Composite Material of PDMS with Interchangeable Transmittance: Study of Optical, Mechanical Properties and Wettability

Mechanical and optical properties assessment of an innovative PDMS/beeswax composite for a wide range of applications

Polydimethylsiloxane (PDMS) is an elastomer that has received primary attention from researchers due to its excellent physical, chemical, and thermal properties, together with biocompatibility and high flexibility properties. Another material that has been receiving attention is beeswax because it is a natural raw material, extremely ductile, and biodegradable, with peculiar hydrophobic properties. These materials are applied in hydrophobic coatings, clear films for foods, and films with controllable transparency. However, there is no study with a wide range of mechanical, optical, and wettability tests, and with various proportions of beeswax reported to date. Thus, we report an experimental study of these properties of pure PDMS with the addition of beeswax and manufactured in a multifunctional vacuum chamber. In this study, we report in a tensile test a 37% increase in deformation of a sample containing 1% beeswax (BW1%) when compared to pure PDMS (BW0%). The Shore A hardness test revealed a 27% increase in the BW8% sample compared to BW0%. In the optical test, the samples were subjected to a temperature of 80 °C and the BW1% sample increased 30% in transmittance when compared to room temperature making it as transparent as BW0% in the visible region. The thermogravimetric analysis showed thermal stability of the BW8% composite up to a temperature of 200 °C. The dynamic mechanical analysis test revealed a 100% increase in the storage modulus of the BW8% composite. Finally, in the wettability test, the composite BW8% presented a contact angle with water of 145°. As a result of this wide range of tests, it is possible to increase the hydrophobic properties of PDMS with beeswax and the composite has great potential for application in smart devices, food and medicines packaging films, and films with controllable transparency, water-repellent surfaces, and anti-corrosive coatings.

Comparative Study of Electrospun Polydimethylsiloxane Fibers as a Substitute for Fluorine-Based Polymeric Coatings for Hydrophobic and Icephobic Applications

The development of superhydrophobic, waterproof, and breathable membranes, as well as icephobic surfaces, has attracted growing interest. Fluorinated polymers like PTFE or PVDF are highly effective, and previous research by the authors has shown that combining these polymers with electrospinning-induced roughness enhances their hydro- and ice-phobicity. The infusion of these electrospun mats with lubricant oil further improves their icephobic properties, achieving a slippery liquid-infused porous surface (SLIPS). However, their environmental impact has motivated the search for fluorine-free alternatives. This study explores polydimethylsiloxane (PDMS) as an ideal candidate because of its intrinsic properties, such as low surface energy and high flexibility, even at very low temperatures. While some published results have considered this polymer for icephobic applications, in this work, the electrospinning technique has been used for the first time for the fabrication of 95% pure PDMS fibers to obtain hydrophobic porous coatings as well as breathable and waterproof membranes. Moreover, the properties of PDMS made it difficult to process, but these limitations were overcome by adding a very small amount of polyethylene oxide (PEO) followed by a heat treatment process that provides a mat of uniform fibers. The experimental results for the PDMS porous coating confirm a hydrophobic behavior with a water contact angle (WCA) ≈ 118° and roll-off angle (αroll-off) ≈ 55°. In addition, the permeability properties of the fibrous PDMS membrane show a high transmission rate (WVD) ≈ 51.58 g∙m-2∙d-1, providing breathability and waterproofing. Finally, an ice adhesion centrifuge test showed a low ice adhesion value of 46 kPa. These results highlight the potential of PDMS for effective icephobic and waterproof applications.

Reproducible Superinsulation Materials: Organosilica-Based Hybrid Aerogels with Flexibility Control

In this study, we report highly crosslinked hybrid aerogels with an organic backbone based on vinylmethyldimethoxysilane (VMDMS) with tuneable properties. For an improved and highly reproducible synthesis, a prepolymer based on 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (D4V4) and VMDMS as monomers was prepared and purified. Di-tert-butylperoxide (DTBP) concentrations of 1 mol% initiate the radical polymerization of the mentioned monomers to achieve high yields of polymers. After purification, the obtained viscous polyorganosilane precursor could be reproducibly crosslinked with dimethyldimethoxysilane (DMDMS) or methyltrimethoxysilane (MTMS) to form gels in benzylic alcohol (BzOH), water (H2O) and tetramethylammonium hydroxide (TMAOH). Whereas freeze-drying these silica-based hybrid aerogels led to high thermal conductivity (>20 mW m-1K-1) and very fragile materials, useful aerogels were obtained via solvent exchange and supercritical drying with CO2. The DMDMS-based aerogels exhibit enhanced compressibility (31% at 7 kPa) and low thermal conductivity (16.5 mW m-1K-1) with densities around (0.111 g cm-3). The use of MTMS results in aerogels with lower compressibility (21% at 7 kPa) and higher density (0.124 g cm-3) but excellent insulating properties (14.8 mW m-1K-1).

