High Molecular Weight Polydimethylsiloxane Research Articles

Condensation of α,ω-Oligodimethylsiloxanols in Ammonia: A New Method for the Synthesis of Polydimethylsiloxanes with the Low Content of Cyclosiloxanes

A new efficient approach to the production of high molecular weight polydimethylsiloxanes (PDMSs) is developed based on the condensation of α,ω-oligodimethylsiloxanols in ammonia. This method provides control over the molar mass of the target PDMSs in a wide range (Mw = 11.0–82.0 kg/mol). The use of cyclic silazanes as dehydrating agents allows for suppressing the side cyclization processes.

A super‐stretched self‐healing silicone elastomer based on high molecular weight polydimethylsiloxanes through intermolecular quadruple hydrogen bonding

AbstractSupramolecular silicone elastomer is a promising material for sustainable development. Here, quadruple hydrogen bonded ureidopyrimidinone (UPy) units were grafted onto high molecular weight polydimethylsiloxanes (PDMS), providing a novel supramolecular silicone elastomer. The high molecular weight PDMS precursor was prepared through a cyclic trimeric phosphazene base catalyzed ring‐opening polymerization of Octamethylcyclotetrasiloxane (D4) and tetravinyltetramethylcyclotetrasiloxane (V4) with a well‐controlled vinyl content. Its side chains were modified by hydroxyl groups via a thiol‐ene reaction. Then UPy units were further grafted on the side chain of high molecular weight PDMS by the isocyanate‐hydroxy reaction. The mechanical and self‐healing properties of the elastomer can be controlled by adjusting the molecular weight of PDMS and the amount of UPy units. The obtained PDMS elastomers displayed excellent stretchability (elongation at break of 1513%), good self‐healing efficiency (76% at 60°C for 2 h), and high resilience (elastic recovery of 93% under 300% strain cyclic stretching).

Lewis Pairs as Highly Tunable Dynamic Cross-Links in Transient Polymer Networks.

Classical Lewis pairs (LPs) between unhindered electron-poor Lewis acids (LAs) and electron-rich Lewis bases (LBs) present an overlooked motif with tremendous potential as dynamic cross-links in transient polymer networks (TPNs) for self-healing and stimuli-responsive applications. We demonstrate that simple and intuitive matching of weak/strong organoborane LA and amine LB pairs offers access to a large set of binding equilibrium constants, Keq, that span ∼6 orders and dissociation rate constants, kdiss, that span ∼7 orders of magnitude. The implementation in polystyrene (PS)/polydimethylsiloxane (PDMS) blends results in dynamically cross-linked networks with bulk thermomechanical properties that are directly correlated with the strength and kinetic parameters for the LP interactions. The LP dynamic cross-link design is highly versatile and broadly applicable to different polymer architectures as demonstrated in the formation of reprocessable elastomers from Lewis base-decorated high molecular weight PDMS in combination with Lewis acid-decorated PS when reinforced with fumed silica as a filler.

Ceramic nanotubes-based elastomer composites for applications in electromechanical transducers

Titanium dioxide-based nanoparticles with nanotube morphology were synthesised and used in low fractions (2 and 5wt%) as dielectric permittivity enhancers for high molecular weight polydimethylsiloxane matrix (Mn=350,000gmol−1). In order to ensure good dispersability in, and compatibility with the silicone matrix, the filler was treated with hexamethyldisilazane. The polymer composites were processed as films and stabilized by condensation of the chain ends with a trifunctional silane, resulting in low density cross-linking elastomers. The films were characterized for morphology, dielectric and mechanical behavior. The evaluated properties were compared with those measured in similar conditions for a pure cross-linked silicone matrix and a commercial dielectric elastomer taken as references. The results indicated an increase in dielectric permittivity (up to 33%), as a result of filler incorporation without significantly damaging dielectric strength (>60Vμm−1) and mechanical properties (elastic modulus as low as 0.4MPa and high strains at break up to 600%). Electromechanical measurements revealed the best performance in actuation for the composite containing 2wt% nanotubes (4.2% lateral strain at 40V·μm−1). Initial tests for energy harvesting performed with dielectric elastomer generators built with the same material sandwiched between two highly compliant electrodes yielded promising energy conversion efficiency (up to 8.84% at 2.5V·μm−1).

