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Polymer and carbon nanotubes (CNTs) nanocomposites exhibit many properties that are not present in either the pure polymer or CNTs. The polymer crystallization kinetics and crystal forms are greatly changed by dispersion of CNTs nanotubes. In addition to various thermophysical properties, CNTs are metallic or semiconductive and highly anisotropic while the polymer hosts are typically excellent insulators and isotropic. In this work, measurements of the electrical conductivity, σ, of thin-film nanocomposites of isotactic polypropylene (iPP) and CNTs as a function of CNT concentration (0, 1, 2, and 5% by weight of CNT) and melt-shearing induced anisotropy in σ between parallel and perpendicular to the shearing axis is presented and compared them with analogous data for their thermal transport properties. The iPP host is itself one of the most widely used polymers, has liquid crystalline phases, and it is expected that iPP/CNT nanocomposites will be widely used in many polymer applications, some of which are for flexible electrodes, medical and electronics packaging and chemical sensors. The effect of melt-shearing is expected to induce anisotropy in the various properties of the iPP/CNT thin film that should be particularly apparent in the electrical conductivity of the polymer films, higher in the direction of the alignment and lower in the direction perpendicular to it. With increasing CNT content, the average conductivity 〈σ〉 increases slightly from 0 to 2% CNT then dramatically increases by eight orders of magnitude for 5%. The shear induced property of anisotropy, δσs = (σ‖ − σ┴) / 〈σ〉, overall increases with CNT content, revealing a large spike for the 1% sample, indicating the possibility of enhanced δσs due to optimized orienting procedures.
Most advances and commercial successes of polymer/inorganic nanocomposites rely only on the dispersion of nanoparticles in a polymer matrix. Such approaches leave untapped opportunities where performance can be improved by controlling the larger length-scale structures. Here, we review selected examples where the hierarchical structure (from millimeter to nanometer) is tailored to control the transport properties of the materials, giving rise to marked property enhancements, relevant to dielectric materials for power capacitors. These examples address composite structures that are self-assembled, both at the nm and the micron scales, and, thus, can be produced using standard industrial practices. Specifically, polyethylene (PE) blends or poly(vinylidene fluoride) (PVDF) copolymers are reinforced with nanofillers; these composites are designed with high filler orientation, which yielded marked improvements in electric-field breakdown strength and, consequently, large improvements in their recoverable energy densities.
The focus of this research is on network formation and electrical conduction in carbon nanotube polydimethylsiloxane nanocomposites. Carbon nanotube network formation prior to and during polymerization was monitored by means of simultaneous electrical and rheological characterization. Processing induced network formation at filler concentrations below statistical percolation was observed, and both the electrical resistivity and the voltage dependence of the sample resistance were found to increase with the degree of polymerization, indicating carbon nanotube separation during polymer cure. Electron tunnelling through insulating polymer layers was identified as the main conduction mechanism. Information about the final network structure and further details about electrical conduction were obtained from the piezoresistive response of the material. Electron tunnelling was found to be dominant at filler concentrations close to the percolation threshold. With increasing filler concentration a densification of the carbon nanotube network was observed, and the resistance behaviour at high filler content was better described by the behaviour of a parallel circuit.
Proton exchange membrane with interconnected H+-transfer channels in submicron scale has been synthesized by means of pore filling polymerization. Polysulfone (PSU) membrane containing densely distributed pores is synthesized using the phase inversion approach. The membrane is then filled up with a designed formula consisting of monomers (e.g. 2-acrylamido-2-methylpropane sulphonic acid and N, N’-Methylenebisacrylamide) and a binary solvent. It is undertaken through solution diffusion of the monomer formula into the pores impregnated with the bore liquid. When the PSU matrix loaded with monomers is subjected to polymerization, a uniform distribution of interconnected H+-transfer channels is realized. This special membrane structure gives rise to a maximum ionic exchange capacity of 2.43 meq/g and the highest proton conductivity of 0.2 S/cm. Compared to the commercial Nafion® membrane, the pore-filled membrane significantly enhances the power output of H2-PEM fuel cell.
A carbon nanotube polymer composite has been used to develop a flexible multi-touch tactile sensor device. Rather than employing the inherent bulk piezoresistive properties of the composite, the contact resistance between polymer and electrode was exploited to achieve finger pressure measurement with fast response. We have synthesized a series of multi-walled nanotube (MWNT) silicone composites to test the feasibility of a force sensor based on the change in surface contact resistance as a function of applied force. A single layer MWNT/polydimethyl-siloxane (PDMS) composite in the range of 1.5-3.0 % w/w nanotubes was employed as a force sensor material in an array of electrodes. It was determined that sensors based on these materials are viable as tactile sensing systems for finger-touch forces in the range of 1-100 N.
