‐angle X‐ray diffraction (WAXD), and differential scanning calorimetry (DSC). The results revealed the effects of GNS on the microstructure, morphology, and crystallization properties of PVA/GNS composite nanofibers. The addition of GNSs in PVA solution increased the viscosity and conductivity of the solution. The electrospun fiber diameter of the PVA/GNS composite nanofiber was smaller than that of neat PVA nanofiber. GNSs were not only embedded at the fibers but also formed protrusions on the fibers. In addition, the crystallinity of PVA/GNS fiber decreased with higher GNS content. The possible application of PVA/GNS fibers in tissue engineering was also evaluated. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41891.     

Mechanical,electrical, and thermal properties of graphene nanosheet/aluminum nitride composites

Graphene nanosheet (GNS)/aluminum nitride (AlN) composites were prepared by hot-pressing and effects of GNSs on their microstructural, mechanical, thermal, and electrical properties were investigated. At 1.49 vol% GNSs content, the fracture toughness (5.09 MPa m^1/2) and flexural strength (441 MPa) of the composite were significantly increased by 30.17% and 17.28%, respectively, compared to monolithic AlN. The electrical conductivity of the composites was effectively enhanced with the addition of GNSs, and showed a typical percolation behavior with a low percolation threshold of 2.50±0.4 vol%. The thermal conductivity of the composites decreased with the addition of GNSs.     

Comparison of electrical properties between multi-walled carbon nanotube and graphene nanosheet/high density polyethylene composites with a segregated network structure

Multi-walled carbon nanotube (MWCNT)/high density polyethylene (HDPE) and graphene nanosheets (GNS)/HDPE composites with a segregated network structure were prepared by alcohol-assisted dispersion and hot-pressing. Instead of uniform dispersion in polymer matrix, MWCNTs and GNSs distributed along specific paths and formed a segregated conductive network, which results in a low electrical percolation threshold of the composites. The electrical properties of the GNS/HDPE and MWCNT/HDPE composites were comparatively studied, it was found that the percolation threshold of the GNS/HDPE composites (1 vol.%) was much higher than that of the MWCNT/HDPE composites (0.15 vol.%), and the MWCNT/HDPE composite shows higher electrical conductivity than GNS/HDPE composite at the same filler content. According to the values of critical exponent, t, the two composites may have different electrical conduction mechanisms: MWCNT/HDPE composite represents a three-dimensional conductive system, while the GNS/HDPE composite represents a two-dimensional conductive system. The improving effect of GNSs as conducting fillers on the electrical conductivity of their composites is far lower than theoretically expected.     

Thermal,electrical, and mechanical properties of polyethylene–graphene nanocomposites obtained by in situ polymerization

In this study, we investigated the thermal, dynamic mechanical, mechanical, and electrical properties of polyethylene (PE)–graphene nanosheet (GNS) nanocomposites, with GNS amounts from 0 to 20 wt %, prepared by in situ polymerization. The thermal stability was evaluated by thermogravimetric analysis (TGA) and showed that the addition of GNSs to the polyolefin matrix increased the onset degradation temperature by 30°C. The electrical conductivity, measured by the impedance technique, presented a critical percolation threshold of 3.8 vol % (8.4 wt %) of GNS. A slight decrease in the tensile strength was found. On the other hand, dynamic mechanical analysis showed an increase in the storage modulus of the nanocomposites compared with that of neat PE. The glass‐transition temperature value increased from ?111°C (neat PE) to ?106°C (PE/6.6 wt % GNS). All of these results show that PE became stiffer and thermally more stable and could be transformed from an insulator to a semiconductor material in the presence of GNSs. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Crystallization of isotactic polypropylene inside dense networks of carbon nanofillers

