The mechanical properties of particulate nanocomposites strongly depend upon the particle dispersion, as well as on the closely related properties in thin polymer films covering the particle surface. The length scale of such changes is relevant for the understanding of particle−particle interactions, which ultimately dominate the mechanical response. Using well-defined 44 nm diameter silica nanoparticles dispersed in poly(ethylene glycol), we focus on surface-induced changes in polymer dynamics. Using proton time-domain NMR, we distinguish three polymer phases of different mobility, i.e., a strongly adsorbed, solid-like fraction, a fraction with intermediate relaxation times and a highly mobile fraction. We explore how these fractions change as we vary polymer molecular weight from 300 to 20 000 and particle volume fraction up to 0.3. A multiple-quantum experiment enables a closer analysis of the mobile component which we show consists of two fractions, one resembling the bulk melt-like and another one showing network-like properties. We demonstrate that above a polymer molecular weight-dependent volume fraction, polymers form elastically active links between particles, resulting in the physical gelation observed in such systems. Our results provide a quantitative picture of network formation, which is described by the amount and length of network-like chains as well as heterogeneities in the polymer dynamics. We relate changes in polymer dynamics to particle microstructure obtained from small angle neutron scattering.
Low-field proton nuclear magnetic resonance (NMR) methods were used to assess the phase fractions, domain thicknesses, T 1 relaxation properties and spin diffusion (SD) coefficients D of different phases in nanophase-separated polystyrenepolybutadiene block copolymers. At low field, SD experiments are challenged by rather short longitudinal relaxation times (T 1 ), requiring careful consideration of the interplay of T 1 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 nm 2 ms À1 , and we propose a rescaling of a literature calibration correlating D for the mobile phase with its T 2 relaxation time.
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