Self-assembled multi-layered carbon nanofiber nanopaper for significantly improving electrical actuation of shape memory polymer nanocomposite

This study presents an effective approach to significantly improve the electrical properties of shape memory polymer (SMP) nanocomposites that show Joule heating triggered shape recovery. Carbon nanofibers (CNFs) were self-assembled to form multi-layered nanopaper to enhance the bonding and shape recovery behavior of SMP, respectively. It was found that both glass transition temperature (Tg) and electrical properties of the SMP nanocomposites have been improved by incorporating multi-layers of self-assembled nanopapers. The electrically actuated shape recovery behavior and the temperature profile during the actuation were monitored and characterized at a voltage of 30 V.     

Significantly improving infrared light-induced shape recovery behavior of shape memory polymeric nanocomposite via a synergistic effect of carbon nanotube and boron nitride

The present work studies the thermomechanical properties and infrared light-induced shape memory effect (SME) in shape memory polymer (SMP) nanocomposite incorporated with carbon nanotube (CNT) and boron nitride. The combination of CNT and boron nitride results in higher glass transition temperature, mechanical strength and thermomechanical strength. While CNTs are employed to improve the absorption of infrared light and thermally conductive property of SMP, boron nitrides facilitate heat transfer from CNTs to the polymer matrix and thus to enable fast response. A unique synergistic effect of CNT and boron nitride was explored to facilitate the heat transfer and accelerate the infrared light-induced shape recovery behavior of the shape memory polymeric nanocomposite.     

Controlling Au electrode patterns for simultaneously monitoring electrical actuation and shape recovery in shape memory polymer

This paper presents an effective approach to achieve efficient electrical actuation and monitoring of shape recovery based on patterned Au electrodes on shape memory polymer (SMP). The electrically responsive shape recovery behavior was characterized and monitored by the evolution change in electrical resistance of patterned Au electrode. Both electrical actuation and temperature distribution in the SMP have been improved by optimizing the Au electrode patterns. The electrically actuated shape recovery behavior and temperature evolution during the actuation were monitored and characterized. The resistance changes could be used to detect beginning/finishing points of the shape recovery. Therefore, the Au electrode not only significantly enhances the electrical actuation performance to achieve a fast electrical actuation, but also enables the resistance signal to detect the free recovery process.     

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.     

Electrostatic self-assembled carbon nanotube/nano carbon black composite fillers reinforced cement-based materials with multifunctionality

Electrostatic self-assembled carbon nanotube (CNT)/nano carbon black (NCB) composite fillers are added into cement mortar to fabricate smart cement-based materials. The grape bunch structure of CNT/NCB composite fillers is beneficial for dispersing CNT/NCB in cement mortar matrix and achieving cooperative improvement effect. The mechanical, electrically conductive, and piezoresistive behaviors of the cement mortar are investigated. The CNT/NCB composite fillers can effectively enhance the flexural strength and electrical conductivity of cement mortars, and endow stable and sensitive piezoresistivity to cement mortar at a low filler content. However, they weaken the compressive strength of cement mortar to some extent. The percolation threshold zone of cement mortar with CNT/NCB composite fillers ranges in the amount of 0.39–1.52 vol.%. The optimal content of CNT/NCB composite fillers is 2.40 vol.% for piezoresistivity and the stress and strain sensitivities can reach 2.69% MPa⁻¹ and 704, respectively.     

Effect of a multiscale reinforcement by carbon fiber surface treatment with graphene oxide/carbon nanotubes on the mechanical properties of reinforced carbon/carbon composites

An effective carbon fiber/graphene oxide/carbon nanotubes (CF-GO-CNTs) multiscale reinforcement was prepared by co-grafting carbon nanotubes (CNTs) and graphene oxide (GO) onto the carbon fiber surface. The effects of surface modification on the properties of carbon fiber (CF) and the resulting composites was investigated systematically. The GO and CNTs were chemically grafted on the carbon fiber surface as a uniform coating, which could significantly increase the polar functional groups and surface energy of carbon fiber. In addition, the GO and CNTs co-grafted on the carbon fiber surface could improve interlaminar shear strength of the resulting composites by 48.12% and the interfacial shear strength of the resulting composites by 83.39%. The presence of GO and CNTs could significantly enhance both the area and wettability of fiber surface, leading to great increase in the mechanical properties of GO/CNTs/carbon fiber reinforced composites.     

