In order to rationally design new smart materials, it is pivotal to understand their phase behaviour. This means being able to predict under which conditions (e.g. temperature, pressure, concentration) such a material is a fluid, a disordered solid, or a crystal. Colloidal dispersions are suitable model systems to reproduce the behaviour of atomic systems and complex fluids on easily accessible length scales. A particular class of colloids is represented by microgels. Microgels are polymeric crosslinked networks which form spherical particles when swollen in a good solvent. Two of the most important characteristics of microgels are their capability to dramatically change their volume depending on changes in external stimuli (e.g. temperature or pH), and their softness. Indeed, softness (that is, the capability to deswell, deform, or interpenetrate) has a strong impact on the phase transitions and rheological properties of solutions of spheres. However, to date, an ideal model system for really neutral soft spheres is missing. Here, we show that solutions of super-soft microgels are a valuable candidate for this aim.
In this study, we use solutions of super-soft microgels to explore the phase behaviour of soft spheres. The liquid-to-solid phase transition is shifted to higher concentrations with respect to both hard incompressible spheres and conventionally cross-linked microgels. Furthermore, the phase behaviour shows a stable body centred cubic (bcc) lattice, which is not observed for other colloids. The coexistence of bcc lattice with face centred cubic (fcc) lattice, expected for hard spheres and other microgels, is rationalized by means of small-angle scattering experiments. We probe both the microgel-to-microgel arrangement in solution (x rays, small-angle x-ray scattering, SAXS) and the structure of the single microgels as a function of concentration (neutrons, small-angle neutron scattering, SANS, with contrast variation. Besides its fundamental interest, this work can be taken as an example of the complementarity between SAXS and SANS with contrast variation. In general, in a scattering experiment, the scattered intensity, I(q), is proportional to the product between the form factor, P(q), and the structure factor, S(q), which equals 1 in the limit of infinite dilution. The form factor contains all the information (size, polydispersity, shape, internal architecture) about the single scattering object. Here by means of SAXS we probe the variation of the S(q) and gain information on the global structure of our samples. Then, we have taken the very same samples and we have measure by means of SANS. Here the “trick” is that the majority of the microgels used are synthesized using a deuterated monomer. The use of a mixture between water and heavy water allows us to mask the contribution of the deuterated particles, and, therefore, the S(q) of the samples. In this way we use SANS with contrast variation to directly access the complementary information on the shape and structure of the single microgel in crowded environment. The comparison between the results of these techniques shows that bcc crystals, unexpected for neutral colloids, appear at concentrations where the collapse of the microgels competes with their capability to interpenetrate their neighbours.
Figure (a) Examples of samples in the crystalline phase. (b) Scattering curve measured with concentrated samples by means of SANS with contrast variation. (c) Comparison between the microgel-to-microgel distance, dnn, measured with SAXS and the microgel diameter, 2RSANS, measured with SANS.
We believe that thanks to the easy synthetic protocol and the fine control over the size and size polydispersity, ultra-soft microgels represent valid model systems for soft spheres to investigate the role of softness on the crystallization process. They can be used in the future to investigate the prediction of the nucleation rate for soft spheres and compare it to the prediction for hard spheres. Furthermore, we recently demonstrated ultra-soft microgels can be used to investigate the glass and jammed states. Even in this case, the advantage of using ultra-soft microgels over other microgels is that the effects of charge and of osmotic deswelling are minimized. Finally, we mention that this system might be used as model for drug delivery nano-capsules and bio-inspired materials, such as synthetic platelets. Even for these applications, it is important to understand the effects of softness on properties of the individual colloid, and on the global properties of the solution.
This work results from the collaboration between Dr. J. E. Houston (ESS) and Dr. A. Scotti (RWTH-Aachen University). These activities were financed by the Deutsche Forschungsgemeinschaft (DFG) Project No. 191948804 within SFB 985 Functional Microgels and Microgel Systems and by the International Helmholtz Research School of Biophysics and Soft Matter (IHRS BioSoft). The SANS measurements were performed at the D11 instrument at the Institute Laue-Langevin (ILL), Grenoble, France, and at the KWS-1 and KWS-2 instruments operated by JCNS at the Heinz Maier-Leibnitz Zentrum (MLZ), Garching, Germany. The SAXS measurements were performed on the cSAXS beamline of the Swiss Light Source, Paul Scherrer Institut.
Check out the full article at the Physical Review E, 102, 052602 (2020).