Well-defined silica core-poly(vinyl pyrrolidone) shell nanoparticles: Interactions and multi-modal glass transition dynamics at interfaces

Interfacial interactions/dynamics were studied in silica core-poly(vinyl pyrrolidone) (PVP) shell nanoparticles with the shell thickness of 1-2 (monolayer), 2-5, 5-15, and 8-25 nm using far- and mid-IR spectroscopy and DSC activation analysis of the glass transition. Particles were prepared using a pseudo-liquid state procedure. Their geometry, structural organization and shell uniformity were characterized comprehensively by LTNA, AFM, QELS, FT-IR and densitometry techniques. As revealed, strong core-shell interfacial interactions, including hydrogen bonding and Lewis acid-base ones, resulted in the quite different, multi-modal glass transition dynamics in PVP nano shells. For 1-2 nm thick monomolecular PVP shell, only several “abnormal” modes, including constrained cooperative and non-cooperative as well as ultra-fast non-cooperative ones, constituted segmental dynamics in the glass transition; the latter covered the range from 70 to 230 °C, with varying apparent activation energy by one order of magnitude. Ultra-fast dynamic mode was caused by collapse of motional cooperativity (α- to β-relaxation transformation).