Polydimethylsiloxane Composites Research Articles

Flexible dry electrode based on carbon nanotube/polymer hybrid micropillars for biopotential recording

A novel flexible dry electrode based on carbon nanotube (CNT) and polydimethylsiloxane (PDMS) is proposed for recording biopotentials. Because the homogeneous dispersion of CNTs in PDMS is challenge due to the high viscosity of PDMS and aggregation of CNTs, a novel process is developed through addition of an organic solvent to disentangle CNTs and reduce the viscosity of PDMS. The electrical performance of the composite of CNTs and PDMS as the function of CNT concentration was characterized. The optimized 10wt% MWCNTs is dispersed in PDMS as the material of flexible dry electrode. In order to apply for long-term, wearable biopotential recording devices, the flexible dry electrode with micropillar array is designed and fabricated by MEMS process. The testing result shows that the skin-electrode contact impedance of the flexible mciropillar electrode is lower an order magnitude than that of the flexible flat electrode without micropillar array structures. Moreover, the contact impedance of this fabricated electrode was stable during two-day continuous testing, which indicates the flexible dry electrode is suitable for long-term measurement. In order to investigate the effect of motion artifact on the ECG signal, ECG signals are recording under two statuses of resting and walking. The ECG signals measured by the fabricated micropillar electrode were good fidelity, and did not degrade because of the motion.

A transparent silica colloidal crystal/PDMS composite and its application for crack suppression of metallic coatings

A silica colloidal crystal (SCC)–polydimethylsiloxane (PDMS) composite with a heterogeneous surface of silica and PDMS was prepared by spreading a premixed PDMS solution on the 3D structured SCCs and curing the solution in-situ. Although the SCCs had a light blue color, the obtained composite of SCC and PDMS, due to the close effective refractive indexes of the materials, was colorless and transparent; the UV–vis spectra indicated a negligible effect of the added SCC on the transmittance of the PDMS sheet (1% reduction). Interestingly, the transparent composite sheet became translucent under stress and became clear again when relaxed. It was found that the wrinkles formed on the surface under stress were responsible for the optical change; and, the formation of the wrinkles was ascribed to the rigid nature of the SCC layer embedded in PDMS. We had applied this SCC/PDMS composite as a substrate to support a thin gold film of nanoscale thickness and found that the embedded SCC layer worked well as a transitional interface for bonding materials of mismatched mechanical properties. The incorporation of SCC layer significantly suppressed the crack generation and propagation of the gold film. The results demonstrated a potential approach for fabricating compliant and crackfree metallic films on polymeric substrates.

Preparation and characterization of novel addition cured polydimethylsiloxane nanocomposites using nano-silica sol as reinforcing filler

A series of novel addition cured polydimethylsiloxane (PDMS) nanocomposites with various amounts of nano-silica sol were prepared via hydrosilylation for the first time. The influence of various amounts of nano-silica sol on the morphology, thermal behavior, mechanical and optical properties of these PDMS nanocomposites was studied in detail. It was found that with an increment in the amount of nano-silica sol the reinforcing effect of the nano-silica sol on the thermal and mechanical properties of the PDMS nanocomposites was very noticeable compared with the reference material. The prominent improvements in resistance to thermal degradation and mechanical properties can probably be attributed to the strong interaction of PDMS chains and uniformly dispersed particles resulting from the nano-silica sol. However, the transparency of the PDMS nanocomposites slightly decreased with an increment in weight fraction of nano-silica, compared with that of PDMS composite without nano-silica (Sol-0), which can probably be ascribed to an increasing size of the aggregated particles in the PDMS nanocomposites. The optimum amount of nano-silica sol for preparing novel addition curing PDMS nanocomposites was about 15 wt%. © 2015 Society of Chemical Industry

Multifold Increases in Thermal Conductivity of Polymer Nanocomposites through Microwave Welding of Metal Nanowire Fillers

A 40-fold thermal conductivity increase in silver nanowire-filled polydimethylsiloxane composites is demonstrated by in situ microwave welding of nanowire fillers. Even for nanowire fractions ≤0.04, welding counteracts thermal transport bottlenecks associated with low polymer thermal conductivity and high polymer–filler interface thermal resistances. The microwave transparency of the polymer precludes thermal degradation, and the composites retain high compliance.

