Polydimethylsiloxane Composites Research Articles

Mechanical, dielectric, and thermal properties of polydimethylsiloxane/polysilsesquioxane nanocomposite for sealant application

ABSTRACTPolysilsesquioxanes (PS) powder was synthesized from methyltrimethoxysilane and vinyltrimethoxysilane by hydrolytic condensation method in aqueous phase. The prepared PS powder was characterized using Fourier transform infrared spectroscopy, particle size, and polydispersity of the powders was determined using dynamic light scattering technique. Polydimethylsiloxane (PDMS) nanocomposites and the sealant were synthesized using different weight percentage (1–4 wt %) of PS powder and hydrophobic fumed silica. Tensile strength and thermal stability of the nanocomposites showed perceptible enhancement on increasing the filler ratio when compared to pristine PDMS composites. The surface morphology and the extent of filler dispersion were visualized from the scanning electron microscopy images. Dielectric strength of the composites also showed significant improvement on increasing the filler loading. Poly(methyl/vinyl)silsesquioxane (PMVS) (4 wt %) reinforced composites showed a considerable enrichment in properties when compared to other formulations. Adhesive strength of silicone sealant on both the alumina and mild steel substrate was studied by conducting lap shear test. PMVS‐loaded nanocomposites exhibited more adhesive strength on both the substrates and hence it can be used as a sealant in electronic assemblies. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47228.

A novel method to prepare homogeneous biocompatible graphene‐based PDMS composites with enhanced mechanical, thermal and antibacterial properties

A negative pressure assisted mixing method was applied to prepare homogeneous graphene‐based polydimethylsiloxane (PDMS) composite. The problem of graphene agglomeration in polymer matrix was well overcome by using the graphene microsphere aerogels. They possess unique porous structure and excellent adsorption capacity, enabling the PDMS solution can uniformly fill into the reduced graphene oxide aerogel microspheres (rGOAMs) or GO aerogel microspheres (GOAMs) under reduced pressure. Raw reduced graphene oxide (rGO) and graphene oxide (GO) were added into PDMS by a simple ultrasonic assisted mixing method as a comparison. rGOAMs‐PDMS and GOAMs‐PDMS composites displayed higher tensile strength, Young's modulus and storage modulus than rGO–PDMS and GO–PDMS composites. In addition, the thermal stability of graphene‐based PDMS composites was also significantly improved. Finally, rGOAMs‐PDMS and GOAMs‐PDMS composites performed much better antibacterial activity than that of rGO‐PDMS and GO‐PDMS composites, which demonstrated that this kind of graphene‐based PDMS composites could display a great potential application in biomedical field. POLYM. COMPOS., 40:E1397–E1406, 2019. © 2018 Society of Plastics Engineers

Change in surface properties of polydimethylsiloxane/polyurethane/carbon nanotubes elastomeric coatings

Addition of polydimethylsiloxane (PDMS) to polyurethane (PU) improves mechanical, surface and bulk properties. PDMS plays the role of a soft segment in PU because of its low surface tension and glass transition temperature. This polymer blend shows highly segregated morphology allowing production of thermoplastic elastomers with properties in between a crosslinked elastomer and thermoplastic depending on composition. In this study, PDMS/PU blends were synthesized in situ and the effects of PDMS composition, the type and content of catalyst, solvent and isocyanates molecular weight as well as carbon nanotubes (CNTs) incorporation on the surface properties of the resulting elastomeric coatings were discussed. Different measurements including contact angle, SEM, AFM, FTIR and elemental composition analyses were used for interpretations. The competition between OH terminating groups of PDMS and polycaprolactone was the key controlling over the hydrophilicity of the resulting PDMS/PU coatings. Such effect was studied by monitoring the changes in surface properties of the cured coatings. The results revealed that by adjusting the above-mentioned parameters, surface chemistry of the cured films were changed. PDMS introducing to PU resulted in a rise in water contact angle from 93° to 113°, which indicated a considerable surface hydrophobicity. Moreover, solvent selection exhibited a crucial role on surface roughness. CNTs incorporation resulted in surface hydrophobicity enhancement, as featured by a rise in water contact angle from 100 to 135°, suggesting the possibility of surface tailoring by optimizing PDMS and CNTs concentrations.