Transport properties of polymer nanocomposites become increasingly important for range applications with many outstanding questions remaining. Thermal conductivity is especially important in applications like temperature sensing and packaging. We chose isotactic PolyPropylene (iPP) as one of the most widely used polymers and created nano-colloidal dispersions at different weight percent concentration of carbon nanotubes (CNTs). We oriented the thin-film samples using melt-shear at 200°C and 1Hz in a Linkam microscope shearing hot stage. Thermal conductivity measurements were performed at room temperature on two iPP/CNT sheared thin-film samples (1% and 5% CNT content) both parallel and perpendicular to the shear direction as well as a pure iPP sheared thin-film, prepared using the same process. Our findings indicate that the CNTs enhance kappa by 12% for the 1% CNT sample and 35% by the 5% CNT sample compared to that measured for pure iPP. Additionally, the CNTs under shear induce a novel anisotropy to the thermal conductivity in iPP/CNTs nano-composites. We introduce an approach to extract the shear induced orientational order of thermal conductivity by the dispersed CNTs.
Polymer/carbon nanotube (CNT) composites show next to improved mechanical, thermal, and electrical properties also sensitivity to external stimuli. A detection of environmental condition changes is possible, if it affects the electrically conductive CNT network inside the insulating polymer matrix. In case of liquid sensing, swelling of the polymer matrix due to contact with organic liquids and interactions between solvent molecules and CNTs result in a local gap enlargement between individual CNTs and/or CNT clusters, resulting in a detectable increase of electrical resistance. Accordingly, CNT based conductive polymer composites (CPCs) represent very promising candidates for the design of smart components capable of integrated monitoring. In this presentation we focus on their use as leakage detectors for organic solvents.
The sensor concept, as well as the underlying mechanism, is demonstrated for polycarbonate (PC)/CNT based CPCs on compression-molded samples. The selectivity as an important sensor property will be discussed in context with the Hansen solubility parameters and the solvent molecule’s size. The time dependent electrical response characteristic upon immersion depends on the diffusion kinetics of the specific solvent molecules into the CPC. A model allowing the calculation of the time dependent relative resistance (Rrel) change is presented considering several factors like the diffusion parameters, composite characteristics like initial resistance and geometrical values of the sensing sample. Using this model, Rrel curves of PC/MWCNT composites were simulated which fit very well the experimental data.
In order to examine the production of first prototype large area sensors, fibers and textiles based on composites of different polymer matrices with multi-walled carbon nanotubes (MWCNT) were produced. MWCNT containing fibers based on poly lactic acid (PLA) and polycaprolactone (PCL)/ polypropylene (PP) blends were produced by melt spinning and textile fabrication was performed for PCL/PLA blends. For all presented composite systems the electrical response characteristics was analyzed for various organic solvents.
A bottom-up approach for poly(vinyl alcohol) (PVA) - graphene oxide (GO) nanocomposites using a spraying method is presented. Very simple and versatile, spraying allows to build-up uniform layered composite films with good control on the structure of each layer. 150 bi-layers were deposited to create a transparent film with improved mechanical properties at a loading of 5.4 wt.% GO. The Young’s modulus and strength of these films doubled or nearly doubled which is believed to be due to a synergic effect as a result of the nanoscale organization of the composite by the 2D nanofiller, and hydrogen bonding between the PVA and the GO.
Optical transport through Isotactic Polypropylene (iPP) and multiwall carbon nanotubes (MWCNTs) nanocomposite thin films is important to many applications where optical transmission or polarization are used. Especially interesting is the case where the optical properties are anisotropic as in oriented thin films and the optical transport is different in the direction of orientation and perpendicular to it. Changing the orientation of the film or the polarization of the light can change the way in which the nanocomposite film interacts with light. Our polymer of choice, Isotactic Polypropylene, is one of the most widely used polymers which will increase the applicability of our results. We blended iPP with different concentration of carbon nanotubes (CNTs): 1%, 2% and 5% and oriented the thin film samples using melt-shear at 200°C and 1Hz in a Linkam microscope sharing hot stage. We measured that the index of refraction of the nanocomposites slightly decreased when CNTs are added and that when nanocomposites were shear-oriented at low loading of CNTs the index of refraction showed small difference in directions parallel and perpendicular to the direction of orientation. The extinction coefficient increased therefore it’s tuning in the nanocomposite films by the content of the carbon nanotubes can help devise new materials with the desired values of this property.
Polymethylphenylsilicone (PMPS), a siloxane polymer with a phenyl group, was first successfully electrospun to fabricate different diameters of silicone fibers ranging from 500 nm to 10 μm by considering solubility parameters of 12 different solvents. The resulting PMPS fibers were mixed with polydimethylsiloxane (PDMS) by retaining their original nanofiber structures to produce a polysiloxane-based nanofibrous composite. As for the mechanical properties, the PMPS/PDMS composite presented higher Young’s modulus and higher fracture strain than pure PDMS. The gas permeability test revealed that the PMPS/PDMS composite exhibited higher CO2 permeability than the pure PDMS membrane. Moreover, CO2 permeability gradually increased by raising the compounding ratio of PMPS-fibers in the PMPS/PDMS composite and by decreasing the diameter of PMPS-fibers. The enhancement mechanism observed in both mechanical properties and CO2 permeability was discussed from the viewpoint of the interface between PMPS and PDMS along with the nanofiber network structures.