Small-angle neutron scattering is the perfect tool to study the average conformation of macromolecules in about any environment, from solvents to other polymer molecules. The power of the method is intimately related to the length scale of observation, the virtual absence of artefacts due to the averaging, and most importantly, the possibilities to adjust contrasts and highlight chosen parts of the samples. In this talk, some recent applications will be reviewed. During a stay with Arantxa Arbe and Juan Colmenero (San Sebastian), we have studied the structure of synthetic single-chain nanoparticles (SCNPs) in presence of linear polystyrene crowding molecules under contrast-matching of the crowders [1]. A model describing the scattering of aggregating polydisperse SCNPs has been developed, resulting in the determination of the potentially squeezed size of the individual SCNPs within aggregates, their local chain statistics, and the average aggregation number, as a function of crowding. Secondly, in collaboration with Marian Cors and Thomas Hellweg (Bielefeld), we have investigated the internal structure of core-shell microgel particles in suspension. Using selective deuteration, core- and shell monomers could be measured independently, and the corresponding monomer profile described by a reverse Monte Carlo approach [2]. In the last part, an application to polymer nanocomposites will be discussed. Rubber-based nanocomposites show a large variety of spatial organization of the filler nanoparticles, which can be studied by small-angle scattering, and analyzed by large-scale reverse Monte Carole simulation [3]. A key feature of rubber nanocomposites is linked to the influence of the filler surfaces on the polymer structure and dynamics, [4]. In particular, we have studied blends of short and long chains, where one chain type is deuterated, by small-angle neutron scattering. Different degrees of spatial segregation could be identified, including a peculiar, “fish-shaped” interfacial gradient. [1] J. Oberdisse, M. González-Burgos, A. Mendia, A. Arbe, A. J. Moreno, J. A. Pomposo, A. Radulescu and J. Colmenero, Macromolecules, 2019, 4295.[2] M. Cors, O. Wrede, L. Wiehemeier, A. Feoktystov, F. Cousin, T. Hellweg, J. Oberdisse, Scientific Reports 2019, 9, 13812.[3] D. Musino, A.-C. Genix, T. Bizien, J. Oberdisse, Nanoscale, 2020, 12, 3907.[4] A.C. Genix, V. Bocharova, B. Carroll, P. Dieudonné-George, M. Sztucki, R. Schweins, A. P. Sokolov, and J. Oberdisse, ACS Applied Materials and Interfaces 2021, 13, 36262.

A few examples of experimental studies of soft matter systems will be presented toillustrate some aspects of the personal and scientific journey of Isabelle Grillo. I have metIsabelle for the first time almost twenty-five years ago in the Bombannes school, and sincethen a combination of geographic coincidence, common scientific topics – from surfactantsystems to polymer nanocomposites -, and a shared interest in a technique, neutronscattering, made us walk some steps of this journey together. While the extra-ordinarystructure of many of the systems Isabelle took interest in will be presented by world-leadingexperts in this symposium, my presentation will shortly highlight vesicular structures, andthen bridge the gap between PhD work supervised by us on nanocomposites to our currentunderstanding of the structure of such nanomaterials.

The structure of the nematic (cholesteric) drops that form at the clearing temperature of a mixture of the bent-core molecule CB7CB and the rod-like molecule 8CB doped with a surfactant is optically determined. Using experimental observations and numerical simulations, it is demonstrated that the director field inside these drops is not escaped-concentric as previously proposed, but twisted-bipolar. The Lehmann rotation of these drops in the presence of a temperature gradient is described. Their rotation velocity is shown to be proportional to the temperature gradient and to the surface twist angle of the director field and inversely proportional to the drop radius, thus revealing a fundamental scaling law for the Lehmann effect of nematic/cholesteric twisted bipolar droplets.

Understanding the origin of the unique rheological properties of wheat gluten, the protein fraction of wheat grain, is crucial in bread-making processes and has raised questions of scientists for decades. Gluten is a complex mixture of two families of proteins, monomeric gliadins and polymeric glutenins. To better understand the respective role of the different classes of proteins in the supramolecular structure of gluten and its link to the material properties, we investigate here concentrated dispersions of gluten proteins in water with a fixed total protein concentration but variable composition in gliadin and glutenin. Linear viscoelasticity measurements show a gradual increase in the viscosity of the samples as the glutenin mass content increases from 7 to 66%. While the gliadin-rich samples are microphase-separated viscous fluids, homogeneous and transparent pre-gel and gels are obtained with the replacement of gliadin by glutenin. To unravel the flow properties of the gluten samples, we perform shear startup experiments at different shear-rates. In accordance with the linear viscoelastic signature, three classes of behavior are evidenced depending on the protein composition. As samples get depleted in gliadin and enriched in glutenin, distinctive features are measured: (i) viscosity undershoot suggesting droplet elongation for microphase-separated dispersions, (ii) stress overshoot and partial structural relaxation for near-critical pre-gels, and (iii) strain hardening and flow instabilities of gels. We discuss the experimental results by analogy with the behavior of model systems, including viscoelastic emulsions, branched polymer melts, and critical gels, and provide a consistent physical picture of the supramolecular features of the three classes of protein dispersions.

Solitons attract a great deal of interest in many fields, ranging from optics to fluid mechanics, cosmology, particle physics and condensed matter. However, solitons of these very different types rarely coexist and interact with each other. Here we develop a system that hosts optical solitons coexisting with topological solitonic structures localised in the molecular alignment field of a soft birefringent medium. We experimentally demonstrate and theoretically explain optomechanical interactions between such optical and topological solitons, mediated by the local transfer of momentum between light and matter and the nonlocal orientational elasticity of the liquid-crystal phase used in our system. We show that the delicate balance arising from these different contributions to the optomechanical force enables facile dynamical control and spatial localisation of topological solitons. Our findings reveal unusual solitonic tractor beams and emergent light-matter self-patterning phenomena that could aid in creating new breeds of nonlinear photonic materials and devices.

For small tension the response of a solid to an applied stress is given by Hooke’s law. Outside this linear regime the relation between stress and strain is no longer universal and at present there is no satisfactory insight on how to connect for disordered materials the stress-strain relation to the microscopic properties of the system. Here we use atomistic computer simulations to establish this connection for the case of silicate glasses containing alkali modifiers. By probing how in the highly non-linear regime the stress-strain curve depends on composition, we are able to identify the microscopic mechanisms that are responsible for the complex dependence of stress on strain in these systems, notably the presence of an unexpected quasi-plateau in the tangent modulus. We trace back this dependence to the mobility of the modifiers which, without leaving their cage or modifying the topology of the network, are able to relieve the local stresses. Since the identified mechanism is general, the results obtained in this study will also be helpful for understanding the mechanical response of other disordered materials.

The interfacial region formed in a polymer matrix around nanoparticles (NPs) controls many macroscopic properties of polymer nanocomposites (PNCs). However, understanding the factors controlling the structure and properties of the interfacial region remains a challenge. We demonstrated that the initial trapping of polymer chains at surfaces of NPs in solution strongly affects the macroscopic properties of PNCs. The most surprising result is that the differences in properties of PNCs persist even after an extremely long thermal annealing time. We ascribe the observed changes to the formation of an interfacial layer that is trapped in a deep metastable state already in solution. Furthermore, the presented analysis suggests that the PNC equilibration time is defined by the chain desorption time that can be extremely long and, in some cases, even not accessible on a reasonable experimental time scale. These results highlight the importance of polymer solution concentrations on the formation of an interfacial layer and the macroscopic properties of PNCs.