Shape memory and thermo-mechanical properties of shape memory polymer/carbon fiber composites

In this study, chopped carbon fiber reinforced trans-1, 4-polyisoprene (TPI) was developed via a proposed new manufacturing process with the aim of improving weak mechanical properties of bulk TPI bulk. Specimens of the developed shape memory polymer (SMP) composites were fabricated with carbon fiber weight fraction of 5%, 7%, 9%, 11% and 13%, respectively. Measured are the effects of chopped carbon fiber and temperature on: (a) shape recovery ratio and rate; (b) stress–strain relationship; (c) maximum tensile stress, strain and Young’s modulus; and (d) maximum stress and residual strain under a constant strain cyclic loading. In addition, SEM micrographs were also presented to illustrate the fracture surface. The present experimental results show that the SMP with 7% carbon fiber weight fraction appears to perform best in all the tests. This indicates that the 7% carbon fiber weight fraction could be the optimum value for the SMP developed using the proposed manufacturing process.     

Evaluation of surface properties of epoxy–nanodiamonds composites

Epoxy–matrix reinforced with nanodiamond (ND) particles, with ND content up to 5 wt%, were synthesized. Characterization of NDs by field emission scanning electron microscopy (FE-SEM) and Raman spectroscopy was conducted, while composites were characterized through contact angle, nanoindentation, nanoscratch and scanning probe microscopy. The assessed properties evaluated were hardness, elastic modulus, contact angle, deformation mechanisms, creep deformation, coefficient of friction and adhesion, namely. Results showed that even small additions of ND lead to significant enhancement in the hardness and elastic modulus of epoxy matrix, while properties of composites present a switch in behavior after passing a concentration threshold; this threshold was identified and discussed.     

Effects of CNT diameter on mechanical properties of aligned CNT sheets and composites

Drawing, winding, and pressing techniques were used to produce horizontally aligned carbon nanotube (CNT) sheets from free-standing vertically aligned CNT arrays. The aligned CNT sheets were used to develop aligned CNT/epoxy composites through hot-melt prepreg processing with a vacuum-assisted system. Effects of CNT diameter change on the mechanical properties of aligned CNT sheets and their composites were examined. The reduction of the CNT diameter considerably increased the mechanical properties of the aligned CNT sheets and their composites. The decrease of the CNT diameter along with pressing CNT sheets drastically enhanced the mechanical properties of the CNT sheets and CNT/epoxy composites. Raman spectra measurements showed improvement of the CNT alignment in the pressed CNT/epoxy composites. Research results suggest that aligned CNT/epoxy composites with high strength and stiffness are producible using aligned CNT sheets with smaller-diameter CNTs.     

Effect of graphene oxide nanosheets on the physico-mechanical properties of chitosan/bacterial cellulose nanofibrous composites

In this work, novel chitosan/bacterial cellulose (CS/BC) nanofibrous composites reinforced with graphene oxide (GO) nanosheets are introduced. As cell attachment and permeability of nanofibrous membranes highly depend on their fiber diameter, the working window for successful electrospinning to attain sound nanofibrous composites with a minimum fiber diameter was determined by using the response surface methodology. It is shown that the addition of GO nanosheets to CS/BC significantly reduces the average size of the polymeric fibers. Their mechanical properties are also influenced and can be tailored by the concentration of GO. Fourier transform infrared spectroscopy reveals hydrogen bonding between the GO nanosheets and the polymer matrix. A decrease in the hydrophilicity of the electrospun nanofibers and their water vapor permeability with the addition of GO are also reported. The prepared nanofibrous composites are potentially suitable candidates for biomedical applications such as skin tissue engineering and wound dressing.     

Nanotip-induced ultrahigh pressure-sensitive composites: Principles,properties and applications

Spiky spherical nickel powders with sharp nanotips on their surface are excellent fillers for developing pressure-sensitive composites. The sharp nano-tips are responsible for generating field-assisted tunneling conduction, which leads to the strong responses of electrical conductivity of the composites to external force or deformation. The nanotip-induced ultrahigh pressure-sensitive composites can be used to develop new sensors, switches and controls for a wide range of applications including electronics, transport, space, medicine, defense, textiles, oil and gas, and civil engineering. In this paper, we examine a systematical review of research progress on the nanotip-induced ultrahigh pressure-sensitive composites, with attentions to mechanism of pressure-sensitivity, sensing performances, and applications of the composites. Future challenges in the development and application of the nanotip-induced ultrahigh pressure-sensitive composites are also discussed.     

Overall behavior and microstructural deformation of R-CNT/polymer composites

Carbon nanotubes (CNTs) are an excellent candidate for the reinforcement of composite materials owing to their distinctive mechanical and electrical properties. Reticulate carbon nanotubes (R-CNTs) with a 2D or 3D configuration have been manufactured in which nonwoven connected CNTs are homogeneously distributed and connected with each other. A composite reinforced by R-CNTs can be fabricated by infiltrating a polymer into the R-CNT structure, which overcomes the inherent disadvantages of the lack of weaving of the CNTs and the low strength of the interface between CNTs and the polymer. In this paper, a 2D plane strain model of a R-CNT composite is presented to investigate its micro-deformation and effective stiffness. Using the two-scale expansion method, the effective stiffness coefficients and Young’s modulus are determined. The influences of microstructural parameters on the micro-deformation and effective stiffness of the R-CNT composite are studied to aid the design of new composites with optimal properties. It is shown that R-CNT composites have a strong microstructure-dependence and better effective mechanical properties than other CNT composites.     

The performance of stress-sensing smart fiber reinforced composites in moist and sodium chloride environments

The addition of carbon nano-fibers (CNF) to fiber reinforced composites (FRC) based on polyvinyl alcohol fibers can improve the flexural strength of composites. Depending on applied stress, moisture content, and exposure to chloride solutions, the developed CNF composites exhibit specific levels of electrical conductivity. Reported research has demonstrated a strong dependency of electrical response of composite to crack formation in moist and NaCl environments. It was demonstrated that the sensitivity to strain and chloride solution can be enhanced by CNF. The developed technology and smart composite material are scalable for application in nondestructive monitoring of concrete structures that require improved integrity under service loads and stability in harsh environments.     

PHBV nanocomposites based on organomodified montmorillonite and halloysite: The effect of clay type on the morphology and thermal and mechanical properties

The aim of this study was to evaluate the effect of the addition of two types of nanoparticles, organomodified montmorillonite Cloisite® 30B (C-30B), and a tubular like clay, halloysite (HNT), on the morphology and thermal and mechanical properties of poly(hydroxybutyrate-co-hydroxyvalerate) – PHBV nanocomposites. TEM and WAXD results showed a combination of a few tactoids and a partially exfoliated structure for PHBV/C-30B nanocomposites and a good dispersion of HNT in the PHBV matrix. DSC analysis indicated a lower nucleation density with the addition of nanoparticles. Furthermore, the presence of C-30B led to the formation of double melting peaks, related to different crystalline phases. However, a higher melting temperature was obtained for PHBV/HNT nanocomposites. A general increase in the Young’s modulus was observed. However, for PHBV/C-30B nanocomposites, this enhancement was at the expense of the strain at break and impact strength, probably due to the degradation of the polymer during processing.     

Characterization of polyamide 6/carbon nanotube composites prepared by melt mixing-effect of matrix molecular weight and structure

Effects of molecular weight and structure of polyamide 6 (PA6) on morphology and properties of PA6/MWCNT prepared by melt mixing were investigated. Microscopic analysis showed fine dispersion of MWCNT within low viscosity PA6s due to domination of melt infiltration into MWCNT agglomerate at low viscosity matrices with linear structure. Rheological data indicated good interfacial interaction with no percolation of MWCNT up to 2 wt% loading. DSC thermograms showed nucleating role of MWCNT on crystallization of PA6s with marginal effect on crystallinity. Experimental data supported with micromechanical model showed limited improvement on mechanical properties, but it was closely consistent with degree of dispersion of MWCNT.     

In situ polymerized nanocomposites: Polystyrene/CNT and Poly(methyl methacrylate)/CNT composites

Carbon nanotubes (CNTs) were incorporated into polystyrene (PS) and poly(methyl methacrylate) (PMMA) matrices via in situ emulsion and emulsion/suspension polymerization methods. The polymerizations were carried out using various initiators, surfactants, and carbon nanotubes to determine their influence on polymerization and on the properties of the composites. The loading of CNTs in the composites varied from 0 to 15 wt.%, depending on the CNTs used. Morphology and dispersion of the CNTs were analyzed by transmission and scanning electron microscopy techniques. The dispersion of multi-walled carbon nanotubes (MWCNT) in the composites was excellent, even at high CNT loading. The mechanical properties, and electrical and thermal conductivities, of the composites were also analyzed. Both electrical and thermal conductivities were improved.     

Design and analysis of sandwiched fullerene-graphene composites using molecular dynamics simulations

Computational design of a novel carbon based hybrid material that is composed of fullerene units covalently sandwiched between parallel graphene sheets is presented. In this regard, atomistic models for the proposed novel material structure are generated via a systematic approach by employing different fullerene types (i.e. C₁₈₀, C₃₂₀, C₅₄₀ and C₇₂₀) as sandwich cores. Then, thermodynamic stability of the atomistic structures is checked by monitoring free energy profiles and junctional bond configurations which are obtained through classical molecular dynamics (MD) simulations. Thermodynamic feasibility of all atomistic specimens with different fullerene types is suggested by the energy profiles, because total configuration energies for all systems are minimized and remained stable over a long period of time. Furthermore, mechanical behavior of the nano-sandwiched material system is investigated by performing compression tests via MD simulations and basic deformation mechanisms underlying the compressive response are determined. By detailed examination, it is shown that proposed nano-sandwiched material can be identified as quasi-foam material due to comparable energy absorbing characteristics. Furthermore, regarding the effect of fullerene size on the compressive response, it is found that for a given stress level, specimens with larger fullerenes exhibit higher energy absorbing capacity.     

Dispersion evaluation,processing and tensile properties of carbon nanotubes-modified epoxy composites prepared by high pressure homogenization

In this paper, high pressure homogenization (HPH) served as the main dispersion method for making carbon nanotubes (CNTs)-modified composite materials. A new quantitative approach was proposed to evaluate the dispersion of CNTs in epoxy resin by combining microscopic images with maximum likelihood estimation theory. The results were in good agreement with those obtained from optical observation and ultraviolet–visible (UV–vis) absorbance measurement. Moreover, the dispersion process specific to HPH processing was analyzed with the aid of the proposed approach. The changes of the morphologies and lengths of CNTs in HPH were illustrated. It was found that, in the first few cycles, the lengths of CNTs fell significantly and the entanglements remained severe. In the following cycles, individual CNTs were gradually separated from agglomerates and the lengths experienced a slow decline. After homogeneous dispersion, the transverse tensile properties of CNTs-modified carbon fiber reinforced epoxy composites (CFRP) got obviously ameliorated.     

The transverse elastic properties of chiral honeycombs

This work describes the out-of-plane linear elastic mechanical properties of trichiral, tetrachiral and hexachiral honeycomb configurations. Analytical models are developed to calculate the transverse Young’s modulus and the Voigt and Reuss bounds for the transverse shear stiffness. Finite Element models are developed to validate the analytical results, and to identify the dependence of the transverse shear stiffness vs. the gauge thickness of the honeycombs. The models are then validated with experimental results from flatwise compressive and simple shear tests on samples produced with rapid prototype (RP)-based techniques.     

Actuation of electrochemical, electro-magnetic, and electro-active actuators for carbon nanofiber and Ni nanowire reinforced polymer composites

Actuation of electrochemical, electro-magnetic, and electro-active actuators composed of CNF and/or Ni nanowire/polymer nanocomposites was evaluated with different materials and preparation processes. The actuated strain was compared with frequency, applied voltage, and wave type. The hysteresis of the actuated strain was continuously delayed in electrochemical actuators, whereas the strain was uniformly actuated in the electro-magnetic actuators. The actuated strain decreased with increasing frequency in both electrochemical and electro-magnetic actuators. In magnetic fields the actuated strain increased with increasing Ni nanowire content whereas the current increased with applied voltage. Ni nanowire/cellulose actuators in a magnetic field responded better at high frequencies, compared to the other actuators studied. Actuated strain of cellulose or Ni nanowire/cellulose nanocomposite in air was larger than either PVDF or PVDF/cellulose nanocomposite. In Ni nanowire/cellulose nanocomposite, actuated strain also decreased with increasing frequency and increased with increasing voltage. Electro-active actuators responded well in air when low voltages and high frequencies were applied compared to the other two actuators. Electro-active actuators in this paper have unique advantages for many practical applications, including easy fabrication, lightweight and low application voltages.     

Free-volume correlation with mechanical and dielectric properties of natural rubber/multi walled carbon nanotubes composites

The microstructure, mechanical strength, dielectric properties, Doppler broadening measurements and positron life time studies of the composites containing multi walled carbon nanotubes (MWCNTs) and natural rubber (NR) are investigated. The uniform distribution of MWCNTs in the elastomer medium is studied by Raman spectroscopy and the electron microscopy images show the composite’s internal microstructure. Free volume sizes and interstitial mesopore sizes of the nanocomposites are determined by positron annihilation lifetime spectroscopy (PALS). PALS investigates the influence of the nanotubes in regulating the interphase nanoscale character. Strong interfacial interaction causes an apparent reduction of the free-volume fraction of NR probably by depressing the formation of free-volume holes in the interfacial region. The mechanical percolation and percolation observed from the dielectric measurements are correlated with the life time values. It is established that the sub-nano level free volumes and nano level structure of the composites have significant roles in regulating the mechanical properties.     

Self-sensing and dispersive evaluation of single carbon fiber/carbon nanotube (CNT)-epoxy composites using electro-micromechanical technique and nondestructive acoustic emission

Self-sensing and interfacial evaluation were investigated with different dispersion solvents for single carbon fiber/carbon nanotube (CNT)-epoxy composites by electro-micromechanical technique and acoustic emission (AE) under loading/subsequent unloading. The optimized dispersion procedure was set up to obtain improved mechanical and electrical properties. Apparent modulus and electrical contact resistivity for CNT-epoxy composites were correlated with different dispersion solvents for CNT. CNT-epoxy composites using good dispersion solvents exhibited a higher apparent modulus because of better stress transferring effects due to the relatively uniform dispersion of CNT in epoxy and enhanced interfacial adhesion between CNT and the epoxy matrix. However, good solvents exhibited a higher apparent modulus but lower thermodynamic work of adhesion, W_a for single carbon microfiber/CNT-epoxy composite. It is attributed to the fact that hydrophobic behavior with high advanced contact angle was observed for CNT-epoxy in the good solvent, which might not be compatible well with the carbon microfiber. Damage sensing was also detected simultaneously using AE combined with electrical resistance measurement. Electrical resistivity increased stepwise with progressing fiber fracture due to the decrease in electrical contact by the CNT.     

Embedding thin-film lithium energy cells in structural composites

This paper reports on the recent progress towards the development of power composite structures capable of energy harvesting and storage in addition to load bearing. The process of physically embedding all-solid-state thin-film lithium energy cells into a carbon fiber reinforced plastic (CFRP) and the performance of the resulting power composites are reported. The embedded thin-film lithium-ion energy cells did not significantly alter the mechanical properties of the composite (modulus and strength) under quasi-static uniaxial loading conditions. The embedded energy cells performed at baseline charge/discharge levels up to a loading of about 50% of the CFRP tensile strength.     

Mechanical and electrical property improvement in CNT/Nylon composites through drawing and stretching

The excellent mechanical properties of carbon nanotubes (CNTs) make them the ideal reinforcements for high performance composites. The misalignment and waviness of CNTs within composites are two major issues that limit the reinforcing efficiency. We report an effective method to increase the strength and stiffness of high volume fraction, aligned CNT composites by reducing CNT waviness using a drawing and stretching approach. Stretching the composites after fabrication improved the ultimate strength by 50%, 150%, and 190% corresponding to stretch ratios of 2%, 4% and 7%, respectively. Improvement of the electrical conductivities exhibited a similar trend. These results demonstrate the importance of straightening and aligning CNTs in improving the composite strength and electrical conductivity.     

Effects of stretching on mechanical properties of aligned multi-walled carbon nanotube/epoxy composites

Composites based on epoxy resin and differently aligned multi-walled carbon nanotube (MWCNT) sheets have been developed using hot-melt prepreg processing. Aligned MWCNT sheets were produced from MWCNT arrays using the drawing and winding technique. Wavy MWCNTs in the sheets have limited reinforcement efficiency in the composites. Therefore, mechanical stretching of the MWCNT sheets and their prepregs was conducted for this study. Mechanical stretching of the MWCNT sheets and hot stretching of the MWCNT/epoxy prepregs markedly improved the mechanical properties of the composites. The improved mechanical properties of stretched composites derived from the increased MWCNT volume fraction and the reduced MWCNT waviness caused by stretching. With a 3% stretch ratio, the MWCNT/epoxy composites achieved their best mechanical properties in this study. Although hot stretching of the prepregs increased the tensile strength and modulus of the composites considerably, its efficiency was lower than that of stretching the MWCNT sheets.     

Time discretization effect on the nonlinear vibration of embedded SWBNNT conveying viscous fluid

In this study, the effect of time discretization on the nonlinear transverse vibration and instability of single-walled boron nitride nanotube (SWBNNT) conveying viscous fluid is investigated based on the nonlocal piezoelasticity theory. SWBNNT is considered as an Euler–Bernoulli beam and is subjected to combined mechanical loading, thermal changes and electrical field. The elastic medium is simulated as Winkler and Pasternak foundation. The interaction between the inner viscous fluid and SWBNNT is obtained using Navier–Stokes equation. The axial inertia is neglected and a new approach is introduced to decouple the mechanical and electrical fields considering charge equation. Motion equations are derived by Hamilton’s principle using the Von-Kármán nonlinearity theory. In the first approach, time and space domains are discretized using the method of multiple scale (MMS) and Galerkin procedure respectively, and in the second one differential quadrature method (DQM) is utilized to space discretization. Good agreement is shown between the results of first and second approach. Numerical results indicate the significant effects of aspect ratio, elastic medium and nonlocality on the frequency and instability of the SWBNNT.