Resolving the different bulk moduli within individual soft nanogels using small-angle neutron scattering

Abstract

The bulk modulus, K , quantifies the elastic response of an object to an isotropic compression. For soft compressible colloids, knowing K is essential to accurately predict the suspension response to crowding. Most colloids have complex architectures characterized by different softness, which additionally depends on compression. Here, we determine the different values of K for the various morphological parts of individual nanogels and probe the changes of K with compression. Our method uses a partially deuterated polymer, which exerts the required isotropic stress, and small-angle neutron scattering with contrast matching to determine the form factor of the particles without any scattering contribution from the polymer. We show a clear difference in softness, compressibility, and evolution of K between the shell of the nanogel and the rest of the particle, depending on the amount of cross-linker used in their synthesis.

Harnessing the polymer-particle duality of ultra-soft nanogels to stabilise smart emulsions

“…The ULC nanogels used in this study were synthesised following the same protocol, have the identical hydrodynamic size, swelling behaviour, and viscometry conversion constant as the ones studied at a flat decane-water interface in a previously published paper 46 . This means they have an identical softness phase behaviour compared to the ULC nanogel we studied previously 1,39,42,43,46,71 . Consequently, we can directly compare the nearest-neighbour distance between ULC nanogels at the droplet surface to the d nn between ULC nanogels of Ref.…”

“…The interfacial viscoelasticity of linear pNIPAM, ULC, and BIS-crosslinked nanogels is very similar and, therefore, cannot be the reason for the different properties of the emulsions. In contrast, the extreme softness of the ULC microgels 39 makes them very stretchable and able to cover the surface uniformly as a linear polymer 46 . At the same time, unlike linear polymer these nanogels possess a crosslinked network, which can protrude into the aqueous phase of the emulsion 46,48 and provide a steric barrier and a strong response to the variation of temperature 48 .…”

“…The softest pNIPAM-based nanogels that can be obtained via precipitation polymerisation are the so-called ultra-low crosslinked (ULC) nanogels 27,39 . They are synthesised without any crosslinking agent 40 .…”

In-situ study of the impact of temperature and architecture on the interfacial structure of microgels

“…52 As with the standard microgels, we use a partially deuterated monomer in which the vinyl group is deuterated 52 to increase and vary the contrast in neutron reflectometry. Given the absence of cross-linker agent, these ultra-low cross-linked (ULC) microgels are ultra-soft 53,54 and have an almost uniform, albeit very low, internal density of polymer segments. 39 Nonetheless, such particles remain fundamentally different from linear polymers.…”

“…For instance, the presence of a more cross-linked and denser core inhibits large compression in bulk, 56 whereas the poorly cross-linked network of the ULC microgels is easy to compress in crowded solutions. 53,57 While compressibility is the key aspect for the three-dimensional response of microgels, their deformability is pivotal once they are confined in two dimensions, i.e. onto liquid or solid interfaces.…”