Revisit to phase diagram of poly(N-isopropylacrylamide) microgel suspensions by mechanical spectroscopy

Abstract

Microgels are soft particles that can be deformed and compressed, which would induce intriguing phase behaviors at high packing fractions. Poly(N-isopropylacrylamide) (PNIPAM) microgels, with a lower critical solution temperature (LCST) of 33 °C, have attracted considerable interests as model colloids, since the volume of them and the interaction between the microgels can be tuned precisely by temperature. In this work, the linear viscoelastic properties of PNIPAM microgel suspensions have been investigated using mechanical spectroscopy. A particular attention is focused on the phase behaviors at high concentrations. With increasing concentration the system undergoes a repulsive glass-to-gel transition below the LCST, while, as temperature is raised across the LCST, the system undergoes a gel-to-attractive glass transition. A mechanism of these transitions for the microgels is proposed based on the directional interaction between the particles. In moderate concentration or de-swelling microgels the interaction is isotropic leading to the glass phase, while in concentrated and deformed microgels the interaction is directional leading to the gel phase. Our results enrich the current understanding of the phase transition in microgel systems and shed new light on the phase diagram of colloidal suspensions in general.

A review of the slow relaxation processes in the glass–rubber transition region of amorphous polymers

“…[88] Because the shapes of colloids have large impact on their dynamics, they have been usually studied with the ageing of polymer colloids. [89,90] However, the dynamics of polymeric colloids are of short-time dynamic signature,[91] so we do not comment further herein.…”

Swelling of responsive-microgels: experiments versus models

“…This typical swelling/shrinking behavior has been shown to be the driving mechanism of the phase behavior of aqueous suspensions of PNIPAM microgels, since it affects the interactions between particles and provides good tunability of both softness and volume fraction as a function of temperature [1-3, 13, 26]. It has been recently shown that the microgel swelling/shrinking behavior can be strongly affected by concentration [13,27], solvents [28] and internal structure and composition (such as number and distribution of cross-linking points [29,30] and core-shell structure [31,32]) or by introducing additives into the PNIPAM network [31].In this context PNIPAM microgels containing another specie as co-monomer or interpenetrated polymer are even more interesting, as a more complex scenario comes out. In particular, addition of poly(acrilic acid) (PAAc) to PNIPAM microgel provides an additional pH-sensitivity to the thermoresponsive microgel.…”

Mechanical Spectroscopy: Some Applications On Structural Changes And Relaxation Dynamics In Soft Matter

“…In moderate concentration or de-swelling microgels the interaction is isotropic leading to the glass phase, while in concentrated and deformed microgels the interaction is directional leading to the gel phase. The results enrich the current understanding of the phase transition in microgel systems and shed new light on the phase diagram of colloidal suspensions in general [22].…”

“…Among most microgels, poly(N-isopropylacrylamide) (PNIPAM) microgels, with a lower critical solution temperature (LCST) of 33°C, have attracted considerable interests as model colloids, since the volume of them and the interaction between the microgels can be tuned precisely by temperature [20,21]. By using mechanical spectroscopy, the liquid, glass and gel transition in suspensions of PNIPAM microgel with three cross-link densities have been studied [22]. In Fig.…”