Poly Dimethylsiloxane Networks Research Articles

Balancing Interfacial Toughness and Intrinsic Dissipation for High Adhesion and Thermal Conductivity of Polymer-Based Thermal Interface Materials.

In recent years, adhesive thermal interface materials have attracted much attention because of their reliable adhesion properties on most substrates, preventing moisture, vibration impact, or chemical corrosion damage to components and equipment, as well as solving the heat dissipation problem. However, thermal interface materials have a huge contradiction between strong adhesion and high thermal conductivity. Here, we report a polymer-based thermal interface material consisting of polydimethylsiloxane/spherical aluminum fillers, which possesses both adhesion properties (adhesion strength of 3.59 MPa and adhesion toughness of 1673 J m-2 and enhanced thermal conductivity of 3.90 W m-1 K-1). These excellent properties are attributed to the modified chain structure by introducing acrylate accelerators into the polydimethylsiloxane network, thereby striking a balance between interfacial toughness and intrinsic dissipation. The addition of thermally conductive aluminum fillers not only increases the thermal conductivity but also improves the bulk energy dissipation of the thermal interface material. This work provides a novel strategy for designing a novel thermal interface material, leading to new ideas in long-term applications in high-power electronics.

Thermal Interface Materials with High Adhesion and Enhanced Thermal Conductivity via Optimization of Polydimethylsiloxane Networks and Aluminum Fillers

Thermal Interface Materials with High Adhesion and Enhanced Thermal Conductivity via Optimization of Polydimethylsiloxane Networks and Aluminum Fillers

Mechanistically Different Mechanochromophores Enable Calibration and Validation of Molecular Forces in Glassy Polymers and Elastomeric Networks.

Sterically distorted donor-acceptor π-systems, termed DA springs, can be progressively planarized under mechanical load causing a bathochromic shift of the photoluminescence (PL) spectrum. By combining theory and experiment, we here use a simple linear force calibration for two different conformational mechanochromophores to determine molecular forces in polymers from the mechanochromic shift in PL wavelength during multiple uniaxial tensile tests. Two systems are used, i) a highly entangled linear glassy polyphenylene and ii) a covalent elastomeric polydimethylsiloxane network. The mean forces estimated by this method are validated using known threshold forces for the mechanochemical ring-opening reactions of two different spiropyran force probes. The agreement between both approaches underlines that these DA springs provide the unique opportunity for the online monitoring of local molecular forces present in diverse polymer matrices.

Spider silk-inspired supramolecular polydimethylsiloxane network with prominent mechanical robustness for bifunctional flexible electronics

Self-healing and recyclable polymer elastomers are being developed to provide new prospects in building sustainable societies. Nevertheless, the synthesis of such materials still poses a significant difficulty because of the conflicting requirements between the self-healing ability and mechanical robustness for the dynamicity of crosslinks. Herein, a self-healing and recyclable supramolecular polydimethylsiloxane with outstanding tensile stress and ultrahigh mechanical toughness is constructed by synergistically incorporating dynamic disulfide bonds and multiple hydrogen bonds into a siloxane chain. The adipic dihydrazide (AD) bearing two hydrazide groups is appropriate introduced to provide multiple hydrogen bonding sites between polymer chains and improve the mechanical robustness of the crosslinked structure. The aromatic disulfide bonds contribute to shorter healing time due to the elevated dynamicity of the crosslinked network. The resulting elastomers exhibit high tensile strength (6.44 ± 0.24 MPa), distinguished toughness (37.00 ± 0.85 MJ m−3), superior elastic restorability, outstanding self-healing capability (∼86 %) and multiple recyclability. Furthermore, a bifunctional sensor based on this elastomer is constructed to monitor human motions and pressure changes, demonstrating the potential application in flexible stretchable electronics.

