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Viscoelastic Behavior of Unentangled POSS–Styrene Nanocomposites and the Modification of Macromolecular Dynamics

Romo-Uribe, Angel
Macromolecules 2017 v.50 no.18 pp. 7177-7189
activation energy, differential scanning calorimetry, glass transition temperature, melting, molecular weight, nanocomposites, polystyrenes, viscoelasticity, viscosity
The viscoelastic properties of molten unentangled hybrid nanocomposites poly[(propylmethacryl-heptaisobutyl-POSS)-co-styrene], denoted POSS-sty, were investigated; POSS content varied up to 45 wt %. Unentangled polystyrene (PS) was also investigated. Differential scanning calorimetry (DSC) showed that the glass transition temperature, Tg, significantly decreased relative to neat PS, i.e., ΔTg < 0, and this effect was not associated with molecular weight. Master curves were constructed using the time–temperature superposition (TTS) principle. The shift factors obeyed Arrhenius-type relationship, and the calculated flow activation energy Eₐ first increased and then rapidly decreased as POSS content increased. The viscoelastic spectra of PS and the nanocomposites exhibited only the terminal regime (G′′ > G′), whereas PS exhibited the typical Rouse behavior; POSS-sty melts did not obey the scaling G′′ ∼ ω². Strikingly, the mechanical damping tan δ exhibited minima at the longest relaxation time, not seen in melts of unentangled flexible polymers. This suggests (weak) interactions/associations between the bulky POSS groups giving rise to dynamics retardation. The fractional free volume fg exhibited considerable increase, and the zero-shear viscosity η₀ decreased an order of magnitude, relative to the neat polymer. Analysis of several POSS-based nanocomposites showed that ΔTg was positive or negative, in all cases produced viscosity reduction, and molecular weight was not a factor. As entanglements effects were removed in this styudy, the results suggest a modified polymer dynamics driven by interactions/associations of the bulky POSS macromers, thus opening opportunities for tunability of bulk properties in nanocomposites and the development of advanced functional materials.