Responsive microgels: from individual systems to their collective properties
Jérôme Crassous, Institute of Physical Chemistry, RWTH Aachen University,52074 Aachen, Germany
Although poly(N-isopropylacrylamide) microgels have been widely used as model systems for soft colloids, their properties are still far to be completely understood. This stems from their heterogeneous structure strongly differing from an idealized polymeric network. These features can be reproduced in computer simulations using the so-called in silico synthesis to capture the structural properties obtained with scattering techniques over a wide range of temperature and provide new insights on their interactions.
At higher volume fractions, depending on their degree of crosslinking, such soft microgels may further interpenetrate, deform and facet with direct consequences on their rheological properties. In particular, their ability to change size and thus volume fraction with temperature has made them highly attractive model systems for the investigation of liquid solid transitions such as the glass or jamming transition. This generally assumes that the interparticle interactions between microgels does not change with temperature, and there seems to be common agreement that this is valid below at temperatures below the collapse or volume phase transition temperature. Here we now critically re-examine this assumption, and present results from a systematic study of the effect of number density and temperature on the structural and dynamic properties of PNIPAM microgel suspensions.
Based on the acquired knowledge, we finally discuss how the interplay of molecular and colloidal scales controls drying of microgel dispersions. This is achieved by monitoring the drying at the end of a capillary exposed to a controlled humidity with constant particle feeding ensures by its connection to a reservoir. Hereby, the water evaporation and diffusion set a flow driving the particles at the drying front where they build up crystals. Whether or not the particles are interpenetrable is characterized by the difference of scaling law in the time evolution of the drying front. We evidence an original drying behavior intermediate between colloidal and solution drying, in which a diffusional scaling is observed together with a weak dependence on the air relative humidity. Mapping composition and structuration gradients using Raman spectroscopy and small-angle scattering techniques, we show that this behavior stems from the ability of microgels to both interpenetrate and compact. As a result, water activity and transport is drastically decreased in the vicinity of the air/liquid interface. This mechanism will be at play in a large diversity of complex colloidal systems and is pivotal for the mastering of drying processes.
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