Photoacoustic method for measuring the elasticity of polydimethylsiloxane at various mixing ratios

Measuring elasticity without physical contact is challenging, as current methods often require deconstruction of the test sample. This study addresses this challenge by proposing and testing a photoacoustic effect-based method for measuring the elasticity of polydimethylsiloxane (PDMS) at various mixing ratios, which may be applied on the wide range of applications such as biomedical and optical fields. A dual-light laser source of the photoacoustic (PA) system is designed, employing cross-correlation signal processing techniques. The platform systems and a mathematical model for performing PDMS elasticity measurements are constructed. During elasticity detection, photoacoustic signal features, influenced by hardness and shapes, are analyzed using cross-correlation calculations and phase difference detection. Results from phantom tests demonstrate the potential of predicting Young's modulus using the cross-correlation method, aligning with the American Society for Testing and Materials (ASTM) standard samples. However, accuracy may be affected by mixed materials and short tubes. Normalization or calibration of signals is suggested for aligning with Young's coefficient.

Stress Concentration on PDMS: An evaluation of three numerical constitutive models using digital image correlation

The examination of hyperelastic materials' behavior, such as polydimethylsiloxane (PDMS), is crucial for applications in areas as biomedicine and electronics. However, the limitations of hyperelastic models for specific stress scenarios, with stress concentration, are not well explored on the literature. To address this, firstly, three constitutive models were evaluated (Neo-Hookean, Mooney-Rivlin, and Ogden) using numerical simulations and Digital Image Correlation (DIC) analysis during a uniaxial tensile test. The samples were made of PDMS with stress concentration geometries (center holes, shoulder fillets, and edge notches). Results of ANOVA analysis showed that any of the three models can be chosen for numerical analysis of PDMS since no significant differences in suitability were found. Finally, the Ogen model was chosen to obtain the stress concentration factors for these geometries, a property which characterize how discontinuities change the maximum stress supported by an element. Our study provides new values for variables needed to analyze and design hyperelastic elements and produce a foundation for understanding PDMS stress-strain behavior.

Long-lasting biofouling formation on transparent fouling-release coatings for the construction of efficient closed photobioreactors

In order to build an efficient closed-photobioreactor (PBR) in which biofouling formation is avoided, a non-toxic coating with high transparency is required, which can be applied to the interior surface of the PBR walls. Nowadays, amphiphilic copolymers are being used to inhibit microorganism adhesion, so poly(dimethylsiloxane)-based coatings mixed with poly(ethylene glycol)-based copolymers could be a good option. The 7 poly(dimethylsiloxane)-based coatings tested in this work contained 4% w/w of poly(ethylene glycol)-based copolymers. All were a good alternative to glass because they presented lower cell adhesion. However, the DBE-311 copolymer proved the best option due to its very low cell adhesion and high transmittance. Furthermore, XDLVO theory indicates that these coatings should have no cell adhesion at time 0 since they create a very high-energy barrier that microalgae cells cannot overcome. Nevertheless, this theory also shows that their surface properties change over time, making cell adhesion possible on all coatings after 8 months of immersion. The theory is useful in explaining the interaction forces between the surface and microalgae cells at any moment in time, but it should be complemented with models to predict the conditioning film formation and the contribution of the PBR's fluid dynamics over time.

Laser-induced heating of polydimethylsiloxane-magnetite nanocomposites for hyperthermic inhibition of triple-negative breast cancer cell proliferation

This paper presents the results of an experimental and computational study of the effects of laser-induced heating provided by magnetite nanocomposite structures that are being developed for the localized hyperthermic treatment of triple-negative breast cancer. Magnetite nanoparticle-reinforced polydimethylsiloxane (PDMS) nanocomposites were fabricated with weight percentages of 1%, 5%, and 10% magnetite nanoparticles. The nanocomposites were exposed to incident Near Infrared (NIR) laser beams with well-controlled powers. The laser-induced heating is explored in: (i) heating liquid media (deionized water and cell growth media [Leibovitz L15+]) to characterize the photothermal properties of the nanocomposites, (ii) in vitro experiments that explore the effects of localized heating on triple-negative breast cancer cells, and (iii) experiments in which the laser beams penetrate through chicken tissue to heat up nanocomposite samples embedded at different depths beneath the chicken skin. The resulting plasmonic laser-induced heating is explained using composite theories and heat transport models. The results show that the laser/nanocomposite interactions decrease the viability of triple-negative breast cancer cells (MDA-MB-231) at temperatures in the hyperthermia domain between 41 and 44°C. Laser irradiation did not cause any observed physical damage to the chicken tissue. The potential in vivo performance of the PDMS nanocomposites was also investigated using computational finite element models of the effects of laser/magnetite nanocomposite interactions on the temperatures and thermal doses experienced by tissues that surround the nanocomposite devices. The implications of the results are then discussed for the development of implantable nanocomposite devices for localized treatment of triple-negative breast cancer tissue via hyperthermia.

