XPS INVESTIGATIONS OF ACID-BASE INTERACTIONS IN ADHESION .3. EVIDENCE FOR ORIENTATION OF CARBONYL GROUPS FROM POLY(METHYLMETHACRYLATE) (PMMA) AT THE PMMA GLASS AND PMMA-SIO2 INTERFACES

The effect of the substrate acidity/basicity on the orientation of the carbonyl group (C=O) from poly(methylmethacrylate) (PMMA) has been investigated by X-ray photoelectron spectroscopy (XPS). Analyses of the underside of PMMA films cast onto glass microscope slides, and silicon dioxide thermally grown on a silicon wafer, indicated an increasing intensity ratio O1s(C=O)/O1s(OCH3) with the substrate acidity. The XPS measure of substrate acidity was monitored by an O1s + Si2p binding energy parameter (SIGMABE) or an O1s - Si2p binding energy (DELTABE). Both SIGMABE and DELTABE increase with the acidity of the pretreatment solutions (low pH). The PMMA XPS results suggest a preferential orientation of the carbonyl group at the interface when PMMA is solvent cast onto the substrate, a phenomenon which is most pronounced for the very acidic substrate. This orientation is most probably related to an interfacial acid-base interaction between the surface acidic sites of the substrate and the oxygen of the basic carbonyl group from PMMA.