Silica coating onto graphene for improving thermal conductivity and electrical insulation of graphene/polydimethylsiloxane nanocomposites

Graphene possess extremely high thermal conductivity, and they have been regarded as prominent candidates to be used in thermal management of electronic devices. However, addition of graphene inevitably causes dramatic decrease in electrical insulation, which is generally unacceptable for thermal interface materials (TIMs) in real electronic industry. Developing graphene-based nanocomposites with high thermal conductivity and satisfactory electrical insulation is still a challenging issue. In this study, we developed a novel hybrid nanocomposite by incorporating silica-coated graphene nanoplatelets (Silica@GNPs) with polydimethylsiloxane (PDMS) matrix. The obtained Silica@GNP/PDMS composites showed satisfactory electrical insulation (electrical resistivity of over 1013 Ω cm) and high thermal conductivity of 0.497 W m−1 K−1, increasing by 155% compared with that of neat PDMS, even higher than that of GNP/PDMS composites. Such high thermal conductivity and satisfactory electrical insulation is mainly attributed to the insulating silica-coating, good compatibility between components, strong interfacial bonding, uniform dispersion, and high-efficiency heat transport pathways. There is great potential for the Silica@GNP/PDMS composites to be used as high-performance TIMs in electronic industry.

Evaluation of Photoacoustic Transduction Efficiency of Candle Soot Nanocomposite Transmitters

Candle soot nanoparticles (CSNP) and polydimethyl-siloxane (PDMS) composite has shown the highly efficient photoacoustic transduction owing to their high light absorption coefficient and low interfacial thermal resistance. In this study, we report the effect of candle soot structure and thickness on the photoacoustic transduction efficiency. Optical properties of the CSNP/PDMS nanocomposites were characterized through both experimental measurements and finite difference time domain analysis in the visible wavelength range, indicating that the carbon volume fraction and thickness of CS/PDMS composite are highly relevant with light absorption. With a low laser energy input ( $ ), the CS/PDMS composite with 2.15 μ m thickness exerts an output pressure of 3.78 MPa and a conversion efficiency of ${\text{9.69}}\,\times \,{\text{10}}^{- 3}$ , which is two orders of magnitude higher than previously reported results.

Highly selective, reusable electrochemical impedimetric DNA sensors based on carbon nanotube/polymer composite electrode without surface modification

We fabricated a composite of multi-walled carbon nanotube and polydimethylsiloxane and utilized it as an electrode for DNA sensing using electrochemical impedance spectroscopy. Without any surface modification or probe immobilization, often necessary for other electrodes, this electrode also acts as a recognition layer for DNA via π-π interactions between the multi-walled carbon nanotube and DNA. This electrode is easily reusable via a simple cleansing process, because there are no covalently bonded adsorbates on the electrode. Compared to previous DNA detection based on differential pulse voltammetry using a similar electrode, the measurement time was reduced from 1 h to less than 30 min, and the limit of detection (25 pM) was reduced by a factor of more than five. In addition, our system can detect the single-base mismatch between the target and probe. Our results indicate that electrochemical impedance spectroscopy is promising for utilizing the multi-walled carbon nanotube and polydimethylsiloxane electrode as a DNA sensor.

Actuation behavior of PDMS dielectric elastomer composites containing optimized graphene oxide

Actuation behavior of polydimethylsiloxane composites containing conductive fillers is of great interest but faces challenges such as increased elastic modulus and dielectric loss. In this work, thermal reduction of graphene oxide (GO) was utilized as an alternative to chemical modification in order to optimize dielectric properties and enhance actuation strain of the composites containing reduced graphene oxides (rGOs) near percolation threshold. During thermal reduction, the ratio of carbon to oxygen on the surface of rGOs was systematically altered in order to design particles with different surface properties. Dielectric behavior of composites containing rGOs prepared at four reduction temperatures (150 °C, 200 °C, 300 °C, and 400 °C) and employed at three volume fractions (0.25%, 0.5%, and 1%) was measured. The optimized condition for maximum dielectric permittivity and minimum dielectric loss was introduced as 0.52 vol% rGO reduced at 283 °C. The optimized condition was verified by measuring planar actuation strain and comparing with theoretical results. It was discussed that for dielectric elastomer composites containing conductive fillers near electrical percolation threshold, optimization based on dielectric properties provides better results than predictions based on the electromechanical sensitivity.

