Short Polydimethylsiloxane Research Articles

Meltable copolymeric elastomers based on polydimethylsiloxane with multiplets of pendant liquid-crystalline groups as physical crosslinker: A self-healing structural material with a potential for smart applications

Elastomers with strong physical crosslinks were prepared, based on alternating polydimethylsiloxane (PDMS) spacer segments and pendant quartets of mesogenic building blocks (LC) of azobenzene type. They are structurally related to the well-studied polymers with pendant-chain LC units (light-sensitive actuators), but are generally highly different: The LC units make up only a small volume fraction in our materials and they do not generate elastic energy upon irradiation, but they act as physical crosslinkers with thermotropic properties. Our elastomers lack permanent chemical crosslinks – their structure is fully linear (with some dangling units). The aggregation of the relatively rare and spatially separated LC quartets (of small mesogen units) nevertheless proved to be an efficient crosslinking mechanism: The most attractive product displays a rubber plateau extending over 100 °C, melts near 70 °C and is soluble in organic solvents. The LC nano-aggregates were also found to be responsible for a continuous temperature region of phase transitions, e.g. two gel points observed by rheology. The physical crosslinks are reversibly disconnected by large mechanical strain at room temperature, but they undergo self-healing, also after sample disruption. The elastomers might be of interest for the development of passive smart materials (e.g. meltable rubbers for 3D-printing, or thermo-reversible visco-elastic mechanical coupling). Our study focuses on the comparison of physical properties and structure-property relationships in two systems, with long and with short PDMS spacer segments.

Highly micro- and mesoporous oxycarbide derived materials from HF etching of silicon oxycarbide materials

Abstract Highly micro-mesoporous silicon oxycarbide-derived carbon materials (SiOC-DC) with elevated specific surface area up to 895 m 2 g -1 and high pore volumes of mesopores (0.3 cm 3 g -1 ) and micropores (0.2 cm 3 g -1 ) have been obtained by direct etching of SiOC with HF. Initial organic-inorganic hybrids were synthesized by the sol-gel method employing triethoxysilane and polydimethylsiloxane (PDMS) of different molecular weights (550, 1750 and 4200 gmol -1 ). These hybrids were pyrolyzed from 1100 to 1400 °C. Short PDMS chains (550 gmol -1 ) within the hybrid structure rendered dense SiOC materials after pyrolysis, however long PDMS chains (1750 and 4200 gmol -1 ) produced highly micro-mesoporous SiOC materials up to 1300 °C. The etching of SiOC employing HF produced the selective removing of silica nano-domains and as a result highly micro-mesoporous SiOC-DC materials were created. In dense SiOC the HF attack is favored at the highest temperatures (1400 °C) when the SiOC is more phase separated. On the other hand, in porous SiOC there is an agreement between the pyrolysis temperature and the porosity and, therefore the SiOC materials are etched easier at 1200 °C. SiOC-DC materials are composed by a porous glassy network enriched with carbon (graphene-like carbon and turbostratic carbon) and SiC which present an incipient crystallization.

Effect of nano-silica filler on microstructure and mechanical properties of polydimethylsiloxane-based nanocomposites prepared by “inhibition-grafting” method

Silica/polydimethylsiloxane nanocomposites (denoted as SiO2/PDMS) prepared by physical mixing exhibit poor processing flexibility and strength associated with the high viscosity effect and low addition amount of silica during the fabrication of room-temperature vulcanized PDMS elastomer. Thus a facile and scalable one-step “inhibition-grafting” method was established to graft polydimethylsiloxane (PDMS) onto the surface of DNS-2 (a kind of dispersible nano-silica with network structure) to yield nano-SiO2/PDMS high-performance nanocomposites. Their microstructure and chemical structure were characterized by TEM, GPC, FTIR and TGA. The viscosity and rheological properties were evaluated, and their mechanical properties of the as-prepared nano-SiO2/PDMS elastomers were measured as well. Findings indicate that PDMS chains are grafted on the silica surface via covalent bonding and the chains either grafted on the silica or in free state interpenetrated silica network thereby forming a kind of interpenetrating network. This kind of interpenetrating network and short PDMS chains can provide more crosslinking sites, leading to low viscosity and high mechanical properties of SiO2/PDMS composites. Besides, the nano-SiO2/PDMS elastomers containing over 16 phr (phr: parts of silica per hundred parts of PDMS by weight) of nano-silica exhibit shear thinning behavior, which corresponds to the transformation from Newtonian fluids to non-Newtonian fluids associated with the formation of whole interpenetrating network between nano-silica and PDMS chains. In summary, the nano-SiO2/PDMS elastomers exhibit a low viscosity and good mechanical properties, which is favorable for promoting their applications in the industry of high performance silicone materials.

Sensing adsorption kinetics through slip velocity measurements of polymer melts.

