Deactivation by polysiloxane and phenyl containing disilazane: A29SI CP-MAS NMR study after the formation of polysiloxane chains at the surface

Summary A high degree of deactivation of glass and fused-silica capillary column walls is attainable by means of high temperature silylation (HTS) with or without a preceding leaching process. HTS with a phenyl containing disilazane, diphenyltetramethyldisilazane (DPTMDS), and polydimethylsiloxane (PDMS) are studied onCab-O-Sil,afumedsilica,asamodelsubstrate.Using29SiCPMAS NMR, it was shown that no dirnethylsiloxane chains were formed upon silylation with DPTMDS under different conditions of humidity and stoichiometry at 377OC. With DPTMDS deactivation it is possible that amino trisiloxy silane groups areformed, these groups add extra activity to the surface. Silylation with a PDMS, OV 101, at various temperatures between 30Oo42O0C did show that dimethylsiloxane chains were bonded at the surface. Using the 29Si CP-MAS NMR technique with variable contact times to reveal siloxy group mobility, the degradation of dimethylsiloxane chains at the surface was studied. PDMS degradation at an optimal temperature gives a more effective diminuation of the silane activity caused by chemical reaction with thesilanolgroupsandtheeffectivescreeningof theremaining silanol groups with anchored polydimethylsiloxane chains and small cyclodimethylsiloxane ring structures at the surface.