Study on surface structure of plasma‐treated polydimethylsiloxane (PDMS) elastomer by slow positron beam

In this paper, the variations in surface structure of polydimethylsiloxane elastomers before and after argon plasma treatments have been investigated by X‐ray photoelectron spectroscopy, slow positron beam, and scanning electron microscope. An inorganic silica‐like layer was probed by X‐ray photoelectron spectroscopy after 3 minutes or longer time of treatments, and the sample surface turned into totally hydrophilic. Short time (1 and 2 min) plasma exposure mainly removed preexisting low molecular weighted (LMW) siloxanes on sample surface. By using slow positron beam, the thicknesses of silica‐like layer for 3‐, 5‐, and 10‐minute–treated samples were estimated to be around 30, 66, and 91 nm, respectively. Beneath the silica‐like layer, a loose polymeric structure was also detected, which was ascribed to the accumulation of LMW siloxanes. Scanning electron microscope images showed that the silica‐like layer cracked after 10 minutes of plasma treatment, which provided direct diffusion pathways for LMW siloxanes. Hence, 10‐minute–treated sample showed rather low organic composition near surface. Slow positron beam provides valuable depth profile information for evaluating the surface aging condition of polydimethylsiloxane composite.

The effect of dual-scale carbon fibre network on sensitivity and stretchability of wearable sensors

Sensitivity and stretchability are two key characteristics of wearable sensors and tactile sensors for soft robotics. Here, we present a new technique to increase the sensitivity of wearable sensors by creating a conductive network of dual-scale carbon fibres, i.e., carbon nanofibres (CNFs) and short carbon fibres (SCFs), in a polydimethylsiloxane (PDMS) matrix. To quantify the effects of this dual-scale carbon fibre network on the stretchability, sensitivity, and stability under repeated loading, comprehensive experiments were conducted to characterise the electrical conductivity, mechanical properties, and piezoresistivity of the resultant PDMS composites with varying concentrations of carbon fibres. The Prony series model of viscoelasticity was adapted to model the strain-rate dependent behaviour of the new sensors. The results reveal that this dual-scale network is able to significantly lower the percolation threshold below that of either of the single-scale composites containing only SCFs or CNFs, indicating a strong synergistic effect. Furthermore, the dual-scale carbon network exhibits higher piezoresistive sensitivity than the CNF-reinforced composite while retaining similar stretchability, thus offering a new technique for creating highly sensitive wearable sensors and tactile sensors for soft robotics.

Treatment of Atmospheric-Pressure Radio Frequency Plasma on Boron Nitride for Improving Thermal Conductivity of Polydimethylsiloxane Composites

Boron nitride (BN) particles were treated with atmospheric-pressure radio frequency plasma and then modified using a triethoxyvinyl silane coupling agent in order to enhance the thermal conductivity of polydimethylsiloxane (PDMS) composites. Raw BN, silane-treated BN, and silane-plasma-treated BN were incorporated into PDMS matrix. The surface modification by plasma and coupling agent was verified. The composites filled with treated BN showed a superior effect on thermal conductivity compared to the raw BN filled composite at the same filler loading. Specifically, using silane-plasma-treated BN at 40 wt% of BN increased the thermal conductivity from 1.7 to 3.4 W/mK.