The evolution over time of the nonlinear slip behavior of a polydimethylsiloxane (PDMS) polymer melt on a weakly adsorbing surface made of short non-entangled PDMS chains densely end-grafted to the surface of a fused silica prism has been measured. The critical shear rate at which the melt enters the nonlinear slip regime has been shown to increase with time. The adsorption kinetics of the melt on the same surface has been determined independently using ellipsometry. We show that the evolution of slip can be explained by the slow adsorption of melt chains using the Brochard-de Gennes's model.

Friction of Polymers: from PDMS Melts to PDMS Elastomers.

The slip behavior of polydimethylsiloxane (PDMS) polymer melts flowing on weakly adsorbing surfaces made of short nonentangled PDMS chains densely end-grafted to silica has been characterized. For high enough shear rates, slip lengths proportional to the bulk fluid viscosity have been observed, in agreement with Navier's interfacial equation and demonstrating that the interfacial Navier's friction coefficient is a local quantity, independent of the polymer molecular weight. Comparing the interfacial shear stresses deduced from these measured slip lengths to available friction stress measured for cross-linked PDMS elastomers, we further demonstrate the local character of the friction coefficient and compare its value to the monomer-monomer friction.

In-line sequential injection-based hollow-fiber sorptive microextraction as a front-end to gas chromatography–mass spectrometry: a novel fully automatic sample processing technique for residue analysis

A novel and affordable analytical setup is herein reported for automatic flow-through sorptive microextraction of organic contaminants, exploiting polydimethylsiloxane (PDMS) as a front-end to gas chromatography-ion trap-tandem mass spectrometry. The analytical procedure involves a short single-strand PDMS hollow fiber integrated in a sequential injection (SI) network for automatic fluidic handling by programmable flow. The target species are in-line extracted from 10 mL of sample containing 20% (v/v) methanol followed by elution with a metered volume of organic solvent, which is whereupon quantitatively transferred into the programmed temperature vaporization (PTV) injector of the GC. Diffusional resistance to mass transfer was overcome by effecting the overall concentration and stripping steps at a single PDMS tubing interface. The proof of concept of the novel hyphenated system was demonstrated for extraction and determination of organochlorine pesticides (OCPs), namely, heptachlor, dieldrin, endrin, endosulfan, p,p'-dichlorodiphenyldichloroethane, p,p'-dichlorodiphenyltrichloroethane, dichlorodiphenyldichloroethylene, and endrin ketone, taken as model analytes, in environmental and industrial waters. Four organic solvents with a broad spectrum of polarity were investigated as eluents in the SI-based assembly, namely, ethyl acetate, methyl tert-butyl ether, hexane, and chloroform. Chloroform was proven the most suitable solvent for expedient elution and fast evaporation in the PTV injector. Under the selected experimental variables, limits of detection (signal-to-noise ratio (S/N) = 3) within the range of 0.3-1.1 ng L(-1), limits of quantification (S/N = 10) of 1.0-3.6 ng L(-1), and method repeatabilities spanning from 1.7 to 4.7% were obtained for the suite of OCPs. The hyphenated flow analyzer was harnessed to the analysis of samples of varying matrix complexity with good relative recoveries (86-112%) in drinking water, surface water, and influent and effluent wastewaters, with quantification limits far below those endorsed by WHO and EU drinking water directives setting maximum allowed concentrations at ≤100 ng L(-1) OCPs.

Interfacial Friction and Adhesion of Cross-Linked Polymer Thin Films Swollen with Linear Chains

The preparation and interfacial properties of a new type of tethered, thin-film lubricant coating are presented. These coatings are composed of three components: a dense self-assembled monolayer (SAM) underlayer that presents reactive vinyl groups at its surface; a cross-linked polydimethylsiloxane (PDMS) overlayer that is covalently tethered to the SAM; and free, mobile linear PDMS chains dispersed in the network. We investigate the influence of the molecular weight (Ms) and concentration of the free PDMS chains on the structure and equilibrium swelling properties of the cross-linked films. Using a bead-probe lateral force microscopy measurement technique, we also quantify the interfacial friction and adhesion characteristics of surfaces functionalized with these coatings. We find that both the volume fraction and the molecular weight of free PDMS molecules in the coatings influence their interfacial friction and adhesion properties. For example, the addition of short PDMS chains in dry, cross-linked PDMS thin films yields tethered surface coatings with ultralow friction coefficients (mu = 5.2 x 10(-3)). An analysis based on classical lubrication theory suggests that the reduction in friction force produced by free polymer is a consequence of the gradual separation of asperities on opposing surfaces and the consequent substitution of solid-solid friction by viscous drag of the free polymer chains in the network.

