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

The response of a silicone polymer fragment to external stresses is conside red in terms of a mechanochemical reaction. The quantum chemical realizatio n of the approach is based on a coordinate-of-reaction concept for the purp ose of introducing a mechanochemical internal coordinate (MIC) that specifi es a deformational mode. The related force of response is calculated as the energy gradient along the MIG, 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 Si-n with n = 4, 5, and 10 subjected to uniaxial tension, followed by the molecule brea king and a postfracture relaxation. Three stages of deformation, differing by structural transformation, have been detected. The observed peculiaritie s of the oligomer mechanical behavior are well attributed to the characteri stic modes of vibrational spectra, The oligomer strength and the related Yo ung's moduli are obtained. A cooperative radical-driven mechanism of silico ne polymer fracture is suggested.