Synthesis of PDMS Chain Structure with Introduced Dynamic Covalent Bonding for High‐Performance Rehealable Tactile Sensor Application

AbstractIn addressing the increasing demand for wearable sensing systems, the performance and lifespan of such devices must be improved by enhancing their sensitivity and healing capabilities. The present work introduces an innovative method for synthesizing a healable disulfide bond contained in a polydimethylsiloxane network (PDMS−SS) that incorporates ionic salts, which is designed to serve as a highly effective dielectric layer for capacitive tactile sensors. Within the polymer network structure, the cross‐linking agent pentaerythritol tetrakis 3‐mercaptopropionate (PTKPM) forms reversible disulfide bonds while simultaneously increasing polymer softness and the dielectric constant. The incorporation of dioctyl sulfosuccinate sodium salt (DOSS) significantly improves the capacitance and sensing properties by forming an electrical double‐layer through interactions between the electrode charge and salt ions at the contact interface. The developed polymer material‐based tactile sensor shows a strong response signal at low pressure (0.1 kPa) and maintains high sensitivity (0.175 kPa−1) over a wide pressure range (0.1–10 kPa). It also maintains the same sensitivity over 10 000 repeated applications of external pressure and is easily self‐healed against mechanical deformation due to the dynamic disulfide covalent bonding, restoring ≈95% of its detection capacity.

Facile Preparation of a Transparent, Self-Healing, and Recyclable Polysiloxane Elastomer Based on a Dynamic Imine and Boroxine Bond.

Transparent polysiloxane elastomers with good self-healing and reprocessing abilities have attracted significant attention in the field of artificial skin and flexible displays. Herein, we propose a simple one-pot method to fabricate a room temperature self-healable polysiloxane elastomer (HPDMS) by introducing dynamic and reversible imine bonds and boroxine into polydimethylsiloxane (PDMS) networks. The presence of imine bonds and boroxine is proved by FT-IR and NMR spectra. The obtained HPDMS elastomer is highly transparent with a transmittance of up to 80%. The TGA results demonstrated that the HPDMS elastomer has good heat resistance and can be used in a wide temperature range. A lower glass transition temperature (Tg, -127.4 °C) was obtained and revealed that the elastomer is highly flexible at room temperature. Because of the reformation of dynamic reversible imine bonds and boroxine, the HPDMS elastomers exhibited excellent autonomous self-healing properties. After healing for 3 h, the self-healing efficiency of HPDMS reached 96.3% at room temperature. Moreover, the elastomers can be repeatedly reprocessed multiple times under milder conditions. This work provides a simple but effective method to prepare transparent self-healable and reprocessable polysiloxane elastomers.

Highly Recyclable and Tough Elastic Vitrimers from a Defined Polydimethylsiloxane Network

AbstractDespite intensive research on sustainable elastomers, achieving elastic vitrimers with significantly improved mechanical properties and recyclability remains a scientific challenge. Herein, inspired by the classical elasticity theory, we present a design principle for ultra‐tough and highly recyclable elastic vitrimers with a defined network constructed by chemically crosslinking the pre‐synthesized disulfide‐containing polydimethylsiloxane (PDMS) chains with tetra‐arm polyethylene glycol (PEG). The defined network is achieved by the reduced dangling short chains and the relatively uniform molecular weight of network strands. Such elastic vitrimers with the defined network, i.e., PDMS‐disulfide‐D, exhibit significantly improved mechanical performance than random analogous, previously reported PDMS vitrimers, and even commercial silicone‐based thermosets. Moreover, unlike the vitrimers with random network that show obvious loss in mechanical properties after recycling, those with the defined network enable excellent thermal recyclability. The PDMS‐disulfide‐D also deliver comparable electrochemical signals if utilized as substrates for electromyography sensors after the recycling. The multiple relaxation processes are revealed via a unique physical approach. Multiple techniques are also applied to unravel the microscopic mechanism of the excellent mechanical performance and recyclability of such defined network.

Highly Recyclable and Tough Elastic Vitrimers from a Defined Polydimethylsiloxane Network.

