Structure of a PDMS layer grafted onto a silica surface studied by means of DSC and solid-state NMR

The structure of a poly(dimethylsiloxane) (PDMS) layer at the surface of hydrophilic silica has been studied by means of DSC, proton NMR T-2 relaxation experiments, and H-1 and Si-29 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 T-g 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.