Viscoelasticity and microstructure of POSS-methyl methacrylate nanocomposites. Dynamics and entanglement dilution

Add time:07/23/2019    Source:sciencedirect.com

The linear viscoelastic behavior of molten hybrid nanocomposites poly [(propylmethacryl-heptaisobutyl-POSS)-co-methyl methacrylate], denoted POSSn-MMA, was investigated; POSS content varied up to n = 45 wt%. The nanocomposites were amorphous and ΔTg ≈ −20 K. The melts exhibited linear viscoelastic behavior, and master curves were constructed using time-temperature superposition (TTS) at Tref = Tg+50 K. The viscoelastic response covered terminal, rubber-like and transition regimes. Strikingly, POSS induced considerable fast dynamics with over three orders of magnitude reduction of terminal relaxation time τt. Furthermore, POSS induced entanglement dilution where the rubbery modulus Ge decreased three-fold (i.e., packing length p increased). The melt viscosity exhibited Newtonian and shear thinning regimes, and the zero-shear viscosity ηο decreased up to three orders of magnitude. The unusual viscosity reduction suggests that the viscoelastic response arises from (relatively weak) POSS-POSS associations and not chain entanglements as in linear polymers. Hence, shear would easily break the associations and rather low viscosity would be experienced. At Angstrom-scale POSS modified the structure of PMMA but did not crystallize, consistent with DSC results. At nm-scale POSS-POSS associations were detected by small-angle X-ray scattering, and the size of aggregates increased with POSS content.

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