9 research outputs found
Mechanical, electrical and electro-mechanical properties of thermoplastic elastomer styrene–butadiene–styrene/multiwall carbon nanotubes composites
Composites of styrene-butadiene-styrene (SBS) block copolymer with multiwall carbon
nanotubes (MWCNT) were processed by solution casting in order to investigate the
influence of filler content, the different ratio of styrene/butadiene in the copolymer and
the architecture of the SBS matrix on the electrical, mechanical and electro-mechanical
properties of the composites. It was found that filler content and elastomer matrix
architecture influence the percolation threshold and consequently the overall composite
electrical conductivity. The mechanical properties are mainly affected by the styrene
and filler content. Hopping between nearest fillers is proposed as the main mechanism
for the composite conduction. The variation of the electrical resistivity is linear with the
deformation. This fact, together with the gauge factor values in the range of 2 to 18,
results in appropriate composites to be used as (large) deformation sensors.This work was funded by FEDER funds through the "Programa Operacional Factores de
Competitividade – COMPETE" and by national funds by FCT - Fundação para a
CiĂŞncia e a Tecnologia, through project references PTDC/CTM/69316/2006,
PTDC/CTM/73465/2006, PTDC/CTM-NAN/112574/2009, and NANO/NMed-
SD/0156/2007. PC, JS and VS also thank FCT for the SFRH/BD/64267/2009,
SFRH/BD/60623/2009 and SFRH/BPD/63148/2009 grants, respectively. The authors
also thank support from the COST Action MP1003 ”European Scientific Network for
Artificial Muscles” and the COST action MP0902 “Composites of Inorganic Nanotubes
and Polymers (COINAPO)
Electric field versus surface alignment in confined films of a diblock copolymer melt
The dynamics of alignment of microstructure in confined films of diblock copolymer melts in the presence of an external elec. field was studied numerically. We consider in detail a sym. diblock copolymer melt, exhibiting a lamellar morphol. The method used is a dynamic mean-field d. functional method, derived from the generalized time-dependent Ginzburg-Landau theory. The time evolution of concn. variables and therefore the alignment kinetics of the morphologies are described by a set of stochastic equations of a diffusion form with Gaussian noise. We investigated the effect of an elec. field on block copolymers under the assumption that the long-range dipolar interaction induced by the fluctuations of compn. pattern is a dominant mechanism of elec.-field-induced domain alignment. The interactions with bounding electrode surfaces were taken into account as short-range interactions resulting in an addnl. term in the free energy of the sample. This term contributes only in the vicinity of the surfaces. The surfaces and the elec. field compete with each other and align the microstructure in perpendicular directions. Depending on the ratio between elec. field and interfacial interactions, parallel or perpendicular lamellar orientations were obsd. The time scale of the elec.-field-induced alignment is much larger than the time scale of the surface-induced alignment and microphase sepn. [on SciFinder (R)
Continuum percolation of carbon nanotubes in polymeric and colloidal media
We apply continuum connectedness percolation theory to realistic carbon nanotube systems and predict how bending flexibility, length polydispersity, and attractive interactions between them influence the percolation threshold, demonstrating that it can be used as a predictive tool for designing nanotube-based composite materials. We argue that the host matrix in which the nanotubes are dispersed controls this threshold through the interactions it induces between them during processing and through the degree of connectedness that must be set by the tunneling distance of electrons, at least in the context of conductivity percolation. This provides routes to manipulate the percolation threshold and the level of conductivity in the final product. We find that the percolation threshold of carbon nanotubes is very sensitive to the degree of connectedness, to the presence of small quantities of longer rods, and to very weak attractive interactions between them. Bending flexibility or tortuosity, on the other hand, has only a fairly weak impact on the percolation threshold
On the influence of the processing conditions on the performance of electrically conductive carbon nanotube/polymer nanocomposites
We prepared multiwalled carbon nanotube/polystyrene (MWCNT/PS) nanocomposites using a latex-based process, the main step of which consists of directly mixing an aqueous suspension of exfoliated MWCNTs and a PS latex, both stabilized by an anionic surfactant. After freeze drying and compression molding homogeneous polymer films with well-dispersed carbon nanotubes were produced as evidenced by scanning electron microscopy. Conductivity measurements performed on our nanocomposite films show that they have a low percolation threshold and exhibit high levels of electrical conductivity above this threshold. We observe that both these properties are influenced by the applied processing conditions, i.e., temperature and time, and provide a plausible explanation based on the diffusive motion of the MWNTs in the polymer melt during the compression molding stage
Controlling electrical percolation in multicomponent carbon nanotube dispersions
Carbon nanotube reinforced polymeric composites can have favourable electrical properties, which make them useful for applications such as flat-panel displays and photovoltaic devices. However, using aqueous dispersions to fabricate composites with specific physical properties requires that the processing of the nanotube dispersion be understood and controlled while in the liquid phase. Here, using a combination of experiment and theory, we study the electrical percolation of carbon nanotubes introduced into a polymer matrix, and show that the percolation threshold can be substantially lowered by adding small quantities of a conductive polymer latex. Mixing colloidal particles of different sizes and shapes (in this case, spherical latex particles and rod-like nanotubes) introduces competing length scales that can strongly influence the formation of the system-spanning networks that are needed to produce electrically conductive composites. Interplay between the different species in the dispersions leads to synergetic or antagonistic percolation, depending on the ease of charge transport between the various conductive components
Role of the particle size polydispersity in the electrical conductivity of carbon nanotube-epoxy composites
Benign reduction of carbon nanotube agglomerates using a supercritical carbon dioxide process
Magnetic assembly of transparent and conducting graphene-based functional composites
Innovative methods producing transparent and flexible electrodes are highly sought in modern optoelectronic applications to replace metal oxides, but available solutions suffer from drawbacks such as brittleness, unaffordability and inadequate processability. Here we propose a general, simple strategy to produce hierarchical composites of functionalized graphene in polymeric matrices, exhibiting transparency and electron conductivity. These are obtained through protein-assisted functionalization of graphene with magnetic nanoparticles, followed by magnetic-directed assembly of the graphene within polymeric matrices undergoing sol–gel transitions. By applying rotating magnetic fields or magnetic moulds, both graphene orientation and distribution can be controlled within the composite. Importantly, by using magnetic virtual moulds of predefined meshes, graphene assembly is directed into double-percolating networks, reducing the percolation threshold and enabling combined optical transparency and electrical conductivity not accessible in single-network materials. The resulting composites open new possibilities on the quest of transparent electrodes for photovoltaics, organic light-emitting diodes and stretchable optoelectronic devices