Revealing the Shear Effect on the Interfacial Layer in Polymer Nanocomposites through Nanofiber Reorientation

For polymer nanocomposites with attractive particle–polymer interactions, the interfacial layer consists of anchored and interpenetrating unanchored chains. The interfacial layer increases the effective hydrodynamic size of the nanoparticles and plays a critical role in mechanical reinforcement. Although it is clear that shear flow can lead to bonding-debonding of adsorbed chains, the effect of fast flow on the interfacial layer remains elusive. In this work, we adopted nanofiber-filled polymer nanocomposites with attractive fiber-polymer interactions to reveal the shear effect on the interfacial layer. We found a resting time-dependent stress overshoot in the reversal shear step of the preshear-resting-reversal shear protocol. Such a phenomenon disappeared either when nanofibers were surface-treated to reduce the attractive interaction or when the polymer matrix was replaced with one without attractive interactions. We ascribed the stress overshoot in the reversal shear to the collision and reorientation of nanofibers, and the decrease of overshoot strain with the resting time resulted from the decrease of nanofibers’ aspect ratio instead of Brownian motion and the relaxation of stretched interfacial chains. Because of the retarded relaxation in the interfacial layer, weak shear was sufficient to disentangle the nonadsorbed chains from the adsorbed ones, while a slow process was needed for the free chains re-interpenetrate, whose characteristic time matched the re-entanglement time of free polymer chains.