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.