In this study, we performed the crystallization of carbon nanotube (CNT)/isotactic polypropylene (iPP) and graphene nanosheet (GNS)/iPP composites with very high nanofiller loadings; these are frequently used in polymer composites for electromagnetic interference shielding and thermal conductivity. Rheology testing indicated that both the high‐loading CNTs and GNSs formed dense networks in the iPP matrix, and transmission electron microscopy showed that their connection types were completely different: the CNTs contacted one another in a dot‐to‐dot manner, whereas the GNSs linked reciprocally in a plane‐to‐plane manner. The carbon nanofiller networks brought about two opposite effects on iPP crystallization: a nucleation effect and a confinement effect. The CNT network showed a stronger nucleation effect; however, the CNT network also revealed a more powerful confinement effect because the CNT network was denser than the GNS network. With increasing content of the carbon nanofillers, the crystallization rates of both the CNT and GNS composites first increased, then decreased, and showed a very high saturation concentration at 50 wt %; this resulted from the competitive relationship between the nucleation effect and confinement effect. The crystallization was facilitated when the carbon nanofiller concentration was below saturation, where the nucleation effect invariably played a dominant role. Although the crystallization was depressed when the carbon nanofiller concentration was above saturation, the nucleation effect was subdued, and the confinement effect was extensive. Compared to the GNS/iPP composites, the CNT/iPP composites showed a more depressed crystallization. The suppression mechanism is discussed with consideration of the local topological structure constructed by those two carbon nanofillers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39505.     

Effect of multiwall carbon nanotubes on thermomechanical and electrical properties of poly(trimethylene terephthalate)

Multiwalled carbon nanotubes (MWCNTs) were synthesized using chemical vapor deposition and poly(trimethylene terephthalate) (PTT)/MWCNT composites with varying amounts of MWCNTs were prepared by melt compounding using DSM micro‐compounder. Morphological characterization by SEM and TEM showed uniform dispersion of MWCNTs in PTT matrix upto 2% (w/w) MWCNT loading. Incorporation of MWCNTs showed no effect on percent crystallinity but affected the crystallite dimensions and increased the crystallization temperature. Dynamic mechanical characterization of composites showed an increase in storage modulus of PTT upon incorporation of MWCNTs above glass transition temperature. The electrical conductivity of PTT/MWCNT composites increased upon incorporation of MWCNTs and percolation threshold concentration was obtained at a loading of MWCNTs in the range of 1–1.5% (w/w). © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of the filler structure of carbon nanomaterials on the electrical,thermal, and rheological properties of epoxy composites

Carbon nanotubes (CNTs) and graphene nanosheets (GNSs) were used as fillers in epoxy composites with the aim of increasing the electrical and thermal conductivities of the composites. The filling of pristine CNTs produced the highest electrical conductivity (σ), whereas a high CNT functionalization and the two‐dimensional planar structure of GNSs were promising for improving the thermal conductivity. A combination of CNTs and GNSs exploited the advantages of both. When the CNT fraction was larger than 50 wt %, a higher σ was obtained. When a small amount of functionalized CNTs was added to the GNSs, the thermal conductivity was also increased. The rheological measurements revealed the lowest complex viscosity for the GNS filling and showed the exciting advantages of an easy processing. As a result, the mixed filling also exhibited a much lower viscosity than the pure CNT fillings. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

In situ fast polymerization of graphene nanosheets‐filled poly(methyl methacrylate) nanocomposites

A series of graphene nanosheets‐filled poly(methyl methacrylate) nanocomposites (GNS/PMMA) is successfully prepared by an in situ fast polymerization method with graphene weight fractions from 0.1 to 2.0 wt %. In situ polymerization is effective in well dispersing of GNS in matrixes and suitable for both low and high content of GNS. The synthesis processes of polymer composites could be simplified and fast by using industrial grade graphene. The GNS fillers are found to disperse homogeneously in the PMMA matrix. The maximum electrical conductivity of the composites achieves 0.57 S m^?1, with an extremely low percolation threshold of 0.3 wt %. The electrical conductivities are further predicted by percolation theory and found to agree well with the experimental results. The results indicate that the microstructures, thermal, electrical, and mechanical properties of PMMA polymer are significantly improved by adding a low amount of graphene nanosheets. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43423.     

Preparation of chitosan–graphene nanosheet composites with enhanced electrochemical and mechanical properties

In this study, we proposed an alternative way to produce graphene nanosheet (GNS)/chitosan (CS) composites. Graphite‐intercalating compounds (GICs) were prepared by graphite and m‐chloroperbenzion acid (M‐CPBA) as starting materials. The GNS–CS composites were produced through the mixing of the GICs into CS and the removal of impurities in the final step. X‐ray diffraction, atomic force microscopy, and transmission and scanning electron microscopy analyses showed that graphite could be effectively exfoliated by an intercalating agent (M‐CPBA) and the GNSs were homogeneously dispersed into the CS matrix. Furthermore, we demonstrated that the electrochemical behavior and mechanical properties of the GNS–CS composites were significantly improved with a low level of GNSs in CS. The redox peak current increased 509%, and the modulus and hardness of the GNS–CS composites increased by 2.7 and 0.15 GPa, respectively, when the GNSs addition was 0.6 wt %. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45104.     