Thermal stability, mechanical and optical properties of novel addition cured PDMS composites with nano-silica sol and MQ silicone resin

A series of novel addition cured polydimethylsiloxane (PDMS) composites with a given weight amount of nano-silica sol were prepared using MQ silicone resin as reinforcing agent by hydrosilylation for the first time. Influences of the used amount of MQ silicone resin on morphology, thermal stability, mechanical and optical properties of these novel PDMS composites were mainly studied. It was found that thermal stabilities and transparencies of the novel PDMS composites with a given amount of nano-silica sol decreased with an increment in weight fraction of MQ silicone resin, as compared with that of PDMS composites without MQ silicone resin (SMQ-0), which were likely ascribed to an increasing amount of remained silanols and the increasing size of aggregated particles in PDMS composites, respectively. However, mechanical properties of the novel PDMS composites with a given weight of nano-silica sol were improved significantly with amount of MQ silicone resin increasing, especially their elongations at break, which were up to 10 times. This was a great breakthrough for PDMS composites. The prominent improvements in mechanical properties were probably attributed to strong interaction of PDMS chains and uniformly dispersed particles resulted from crosslink of MQ silicone resin and nano-silca sol. This strategy for preparing novel addition curing PDMS composites provides a guide method to develop high elastic PDMS composites with desirable properties in industry applications.

Core/Shell Iron/Oxide Nanoparticles for Improving the Magneto‐Dielectric Properties of Polymer Composites

A simple method to obtain high magnetic permeability (μr), high dielectric permittivity (ϵr), and low loss polymer composites with core/shell iron/metal oxide particles is developed. Magnetic iron (Fe0) particles with protective SiO2 shells are synthesized in aqueous environment at room temperature. The shells provide electrical insulting layers which decrease energy loss and also prevent the possibility of μr decrease due to Fe0 oxidation. Furthermore, Fe0 particles could also be coated with TiO2 shells which allow ϵr to be tuned. The ϵr and μr of the resulting polydimethylsiloxane composites are optimized for core‐shell particle doping. The value of ϵr is up to 9.8 and most importantly, the μ is close to 2 at 1 GHz with 43 wt% doping of core‐shell particles. The dielectric loss of the composites is less than 0.02.

Tuning the Friction Characteristics of Gecko-Inspired Polydimethylsiloxane Micropillar Arrays by Embedding Fe₃O₄ and SiO₂ Particles.

In order to improve stiffness of polydimethylsiloxane (PDMS) pillars while maintaining high friction, the effects of embedding Fe3O4 and SiO2 particles on the friction behavior of PDMS micropillars are studied. Both types of added particles increase the stiffness of the PDMS composite, but affect the friction behavior differently. The frictional force of the fibrillar array fabricated with Fe3O4/PDMS composite decreases initially, then increases as the particle content increases. For silica/PDMS composite pillars, the frictional force is independent of the particle density. Characterization by scanning electron microscopy shows that Fe3O4 particles are distributed uniformly in the PDMS matrix at low concentration, but heterogeneous distribution is observed at high particle loading, with particles being hindered from penetrating into the pillars. For silica/PDMS composite pillars, the particles distribute homogeneously inside the pillars, which is attributed to the formation of hydrogen bonding between silica particles and PDMS. The difference in particle distribution behavior is used to explain the observed difference in the friction response of these two composite systems.