Despite intensive research on sustainable elastomers, achieving elastic vitrimers with significantly improved mechanical properties and recyclability remains a scientific challenge. Herein, inspired by the classical elasticity theory, we present a design principle for ultra-tough and highly recyclable elastic vitrimers with a defined network constructed by chemically crosslinking the pre-synthesized disulfide-containing polydimethylsiloxane (PDMS) chains with tetra-arm polyethylene glycol (PEG). The defined network is achieved by the reduced dangling short chains and the relatively uniform molecular weight of network strands. Such elastic vitrimers with the defined network, i.e., PDMS-disulfide-D, exhibit significantly improved mechanical performance than random analogous, previously reported PDMS vitrimers, and even commercial silicone-based thermosets. Moreover, unlike the vitrimers with random network that show obvious loss in mechanical properties after recycling, those with the defined network enable excellent thermal recyclability. The PDMS-disulfide-D also deliver comparable electrochemical signals if utilized as substrates for electromyography sensors after the recycling. The multiple relaxation processes are revealed via a unique physical approach. Multiple techniques are also applied to unravel the microscopic mechanism of the excellent mechanical performance and recyclability of such defined network.

Tuning moduli of hybrid bottlebrush elastomers by molecular architecture

Polymeric elastomers made of bottlebrush chains are soft, solvent-free materials with precisely controllable moduli by network topology. However, fabrication methods of bottlebrush networks with precisely programmable topology are lacking. The aim of this study is to achieve precise control over the modulus of hybrid elastomers containing bottlebrush and linear strands by integrating a versatile experimental synthesis approach with Molecular Dynamics (MD) simulations. In this study, polydimethylsiloxane (PDMS) networks (G′=10−328 kPa) are made by crosslinking prepared and purified bottlebrush molecules where the precursors, side chains and backbone, are synthesized using well-controlled living anionic polymerization. This approach allows independent programming of the crucial topological parameters of bottlebrushes, such as length of both sides chain and backbone and grafting density. In addition to this versatile synthesis, MD simulations reveal the influence of network defects inevitably present in all polymer networks. This novel combining of fabrication method and simulation yields experimentally-measured structural shear and Young's moduli which are in better agreement with theoretical predictions. Importantly, these bottlebrush-containing networks have no solvent or additives and notably, by removing ungrafted side chains impurities before network formation, results in soft, solvent-free elastomers with Young's moduli as low as E∼30 kPa with adhesiveness far lower than commercial silicone products of similar stiffness.

Self-Healable and Reprocessable Silicon Elastomers Based on Imine-Boroxine Bonds for Flexible Strain Sensor.

Silicon elastomers with excellent self-healing and reprocessing abilities are highly desirable for the advancement of next-generation energy, electronic, and robotic applications. In this study, a dual cross-linked self-healing polysiloxane elastomer was facilely fabricated by introducing an exchangeable imine bond and boroxine into polydimethylsiloxane (PDMS) networks. The PDMS elastomers exhibited excellent self-healing properties due to the synergistic effect of dynamic reversible imine bonds and boroxine. After healing for 2 h, the mechanical strength of the damaged elastomers completely and rapidly recovered at room temperature. Furthermore, the prepared PDMS elastomers could be repeatedly reprocessed multiple times under milder conditions without significant degradation in mechanical performance. In addition, a stretchable and self-healable electrical sensor was developed by integrating carbon nanotubes (CNTs) with the PDMS elastomer, which can be employed to monitor multifarious human motions in real time. Therefore, this work provides a new inspiration for preparing self-healable and reprocessable silicone elastomers for future flexible electronics.

Novel template digestive fabrication of multi-helical flow reactors

Embedded template-assisted fabrication by template removal technique is a simple and cost-effective method for preparation of complex 2D and 3D microchannels in soft polydimethylsiloxane (PDMS) platforms. However, employing this technique for fabrication of macroscale flow channels is impractical due to the requirement of a high swelling ratio of the PDMS network and other difficulties arising with the usage of millimeter-sized monofilament threads. The current work introduces a novel template digestion technique for fabricating topologically complex and transparent flow channels. The technique involves preparation of templates using flexible polyurethane (PU) tubes and their subsequent digestion by hot dimethyl sulfoxide (DMSO) within the embedding PDMS network. It is demonstrated for fabrication of various multi-helical channels (MHCs) with different geometrical features and compact modules of multiple MHCs which cater to process integration and intensification operations. Finally, an integrated operation of liquid-liquid extraction with subsequent phase separation is also demonstrated through typical parallel flow observed in MHCs.