Structural variations and morphological features of polyethylene/carbon black conductive composites after processing in an internal mixer

Polyethylene (PE) composites filled with carbon black (CB) were prepared using an internal mixer. Several analytical techniques, including rheometry, gel permeation chromatography, electrical conductivity measurements, differential scanning calorimetry, wide‐angle X‐ray diffraction, and transmission electron microscopy (TEM), were used to reveal the structural variations, thermal degradation, morphological features, and crystallization of the PE/CB conductive composites. It was found that the PE polymer chains were degraded, forming long‐chain branching structures after over 30 min of internal mixing. The electrical conduction of the PE/CB composites was determined by the filler content and distribution. The electrical percolation threshold of the PE/CB composites was determined to be between 20 and 30 wt %. The addition of CB had no significant influence on the crystallinity of the PE/CB composites. In contrast, the electron‐beam radiation dose had a significant effect on crystallinity. TEM micrographs of the PE/CB composites exhibit a random four‐phase morphology, including PE lamellae, PE amorphous, CB particles, and voids at the PE/CB interface. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1038‐1046, 2013     

Preparation and electrical properties of graphene nanosheet/Al2O3 composites

Fully dense graphene nanosheet(GNS)/Al₂O₃ composites with homogeneously distributed GNSs of thicknesses ranging from 2.5 to 20 nm have been fabricated from ball milled expanded graphite and Al₂O₃ by spark plasma sintering. The percolation threshold of electrical conductivity of the as-prepared GNS/Al₂O₃ composites is around 3 vol.%, and this new composite outperforms most of carbon nanotube/Al₂O₃ composites in electrical conductivity. The temperature dependence of electrical conductivity indicated that the as-prepared composites behaved as a semimetal in a temperature range from 2 to 300 K.     

Preparation and physical properties of functionalized graphene/waterborne polyurethane UV‐curing composites by click chemistry

Functionalized graphene nanoplatelets (f‐GNS) were modified with (3‐mercaptopropyl)trimethoxysilane (MPTMS) to enhance their compatibility with the polyurethane coating matrix. The results of Fourier transform infrared spectroscopy, AFM, Raman and XRD showed that the MPTMS was successfully attached onto the surface of the graphene nanoplatelets. Functionalized graphene/waterborne polyurethane acrylate (f‐GNS/WPUA) nanocomposites were fabricated by UV‐curing technology. The SEM and TEM images indicated that f‐GNS could be well dispersed in the polymer matrix and improved the interfacial adhesion. With the incorporation of 1 wt% f‐GNS, the thermal decomposition temperature of the composites was increased by 25 °C. Meanwhile, the conductivity, hydrophobicity and tensile strength were increased. When the load was further increased, the performance of the composites showed varying degrees of reduction. However, the dielectric loss tangent (tan δ) could be maintained at 0.08 or less and the electromagnetic shielding factor of the composites reached from 5 to 36 dB, showing a good electromagnetic shielding effect at a high content (2.5 wt% f‐GNS). It was considered that f‐GNS could disperse in the waterborne polyurethane well and crosslink with the polyurethane. © 2016 Society of Chemical Industry     

Graphene nanosheets‐filled epoxy composites prepared by a fast dispersion method

Graphene nanosheets‐filled epoxy composites (GNS/Epoxy) were prepared at different filler loading levels from 0.25 to 3.00 wt %. A fast dispersion method as short as 5 min is employed to disperse GNS in epoxy matrix, which was enough for the homogeneous dispersion of GNS with the help of high ultrasonic frequency of 100 kHz and power of 200 W and high heat treatment temperature of 70 °C. The maximum electrical conductivity and thermal conductivity of the composites achieved 0.058 S m^?1 and 0.57 W m^?1 K^?1, respectively, with a low electrical percolation threshold of 1.50 wt %. The electrical conductivities were further predicted by percolation theory and found to agree well with the experimental results, which indicated that the graphene nanosheets dispersed very well in the matrix even at very short processing time. The results showed that the microstructures, thermal, electrical, and mechanical properties of epoxy polymer were significantly improved by adding a low amount of graphene nanosheets. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45152.     

