Effects of polydispersity on the glass transition dynamics of aqueous suspensions of soft spherical colloidal particles

Abstract

Thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) particles of a nearly constant swelling ratio and with polydispersity indices (PDIs) varying over a wide range (7.4% -48.9%) are synthesized to study the effects of polydispersity on the dynamics of suspensions of soft PNIPAM colloidal particles. The PNIPAM particles are characterized using dynamic light scattering (DLS) and scanning electron microscopy (SEM). The zero shear viscosity (η0) data of these colloidal suspensions, estimated from rheometric experiments as a function of the effective volume fraction φ ef f of the suspensions, increases with increase in φ ef f and shows a dramatic increase at φ ef f = φ0. The data for η0 as a function of φ ef f fits well to the Vogel-Fulcher-Tammann (VFT) equation. It is observed that increasing PDIs results in increasingly fragile supercooled liquid-like behavior, with the parameter φ0, extracted from the fits to the VFT equation, shifting towards higher φ ef f . The observed increase in fragility is attributed to the prevalence of dynamical heterogeneities (DHs) in these polydisperse suspensions, while the simultaneous shift in φ0 is ascribed to the decoupling of the dynamics of the smallest and largest particles. Finally, it is observed that the intrinsic nonlinearity of these suspensions, estimated at the third harmonic near φ0 in Fourier transform oscillatory rheological experiments, increases with increase in PDIs. Our results are in agreement with theoretical predictions and simulation results for polydisperse hard sphere colloidal glasses and clearly demonstrate that jammed suspensions of polydisperse colloidal particles can be effectively fluidized with increasing PDIs. Suspensions of these particles are therefore excellent candidates for detailed experimental studies of the effects of polydispersity on the dynamics of glass formation.

Relaxation Dynamics, Softness, and Fragility of Microgels with Interpenetrated Polymer Networks

“…where C w0 sets the apparent divergence, D C w controls the growth of the structural relaxation time and τ 0 is the characteristic structural relaxation time in the high dilution limit. This result points out that, by changing PAAc content, different dynamical Recently, the unifying concept of fragility has been extended to colloidal suspensions (33,34,38,49,50) and many efforts have been devoted to understand the effect of softness on fragility (38,(49)(50)(51)(52). In this context the slope of the data at C w = C * w can be used to define a kinetic fragility as:…”

“…The most studied responsive microgels are those based on the thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM), with a reversible Volume Phase Transition (VPT) around 305 K. These systems have been largely investigated both theoretically and experimentally (30)(31)(32)(33)(34)(35)(36). In this work we study a PNIPAM-based microgel with a second interpenetrated polymer network (IPN) of poly(acrylic acid) (PAAc) (37-42) that provides additional pH sensitivity, topological constraints to the particles and extra charges to the system.…”

Structural relaxation, softness and fragility of IPN Microgels

“…The most studied responsive microgels are those based on the thermo-sensitive poly(Nisopropylacrylamide) (PNIPAM), with a reversible Volume Phase Transition (VPT) at about 305 K. It has been largely investigated both theoretically and experimentally [39][40][41][42][43][44][45][46][47]. In this work we will study a PNIPAM-based microgel with a second interpenetrated polymer network (IPN) of poly(acrylic acid) (PAAc) [48][49][50][51][52][53][54][55][56] that provides additional topological constraints to the particles and extra charges to the system.…”

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

“…Therefore, the glass transition of such dispersions has been studied typically at constant temperature, changing the volume fraction only [11,12]. Several similarities between the glass transition of hard and soft spheres have been reported [28], however, many properties remain unclear or controversial, such as the fragility, the role of softness, and the relation to gelation and jamming transitions [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44].…”