Synthesis of ultrahydrophobic and thermally stable inorganic–organic nanocomposites for self-cleaning foul release coatings

A conformal novel and low-cost series of elastomeric high-molecular weight polydimethylsiloxane (PDMS)/controlled SiO2-doped ZnO nanocomposites were accurately synthesized via hydrosilation curing. Different concentrations of doped nanospheres were inserted in the nanocomposite via in situ technique. The synergetic effect of micronanobinary scale roughness and controlled fouling on different kinds of substrates was determined. The hydrophobicity, roughness, and free-energy properties were investigated as self-cleaning and fouling release (FR) factors. The nanocomposites were also subjected to various tests on surface adhesion and mechanical properties, such as impact, T-bending, crosscut, and abrasion resistance. The anticorrosive features of nanocomposites were investigated through salt spray test. The mechanical tests and salt spray test exhibited the most profound effect by incorporation of 0.5% SiO2-doped ZnO nanospheres, indicating well distributed SiO2-doped ZnO nanofillers (0.5%). Results indicate that the nanocomposites retained the nanostructure characteristics under thermal and irradiation treatments. Furthermore, microfoulants of chosen bacterial progenies were applied on vinyl-ended PDMS/spherical SiO2-doped ZnO nanocomposites for about one month of laboratory assessments. These studies indicated the importance of good distribution of doped nanofillers on enhancing FR ability in the modeled nanocoatings. A particular increase in contact angle (CA, 167°±2) and the decrease in free energy of surface (9.24mN/m) and microroughness indicated the FR functionality of these nanocomposites. Our findings show evidence that the developed nanocomposites demonstrated inert and nonwettable properties, superior physical characteristics, surface innerness and lotus effect, long-term durability under UV radiation, and thermal stability and resistance against a wide range of pH solutions, making them promising as efficient environment-friendly self-cleaning for coating of ship hulls.

Assessment of solubility and viscosity of ultra-high molecular weight polymeric thickeners in ethane, propane and butane for miscible EOR

Natural gas liquid (NGL), a mixture consisting primarily of ethane, propane, and butane, is an excellent enhanced oil recovery (EOR) solvent. However, NGL is typically about ten times less viscous than the crude oil within the carbonate or sandstone porous media, which causes the NGL to finger through the rock toward production wells resulting in low volumetric sweep efficiency in five-spot patterns or during a linear drive displacement. The viscosity of candidate polymeric NGL thickeners is measured with a windowed, close-clearance falling ball viscometer, and an expression for the average shear rate associated with this type of viscometer is derived. High molecular weight polydimethyl siloxane (PDMS, Mw 9.8 105) can thicken ethane, propane and butane, but the viscosity enhancement is very modest (e.g. a doubling of butane viscosity with 2% PDMS at 7MPa and 25°C), making field application of PDMS unlikely. A dilute concentration of a drag-reducing agent (DRA) poly-α-olefin that has an average molecular weight greater than 2.0 107 is more promising as a potential thickener for liquid butane, liquid propane and liquid or supercritical ethane. The DRA polymer, which is introduced as an extremely viscous 1% or 2% solution in hexane, is soluble in butane and propane at 25–60°C and concentrations up to at least 0.5wt% at pressures slightly above the vapor pressure of butane or propane. The DRA polymer is much more difficult to dissolve in ethane, however, requiring pressures of more than 20MPa. The DRA polymer is especially effective for thickening butane (e.g. a 4.8-fold viscosity increase at 25°C, 55.16MPa and 0.2wt% DRA). The DRA is less effective for increasing propane viscosity (e.g. a 2.3-fold viscosity increase at the same conditions), and even less effective for thickening ethane. In general, viscosity enhancement increases with decreasing temperature, increasing pressure, and an increase in the carbon number of the light alkane, which are reflective of increased NGL solvent strength at low temperature and high pressure. Practical application of DRA during EOR may be hindered, however, by the relatively high concentration (~5000ppm) of DRA polymer required for order-of-magnitude viscosity increases, very high pressure requirements for DRA dissolution if the ethane content of the NGL is high, and the large amount of hexane that would have to be introduced if the DRA polymer if it is introduced as a solution in hexane.

