41. Dynamic nuclear polarization fast field cycling method for selective study of molecular dynamics in block copolymers
Dynamic nuclear polarization (DNP) is one of the most useful methods for increasing sensitivity in NMR. It is based on the transfer of magnetization from an electron to the nuclear spin system. Based on previous work demonstrating the feasibility of integrating DNP with Fast Field Cycling (FFC) relaxometry, and the possibility to distinguish between different mechanisms such as Overhauser Effect (OE) and Solid Effect (SE), the first FFC study of the differential relaxation properties of a copolymer is presented. For this purpose, concentrated solution of polystyrene-block-polybutadiene-block-polystyrene (SBS) triblock copolymer and their corresponding homopolymers were investigated. T1-T2 relaxation data are discussed in terms of molecular mobility and the presence of radicals. The DNP selective data indicate a dominant solid effect contribution to the enhancement of the NMR signal for both blocks of the triblock copolymer as well as for the homopolymer solutions. The enhancement factors are different for both polymer types and in the copolymer, which is explained by the individual 1H T1 relaxation times and different electron-nucleus coupling strength. T1 relaxation dispersion measurements of the SE enhanced signal were performed, leading to improved signal-to-noise ratios that allowed the site-specific separation of relaxation times and their dependence on Larmor frequency with a higher accuracy.