Uniformly dispersed polymeric nanofiber composites by electrospinning: Poly(vinyl alcohol) nanofibers/polydimethylsiloxane composites

A method for the fabrication of homogeneous and well-dispersed polymeric nanofiber composites was investigated. Nanofiber fillers can be used to produce polymeric nanocomposites by mixing the fillers to base polymers, eventually enhancing the mechanical property of the matrix polymers. To produce such composites, nanofibers were usually sandwiched by molten matrix polymers at high temperature before molding. The traditional so-called sandwich method, however, was found to produce rather biased and inhomogeneous composites due largely to the solid entanglement of the nanofibers. In this work, unwoven polymer nanofibers were synthesized through electrospinning by controlling the electrostatic repulsion of the nanofibers. We modified the electrospinning apparatus for the direct synthesis of homogenous composites: nanofibers were electrospun and directly ejected from the electrospinning syringe to the matrix polymer solution (not in a solid state), where a regular metal electrode plate was replaced by an optimized metal container containing the base polymer solution. It was found that this new fabrication method could realize homogeneous mixing of the nanofibers that were eventually uniformly dispersed in the polymer solution. Poly(vinyl alcohol) (PVA) was used for nanofibers and polydimethylsiloxane (PDMS) was used for polymer matrix. The field emission scanning electron microscopy (FE-SEM) revealed the homogeneous and well-dispersed PVA nanofibers in the resulting PDMS composites. The composites also presented higher mechanical properties as compared with the composites fabricated by the traditional sandwich method.

Study of the absorption coefficient of graphene-polymer composites

In this work, we have prepared a series of polydimethylsiloxane (PDMS) composites containing various graphene flakes loadings (0.02–2 wt%), and their broadband optical properties are being investigated. We demonstrate the tunability and evolution of transmittance and reflection spectra of the composites in a wide spectral range (0.4–200 μm) as a function of graphene content. Using these data we derive the broadband wavelength-dependent absorption coefficient (α) values. Our results show that α is roughly constant in the visible and IR ranges, and, surprisingly, is approximately one order of magnitude lower in the terahertz regime, suggesting different terahertz radiation scattering mechanism in our composite. Our material could be useful for applications in optical communication, sensing or ultrafast photonics.

Stretchable strain sensors based on PDMS composites with cellulose sponges containing one- and two-dimensional nanocarbons

Here we present a new technique to create stretchable strain sensors by using porous cellulose composites with a low content of nanocarbon materials. In this method, highly porous cellulous microfibre sponges containing nanocarbon materials are first created by a freeze-drying method. The resullting cellulose sponges are highly conductive due to the spatial confinement of the reduced graphene oxide (rGO) or its hybrid with carbon nanofibers (CNFs) in the fibrous skeleton. Infiltrating this highly porous sponge with polydimethylsiloxane (PDMS) then creates a stretchable composite containing hierarchical conductive network with a very low loading of nanocarbons (∼0.1 wt% in the resultant PDMS composites). Experimental results reveal that the hybridisation of rGO with a small amount of CNFs increases the electrical conductivity and piezoresistive sensitivity of the composite sensor. Moreover, upon increasing the CNFs content (rGO:CNFs mass ratio varies from 1:0 to 1:1), the electrical conductivity increases up to 9.2 × 10−3 S/m, the modulus increases while strength and elongation at break decrease, and the gauge factor increases initially from approximately 3.4 to 9.4 (at rGO:CNFs = 1:0.1) then decreases thereafter. When subjected to cyclic loading-unloading up to 10 000 cycles, the new composite sensors show excellent durability with very little drift over a large number of cycles. The piezoresistive response exhibits negligible strain-rate dependence up to 0.2 s−1. Finally, an example is presented to highlight the potentials of the composite sensor as wearables in monitoring the movement of human joints, e.g., the bending of a finger joint.

