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

Abstract

We study the structure and dynamics of colloidal particles with a spherical hard core and a thermo-responsive soft shell over the whole phase diagram by means of small-angle X-ray scattering and X-ray photon correlation spectroscopy. By changing the effective volume fraction by temperature and particle concentration, liquid, repulsive glass and attractive gel phases are observed. The dynamics slow down with increasing volume fraction in the liquid phase and reflect a Vogel-Fulcher-Tamann behaviour known for fragile glass formers. We find a liquid-glass transition above 50 vol.% that is independent from the particles' concentration and temperature. In an overpacked state at effective volume fractions above 1, the dispersion does not show a liquid phase but undergoes a gel-glass transition at an effective volume fraction of 34 vol.%. At the same concentration, extrema of subdiffusive dynamics are found in the liquid phase at lower weight fractions.We interpret this as dynamic precursors of the glass-gel transition.

In situ characterization of crystallization and melting of soft, thermoresponsive microgels by small-angle X-ray scattering

“…1,2,8 These complex interactions and their tunability render microgels as the ideal model system to study soft colloidal interactions and phase behaviour. In this role microgels have been used to study important fundamental phenomena, such as phase transitions, defect formation, as well as the glass transition or jamming in dense soft particle systems [9][10][11][12][13][14][15][16][17][18][19][20][21] . In particular, studies from the last few years addressed the microgel structure in densely packed systems, revealing that microgels at sufficiently high particle densities can experience different phenomena such as interpenetration [22][23][24] and/or deswelling.…”

“…22,[25][26][27][28][29] When microgels are prepared from poly-Nisopropylacrylamide (PNIPAM), the size and volume fraction of microgels can be controlled in-situ by temperature variations 30 . This makes PNIPAM microgels of particular interest for in situ investigations of phase transitions, such as crystallization and melting 10,15,17 . The temperature behaviour is related to the lower critical solution temperature (LCST) of PNIPAM in water below which polymer-solvent interactions are favoured.…”

“…One of the main reasons why the temperature response of PNIPAM microgels is hard to address, is the fact that upon close contact the microgels start to overlap and cannot be resolved individually. This explains why most optical (fluorescent) microscopy studies have focused on dilute systems 34 or crystalline systems in which the periodic order helps to resolve particle centers 10,15,17 and only intensive experimental optimization such as specific fluorescent labelling and super-resolution methods provide enough resolution to resolve the microgels in dense states 22 . Also, for scattering methods using e.g.…”

“…In this role microgels have been used to study important fundamental phenomena, such as phase transitions, defect formation, as well as the glass transition or jamming in dense soft particle systems. 9–21 In particular, studies from the last few years addressed the microgel structure in densely packed systems, revealing that microgels at sufficiently high particle densities can experience different phenomena such as interpenetration 22–24 and/or deswelling. 22,25–29…”

“…30 This makes PNIPAM microgels of particular interest for in situ investigations of phase transitions, such as crystallization and melting. 10,15,17 The temperature behaviour is related to the lower critical solution temperature (LCST) of PNIPAM in water below which polymer–solvent interactions are favoured. Above the LCST polymer–polymer interactions dominate leading to chain collapse into globules and results in the pronounced volume phase transition (VPT) behaviour.…”