Proton NMR spin-diffusion studies of PS-PB block copolymers at low field: two- vs three-phase model and recalibration of spin-diffusion coefficients

Abstract

Low-field proton nuclear magnetic resonance (NMR) methods were used to assess the phase fractions, domain thicknesses, T1 relaxation properties and spin diffusion (SD) coefficients D of different phases in nanophase-separated polystyrene-polybutadiene block copolymers. At low field, SD experiments are challenged by rather short longitudinal relaxation times (T1), requiring careful consideration of the interplay of T1 relaxation and SD effects. Building on earlier work, we used a numerical fitting procedure for a separate as well as combined analysis of phase-resolved rigid- and mobile-phase filtered SD, as well as saturation recovery curves taken on a well-defined lamellar sample. We demonstrate the advantages in using three-component model, distinguishing a rigid and a mobile, as well as an interphase that can be resolved by fits to the refocused free-induction decay. We further use domain sizes from small-angle X-ray scattering as a gauge and find that SD coefficients from literature calibrations are overestimated. Under static low-field conditions, D for the rigid polystyrene phase is found to be 0.38±0.06 nm2 ms−1, and we propose a rescaling of a literature calibration correlating D for the mobile phase with its T2 relaxation time.