Anomalous chain diffusion in unentangled model polymer nanocomposites

Abstract

We studied unentangled poly(ethylene-alt-propylene) (PEP) in a composite with hydrophobic silica particles as a function of the filler concentration. Our neutron spin echo (NSE) experiments cover both the internal dynamics as well as the center of mass diffusion beyond the Rouse time. The key experimental results are (i) all of the chains are equally mobile, (ii) the basic segmental (Rouse) relaxation rate is unaffected even at highest filler concentrations, and (iii) apparently the obstacles reduce significantly the translational center of mass motion. This happens, even in the case when the particles do not significantly confine the polymer. (iv) A transition from regular to anomalous diffusion in the Rouse regime at the highest particle fractions is clearly evidenced. In order to understand the microscopic mechanisms underlying the experimental observations, we performed coarse grained simulations. We demonstrate that the geometrical confinement only affects the dynamics at a long time scale outside the experimental window and therefore it is not able to explain the results found in the NSE experiments. The consideration of inter-chain interactions, however, results in a significant influence even at shorter times and a quantitative agreement between the experiments and simulations was found. The simulations clearly demonstrate that the interfaces cause a deceleration of the chains in their close vicinity. Then the interchain interactions carry this slowing down to the other chains at a time-scale of the Rouse relaxation time. Hence, in the experimental datasets an overall slowing down is observed.

Polymer and spherical nanoparticle diffusion in nanocomposites

“…The confined polymer was found to be surprisingly mobile and did not exhibit glassy behavior 29,30 . For unentangled poly(ethylene-alt-propylene) (PEP) there is a strong deceleration of the polymer center of mass motion, but even at the highest silica volume fraction, the segmental chain dynamics is not affected 28 .…”

“…The tracer diffusivity decreases across a wide range of polymer molecular weight, nanoparticle size 23,24 , and unexpectedly decreases more strongly at higher temperatures 25 . In addition, Richter and co-workers explored the dynamics of entangled 26,27 and unentangled poly(ethylene-altpropylene) (PEP) polymers 28 in nanocomposites by neutron spin echo (NSE) experiments. For entangled polymers it was found that the initial Rouse relaxation rate was unaffected by the hydrophobic silica nanoparticles.…”

Interphases in Polymer Solid-Contacts and Nanocomposites Probed by Positron Annihilation Lifetime Spectroscopy

“…Because of a finite center of mass diffusion and a low radius of gyration, the interstices between particles were effectively filled and hence a most suitable model system without interphase was obtained. The absence of an interphase was confirmed by neutron scattering and neutron spin echo measurements (38,40). DSC experiments showed a constant glass S138 Ohrt et al transition temperature T g and a decrease in c p at T g with increasing filler concentration as expected from a simple mixing rule (26).…”

“…Within a joint effort, we aim to understand the properties of polymers in nanocomposites using a collection of techniques (9,26,27,(37)(38)(39)(40)(41). In the present paper, we focus on poly(ethylene-altpropylene) PEP as soft phase.…”

“…In the present paper, we focus on poly(ethylene-altpropylene) PEP as soft phase. The synthesis and properties are described in detail in the literature (26,(38)(39)(40)(41).…”