Probing Interfaces and Surface Reactions of Zirconium Phosphate/Phosphonate Multilayers Using 31P NMR Spectrometry

Abstract

We report our 31P magic angle spinning (MAS) NMR characterization of zirconium phosphate/phosphonate (ZP) multilayer assemblies grown on SiOx. The reaction of silica with excess POCl3 and treatment with collidine results in both physisorbed and chemisorbed HxPO4(x-3) being present at the SiOx surface. The 31P NMR spectrum of zirconium phosphate grown from silica shows no residual Cl-containing species, indicating essentially complete hydrolysis. The phosphate 31P resonances are broadened substantially upon complexation with Zr4+, and T1 data demonstrate the broadening to be inhomogeneous in nature. Data on maleimide−vinyl ether copolymer layers we reported recently [Langmuir 1999, 15, 1418] confirm that hydrolysis/deprotection by bromotrimethylsilane is efficient in converting phosphoesters to the corresponding phosphorus oxyacid in the synthesis of multilayers. Comparison of polymer multilayers with the relatively more ordered bisphosphonic acid-based multilayers indicates that essentially the same interlayer bonding chemistry is seen for both systems, with subtle differences arising as a consequence of the greater available free volume and more extensive disorder within the polymer.