How Softness Matters in Soft Nanogels and Nanogel Assemblies

Abstract

Softness plays a key role in determining the macroscopic properties of colloidal systems, from synthetic nanogels to biological macromolecules, from viruses to star polymers. However, we are missing a way to quantify what the term “softness” means in nanoscience. Having quantitative parameters is fundamental to compare different systems and understand what the consequences of softness on the macroscopic properties are. Here, we propose different quantities that can be measured using scattering methods and microscopy experiments. On the basis of these quantities, we review the recent literature on micro- and nanogels, i.e. cross-linked polymer networks swollen in water, a widely used model system for soft colloids. Applying our criteria, we address the question what makes a nanomaterial soft? We discuss and introduce general criteria to quantify the different definitions of softness for an individual compressible colloid. This is done in terms of the energetic cost associated with the deformation and the capability of the colloid to isotropically deswell. Then, concentrated solutions of soft colloids are considered. New definitions of softness and new parameters, which depend on the particle-to-particle interactions, are introduced in terms of faceting and interpenetration. The influence of the different synthetic routes on the softness of nanogels is discussed. Concentrated solutions of nanogels are considered and we review the recent results in the literature concerning the phase behavior and flow properties of nanogels both in three and two dimensions, in the light of the different parameters we defined. The aim of this review is to look at the results on micro- and nanogels in a more quantitative way that allow us to explain the reported properties in terms of differences in colloidal softness. Furthermore, this review can give researchers dealing with soft colloids quantitative methods to define unambiguously which softness matters in their compound.

Fluid interface-assisted assembly of soft microgels: recent developments for structures beyond hexagonal packing

“…This gap is most likely related to the different behavior at liquid interfaces as compared to bulk, where microgels are isotropic over a broad range of concentration and the softness of microgels can be easily quantified through rather standard experiments. 53 The role of shell architecture and the resulting interaction potential in complex interfacial self-assembly has been recently demonstrated by using core–shell particles with a linear, brush-like polymer shell rather than crosslinked microgels. 79 In the future, exploring the role of electrostatic interactions, for example in core–shell microgels with strongly charged cores, might be interesting for exploring an even broader parameter space and getting access to more complex interactions and potentially microstructures.…”

“…In any case, when it comes to interparticle interactions one needs to distinguish between mechanical stiffness, i.e. rigid as in case of inorganic nanoparticles and soft as in case of (most) microgels 53 and interactions related to steric and/or electrostatic interactions.…”

“…51 The degree of this stretching is limited by the bulk elasticity 58 and can be used as a measure for the softness. 53 With increasing crosslinker density, microgels become more rigid and stretching is less pronounced. Thus, the diameter at the fluid interfaces approaches the diameter in bulk.…”

“…In any case, when it comes to interparticle interactions one needs to distinguish between mechanical stiffness, i.e. rigid as in case of inorganic nanoparticles and soft as in case of (most) microgels 53 and interactions related to steric and/or electrostatic interactions. Electrostatic interactions are relevant to (most) rigid nanoparticles in aqueous environments and result in soft repulsive interactions at distances larger than the particle diameter – despite the particles themselves being rigid.…”

Micron-Sized Silica-PNIPAM Core-Shell Microgels with Tunable Shell-To-Core Ratio

“…They belong to a unique class of materials because they exhibit both solid- and liquid-like behavior and have the capability to respond to external stimuli, e.g., temperature [ 3 ], pH, and ionic strength [ 4 ]. These properties can be tuned by engineering the morphologies [ 5 ], composition [ 6 ], porosity, and elasticity of the microgels [ 7 ]. Due to their tailorable stimuli sensitivity, high colloidal stability, and a broad range of possibilities for various functionalization, the past decades have seen a steadily increasing interest in microgels in applications such as biomedicine, photonic and process technology as well as in fundamental research across disciplines [ 1 ].…”