Increasing the mechanical stability of artificial polymer materials is an important task in materials science, and for this a profound knowledge of the critical mechanoelastic properties of its constituents is vital. Here, we use AFM-based single-molecule force spectroscopy measurements to characterize the rupture of a single silicon-oxygen bond in the backbone of polydimethylsiloxane as well as the force-extension behavior of this polymer. PDMS is not only a polymer used in a large variety of products but also an important model system for highly flexible polymers. In our experiments, we probe the entire relevant force range from low forces dominated by entropy up to the rupture of the covalent Si-O bonds in the polymer backbone at high forces. The resulting rupture-force histograms are investigated with microscopic models of bond rupture under load and are compared to density functional theory calculations to characterize the free-energy landscape of the Si-O bond in the polymer backbone.

We present first principles molecular dynamics simulations of stretched siloxane oligomers in an environment representative of that present in single molecule atomic force microscopy experiments. We determine that the solvent used (hexamethyldisiloxane) does not influence the stretching of the siloxane in the high force regime or the rupture process, but trace amounts of water can induce rupture before the maximum siloxane extension has been attained. This would result in a significantly lower rupture force. The simulations show that the rupture of a covalent bond through a reaction with a molecule from the environment, which would not normally occur between the species when the polymer is not stressed, is possible, opening a route to mechanically induced chemical reactions. The attack of the normally hydrophobic siloxane by water when it is stretched has wider implications for the material failure under tensile stress, where trace amounts of water could induce tearing of the material.

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Siloxanes are versatile elastomers with an exceptional chemical and physical stability that allows them to be used as adhesives, coatings, and sealants [1] in applications ranging from biomedical to aerospace. Although these materials are exceptionally strong, they are limited by the ease of propagation of cracks through the elastomer when subjected to tensile stress. The current method of improving the material strength is to add silica filler particles, [2,3] which hinder tearing in the bulk elastomer. However, the chemical mechanism that facilitates crack propagation and the way in which the filler particles hinder it have not been defined at the molecular level. Understanding these processes entails a full description of the electronic structure of a system during the process of bond rupture and the subsequent reactions between ruptured fragments to correctly determine the underlying chemistry.Information on the response of individual chemical bonds subjected to a tensile load is accessible via single-molecule atomic force microscopy (AFM) experiments, [4][5][6] which can be interpreted with theoretical studies. [7][8][9][10][11][12][13][14][15][16][17][18][19] In the experiments, a single polymer is stretched between a substrate and an AFM tip until one of the backbone bonds ruptures. Factors that can affect the magnitude of the measured rupture force, such as the polymer length and pulling velocity, [15,17] the solvent, [16,18] the presence of knots in the polymer chain, [11,12] and the pulling of a molecule from a substrate, [13,14,19] have been examined by using Car-Parrinello molecular dynamics (CPMD) simulations. These CPMD studies provide valuable insights into the characteristics of bond rupture within single molecules, because a full electronic structure calculation is performed on the fly for a molecular dynamics trajectory, which allows a complete description of the stretching of an oligomer in the highforce regime.Of interest in understanding stress-induced material failure in the bulk elastomer is what subsequently happens to the rupture products-in particular, whether chemical reactions between the rupture products lead to permanent weakening of the material or not. Building on the results of our previous studies of the rupture of isolated siloxane oligomers, here we investigate what happens on the molecular scale when neighboring oligomers are ruptured simultaneously.CPMD simulations have previously been used to examine how rupture occurs at a knot in polyethylene chains when stretched, and how the rupture products can further react with neighboring chains to cause a tear.[20] The CÀC bonds of the backbone ruptured via a radical mechanism and then reacted with neighboring alkane chains to induce further bond rupture. Also, a disproportionation involving hydrogen transfer was observed. In our previous CPMD study of the rupture of isolated polydimethylsiloxane (PDMS) oligomers, we determined that as the siloxanes are stretched, the backbone SiÀO bonds become increasingly polarized, until rupture ...

The photoelectron (PE) spectra of some ortho-disubstituted quinoxalines were measured and assigned mainly on the basis of HAM/3, INDO, and CNDO MO calculations. In the case of cNoro derivatives, the given assignment takes also into account the arguments based on a comparison of HeIUHeI intensities.

Zusammenfassung Silicone decken ebenso wie Kunststoffe einen breiten Anwendungsbereich ab. Der Beitrag stellt die Herstellung der Silicone ausgehend von Quarz vor und führt in die Chemie der Silicone ein. Anhand ausgewählter Anwendungsbeispiele werden Struktur und die daraus resultierenden Eigenschaften der Verbindungen erläutert. Im Chemieunterricht lassen sich ausgehend von Kenntnissen aus der anorganischen und organischen Chemie Eigenschaften ausgewählter Silicone aus deren chemischer Struktur ableiten. Im Experiment „Brennverhalten“ kann aus den Verbrennungsprodukten auf die Hauptbestandteile Silicium (Siliconprobe) bzw. Kohlenstoff (Kunststoffprobe) zurückgeschlossen werden. Im Unterricht kann deshalb das Struktur‐Eigenschafts‐Konzept auch am Beispiel der Silicone erarbeitet werden.

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