Dynamic nuclear polarization enhanced nuclear magnetic resonance of polymer-blend interfaces

Abstract Heterogeneous blends of [3,3′- 13 C 2 ]polycarbonate and [uniform-ring- 12 C 6 ]polystyrene were formed by serial film casting. The polystyrene phase of each blend was homogeneously doped with 2% by weight of a bis (diphenylene) phenylalyl 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 13 C nuclear magnetic resonance. The 13 C signal from polycarbonate arises exclusively from chains which are at the polycarbonate-polystyrene interface. Signals from bulk polycarbonate were suppressed by differencing techniques. The dominant mechanism of polarization transfer from the electrons in the polystyrene phase to the protons in the polycarbonate phase is by direct polarization transfer. The interface signal arises from a 60 A region which is 2% of the film thickness. As monitored by dynamic nuclear polarization selected dipolar rotational spin-echo 13 C nuclear magnetic resonance, polycarbonate chains have less motion at the interface than in the bulk.