Selective Observation of the Interface of Heterogeneous Polycarbonate/Polystyrene Blends by Dynamic Nuclear Polarization C-13 NMR

Heterogeneous blends of polycarbonate and polystyrene were formed by serial film casting. The polystyrene phase of each blend was homogeneously doped with 2 by weight of a bisdiphenylenephenylallyl free radical complex with benzene. Proton polarization enhanced by dynamic nuclear polarization was generated in the polycarbonate phase by dipolar coupling to electrons in the polystyrene phase under 39-GHz microwave irradiation at the difference of the electron and proton Larmor frequencies. Proton magnetization was then transferred to carbons under matched, spin lock conditions for detection with chemical shift selectivity by magic-angle spinning 13C NMR. The 13C signal from polycarbonate arises exclusively from chains which are at the polycarbonate polystyrene interface. Signals from bulk polycarbonate were suppressed by differencing techniques. Attempts at direct polarization transfers from electrons in the polystyrene phase to carbons in the polycarbonate phase failed.