The structure of a poly(dimethylsiloxane) (PDMS) layer at the surface of hydrophilic silica has been studied by means of DSC, proton NMR T2 relaxation experiments, and 1 H and 29 Si NMR spectroscopy. The samples were prepared using (1) the adsorption of PDMS from a PDMS solution onto the silica surface followed by a thermal treatment and (2) mechanical mixing of PDMS with the silica followed by the separation of bound rubber. It was shown that these procedures caused the formation of chain loops chemically attached to the silica surface at both chain ends. The average length of the grafted chains varied from about four to eight Si-O bonds. The experiments provided information on the mobility of the grafted chains, which is related to the structure of the grafted layer. The grafted PDMS layer was found to consist of immobilized chain segments at the PDMS-silica interface and mobile chain portions outside the interface. The chain immobilization at the interface caused a substantial decrease in the heat capacity at Tg and suppressed crystallinity of the grafted PDMS. The interface fraction increased proportionally with a decreasing chain length. About four dimethylsiloxane pendant chain units next to the grafting site were immobilized due to chain anchoring to the silica surface. A small fraction of -SiO(CH3)2-chain units was immobilized as a result of physical adsorption at the silica surface. The fraction of physically adsorbed chain units appears to be proportional to the number of residual silanol groups at the silica surface. The mobility of the chain portions outside the interface was found to differ significantly in the various samples studied and to increase with an increasing average length of the grafted chains. The NMR method allowed us to make a distinction between a dense "brushlike" structure of the grafted layer containing grafted chains of a fairly uniform length and a layer containing a significant fraction of long chain loops outside the densely grafted layer. It was found that the structure of the grafted layer is to a great extent dependent on the grafting procedure employed.
The heterogeneity in segmental chain order of grafted polymers was investigated by 1 H multiple-quantum NMR. The measurements were performed on a series of samples of poly(dimethylsiloxane) (PDMS) layers chemically attached to the surface of hydrophilic silica. These grafted PDMS layers consist of partially immobilized chain segments at the PDMS-silica interface and mobile chain portions outside the interface which is supported by measurements of homonuclear double-and triple-quantum buildup curves. These curves reveal a bimodal distribution of the residual dipolar couplings along the PDMS chain. The segmental orientations detected in the interface and mobile regions are related to the average chain length and reflect the competing effects of the surface-induced orientation and chain conformations. The different behavior of the two chain fragments is evident from temperature-dependent measurements.
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 Si
n
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.
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