Electromechanical response of semiconducting carbon–polyimide nanocomposite thin films

Electromechanically responsive polymer nanocomposite thin films can provide embedded microscale sensing elements for unobtrusive monitoring of strain, torque and pressure particularly for composite structures. Thin nanocomposite carbon–polyimide films with thicknesses up to 90 μm were produced with carbon contents that yield semiconducting behaviour attributable to distance dependent electron hopping between isolated nanoparticles. The tensile modulus and the strain at break indicated minimum interaction between polymer and nanoparticle surfaces. A decreasing storage modulus with increasing temperature indicated increasing free volume inducing polymer chain motions.     

Interfacial evaluation and self-sensing on residual stress and microfailure of toughened carbon fiber/epoxy-amine terminated (AT)-polyetherimide (PEI) composites

Interfacial evaluation and self-sensing of tensile loading/subsequent unloading and microfailure detection of the carbon fiber/epoxy-amine terminated (AT)-polyetherimide (PEI) composites were investigated using micromechanical test and electrical resistance measurement with an aid of acoustic emission (AE). As AT-PEI content increased, both fracture toughness of epoxy-AT-PEI matrix and interfacial shear strength (IFSS) increased due to the optimized matrix modulus for energy absorption. With increasing curing temperature and time, the IFSS increased and then decreased. During curing process, the change in electrical resistance, ΔR increased gradually with adding AT-PEI contents because of different thermal and curing shrinkage of epoxy matrices. Moisture adsorption under durability test could cause to the change in matrix modulus and thus resulted in the change in electrical resistivity correspondently. Under changeable cyclic loading/subsequent unloading, apparent modulus and electrical resistivity during curing process were consistent well with the fracture toughness of epoxy modified with AT-PEI. In compressive test, the electrical resistivity decreased gradually initially and then increased rapidly during subsequent progress of microfailure including fiber fracture showing the buckling pattern.     

Bond behavior of superelastic shape memory alloys to carbon fiber reinforced polymer composites

In this research pull-out specimens were tested to investigate the bond behavior of superelastic NiTi (Nitinol) SMA wires to carbon fiber reinforced polymers (CFRP). A total of 45 pull-out specimens were tested monotonically up to failure. The test parameters considered include the wire diameter and embedment length. A digital image correlation (DIC) system was used to identify the onset and propagation of debonding. Based on the experimental observations two debonding mechanisms were observed: complete debonding after the onset of martensitic transformation of SMA wire, and complete debonding before the onset of wire transformation. The former mechanism predominated, while the latter mechanism governed for larger diameter wires with shorter embedment lengths. A 3-D non-linear finite element model (FEM) was developed to predict the pull-out behavior. A cohesive zone model (CZM) was used to model the interface. A parametric study was conducted using the FEM to quantify the parameters of the cohesive zone model. The results demonstrate that the proposed modeling approach can be used to characterize the bond behavior of superelastic SMA wires embedded in FRP composites.     

Effects of modified and non-modified clay on the rheological behavior of high density polyethylene

HDPE nanocomposites containing 2.5, 5 and 10 wt.% of non-modified and modified clays (NMC and MC) were prepared by melt extrusion in a twin screw extruder. Compression molded samples were prepared. Transmission electron microscopy (TEM) indicated a partial intercalation of the modified clay nanofiller within the HDPE matrix comparing to that of non-modified clay. The moduli of nanocomposites increased with increase in nanofiller concentration; but this increase was greater in the low frequency region. The non-modified clay had a greater increase in the elastic behavior, while the modified clay increased viscose behavior because of more interactions with the matrix and partial intercalation. The rheological behaviors of both HDPE/NMC and HDPE/MC nanocomposites are more sensitive to nanoparticles’ concentration at low frequencies. The HDPE/MC nanocomposites showed semi-circle shapes comparing to HDPE/NMC nanocomposites. While the Cole–Cole plot of HDPE/NMC nanocomposite had more departure of semi-circle shape. The agglomerated particles could concentrate the imposed stress so the yield stress reached at lower shear rates comparing to pure HDPE and HDPE filled 2.5 wt.% NMC nanocomposite. Study of suspension models showed that the Eilers-Van Dijck and Einsten models fitted to almost experimental data satisfactorily.     