Thermotropic Optical Response of Silicone-Paraffin Flexible Blends

Organic phase change materials, e.g., paraffins, are attracting increasing attention in thermal energy storage (TES) and thermal management applications. However, they also manifest interesting optical properties such as thermotropism, as they can switch from optically opaque to transparent reversibly and promptly at the melting temperature. This work aims at exploiting this feature to produce flexible silicone-based blends with thermotropic properties for applications in glazed windows or thermal sensors. Blends are produced by adding paraffin (T_m = 44 °C, up to 10 phr) to a silicone bicomponent mixture, and, for the first time, cetyltrimethylammonium bromide (CTAB) is also added to promote paraffin dispersion and avoid its exudation. CTAB is proven effective in preventing paraffin exudation both in the solid and in the liquid state when added in a fraction above 3 phr with respect to paraffin. Rheological results show that paraffin decreases the complex viscosity, but neither paraffin nor CTAB modifies the curing behavior of silicone, which indicates uniform processability across the investigated compositions. On the other hand, paraffin causes a decrease in the stress and strain at break at 60 °C, and this effect is amplified by CTAB, which acts as a defect and stress concentrator. Conversely, at room temperature, solid paraffin only slightly impairs the mechanical properties, while CTAB increases both the elastic modulus and tensile strength, as also highlighted with ANOVA. Finally, optical transmittance results suggest that the maximum transmittance difference below and above the melting temperature (65-70 percentage points) is reached for paraffin amounts of 3 to 5 phr and a CTAB amount of max. 0.15 phr.

Low-Cost Multifunctional Vacuum Chamber for Manufacturing PDMS Based Composites

Polydimethylsiloxane (PDMS) is one of the best known elastomers and has been used in several areas of activity, due to its excellent characteristics and properties, such as biocompatibility, flexibility, optical transparency and chemical stability. Furthermore, PDMS modified with other materials promotes the desired changes to broaden its range of applications in various fields of science. However, the heating, mixing and degassing steps of the manufacturing process have not received much attention in recent years when it comes to blending with solid materials. For instance, PDMS has been extensively studied in combination with waxes, which are frequently in a solid state at room temperature and as a result the interaction and manufacturing process are extremely complex and can compromise the desired material. Thus, in this work it is proposed a multifunctional vacuum chamber (MVC) with the aim to improve and accelerate the manufacturing process of PDMS composites combined with additives, blends and different kinds of solid materials. The MVC developed in this work allows to control the mixing speed parameters, temperature control and internal pressure. In addition, it is a low cost equipment and can be used for other possible modifications with different materials and processes with the ability to control those parameters. As a result, samples fabricated by using the MVC can achieve a time improvement over 133% at the heating and mixing step and approximately 200% at the last degassing step. Regarding the complete manufacturing process, it is possible to achieve an improvement over 150%, when compared with the conventional manufacturing process. When compared to maximum tensile strength, specimens manufactured using the MVC have shown a 39% and 65% improvement in maximum strain. The samples have also shown a 9% improvement in transparency at room temperature and 12% at a temperature of about 75 °C. It should be noted that the proposed MVC can be used for other blends and manufacturing processes where it is desirable to control the temperature, agitation speed and pressure.

Properties and Applications of PDMS for Biomedical Engineering: A Review

Polydimethylsiloxane (PDMS) is an elastomer with excellent optical, electrical and mechanical properties, which makes it well-suited for several engineering applications. Due to its biocompatibility, PDMS is widely used for biomedical purposes. This widespread use has also led to the massification of the soft-lithography technique, introduced for facilitating the rapid prototyping of micro and nanostructures using elastomeric materials, most notably PDMS. This technique has allowed advances in microfluidic, electronic and biomedical fields. In this review, an overview of the properties of PDMS and some of its commonly used treatments, aiming at the suitability to those fields' needs, are presented. Applications such as microchips in the biomedical field, replication of cardiovascular flow and medical implants are also reviewed.

Polydimethylsiloxane Composites Characterization and Its Applications: A Review

Polydimethylsiloxane (PDMS) is one of the most promising elastomers due its remarkable proprieties such as good thermal stability, biocompatibility, corrosion resistance, flexibility, low cost, ease of use, chemically inertia, hyperplastic characteristics, and gas permeability. Thus, it can be used in areas such as microfluidic systems, biomedical devices, electronic components, membranes for filtering and pervaporation, sensors, and coatings. Although pure PDMS has low mechanical properties, such as low modulus of elasticity and strength, it can be improved by mixing the PDMS with other polymers and by adding particles or reinforcements. Fiber-reinforced PDMS has proved to be a good alternative to manufacturing flexible displays, batteries, wearable devices, tactile sensors, and energy harvesting systems. PDMS and particulates are often used in the separation of liquids from wastewater by means of porosity followed by hydrophobicity. Waxes such as beeswax and paraffin have proved to be materials capable of improving properties such as the hydrophobic, corrosion-resistant, thermal, and optical properties of PDMS. Finally, when blended with polymers such as poly (vinyl chloride-co-vinyl acetate), PDMS becomes a highly efficient alternative for membrane separation applications. However, to the best of our knowledge there are few works dedicated to the review and comparison of different PDMS composites. Hence, this review will be focused on PDMS composites, their respective applications, and properties. Generally, the combination of elastomer with fibers, particles, waxes, polymers, and others it will be discussed, with the aim of producing a review that demonstrates the wide applications of this material and how tailored characteristics can be reached for custom applications.