A solid state NMR study of polycarbonate oligomer grafted onto the surface of amorphous silica

Abstract

High resolution solid state 13C and 29Si CP/MAS NMR was used to investigate the grafting mechanism, morphology and interfacial mobility of polycarbonate (PC) oligomer and bisphenol A grafted onto silica surfaces. It was previously shown that interface modification via grafting led to composites with increased hydrolytic stability and interfacial toughness. The NMR experiments were carried out to determine the nature of the bonding of the reactants to the glass surface and to characterize the relaxation properties of the reacted species. The NMR spectra demonstrate differences between the neat and grafted PC oligomer that suggest strong bonding. A model compound, bisphenol A, was used to resolve signal overlaps caused by repeat units and to verify the formation of primary bonding at the silica surface by the existence of a downfield shift of the C 4 resonance peak and other changes in the spectrum. Proton spin-lattice relaxation times in the rotating frame offer secondary evidence of the formation of Si–O–C bonds on the silica surface. The proton spin-lattice relaxation of the grafted molecules were characterized by a bimodal distribution of relaxation times, while unreacted molecules were represented by a single relaxation time. Temperature dependent studies show that the oligomer loses mobility as a result of grafting, and that the transition responses of the material are lost. The grafted material is visualized as a low density monomolecular layer of covalently bonded material.