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Morphology, Molecular Dynamics, and Interfacial Phenomena in Systems Based on Silica Modified by Grafting Polydimethylsiloxane Chains and Physically Adsorbed Polydimethylsiloxane

Author:
Klonos, Panagiotis A., Goncharuk, Olena V., Pakhlov, Eugeniy M., Sternik, Dariusz, Deryło-Marczewska, Anna, Kyritsis, Apostolos, Gun’ko, Volodymyr M., Pissis, Polycarpos
Source:
Macromolecules 2019 v.52 no.7 pp. 2863-2877
ISSN:
1520-5835
Subject:
adsorption, dielectric spectroscopy, differential scanning calorimetry, glass transition, models, molecular dynamics, molecular weight, nanocomposites, nanoparticles, nitrogen, polydimethylsiloxane, silica
Abstract:
This study deals with effects on structure, molecular mobility, and interfacial polymer–nanoparticles (NPs), arising from modification of silica NPs by grafting of small polydimethylsiloxane (PDMS) chains, via siloxane bond breaking, in combination with polymer molecular weight, Wₘ, both below and above the entanglement threshold (Wₘ,ₑ ≈ 8k). According to infrared (IR), upon grafting of PDMS, the coverage of surface silica hydroxyls is almost complete (100%) and uniform for short chains, whereas it is lower, ∼60%, for longer chains (non-uniform). The combination of results by nitrogen adsorption–desorption and calorimetry (differential scanning calorimetry, DSC) indicates that the nanoparticle surface accessibility by the polymer is dynamic, as it changes non-monotonically with the gradual increasing of polymer adsorption. The grafted chains show no mobility (rigid or dead layer) neither by DSC nor by broadband dielectric spectroscopy (BDS), due to the severe fragmentation during the siloxane breaking process. On the contrary, upon physical adsorption of PDMS of Wₘ ≈ 40k (i.e. ≫Wₘ,ₑ) onto both modified and unmodified NPs, the polymer exhibits both bulk (glass transition, α relaxation) and retarded interfacial dynamics in BDS (αᵢₙₜ relaxation). For adsorbed PDMS > 40%, the bulk-like dynamics is governed by entanglements between bulk polymer chains, whereas for PDMS < 40% by polymer–particles interactions and the presence of interfacial polymer. Comparing to previous findings in similar nanocomposites, αᵢₙₜ exhibits relatively short relaxation times and a non-cooperative character, both independent from the type of particles grafting. αᵢₙₜ exhibits a large strength, which is, however, reduced for the grafted particles. Adopting here previously applied models, effects were explained in terms of formation of interfacial PDMS loops of large height (origin of large dielectric strength) and eliminated ability for cooperative motions due to interfacial chain entanglements.
Agid:
6365261