Silicone dielectric elastomers based on radical crosslinked high molecular weight polydimethylsiloxane co-filled with silica and barium titanate

A strategy, consisting in the use in tandem of two active fillers in different ratios with complementary effects: silica mainly as a reinforcing agent and barium titanate as dielectric permittivity enhancer, was proposed to optimize the electromechanical properties of the silicone elastomers. A high molecular mass polydimethylsiloxane-α,ω-diol (M w = 642000 g mol−1) was used as a matrix to prepare silicone composites which further were processed as films and crosslinked at high temperature. The morphological, thermal, and moisture behaviors of the films were studied by adequate techniques. The mechanical properties were estimated on the basis of normal and cyclic stress–strain curves. Dielectric spectra were recorded in the frequency range of 1–106 Hz at normal temperature. The voltages generated by a mechanical impulses created by falling of a metal ball of 7.1 g from a height of one hundred millimeters on the surface of the films placed between two electrodes ranged between 28.4 and 157.3 V mm−1.

Development of emulsification resistant heavier-than-water tamponades using high molecular weight silicone oil polymers.

Developing new blends of heavier-than-water silicone oil tamponade agents containing high molecular weight polydimethylsiloxane polymer for use in vitreoretinal surgery. The viscoelastic properties of heavier-than-water silicone oil blends (30.5% F6H8 + 69.5% polydimethylsiloxane) containing high molecular weight polymer additive at increasing concentrations were measured using a controlled-stress rheometer (TA Instruments Rheolyst AR 1000 N). Emulsification of the blends was induced using a sonication device and a pluronic surfactant as a strong emulsifier. The percentage emulsion area was photographed and measured using ImageJ software. In a second in vitro emulsification assessment, silicone oil blends were dispersed using a high shear homogenizer and the oil-in-water droplets were counted using a coulter counter particle analyser. The addition of the high molecular weight polymer increased shear viscosity and viscoelasticity of the oil blends, which were measureable and to some extent predictable. The in vitro emulsification models produced contradictory results. This demonstrates the difficulty of designing and using in vitro models to evaluate the emulsification tendency of tamponade agents in vivo. Addition of a high molecular weight polymer to heavy silicone oil can increase the viscoelasticity. These findings might contribute to the development of emulsification resistant heavy silicone oils.

Effects of polydimethylsiloxane (PDMS) molecular weight on performance of PDMS/ceramic composite membranes

The objective of this work was to investigate the effect of polydimethylsiloxane (PDMS) molecular weights on the pervaporation performance and the confinement behavior of PDMS/ceramic composite membranes for the separation of ethanol from aqueous solutions by pervaporation. PDMS with three different molecular weights was used to prepare PDMS/ceramic composite membranes. Due to the fact that high molecular weight PDMS tends to suppress the formation of defects, and as a result improves the constrained degree of the membrane, the PDMS/ceramic composite membrane fabricated with the highest molecular weight PDMS exhibited a better performance than the other two counterparts prepared with lower molecular weight PDMS. The total flux and the separation factor of a PDMS/ceramic composite membrane with a thin defect-free PDMS layer of 5 μm were 1.6 kg/m 2 h and 8.9, respectively, at 40 °C when 5 wt.% of ethanol was used in the feed solution. Furthermore, the ceramic support showed a unique advantage in constraining the swelling of the separation layer, when compared with the conventional polymer support, which significantly improved the membrane stability, especially for PDMS with high molecular weights.

