Dynamics and Internal Stress at the Nanoscale Related to Unique Thermomechanical Behavior in Polymer Nanocomposites

Abstract

A small amount of alumina nanoparticles in polymethylmethacrylate causes a sharp depression of the glass transition temperature (Tg) accompanied by a toughening of the composite. We investigated this phenomenon using multispeckle x-ray photon correlation spectroscopy. Measurements reveal a dynamic structure factor that has the form exp[-(t/taua)beta], with beta greater than 1. We show for the first time that beta(T) tracks the internal stress at the polymer-particle interface. The internal stress, which we propose arises due to the entropic penalty that the polymer faces in the presence of the nanoparticles, engenders temporally heterogeneous dynamics. In the jammed glassy state, we show that the dominant fast relaxation mode--taumax--aided by a weak dewetting interface relieves the stress and follows the variations in Tg.

Analytical Description of Degradation-Relaxation Transformations in Nanoinhomogeneous Spinel Ceramics

“…In case of photo-switchable α-helix [14], the change in the folding kinetics from SER (with β  = ~0.7) to CER (with β  = ~1.3) was explained by protocol of experimental measurements performed with respect to most sensitive employed observables (so the stronger downhill force results in larger stretching factor β and correspondingly more compressed relaxation kinetics). In polymer nanocomposites such as Au [6] or Al 2 O 3 [7] nanoparticles embedded in polymethylmethacrylate, the CER kinetics with non-exponentiality index β close to 1.4–1.9 (in dependence on nanoparticle content) appears at lowering the temperature below glass transition. It is noteworthy that the same order of β (which serves as a measure of internal stress) defined by stretched-to-compressed exponential kinetics crossover for our thermally relaxing thick-film-Ag system, photo-switchable folded α-helix [14], or polymer nanocomposites [6, 7] testifies in a favor of close similarity in the responsible phenomenological models.…”

“…[1–7]. These materials realize their functionality due to specific structural inhomogeneities frozen at atomic and/or sub-atomic length scales, thus significantly modifying their physical-chemical properties, as well as materials’ response to different degradation-relaxation inputs.…”

Rheological and structural details of biocidal iPP-TiO2 nanocomposites

“…Insight about how nanoparticles influence the associated morphological structure [10][11][12] and system dynamics [13][14][15] of polymer-nanoparticle mixtures is only beginning to emerge, and the advancement of knowledge in these areas will be a key factor to develop design rules to engineering materials with desired properties. The study of the rheological behavior of these polymeric nanocomposites with extremely good biocidal properties turns out to be necessary in order to learn how different their processability can be in comparison with that exhibited by the neat iPP homopolymer.…”

Load relaxation behavior of a Zr41.2Ti13.8Cu12.5Ni10 Be22.5 bulk metallic glass

“…where s c and b represent the critical stress relaxation time and a stretched exponential parameter indicating multiple relaxation processes, respectively [35,36]. The linear relationship between log (r(t)/r 0 ) and log t in the second relaxation step, on the other hand, can be formulated as a simple power law,…”