Mechanically robust honeycomb graphene aerogel multifunctional polymer composites

This work reports the fabrication and characterization of three-dimensional (3D) graphene aerogel (GA)–polydimethylsiloxane (PDMS) composites (GAPC) with outstanding mechanical, electrical and thermal properties. GAPC was fabricated by impregnating 3D GA frameworks with PDMS via ice-bath-assisted infiltration and vacuum curing processes. Because of the well-interconnected 3D GA frameworks, GAPC exhibits extremely large deformability (compressive strain=80% and tensile strain=90%), high electrical and thermal conductivities (1S/cm and 0.68W/(mK), respectively), a stable piezo-resistance effect, rapid electric Joule heating performance ((dT/dt)max>3°C/s under a heating power of 12W/cm3), and high hydrophobicity (contact angle=135°). Furthermore, GAPC exhibits a negative temperature coefficient of expansion with decreased electrical resistivity over a broad temperature range, indicating a typical semiconducting behavior and a dual two-dimensional/3D hopping conduction mechanism.

Rigidity-tuning conductive elastomer

We introduce a conductive propylene-based elastomer (cPBE) that rapidly and reversibly changes its mechanical rigidity when powered with electrical current. The elastomer is rigid in its natural state, with an elastic (Young’s) modulus of 175.5 MPa, and softens when electrically activated. By embedding the cPBE in an electrically insulating sheet of polydimethylsiloxane (PDMS), we create a cPBE–PDMS composite that can reversibly change its tensile modulus between 37 and 1.5 MPa. The rigidity change takes ∼6 s and is initiated when a 100 V voltage drop is applied across the two ends of the cPBE film. This magnitude of change in elastic rigidity is similar to that observed in natural skeletal muscle and catch connective tissue. We characterize the tunable load-bearing capability of the cPBE–PDMS composite with a motorized tensile test and deadweight experiment. Lastly, we demonstrate the ability to control the routing of internal forces by embedding several cPBE–PDMS ‘active tendons’ into a soft robotic pneumatic bending actuator. Selectively activating the artificial tendons controls the neutral axis and direction of bending during inflation.

TiO 2 –SiO 2 –PDMS nanocomposites with self-cleaning properties for stone protection and consolidation

Abstract This study deals with the design of two hydrophobic hybrid SiO 2 –TiO 2 materials and their application in the field of monument conservation. The nanocomposite consolidants were based on the modification of tetraethoxysilane (TEOS) with the incorporation of both TiO 2 nanoparticles and hydroxyl-terminated polydimethylsiloxane (PDMS). The TiO 2 nanoparticles have been prepared from the hydrolysis of titanium tetraisopropoxide (TTIP) in the presence of oxalic acid as a catalyst. The physico-chemical properties of the designed hydrophobic nanocrystalline SiO 2 –TiO 2 –PDMS composites have been extensively studied before their application as consolidants on a limestone. Overall, the effectiveness of the nanocomposites was evaluated by the comparative characterization of untreated and treated stone specimens. The results of both capillary water absorption and contact angle measurements point out that the TiO 2 –SiO 2 –PDMS nanocomposites functioned as hydrophobic coatings. Furthermore, the results obtained from microdrilling resistance and Fourier Transform Infrared spectroscopy (FTIR) revealed a penetration depth of the nanocomposites within the stone of c. 15 mm. The innovation of these syntheses pertains to the application of hydrophobic semi-transparent and transparent nanocomposites with self-cleaning and strengthening properties, without altering the colour of the stone surface and the water vapour permeability.

New Attenuation Predictive Model for Carbon-Based Nanocomposites

The use of carbon nanofibers (CNF) reinforced composites is popular among several industries such as healthcare, aerospace and defense as they have enhanced mechanical and thermal properties. CNF and carbon nanotubes (CNT) in the composites also improve damping and attenuation. We have been investigating its application for scaffold implants in human airways which undergoes vibratory stress and requires weight-sensitive sound proofing. This paper proposes a predictive model for the attenuation of sound waves through the composite that takes into consideration the Rayleigh scattering function, absorption, resonance and interfacial friction of the embedded fibers. These factors are dependent on the size, thickness, density, porosity, Young Modulus and volume fraction of the nanofibers or nanotubes. CNF reinforced poly-di-methyl-siloxane (PDMS) and single-walled CNT reinforced PDMS composites were investigated. Ultrasonic testing and measurement of sound wave attenuations through the material were done to validate the proposed model and results are shown to be consistent.