Ultrastable, highly luminescent quantum dot composites based on advanced surface manipulation strategy for flexible lighting-emitting

Although quantum dots (QDs) have remarkable potential application in flexible light emitting diodes (LED), the loss of solvent-protected QDs leads to low quantum yield (QY) and poor stability, severely restricting the development. Flexible QD LEDs (Q-LEDs) with three primary colors were fabricated by mixing CdS/ZnS, CdSe@ZnS/ZnS, and CdSe/CdS QDs with polydimethylsiloxane (PDMS) by in situ hydrosilylation based surface manipulation strategy, which endows the device with highly ultrastable and luminescent performance. The surface manipulation strategy mainly includes the control of solvent dosage, purification times of QDs, concentration of QDs in PDMS, and oxidation on the preparation process of the QDs and PDMS composites. The highest QY of CdSe@ZnS/ZnS-PDMS composite is 82.03%, higher than the QY (80%) of the QD solution. After UV bleaching, organic solvents (acetone, ethanol and water), and heating treatment, the QYs of the QDs and PDMS maintain a high value, manifesting their good stability. Q-LED hybrid light-emitting devices were further fabricated by a molding technique demonstrating satisfied current and thermal stability. Flexible Q-LEDs can be expended to other shapes, such as fibers and blocks, indicating the huge potential of QD-polymer composites for light sources and displays etc.

Antibacterial bioelastomers with sustained povidone-iodine release

A serious problem in biomedical applications is the contamination of medical equipment, that increases the risk of medical failure. A longstanding effort is thus dedicated to develop antibacterial materials that can withstand bacterial growth. Here, we have developed a new material, namely a polydimethylsiloxane (PDMS) and starch bioelastomeric composite that contains an antiseptic compound, namely povidone-iodine (PVPI). The PVPI binds to the bioelastomer via the iodine–starch complexation, only after immersion of the samples in water. Nevertheless, for prolonged water immersion, some PVPI escapes into the water, rendering it yellow. FTIR and μRaman measurements demonstrate the complexation of the iodine to the starch. The sustained release of the PVPI is studied, for different PVPI concentrations, and quantified by UV–vis spectrometry. The antibacterial properties of the material are demonstrated against E. coli, by the agar test. Finally, we performed an experiment adapted from the broth dilution test, normally used for antibiotics, to show the bacteriostatic effect of the PVPI loaded bioelastomers.

Effects of GnF Concentration on the Mechanoelectrical Properties and Surface Morphology of GnF/PDMS Composites

The graphite nanoflake (GnF)-reinforced polydimethylsiloxane (PDMS) composites (GnF/PDMS composites) are developed as new polymer matrix composites (PMCs) with controllable mechanoelectrical properties. Here, we investigate the effect of GnF concentration on the mechanoelectrical properties (i.e., elastic modulus, fracture strain, and conductivity) of GnF/PDMS composites; the change in the surface morphology of GnF/PDMS composites caused by a variation in GnF concentration is also explored. The mechanoelectrical properties are measured by performing tensile tests on the GnF/PDMS composite specimens with different GnF concentrations of 5.0, 10.0, 12.5, 15.0, 20.0, and 25.0 wt.%. The surface morphology is analyzed in terms of internal void formation and surface roughness. The elastic modulus is measured to be in the range of 1.62 to 13.8 MPa which is proportional to GnF concentration, while the fracture strain and electrical conductivity are respectively characterized to be in ranges of 0.09 to 2.09 and 0.3 to 221.0 S/m which are in inverse proportion to GnF concentration. An increase in GnF concentration leads to increases in internal voids’ amount and surface roughness. The GnF/PDMS composites can be used as sensing materials for detecting both small and large deformations in a variety of engineering applications.

Piezoresistive strain sensor array using polydimethylsiloxane-based conducting nanocomposites for electronic skin application

