Siloxane Backbone

The prime role of the siloxane backbone is to present the available methyl groups to their best advantage and it does this by virtue of its unique flexibility. In most hydrocarbons, the bond angles are very fixed and steric packing considerations often prevent the available methyls from adopting lowest surface energy orientations. In silicones, the Si—O bond length is significantly longer and the Si—O—Si bond angle flatter than comparable C—C and C—O bonds resulting in a very low barrier to rotation and making the polymer chains very flexible. This flexibility makes many orientations possible and provides ''free space'' to accommodate different sized substituents or to allow easy diffusion of gaseous molecules; a property useful in the formation of ''breathable'' films. Coupled with the low intermolecular forces between methyl groups, this flexibility also has a profound effect on the bulk as well as the surface properties of silicones. This is seen in the small variation of physical parameters with temperature and molecular weight, the low freezing and pour points of fluids, the low boiling points, the high compressibility and the retention of liquid nature to unusually high molecular weights. It also makes a number of structural and compositional variations possible, resulting in many families of silicones, including linear and cyclic structures, a wide range of molecular weights and varying degrees of branching or cross linking. Additionally, the siloxane bond is exceptionally strong providing the 9

polymer with a high degree of thermal and oxidative stability and ensuring stability when formulated [1-3]. o

Dimethicône pulydimethylsiloxane (POMS) Figure 1 Unique chemical structure of silicones.

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