Exceptionally recyclable, extremely tough, vitrimer-like polydimethylsiloxane elastomers via rational network design

•The elastic networks achieve an unprecedented recyclability of 70 cycles •The PDMS network possesses superior mechanical performance with toughness of 66 MJ/m3 •It provides fresh insight into the structure-property relationship Recently developed elastomers with an adaptable covalent network, e.g., elastic vitrimers, have great potential as sustainable materials, although most of them suffer from limited recyclability and unsatisfied mechanical properties. Here, we report a polydimethylsiloxane (PDMS)-based recyclable elastic network, combining permanent chemical and high-density dynamic physical crosslinks utilizing side-chain reactions, which provides energy dissipation for superb toughness and enables efficient chain rearrangement for excellent recyclability. The optimal elastic network achieves an unprecedented recyclability of 70 cycles, one order of magnitude higher than existing elastic vitrimers, extraordinary toughness of 66 MJ/m3, significantly higher than previously reported PDMS elastomers, excellent chemical resistance, and vitrimer-like rheological behavior. Physical analysis is also performed to provide the accurate estimation of bond-exchange dynamics excluding the often-neglected chain dynamics and to offer insight into the superb performance. We anticipate that such a design principle enables an alternative approach for developing superior recyclable polymer networks. Recently developed elastomers with an adaptable covalent network, e.g., elastic vitrimers, have great potential as sustainable materials, although most of them suffer from limited recyclability and unsatisfied mechanical properties. Here, we report a polydimethylsiloxane (PDMS)-based recyclable elastic network, combining permanent chemical and high-density dynamic physical crosslinks utilizing side-chain reactions, which provides energy dissipation for superb toughness and enables efficient chain rearrangement for excellent recyclability. The optimal elastic network achieves an unprecedented recyclability of 70 cycles, one order of magnitude higher than existing elastic vitrimers, extraordinary toughness of 66 MJ/m3, significantly higher than previously reported PDMS elastomers, excellent chemical resistance, and vitrimer-like rheological behavior. Physical analysis is also performed to provide the accurate estimation of bond-exchange dynamics excluding the often-neglected chain dynamics and to offer insight into the superb performance. We anticipate that such a design principle enables an alternative approach for developing superior recyclable polymer networks.

Flexible Multimodal Sensors Based on Fibrous Porous Networks of Multiwalled Carbon Nanotubes and Polydimethylsiloxane for Sensing and Distinguishing Vertical and Shear Force

Wearable sensors are one of the key components in applications such as motion monitoring, smart medical care, and human-computer interaction systems. In the present work, we focus on the simultaneous improvements of flexible sensors in both performance and functionality. Here, with a porous fiber network of multi-walled carbon nanotubes (MWCNTs)–polydimethylsiloxane (PDMS) as an active layer, a flexible pressure sensor with ultra-high sensitivity and superior capability of identifying transverse shear force and temperature signals is designed and constructed. The fibral MWCNTs–PDMS piezoresistive layer was formed with a scouring pad (SP) fiber network as skeleton, and the PDMS thin layer formed by self-assembly can effectively increase the initial resistance. Attributing to the unique compressibility of the fibral porous network and the adjustable nanoscale contacts, the MWCNT-PDMS/SP sensor exhibits a wide detection range (0–360 kPa), and in particular, an ultra-high sensitivity (84,818.2 kPa–1 when <100 pa). Beyond the highly sensitive characteristics, the sensor also has a high shear-force sensitivity (GF = 6.15 under 0.05 N). The sensor can still maintain a relatively stable performance even after operating for more than 10,000 pressure cycles, showing as a potential candidate for the applications of electronic skin, intelligent robots, etc.

Siloxane Molecules: Nonlinear Elastic Behavior and Fracture Characteristics.