PurposeThis paper aims to report a stretchable piezoresistive strain sensor array that can detect various static and dynamic stimuli, including bending, normal force, shear stress and certain range of temperature variation, through sandwiching an array of conductive blocks, made of multiwalled carbon nanotubes (MWCNTs) and polydimethylsiloxane (PDMS) composite. The strain sensor array induces localized resistance changes at different external mechanical forces, which can be potentially implemented as electronic skin.Design/methodology/approachThe working principle is the piezoresistivity of the strain sensor array is based on the tunnelling resistance connection between the fillers and reformation of the percolating path when the PDMS and MWCNT composite deforms. When an external compression stimulus is exerted, the MWCNT inter-filler distance at the conductive block array reduces, resulting in the reduction of the resistance. The resistance between the conductive blocks in the array, on the other hand, increases when the strain sensor is exposed to an external stretching force. The methodology was as follows: Numerical simulation has been performed to study the pressure distribution across the sensor. This method applies two thin layers of conductive elastomer composite across a 2 × 3 conductive block array, where the former is to detect the stretchable force, whereas the latter is to detect the compression force. The fabrication of the strain sensor consists of two main stages: fabricating the conducting block array (detect compression force) and depositing two thin conductive layers (detect stretchable force).FindingsCharacterizations have been performed at the sensor pressure response: static and dynamic configuration, strain sensing and temperature sensing. Both pressure and strain sensing are studied in terms of the temporal response. The temporal response shows rapid resistance changes and returns to its original value after the external load is removed. The electrical conductivity of the prototype correlates to the temperature by showing negative temperature coefficient material behaviour with the sensitivity of −0.105 MΩ/°C.Research limitations/implicationsThe conductive sensor array can potentially be implemented as electronic skin due to its reaction with mechanical stimuli: compression and stretchable pressure force, strain sensing and temperature sensing.Originality/valueThis prototype enables various static and dynamic stimulus detections, including bending, normal force, shear stress and certain range of temperature variation, through sandwiching an array of conductive blocks, made of MWCNT and PDMS composite. Conventional design might need to integrate different microfeatures to perform the similar task, especially for dynamic force sensing.

Flame Retardant Effect of Nano Fillers on Polydimethylsiloxane Composites.

Polydimethylsiloxane has exceptional fire retardancy characteristics, which make it a popular polymer in flame retardancy applications. Flame retardancy of polydimethylsiloxane with different nano fillers was studied. Polydimethylsiloxane composite fire property varies because of the shape, size, density, and chemical nature of nano fillers. In house made carbon and bismuth oxide nano fillers were used in polydimethylsiloxane composite. Carbon from biochar (carbonised bamboo) and a carbon by-product (carbon soot) were selected. For comparative study of nano fillers, standard commercial multiwall carbon nano tubes (functionalised, graphitised and pristine) as nano fillers were selected. Nano fillers in polydimethylsiloxane positively affects their fire retardant properties such as total smoke release, peak heat release rate, and time to ignition. Charring and surface ceramization are the main reasons for such improvement. Nano fillers in polydimethylsiloxane may affect the thermal mobility of polymer chains, which can directly affect the time to ignition. The study concludes that the addition of pristine multiwall carbon nano tubes and bismuth oxide nano particles as filler in polydimethylsiloxane composite improves the fire retardant property.

Mechanical properties of nanocomposites reinforced by carbon nanotube sponges

Carbon nanotube (CNT) sponge exhibits unique porous and hierarchical structure that are beneficial to the design of ultralight and tough composites. In this study, CNT sponges (undoped and boron doped) reinforced polydimethylsiloxane (PDMS) composites were fabricated. Mechanical properties of the composite, including compressive modulus, rate-dependent modulus, stress relaxation behaviors, dynamic viscoelastic properties, and their dependency on temperature, were systematically investigated. A micromechanical model, Mori-Tanaka model, was validated to describe the mechanical behaviors of CNT sponge reinforced composites. By coupling with boron-doped CNT sponge, PDMS composites showed remarkable improvement of mechanical properties, including compressive modulus (70%), viscous modulus (243%) and damping capacity (50%). Such reinforcement effects can be controlled by the morphology of CNT sponges, as the boron-doped and undoped nanocomposites showed distinct viscoelastic behaviors. The results proved that CNT sponge reinforcement is a promising strategy to develop engineering composites with both outstanding mechanical stiffness and controllable viscoelastic performances.

Ultrasound Generation: Polydimethylsiloxane Composites for Optical Ultrasound Generation and Multimodality Imaging (Adv. Funct. Mater. 9/2018)

Ultrasound Generation: Polydimethylsiloxane Composites for Optical Ultrasound Generation and Multimodality Imaging (Adv. Funct. Mater. 9/2018)