Origins of Material Failure in Siloxane Elastomers from First Principles

Abstract

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 ...

Disulfide Bond Cleavage: A Redox Reaction Without Electron Transfer

“…In the past we used the CPMD approach to simulate the mechanically induced cleavage of polymer chains 11. 12, 13, 14 The observed reactions could be classified as homolytic or heterolytic reactions, that is, either two unpaired electrons were formed upon bond rupture or all electrons remained paired. The homolytic reactions simply lead to radical fragments.…”

Conservation of Orbital Symmetry can be Circumvented in Mechanically Induced Reactions

“…This approach allows an analysis of the reaction mechanism without further assumptions concerning the course of the reaction and was used in many previous studies for investigating mechanically induced processes. [23][24][25][26][27] …”

Anisotropic Hydrolysis Susceptibility in Deformed Polydimethylsiloxanes

“…20,26−32 The chemistry of polysiloxanes under tension has received some attention at various levels of quantum chemical theory. 20,26,28,29,31 Most of these studies have focused on understanding the response of single or multiple PDMS chains in the absence of environmental gases. A consistent finding is that dry chains broken through purely mechanical means typically have highly reactive termini at the cleavage site, which can go on to induce further degradation.…”

“…Detailed reaction mechanisms and energetics for chemical degradation processes can be difficult to elucidate from experiments alone. Atomistic modeling based on electronic structure theory can predict these properties and provides a robust and accurate means to probe silicon chemistry. ,− The chemistry of polysiloxanes under tension has received some attention at various levels of quantum chemical theory. ,,,, Most of these studies have focused on understanding the response of single or multiple PDMS chains in the absence of environmental gases. A consistent finding is that dry chains broken through purely mechanical means typically have highly reactive termini at the cleavage site, which can go on to induce further degradation.…”