Yohei Noda

Mesoscale spatial distribution of electron spins studied by time-resolved small-angle and ultrasmall-angle neutron scattering with dynamic nuclear polarization: a case of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) doped in high-density polyethylene.

We carried out time-resolved small-angle neutron scattering (SANS) and ultrasmall-angle neutron scattering (USANS) studies of dynamically polarized high-density polyethylene (HDPE) doped with 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) persistent free radicals. We observed a remarkable enhancement of the scattering intensity shortly after a switching of microwave frequency from positive (negative) to negative (positive) dynamic nuclear polarization (DNP). The enhancement was found to be due to spatially heterogeneous proton-spin polarization generated as a result of heterogeneously distributed TEMPO in the HDPE sample. The spatial fluctuation of the polarization ranged up to the length-scale of > or = 100 nm. This result strongly suggests that the TEMPO free radicals are localized more in nonfibrils but less in fibrils of HDPE. In this way, we propose that the time-resolved DNP-SANS and DNP-USANS be general techniques to determine mesoscale spatial distribution of electron spins in dielectric materials.

Dynamic nuclear polarization of electron-beam irradiated polyethylene by pairs of alkyl free radicals

We compared dynamic nuclear polarization (DNP) behavior of electron-beam irradiated polyethylene (e-PE) with that of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-doped polystyrene (T-PS). Growth and decay rates of nuclear polarization P(n) of e-PE increased linearly with increasing concentration C(e) of free radicals, whereas those of T-PS increased with square of C(e). From these results, we suggest that the pair of alkyl radicals produced by the radiolysis, -CH(2)-CH(2)-CH(2)-→-CH(2)-CH()-CH(2)-+H(), and the subsequent hydrogen abstraction reaction, H()+-CH(2)-CH(2)-CH(2)-→H(2)+-CH(2)-CH()-CH(2)- in e-PE plays an important role on DNP by the cross effect, whereas T-PS is polarized by a pair of unspecified TEMPO radicals accidentally located nearby. We propose that higher P(n) would be expected by optimizing the inter-radical distance within the pair in e-PE and other irradiated samples.

Dynamic nuclear polarization study of UV-irradiated butanol for hyperpolarized liquid NMR.

We have carried out a dynamic nuclear polarization (DNP) study of proton spins of UV-irradiated solid butanol containing phenol at 0.1-1%. By the aid of butyl free radicals produced by the photolysis, the butanol sample was dynamically polarized up to the polarization of 1.4+/-0.1% at 1.5 K and 1.2 T, which is about 4000 times larger than that at the thermal equilibrium at an ambient temperature. Unlike persistent free radicals such as nitroxy and trityl free radicals commonly used for DNP, the butyl free radicals immediately decay by melting. We propose that free radicals produced by UV-photolysis are applicable to the DNP-enhanced hyperpolarized liquid-state NMR (Ardenkjaer-Larsen et al. [6]). The hyperpolarization of the UV-irradiated samples should remain longer than that of the samples having persistent free radicals, because of the absence of the free radicals after the melting in the UV-irradiated samples.