Fracture phenomena in soft materials span multiple length and time scales. This poses a major challenge in computational modeling and predictive materials design. To pass quantitatively from molecular to continuum scales, a precise representation of the material response at the molecular level is vital. Here, we derive the nonlinear elastic response and fracture characteristics of individual siloxane molecules using molecular dynamics (MD) studies. For short chains, we find deviations from classical scalings for both the effective stiffness and mean chain rupture times. A simple model of a nonuniform chain of Kuhn segments captures the observed effect and agrees well with MD data. We find that the dominating fracture mechanism depends on the applied force scale in a nonmonotonic fashion. This analysis suggests that common polydimethylsiloxane (PDMS) networks fail at cross-linking points. Our results can be readily lumped into coarse-grained models. Although focusing on PDMS as a model system, our study presents a general procedure to pass beyond the window of accessible rupture times in MD studies employing mean first passage time theory, which can be exploited for arbitrary molecular systems.

PDMS networks meet barnacles: a complex and often toxic relationship

The biological impact of chemical formulations used in various coating applications is essential in guiding the development of new materials that directly contact living organisms. To illustrate this point, an investigation addressing the impact of chemical compositions of polydimethylsiloxane networks on a common platform for foul-release biofouling management coatings was conducted. The acute toxicity of network components to barnacle larvae, the impacts of aqueous extracts of crosslinker, silicones and organometallic catalyst on trypsin enzymatic activity, and the impact of assembled networks on barnacle adhesion was evaluated. The outcomes of the study indicate that all components used in the formulation of the silicone network alter trypsin enzymatic activity and have a range of acute toxicity to barnacle larvae. Also, the adhesion strength of barnacles attached to PDMS networks correlates to the network formulation protocol. This information can be used to assess action mechanisms and risk–benefit analysis of PDMS networks.

Development of an Innovative Soft Piezoresistive Biomaterial Based on the Interconnection of Elastomeric PDMS Networks and Electrically-Conductive PEDOT:PSS Sponges.

A deeply interconnected flexible transducer of polydimethylsiloxane (PDMS) and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) was obtained as a material for the application of soft robotics. Firstly, transducers were developed by crosslinking PEDOT:PSS with 3-glycidyloxypropryl-trimethoxysilane (GPTMS) (1, 2 and 3% v/v) and using freeze-drying to obtain porous sponges. The PEDOT:PSS sponges were morphologically characterized, showing porosities mainly between 200 and 600 µm2; such surface area dimensions tend to decrease with increasing degrees of crosslinking. A stability test confirmed a good endurance for up to 28 days for the higher concentrations of the crosslinker tested. Consecutively, the sponges were electromechanically characterized, showing a repeatable and linear resistance variation by the pressure triggers within the limits of their working range ( max = 80% for 1–2% v/v of GPTMS). The sponges containing 1% v/v of GPTMS were intertwined with a silicon elastomer to increase their elasticity and water stability. The flexible transducer obtained with this method exhibited moderately lower sensibility and repeatability than the PEDOT:PSS sponges, but the piezoresistive response remained stable under mechanical compression. Furthermore, the transducer displayed a linear behavior when stressed within the limits of its working range. Therefore, it is still valid for pressure sensing and contact detection applications. Lastly, the flexible transducer was submitted to preliminary biological tests that indicate a potential for safe, in vivo sensing applications.

Superhydrophobic and flame retardant polydimethylsiloxane coatings with layered double hydroxide and ammonium polyphosphate

The development of coatings with flame retardancy and superhydrophobicity is of great importance in practical application. Herein, a fluorine-free superhydrophobic and halogen-free flame retardant coating was constructed via simple dip-coating and efficient ultraviolet (UV) curing methods, using heavy cross-linking polydimethylsiloxane networks (HCPDMS) as low-surface-energy materials and hierarchical-structured layered double hydroxide-ammonium polyphosphate (LDH-APP) as organic-inorganic composite flame retardants. Owing to the such combination of hierarchical structure and low surface energy, these prepared coatings show striking repellency to various liquids, great self-cleaning and antifouling ability. Besides, they can resist the abrasion test by sandpaper for 30 cycles, the tape-peeling test for 200 cycles, and repeatedly kneading test. Meanwhile, these coatings offer good thermal stability and flame retardant, exhibiting an enhanced limiting oxygen index (LOI) from 18.0 % to 29.0 %. Moreover, a high oil/water separation efficiency of these coatings is over 95 % even after 16 separation cycles. Equipped with harmless to environment, facile preparation process, and excellent comprehensive performances, these coatings will have the potential for widespread application.