Polydimethylsiloxane composites containing 1,2-naphtoquinone 4-sulphonate as unique dispositive for estimation of casein in effluents from dairy industries

A unique dispositive to determine casein which is the most abundant protein in dairy sewages has been proposed. In this sensing technology, the derivatization reagent 1,2-naphtoquininone 4-sulphonate (NQS) is embedded into a polydimethylsiloxane-tetraethylortosilicate-SiO2 nanoparticles composite (PDMS-TEOS-SiO2NPs). When the composite is immersed into the samples, casein is extracted from the solution and derivatized inside the PDMS matrix after 10min at 100°C. The sensing support changes its color from yellow to orange depending on the casein concentration. Quantitative analysis can be carried out by measuring the absorbance with a reflection probe or by image-processing tool (GIMP). This sensor provides good sensitivity and precision (RSD% <12%). The method validation has been done by applying the biocinchoninic acid method (BCA). Moreover, semiquatitative analysis of casein can be performed by visual observation. Taking into account the advantages of small size, rapidity, simplicity, good stability and high compatibility in aqueous solution, this sensor is expected to have potential practical applications for in-situ determination of casein. Finally the method has been applied to analyze effluents from dairy industries.

Polydimethylsiloxane composites containing core-only lanthanide-doped oleylamine-stabilized LaF3 nanoparticles with high emission lifetimes

This work reports on the synthesis and spectroscopic characterization of oleylamine-stabilized lanthanide-doped LaF3 nanoparticles isolated or dispersed in polydimethylsiloxane (pdms).

Implantable magnetic nanocomposites for the localized treatment of breast cancer

This paper explores the potential of implantable magnetic nanocomposites for the localized treatment of breast cancer via hyperthermia. Magnetite (Fe3O4)-reinforced polydimethylsiloxane composites were fabricated and characterized to determine their structural, magnetic, and thermal properties. The thermal properties and degree of optimization were shown to be strongly dependent on material properties of magnetic nanoparticles (MNPs). The in-vivo temperature profiles and thermal doses were investigated by the use of a 3D finite element method (FEM) model to simulate the heating of breast tissue. Heat generation was calculated using the linear response theory model. The 3D FEM model was used to investigate the effects of MNP volume fraction, nanocomposite geometry, and treatment parameters on thermal profiles. The implications of the results were then discussed for the development of implantable devices for the localized treatment of breast cancer.

Preparation and properties of novel polydimethylsiloxane composites using polyvinylsilsesquioxanes as reinforcing agent

A series of novel polydimethylsiloxane (PDMS) composites were prepared using polyvinylsilsesquioxanes (PVS) as reinforcing agent by hydrolytic condensation in the presence of organotin catalyst for the first time. The cross-linked network, morphology, thermal behavior and mechanical properties of these novel PDMS composites were examined by attenuated total reflection infrared spectroscopy (ATR-IR), scanning electron microscope (SEM), thermogravimetric analysis (TGA) and universal tensile testing machine, respectively. Experimental results showed that both thermal and mechanical properties of the PDMS composites were improved greatly by adding PVS. The prominent improvements in thermal and mechanical properties were likely attributed to the reinforcing interaction of PDMS chains and formed particles resulted from PVS self-crosslink. Meanwhile, we also found that with the increment in loading PVS, the resistance to thermal degradation of the PDMS composites in nitrogen atmosphere was enhanced obviously, but their resistance to thermal oxidation in air was not improved apparently. The unobvious improvement in resistance to thermal oxidation of the novel composites was likely due to the catalysis of oxygen.