Electromechanical Responses of a Crosslinked Polydimethylsiloxane

AbstractA high molecular weight polydimethylsiloxane, PDMS, gel was prepared and investigated as an electroactive polymer actuator. Electromechanical properties of the PDMS gels were measured under an oscillatory shear mode at the temperature of 27 °C to determine the effects of crosslink ratio and electric field strength. The storage modulus, G′, of PDMS gel increases linearly with crosslink density but nonlinearly with electric field. The increase in the storage modulus with crosslink density is due to the increase in the number of junction points and strands. With increasing electric field strength, the storage modulus increases as the electric field induces dipole moments generating the electrostatic forces within the matrices. The gel with the crosslink ratio of 0.01 possesses the highest G′ sensitivity of 41% at 2 kV/mm. The temporal response of PDMS gels upon repeated applications of electric field strength of 2 kV/mm was investigated. For the crosslink PDMS (Nc/Nm = 0.01) system, at the electric field of 2 kV/mm, G′ immediately increases and rapidly reaches a steady‐state value. With electric field off, G′ decreases and nearly recovers its original value. The crosslinked PDMS (Nc/Nm = 0.01) is nearly a reversible system. Finally, we investigated the bending response of the PDMS films, suspended in silicone oil between copper electrodes. From the deformation data, we estimated the dielectrophoresis force, FD, to be a linear function of electric field strength.

Peeling of polydimethylsiloxane adhesives: The case of adhesive failure

AbstractThe adhesion properties of high molecular weight polydimethylsiloxane adhesives are measured using 90°‐peel adhesion tests, in the high velocity range. Such adhesives undergo mainly adhesive failure in this regime. The influence of viscosity (non‐Newtonian), adhesive thickness, peeling velocity, and backing properties are studied, and new unexpected behaviors are shown. The role of rheology and peeling velocity can be explained by an extension of a model already presented for cohesive failure, by using a power‐law fluid for the adhesive. On the other hand, the influence of the backing rigidity reveals to be coupled with the adhesive elastic properties, this effect being correlated to the introduction of a new parameter in the model, the Weissenberg number for viscoelasticity. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2113–2122, 2007

Adhesion study of silicone coatings: the interaction of thickness, modulus and shear rate on adhesion force

Interactions between coating thickness, modulus and shear rate on pseudobarnacle adhesion to a platinum-cured silicone coating were studied using a statistical experimental design. A combined design method was used for two mixture components and two process variables. The two mixture components, vinyl end-terminated polydimethylsiloxanes (V21: MW = 6 kg mole−1 and V35: MW = 4 9.5 kg mole−1, Gelest Inc.) were mixed at five different levels to vary the modulus. The dry coating thickness was varied from 160 – 740 μm and shear tests were performed at four different shear rates (2, 7, 12, and 22 μm s−1). The results of the statistical analysis showed that the mixture components were significant factors on shear stress, showing an interaction with the process variable. For the soft silicone coating based on the high molecular weight polydimethylsiloxane (E = 0.08 MPa), shear stress significantly increased as coating thickness decreased, while shear rate slightly impacted shear force especially at 160 μm coating thickness. As the modulus was increased (E = 1.3 MPa), more force was required to detach the pseudobarnacle from the coatings, but thickness and rate dependence on shear stress became less important.

Vinyl ester resin modified with silicone‐based additives: II. Flammability properties

AbstractSilicone‐based additives have been used as fire retardants for thermoplastics, presenting the advantages of improving processing and impact resistance of the polymers. In this work we used three different silicone‐based additives as modifiers of a thermoset based on a vinyl ester resin. The additives are fine powders made up of about 50 wt % ultra high molecular weight polydimethylsiloxane and 50 wt % silica. The differences between them are the functional groups inserted on the additives and the size and size distribution of the particles. The additives were dispersed in resin containing 35 wt % of styrene. For curing the mixture a conventional catalyst and initiator were used and the reaction was carried out in two ways, differing in the curing temperature, the post curing temperature, and the time, and in the addition of dimethylaniline (DMA) as a promoter of the polyaddition reaction. The samples were characterized by thermogravimetric analyses and swelling experiments. The fire retardances of the samples were evaluated by the determination of the flash‐ignition, self‐ignition, and pyrolysis temperatures (ASTM D1919–91a), and of the oxygen index (ASTM D‐2863–91). The results obtained showed that the silicone‐based additives and the methods used in the preparation of the modified resin influence the flash‐ignition, self‐ignition, and pyrolysis temperatures, but not the oxygen index. Samples cured by different methods present different network characteristics, which influence their thermal decomposition. The volatile species produced by thermal decomposition may be a combination of inert and active species. The network structure may influence only the inert fraction of the volatiles, not the combustibles. These volatile inert species (smoke‐black, water vapor, carbon dioxide, etc.) probably dilute the combustibles in the solid and in the gaseous phase, increasing the flash‐ignition temperature of the samples. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 644–649, 2006