Hybrid coating of polydimethylsiloxane with nano-ZrO2 on magnesium alloy for superior corrosion resistance

To enhance the corrosion resistance of magnesium alloys, an organic-inorganic coating of polydimethylsiloxane (PDMS) hybridized with ZrO2 nanoparticles is developed, where the PDMS is selected as the polymeric matrix for superhydrophobic surface, and nano-ZrO2 particles as a physical barrier to strengthen and densify the polymeric matrix. The nanocomposite materials were characterized by field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectrophotometry (FT-IR), and X-ray diffraction (XRD). Electrochemical, dropping corrosion, and salt spray tests were carried out to evaluate the effect of the amount of ZrO2 on the corrosion property of the PDMS-ZrO2 coatings. Thermal stability, hydrophobicity, and corrosion resistance of the composite coatings are substantially improved by the introducing ZrO2 modifier into PDMS matrix. Depending on the ZrO2 content, the PDMS-ZrO2 hybrids exhibit superior corrosion resistance, where the corrosion current density of PDMS-ZrO2 coatings containing 10% (wt.%) ZrO2 decreases about five orders of magnitude compared with Mg alloy substrate. Furthermore, it is noteworthy that ZrO2 nanoparticles are filled in the PDMS matrix to provide a long-term protection in the salt spray test up to 210 h. The PDMS-ZrO2 coatings demonstrate an excellent shielding ability for the Mg alloy, which is attributed to the strong hydrophobic surface of PDMS and compact network structure separated by the physical barrier of ZrO2 nanoparticles in the PDMS chains. The proposed mixed matrix membrane guides a design direction for highly water-repellent coating with a long-term corrosion protection performance.

Donor–Acceptor Stenhouse Adduct‐Polydimethylsiloxane‐Conjugates for Enhanced Photoswitching in Bulk Polymers

Donor-acceptor Stenhouse adducts (DASAs) are a rapidly emerging class of visible light-activated photochromes and DASA-functionalized polymers hold great promise as biocompatible photoresponsive materials. However, the photoswitching performance of DASAs in solid polymer matrices is often low, particularly in materials below their glass transition temperature. To overcome this limitation, DASAs are conjugated to polydimethylsiloxanes which have a glass transition temperature far below room temperature and which can create a mobile molecular environment around the DASAs for achieving more solution-like photoswitching kinetics in bulk polymers. The dispersion of DASAs conjugated to such flexible oligomers into solid polymer matrices allows for more effective and tunable DASA photoswitching in stiff polymers, such as poly(methyl methacrylate), without requiring modifications of the matrix. The photoswitching of conjugates with varying polymer molecular weight, linker type, and architecture is characterized via time-dependent UV-vis spectroscopy in organic solvents and blended into polymethacrylate films. In addition, DASA-functionalized polydimethylsiloxane networks, accessible via the same synthetic route, provide an alternative solution for achieving fast and efficient DASA photoswitching in the bulk owing to their intrinsic softness and flexibility. These findings may contribute to the development of DASA-functionalized materials with better tunable, more effective, and more reversible modulation of their optical properties.

Dynamic Covalent Bond Cross-Linked Luminescent Silicone Elastomer with Self-Healing and Recyclable Properties.

Two aldehyde-modified tetraphenylene derivatives with different functionality are synthesized and exhibit different fluorescence properties. By incorporating tetraphenylene derivatives into polydimethylsiloxane (PDMS) networks, two elastomers are prepared through dynamic covalent cross-linking. The elastomers show excellent fluorescence properties, mechanical properties, thermal stability as well as self-healing and recycle properties. At the same time, the mechanical properties of the elastomers are influenced by the functionality of the tetraphenylene derivatives and the molecular weight of the PDMS. The self-healing process takes place quickly and the recycling process can be carried out by solution processing and hot pressing. It shows the similar tensile properties between the pristine and healed samples.