Pathways and challenges towards a complete characterization of microgels

Abstract

Due to their controlled size, sensitivity to external stimuli, and ease-of-use, microgel colloids are unique building blocks for soft materials made by crosslinking polymers on the micrometer scale. Despite the plethora of work published, many questions about their internal structure, interactions, and phase behavior are still open. The reasons for this lack of understanding are the challenges arising from the small size of the microgel particles, complex pairwise interactions, and their solvent permeability. Here we describe pathways toward a complete understanding of microgel colloids based on recent experimental advances in nanoscale characterization, such as super-resolution microscopy, scattering methods, and modeling. T he term "microgel" usually refers to a cross-linked polymer network of colloidal size that is swollen in a good solvent and has a diameter between~100 nm and several micrometers 1-3 (Fig. 1). Colloidal hydrogels are a common subtype where the continuous phase is water. Often, but not always, these networks exhibit conformational changes in response to changes in their environment, such as the solvent composition or temperature 1,4,5. The sensitivity to external stimuli, such as temperature, pH, or solvent conditions, is an essential feature of many microgels, in particular for the most widely studied type of microgels, based on pNIPAM (poly-N-isopropylacrylamide) and its derivatives 1,6 (Fig. 1b). Here, we consider homogeneous suspensions of these particles at different densities from the dilute to the highly packed regime, as well as microgels adsorbed at surfaces. It is also possible to synthesize larger, millimeter-sized, gel particles 7 , or to study two-dimensional assemblies at an interface 8,9. Additional functions, for example, for chemical transformation or sensing applications 10,11 , can be added using core-shell particles or by decorating the microgels with inorganic nanoparticles 12,13. Microgels are already used in several applications as viscosity modifiers and lubricants 14 , for CO 2 capturing 15 or 3D bioprinting 16 , as biocompatible additives and delivery vehicles 17 , sensors, or stimuli-responsive color-changing systems 18,19. Despite their popularity and widespread application, the internal structure of microgels is not well-understood. During the microgel synthesis, the degrees of freedom for the polymer strands are intentionally reduced by forming covalent cross-links between polymer chains. The chemical crosslinking itself is a nonequilibrium process, and it does not necessarily progress uniformly, which may lead to spatial fluctuations, gradients, and heterogeneities in the microgel particle architecture 20. For a complete understanding of a microgel colloid as a material building block or a carrier, it is imperative to know the spatial distribution of polymer and cross-link densities on the nanoscale. Initially, many studies assumed that microgels could be described simply as spherical, homogeneous gel particles, as discussed in ref. 1. Based on this assumption, t...

On the Determination of Mechanical Properties of Aqueous Microgels—Towards High-Throughput Characterization

“…Overview of the impact of indentation geometries and elaborations of the theoretical basis for analysis of nanoscale mechanics has been provided [ 63 ], and also, more recently, summarizing, e.g., operation in dynamic modes [ 64 ]. Here we have chosen to focus on applying AFM to characterize aqueous microgels, including also samples made of polyelectrolytes [ 65 , 66 , 67 , 68 , 69 , 70 ]. AFM stands apart from most methods for determining the mechanical properties of microgels since it offers information at the nanoscale.…”

Glass-liquid and glass-gel transitions of soft-shell particles

“…A special class of frequently studied soft particles are microgels or nanogels made from Poly(N-isopropylacrylamide) (PNIPAm). Such PNIPAm particles dispersed in water show a reversible volume phase transition at a lower critical solution temperature (LCST) of 306 K [9][10][11][12][13]. Below this LCST, the polymer is hydrophilic and the particles hence swollen with water.…”

“…As the PNIPAm particles deform and deswell at large volume fractions, an effective volume fraction is conventionally used defined by ζ = N •V (T ), with N the particle number concentration and V (T ) the volume fraction of a particle in a dilute dispersion at temperature T as obtained by dynamic light scattering [39,45,46]. Values around and above ζ = 1 are possible with deformed particles and interpenetration of the PNIPAm shell [12,13,45,[47][48][49][50]. Depending on the softness of pure PNIPAm particles the real volume fraction matches ζ up to approximately ζ = 0.8 [46,51].…”

Microgel Particles with Distinct Morphologies and Common Chemical Compositions: A Unified Description of the Responsivity to Temperature and Osmotic Stress

“…The core of this type of microgel particle does not contain BIS crosslinker; however, we cannot exclude the existence of a thin outermost ring of crosslinks merging with the crosslinked shell, due to the diffusion of KPS. The DC microgels, in analogy to the reported examples in the literature [ 20 ], will have a core with a decreasing crosslink density radial gradient, contrasted by an opposite chain density gradient due to the proton abstraction effect of KPS, but distinct from the fuzzy crown made of freely dangling chains, propagating in the absence of crosslinker, although containing some autocrosslinked units. This picture is qualitatively in agreement with the differential interference contrast (DIC) optical microscopy images of the four types of PNIPAM microgels prepared, after overnight equilibration of the dried microgels in deionized water at room temperature, shown in Figure 1 .…”