Dynamics of Polymer Chains in Poly(ethylene oxide)/Silica Nanocomposites via a Combined Computational and Experimental Approach

Abstract

The dynamics of polymer chains in poly­(ethylene oxide)/silica (PEO/SiO2) nanoparticle nanohybrids have been investigated via a combined computational and experimental approach involving atomistic molecular dynamics simulations and dielectric relaxation spectroscopy (DRS) measurements. The complementarity of the approaches allows us to study systems with different polymer molecular weights, nanoparticle radii, and compositions across a broad range of temperatures. We study the effects of spatial confinement, which is induced by the nanoparticles, and chain adsorption on the polymer’s structure and dynamics. The investigation of the static properties of the nanocomposites via detailed atomistic simulations revealed a heterogeneous polymer density layer at the vicinity of the PEO/SiO2 interface that exhibited an intense maximum close to the inorganic surface, whereas the bulk density was reached for distances ∼1–1.2 nm away from the nanoparticle. For small volume fractions of nanoparticles, the polymer dynamics, probed by the atomistic simulations of low-molecular-weight chains at high temperatures, are consistent with the presence of a thin adsorbed layer that exhibits slow dynamics, with the dynamics far away from the nanoparticle being similar to those in the bulk. However, for high volume fractions of nanoparticles (strong confinement), the dynamics of all polymer chains were predicted slower than that in the bulk. On the other hand, similar dynamics were found experimentally for both the local β-process and the segmental dynamics for high-molecular-weight systems measured at temperatures below the melting temperature of the polymer, which were probed by DRS. These differences can be attributed to various parameters, including systems of different molecular weights and nanoparticle states of dispersion, the different temperature range studied by the different methods, the potential presence of a reduced-mobility PEO/SiO2 interfacial layer that does not contribute to the dielectric spectrum, and the presence of amorphous–crystalline interfaces in the experimental samples that may lead to a different dynamical behaviors of the PEO chains.

Heterogeneous Glass Transition Behavior of Poly(Ethylene oxide)/Silica Nanocomposites via Atomistic MD Simulations

“…30 Details of the simulation model and the equilibration protocol are given elsewhere. 31 Initial atomistic PEO/SiO 2 configurations at a high temperature (T = 400K) were obtained from a previous work. 31 The models were afterward subjected to MD cooling steps of 10 K in the isobaric-isothermal ensemble down to T = 150 K. For each of the investigated systems, cooling was performed for 3 uncorrelated configurations to improve statistics.…”

“…31 Initial atomistic PEO/SiO 2 configurations at a high temperature (T = 400K) were obtained from a previous work. 31 The models were afterward subjected to MD cooling steps of 10 K in the isobaric-isothermal ensemble down to T = 150 K. For each of the investigated systems, cooling was performed for 3 uncorrelated configurations to improve statistics. At each cooling step, MD runs of 10 ns were performed, after the first 5 ns of which the potential energy and the specific volume of the system were stabilized.…”

“…Details of the simulation model and the equilibration protocol are given elsewhere . Initial atomistic PEO/SiO 2 configurations at a high temperature ( T = 400K) were obtained from a previous work .…”

Distribution of Mechanical Properties in Poly(ethylene oxide)/silica Nanocomposites via Atomistic Simulations: From the Glassy to the Liquid State

“…The equilibrium structural and dynamic properties of PEO/SiO 2 systems have been extensively examined by both simulations 28,62,84−86 and experiments. 28,85,87,88 Our methodology for probing the distribution of local mechanical properties is based on a recent multiscale computational approach computing effective (per atom) stress and strain fields within atomistic model PNCs under an applied external field. 65,89 In the rest of the paper, we provide in Section 2 details about the model systems and the simulations focusing on the deformation process and the way we compute the local stress and strain fields.…”

The effect of dynamically heterogeneous interphases on the particle dynamics of polymer nanocomposites

“…Functionalizing nanoparticles through chain adsorption or grafted nanoparticles is a common strategy to control particle dispersion in polymer nanocomposites and their structure, dynamics and mechanical properties have been extensively investigated. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Relating the particle dynamics to the dynamics of polymer grafted chains has importance in understanding the composite dynamics; however, it can only be attained for matrix-free composites. The recent simulation study on grafted particles in unentangled matrixes showed that relaxations of monomers close to particle surfaces are slower than the chains farther away due to graft chain constraints and the confinement of neighboring chains.…”