Influence of molecular weight of polymer matrix on the structure and rheological properties of graphene oxide/polydimethylsiloxane composites

The structure and rheological properties of graphene oxide (GO)/polydimethylsiloxane (PDMS) composites are examined as the molecular weight of PDMS and concentration of GO are varied. Clusters formed by GO sheets get smaller and disperse better with increasing molecular weight of PDMS, which results in the higher critical concentration to form network (Ccr). Moreover, at GO concentration just above Ccr, the plateau modulus of samples decreases with the molecular weight of PDMS. During shear experiments, negative normal stress differences (ΔN) are observed in composites with PDMS molecular weight lower than critical entanglement molecular weight (Mc). However, positive ΔN is found in samples with PDMS molecular weight above Mc. It can be concluded that the vorticity alignment of GO clusters induces the negative ΔN based on the optical shear experiments. The possible mechanism for the positive ΔN is also proposed.

Impedance spectroscopy analysis of the tunnelling conduction mechanism in piezoresistive composites

The piezoresistive behaviour based on the quantum tunnelling conduction of a copper–polydimethylsiloxane composite was studied by means of impedance spectroscopy and confirmed by the fitting with the tunnelling piezoresistance mathematical model. This investigation paves the way to the implementation of the impedance spectroscopy technique in the characterization of piezoresistive pressure sensors for robotic application.

Softening in silver-nanowire-filled polydimethylsiloxane nanocomposites

We report that silver nanowire fillers can result in a three-fold decrease in viscoelastic storage modulus of polydimethylsiloxane composites above a low critical filler fraction of ∼0.5%, contrary to theoretical predictions presaging a modulus increase. Similar fractions of silver nanocube fillers result in no such observable effects. Rheology measurements and calorimetric kinetics analyses reveal that high surface area nanowire filler percolation curtails macromolecular mobility via pre-cure gelation, and hinders crosslinking. Our findings on the effect of metal filler aspect ratio on polymer stiffness at low loading fractions would be important for designing nanocomposites for applications.

A flexible strain gauge exhibiting reversible piezoresistivity based on an anisotropic magnetorheological polymer

A flexible, anisotropic and portable stress sensor (logarithmic reversible response between 40–350 kPa) was fabricated, in which i) the sensing material, ii) the electrical contacts and iii) the encapsulating material, were based on polydimethylsiloxane (PDMS) composites. The sensing material is a slide of an anisotropic magnetorheological elastomer (MRE), formed by dispersing silver-covered magnetite particles (Fe3O4@Ag) in PDMS and by curing in the presence of a uniform magnetic field. Thus, the MRE is a structure of electrically conducting pseudo-chains (needles) aligned in a specific direction, in which electrical conductivity increases when stress is exclusively applied in the direction of the needles. Electrical conductivity appears only between contact points that face each other at both sides of the MRE slide. An array of electrical contacts was implemented based on PDMS-silver paint metallic composites. The array was encapsulated with PDMS. Using Fe3O4 superparamagnetic nanoparticles also opens up possibilities for a magnetic field sensor, due to the magnetoresistance effects.

Development of a flexible three-axis tactile sensor based on screen-printed carbon nanotube-polymer composite

A flexible, three-axis carbon nanotube (CNT)–polymer composite-based tactile sensor is presented. The proposed sensor consists of a flexible substrate, four sensing cells, and a bump structure. A CNT–polydimethylsiloxane (PDMS) composite is produced by a solvent evaporation method, and thus, the CNTs are well-dispersed within the PDMS matrix. The composite is directly patterned onto a flexible substrate using a screen printing technique to fabricate a sensor with four sensing cells. When a force is applied on the bump, the magnitude and direction of force could be detected by comparing the changes in electrical resistance of each sensing cell caused by the piezoresistive effect of the composite. The experimentally verified sensing characteristics of the fabricated sensor exhibit a linear relationship between the resistance change and the applied force, and the measured sensitivities of the sensor for the normal and shear forces are 6.67 and 86.7%/N for forces up to 2.0 and 0.5 N, respectively. Experiments to verify the load-sensing repeatability show a maximum 2.00% deviation of the resistance change within the tested force range.