Supercritical CO 2-assisted electrospinning

Polymer fibers of high molecular weight polydimethylsiloxane (PDMS) and poly ( d, l-lactic acid) (PLA) were produced using only electrostatic forces and without the use of a liquid solvent. The fibers were formed between two electrodes in a high-pressure view cell. A polymer sample was placed on a grounded electrode and a second, counter electrode was placed at high potential. The polymer was observed as the thermodynamic conditions (e.g. temperature, CO 2 pressure) and potential difference were varied. At a CO 2 temperature and pressure above the critical point, but well below the single-phase region, polymer fibers formed between the grounded electrode and the high voltage counter electrode. It is surmised that the supercritical CO 2 reduces the polymer viscosity sufficiently to allow fibers to be electrostatically pulled from an un-dissolved bulk polymer sample.

Adhesion and friction of PDMS networks: molecular weight effects

The objective of this work is to find relations between adherence and friction behaviors of elastomer networks. The chosen approach is based on the parallel study of the initial molecular weight (i.e., the degree of cross-linking) dependence of both adherence and friction. The polymers used are cross-linked polydimethylsiloxane (PDMS) and the substrate is a smooth glass plate. The experimental procedure uses both friction (pin on disk tribometer) and adhesion (tack test) measurements, associated with surface analysis and mechanical and rheological characterizations. Tack results show that high molecular weight PDMS exhibits the greater adherence energy. This can be explained by the role of both chain length and free and pendant chains: more numerous and longer free chains favor the substrate wetting (at a molecular scale) and increases the energy dissipation during separation (extraction and reptation mechanisms). However, friction results indicate a higher friction resistance for low molecular weight PDMS. This result could be quite surprising. An explanation based on interfacial sliding properties of free and pendant chains can be proposed. Elsewhere, for the lower molecular weight polymer, elastic contact present during friction is able to act as a forced wetting, constraining the network and consequently leading to a greater energy dissipation.

A Review of the Fate and Effects of Silicones in the Environment

Silicones are well-known useful materials varying in structure, reactivity, and chemical and physical properties, but they all contain a covalent bond between the silicon atom and an organic group. Most common of these polymers are those based on polydimethylsiloxane (PDMS) having a siloxane (Si–O–Si) repeat unit and two methyl groups on each silicon atom. All these polymers are manmade, and the organosilicon linkage is not found in nature. It was therefore erroneously assumed that these polymers do not degrade naturally in the environment. It is the purpose of this review to refute this myth and to describe the degradation processes of PDMS in the environment and any potential ecological impact on the terrestrial, aquatic, and atmospheric compartments. Although it was found that minor degradation takes place by hydrolysis of PDMS to dimethylsilandiol followed by oxidation of the methyl group to aldehyde and ultimately to CO2 by Arthobacter and Fusarium oxysporium schlechtendahl, the major degradation processes are abiotic. High molecular weight PDMS are initially depolymerized by soil hydrolysis of the siloxane bonds to yield organosilanol terminated oligomers. These organosilanols and low molecular weight linear PDMS and cyclics are evaporated into the atmosphere and are oxidized there by hydroxyl radicals to benign silica, water, and CO2.

