Although the Rouse model has been widely used to describe unentangled polymer dynamics, there are some experimental and simulation results, in which the local chain dynamics is not fully consistent with the Rouse predictions. In this study, molecular simulations by a few different molecular models were conducted to reveal the effects of inter-molecular interactions on the relaxation of Rouse modes. Kremer-Grest bead-spring simulations exhibited that the second and third Rouse modes relaxations deviate from the Rouse prediction even though the first Rouse mode relaxation is fairly consistent with the Rouse behavior. A similar deviation was observed for full-atomistic simulations of polybutadiene and polyisoprene. For dissipative particle dynamics simulations, the magnitude of deviation was much smaller. Additional Kremer-Grest simulations with various molecular weights and full-atomistic simulations without excluded volume interactions suggest that the chain rigidity and the hard-core inter-beads interactions are attributable to the deviation through short-time relaxations induced by inter-beads collisions.
In this paper, we describe a method for directly detecting magnetic resonance (MR) signals using optically pumped atomic magnetometers (OPAMs) in ultra-low field magnetic resonance imaging (ULF-MRI). For direct detection with OPAMs in ULF-MRI, we propose the use of an actively shielded bias field tuning coil to match the resonant frequencies between MR signals and OPAM. Design of the tuning coil was based on both a target field approach and a stream function method. The results of field distribution analyses demonstrated that the resonant frequency of an OPAM could be tuned by the actively shielded bias field tuning coil. In addition, the actively shielded bias field tuning coil minimized the distortion of an external magnetic field in the sample region. These results indicate that actively shielded bias field tuning coil can be designed to match the resonant frequencies of MR signal and an OPAM with the inhomogeneity of sample region less than 0.25%. However, the actively shielded bias field tuning coil must be fabricated with an error of 0.5 mm or less between the centers of the primary and shield coils.Index Terms-optically pumped atomic magnetometer, actively shielded coil, ultra-low field magnetic resonance imaging 1530-437X (c)
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