Using a non-covalent modification to prepare a high electromagnetic interference shielding performance graphene nanosheet/water-borne polyurethane composite

We prepared flexible, lightweight, and high electromagnetic interference (EMI) shielding performance graphene nanosheet (GNS)/water-borne polyurethane (WPU) composites. WPU, with sulfonate functional groups, was used as the polymer matrix. By adsorbing the cationic surfactant (stearyl trimethyl ammonium chloride) on the surface of the GNSs (S-GNSs), restacking and aggregation of the GNSs have been efficiently suppressed, which also attracted sulfonate groups from the WPU matrix. Because of the favorable interfacial interactions arising from electrostatic attraction, the S-GNS exhibited good compatibility with the WPU matrix. Such a homogeneous dispersion contributed to the construction of an electrical conductive network. The S-GNS/WPU composite exhibited a low electrical conductivity percolation threshold and an outstanding enhanced electrical conductivity of approximately 5.1 S/m. A high EMI shielding effectiveness of approximately 32 dB was obtained by the WPU composites with contents of 5 vol.% (approximately 7.7 wt.%) S-GNSs.     

Electrical conductivity,dielectric and microwave absorption properties of graphene nanosheets/magnesia composites

Herein, a novel microwave absorbing material with Graphene nanosheets (GNSs) as microwave absorbing filler and magnesia (MgO) as matrix were prepared by hot-pressing sintering. The composites were highly dense with a homogeneous distribution of GNSs. Electrical conductivity, dielectric and microwave absorption properties in X-band were investigated. The results revealed that the electrical conductivity of the GNSs/MgO composites showed a typical percolation-type behavior with a percolation threshold of 3.34 vol%. With GNSs content increased to 3 vol%, the real permittivity, imaginary permittivity and dielectric loss tangent of the composites increased from ～9, ～0 and ～0 to 26–43, 23–28 and 0.55–0.96, respectively. By adjusting the GNSs content, thickness and frequency, the 2.5 vol% GNSs/MgO composite shows the minimum reflection loss of ?36.5 dB at 10.7 GHz and the reflection loss below ?10 dB (90% absorption) ranges from 9.4 to 11.4 GHz with 1.5 mm thickness, exhibiting excellent microwave absorption properties.     

Thermal conductivity and structure of non-covalent functionalized graphene/epoxy composites

Non-covalent functionalization was used to functionalize graphene nanosheets (GNSs) through π–π stacking of pyrene molecules with a functional segmented polymer chain, which results in a remarkable improvement in the thermal conductivity of GNS-filled polymer composites. The functional segmented poly(glycidyl methacrylate) containing localized pyrene groups (Py-PGMA) was prepared by atom transfer radical polymerization, and Py-PGMA was characterized by nuclear magnetic resonance spectroscopy. Raman spectra, X-ray photoelectron spectroscopy and thermogravimetric analysis reveal the characteristics of Py-PGMA–GNS. Differential scanning calorimetry indicated that the functional groups on Py-PGMA–GNSs can generate covalent bonds with the epoxy matrix, and further form a cross-linked structure in Py-PGMA–GNS/epoxy composites. The Py-PGMA on the GNS surface not only plays an important role to facilitate a homogeneous dispersion in the polymer matrix but also improves the GNS–polymer interaction, which results in a high contact area. Consequently, the thermal conductivity of integrated Py-PGMA–GNS/epoxy composites exhibited a remarkable improvement and is much higher than epoxy reinforced by multi-walled carbon nanotubes or GNSs. The thermal conductivity of 4 phr Py-PGMA–GNS/epoxy has about 20% (higher than that of pristine GNS/epoxy) and 267% (higher than pristine MWCNT/epoxy).     

