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

Abstract

The structural characterization of microgels at interfaces is fundamental to understand both their 2D phase behavior and their role as stabilizers that enable emulsions to be broken on demand. However, this characterization is usually limited by available experimental techniques, which do not allow a direct investigation at interfaces. To overcome this difficulty, here we employ neutron reflectometry, which allows us to probe the structure and responsiveness of the microgels in-situ at the air-water interface. We investigate two types of microgels with different cross-link density, thus having different softness and deformability, both below and above their volume phase transition temperature, by combining experiments with computer simulations of in silico synthesized microgels. We find that temperature only affects the portion of microgels in water, while the strongest effect of the microgels softness is observed in their ability to protrude into the air. In particular, standard microgels have an apparent contact angle of few degrees, while ultra-low cross-linked microgels form a flat polymeric layer with zero contact angle. Altogether, this study provides an in-depth microscopic description of how different microgel architectures affect their arrangements at interfaces, and will be the foundation for a better understanding of their phase behavior and assembly.

Harnessing the polymer-particle duality of ultra-soft nanogels to stabilise smart 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 interplay between these two aspects is responsible for the mixed properties of the emulsions, in between the one stabilised by polymer and by regularly crosslinked nanogels.…”

“…To rationalise this, we can consider the recent literature that studied the architecture of linear pNIPAM, ULC nanogels, and regular nanogels in the direction orthogonal to the interface 48,[60][61][62] . Neutron reflectometry experiments show that linear pNIPAM and ULC nanogels protrude only a few nanometers into the hydrophobic phase and their contact angle is zero, both above and below the VPTT 48,60 . In contrast, regular nanogels protrude for tens of nanometers into air or non-polar oils, such as decane, with a contact angle of few degrees 48,62 .…”

“…On the aqueous side of the interface, the main difference between linear pNIPAM and nanogels is the significant protrusion of the nanogels into the water phase. For both ULC and regular microgels, the extent into the water phase was determined to be in the range of their hydrodynamic radius 48,62 . In contrast, linear pNIPAM is completely flattened and adsorbed onto the interface 60 .…”

“…Neutron reflectivity has been used to probe the responsiveness of ULC nanogel at the interface, revealing that they extend in the water phase where they remain thermoresponsive. Furthermore, it has been observed that they extend in the hydrophobic phase only for few nanometers with a contact angle that is practically zero 48 . Also in this case, their behaviour in the water phase is characteristic of nanogels, while ULC nanogels resemble linear polymers on the interface and in the hydrophobic phase.…”

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

“…The NR raw data used in this study are available in the ILL Data Portal database under accession code 10.5291/ILL-DATA.9-11-1871 91 and 10.5291/ILL-DATA.EASY-462. 92 The raw data, associated data, and derived data supporting the results of this study have been deposited in the RADAR4Chem database under DOI:10.22000/603 93 or are available from the corresponding author at the link http://hdl.handle.net/21.11102/b0e200f4-d196-44bd-874a-2f5f79d22527.…”