Ordered multiphase polymer nanocomposites for high-performance solid-state supercapacitors

Multilayered ordered nanostructures were fabricated by assembling in-situ grown polyaniline nanowire arrays with graphene oxide nanosheets. As-fabricated nanostructure was subsequently impregnated with the (H₃PO₄–Nafion)/polyvinyl alcohol solution to create a multiphase composite, which was used as a solid-state supercapacitor where graphene oxide/polyaniline nanowires served as electrode and (H₃PO₄–Nafion)/polyvinyl alcohol served as solid electrolyte. The ordered polyaniline (PANI) nanostructures facilitated the charge transfer and resulted in the specific capacitance of 83 F/g even if the discharge current was 5 A/g. The efficient charge transportation and electrode–electrolyte interaction resulted in small equivalent series resistance as low as 5.83 Ω, and thus outstanding electrochemical performance. The charge transfer resistance was much smaller than other commonly used solid-state electrolyte and almost negligible. As a result only 7% capacitance loss was found when the frequency increased from 100 to 1000 Hz. The energy density was as high as 26.5 Wh/kg while the power density was ∼3600 W/kg. The energy storage performance was also very stable since 82% specific capacitance was maintained after 1000 cycles.     

Exploiting carbon nanotube networks for damage assessment of fiber reinforced composites

An approach for damage inspection of composite structures utilizing carbon nanotubes (CNT) networks is investigated. CNT are dispersed in an epoxy using a processing technique compatible with commonly employed composite manufacturing techniques and subsequently used as matrix for a structural glass fiber reinforced composite. The developed electrical conductivity of the composite system is verified experimentally. The electrically conductive CNT network within the GFRP is exploited through distributed electrical voltage measurements to sense and, ultimately, locate damage in the plane of the composite plate. Damage in the form of cracks or delamination interrupts the continuity of the CNT network separating and isolating regions of the conductive network. Employing electric potential fields these changes can become measurable and can provide information for inversely locating the damage. Electrical Resistance Tomography (ERT) is formulated and experimentally applied to measure changes in the potential fields and deliver electrical conductivity change maps which are used to identify and locate changes in the CNT networks. These changes are correlated to capture the damage in the composite. Different damage modes are studied to assess the capabilities of the technique. The technique shows sensitivity to very small damages; less than 0.1% of the inspected area. The solution of the inverse ERT problem delivers a conductivity change maps which offers an effective localization with nearly 10% error and an inspection area suppression of around 75%. The proposed methodology to create CNT networks enables the application of ERT for Non-Destructive Evaluation of composite materials, previously not possible due to lack of conductivity, thus offering damage sensing and location capabilities even in-situ.     

Effect of fiber arrangement on shape fixity and shape recovery in thermally activated shape memory polymer-based composites

In the present study, we conducted periodic-cell simulations of the thermomechanical cycle of thermally activated shape memory polymer (SMP)-based composites. The present simulation utilizes a micromechanical model for reproducing the discontinuous fibers and SMP. We analyzed the effect of fiber volume fraction, fiber aspect ratio, and fiber end position on the shape fixity and shape recovery of the composite. The simulated results revealed that fiber elasticity is a key factor for the shape fixity of the composite, while both strain concentration near the fiber ends and fiber elasticity play important roles in the shape recovery properties of the composite.     

Interfacial microstructure and mechanical properties of carbon fiber composites by fiber surface modification with poly(amidoamine)/polyhedral oligomeric silsesquioxane

Polyhedral oligomeric silsesquioxane (POSS) was grafted onto carbon fiber surface using poly(amidoamine) (PAMAM) as a novel coupling agent at mild reaction conditions. Firstly, the reinforcement was designed with propagation of PAMAM on the fiber surface by in situ polymerization to improve the surface activities of carbon fiber. Secondly, the POSS further grafted on the fiber could significantly enhance fiber surface energy and wettability, which would greatly increase the interfacial strength of fiber-matrix. The microstructure and mechanical properties of carbon fiber and the resulting composites were investigated. The results indicated that PAMAM and POSS, which could significantly increase the surface roughness and wettability of carbon fiber, were successfully grafted on the fiber surface. Compared with the desized fiber composites, the interlaminar shear strength and the interfacial shear strength of the modified carbon fiber composites increased by 48% and 89%, respectively.     