Banded spherulites of electrospun poly(trimethylene terephthalate)/carbon nanotube composite mats

The crystallization of poly(trimethylene terephthalate) (PTT) composites containing carbon nanotubes (CNTs) were studied in this work. The electrospinning technology was employed successfully to fabricate thin film samples with well‐embedded CNTs for spherulite observations using atom force microscopy. The results show that the composites present a higher overall crystallization rate than that of the neat PTT due to the nucleation effect of the CNTs. Banded spherulites can be observed on both the neat PTT and the composites. The presence of CNTs does not change the twisting mode of PTT crystal, but reduces band spacing and twist period. This is attributed to the enhanced fold staggering level of lamellae caused by the narrowed lamellae size and accelerated spherulite growth, which is further confirmed by analysis through secondary nucleation theory. Copyright © 2011 Society of Chemical Industry     

Isothermal crystallization kinetics and melting behaviors of nanocomposites of poly(trimethylene terephthalate) filled with nano‐CaCO3

The isothermal crystallization and subsequent melting behavior of poly(trimethylene terephthalate) (PTT) composites filled with nano‐CaCO₃ were investigated at designated temperatures with differential scanning calorimetry. The Avrami equation was used to fit the isothermal crystallization. The Avrami exponents were determined to be 2–3 for the neat PTT and PTT/CaCO₃ composites. The particles of nano‐CaCO₃, acting as nucleating agents in the composites, accelerated the crystallization rate, with the half‐time of crystallization decreasing or the growth rate constant (involving both nucleation and growth rate parameters) increasing. The crystallization activation energy calculated from the Arrhenius formula was reduced as the nano‐CaCO₃ content increased from 0 to 2%, and this suggested that nano‐CaCO₃ made the molecular chains of PTT easier to crystallize during the isothermal crystallization process. Subsequent melting scans of the isothermally crystallized composites exhibited triple or double melting endotherms: the greater the content was of nano‐CaCO₃, the lower the temperature was of the melting peak. The degree of crystallization deduced from the melt enthalpy of composites with the proper concentration of nano‐CaCO₃ was higher than that of pure PTT, but it was lower when the nano‐CaCO₃ concentration was more than 2%. The transmission electron microscopy pictures suggested that the dispersion state of nano‐CaCO₃ particles in the polymer matrix was even when its concentration was no more than 2%, whereas some agglomeration occurred when its concentration was 4%. Polarized microscopy pictures showed that much smaller or less perfect crystals formed in the composites because of the interaction between the molecular chains and nano‐CaCO₃ particles. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Melting behaviors,isothermal crystallization kinetics,and morphology of poly(trimethylene terephthalate)/stainless steel fiber composites

The melting behavior and crystallization kinetics of poly(trimethylene terephthalate) (PTT)/stainless steel fiber (SSF) composites were investigated with differential scanning calorimetry. The morphology was studied with scanning electron microscopy and polarized optical microscopy. Differential scanning calorimetry analysis revealed that the crystallization temperature increased by 27°C with the addition of 1 vol % SSF to the matrix. The Avrami exponents, analyzed in isothermal crystallization kinetics, were determined to be 2–3 for both neat PTT and PTT/SSF composites. SSF, as a nucleating agent in the composites, greatly increased the crystallization rate. The activation energies of the composites were obviously lower than that of pure PTT, and this indicated much easier crystallization of the composites. All these samples exhibited banded spherulites, and the spherulite size gradually decreased with the SSF loading increasing. Subsequent melting behaviors revealed that all of these samples, especially of the composites, exhibited triple melting peaks at all crystallization temperatures studied. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Studies on the isothermal crystallization kinetics and crystal morphology of PTT/SGF composites

The kinetics of isothermal crystallization and crystal morphology of poly(trimethylene terephthalate)/short glass fibers (PTT/SGF) composites were investigated by using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). DSC data was analyzed by the Avrami equation and Hoffman‐Lauritzen theory. The results show that SGF plays a role as the nucleating agent, which largely accelerates the crystallization rate of PTT. SGF below 20% will increase the crystallinity of the composites but 30% SGF will decrease the crystallinity, which is also verified by the crystallinity results calculated from the wide‐angle x‐ray diffraction (WAXD) experiments. The crystal morphology of the composites exhibits so much microcrystallites because of the fast crystallization rate and the strong interaction between SGF and polymers, which is consistent with the results analysized by the Avrami theory. The nuclei exponent K_g is increased sharply as the SGF added into polymer, and SGF makes PTT easier to crystallize during isothermal crystallization process. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers