Further Consideration of Viscoelastic Two Glass Transition Behavior of Nanoparticle-Filled Polymers

Abstract

Tsagaropoulos and Eisenberg [Macromolecules 1995, 28, 396; Macromolecules 1995, 28, 6067] reported a second loss tangent (tan δ) peak in temperature-dependent viscoelastic data for various un-cross-linked polymers filled with nanometer-sized silica particles. This peak, occurring at temperatures as much as 100 °C above the primary tan δ peak (glass-to-rubber softening transition), was ascribed to the glass transition of immobilized chains near the particles. This research is often cited as support for the existence of severely retarded segmental motion of polymer near the surfaces of small particles (glassy polymer shell). We offer a different interpretation of the results from Tsagaropoulos and Eisenberg, reinforced by our recent measurements on particle-filled polybutadiene and consideration of other literature data. Particles restrict the flow of some polymer chains, thus resulting in incomplete terminal relaxation, and partial cross-linking of unfilled polymers produces the same higher temperature tan δ peak. Polymer chains which are adsorbed onto filler surfaces are immobilized from a flow relaxation (chain diffusion/reptation) standpoint, but segmental relaxation (glass transition) is not substantially altered by small particles as a general rule.