Fluid Interfaces Grazing Angles ReflectOmeter

Example 2: Smart Nanogels at the Air/Water Interface

Nanomaterials are receiving increased attention in fundamental studies and biomedical applications. Nanogels are covalently cross-linked polymers, characterised by their small size and high surface-to-volume ratio, combining properties of typical colloids with the soft character and responsiveness of gels. The development of nanogels that exhibit a switchable phase transition close to the normal physiological temperature of 37 °C is of key interest. Their structures at interfaces, however, require a better understanding before they can be used in applications.

A project was carried out using FIGARO to characterize the interfacial structure of nanogels comprising poly(N-isopropyl acrylamide) (PNIPAM) with different degrees of a cross-linker called N,N′-methylenebisacrylamide (MBA). Normal and deuterated nanogels were synthesized specifically for this work to maximize the sensitivity of the measurements to fine details of the interfacial structure. The use of neutrons in this work was of paramount importance to gain an understanding of the factors that affect the structures created.

A three-layer model was found to describe the structures formed: first a densely packed layer in contact with air, then a layer of solvated polymer, and lastly a layer of diffuse polymer chains extending into the solution. The water content in the first layer was related to the ability of the nanogel to change conformation. This was the first experimental evidence of structural changes of nanogels as a function of the degree of cross-linking at the air/water interface.

The work went on to provide a structural characterization with respect to the bulk nanogel concentration. Clear evidence was presented that the nanogels are adsorbed at the interface in a strongly deformed shape and form a multilayer where the thickness increases with nanogel concentration in the bulk. The combination of surface characterisation techniques and bulk studies indicate that interfacial film formation is preferred over bulk aggregation.

It is hoped that the insight gained in the extended project may lead to achievement of the rational, smart design of new materials for biomedical applications.