Fabrication of hybrid membrane of electrospun polycaprolactone and polyethylene oxide with shape memory property

Hybrid materials with nanostructure could exhibit a diverse range of applications as advanced functional materials. This research work, composite membranes with shape memory property based on biocompatible polycaprolactone and polyethylene oxide were successfully fabricated by using electrospinning technique. Electrospun fiber configuration is strongly related to the concentration of polymer and electric field strength. The hydrophilic property of hybrid membrane has been improved and water play a critical role in resulting lower its responsive temperature compared with dry membrane. The mechanism of shape memory PCL/PEO hybrid membrane at wet condition has been proposed.     

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.     

Effect of carbon nanotube modified epoxy adhesive on CFRP-to-steel interface

The effects of carbon nanotube (CNT) modified epoxy adhesive on CFRP-to-steel interfaces were investigated using double strap joints. The bond behaviours studied were failure modes, bond interface at microlevel, bond strength, effective bond length, CFRP strain distribution and bond-slip relationships.For the first time, a novel type of failure in the CFRP-steel joint was discovered, attributable to weak bonding between woven mesh and CFRP fibres. This failure mode prevented exploitation of the full potential of the carbon fibres and the CNT modified epoxy adhesive. Joints bonded with CNT-epoxy adhesive had an effective bond length of about 60 mm, whereas that of joints bonded with pure epoxy was about 70 mm. The CNT-epoxy adhesive can transfer more load from the host structure to the bonded CFRP laminates, consequently modifying bond behaviour. It is therefore expected that CNT-epoxy nanocomposites will assist in the strengthening and rehabilitation of steel infrastructures using CFRP laminates.     

Influence of functionalized carbon nanofibers on the single carbon fiber–epoxy matrix interface

Amine-functionalized carbon nanofibers (A-CNFs) were deposited on the surface of individual sized carbon fibers using electrophoretic deposition (EPD), and the average interfacial shear strength (IFSS) was determined using the single fiber fragmentation test in conjunction with Weibull analysis. The IFSS decreased by 25% for fibers acting as the negative electrode in water without CNFs, and the impact of agglomerates on IFSS estimation is discussed. Further, a 187% IFSS increase was achieved for fibers undergoing a two-stage A-CNF EPD approach.     

Pullout resistance of straight NiTi shape memory alloy fibers in cement mortar after cold drawing and heat treatment

In our study, we found cold drawing to be an effective method for enhancing the pullout resistance of NiTi shape memory alloy (SMA) fibers in concrete. The pullout resistance was observed to be dependent on the contact pressure and friction coefficient at the interface between the fibers and the mortar matrix. The drawing process increased the stiffness and yield stress of the fibers and consequently increased the contact pressure at the interface between the fibers and the mortar matrix. Moreover, heat treatment of the fibers after cold drawing was found to noticeably recover the fiber diameter, thereby significantly enhancing the pullout resistance. The enhancement of the interfacial bond strength by heat treatment verified the crack-closing capabilities of SMA-fiber-reinforced cement composites.     

Micro-infiltration of three-dimensional porous networks with carbon nanotube-based nanocomposite for material design

Epoxy composite beams reinforced with a complex three-dimensional (3D) skeleton structure of nanocomposite microfibers were fabricated via micro-infiltration of 3D porous microfluidic networks with carbon nanotube nanocomposites. The effectiveness of this manufacturing approach to design composites microstructures was systematically studied by using different epoxy resins. The temperature-dependent mechanical properties of these multifunctional beams showed different features which cannot be obtained for those of their individual components bulks. The microfibers 3D pattern was adapted to offer better performance under flexural solicitation by the positioning most of the reinforcing microfibers at higher stress regions. This led to an increase of 49% in flexural modulus of a reinforced-epoxy beam in comparison to that of the epoxy bulk. The flexibility of this method enables the utilization of different thermosetting materials and nanofillers in order to design multifunctional composites for a wide variety of applications such as structural composites and components for micro-electromechanical systems.