https://hal.archives-ouvertes.fr/hal-00378323/document

Flame retardancy of silicone-based materials

To investigate the failure of the poly(dimethylsiloxane) polymer (PDMS) at high temperatures and pressures and in the presence of various additives, we have expanded the ReaxFF reactive force field to describe carbon−silicon systems. From molecular dynamics (MD) simulations using ReaxFF we find initial thermal decomposition products of PDMS to be CH_3 radical and the associated polymer radical, indicating that decomposition and subsequent cross-linking of the polymer is initiated by Si−C bond cleavage, in agreement with experimental observations. Secondary reactions involving these CH_3 radicals lead primarily to formation of methane. We studied temperature and pressure dependence of PDMS decomposition by following the rate of production of methane in the ReaxFF MD simulations. We tracked the temperature dependency of the methane production to extract Arrhenius parameters for the failure modes of PDMS. Furthermore, we found that at increased pressures the rate of PDMS decomposition drops considerably, leading to the formation of fewer CH_3 radicals and methane molecules. Finally, we studied the influence of various additives on PDMS stability. We found that the addition of water or a SiO_2 slab has no direct effect on the short-term stability of PDMS, but addition of reactive species such as ozone leads to significantly lower PDMS decomposition temperature. The addition of nitrogen monoxide does not significantly alter the degradation temperature but does retard the initial production of methane and C_2 hydrocarbons until the nitrogen monoxide is depleted. These results, and their good agreement with available experimental data, demonstrate that ReaxFF provides a useful computational tool for studying the chemical stability of polymers.

Simulations on the thermal decomposition of a poly(dimethylsiloxane) polymer using the ReaxFF reactive force field.

PDMS with designer functionalities—Properties, modifications strategies, and applications

Environmental chemistry of organosiloxanes.

A number of commercialized nanomaterials incorporate TiO2 nanoparticles. Studying their structural stability in media mimicking the environment or the conditions of use is crucial in understanding their potential eco-toxicological effects. We focused here on a hydrophobic TiO2 nanoparticle-based formulation used in cosmetics: T-Lite SF. It is composed of a TiO2 core, coated with two successive protective layers of Al(OH)3, and polydimethylsiloxane. Soon after contact with water (pH = 5, low ionic strength), the T-Lite SF becomes hydrophilic and form aggregates. During this aging, 90%wt of the total Si of the organic layer is desorbed, and the PDMS remaining at the surface is oxidized. The Al(OH)3 layer is also affected but remains sorbed at the surface. This remaining Al-based layer still protects from the production of superoxide ions from the photoactive/phototoxic TiO2 core in our experimental conditions.

S. M. Lomakin

Papers

Thermal polydimethylsiloxane degradation. Part 2. The degradation mechanisms