Measuring SANS and NMR simultaneously: a new tool for transient physics

Abstract : Specific properties of soft condensed matter result from the control of subtle interactions which trigger macroscopic variations. In such (macro)molecular assemblies, tuning molecular interactions via external chemo/thermo/photo (…) stimuli is often the key point to control self-aggregation mechanisms and the visible properties (optical, viscosity, color …). Analysis of such multi-scale transformations requires usually a multi-technique approach to probe both structural and dynamical changes. To this respect, Small Angle Neutron Scattering (SANS) is a central tool to probe nanometric structures while Nuclear Magnetic Resonance (NMR) is a versatile technique which can easily probe dynamical information such as local reorientation mechanisms (relaxation times) and self-diffusion coefficients of molecules. In this talk, we report on an original setup which allows to measure simultaneously in-situ SANS and NMR using a low-field spectrometer [1]. We illustrate the capabilities of alliancing these experimental methods by following the critical temperature-induced phase separation of a concentrated Poly(Methacrylic Acid) (PMAA) solution at its Lower Critical Solution Temperature (LCST). The characteristic size related to the domain growth of the polymer-rich phase of the gel is monitored by the evolution of the SANS spectra, while the dynamics of the sol phase (H 2 O and polymer) is simultaneously characterized by NMR by measuring T 1 , T 2 and the diffusion coefficient. A specific cell was carefully deisigned to optimize thermalization of the sample and in particular its equilibration time. The acquisition time needed to reach good signal-to-noise ratios, for both NMR and SANS, match: it is of the order of one hour. Altogether, we show that in-situ low-field NMR/SANS coupling the NMR is meaningful and is a promising experimental approach. Such multimodal approach is of major interest when a sample experiences transient physical states or evolves rapidly and/or irreversibly.