Correlated heterogeneous dynamics in glass-forming polymers

Abstract

We report x-ray photon correlation spectroscopy experiments on the dynamics of the glass-former polypropylene glycol covering a temperature range from room temperature to the glass transition at T g = 205 K using silica tracer particles. Three temperature regimes are identified: At high temperatures, Brownian motion of the tracer particles is observed. Near T g , the dynamics is hyperdiffusive and ballistic. Around 1.12T g , we observe an intermediate regime. Here the stretching exponent of the Kohlrausch-Williams-Watts function becomes q dependent. By analyzing higher-order correlations in the scattering data, we find that dynamical heterogeneities dramatically increase in this intermediate-temperature regime. This leads to two effects: increasing heterogeneous dynamics and correlated motion at temperatures close to and below 1.12T g .

Dynamics of soft nanoparticle suspensions at hard X-ray FEL sources below the radiation-damage threshold

“…Thus, an effect of size and shape polydispersity on the dynamics that leads dynamical heterogeneities with <1 as well (Andrews et al, 2018) can be neglected. The results are in qualitative agreement with various XPCS studies on polymeric systems where stretched exponentials together with p<2 suggesting super-diffusive dynamics are reported frequently (Nogales & Fluerasu, 2016;Hoshino et al, 2013;Ruta et al, 2014;Conrad et al, 2015;Senses et al, 2017).…”

“…with the relaxation rate ÀðqÞ ¼ 1= c ðqÞ and relaxation time c ðqÞ. The q dependence of c / q Àp and the shape parameter provide information of the sample dynamics, such as diffusion (p ¼ 2; ¼ 1), ballistic motion (p ¼ 1) or cooperative dynamics ( ¼ 2) found in soft-matter glasses (Caronna et al, 2008;Conrad et al, 2015).…”

“…For soft-matter systems XPCS is used whenever the application of light-scattering methods is challenged. This includes, for example, glass formation and gelation of dense polymeric and colloidal systems (Guo et al, 2012;Kwaśniewski et al, 2014;Conrad et al, 2015;Zhang et al, 2017) or soft matter under shear (Leheny et al, 2015).…”

Structure and dynamics of multicellular assemblies measured by coherent light scattering

“…Light scattering is not limited to stucture identification and many light scattering-based methods have emerged in biology [10]. One of them is speckle analysis or dynamic light scattering (DLS), which was already well-established in soft matter [11][12][13]. In the multiple scattering regime, it has been used for functional imaging such as the measure of brain activity [14][15][16].…”

On the use of two-time correlation functions for X-ray photon correlation spectroscopy data analysis

“…The 2-TCF is very useful to analyse the dynamics of non-equilibrium systems (Sutton et al, 2003). The temporal fluctuations and the variance of the 2-TCF are also used to investigate dynamical heterogeneities in glassy systems through the analysis of higher-order correlations and multi-point dynamic susceptibilities [see Orsi et al (2012) and Conrad et al (2015) for recent XPCS work and references therein for details on the use of higher-order correlations to study dynamical heterogeneities].…”