Dynamical crossover between hyperdiffusion and subdiffusion of polymer-grafted nanoparticles in a polymer matrix

Abstract

The dynamical behavior of polystyrene-grafted silica nanoparticles dispersed in an atactic polystyrene matrix was studied using x-ray photon correlation spectroscopy. The time-autocorrelation functions were subjected to fitting analyses based on continuous-time random walk models. The nanoparticles exhibited non-Brownian behavior, and as the temperature increased, the crossover from hyperdiffusion to subdiffusion occurred at 1.25T_{g}, where T_{g} is the glass transition temperature of the matrix polystyrene. Hyperdiffusive behavior is caused by the dynamical heterogeneity of the polymer matrix associated with the glass transition. When the temperature was higher than 1.25T_{g}, the interaction of the grafted polymers with the polymer matrix became relatively significant, and caused a dramatic change in the dynamical behavior of the nanoparticles.

Correlated heterogeneous dynamics in glass-forming polymers

“…Similar results were obtained for slightly lower silica particle concentrations, also obeying the requirements for tracer particles. The change to hyperdiffuse dynamics is in line with recent studies on different glass formers [22][23][24] where such transitions were observed in a temperature region around 1.1T g -1.2T g . However, due to experimental restrictions, these studies only reported the transition from C to B without accessing the low-temperature region A.…”

“…This transition was observed at T ≈ 1.2T g and discussed in the context of cooperative dynamics at low temperatures. Similar observations were made by Guo et al [23] and Hoshino et al [24] studying tracer particles in different polystyrene samples. However, these studies could report only results down to temperatures of 1.1T g -1.2T g and dynamical heterogeneities were not addressed.…”

“…We interpret this crossover as a change of properties of the glass former. In contrast to previous studies [22][23][24], we were able to probe temperatures down to T g (region A) showing hyperdiffuse correlated and heterogeneous dynamics. The increase of temporally heterogeneous dynamics was observed at the onset of hyperdiffusive dynamics.…”

“…However, due to experimental restrictions, these studies only reported the transition from C to B without accessing the low-temperature region A. The transition from C to B was successfully described by a continuous-time randomwalk model [22,24]. Within this model, the q dependences of and γ in particular in region B can be described more quantitatively, yielding the nature of the diffusivity and thus the dynamics of the sample.…”

Grafted polymer chains suppress nanoparticle diffusion in athermal polymer melts

“…Studies show that dispersion in a polymer matrix can be controlled by varying the ratio of the brush to matrix molecular weight (P/N). 1,[26][27][28] Hoshino et al 29 investigated the dynamics of polystyrene (PS)-grafted silica nanoparticles (110 nm) in unentangled PS melts using X-ray photon correlation spectroscopy (XPCS) and observed that nanoparticles followed sub-diffusive behavior when the annealing temperature was greater than 1.25T g , where T g is the glass transition of the PS matrix. This sub-diffusive behavior was attributed to the penetration of short matrix chains into the brush.…”