Single-Molecule Measurement of the Strength of a Siloxane Bond ² Peter Schwaderer, ‡ Enno Funk, § Frank Achenbach, § Johann Weis, | Christoph Bra ¨uchle, ‡ and Jens Michaelis* ,‡ Department of Chemistry and Biochemistry and Center for Nanoscience, Ludwig Maximilians UniVersita ¨t Mu ¨nchen, Butenandtstrasse 11, 81377 Mu ¨nchen, Germany, Wacker Chemie AG, 84489 Burghausen, Germany, and Wacker Chemie AG, Consortium Elektrochemische Industrie, 81379 Mu ¨nchen, Germany ReceiVed August 1, 2007. In Final Form: September 19, 2007 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. Introduction Silicone elastomers are high-performance materials used in a wide field of applications. 1,2 Among their remarkable qualities are low-temperature dependence of the mechanoelastic proper- ties, 3 high resistance against thermooxidation, 4 high flexibility at low temperatures, 4 adjustable release properties, 5 and physi- ological harmlessness. 6 However, silicone elastomers also have some disadvantages, such as relatively low ultimate mechanical strength, 7 vulnerability to hydrolytic reagents, 8 and swelling in nonpolar environment. 9 The mechanical strength of silicone elastomers is of utmost importance for many of its industrial applications, and numerous strategies exist that help to make materials capable of sustaining large deformations. 10 However, there is very little knowledge about how rupture occurs on the molecular level and the ultimate stability of the silicon-oxygen (Si-O) bond. Single-molecule force spectroscopy has been used to study the force-extension behavior of DNA molecules, 11 proteins, 12 and polysaccharides 13 as well as the interaction of polymers with surfaces 14 or among themselves 15 and has helped to elucidate a variety of mechanical properties of polymers. 16,17 In single- molecule experiments, the exerted force can be used to rupture bonds that are extremely stable under normal conditions, with examples include the unfolding of protein domains, 18 the overstretching of DNA, 19 and the breakage of receptor-ligand linkages. 20 The high forces that can be exerted by an AFM cantilever were even used to determine the strength of covalent bonds by covalently bridging a polymer molecule between the cantilever and the glass slide surface. The experiments determined the rupture force for a particular force-loading rate. 21 Moreover, the force-extension data of many polymers can be accounted for by simple statistical physics models, such as the freely jointed chain (FJC), 22 the wormlike chain (WLC), 23 and the freely rotating chain. 24 For higher forces, these models can be expanded by the use of an enthalpic term (such as a hookian spring), yielding the extendible freely jointed chain or wormlike chain model 25 or by means of ab initio molecular dynamics calculations that describe the high force behavior prior to bond rupture. 26,27 For a particular bond, the rupture force depends on the loading rate of the polymer. 28,29 The reason is that the dissociation rate is increased in the presence of force, an effect that can be easily understood in the Kramers theory of reaction kinetics. 30 However, when one varies the loading rate 31 or analyzes the probability ² Part of the Molecular and Surface Forces special issue. * Corresponding author. 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