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

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, W m, both below and above the entanglement threshold (W m,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 W m ≈ 40k (i.e. ≫W m,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.

Gradient of Segmental Dynamics in Stereoregular Poly(methyl methacrylate) Melts Confined between Pristine or Oxidized Graphene Sheets

“…Dielectric relaxation spectroscopy experiments have also been extensively used to study the interfacial polymer dynamics, particularly the interfacial segmental dynamics. Quantities like the thickness of the interfacial polymer layer, the relaxation times, and local T g at the interfacial layer, and the temperature dependence of the interfacial dynamics have been studied by dielectric relaxation spectroscopy measurements [ 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 ]. The results of dielectric spectroscopy experiments are discussed in more detail in the main text and are compared to the simulation results.…”

Nanostructured Amorphous Silicas Hydrophobized by Various Pathways

“…There is a general tendency for an increasing θ value (corresponding to better hydrophobization 62−65 ) with an increase in the reaction temperature from 100 to 250 °C (Figures S11–S14). However, this effect could be decreased with increasing length of PDMS, for example, for AP2D, θ = 0.67 and 0.86, and for AP5D, θ = 0.60 and 0.63 at the reaction temperature of 100 and 200 °C, respectively (Figure S11).…”

“…Note that different aspects of the interactions of PDMS-hydrophobized silicas with water, as well as other adsorbates, have been analyzed elsewhere. 41,50,62−65 Typically, uniform hydrophobic surfaces, for example, modified by PDMS fragments, are characterized by a contact angle for settled water drops not greater than θ c = 120°. Nano/microstructured hydrophobic particles (structures) can provide much larger θ c —up to 170° due to a surface geometry factor.…”

Siloxanes—Versatile Materials for Surface Functionalisation and Graft Copolymers

“…A continuation of this work, limited to the use of dimethyl carbonate and PDMS to functionalise silica nanoparticles, was carried out, focusing on the adsorption of PDMS oil on the modified nanoparticles and on the molecular dynamics of the obtained systems [ 25 ]. PDMS with various molecular weights were used (4–40·kg/mol), yielding a total coverage of the silica grain surfaces in the case of low-MW PDMS and a lower coverage (≈60%) for high-MW (molecular weight) PDMS.…”