Chain disentanglements and oxygen transmission reduction in LDPE/POSS nanocomposites. Influence of POSS size

It was recently shown that molecularly dispersed octaisobutyl-polyhedral oligomeric silsesquioxane (bu-POSS) nanoparticles acted like nanoplugs and reduced O-2 transmission in low-density polyethylene (LDPE) (RomoUribe, A.; et al. Polymer 2019, 165, 61-71, DOI : 10.1016/j.polymer.2019.01.024). Here, the influence of alkyl group on O-2 transmission and chain dynamics was investigated by dispersing octamethyl-POSS nanoparticles (me-POSS) in LDPE. me-POSS was melt mixed with LDPE by twin-screw extrusion. High-resolution transmission electron microscopy (HRTEM) and wide-angle X-ray scattering showed that me-POSS is of ca. 0.8 nm size (bu-POSS size is ca. 1.5 nm), and at concentration c(POSS) <= 5 wt % me-POSS was effectively dispersed into LDPE at a single unit. The chain dynamics was studied by small-strain oscillatory shear and the time-temperature superposition (TTS) principle was used to construct master curves at T-ref = 190 degrees C. me-POSS nanoparticles induced chain disentanglements and reduction of terminal relaxation time tau(t), which translated into an anomalous, non-Einstein-like reduction of zero-shear rate viscosity eta(o). However, me-POSS produced shorter terminal relaxation time tau(t) and induced much greater reduction of eta(o) than bu-POSS. These results suggest that the size of me-POSS is key for inducing faster dynamics and greater viscosity reduction than bu-POSS. Consequently me-POSS intercalated the entangled molecular mesh and produced a nanoplug effect reducing O-2 transmission. However, ca. 5 wt % me-POSS was required to reduce O-2 transmission against only ca. 2 wt % bu-POSS, hence reflecting the influence of POSS size.