Dual-phase twisters: A new approach to headspace sorptive extraction and stir bar sorptive extraction

The fields of applicability of headspace sorptive extraction (HSSE) and stir bar sorptive extraction (SBSE) using polydimethylsiloxane (PDMS) as sorbent have been intensively discussed and widely described. One of the limits of sorptive extraction is that PDMS (i.e. an apolar phase) is the only polymer currently in use making it difficult to recover polar analytes from complex or multi-ingredient matrices and those with very volatile components (C1–C4 analytes). Dual-phase twisters are here introduced as new tools for HSSE and SBSE to overcome the above limits. Dual-phase twisters combine the concentration capabilities of two or more sampling materials operating in different ways (in this case sorption and adsorption). The new twisters consist of a short PDMS tube the ends of which are closed with two magnetic stoppers, thus creating an inner cavity that can be packed with different types of adsorbents like activated carbons. The concentration capability of dual-phase twisters was evaluated by using them for the HSSE and SBSE sampling of a number of matrices in the vegetable, food and environmental fields. The contributions made by different carbons to recovery, repeatability and intermediate precision were also investigated.

Toughening of a polysilsesquioxane network by homogeneous incorporation of polydimethylsiloxane segments

Methods were developed to characterize the fracture behavior of a condensation cure polysilsesquioxane network, and to toughen the network with homogeneously incorporated polydimethylsiloxane (PDMS) segments. Hydroxyl terminated PDMS short chains were end-capped with tetraethoxy silane and the end-capped segments were coupled with the oligomeric silsesquioxane bearing silanol ends. The end-capping and the coupling reactions were investigated by FT-IR, 29Si and 1H NMR, and GPC. The complete end-capping of the PDMS chains was achieved with minimum self-condensation and cyclization, and a complete coupling of the functionalized PDMS with the oligomeric silsesquioxane was achieved with no self-condensation of the PDMS chains. To toughen the network such a coupling reaction was necessary, otherwise PDMS chains formed a separate phase which was ineffective. Short PDMS chains and silsesquioxane oligomers were incompatible and a ternary phase diagram with toluene as the third component was constructed to define a concentration window for the coupling reaction. When homogeneously reacted into the resin network, all the PDMS chains of degrees of polymerization (DP) between 8 and 55 increased the fracture toughness, and within this range the longer chains were more effective. Ten parts of PDMS of DP 55 increased the KIc from 0.253 to 0.456MPam1/2, and GIc from 34.1 to 151.11J/m2. TGA showed the thermal stability of the network was retained after PDMS toughening. Enhanced inelastic deformation was responsible for the increased fracture toughness. Upon re-initiation of a crack, the toughened network developed a plastic zone the size of which was consistent with the calculated zone from Irwin's model, while no evidence of such yielding was seen for the untoughened network.

Investigation of Polyimidesiloxanes for Use as Adhesives by Electron Spectroscopy for Chemical Analysis

Angle-dependent electron spectroscopy for chemical analysis (ESCA) was used to examine the air facing surface (20–100 Å thick) composition of polyimidesiloxanes with different processing variations, and of varying polydimethylsiloxane (PDMS) content and block length (number of PDMS repeat unit varies from 1 to 9). Polyimide was clearly detected (due to the nitrogen content) in the 20–100 Å surface regions. This shows that a small amount of PDMS and short PDMS segment lengths in polyimidesiloxanes give a surface region with both PDMS and polyimide present. The surface composition, particularly that in the ca. 100 Å region, was correlated to the peel strength of polyimidesiloxane melt pressed to a metal substrate. Our results suggest that both PDMS and polyimide are essential components to rendering a needed peel strength. PDMS, having a good diffusive ability, readily reaches the substrate upon being pressed, achieving intimate contact, while imide groups interact with the substrate, presumably through chemical bonding; these two factors act synergistically to result in a high peel strength. In addition, the interaction mechanism and the failure mechanism involved in bonding polyimidesiloxane and metal substrate were also elucidated based on the ESCA results.

Model networks of end‐linked polydimethylsiloxane chains. XI. Use of very short network chains to improve ultimate properties

AbstractElastomeric networks were prepared by tetrafunctionally end‐linking mixtures of various proportions of relatively long and very short polydimethylsiloxane (PDMS) chains. The former had a number‐average molecular weight of 18,500 and the latter either 660 or 220 g mole−1. The series of (unfilled) bimodal networks thus prepared were studied in elongation to the rupture point at 25°C, and in swelling equilibrium in benzene at room temperature. Elasticity constants characterizing the Gaussian regions of the stress–strain isotherms, and values of the degree of equilibrium swelling were used to evaluate the most recent molecular theories of rubberlike elasticity. The isotherms also gave values of the elongation at which the modulus begins to increase anomalously because of limited chain extensibility, and values of the elongation and nominal stress at the point of rupture. These results were interpreted in terms of the known configurational characteristics of the constituent PDMS chains. Values of the energy or work required for rupture were used as an overall measure of the “toughness” of the networks. The very short chains were found to give a marked increase in toughness, through an increase in ultimate strength without the usual corresponding decrease in maximum extensibility. A variety of additional experiments will be required in order to elucidate the molecular origins of this important effect.