Deformation of poly(dimethylsiloxane) oligomers under uniaxial tension: Quantum chemical view

The response of a silicone polymer fragment to external stresses is considered in terms of a mechanochemical reaction. The quantum chemical realization of the approach is based on a coordinate-of-reaction concept for the purpose of introducing a mechanochemical internal coordinate (MIC) that specifies a deformational mode. The related force of response is calculated as the energy gradient along the MIC, while the atomic configuration is optimized over all of the other coordinates under the MIC constant-pitch elongation. The approach is applied to a set of linear silicone oligomers Sin with n = 4, 5, and 10 subjected to uniaxial tension, followed by the molecule breaking and a postfracture relaxation. Three stages of deformation, differing by structural transformation, have been detected. The observed peculiarities of the oligomer mechanical behavior are well attributed to the characteristic modes of vibrational spectra. The oligomer strength and the related Young's moduli are obtained. A cooperative radical-driven mechanism of silicone polymer fracture is suggested. © 1999 American Chemical Society.