The linear viscoelastic behavior of highly filled polydimethylsiloxane measured in shear and compression

The frequency and time dependent moduli of up to 60% glass bead filled polydimethyl siloxane (PDMS) were studied for four different molecular weight materials with a rotational, compressional, and lubricated squeezing flow rheometer. Measurements of the storage and loss modulus taken with a conventional rotational instrument show good qualitative and quantitative agreement with those taken with a newly developed oscillatory compressional rheometer for PDMS of the lowest molecular weight studied, and show trends similar to those reported previously for similar materials. The agreement between the two instruments is better for both unfilled and 35% filled material of a higher molecular weight. Good agreement was also observed between measurements of the relaxation modulus in lubricated squeezing flow and measurements of the storage modulus with the new compressional device for a high molecular weight PDMS at all filler levels. The modulus derived from oscillatory measurements showed good quantitative agreement with that measured in lubricated squeezing flow for identical materials. The effect of polymer molecular weight on the relative behavior of the loss and storage modulus with increasing filler content was also studied. It was shown that as the polymer molecular weight increases, the ratio of the loss to storage modulus becomes nearly independent of the filler volume fraction over the frequency range studied. The effect of an increase in filler amount on the elasticity of the material was shown to depend both on the polymer molecular weight and the frequency, and is explained in terms of the influence of the filler on the Deborah number of the system.

Less-flammable liquids used in transformers

Two main types of replacement liquid have been developed, high molecular-weight hydrocarbons and polydimethylsiloxane (silicone). These liquids are formulated to withstand fairly large amounts of electrical arcing, and are less flammable than mineral oils. These properties qualify the liquids for use in installations where less-flammable liquids are required and where the use restrictions associated with the products are followed. The requirements of the National Electrical Code are described and the advantages of using less-flammable liquids are also described. The fault mechanisms of transformers are discussed. The UL investigation of transformer liquids and the limitations of use are also discussed.

Ring-Opening Polymerization of Siloxanes Using Phosphazene Base Catalysts

Phosphazene bases such as {(NMe2)3P=N–)3P=NBut} have been reported in the literature to be strongly basic materials with basicities up to 1×1018 times stronger than that of diazabicycloundecene (DBU) a strong hindered amine base used in organic reactions. A study of these phosphazene bases as catalysts revealed that they can be activated by small amounts of water, which all silicone feed stocks contain, to form an active ionic base catalyst [(NMe2)3P=N–)3P–NHBut]+[OH]−. This paper discusses the use of these types of base catalysts, and their analogues, as ring-opening polymerization catalysts for cyclosiloxanes. Phosphazene base catalysts can be used at low concentrations to make high molecular weight polydimethylsiloxanes with short reaction times over a wide temperature range. Molecular weight can easily be controlled in the presence of suitably functionalized endblockers. Water and carbon dioxide have been shown to have a significant impact on the polymerization rates. Polymers prepared show excellent thermal stability by thermogravimetric analysis (TGA), following neutralization of the catalyst, with decomposition onset temperatures >500°C in some cases. As a result of the extremely low levels of catalyst used, the polymers often do not require filtration.

Wall slip of a polydimethylsiloxane extruded through a slit die with rough steel surfaces: Micrometric measurement at the wall with fluorescent-labeled chains

Upstream instability and wall slip of a high molecular weight polydimethylsiloxane (PDMS) have been studied in a rough stainless-steel slit die. The velocity field at a micrometric scale close to a wall is determined with a fluorescence technique. A film of a mixture of PDMS and small fluorescent-labeled chains (PDMS–NBD) is deposited on the steel surface prior to the high-pressure flow. During the flow, the fluorescence of a small area in the middle of the surface is excited with an argon laser line and measured with a phototube. The signal decays towards an asymptotic low value, due to stray light and thermal current. The slip velocity is inferred from a comparison between the experimental decrease and a theoretical model, which takes diffusion effects into account. It is shown that the upstream instability induces a transverse oscillating velocity in the channel. A precise determination of the residual fluorescence after the wall slip, with a known diffusion coefficient and surface roughness, shows that the wall slip occurs in a plane localized at less than 1 μm from the ridges of the surface roughness.