POSS driven chain disentanglements, decreased the melt viscosity and reduced O2 transmission in polyethylene

Abstract

Untethered octaisobutyl-polyhedral oligomeric silsesquioxane (POSS) molecularly dispersed into low-density polyethylene (LDPE) induced entanglement dilation and dynamics acceleration, reduced the melt viscosity and acted as nano-plugs thus reducing O2 transmission. POSS nanoparticles were melt mixed with LDPE using a twin-screw extruder and adding up to 10 wt% POSS. High-resolution transmission electron microscopy (HRTEM) demonstrated excellent dispersion of POSS into LDPE, at a single POSS unit at low concentrations. The dynamics was probed by shear rheometry constructing master curves at Tref = 190 °C using the time-temperature superposition (TTS) principle. In the low concentration range (cPOSS<5 wt%) the terminal (liquid-like) regime showed that POSS nanoparticles induced accelerated dynamics, decreasing the terminal relaxation time τt. This translated into an anomalous, non-Einstein, reduction of viscosity. Increasing concentration to c∼10 wt% POSS formed clusters of ca. 10 units, the terminal relaxation time increased and the melt exhibited higher viscosity than the neat LDPE, as predicted for particulate filled suspensions. Therefore, at low concentrations the size of POSS (ca. 1.5 nm), smaller than entanglement mesh size of polyethylene (tube diameter dt = 3.28 nm), would drive chain disentanglement and be responsible for the dilution effect. At higher concentrations POSS aggregated and therefore increased the viscosity of the nanocomposite, a behavior akin to Einstein's prediction. Strikingly, the intercalation of POSS into the entangled molecular mesh reduced oxygen transmission, suggesting that POSS acted as nano-plugs by filling the voids in the entangled mesh.